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In two-moment bulk microphysics schemes, the practice of using different weighted fall velocities for the various moments is known to lead to artificial growth in reftectivity values for fast-falling particles, particularly at the downward leading edge of a precipitation column. Two simple correction schemes that prevent these artifacts while still allowing some effects of size sorting are presented. The corrections are obtained by comparing particle number concentrations that result from two or three different sedimentation calculations. The corrections do not conserve particle number concentrations but do prevent spurious reflectivity growth automatically without the need to place ad hoc limits on mean particle size. Multimoment bulk microphysics schemes often have used inconsistent variables in terms of the appropriate advection equation (e.g., mass mixing ratio and particle number concentration). A brief review of consistent advection and turbulent mixing for such variables is presented to provide clarification. DOI: 10.1175/2010JAS3341.1

In previous work, it was argued that a source of radar coherent scatter occurs in the direction perpendicular to the direction of wave propagation because of the presence of grids of enhanced particle concentrations with spatial periodicities in resonance with the radar wavelength. While convincing, the evidence thus far has been indirect. In this work the authors now present direct observations of radar coherent backscattered signals in precipitation in the direction of wave propagation. The theory is developed for the cross-correlation function of the complex amplitudes in the direction of propagation calculated for nearest neighbor range bins. Data are analyzed in snow and in rain. The results agree with the earlier conclusions in the previous work, namely that coherent scatter occurs in both rain and snow, that it is larger in snow than it is in rain, and that it can be significant at times. DOI: 10.1175/2010JAS3488.1

Classical radar theory only considers incoherent backscatter from precipitation. Can precipitation generate coherent scatter as well? Until now, the accepted answer has been no, because hydrometeors are distributed sparsely in space (relative to radar wavelength) so that the continuum assumption used to explain coherent scatter in clear air and clouds does not hold. In this work, a theory for a different mechanism is presented. The apparent existence of the proposed mechanism is then illustrated in both rain and snow. A new power spectrum Z(f), the Fourier transform of the time series of the radar backscattered reflectivities, reveals statistically significant frequencies f of periodic components that cannot be ascribed to incoherent scatter. It is shown that removing those significant fs from Z(f) at lower frequencies greatly reduces the temporal coherency. These lower frequencies, then, are associated with the increased temporal coherency. It is also shown that these fs are also directly linked to the Doppler spectral peaks through integer multiples of one-half the radar wavelength, characteristic of Bragg scatter. Thus, the enhanced temporal coherency is directly related to the presence of coherent scatter in agreement with theory. Moreover, the normalized backscattered power spectrum Z(f) permits the estimation of the fractional coherent power contribution to the total power, even for an incoherent radar. Analyses of approximately 26 000 one-second Z(f) in both rain and snow reveal that the coherent scatter is pervasive in these data. These findings present a challenge to the usual assumption that the scatter of radar waves from precipitation is always incoherent and to interpretations of backscattered power based on this assumption. DOI: 10.1175/2010JAS3336.1

S-band radar surveillance scans of precipitating shallow convection are analyzed. The scans are complied from 52 days of near-continuous measurements in the winter trades of the North Atlantic during the Rain in Cumulus over the Ocean (RICO) field campaign. After being analyzed and filtered to exclude spurious returns, the scans are segmented to identify contiguous returns, or echoes. The echo size and reflectivity statistics are then analyzed. A new normalization method is developed to account for biases associated with the nonuniformity in the native radar grid. The echo area distribution robustly exhibits power-law scaling up until sizes of about 10 [km.sup.2], with a scaling exponent of about -1.1. At larger sizes the scaling behavior breaks down and varies more markedly across subsamples of the data. Conditional sampling suggests that the scaling behavior of the larger echoes does, however, approach that of the smaller echoes as echo coverage increases, which supports the idea of a limiting distribution. Departures from this limiting distribution are argued to reflect finite size effects, modulated by the presence of a capping inversion whose height and strength varies across the samples. DOI: 10.1175/2009JAS3178.1

The 'natural laboratory' of mountainous Dominica (15[degrees]N) in the trade wind belt is used to study the physics of tropical orographic precipitation in its purest form, unforced by weather disturbances or by the diurnal cycle of solar heating. A cross-island line of rain gauges and 5-min radar scans from Guadeloupe reveal a large annual precipitation at high elevation (7 m [yr.sup.-1]) and a large orographic enhancement factor (2 to 8) caused primarily by repetitive convective triggering over the windward slope. The triggering is caused by terrain-forced lifting of the conditionally unstable trade wind cloud layer. Ambient humidity fluctuations associated with open-ocean convection may play a key role. The convection transports moisture upward and causes frequent brief showers on the hilltops. The drying ratio of the full air column from precipitation is less than 1% whereas the surface air dries by about 17% from the east coast to the mountain top. On the lee side. a plunging trade wind inversion and reduced instability destroys convective clouds and creates an oceanic rain shadow.

Results from a 20-yr simulation of a high-resolution AGCM forced with climatological SST, along with simplified model experiments and supplementary data diagnostics, are used to investigate internal feedbacks arising from monsoon-midlatitude interactions during droughts in the Indian summer monsoon. The AGCM simulation not only shows a fairly realistic mean monsoon rainfall distribution and large-scale circulation features but also exhibits remarkable interannual variations of precipitation over the subcontinent, with the 20-yr run showing incidence of four 'monsoon droughts.' The present findings indicate that the internally forced droughts in the AGCM emanate largely from prolonged 'monsoon breaks' that occur on subseasonal time scales and involve dynamical feedbacks between monsoon convection and extratropical circulation anomalies. In this feedback, the suppressed monsoon convection is shown to induce Rossby wave dispersion in the summertime subtropical westerlies and to set up ah anomalous quasi-stationary circulation pattern extending across continental Eurasia in the middle and upper troposphere. This pattern is composed of a cyclonic anomaly over west central Asia and the IndoPakistan region, a meridionally deep anticyclonic anomaly over East Asia (~100 [degrees]E). and a cyclonic anomaly over the Far East. The results suggest that the anchoring of the west central Asia cyclonic anomaly by the stagnant ridge located downstream over East Asia induces anomalous cooling in the middle and upper troposphere through cold-air advection, which reduces the meridional thermal contrast over the subcontinent. Additionally, the intrusion of the dry extratropical winds into northwest India can decrease the convective instability, so that the suppressed convection can in turn weaken the monsoon flow. The sustenance of monsoon breaks through such monsoon-midlatitude feedbacks can generate droughtlike conditions over India.

Large-eddy simulations are used to explore the structure and mesoscale organization of precipitating stratocumulus. The simulations incorporate a simple, two-moment, bulk representation of microphysical processes, which by varying specified droplet concentrations allows for comparisons of simulations that do and do not develop precipitation. The boundary layer is represented over a large (25.6 km x 25.6 km) horizontal domain using a relatively fine mesh, thereby allowing for the development of mesoscale circulations while retaining an explicit representation of cloud radiative, dynamical and microphysical interactions on scales much smaller than the dominant eddy scale. Initial conditions are based on measurements made as part of the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMSII). The simulations show that precipitation is accompanied by sharp reductions in cloudiness and changes in flow topology. Mesoscale features emerge in all of the simulations but are amplified in the presence of drizzle. A cloud albedo of near 75% in the nonprecipitating simulation is reduced to less than 35% in the precipitating case. The circulation transitions from a well-mixed, stationary stratocumulus layer with closed-cellular cloud planforms to a stationary cumulus-coupled layer, with incipient open-cellular cloud planforms and sustained domain-averaged surface precipitation rates near 1 mm [day.sup.-1]. The drizzling simulations embody many other features of observed precipitating stratocumulus, including elevated cloud tops in regions of precipitation and locally higher values of subcloud equivalent potential temperature. The latter is shown to result from the tendency for precipitating simulations to develop greater thermodynamic gradients in the subcloud layer as well as mesoscale circulations that locate regions of upward motion in the vicinity of precipitating cells.

This study investigates the effects of aerosol on clouds, precipitation, and the organization of trade wind cumuli using large eddy simulations (LES). Results show that for this shallow-cumulus-under-stratocumulus case, cloud fraction increases with increasing aerosol as the aerosol number mixing ratio increases from 25 (domain-averaged surface precipitation rate ~0.65 mm [day.sup.-1]) to 100 [mg.sup.-1] (negligible surface precipitation). Further increases in aerosol result in a reduction in cloud fraction. It is suggested that opposing influences of aerosol-induced suppression of precipitation and aerosol-induced enhancement of evaporation are responsible for this nonmonotonic behavior. Under clean conditions (25 [mg.sup.-1]), drizzle is shown to initiate and maintain mesoscale organization of cumulus convection. Precipitation induces downdrafts and cold pool outflow as the cumulus cell develops. At the surface, the center of the cell is characterized by a divergence field, while the edges of the cell are zones of convergence. Convergence drives the formation and development of new cloud cells, leading to a mesoscale open cellular structure. These zones of new cloud formation generate new precipitation zones that continue to reinforce the cellular structure. For simulations with an aerosol concentration of 100 mg the cloud fields do not show any cellular organization. On average, no evidence is found for aerosol effects on the lifetime of these clouds, suggesting that cloud fraction response to changes in aerosol is tied to the frequency of convection and/or cloud size.

Cloud droplet effective radius (DER) and liquid water path (LWP) are two key parameters for the quantitative assessment of cloud effects on the exchange of energy and water. Chang and Li presented an algorithm using multichannel measurements made at 3.7, 2.1, and 1.6 [micro]m to retrieve a cloud DER vertical profile for improved cloud LWP estimation. This study applies the multichannel algorithm to the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) data on the Aqua satellite, which also carries the Advanced Microwave Scanning Radiometer (AMSR-E) for measuring cloud LWP and precipitation. By analyzing one day of coincident MODIS and AMSR-E observations over the tropical oceans between 40[degrees]S and 40[degrees]N for overcast warm clouds (>273 K) having optical depths between 3.6 and 23, the effects of DER vertical variation on the MODIS-derived LWP are reported. It is shown that the LWP tends to be overestimated if the DER increases with height within the cloud and underestimated if the DER decreases with height within the cloud. Despite the uncertainties in both MODIS and AMSR-E retrievals, the result shows that accounting for the DER vertical variation reduces the mean biases and root-mean-square errors between the MODIS- and AMSR-E-derived LWPs. Besides, the manner in which the DER changes with height has the potential for differentiating precipitative and nonprecipitative warm clouds. For precipitating clouds, the DER at the cloud top is substantially smaller than the DER at the cloud base. For nonprecipitating clouds, however, the DER differences between the cloud top and the cloud base are much less.

A method of analyzing radar data is developed and applied to determine whether the X-band radar reflectivity evolution of clouds observed during summertime on the northeast Florida coast during the Small Cumulus Microphysics Study (SCMS) shows distinct differences in precipitation development that can be associated with the clouds' maritime or continental characteristics. For this study, the entire National Center for Atmospheric Research CP2 radar dataset from SCMS was examined, and 38 clouds were used. For these clouds the evolution in X-band radar reflectivity, from the clouds' earliest detection through precipitation, was clearly documented and met specific requirements concerning the clouds' location relative to the coastline and direction of movement. Since cloud condensation nuclei (CCN) and giant and ultragiant nuclei (GN) measurements were not available for the specific clouds used in this study, proxies were used to partition the clouds into four groups based on the cloud location and direction of movement. Specifically, it was assumed that clouds forming over the ocean during onshore flow had maritime characteristics (group 1: low CCN, high GN), clouds forming over land during onshore flow would have modified maritime characteristics (group 2: high CCN, high GN), clouds forming over land during offshore flow would have continental characteristics (group 3: high CCN, low GN), and clouds forming over the ocean during offshore flow would have modified continental characteristics (group 4: high CCN, high GN). These assumptions are based on past measurements presented by Sax and Hudson. Then, these populations were statistically compared using the nonparametric multiresponse permutation procedure developed by Mielke et al. A comparison of groups 1 and 2 provided a test of the role of CCN concentrations in precipitation development in these cloud populations. A comparison of groups 3 and 4 provided a test of the role of GN concentrations in precipitation development in these cloud populations. The two cloud populations that were disjoint at a statistically significant level were groups 1 and 2. For these groups, the analysis showed that the median characteristic total water content of the truly maritime clouds (group 1) was about half that of the modified maritime clouds (group 2) at the time of precipitation formation. The characteristic time to precipitation formation was about 60% smaller for the truly maritime clouds. Thus, the characteristic reflectivity threshold for precipitation development was reached at a much lower altitude above cloud base in a much faster time in the truly maritime clouds. This result supports the conclusions of Hudson and Yum that precipitation development in the SCMS clouds was primarily controlled by CCN concentrations rather than GN concentrations.

Ground-based vertically pointing and airborne/spaceborne nadir-pointing millimeter-wavelength radars are being increasingly used worldwide. Though such radars are primarily designed for cloud remote sensing, they can also be used for precipitation measurements including snowfall estimates. In this study, modeling of snowfall radar properties is performed for the common frequencies of millimeter-wavelength radars such as those used by the U.S. Department of Energy's Atmospheric Radiation Measurement Program ([K.sub.a] and W bands) and the CloudSat mission (W band). Realistic snowflake models including aggregates and single dendrite crystals were used. The model input included appropriate mass--size and terminal fall velocity--size relations and snowflake orientation and shape assumptions. It was shown that unlike in the Rayleigh scattering regime, which is often applicable for longer radar wavelengths, the spherical model does not generally satisfactorily describe scattering of larger snowflakes at millimeter wavelengths. This is especially true when, due to aerodynamic forcing, these snowflakes are oriented primarily with their major dimensions in the horizontal plane and the zenith/nadir radar pointing geometry is used. As a result of modeling using the experimental snowflake size distributions, radar reflectivity--liquid equivalent snowfall rates ([Z.sub.e]-S) relations are suggested for 'dry' snowfalls that consist of mostly unrimed snowflakes containing negligible amounts of liquid water. Owing to uncertainties in the model assumptions, these relations, which are derived for the common [K.sub.a]- and W-band radar frequencies, have significant variability in their coefficients that can exceed a factor of 2 or so. Modeling snowfall attenuation suggests that the attenuation effects in 'dry' snowfall can be neglected at the [K.sub.a] band for most practical cases, while at the W band attenuation may need to be accounted for in heavier snowfalls observed at longer ranges.

The triggering of convective orographic rainbands by small-scale topographic features is investigated through observations of a banded precipitation event over the Oregon Coastal Range and simulations using a cloud-resolving numerical model. A quasi-idealized simulation of the observed event reproduces the bands in the radar observations, indicating the model's ability to capture the physics of the band-formation process. Additional idealized simulations reinforce that the bands are triggered by lee waves past small-scale topographic obstacles just upstream of the nominal leading edge of the orographic cloud. Whether a topographic obstacle in this region is able to trigger a strong rainband depends on the phase of its lee wave at cloud entry. Convective growth only occurs downstream of obstacles that give rise to lee-wave-induced displacements that create positive vertical velocity anomalies [w.sub.c] and nearly zero buoyancy anomalies be as air parcels undergo saturation. This relationship is quantified through a simple analytic condition involving [w.sub.c], [b.sub.c], and the static stability [N.sup.2.sub.m] of the cloud mass. Once convection is triggered, horizontal buoyancy gradients in the cross-flow direction generate circulations that align the bands parallel to the flow direction.

This study combines high-resolution mesoscale model simulations and comprehensive airborne Doppler radar observations to identify kinematic structures influencing the production and mesoscale distribution of precipitation and microphysical processes during a period of heavy prefrontal orographic rainfall over the Cascade Mountains of Oregon on 13-14 December 2001 during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field program. Airborne-based radar detection of precipitation from well upstream of the Cascades to the lee allows a depiction of terrain-induced wave motions in unprecedented detail. Two distinct scales of mesoscale wave-like air motions are identified: 1) a vertically propagating mountain wave anchored to the Cascade crest associated with strong midlevel zonal (i.e., cross barrier) flow, and 2) smaller-scale ( Errors in the model representation of a low-level shear layer and the vertically propagating mountain waves are analyzed through a variety of sensitivity tests, which indicated that the mountain wave's amplitude and placement are extremely sensitive to the planetary boundary layer (PBL) parameterization being employed. The effects of 1) using unsmoothed versus smoothed terrain and 2) the removal of upstream coastal terrain on the flow and precipitation over the Cascades are evaluated through a series of sensitivity experiments. Inclusion of unsmoothed terrain resulted in net surface precipitation increases of ~4%-14% over the windward slopes relative to the smoothed-terrain simulation. Small-scale waves (

Simulations of one maritime and four continental observed cases of deep convection are performed with the Hebrew University Cloud Model that has spectral bin microphysics. The maritime case is from observations made on 18 September 1974 during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE). The continental storm cases are those of summertime Texas clouds observed on 13 August 1999, and green-ocean, smoky, and pyro-clouds observed during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Smoke, Aerosols, Clouds, Rainfall, and Climate (LBA-SMOCC) campaign on 1-4 October 2002. Simulations have been performed for these cases with a detailed melting scheme. This scheme allows calculation of liquid water fraction within each mass bin for the melting of graupel, hail, snowflakes, and crystals, as well as alteration of the sedimentation velocity of ice particles in the course of their melting. The results obtained with the detailed melting scheme are compared with corresponding results from simulations involving instantaneous melting at the freezing (0[degrees]C) level. The detailed melting scheme allows penetration of ice from the freezing level down into the boundary layer by distances ranging from a few hundred meters for the numerous, smaller particles to ~1.5 km for the largest particles, which are much scarcer. In these simulations, most of the mass of ice falling out melts over this short distance of a few hundred meters. The deepening and intensification of the layer of latent cooling enhances the convective destabilization of the troposphere. This effect is especially pronounced under continental conditions, causing significant changes in the accumulated rain amount.

This article is the second of two describing convective lines with parallel stratiform (PS) precipitation. The PS mode appears to be the preferred organizational structure in environments with line-parallel vertical wind shear. This paper presents a detailed analysis of the processes that lead to the development of the PS structure within line-parallel shear, and the positive and negative feedbacks associated with the mature PS structure. As well, the particular importance of line-perpendicular and line-parallel wind shear, line-end effects, inertial stability, and patterns of convective initiation are investigated through a battery of sensitivity tests. Convective lines with PS precipitation develop in environments with both significant line-perpendicular and line-parallel vertical wind shear. Although the studied environments are initially supportive of supercells, the merging of outflows soon renders a predominant linear forcing and the characteristic PS structure. The systems' linearity in the presence of along-line wind shear makes the local wind field more dependent upon the mesoscale structure of the convective system. For example, the along-line transport of hydrometeors is required for the development of a line-parallel precipitation region, and yet this transport does not occur immediately down the convective line's axis because it is interrupted by the pressure maxima associated with other convective cells that are farther down the line. However, the along-line flow within the line's leading and trailing anvils is able to contribute substantially because there are along-line pressure gradient accelerations associated with the tilted mesoscale structure of the system's buoyancy field. This paper concludes the study by synthesizing its dynamical and sensitivity analyses with the overarching structures described in the companion article, yielding perhaps the first consolidated view of these little-studied systems.

The mesoscale organization of precipitating convection is highly relevant to next-generation global numerical weather prediction models, which will have an intermediate horizontal resolution (grid spacing about 10 km). A primary issue is how to represent dynamical mechanisms that are conspicuously absent from contemporary convective parameterizations. A hybrid parameterization of mesoscale convection is developed, consisting of convective parameterization and explicit convectively driven circulations. This kind of problem is addressed for warm-season convection over the continental United States. although it is argued to have more general application. A hierarchical strategy is adopted: cloud-system-resolving model simulations represent the mesoscale dynamics of convective organization explicitly and intermediate resolution simulations involve the hybrid approach. Numerically simulated systems are physically interpreted by a mechanistic dynamical model of organized propagating convection. This model is a formal basis for approximating mesoscale convective organization (stratiform heating and mesoscale downdraft) by a first-baroclinic heating couplet. The hybrid strategy is implemented using a predictor-corrector strategy. Explicit dynamics is the predictor and the first-baroclinic heating couplet the corrector. The corrector strengthens the systematically weak mesoscale downdrafts that occur at intermediate resolution. When introduced to the Betts-Miller-Janjic convective parameterization, this new hybrid approach represents the propagation and dynamical structure of organized precipitating systems. Therefore, the predictor-corrector hybrid approach is an elementary practical framework for representing organized convection in models of intermediate resolution.

During the 1998 and 2001 hurricane seasons of the western Atlantic Ocean and Gulf of Mexico, the Advanced Microwave Precipitation Radiometer (AMPR), the ER-2 Doppler (EDOP) radar, and the Lightning Instrument Package (LIP) were flown aboard the NASA ER-2 high-altitude aircraft as part of the Third Convection and Moisture Experiment (CAMEX-3) and the Fourth Convection and Moisture Experiment (CAMEX-4). Several hurricanes, tropical storms, and other precipitation systems were sampled during these experiments. An oceanic rainfall screening technique has been developed using AMPR passive microwave observations of these systems collected at frequencies of 10.7, 19.35, 37.1, and 85.5 GHz. This technique combines the information content of the four AMPR frequencies regarding the gross vertical structure of hydrometeors into an intuitive and easily executable precipitation mapping format. The results have been verified using vertical profiles of EDOP reflectivity and lower-altitude horizontal reflectivity scans collected by the NOAA WP-3D Orion radar. Matching the rainfall classification results with coincident electric field information collected by the LIP readily identifies convective rain regions within the precipitation fields. This technique shows promise as a real-time research and analysis tool for monitoring vertical updraft strength and convective intensity from airborne platforms such as remotely operated or uninhabited aerial vehicles. The technique is analyzed and discussed for a wide variety of precipitation types using the 26 August 1998 observations of Hurricane Bonnie near landfall.

The Keys Area Microphysics Project (KAMP), conducted as part of NASA's Fourth Convective and Moisture Experiment (CAMEX-4) in the lower Keys area, deployed a number of ground radars and four arrays of rain gauge and disdrometer clusters. Among the various instruments is an X-band dual-polarization Doppler radar on wheels (XPOL), contributed by the University of Connecticut. XPOL was used to retrieve rainfall rate and raindrop size distribution (DSD) parameters to be used in support of KAMP science objectives. This paper presents the XPOL measurements in KAMP and the algorithm developed for attenuation correction and estimation of DSD model parameters. XPOL observations include the horizontal polarization reflectivity [Z.sub.H], differential reflectivity [Z.sub.DR], and differential phase shift [[PHI].sub.DP]. Here, [Z.sub.H] and [Z.sub.DR] were determined to be positively biased by 3 and 0.3 dB, respectively. A technique was also applied to filter noise and correct for potential phase folding in [[PHI].sub.DP] profiles. The XPOL attenuation correction uses parameterizations that relate the path-integrated specific (differential) attenuation along a radar ray to the filtered-[[PHI].sub.DP] (specific attenuation) profile. Attenuation-corrected [Z.sub.H] and specific differential phase shift (derived from filtered [[PHI].sub.DP] profiles) data are then used to derive two parameters of the normalized gamma DSD model, that is, intercept ([N.sub.w]) and mean drop diameter ([D.sub.0]). The third parameter (shape parameter [mu]) is calculated using a constrained [mu]-A relationship derived from the measured raindrop spectra. The XPOL attenuation correction is evaluated using coincidental nonattenuated reflectivity fields from the Key West Weather Surveillance Radar-1988 Doppler (WSR-88D), while the DSD parameter retrievals are statistically assessed using DSD parameters calculated from the measured raindrop spectra. Statistics show that XPOL DSD parameter estimation is consistent with independent observations. XPOL estimates of water content and [N.sub.w] are also shown to be consistent with corresponding retrievals from matched ER-2 Doppler radar (EDOP) profiling observations from the 19 September airborne campaign. Results shown in this paper strengthen the applicability of X-band dual-polarization high resolution observations in cloud modeling and precipitation remote sensing studies.

The fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) is used to simulate Hurricane Bonnie at high resolution (2-km spacing) in order to examine budgets of water vapor, cloud condensate, and precipitation. Virtually all budget terms are derived directly from the model (except for the effects of storm motion). The water vapor budget reveals that a majority of the condensation in the eyewall occurs in convective hot towers, while outside of the eyewall most of the condensation occurs in weaker updrafts, indicative of a larger role of stratiform precipitation processes. The ocean source of water vapor in the eyewall region is only a very small fraction of that transported inward in the boundary layer inflow or that condensed in the updrafts. In contrast, in the outer regions, the ocean vapor source is larger owing to the larger area, counters the drying effect of low-level subsidence, and enhances the moisture transported in toward the eyewall. In this mature storm, cloud condensate is consumed as rapidly as it is produced. Cloud water peaks at the top of the boundary layer and within the melting layer, where cooling from melting enhances condensation. Unlike in squall lines, in the hurricane, very little condensate produced in the eyewall convection is transported outward into the surrounding precipitation area. Most of the mass ejected outward is likely in the form of small snow particles that seed the outer regions and enhance in situ stratiform precipitation development through additional growth by vapor deposition and aggregation. This study also examines artificial source terms for cloud and precipitation mass associated with setting to zero negative mixing ratios that arise from numerical advection errors. Although small at any given point and time, the cumulative effect of these terms contributes an amount of mass equivalent to 13% of the total condensation and 15%-20% of the precipitation. Thus, these terms must be accounted for to balance the model budgets, and the results suggest the need for improved model numerics.

Observations from the East Pacific Investigation of Climate (EPIC) 2001 field campaign are well suited for exploring the relationships among the diurnal cycle, mesoscale (10-100 km) structure, and precipitation in the stratocumulus region in the southeast Pacific. Meteorological time series and observations from a scanning C-band radar, vertically pointing cloud radar, and ceilometer, as well as satellite data, are used to show that drizzle is associated with increased variability in cloud and boundary layer properties compared to nondrizzling periods. The stratocumulus-topped boundary layer is typically well mixed at night, transitioning to less well mixed in the afternoon, with drizzle most frequently occurring in the early morning. Coherent patches of drizzle, or 'cells,' can have large areas with radar reflectivities of greater than 5 dBZ of up to about 100 [km.sup.2]. Individual cells have long lifetimes, up to 2 h, and appear to be replenished by moisture in the boundary layer.

This paper compares airborne in situ observations of cloud microphysical parameters with the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) simulations, using the Reisner-2 bulk microphysical parameterization, for a heavy precipitation event over the Oregon Cascades on 13-14 December 2001. The MM5 correctly replicated the extent of the snow field and the growth of snow particles by vapor deposition measured along aircraft flight tracks between altitudes of 4.9 and 6 km, but overpredicted the mass concentrations of snow. The model produced a broader number distribution of snow particles than observed, overpredicting the number of moderate-to-large-sized snow particles and underpredicting the number of small particles observed along the aircraft flight track. Over the mountain crest, the model overpredicted depositional growth of snow and mass concentrations of snow, but underpredicted the amount of cloud liquid water and conversion of snow to graupel. The misclassification of graupel as snow and excessive amounts of snow resulted in the model overpredicting precipitation on the lee slopes and in localized areas along the foothills of the Cascades. The model overpredicted cloud liquid water over the lower windward slopes and foothills, where accretion of cloud liquid water by rain was the primary precipitation-producing mechanism.

This paper describes the large-scale synoptic and mesoscale features of a major precipitation event that affected the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) study area on 13-14 December 2001. The fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) was used to simulate both the synoptic and mesoscale features of the storm. Extensive model verification was performed utilizing the wealth of observational assets available during the experiment, including in situ aircraft measurements, radiosondes, radar data, and surface observations. The 13-14 December 2001 storm system was characterized by strong low-level cross-barrier flow, heavy precipitation, and the passage of an intense baroclinic zone. The model realistically simulated the three-dimensional thermodynamic and kinematic fields, the forward-tilted vertical structure of the baroclinic zone, and the associated major precipitation band. Deficiencies in the model simulations included an attenuated low-level jet accompanying the middle-level baroclinic zone and the lack of precipitation associated with the surface front; NOAA P-3 aircraft in situ data indicated that the model required 1.33-km grid spacing to capture realistically the complex mesoscale forcing related to terrain features. Despite the relatively skillful portrayal of mesoscale and synoptic structures, the model overpredicted precipitation in localized areas on the windward slopes and over a broad area to the lee of the Oregon Cascades.

This manuscript describes numerical experiments investigating the influence of 2-30-km striplike heterogeneity on wet and dry convective boundary layers coupled to the land surface. The striplike heterogeneity is shown to dramatically alter the structure of the convective boundary layer by inducing significant organized circulations that modify turbulent statistics. The impact, strength, and extent of the organized motions depend critically on the scale of the heterogeneity A relative to the boundary layer height [z.sub.i]. The coupling with the land surface modifies the surface fluxes and hence the circulations resulting in some differences compared to previous studies using fixed surface forcing. Because of the coupling, surface fluxes in the middle of the patches are small compared to the patch edges. At large heterogeneity scales ([lambda][z.sub.i] ~18) horizontal surface-flux gradients within each patch are strong enough to counter the surface-flux gradients between wet and dry patches allowing the formation of small cells within the patch coexisting with the large-scale patch-induced circulations. The strongest patch-induced motions occur in cases with 4 < [lambda]/[z.sub.i] Velocity and scalar fields respond differently to variations of heterogeneity scale. The patch-induced motions have little influence on total vertical scalar flux, but the relative contribution to the flux from organized motions compared to background turbulence varies with heterogeneity scale. Patch-induced motions are shown to dramatically impact point measurements in a free-convective boundary layer. The magnitude and sign of this impact depends on the location of the measurement within the region of heterogeneity.

A new double-moment bulk microphysics scheme predicting the number concentrations and mixing ratios of four hydrometeor species (droplets, cloud ice, rain, snow) is described. New physically based parameterizations are developed for simulating homogeneous and heterogeneous ice nucleation, droplet activation, and the spectral index (width) of the droplet size spectra. Two versions of the scheme are described: one for application in high-resolution cloud models and the other for simulating grid-scale cloudiness in larger-scale models. The versions differ in their treatment of the supersaturation field and droplet nucleation. For the high-resolution approach, droplet nucleation is calculated from Kohler theory applied to a distribution of aerosol that activates at a given supersaturation. The resolved supersaturation field and condensation/deposition rates are predicted using a semianalytic approximation to the three-phase (vapor, ice, liquid) supersaturation equation. For the large-scale version of the scheme, it is assumed that the supersaturation field is not resolved and thus droplet activation is parameterized as a function of the vertical velocity and diabatic cooling rate. The vertical velocity includes a subgrid component that is parameterized in terms of the eddy diffusivity and mixing length. Droplet condensation is calculated using a quasi-steady, saturation adjustment approach. Evaporation/deposition onto the other water species is given by nonsteady vapor diffusion allowing excess vapor density relative to ice saturation.

The western and central United States experience a pronounced diurnal cycle in rainfall during the warm season. Over the higher terrain west of 105[degrees]W, most precipitation occurs in the afternoon, whereas the central United States experiences more nocturnal events. This coherent phase transition between the Rocky Mountains and the U.S. Great Plains is well defined for all warm seasons between 1996 and 2003, provided that the rainfall observations are remapped relative to the elevated terrain in the western United States prior to north south averaging. Due to the westward shift of the Continental Divide north of 42[degrees]N and its intersection with the warm season storm track for 2002, the diurnal coherence greatly improves after remapping the 2002 rainfall observations. This speaks to the long-range influence of orography on precipitation frequency and suggests that the primary east-west corridor of precipitation for an individual warm season intersects the cordillera over a fairly narrow latitude range.

Observations by Borys, Lowanthal, Cohn, and Brown in midlatitude orographic clouds show that for a given supercooled liquid water content, both the riming and the snowfall rates are smaller if the supercooled cloud has more cloud droplets as, for example, caused by anthropogenic aerosols. The climatic implication of this effect was studied in global climate model simulations by replacing the constant riming efficiency with a size-dependent one appropriate for planar crystals and aggregates, respectively. In the model simulations that use a size-dependent riming collection efficiency, the pollution-induced decrease in cloud droplet size causes a decrease in the riming rate in stratiform clouds despite larger liquid water contents in polluted clouds. Contrary to the above-mentioned observations, in all model simulations the snowfall rate increases because of feedbacks in the climate system. Anthropogenic aerosol particles increase the aerosol and cloud optical thickness, which reduces the solar radiation at the top of the atmosphere and the surface. This in turn causes a cooling in Northern Hemisphere midlatitudes that favors precipitation formation via the ice phase.

Simple numerical experiments using a two-dimensional model were conducted to investigate the diurnal variation of water vapor in the lee of a mountain range, which strongly affects nighttime precipitation. In the daytime, moisture increases around the convergence zone in the lee of the mountain were formed by thermally induced local circulations in moderate ambient wind. Then, a region of enhanced precipitable water vapor (PWV) was formed. The region of enhanced PWV was transported to the lee side of the mountain by the ambient wind and finally reached the plain during the night. The convective instability in the lower troposphere and the possibility of deep convection were controlled by the advection of water vapor in the lee of the mountain, especially by the position of the enhanced PWV region. As a result, the diurnal cycle of the convective instability showed a phase difference between the mountainous region and the plain. The profile of equivalent potential temperature tended to be unstable in the lee of the mountain at night and contributed to the production of nighttime precipitation. The propagation speed of the enhanced PWV region was roughly determined by the ambient wind speed. Some sensitivity experiments using bulk microphysics have suggested that the propagation speed of the enhanced PWV region is insensitive to the occurrence of a deep convection. The enhanced PWV region generated over a wider mountain range tends to travel farther, retaining its magnitude even until night. The water vapor transport process presented in this study agrees well with the actual diurnal variations of rainfall observed in many regions over land.

Satellite-derived brightness temperature has been used to estimate tropical precipitation. Ricciardulli and Garcia applied it to quantify forcing of atmospheric waves that are excited by tropical cumulus convection and propagate into the middle atmosphere. Because of the broad coverage of the satellite data, this method provides exclusively dense information on wave forcing and is especially valuable for middle-atmosphere modeling. However, the validity of the method has not been investigated, which is done in this study using radar-derived precipitation during the Tropical Ocean and Global Atmosphere Coupled Ocean--Atmosphere Response Experiment (TOGA COARE) field experiment. The method is shown to overestimate the variance of precipitation so that the wave forcing derived with it is too strong. The overestimation is most severe at coarse resolution, reaching nearly an order of magnitude at a grid scale of 2[degrees], which is comparable to typical resolutions of current global climate models. Although the comparison was made using data from a limited region in the western Pacific, it is suggested that the method overestimates wave forcing globally. The probability distribution of the mean radar precipitation on the meso-[beta] scale fits well to the gamma distribution, while that for the satellite-derived precipitation does not. The latter shows a high probability of extreme grid mean precipitation, and this contributes to the overestimation. The frequency spectra of radar and satellite precipitation showed some similarity in shape, but differences are evident at subdiurnal frequencies. In addition to the satellite method, the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI) is also investigated. GPI shows a similar, but better, performance to estimate wave forcing.

Laboratory measurements were made of the collision cross section for water drops freely falling in air to evaluate coalescence efficiencies for cloud drops of 55-105-[micro]m radius in a radius ratio range of 0.5-1.0. The charge on drops was minimized by control of the electric field at the location of drop formation and was further reduced using bipolar ions. The drop charge was below values expected in the developing stage of warm cloud precipitation and well below thresholds needed to promote coalescence. The horizontal distance between drop centers, measured from photographs, provided data on collision offsets that were used to determine coalescence efficiencies. Experiment uncertainty was calculated from numerical trials using random offsets with variance from measurement errors. The analysis demonstrated that the coalescence efficiency for collisions between cloud drops is certainly greater than 91% and likely greater than 95%. Because the present experiment shows that colliding cloud drops with minimal charge will coalesce, rather than bounce apart, the rate of growth of cloud drops by collision-coalescence can be calculated using available theoretical collision efficiencies based on the interactions of uncharged spheres.

Recent studies have led to the statistical characterization of the spatial (temporal) distributions of cloud (precipitation) particles as a doubly stochastic Poisson process. This paper arrives at a similar conclusion (larger-than-Poissonian variance) via the more fundamental route of statistical physics and significantly extends previous findings in several ways. The focus is on the stochastic structure in the spatial distribution of cloud particles. A new approach for exploring the stochastic structure of clouds is proposed using a direct relation between number density variance and the pair correlation function. In addition, novel counting diagrams, particularly useful for analyzing counts at low data rates, demonstrate droplet clustering and striking deviations from Poisson randomness on small (centimeter) scales. These findings are shown to agree with pair correlation functions calculated for droplet counts obtained from an aircraft-mounted cloud probe. Time series of the arrival of each droplet are used to bin the data evenly so as to avoid corruption of the statistics through the operations of multiplication and division. Furthermore, it is shown that statistically homogeneous series of particle counts exhibit super-Poissonian variance. Since it is not always practical or feasible to obtain such direct measurements, the possibility of studying cloud texture using a revival of the idea of coherent microwave scatter from cloud droplets is discussed, including a more complete interpretation of Bragg scatter that seems to explain some recent observations in clouds. Finally, the appearance of clustering and the subsequent geometric distribution of droplet counts are interpreted using basic considerations of turbulence.

The purpose of this study is to assess the relative importance of collision and coalescence efficiencies as reported in the literature in different drop size regimes for the development of precipitation via the condensation-coalescence process. The stochastic growth of cloud droplet distributions due to collection processes is studied using a detailed microphysical parcel model. The evolution of rainwater content ([L.sub.R]) and the radar reflectivity factor (Z) are plotted in order to trace the progress of transfer of cloud water into rainwater and determine the importance of droplet collection in different size ranges. The results indicate that the van der Waals forces are effective in enhancing droplet collision when the droplets are small and the distributions are narrow. Wake capture is negligible for clouds forming in a continental air mass with low liquid water contents. However, it is effective when coalescence becomes the dominant growth process and rainwater content has reached high values. When nonunity coalescence efficiencies are used, the drop growth and cloud water to rainwater conversion is reduced compared to the traditional unity coalescence efficiencies used in previous modeling studies. However, the major difference between the results using nonunity and unity coalescence efficiencies is due to the extrapolation of coalescence efficiencies measured in laboratory to size domains outside the domain of the measurements.

For M drop size categories, rain is frequently viewed simply as the superposition of M, statistically independent Poisson-distributed drop fluxes each described by its own mean concentration. Implicit in such a Poissonian model is the assumption of uncorrelated counts among the drops. However, it is well known that drop size distributions are the result of the processes of collision, coalescence, and breakup, which should lead to correlations. This inconsistency is resolved in this work. Using 1-min disdrometer measurements, two-point cross-correlation functions are used to show that drop counts at different sizes are correlated rather than independent. Moreover, it is argued that it is more appropriate to characterize rain statistically as a doubly stochastic Poisson process (Poisson mixture) among a collection of M correlated random variables (fluxes) each having its own probability distribution of unpredictable (random) mean values and its own coherence time, [[Tau].sub.M]. It is also shown that a drop size distribution has a characteristic coherence time, [Tau]. It is then argued that in order to preserve the purity of a size distribution of interacting drops, [Tau] must be equivalent to the shortest [[Tau].sub.M]. For sampling intervals much shorter than [Tau] and when the observation time, T, is less than [Tau], the drop counts remain correlated and the drop size distribution assumes the definition of a collection of physically interacting drops. On the other hand, when T [much greater than] [Tau], the drop counts decorrelate and the concept of the drop size distribution changes to a formal relation among the M observed drop concentrations averaged over several different size distributions. Moreover, when T is between the longest and shortest [[Tau].sub.M], part of the observed distribution will represent the distribution of interacting drops and the other part will represent a mixture of drops from different distributions. Finally, this work suggests using multiple time series analysis techniques for estimating mean drop concentrations in order to use all the available information and to help reduce drop size distribution mixing associated with the conventional analysis based on fixed time intervals.

Broadening of the cloud droplet (diameter < 50 [[micro]meter]) spectrum with increased droplet size was found to depend on the vertical profiles of cloud water. Clouds with liquid water profiles resembling adiabatic conditions displayed constant spectral widths. Other clouds displayed broader droplet spectra and increasing broadness with mean droplet sizes. Less than adiabatic cloud liquid water profiles may be accounted for by conversion to drops (diameter > 50 [[micro]meter], i.e., drizzle). Broad droplet spectra were most closely associated with drizzle drops. Both the concentration, C, and slope, k, of the cloud condensation nuclei (CCN) spectra were theoretically found to affect droplet spectral width. For individual cloud parcels a higher C and lower k each contributed to broader droplet spectra. When mixing among cloud parcels with different updrafts was considered, the predictions deviated especially at larger mean droplet diameters. Variations in updraft velocity result in differences in droplet concentrations and mean droplet sizes. The predictions for this internal mixing process showed greater droplet spectral widths for CCN spectra with higher k, especially at the larger mean droplet diameters. Instead of the individual parcel predictions of narrower droplet spectra at larger mean droplet sizes, internal mixing predicted increasing droplet spectral width with increasing mean droplet size. These predictions are consistent with the observations. First, when only cloud parcels with small mean droplet diameters (< 11 [[micro]meter]) were considered, the polluted clouds that formed on CCN with higher C and lower k displayed broader droplet spectra than clean clouds. Cloud parcels with large mean droplet diameters (> 12 [[micro]meter]) and large [[Sigma].sub.c] were observed only in clean conditions where k was high. Increasing droplet spectral width with mean droplet diameter (especially > 12 [[micro]meter]) is typical of many observations here and elsewhere.

This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW-SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a 'rear-inflow' that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with height and reflectivity bands extending rearward transverse to the principal convective line. Vertical motions within the active convective region of the squall line system were determined using a new approach that utilized near-simultaneous observations by the Doppler radars on two aircraft with up to four Doppler radial velocity estimates at echo top. Echo-top vertical motion can then be derived directly, which obviates the traditional dual-Doppler assumption of no vertical velocity at the top boundary and results in a more accurate estimate of tropospheric vertical velocity through downward integration of horizontal divergence. Low-level flight-level observations of temperature, wind speed, and dew point collected rearward of the squall line are used to estimate bulk fluxes of dry and moist static energy. The strong near-surface fluxes, due to the warm sea and high winds, combined with estimates of mesoscale advection, are used to estimate boundary layer recovery time; they indicate that the boundary layer could recover from the effects of the cold dome within about 3 h of first cold air injection if the observed near-surface winds were maintained. However, the injection and spreading of air from above leads to cooling at a fixed spot [approximately]20 km rearward of the convective line (surface [[Theta].sub.e] minimum point), suggesting that the cold pool could be still intensifying at the time of observation. Recovery time at a point is probably similar to that measured in previous studies.

In this paper, the authors investigate the momentum budget of a squall line with trailing stratiform precipitation by examining how the momentum balance varies with respect to the storm's internal structure. In particular, the authors determine differences between the momentum budgets of the convective and stratiform precipitation regions, which are physically distinct parts of the storm. The results from a high-resolution nonhydrostatic numerical simulation of the two-dimensional segment of the 10-11 June 1985 PRE-STORM squall line are used. The momentum equation is averaged over a 300-km-wide large-scale area for time periods of 1 h. On the 1-h timescale, the convective-scale temporal variations of horizontal and vertical velocities are nearly uncorrelated, and thus their contribution to the momentum flux is negligible. The remaining standing-eddy and mean-flow circulations account for the momentum flux on this timescale. The combination of the standing eddy and mean flow behave almost exactly like Moncrieff's idealization of two-dimensional steady-state squall line flow. Because the standing-eddy circulation and the pressure-gradient acceleration vary from one part of the storm to another, the interplay of forces leading to the large-scale momentum tendency also differs strongly from one subregion to another. The convective precipitation region dominates the momentum budget at low levels, where the standing-eddy flux convergence produces a forward acceleration that slightly outweighs the rearward pressure-gradient acceleration. At midlevels, both the convective and stratiform precipitation regions contribute to the net large-scale momentum tendency. The pressure-gradient forces in the convective and stratiform precipitation regions are both strong but oppositely directed; however, the rearward standing-eddy flux convergence in the convective precipitation region is also strong; thus, the net large-scale momentum tendency at midlevels is rearward. At upper levels, the momentum budget is completely dominated by the stratiform precipitation region, where a strong forward-directed pressure-gradient acceleration dominates the net large-scale momentum tendency. These differences between the momentum budgets of the convective and stratiform precipitation regions suggest that rather different large-scale momentum tendencies can arise as a function of storm structure; storms with strong convective precipitation regions and weak stratiform precipitation regions would produce momentum tendencies quite different from storms with well-developed stratiform precipitation regions.

Intraseasonal variability of tropical mean tropospheric temperature and precipitation are investigated using newly available gridded datasets from the satellite-based microwave sounding unit (MSU). Lag regression and cross-spectrum analysis reveal a strong, dynamically consistent, zonally symmetric component of the tropical 40-50 day oscillation. A fast eastward propagating signal is found in upper-tropospheric temperature and zonal wind, emanating from the region of enhanced precipitation associated with the 40-50 day oscillation. The temperature signal is consistent with an interpretation in terms of the transient tropospheric response to the switch-on of an equatorial heat source, in the form of a Kelvin wave front, and a recently detected very fast ([approximately] 40 m [s.sup.-1]) tropical tropospheric signal, with a vertical structure suggestive of the first baroclinic mode. Such fast moving wave fronts are hypothesized to be instrumental in maintaining the observed high degree of homogeneity in tropical temperature fields by effectively redistributing the warming generated by latent heat released in regional convective systems over the entire Tropics.

Previous studies have shown that a surface cold front often coincides with a heavy band of precipitation commonly designated as a narrow cold-frontal rainband. The maximum rainfall rate within this band can exceed 100-200 mm [h.sup.-1]. This study uses a nonhydrostatic two-dimensional cloud model with ice microphysics to investigate the precipitation processes within this type of rainband. Despite the relatively simple initialization and two-dimensionality, many aspects of these storms were well simulated. In these simulations, the intense but shallow updrafts produced large amounts of cloud water that were transformed primarily into rain and graupel within the zone of heavy precipitation and, to a lesser extent, into snow. The graupel and snow produced a zone of trailing stratiform precipitation. While the heavy rainfall could be represented in a warm rain model of the storm, an ice phase was needed in order to replicate the stratiform precipitation. Feedbacks of microphysical processes upon the dynamics of the flow were investigated. Sublimation and melting of frozen hydrometeors produced a pronounced cooling within the cold air mass, which slowly increased the depth and intensity of the cold air mass. This diabatic cooling within the cold air could potentially play a role in maintaining or even intensifying the circulations that lead to these rainbands. Previous studies of these types of fronts have instead concentrated on the role of melting in maintaining these structures through producing a stable layer across the cold air interface that could inhibit mixing.

The authors derive a relationship between the vertical Doppler spectrum of the rain just below the radar bright band and that of the snow just above. It neglects vertical air motions and assumes that each snowflake simply melts to form a raindrop of the same mass, disregarding other possible effects such as aggregation to form larger particles or breakup to create smaller ones. The relationship shows that, regardless of the dependence of particle fallspeed on size, the product of the equivalent reflectivity factor and the mean Doppler velocity of the snow is proportional to the same product for the rain, with a constant proportionality factor of 0.23, which equals the ratio of the dielectric factors of ice and water. Observed values of the reflectivity and mean Doppler velocity above and below the melting layer sometimes agree with this theoretical prediction but more often deviate from it in ways that may be interpreted as indicating the predominance of either aggregation or breakup processes. The data suggest that aggregation is occurring much of the time in the melting layer but that breakup effects become dominant in heavy precipitation. The analysis is extended by assuming relations between particle size and fallspeed for rain and snow. This enables the comparison of measured spectra with those derived theoretically. A simple allowance for aggregation or breakup in the spectral transformation from snow to rain is found to give improved spectral agreement in cases where these effects are indicated.

Observations of the effects of relative humidity on coalescence are limited to studies using supported drops or streams of drops, and the results are contradictory. In this paper, findings are presented on the effect of high and low relative humidity on collisions between freely falling drops. Comparisons between the collision outcomes (coalescence, bounce, and temporary coalescence with and without satellite drops) for high-humidity (RH > 95%) and low-humidity (RH [approximately equal to] 30%) experiments were made for small precipitation drops at terminal velocity and with minimal electric charge. Coalescence begins after the air-film between colliding drops is drained sufficiently to allow the drops to make contact. For temporary coalescence, the union of the two drops is not permanent because the rotational energy caused by a non-head-on collision is sufficient to pull the coalesing drops apart. One or more satellite drops form during a temporary coalescence when water filament between the separating drops breaks in more than one location. Experiments with higher drop charge were used to examine further the influence of humidity on coalescence. Our results show that relative humidity does not affect the coalescence efficiency for small precipitation drops. The effect of humidity is limited to collisions where permanent coalescence does not occur, and the collision outcome can be temporary coalescence. In cases where bounce is also a possible outcome, it was found that the probability of bounce is enhanced at the expense of temporary coalescence when relative humidity is decreased. For two of the comparisons between high-humidity and low-humidity results, the fraction of temporary coalescence collision outcomes halved at low humidity. Since the colliding drops are at the wet-bulb temperature, this effect is traced to the colder air gap that drains more slowly and retards coalescence at lower humidities. At high relative humidity the number of satellite drops about doubles with the increased probability of temporary coalescence. Other experiments showed that the increase in satellite drops at higher relative humidities also occurs for cases where collision outcomes are limited to coalescence or temporary coalescence. Since there are more temporary coalescence outcomes at the higher relative humidities in clouds, there are also more satellite drops that can act as embryos for new raindrops. These results apply to rain shafts within and below clouds.

Inhomogeneous distributions of liquid water like those observed in real clouds generally reflect less solar radiation than idealized uniform distributions assumed in plane-parallel theory. Here the authors determine cloud reflectivity and the associated plane-parallel albedo bias from distributions of liquid water path derived from 28 days of microwave radiometer measurements obtained on Porto Santo Island in the Madeiras during June 1992 as part of the Atlantic Stratocumulus Transition Experiment (ASTEX). The distributions are determined for each hour of the day, both for composites of the full set of 28 days and for a subset of 8 days having a high fraction of relatively thick cloud. Both sets are compared with results obtained from California stratocumulus during FIRE [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment]. In FIRE the albedo bias was dominated by the variability of the cloud optical depth, as measured by a fractal parameter, 0 [less than or equal to] f [less than or equal to] 1, while the ASTEX results are more complex. Mean cloud fraction above a 10 g [m.sup.-2] threshold is about 50% in the 28-day set, compared to 76% in the 8-day subset and 82% in FIRE. Cloud fraction is sensitive to the threshold for the 28 ASTEX days, probably due to a large fraction of thin cloud below the threshold, but this is not the case for the 8-day subset or for FIRE. Clear fractions during ASTEX are generally of shorter duration than those in FIRE, as are those in the 8-day subset. The diurnal mean fractal parameter is about 0.6 in ASTEX compared to 0.5 in FIRE, while the 8-day subset has nearly the same mean but a wider range. The diurnal cycle in cloud albedo and albedo bias is computed from the cloud parameters for both sets, assuming zero clear-sky albedo. The total absolute albedo bias rises to values above 0.3 at sunrise and sunset, but since there is little incident energy at that time, the reflected flux is more affected by the midday bias. The total albedo bias has a 1000 LST maximum of about 0.3, largely due to a cloud fraction contribution of 0.2, absent in FIRE because in that case cloud fraction remains near 100% until after 1000 LST. The albedo bias has a second maximum of about 0.2 at noon, again mainly from cloud fraction, and then drops to a minimum of about 0.1 at 1400 LST, when cloud fraction and fractal structure contribute about equally. Finally, a third maximum due to cloud fraction occurs at 1600 LST. In the 8-day subset, the 1000 LST maximum becomes dominated by the fractal structure, since the cloud fraction remains near 100% until 1000 LST, as in FIRE. The noon maximum receives roughly equal contributions, while the 1400 LST minimum bias is mainly due to fractal structure. Finally, the 1600 LST maximum and the evening limb bias are similar to those of the full 28-day set. These results show that cloud fractal and radiative properties can vary considerably from one site and time to another, and at different times within the same site, as meteorological conditions change.

The evolution of raindrop size distribution by coalescence, collisional breakup, and evaporation is studied using the Low and List parameterization for collisions. The authors consider two models of the development of raindrop size distribution; model 1 is a spatially homogeneous, time-dependent model, and model 2 is a ID (vertical) time-dependent model. The authors present the governing equations for the drop size distribution, balance equations for the rainwater content and rainfall rate, and scaling relationships. The authors demonstrate that the two models are intimately related. For model 1, the authors find that under the action of coalescence and breakup the size distribution attains a steady equilibrium form with three peaks at small drop sizes and an exponential tail at large drop sizes with a slope of approximately 65 [cm.sup.-1]. Under the action of coalescence, breakup, and evaporation, the evolution of a size distribution with a high enough rainwater content can be divided into two phases. In the initial phase, the evolution is collision controlled and the distribution evolves rapidly to approximate the shape of the equilibrium distribution without evaporation. The second phase starts after the rainwater content has been sufficiently reduced by evaporation. In this phase the evolution is evaporation controlled, the peaks at small drop sizes are smoothed, but the tail of the distribution remains exponential and its slope changes only slowly with time. In model 2, with a steady input of raindrops at the top of the rain shaft, a steady distribution is attained at any height after the lapse of a sufficiently long time. After a sufficient distance of fall, the steady distribution becomes approximately invariant with distance of fall and very close to the equilibrium distribution of model 1. With evaporation also occurring, the evolution is similar to that for model 1, with distance of fall taking on the role of time of evolution. A comparison of the results of the calculations with observed raindrop size distributions, at high rainfall rates, supports the idea that the observed distributions are in or near collisional equilibrium. However, the slope (20-25 [cm.sup.-1]) of the tail of the observed distributions is much smaller than the slope (65 [cm.sup.-1]) of the computed equilibrium distribution. This discrepancy suggests that either the Low and List parameterization is greatly overestimating drop breakup and/or the number of fragments formed by collisional breakup, or processes other than coalescence, breakup, and evaporation are strongly controlling the observed raindrop size distributions. Another possible factor contributing to the discrepancy may be the averaging of observed raindrop size distributions over considerable periods of time. It is recommended that in future observations attempts be made to obtain reliable drop size distributions in much shorter periods of time.

A series of GCM experiments is performed in order to examine the dynamics of the time-averaged distribution of precipitation and Hadley circulation in low-latitude areas. As an extension to Part I of this study, the sensitivity to the latitudinal distribution of the globally prescribed sea surface temperature (SST) is examined. When the peak of SST is not at the equator but nearby, a single precipitation peak appears on the opposite side of the equator. When the latitude of the SST peak becomes high enough (about 15 [degrees]), an abrupt jump of the precipitation peak is observed. This behavior is completely different if the evaporation from the ocean is estimated independent of the surface wind speed. Factors controlling the latitude of the precipitation zone are discussed. It is found that one essential component is the convergent flow in the planetary boundary layer, which is driven by the SST gradient. Another important factor is the net moist static energy input into the atmosphere, which is the difference between the energy input from the surface and the radiative loss. The near-neutral stratification of the atmosphere, which may be partially determined by the distribution of the SST, is also considered to be important because it controls the moist static energy balance of the atmosphere.

This paper describes the evolution of global angular momentum (GAM) on intraseasonal timescales in data from two general circulation model (GCM) runs: an aquaplanet GCM and a fully 'realistic' GCM that includes continents, topography, and observed climatological sea surface temperatures. For both GCMs, the angular momentum budget is quite well balanced. Composites of various quantities are calculated at different lags relative to the maximum GAM and GAM tendency. In both GCMs, this composite analysis shows that the GAM tendency is largest as a precipitation anomaly propagates eastward along the equator. Associated with this precipitation anomaly is a tropical circulation that shows some of the characteristics of the Gill model, particularly in the aquaplanet GCM, and a Rossby wave train that propagates from the Tropics into midlatitudes. It is the anomalous midlatitude surface wind field associated with this Rossby wave train that is primarily responsible for the anomalous friction torques in both models. In the realistic GCM, this Rossby wave train has the appropriate structure to induce a large mountain torque, particularly at the Rocky Mountains. Also, it is found that the friction and mountain torques contribute about equally to the intraseasonal evolution of GAM, with the anomalous friction torque leading the anomalous mountain torque by three days. After the precipitation anomaly weakens, the Rossby wave train completely propagates out of the Tropics and leaves behind a pattern resembling that of a Kelvin wave. Consistent with this wave propagation, in both GCMs, eddies transport the angular momentum gained at the surface in midlatitudes toward the equator. Lastly, the effects of zonal inhomogeneities on the wave dynamics associated with GAM evolution are discussed.

Results from laboratory observations of isolated collisions between small precipitation-size drops falling freely at terminal velocity in a refrigerated collision chamber are presented. The average radii of the size pair studied were 353 and 306 [[micro]meter]. Air temperatures ranged from 20 [degrees] to - 15 [degrees] C. Drop temperatures ranged from 20 [degrees] to approximately 2 [degrees] C. Experimentation revealed that the coalescence efficiency increased from approximately 42% for mean drop temperatures between 20 [degrees] and 10 [degrees] C to about 81% for mean drop temperatures between 10 [degrees] and 2 [degrees] C. A particularly interesting finding was an abrupt, rather than gradual, increase in coalescence efficiency at mean drop temperature of about 10 [degrees] C. A reduction in drop deformation during impact due to a substantial increase in viscosity with decreasing temperature is considered as a mechanism that can act to promote coalescence. The apparent abrupt increase in coalescence efficiency requires further investigation. The extent to which these results may be extended to collection processes in clouds remains uncertain because of the effect that reduced pressure can have on deformation through drop fall speed and interaction time and because the drops were not at thermal equilibrium. The results of this experiment point to the need for further investigation in which free-fall drop collisions are produced at thermal equilibrium and at lower pressures and suggest that precipitation processes involving drizzle and raindrops may be considerably more complicated than previously suggested by experiment.

This study focuses on the effects of drizzle in a one-dimensional third-order turbulence closure model of the nocturnal stratus-topped marine boundary layer. When the simulated drizzle rate is relatively small (maximum approximately 0.6 mm day to the -1), steady-state solutions are obtained. The boundary layer stabilizes essentially because drizzle causes evaporative cooling of the subcloud layer. This stabilization considerably reduces the buoyancy flux and turbulence kinetic energy below the stratus cloud. Thus, drizzle tends to decouple the cloud from the subcloud layer in the model, as suggested by many observational studies. In addition, the evaporation of drizzle in the subcloud layer creates small scattered clouds, which are likely to represent cumulus clouds, below the solid stratus cloud in the model. The sensitivity experiments show that these scattered clouds help maintain a coupled boundary layer. When the drizzle rate is relatively large (maximum approximately 0.9 mm day to the -1), the response of the model becomes transient with bursts in turbulent fluxes. This phenomenon is related to the formation of the scattered cloud layer below the solid stratus cloud. It appears that the model is inadequate to represent the heat and moisture transport by strong updrafts covering a small fractional area in cumulus convection.

Surface, aircraft, and satellite observations are analyzed for the 21-day 1989 intensive field campaign of the First ISLSCP Field Experiment (FIFE) to determine the effect of precipitation, vegetation, and soil moisture distributions on the thermal properties of the surface including the heat and moisture fluxes, and the corresponding response in the boundary-layer circulation. Mean and variance properties of the surface variables are first documented at various time and space scales. These calculations are designed to set the stage for Part II, a modeling study that will focus on how time-space dependent rainfall distribution influences the intensity of the feedback between a vegetated surface and the atmospheric boundary layer. Further analysis shows strongly demarked vegetation and soil moisture gradients extending across the FIFE experimental site that were developed and maintained by the antecedent and ongoing spatial distribution of rainfall over the region. These gradients are shown to have a pronounced influence on the thermodynamic properties of the surface. Furthermore, perturbation surface wind analysis suggests for both short-term steady-state conditions and long-term averaged conditions that the gradient pattern maintained a diurnally oscillating local direct circulation with perturbation vertical velocities of the same order as developing cumulus clouds. Dynamical and scaling considerations suggest that the embedded perturbation circulation is driven by surface heating/cooling gradients and terrain effects rather than the manifestation of an inertial oscillation. The implication is that at even relatively small scales (

The effects of various microphysical processes and the Coriolis force on the dynamics of squall systems were investigated with a two-dimensional, anelastic numerical model. The incorporation of ice-phase microphysics into the model has been found to be important in the successful simulation of realistic storm structure and evolution of squall lines. The significance of the ice-phase microphysics is largely accounted for by the small terminal velocities of ice particles and cooling by melting. The response of the atmosphere to the cooling by melting is a complicated one and has been shown to play an important role in shaping the kinematic and precipitation characteristics of the observed and modeled squall systems. The interaction between the front-to-rear (FTR) flow and cooling by melting would both intensify (by enhancing the mesoscale updraft and the FTR flow above the melting layer) and limit (by partially driving the rear-to-front flow at the back of the stratiform region) the stratiform precipitation development. The Coriolis force has also been found to have significant effects on the simulated squall systems. The rotational component of the storm flow field constrains the strength of the divergent wind field, which in turn limits the horizontal scale of the mesoscale circulation and the associated stratiform region. The model squall lines seemed to be most sensitive to the variations of f in the range between f = 0.7 X 10 to the -4 s to the -1 and f = 1 X 10 to the -4 s to the -1.

A detailed ice-phase bulk microphysical scheme has been developed for simulating the hydrometeor distributions of convective and stratiform precipitation in different large-scale environmental conditions. The proposed scheme involves 90 distinct microphysical processes, which predict the mixing ratios and the number concentrations of small ice crystals, snow, graupel, and frozen drops/hail, as well as the mixing ratios of liquid water on wet precipitation ice (snow, graupel, frozen drops). The number of adjustable coefficients has been significantly reduced in comparison with other bulk schemes. Additional improvements have been made to the parameterization in the following areas: 1) representing small ice crystals with nonzero terminal fall velocities and dispersive size distributions, 2) accurate and computationally efficient calculations of precipitation collection processes, 3) reformulating the collection equation to prevent unrealistically large accretion rates, 4) more realistic conversion by riming between different classes of precipitation ice, 5) preventing unrealistically large rates of raindrop freezing and freezing of liquid water on ice, 6) detailed treatment of various rime-splintering ice multiplication mechanisms, 7) a simple representation of the Hobbs-Rangno ice enhancement process, 8) aggregation of small ice crystals and snow, and 9) allowing explicit competition between cloud water condensation and ice deposition rates rather than using saturation adjustment techniques. For the purposes of conserving the higher moments of the particle distributions, preserving the spectral widths (or slopes) of the particle spectra is shown to be more important than strict conservation of particle number concentration when parameterizing changes in ice-particle number concentrations due to melting, vapor transfer processes (sublimation of dry ice, evaporation from wet ice), and conversion between different hydrometeor species. The microphysical scheme is incorporated into a nonhydrostatic cloud model in Part II of this study. The model performed well in simulating the radar and microphysical structures of a midlatitude-continental squall line and a tropical-maritime squall system with minimal tuning of the parameterization, even though the vertical profiles of radar reflectivity differed substantially between these storms.

A high-resolution composite analysis covering the entire breadth of the northern portion of a mature leading-line, trailing stratiform squall-line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft: one at upper levels that was mechanically forced and one at lower levels that was microphysically forced. Both the upper-level and lower-level mean downdrafts in the transition zone appeared to be the average effect of convective-scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line. The structure of individual upper-level convective-scale downdrafts suggested that they may have been partially composed of gravity waves excited by the interaction of the penetrative convective updrafts of the mature and dissipating convective cells with the stable ambient flow. The lower-level mean downdraft extended from midlevels to near the surface but was maximum near the melting level and was associated with air of low equivalent potential temperature. It was likely microphysically driven by cooling associated with melting and evaporation. The upper-level and lower-level subsidence in the transition zone had little effect on the radar reflectivity minimum observed at middle to low levels in the transition zone. The primary microphysical process affecting the development of the reflectivity minimum appears to have been the inability of small ice crystals to form, grow, or persist at midlevels in the transition zone. Consequently, less aggregation could occur in the transition zone just above the melting level than in the secondary band at the same altitude.