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A detailed model of the melting, shedding, and wet growth of spherical graupel and hail is presented. This model is based upon recent experimental studies by Rasmussen et al. and Lesins et al. The model is presented in the form of five easy-to-use tables. Important quantities considered were the heat transfer. terminal velocity behavior, and shedding of liquid water.

Equations were developed to calculate the growth of the ice phase in cirrus clouds. Calculations indicated that nucleation of ice crystals in cirrus uncinus heads forming at temperatures lower than −35°C generally should occur near the upwind base of the head, and in cirrostratus clouds at the top of the cloud. The growth of ice crystals and the resulting shape of cirrus uncinus clouds with an updraft velocity of 100 cm s−1 were calculated. With an initial crystal concentration of 0.025 cm −2 and a nucleation temperature of −40°C, crystals of 0.45 mm length, and a maximum ice water content of 0.3 g m −3 were predicted. Latent beat release due to the ice crystal growth increased the initial updraft velocity only slightly. A downdraft velocity comparable in magnitude to the original updraft velocity was calculated to occur in the downshear part of the cirrus uncinus head.

The accuracy of various techniques for calculating the collection efficiency, accretional density and terminal velocity of graupel particles is examined, using for verification experimental graupel growth data from Pflaum and Pruppacher. An approach to calculating the collection efficiency is recommended, based on theoretical considerations and supported by the experimental data. Techniques for calculating the density during accretion and the terminal velocities of graupel which grow from frozen drops are developed.

This study examines the aircraft observations and theoretical evolution of particles above, through, and below the melting layer in the stratiform region associated with a mesoscale convective system (MCS). The aircraft data were obtained from an advecting spiral descent where the descent rate approximately corresponded to the typical hydrometeor fall speeds. The microphysical and thermodynamic measurements not only allowed us to characterize the particle evolution, but also enabled us to compare them with the theoretical evolution of the particles in the melting layer and to quantify the associated heating and cooling rates. Even though complete melting requires a fairly deep layer, most of the mass melts, and thus most of the cooling occurs in a thin layer above the location of the radar bright band. Based upon the magnitude of vertical velocity fluctuations, the layers below the melting layer appear to be decoupled from those above. The ice water content above the melting layer is 2–3 times th...

The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements. The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region f...

Relationships between radar reflectivity and ice water content are derived from pendmtions into thundemonn anvils in Montana on seven days during the Cooperative Convective Precipitation Experiment (CCOPE), using aircraft data and radar reflectivity, based upon an approach which minimizes the errors in converting measured crystal size to mass. Other sources of error do exist, particularly when measurements are taken in the vicinity of convective cells. The effects of truncation of the measured size spectrum due to sampling volume limitations are discussed. Ice water content (IWC) values predicted from the curves for most of the cases investigated are about the same for a given value of the radar reflectivity factor. Derived curves differ significantly in some cases from those applied to thunderstorm anvils in the past. A sensitivity study is performed to develop an improved mass-diameter relationship for anvil crystals. The choice of Z-IWC relationship has a major effect upon the estimate of the ...

Measurements of 461 planar crows and 312 graupel reported by Kajikawa have been used to develop equations for calculating the terminal velocity and mass of lump and conical graupel and of planar crystals of a variety of shapes and degrees of riming, the resultant equations being applicable to numerical modeling studies. Because all of the parameters required for the derivation of particle drag coefficients were known, the equations are more accurate than those previously used. The implications of the changed values for terminal velocity for these particle types are discussed with respect to the growth of particles through accretion.

The transport of water substance (ice and vapor) into the anvils of midlatitude continental thunderstorms is examined. Doppler radar reflectivity fields and horizontal and vertical windfields, in situ anvil measurements, and environmental soundings were used to estimate the anvil water mass flux for approximately five-minute intervals over one hour periods in six moderate to severe storms. Vapor and ice mass fluxes into the anvil time-averaged for the study periods are about equal. Ratios of the time-averaged sums of these fluxes (A¯) to aircraft-derived cloud base influx (from Fankhauser) range from 18% to greater than 100%. Estimated accuracies are ±30 to 40%. Anvil fluxes exceed rainout at cloud base level as derived from radar reflectivity data by Fankhauser for half the storms. It is shown that influx values alone are not reliable predictors of total storm condensation rates. The water mass storage term is evaluated and is found to be unimportant in relation to influx for all but one storm s...

Measurements within continental convective clouds in the Highveld region of South Africa indicate that the first appreciable nucleation of ice occurs between the −9° to −12°C levels as the cloud top rises through these levels. Moreover, these crystals usually take the form of double plates. Frozen drop centers were observed in 30% of the double crystals, with the diameter of the frozen drop between 10 and 20 μm. The growth of these ice crystals is investigated using ice crystal measurements collected in situ as well as a modeling study. New information regarding axial dimensions, bulk densities, and riming characteristics of double-plate crystals is presented. Numerical simulations were performed to compare the growth characteristics of ice crystals nucleated at different temperature levels between −6° and −16°C in updrafts of 1 and 3 m s−1. Crystals nucleated at temperatures ≥−12°C are shown to start riming at smaller sizes than previously thought and these crystals are potentially important as ...

Velocity adjustments are evaluated for altitude changes using Reynolds number-Davies number correlations of the form Re = aXb which have been obtained from empirical fall velocities of ice particles. In general, the altitude adjustment was found to vary with both pressure and temperature, except for a temperature-independent range near b ≈ 0.7. A quantitative evaluation of b, using the drag on a sphere, shows that altitude adjustments for precipitation particles are less sensitive to changes in temperature than pressure, and that the net adjustment is reduced by compensation between the two effects. A comparison between the X-Re method of Heymsfield and Kajikawa (1987) and the Reynolds number method of Beard (1980), developed from drag data using models of hydrometeor shapes, yields similar velocity adjustments for altitude changes. The agreement suggests that X-Re formulas, based on X for ice particles of one type, but different masses, can also be used for altitude adjustments because the shape...

A simple parameterization is presented which allows calculation of surface averaged radial impact velocities for droplets colliding with spheres as a function of the Reynolds and Stokes numbers. These impact velocities are averaged over the collector particle surface, assuming that the incoming droplets are uniformly distributed in space. The results extend the experimental graupel density measurements of Pflaum and Pruppacher to a wide range of graupel masses, sizes, and fallspeeds.

Results are given from analysis of the aggregation of thick plate, columnar, and bullet rosette ice crystals in cirrus. Data were obtained from PMS 2D-C images, oil coated slides, and aircraft meteorological measurements. Crystal size ranged from 100 to 900 microns in temperatures from -30 to -45 C. The results indicate that the ratio of the sizes of aggregating crystals and the difference of their terminal velocities are important in aggregation. The collection efficiency was calculated for the thick plate crystals from the same data.

Microphysical measurements in and near the weak echo region of a supercell thunderstorm are discussed. The observations were made in southeastern Montana with an armored T-28 aircraft, which has the capability to measure hydrometeors over almost the entire spectrum between about 3 μm and 5 cm diameter. The storm exhibited many of the classic supercell characteristics, such as a well-developed weak echo region, overhang, persistent hook echo, and a large high-reflectivity core. Peak updrafts in the weak echo region exceeded 50 m s−1, and a continuous region of updraft extending over a horizontal distance of more than 14 km was observed. The updraft core appeared to be undiluted, but the edges of the updraft were clearly mixed with air from other regions of the storm. Virtually no ice particles were observed in the weak echo region, but the cloud liquid water concentrations exceeded 6 g m−3. Hail larger than 4 cm was encountered in several locations to the west of the weak echo region. The observat...

The physical characteristics of graupel particles were investigated from in in situ collections of ice particles sampled in first-echo summertime cumulus congestus clouds in northeastern Colorado with the NCAR/NOAA sailplane operating during the National Hail Research Experiment. Ice particles were collected in vials containing silicone oil. Each particle was photographed at several different orientations and then melted to determine its equivalent diameter. This permitted the mass and axial dimensions to be determined and the density, axial ratios and terminal velocity to be estimated for each of 125 particles. The mass and terminal velocity of graupel particles were found to be considerably lower than those of equivalent diameter ice spheres. Best-fit equations to the mass-diameter and terminal velocity-diameter data were computed. Graupel densities were typically lower than 0.5 g cm−3 systematic differences were noted between the densities of lump and conical graupel. Calculations indicated th...

The structure and circulations of the cirrus uncinus generating head were determined from aircraft measurements of the temperatures, horizontal wind velocities and particle spectra at different altitudes. Stable layers were found to exist directly above and below the head. The head was found to exist in a region with a dry adiabatic lapse rate. Waves were observed in the stable layer below the head. The head was found to he divided into two regions in active cirrus uncinus. The upshear part of the head is the updraft region, and the downshear part the downdraft region. A region containing almost no crystals was found to separate the up- and downdraft regions. This “hole” was typically 150 m across. The vertical velocities in cirrus uncinus were determined from aircraft and Doppler radar measurements. Typical vertical velocities were estimated to range from 100–200 cm s−1 from aircraft particle measurements, and determined from Doppler radar measurements to range from 120–180 cm s s−1 Typical down...

The processes of development of graupel and had which fell to the ground from a storm in northeastern Colorado on 22 July 1976 are investigated over a one-hour period. The growth and trajectories of 130 000 particles of different types and sizes are calculated in the measured three-dimensional wind fields. The growth conditions that these graupel and hail (spherical ice particles smaller than and larger than 1 cm, respectively) experience are presented and their trajectories are described. Particles which become hail are those whose terminal velocities are nearly equal to the vertical velocities of the air parcels in which they develop. This enables them to fall into regions of relatively high liquid water content in the main updraft cores. The terming velocities of the particles which become graupel are not as well-matched to the parcel velocities; particles grow with lower liquid water contents. Embryos of the graupel and hail are found to be aggregates (snowflakes) of 0.5–1.5 cm in diameter. E...

Equations are developed to compute the size of particles partially contained within the sampling area of the PMS two-dimensional particle size spectrometer. This technique permits the effective sampling volume of the device to be increased considerably over the sampling volume obtained by the two other techniques in common usage. Size spectra obtained by this technique are compared with size spectra obtained by the two other techniques and with spectra from two other probes on the T-28 aircraft in a thunderstorm in northeast Colorado.

Measurements from the National Center for Atmospheric Research Sabreliner aircraft are combined with a multiple Doppler radar synthesis of the wind field to investigate particle growth processes in the anvil region of a severe thunderstorm. The aircraft measurements, obtained in mid- to lower anvil levels at temperatures from −25° to −36°C, show than the size spectra broaden with decreasing altitude, yet ice water content values and other measurements indicate this to be a zone of evaporation. Aggregation—allowing particles to develop to sizes as 1 cm—accounts for this observed growth. Intensification of the storm over the observational period results in the development of increasingly large aggregates. Growth histories and trajectories of anvil particles are calculated using the wind field measurements and are compared to the hydrometeor measurements. Calculations show that particles forming the outer flanks of the anvil initiate in the upwind (relative to the midlevel wind) portions of the updr...

A laboratory experiment was initiated to grow the banded columnar crystals found in cirrus clouds and to determine if they nucleated through a freezing nucleus. Banded columnar crystals were collected in cirrus clouds and also grown in a laboratory cold box over a temperature range of -20 to -46C. They were observed to grow from a frozen droplet through several distinct crystalline transitions. The first distinct crystalline form observed following the growth of a frozen droplet was a polyhedral crystal. With further growth, the polyhedral crystal developed end plates, and it appeared to be a 'double plate.' Then the separation between the plates nearly filled in to leave the characteristic band. Certain columns (-6 to -10C) and plates (-10 to -20C) were also observed to grow from a frozen droplet and showed similar crystalline transitions.

Terminal velocities of different ice crystal forms were calculated, using the most recent ice crystal drag coefficients, aspect ratios, and densities. The equations derived were primarily for use in calculating precipitation rates by sampling particles with an aircraft in cirrus clouds, and determining particle size in cirrus clouds by Doppler radar. However, the equations are sufficiently general for determining particle terminal velocity at any altitude, and almost any crystal type. Two sets of equations were derived. The 'general' equations provide a good estimate of terminal velocities at any altitude. The 'specific' equations are a set of equations for ice crystal terminal velocities at 1000 mb. The calculations are in good agreement with terminal velocity measurements. The results from the present study were also compared to prior calculations by others and seem to give more reasonable results, particularly at higher altitudes.

What is new in this manuscript is a method of using aircraft observations from a long horizontal path through an ice cloud to produce properly correlated 2D fields of particle counts consistent with the observations, including all null values, at several different sizes for use in algorithm development and in scientific studies. A Bayesian approach is used to define the distributions of average counts, P(C), at every size. These are, in turn, used to expand the number of values at each particle size by a factor of 50. These data, then fill a square 2D grid of 20 × 20 km at 100-m resolution. At each grid point, the number concentrations corresponding to each particle size define the particle size distributions (PSD). A method for assuring the proper correlation of counts at each size over the entire grid is devised and discussed. These PSD can then be integrated to yield a number of different quantities over the entire grid such as radar reflectivities and ice water contents. From this perspective, one can then consider the set of observations as just one realization from a much larger ensemble of possible realizations by giving fuller expression to all of the information contained within the observed correlation functions and P(C)s. The in situ observations, however, remain crucial since this method does not ‘make-up’ new meteorology but simply gives wider expression to the meteorology contained within the observations.

A system for interactively editing Particle Measuring Systems (PMS) two-dimensional (2-D) probe data, which has been processed using automated techniques is described. The system allows for reassignment of particle habits and sizes using a touch-screen computer terminal, which greatly simplifies editing of the processed 2-D data. This system, or simpler systems using graphics terminals alone, when used in conjunction with laboratory experiments that are being conducted to relate 2-D images to their actual (photograhic) shape, can lead to improvements in the interpretations from the data.

The growth of ice particles through aggregation is investigated for seeded clouds using currently available field data and a numerical particle-growth model. Observations indicate that the aggregation process is fairly common, even when moderate liquid water contents, ~0.5 g M−3, are available for particle growth through accretion. The modeling study suggests that certain temperature ranges are especially conducive to aggregate formation.

The link between stratiform precipitation microphysics and multifrequency radar observables is thoroughly investigated by exploiting simultaneous airborne radar and in situ observations collected from two aircraft during the OLYMPEX/RADEX (Olympic Mountain Experiment/Radar Definition Experiment 2015) field campaign. Above the melting level, in situ images and triple-frequency radar signatures both indicate the presence of moderately rimed aggregates. Various mass-size relationships of ice particles and snow scattering databases are used to compute the radar reflectivity from the in situ particle size distribution. At Ku and Ka band, the best agreement with radar observations is found when using the self-similar Rayleigh-Gans approximation for moderately rimed aggregates. At W band, a direct comparison is challenging because of the non-Rayleigh effects and of the probable attenuation due to ice aggregates and supercooled liquid water between the two aircraft. A variational method enables the retrieval of the full precipitation profile above and below the melting layer, by combining the observations from the three radars. Even with three radar frequencies, the retrieval of rain properties is challenging over land, where the integrated attenuation is not available. Otherwise, retrieved mean volume diameters and water contents of both solid and liquid precipitation are in agreement with in situ observations and indicate local changes of the degree of riming of ice aggregates, on the scale of 5 km. Finally, retrieval results are analyzed to explore the validity of using continuity constraints on the water mass flux and diameter within the melting layer in order to improve retrievals of ice properties.

This study addresses whether high concentrations of small ice crystals in the upper 1 km or so of high, thick tropical cirrus clouds are principally responsible for the highly reflective clouds observed over the equatorial Pacific “warm pool.” This region of the tropics has recently been shown to contain extensive shields of cirrus clouds which significantly influence the global climate through their effect on the radiation budget of the tropics. In-situ and remote sensing measurements of cloud microphysical and radiative properties from field programs in the central and western tropical Pacific and radiative transfer calculations are used to derive distributions of cloud microphysical properties with height and their relationship to cloud radiative properties. Clouds associated directly with convection are shown to have sufficiently high optical depths near cloud top to produce localized areas of bright or optically “thick” cirrus, reflecting more than 40% of the incoming solar radiation. However, in general the upper parts of cirrus cannot alone account for the high albedos (fraction of incoming solar energy reflected) but do contribute substantially when high albedos are observed. The lower parts of the cirrus, in some cases extending down to the melting layer or below when they are called stratiform cloud regions, are usually necessary to produce high albedos.

Profiles of in situ measurements of ice crystal size distribution of subtropical cirrus were used to calculate solar spectral irradiances above and below the clouds. Spheres and nonspherical ice crystal habits (columns, hollows, plates, bullets, and aggregates) were assumed in the calculations. The simulation results were compared to irradiance measurements from the NASA Solar Spectral Flux Radiometer. The microphysical and radiation data were collected by three aircraft during CRYSTAL-FACE. Two cirrus cases (optical thickness of about 1 and 7) from two mission dates (26 and 23 July 2002) were investigated in detail. The measured downwelling and upwelling irradiance spectra above the cirrus could mostly be reproduced by the radiation model to within +/- 5-10% for most ice crystal habits. Below the cirrus the simulations disagreed with the measured irradiances due to surface albedo variability along the flight track, and nonoptimal colocation between the microphysical and irradiance measurements. The impact of shape characteristics of the crystals was important for the reflected irradiances above the optically thin cirrus, especially for small solar zenith angles, because in this case single-scattering dominated the solar radiation field. For the cirrus of moderate optical thickness the enhanced multiple scattering tended to diminish particular shape features caused by nonspherical single-scattering. Within the ice absorption bands the shape-related differences in the absorption characteristics of the individual nonspherical ice crystals were amplified if multiple scattering prevailed. Furthermore, it was found that below the cloud the shape sensitivity of the downwelling irradiance spectra is larger compared to the nonsphericity effects on reflected irradiances above the cirrus. Finally, it was shown that the calculated cirrus solar radiative forcing could vary by as much as 26% depending on the ice crystal habit.

A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions.

Measurements of ice water content (IWC) and mean ice-crystal size and concentration made by two in-situ probes, CVI and PVM, were compared on the DC-8 aircraft during SUCCESS flights in orographic ice clouds. The comparison of IWC in these wave clouds, that formed at temperatures of about −38 °C on April 30 and −62 °C on May 2, 1996, showed good agreement. The comparison of ice crystal concentrations agreed better for the April-30 clouds than for the May-2 clouds; and the effective radius compared for both probes and for remote retrievals from aircraft and satellite for a segment of the Berthoud wave cloud (May 2) agreed within 30%. The measured parameters of the ice crystals were similar to earlier measurements and recent modeling of cold wave clouds.

As the near-surface atmosphere over the Antarctic Plateau is cold and pristine, its physico-chemical conditions resemble to a certain extent those of the high-troposphere where cirrus clouds form. In this paper, we carry out an observational analysis of two shallow fog clouds forming at cirrus-temperatures - that is, temperatures lower than 235 K - at Dome C, inner Antarctic Plateau. The combination of lidar profiles with temperature and humidity measurements from advanced thermo-hygrometers along a 45-m mast makes it possible to characterise the formation and development of the fog. High supersaturations with respect to ice are observed before the initiation of fog and the values attained suggest that the nucleation process at play is the homogeneous freezing of solution aerosol droplets. To our knowledge, this is the first time that in situ observations show that this nucleation pathway can be at the origin of an ice fog. Once nucleation occurs, the relative humidity gradually decreases down to subsaturated values with respect to ice in a few hours, likely owing to vapour deposition onto ice crystals and turbulent mixing. The development of fog is tightly coupled with the dynamics of the boundary-layer which, in the first study case, experiences a weak diurnal cycle while in the second case, transits from a very stable to a weakly stable dynamical regime. Overall, this paper highlights the potential of the site of Dome C for carrying out observational studies of cloud microphysical processes in natural conditions and using in-situ ground-based instruments.

This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water contents and ice crystal effective radii decreased with time. Meanwhile, the anvil thinned vertically and maintained a stratification similar to its environment. Because effective radii were small, radiative heating and cooling were concentrated in layers approximately 100 m thick at the anvil top and base. A simple analysis suggests that the anvil cirrus spread laterally because mixing in these radiatively driven layers created horizontal pressure gradients between the cloud and its stratified environment. The TTC layer also spread but, unlike the anvil, did not dissipate—perhaps because the anvil shielded the TTC from terrestrial infrared heating. Calculations of top-of-troposphere radiative forcing above the anvil and TTC showed strong cooling that tapered as the anvil evolved.

Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud satellite radar (CloudSat) data. Integrating these data with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data may further increase accuracy. Evaluations of the accuracy of IWC retrieved from radar alone and together with other measurements are now essential. A forward model employing aircraft Lagrangian spiral descents through mid- and low-latitude ice clouds is used to estimate profiles of what a lidar and conventional and Doppler radar would sense. Radar reflectivity Ze and Doppler fall speed at multiple wavelengths and extinction in visible wavelengths were derived from particle size distributions and shape data, constrained by IWC that were measured directly in most instances. These data were provided to eight teams that together cover 10 retrieval methods. Almost 3400 vertically distributed points from 19 clouds were used. Approximate cloud optical depths ranged from below 1 to more than 50. The teams returned retrieval IWC profiles that were evaluated in seven different ways to identify the amount and sources of errors. The mean (median) ratio of the retrieved-to-measured IWC was 1.15 (1.03) ± 0.66 for all teams, 1.08 (1.00) ± 0.60 for those employing a lidar–radar approach, and 1.27 (1.12) ± 0.78 for the standard CloudSat radar–visible optical depth algorithm for Ze > −28 dBZe. The ratios for the groups employing the lidar–radar approach and the radar–visible optical depth algorithm may be lower by as much as 25% because of uncertainties in the extinction in small ice particles provided to the groups. Retrievals from future spaceborne radar using reflectivity–Doppler fall speeds show considerable promise. A lidar–radar approach, as applied to measurements from CALIPSO and CloudSat, is useful only in a narrow range of ice water paths (IWP) (40 < IWP < 100 g m−2). Because of the use of the Rayleigh approximation at high reflectivities in some of the algorithms and differences in the way nonspherical particles and Mie effects are considered, IWC retrievals in regions of radar reflectivity at 94 GHz exceeding about 5 dBZe are subject to uncertainties of ±50%.