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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.

Early progress in weather modification is attributed to a healthy interaction between theory and experiment. During the 1970s, a divergence of approaches took place. A 'theoretical/experimental' approach, exemplified by the Cascade Project, focused on testing scientific hypotheses; an 'observational/experimental' approach, exemplified by the Colorado River Basin Pilot Project, sought to enhance understanding of the seeding process through more detailed observations. The theoretical/experimental school soon came to focus almost exclusively on natural cloud processes, leaving the field of weather modification nearly devoid of a theoretical component. It is suggested that this theoretical component is necessary to revitalize the field of weather modification. Key questions are addressed. These include 1) identification of clouds that are amenable to seeding; 2) glaciogenic versus hygroscopic seeding; 3) optimizing critical seeding variables, such as seed particle concentration for glaciogenic seeding and seed particle size for hygroscopic seeding; and 4) seeding for hail suppression.

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.

Three-dimensional tropical squall-line simulations from the Goddard cumulus ensemble (GCE) model are used as input to radiative computations of upwelling microwave brightness temperatures and radar reflectivities at selected microwave sensor frequencies. These cloud/radiative calculations form the basis of a physical cloud/precipitation profile retrieval method that yields estimates of the expected values of the hydrometeor water contents. Application of the retrieval method to simulated nadir-view observations of the aircraft-borne Advanced Microwave Precipitation Radiometer (AMPR) and NASA ER-2 Doppler radar (EDOP) produce random errors of 23%, 19%, and 53% in instantaneous estimates of integrated precipitating liquid, integrated precipitating ice, and surface rain rate, respectively. On 5 October 1993, during the Convection and Atmospheric Moisture-Experiment. (CAMEX), the AMPR and EDOP were used to observe convective systems in the vicinity of the Florida peninsula. Although the AMPR data alone could be used to retrieve cloud and precipitation vertical profiles over the ocean, retrievals of high-resolution vertical precipitation structure and profile information over land required the combination of AMPR and EDOP observations. No validation data are available for this study; however, the retrieved precipitation distributions from the convective systems are compatible with limited radar climatologies of such systems, as well as being radiometrically consistent with both the AMPR and EDOP observations. In the future, the retrieval method will be adapted to the passive and active microwave measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite sensors.

This paper describes experiments on a refrigerating catalyst - liquid nitrogen (LN) - in different cloud chambers and their results. The nucleation threshold temperature of liquid nitrogen is 0 [degrees] C, and when the temperature less than -2 [degrees] C, the ice nucleation rate is about [10.sup.12] [g.sup.-1]. This indicates that LN can be employed in artificial precipitation enhancement and artificial supercooled fog dispersal activities. It is found in the experiments that in fogs with a temperature slightly above 0 [degrees] C, liquid nitrogen can play a role of broadening the fog drop spectrum, increasing the number of big droplets and decreasing the number of small ones. In addition, the application of microvideo and IR technology in cloud chamber experiments is also described.

This paper presents a computationally simple technique for retrieving the precipitation and vertical hydrometeor profiles from downward viewing radiometers. The technique is computationally much less expensive than previous profiling schemes and has been designed specifically to allow for tractability of assumptions. In this paper, the emphasis is placed upon passive microwave applications, but the combination of passive with active microwave sensors, infrared sensors, or other a priori information can be adapted easily to the framework described here. The technique is based upon a Bayesian approach. Here, we use many realizations of the Goddard Cumulus Ensemble model to establish a prior probability density function of rainfall profiles. Detailed three-dimensional radiative transfer calculations are used to determine the upwelling brightness temperatures from the cloud model to establish the similarity of radiative signatures and thus the probability that a given profile is actually observed. In this study, we show that good results may be obtained by weighting profiles from the prior probability density function according to their deviation from the observed brightness temperatures. Examples of the retrieval results are shown for oceanic as well as land situations. Microwave data from the Advanced Microwave Precipitation Radiometer (AMPR) instrument are used to illustrate the retrieval structure results for high-resolution data while SSM/I is used to illustrate satellite applications. Simulations are performed to compare the expected retrieval performance of the SSM/I instrument with that of the upcoming TMI instrument aboard the Tropical Rainfall Measuring Mission (TRMM) to be launched in August 1997. These simulations show that correlations of [approximately]0.77 may be obtained for 10-km retrievals of the integrated liquid water content based upon SSM/I channels. This correlation increases to [approximately]0.90 for the same retrievals using the TMI channels and resolution. Due to the lack of quantitative validation data, hydrometeor profiles cannot be compared directly but are instead converted to an equivalent reflectivity structure and compared to existing radar observations where possible.

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.

During the Joint Tropical Rain Experiment of the Malaysian Meteorological Service and the University of Toronto, pulsating raindrop ensembles, hereafter pulses, were observed in and around Penang Island. Using a Doppler radar on 25 October 1990, a periodic variation of precipitation aloft 30 km from the radar site, with an approximate 8-min period, was established and seemed to be caused by the evolution and motion of horizontal inhomogeneities existing within the same cell. On 30 October 1990, using a new volume scanning strategy with a repetition cycle of 3.5 min, pulsations of the same frequency were observed up to 3 km above the radar and at the ground by a disdrometer. High concentrations of large drops were followed by high concentrations of successively smaller drops at the ground. This provides observational evidence to support the recent argument for using a time-varying release of precipitation-sized particles to model observed pulsating rainfall. Many cases of nonsteady rain from convective clouds displayed repetition periods of between 8 and 25 min.

Potential shifts in summer precipitation due to an enhanced greenhouse effect indicate the possibility of more rain days and heavier rains in the Midwest, and this study assessed the effects of such changes on transportation in Chicago using a 3-year period of data. Traffic accidents in the metropolitan area doubled on rainy days, with 30% more accidents in more densely populated urban areas than in suburban-rural areas. During rain events accident severity (number of injuries) was 55% higher in suburban and rural areas where less dense but higher speed traffic flows exist than in the city, however. Rain days during dry months produced more accidents and injuries than during normal or wet months. Three times as many accidents occurred during heavy rain periods (> 12.8 mm) as during nonrain conditions. Rain had a negligible influence on weekday traffic volume on busy highways but there was a 9% decrease in traffic volume on rainy weekends. A 3-5% decrease in ridership of public transportation occurred on rainy days, with most decreases during midday. Nationally, 27% of all fatality-producing aircraft accidents occurred during rainy weather conditions, as did 57% of the 30-min flight delays at Chicago's O'Hare Airport. Results suggest that given continued transportation use patterns extend into the future, a future climate with more summer rain days, somewhat higher rain rates, and more storms would mean more total vehicular accidents, more total injuries in vehicular accidents, decreased ridership on public transportation systems, and more aircraft accidents and delays. A drier climate would likely experience fewer moderate to heavy rain events but results show that rain events during drier conditions produced a greater frequency of accidents and injuries per event than during wetter conditions.

Stochastic models have been proposed as one technique for generating scenarios of future climate change. One particular daily stochastic weather generator, termed Richardson's Model or WGEN, has received much attention. Because it is expressed in a conditional form convenient for simulation (e.g., temperature is modeled conditional on precipitation occurrence), some of its statistical characteristics are unclear. In the present paper, the theoretical statistical properties of a simplified version of Richardson's model are derived. These results establish that when its parameters are varied, certain unanticipated effects can be produced. For instance, modifying the probability of daily precipitation occurrence not only changes the mean of daily temperature, but its variance and autocorrelation as well. A prescription for how best to adjust these model parameters to obtain the desired climate changes is provided. Such precautions apply to conditional stochastic models more generally.

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.

Analysis of the high-frequency temperature and precipitation data indicates that a warmer climate results in the decrease of high-frequency temperature variability and an increase in the proportion of precipitation occurring in the extreme events. The data was obtained from hundreds of sites spread over Australia, China, the former Soviet republics and the US over the past 30 to 80 years. Temperature variability on a day-to-day basis has reduced in the Northern Hemisphere while the US and Australia have experienced an increase in the total precipitation contributed by extreme events.

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.

A summary of the North Atlantic Treaty Organization's Advanced Research Workshop Global Precipitations and Climate Change that reviewed the current understanding of global precipitation distribution, as related to climate evolution, on various timescales, is presented. The workshop, which took place at L'Agelonde, France, 27 September - 1 October 1993, was held to assess the availability and quality of data that could be used to improve the understanding of the processes determining precipitation changes through initialization and validation of existing climate models. Furthermore, several examples of the use of precipitation data were presented. A summary of the recommendations for future studies, which arose from the exchanges between the modeling and observational communities, is included.

Historical records of warm- and cold-season floods and associated heavy-precipitation events during 1921-1985 in the Midwest were examined for temporal fluctuations and trends. Floods in basins in the northern Midwest exhibited upward trends in both seasons but no statistically significant temporal changes existed elsewhere. The incidence of heavy-precipitation events also increased in this same area, a region where thunderstorm incidences and cyclone frequencies have also been on the increase since 1920. Pentads of high flood incidences all occurred in major wet periods (generally the 1970s and 1980s), and pentads of lowest flood incidences occurred in notable droughts (1930s and 1951-1965). The times of these droughts (early) and wet periods (late) further help explain the tendency for floods to increase during the 1921-1985 study period. When precipitation over 5-year periods decreased 8% or more below average, or increased by 7% above average, the number of floods was greatly reduced or increased. This suggests that future significantly drier climate conditions in the Midwest could have few floods, and significantly wetter conditions could have increased flood activity.

The Special Sensor Microwave/Imager, a microwave sensor, uses the polarization corrected temperature, PCT(sub 85) algorithm for measuring precipitation over Britain and surrounding regions. The algorithm correlates the readings of the device with a standard radar scale for deriving instantaneous rain rates. However calculations are often erroneous due to the limitations of the device in measuring rainfall induced by mountains and other surface conditions showing the need for better empirical calibration of passive microwave algorithms for greater sensitivity.

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.

Theoretical studies using Bayes theorem help develop a multichannel passive microwave precipitation remote sensing algorithm that retrieves hydrometeor profiles. Maximization of the posterior probability density for each vector of observations helps retrieve hydrometeor profiles. Application of this algorithm reveals that hydrometeor profiles retrieved for land have higher error than those of water. This method retrieves integrated hydrometeor contents more efficiently than profiles. Errors associated with the method and future improvements are discussed.

Examination of orthogonal spatial patterns of daily and monthly precipitation on a mountainous watershed in Idaho by rotated principal component analysis helps derive the influence of topography, geographic location and atmospheric variability on the spatial patterns. Regional boundaries derived from monthly data exhibit close alignment with topography, suggesting the integration of various storm events on monthly timescales. Topography and geographic location exhibit significant correlation with spatial fields. Data from a single location is enough to predict spatial pattern changes and regional precipitation fluctuation, provided sufficient knowledge about local climate is available.

An algorithm from Special Sensor Microwave/Imager (SSM/I) to estimate precipitation is developed from radiative transfer calculations through an atmospheric cloud model. This algorithm captures both emission and scattering rainfall and operates over a variety of climatological conditions, and is found to accurately describe both localized instantaneous rainfall and global monthly rainfall over land and ocean. Comparisons between the SSM/I algorithm estimates and cloud model and precipitation radar show that the difference in instantaneous rainfall are less than the mean rain rate.

Nonhomogeneous hidden Markov models (NHMM), applied to a network of four stations in Washington for 24 years of rainfall, provide a method to link particular patterns in the atmospheric data with particular rainfall patterns. It defines weather states that correspond to particular patterns of precipitation at the states which is in contrast to the weather state models described previously. NHMM models can be used in several ways and are based on the assumption that conditional on weather state, the precipitation occurrence processes are spatially independent.

A study conducted using the Colorado State University Regional Atmospheric Modelling System to determine the effect of landscape variability on cloud formation and precipitation showed that land surface moisture affects the timing of onset of clouds and the intensity and distribution of precipitation, which is dominated by turbulent motions or mesoscale circulations. Cloud and precipitation are affected by the horizontal distribution of land surface wetness in a dry atmosphere, and the local Rossby radius of deformation is the optimal wavelength of land surface forcing producing maximum precipitation. Maximum latent heat flux increases with precipitation because of the relationship between mesoscale latent heat and mesoscale circulations, and the effects of other parameters such as atmospheric moisture and background wind should also be considered in large scale models.

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.

Rainfall and a rain-hail discriminator are estimated by using a specific differential polarization phase shift K(DP), modelled on the complex multivariate Gaussian model used in statistical optics and communications theory. K(DP) is produced by precipitation and the cumulative differential phase fluctuation of precipitation is obtained by using the marginal phase probability function. The estimation provides suboptimal and simple expressions for K(DP).

We present observations of the copolar correlation coefficient between horizontally and vertically polarized echoes [vertical bar][[Rho].sub.hv](0)[vertical bar]. These were made with ground-based and airborne weather radars at nearly vertical incidence. A sharp decrease of [vertical bar][[Rho].sub.hv](0)[vertical bar] occurs at the bright band bottom, and is attributed to a varying mixture of hydrometeors with diverse shape, size, and thermodynamic phase. The largest contribution to decorrelation seems to come from wet aggregates; this is substantiated by consideration of two simple models. One consists of randomly oriented wet prolate spheroids, and the other considers an ensemble of distorted spheres. Prolates with axis ratios of 3 or distorted spheres with rms roughness equal to 15% of the diameter decrease the correlation to 0.8 at S band. At Ku band and for the size range encountered in the bright band, the decrease is a function of equivalent diameter because scattering is in the Mie regime. [vertical bar][[Rho].sub.hv](0)[vertical bar] measurement at 13.8 GHz and from the aircraft are the first ever. Also, differential phase and differential reflectivity at a 10 [degrees] off nadir are the first of its kind. These last two variables showed a distinct signature in the bright band. This is significant because it might lead to applications on airborne or spaceborne platforms.

Cloud water pH observations and cloud-droplet spectra analysis of ten-winter clouds, to detect possible physical and chemical properties relations, reveal that the clouds with the least pH values contain the largest droplet concentration. The minimal mean droplet diameters have the highest ion concentrations and large pH value clouds contain the smallest droplet concentrations. Cloud condensation nucleus composition accounts for the variations in the droplet and ion concentrations in the clouds that contain homologous liquid water levels.

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 melting layer model related to the physical constants and meteorological parameters is employed in this investigation. The specific phase shift, together with the specific attenuation, is computed at 1-100 GHz by using the Mie theory. The additional zenith attenuation, which is the difference between zenith attenuation due to the melting layer and attenuation due to the same thickness of the resulting rain, is comprehensively studied. The ratio of the difference to rain zenith attenuation may be over 1 at 1-5 GHz although the difference is much less than 1 dB. The difference can be over 1 dB at frequencies above 20 GHz. A minimum of the ratio is below 0.05 at frequencies about 40-60 GHz but the ratio can become a value of about 0.1 at 100 GHz. The additional attenuation should be taken into account in satellite-earth communications and radar remote sensing. The power law parameters of the average specific attenuation of the melting layer and rain specific attenuation are tabulated for three raindrop size distributions at rain rates of below 25 mm/h. The power law method could be utilized in the additional attenuation calculation. It is a good approximation of the Mie theory results at 1-50 GHz and a useful estimate at 50-100 GHz.

Decreased solar activity correlates with positive cosmogenic isotope anomalies, and with cool, wet climate in temperate regions of the world. The relationship of isotope anomalies to climate may be the opposite for areas influenced by monsoonal precipitation, i.e., negative anomalies may be wet and warm. Petersen (1988) has found evidence for increased summer precipitation in the American Southwest that can be shown to be coincident with negative 14C anomalies during the Medieval Warm Period. The present study compares palynological indicators of lake level for the Southwest with Petersen's data and with the 14C isotope chronology. Percentages of aquatic pollen and algae from three sites within the Arizona Monsoon record greater lake depth or fresher water from A.D. 700-1350, between the Roman IV and Wolf positive isotope anomalies, thereby supporting Petersens's findings. Maximum summer moisture coincides with maximum population density of prehistoric people of the Southwest. However, water depth at a more northern site was low at this time, suggesting a climate-isotope relationship similar to that of other temperate regions. Further analysis of latitudinal patterns is hampered by inadequate 14C dating.

The Arizona monsoon, a major source of precipitation in the Southwest, shares many features with the monsoons of other continents. Computer modeling and fossil data indicate maximum extent of the African and Asian monsoons 9000 years ago. Fossil data indicate increased summer precipitation 9000 years ago, synchronous with the maxima of the African and Asian monsoons and, paradoxically, with the early-Holocene xerothermic of the Pacific Northwest. Climate model runs for 6000, 9000, 11,500, 13,000, and 18,000 years ago indicate increased summer precipitation 9000 years ago and a reciprocal relationship between precipitation in the Northwest and Southwest, but they relegate insolation to a role secondary to the North American ice sheet in regulating climate, and suggest a non-monsoon source for much of the summer precipitation in the Southwest prior to 9000 years ago.

The Global Precipitation Climatology Project (GPCP) was established by the World Climate Research Programme to produce global analyses of area- and time-averaged precipitation for use in climate research. To achieve the required spatial coverage, the GPCP uses simple rainfall estimates derived from IR and microwave satellite observations. In this paper, we describe the GPCP and its first Algorithm Intercomparison Project (AIP/1), which compared a variety of rainfall estimates derived from Geostationary Meteorological Satellite visible and IR observations and Special Sensor Microwave/Imager microwave observations with rainfall derived from a combination of radar and raingage data over the Japanese islands and the adjacent ocean regions during the June and mid-July through mid-August periods of 1989. To investigate potential improvements in the use of satellite IR data for the estimation of large-scale rainfall for the GPCP, the relationship between rainfall and the fractional coverage of cold clouds in the AIP/1 dataset is examined. Linear regressions between fractional coverage and rainfall are analyzed for a number of latitude-longitude areas and for a range of averaging times. The results show distinct differences in the character of the relationship for different portions of the area. In general, to the south and east of the mountainous axis of Japan, rainfall and fractional coverage are highly correlated for thresholds colder than 245 K, and correlations can be increased by averaging in space and in time up to the dominant period of the precipitation events. To the north and west of the axis, the correlations between rainfall and fractional coverage, while generally smaller for all scales, are highest for thresholds warmer than 245 K. The proportional coefficients relating rainfall to fractional coverage at cold thresholds, however, differ greatly between the two periods and both differ significantly from those found for the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment. These results suggest that the simple IR-based estimation technique currently used in the GPCP can be used to estimate rainfall for global tropical and subtropical areas, provided that a method for adjusting the proportional coefficient for varying areas and seasons can be determined.

The processing of composite samples by the station-year approach either coupled or not coupled to the systematic use of Hershfield's formula can be very helpful when studying design floods. Therefore, a station-year approach is applied to rainfall and discharge data from Indonesia, Malaysia, and the Philippines to define regional distributions for observed maxima. After a cluster analysis defining groups of homogeneous stations, a double distribution concept is used to properly take into account the existence of outliers. Results are compared with information available on probable maximum precipitation (PMP) and design floods. Concerning Indonesian rainfall data, the estimate of 10,000-year daily rainfall was consistent with the application of Hershfield's method for computing PMP. Hershfield's PMP was between 73% and 115% of the 10,000-year daily rainfall. The processing of discharge data yields results that are confirmed by project inflow design floods. The method provides a bridge between statistical and hydrometeorological approaches. Regional analysis obliges to weigh the occurrence of outliers. Data clustering is recommended for applications involving large amounts of information.

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.

Electric field (E) soundings in the stratiform regions and transition zones of mesoscale convective systems (MCSs) are reported. Most of the E soundings were made during the 1991 Cooperative Oklahoma Profiler Studies (COPS-91). Multiple E soundings were made in several MCSs. All of the E soundings collected here can be grouped into one of two types. Within each type the soundings and the inferred charge structures are remarkably similar from one place in an MCS to another and from one MCS to another. The charge regions inferred from the E soundings are hundreds of meters thick and have charge densities up to 5.3 nC m to the -3. Typically, the maximum E in the soundings is about 100 kV m to the -1. Here, E soundings from three classes of MCSs are discussed. The bow-echo MCSs have simpler vertical charge structures with four main charge regions, while squall-line MCSs and predominantly stratiform MCSs have five main charge regions. In all of the E soundings there is a substantial region of charge and a large E at or near 0 degree C.

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.

With the recent advances of dual-polarized radar techniques in meteorology it is now possible to deduce precipitation microphysical characteristics in far more detail than possible with reflectivity measurements alone. Radar parameters such as differential reflectivity and differential phase between horizontal and vertical polarizations have been studied in detail as well as linear depolarization ratio, copolar correlation coefficient, and backscatter differential phase. While these parameters can be linked to certain microphysical properties of specific classes of precipitation such as raindrops or hail, very little study has been directed at the practically important cases of mixtures of different types of precipitation particles such as rain, hail, graupel, ice crystals, and snow. Each type can have different size, shape, orientation, and dielectric constant distributions. The treatment here is rigorous and is based on the Mueller matrix formulation. Radar parameters are derived from the averaged Mueller matrix computations. Careful consideration is given to the orientation and size distributions of the different particle types. After calculating single particle scattering characteristics, some simple two-component mixtures such as rain/hail and ice crystals/snow are considered. Finally, a 2D numerical cloud model is used to simulate the rain, hail/graupel, and snow fields of an evolving convective storm from which the radar parameters are derived for the initial, peak, and dissipating stages of the storm. Model computations are performed at C and S-band frequencies.

Trajectories of 500- and 1000-micrometer diameter particles are calculated as they fall through the spatially varying flow field above sinusoidal terrain for various combinations of atmospheric stability, wind speed, and terrain wavelength. In each case, a set of 20 uniformly spaced particles are released simultaneously above sinusoidal topography and their trajectories are obtained numerically by coupling a linear wave solution for flow over sinusoidal topography with equations for particle motion. The flow field and the associated patterns of deposition are shown to be strongly influenced by atmospheric stratification. For strong stratification, the presence of vertically propagating waves produces relatively concentrated 'particle streams.' For less stratified conditions with evanescent waves, little focusing of particle trajectories is apparent. The ability of the atmosphere to focus or concentrate falling particles may ultimately produce regions along the surface with enhanced deposition.

Cloud droplets may take a variety of paths to reach some location in a cloud and therefore may encounter varying growth conditions. This paper examines the effect of these variations on broadening of the droplet size distribution in cumuli. Diffusional growth of a collection of droplets is studied in detail using measurements obtained during the Cooperative Convective Precipitation Experiment (CCOPE) from the University of Wyoming's King Air research aircraft. A technique is described by which these measurements may be converted to the frame of reference of a moving droplet. Entrainment processes that are associated with variations in vertical velocity appear to have the effect of intensifying fluctuations in integral radius, which in turn produce variations in the supersaturation encountered by the droplets and thus opportunities for variations in growth. Contributions to spectral broadening arise from turbulent structures in the cloud, producing variations in integral radius as well as in the correlation between integral radius and vertical velocity. A relation among these variations and the resultant spectral broadening is developed and applied to measurements in growing cumuli. In unmixed updrafts, the resulting spectral broadening is small. Calculations based upon measurements obtained in mixed parcels produce reasonable qualitative and quantitative agreement with the observed dispersions. The relation of recent studies on spectral broadening to this work is explored.

The degree to which changes in temperature and precipitation over the past hundred years can be seen in the data for Sheffield is examined by data inspection, tabulation, graphical plots and simple statistical methods. Whilst illustrating the difficulty (if not impossibility) of thus establishing evidence for anthropogenically-enhanced global warming and associated precipitation changes, this analysis indicates ways in which local records could be examined by individuals or groups at school.

A series of numerical experiments is performed using a general circulation model with an idealistic ocean-covered boundary condition. The meridional structure of the Hadley circulation system, which is a combined structure of the Hadley circulation and the tropical precipitation zone, is examined from the standpoints of the water vapor and the energy budgets. Although the prescribed SST distribution has a broad peak centered at the equator, the distribution of the precipitation has two peaks straddling the equator. The distribution of the evaporation rate is revealed to be an important factor in the formation of this structure. The evaporation rate is smaller near the equator than in the subtropics because of its dependence upon the wind speed. If this dependence is removed from the parameterization of the evaporation, the latitudinal distribution of evaporation becomes flat and the precipitation concentrates at the equator to form a single band structure. Qualitatively similar results are obtained in the experiments with axisymmetric two-dimensional model without the effect of disturbances. The net supply of the moist static energy, which is the difference between the supply of energy mainly through evaporation and the loss of energy through radiative cooling, must balance with the divergence of the meridional transport of moist static energy. It is considered to be the quantity controlling the meridional structure of the system. Since the latent energy supply and the radiative loss tend to cancel each other, a small fractional change in the evaporation rate causes a large change in the net energy supply and thus results in a strong modification of the meridional structure of the system. In other words, a strong positive feedback between the strength of the Hadley circulation and the latitudinal concentration of the precipitation provides the high sensitivity of the Hadley circulation system to the distribution of the evaporation, which is the principal energy source of the system. A set of comparative experiments with different cumulus parameterization schemes is performed. It is shown that the difference in the vertical stratification significantly modifies the energy budget of the Hadley circulation system and causes a large difference in the distribution of precipitation.

A study was conducted to evaluate a technique used for convection forecasting. The method was used as part of the 1989 Precipitation Augmentation for Crops Experiment cloud-seeding tests. Results show that maximum echo height groupings were conditioned by temperature of the convective condensation level and potential buoyancy. The two factors also provide a basis for precipitation studies.