Graupel Particles Research Articles

Anvil clouds of tropical mesoscale convective systems in monsoon regions

AbstractThe anvil clouds of tropical mesoscale convective systems (MCSs) in West Africa, the Maritime Continent and the Bay of Bengal have been examined with TRMM and CloudSat satellite data and ARM ground‐based radar observations. The anvils spreading out from the precipitating cores of MCSs are subdivided into thick, medium and thin portions. The thick portions of anvils show distinct differences from one climatological regime to another. In their upper portions, the thick anvils of West Africa MCSs have a broad, flat histogram of reflectivity, and a maximum of reflectivity in their lower portions. The reflectivity histogram of the Bay of Bengal thick anvils has a sharply peaked distribution of reflectivity at all altitudes with modal values that increase monotonically downward. The reflectivity histogram of the Maritime Continent thick anvils is intermediate between that of the West Africa and Bay of Bengal anvils, consistent with the fact this region comprises a mix of land and ocean influences. It is suggested that the difference between the statistics of the continental and oceanic anvils is related to some combination of two factors: (1) the West African anvils tend to be closely tied to the convective regions of MCSs while the oceanic anvils are more likely to be extending outward from large stratiform precipitation areas of MCSs, and (2) the West African MCSs result from greater buoyancy, so that the convective cells are more likely to produce graupel particles and detrain them into anvils. Copyright © 2009 Royal Meteorological Society

Electrization and spatial charge separation at the air bubbles exhalation that occurs during the coagulation growth of hail particles in a cloud. Part 2. Generation of thunderstorm electricity due to the charged bubbles exhalation caused by freezing of overcooled cloud drops on the hail particles surface

It has been found that air bubbles, which exhale during freezing of overcooled cloud drops on the hail or graupel particles surface, carry away a great amount of positive charge to the surrounding space while the compensating negative charge remains on the solid particles of hydrometeors. This conception was used for computation of rates of charge accumulation on the hail particles and generation of thunderstorm electricity in deep convective clouds. It is shown that this conception describes the most powerful mechanism of thunderstorm electricity generation and satisfies all the requirements for the theory of thunderstorm electricity.

Electrization and spatial charge separation at the air bubbles exhalation that occurs during the coagulation growth of hail particles in a cloud. Part 1. Kinetics of the bubbles exhalation process caused by increasing temperature of overcooled cloud drops

Kinetics of the air bubbles exhalation process, which occurs in overcooled cloud drops when they hit the hail or graupel particles surface. Analytical relationship between a number of bubbles generated in a drop and their size depending on the degree of overcooling of cloud drops is revealed. It is shown that the air bubbles exhalation, which accompanies the coagulation growth of hail particles, occurs on the principle of “ebullition” of the liquid on the surface of a hail particle.

The development of ice in a cumulus cloud over southwest England

An experiment involving the FAAM BAe 146 aircraft, called the ICE and Precipitation Initiation in Cumulus (ICEPIC) project, was conducted in order to measure the microphysical properties of UK summertime cumulus clouds. A line of clouds was penetrated near the ascending tops. Higher concentrations of ice particles than expected from activation on typical ice nuclei using the Meyers formula were observed at relatively high temperatures (T>-10 °C). The observations of numerous ice particles and the coexistence of both small and large cloud droplets, pristine ice columns and graupel pellets within the temperature zone of -3 to -9 °C strongly suggested the Hallett–Mossop (HM) process of splintering during riming. Agreement between the calculated and observed rates of splinter production supported this suggestion. The Model of Aerosols and Chemistry in Convective Clouds (MAC3) was utilized to establish a quantitative understanding of the observed development of glaciation of this cloud. The results of the model confirmed the important role of the HM process. They also showed that the warm rain process was fundamental to the production of graupel in the cloud studied, and hence the HM ice particles. A sensitivity test with double the concentration of aerosol particles showed that the concentration of supercooled raindrops decreased as expected, which resulted in fewer graupel particles and a smaller quantity of precipitation, which was delayed by about 5 min. However, the production rate of secondary ice particles generated by the HM process increased due to the increased concentration of small cloud droplets.

A modelling study of the effect of ice particle sizes and relative velocity on ice crystal/graupel collisional charge transfer

The effect of ice particle sizes and relative velocity on the sign of charge transfer during graupel/ice crystal interactions in thunderstorms is considered based on the concept of the Relative Growth Rate Hypothesis that the ice surface growing faster by water vapour diffusion charges positively. Diffusional growth rates of ice crystals and riming graupel particles are calculated for various particle sizes and velocities, and the sign of charging during crystal/graupel collisions is determined. The study reveals that larger graupel charges more negatively, while larger ice crystals, or higher relative velocity, lead to increased positive graupel charging.

New parameterization of non-inductive charge transfer based on previous laboratory experiments

Empirical equations for the sign and magnitude of the separated non-inductive charge during rebounding collisions of ice-crystal and graupel are obtained using discriminant and regression analyses. The original source of data is Takahashi's laboratory experiment [Takahashi, T., 1978. Riming electrification as a charge generation mechanism in thunderstorms. J. Atmos. Sci. 35. 1536–1548], who presented measured charge transfer values as isolines, function of cloud temperature and cloud water content. The simulations show that the use of these equations in a numerical cloud model gives similar values for the total charge density in a simulated thundercloud to those obtained using the lookup table of Takahashi's data as implemented by many authors. Tests reveal that the use of the proposed equations is computationally more efficient for numerical simulations of thunderstorm electrification comparing to the use of the look-up table. In addition, the results show that the different parameterization schemes for the charge transfer to a graupel particle due to a single ice crystal interaction proposed by Takahashi [Takahashi, T., 1984. Thunderstorm electrification — a numerical study. J. Atmos. Sci., 41. 2541–2558] and Saunders et al. [Saunders, C.P.R., Keith, W.D. and Mitzeva, R.P., 1991. The effect of liquid water on thunderstorm charging. J. Geophys. Res. 96. 11007–11017], may affect significantly the total charge density in the simulated cloud case.

Nonspherical and spherical characterization of ice in Hurricane Erin for wideband passive microwave comparisons

In order to better understand the characteristics and physical‐to‐radiative relationships of frozen hydrometeors in hurricane systems, computed brightness temperatures (TB) from 10.7 to 183 ± 10 GHz were compared with radiometric observations of Hurricane Erin (2001) from the NASA ER‐2 aircraft. The focus was on the high frequencies (≥85 GHz) that are particularly sensitive to frozen hydrometeors. In order to initialize the cloud profiles used in the radiative transfer calculations, data from airborne radars, dropsondes, and cloud models were used. Three different ice habit and size parameterizations were used with these cloud profiles to obtain the particle radiative signatures including (1) spherical particles with size distributions derived from in situ observations, (2) spherical “fluffy” snow and graupel particles with modified Marshall‐Palmer size distributions, and (3) a non‐spherical bullet rosette habit where the radiation attributes (scattering, absorption, and asymmetry properties) were computed using the Discrete Dipole Approximation. In addition, three different reflectivity to ice water content (Z‐IWC) relationships were used with the three habit and size parameterizations to provide a measure of the sensitivity of the Z‐IWC relationship. This work showed that both the scattering and asymmetry coefficients, along with the ice water content in each layer, play an important role in determining the resultant high frequency brightness temperatures. All low frequency (<40 GHz) calculations matched the observations with correlation coefficients greater than 0.9. At higher frequencies (> 90 GHz), correlation coefficients ranged from 0.7 to 0.92. Comparing between the three ice habit and size parameterizations showed less than a 0.2 difference in correlation coefficient, while the comparisons between the three Z‐IWC relationships caused changes of up to 0.15 in the correlation coefficients, but they had a significant effect on the mean differences between the observations and the calculations.

A New Method for Representing Mixed-phase Particle Fall Speeds in Bulk Microphysics Parameterizations

Here we present a simple method of improving bulk mixed-phase microphysical schemes to allow for a more realistic representation of partially rimed particles. The new procedure unifies the snow and graupel particles by assigning a single fallspeed to both that is weighted by the mixing ratios, and applying that fallspeed to both sedimentation and accretion processes. This avoids the problem of the species separating out by sedimentation as graupel forms, and the further problem of graupel then accreting snow too quickly because of its high relative fallspeed. Instead the unified graupel/snow moves together and evolves in its relative ratio due to riming, behaving as intermediate or partially rimed particles.Tests of the new method were carried out using the Weather Research and Forecasting (WRF) Single-Moment 6-class (WSM6) microphysics scheme in a high-resolution idealized simulation, and mesoscale heavy precipitation events in the summer and winter over Korea. The effect of the new accretion rates on cloud structure and precipitation was found to be greater than that of the changed sedimentation alone. Verification of these tests showed a much-reduced production of graupel and more snow, influencing the cloud structure and surface precipitation fields. The scheme shows promise in improving precipitation intensity and precipitation type forecasts.

Evaluation of radar multiple scattering effects in Cloudsat configuration

Abstract. MonteCarlo simulations have been performed to evaluate the importance of multiple scattering effects in co- and cross-polar radar returns for 94 GHz radars in Cloudsat and airborne configurations. Thousands of vertically structured profiles derived from some different cloud resolving models are used as a test-bed. Mie theory is used to derive the single scattering properties of the atmospheric hydrometeors. Multiple scattering effects in the co-polar channel (reflectivity enhancement) are particularly elusive, especially in airborne configuration. They can be quite consistent in satellite configurations, like CloudSat, especially in regions of high attenuation and in the presence of highly forward scattering layers associated with snow and graupel particles. When the cross polar returns are analysed [but note that CloudSat does not measure any linear depolarization ratio (LDR hereafter)], high LDR values appear both in space and in airborne configurations. The LDR signatures are footprints of multiple scattering effects; although depolarization values as high as −5 dB can be generated including non-spherical particles in single scattering modelling, multiple scattering computations can produce values close to complete depolarization (i.e. LDR=0 dB). Our simulated LDR profiles from an air-borne platform well reproduce, in a simple frame, some experimental observations collected during the Wakasa Bay experiment. Since LDR instrumental uncertainties were not positively accounted for during that experiment, more focused campaigns with air-borne polarimetric radar are recommended. Multiple scattering effects can be important for CloudSat applications like rainfall and snowfall retrievals since single scattering based algorithms will be otherwise burdened by positive biases.

A modelling study of the effect of cloud saturation and particle growth rates on charge transfer in thunderstorm electrification

Numerical studies have been made of the importance of cloud saturation to the sign of charge transfer during graupel/ice crystal interactions in thunderstorms. Previous laboratory studies led to the idea that the diffusional growth rates of the interacting ice surfaces may influence the sign of the charge transferred during brief collisional contact. The ice crystals grow by vapour diffusion in a supersaturated environment while the graupel surface grows by diffusion under low accretion rate conditions, but will sublimate when heated sufficiently by riming. The graupel surface is also influenced, even under net sublimation conditions, by the vapour released to it from droplets freezing on its surface. In a cloud, the diffusional growth rates are also affected by ventilation when the supercooled droplets and their local environment flow past the riming surface. The diffusional growth rates of ice crystals and riming graupel particles are calculated for various cloud saturation ratios, then the sign of electric charge transferred during crystal/graupel collisions is determined according to the concept of the relative vapour diffusional growth rates, according to Baker et al., 1987 [Baker, B., Baker, M.B., Jayaratne, E.J., Latham, J., Saunders, C.P.R., 1987. The influence of diffusional growth rates on the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones. Quart. J. Roy. Met. Soc. 113, 1193–1215]. It is found necessary, in order to account for the observation of positive charging of riming graupel at high accretion rates, to modify the assumptions of Baker et al. in order to increase the flux of vapour to the graupel surface. The variable growth parameters available may be adjusted to represent the environmental saturation conditions in various laboratory experiments, including the mixing of clouds from regions having different growth conditions, and are used to determine charge sign sensitivity to cloud saturation ratio, temperature and accretion as measured by the cloud effective liquid water content.

Observations of Particle Size and Phase in Tropical Cyclones: Implications for Mesoscale Modeling of Microphysical Processes

Abstract Mesoscale model simulations of tropical cyclones are sensitive to representations of microphysical processes, such as fall velocities of frozen hydrometeors. The majority of microphysical parameterizations are based on observations obtained in clouds not associated with tropical cyclones, and hence their suitability for use in simulations of tropical cyclones is not known. Here, representations of mass-weighted fall speed Vm for snow and graupel are examined to show that parameters describing the exponential size distributions and fall speeds of individual hydrometeors [through use of relations such as V(D) = aDb] are identically important for determining Vm. The a and b coefficients are determined by the composition and shape of snow and graupel particles; past modeling studies have not adequately considered the possible spread of a and b values. Step variations in these coefficients, associated with different fall velocity regimes, however, do not have a large impact on Vm for observed size dis...

Dual-Doppler Radar Observations on Factors Causing Differences in the Structure of Snow Clouds during Winter Monsoon Surges

Various processes of development in a convective cell were investigated in order to clarify the factors causing differences in the structure of snow clouds. Convective cells in snow clouds were observed by two Doppler radars over Ishikari Bay, Hokkaido, Japan, during winter monsoon surges. Seven cells that allowed us to examine structural changes were selected for the investigation. From the time series of maximum reflectivity and vertical mass flux in each cell, the analyzed cells were classified into two groups: “rapid change” and “slow change.” The former was further divided into two types according to the strength of vertical wind shear in the environment: rapid change in weak shear (Rw), and rapid change in strong shear (Rs). The structure of the Rw-type changed greatly, due to the formation and the descent of a high-reflectivity core (> 30 dBZ), and the subsequent dissipation of the updraft. Since the updraft hardly tilted and collapsed when the core descended in the updraft, loading by graupel particles was suggested to be effective to terminate the updraft. The structure of the Rs-type changed greatly due to the short lifetime of the updraft that was tilted toward the downshear side due to the strong vertical shear. In contrast, the cell of slow-change group (S-type) maintained its structure because of a persistent updraft. Since the updraft, which was tilted hardly, was sustained even when a core (≤30 dBZ) descended in the updraft region, the loading effect was thought to be small due to the small amount of graupel particles. From a comparison of the three types, it was found that the different features of the cell’s structure were caused by two factors: the tilt of the updraft, and the effect of loading by graupel particles. Based on these results, we discussed the maintenance process of a snow cloud composed of convective cells. The fact that convective cells in the past observational studies can also be classified in the same way as cells analyzed in the present study, was also discussed. The relationship between the growth of graupel particles, and the temperature in the environment, was also discussed.

Shear strength and angle of repose of snow layers including graupel

AbstractSnow avalanches with failure layers including graupel are sometimes observed in Japan. A snow layer including graupel may be formed close to the melting point (0°C) after passage of a cold front with cumulus clouds. Graupel particles have a large diameter and a spherical shape, so they have a relatively small contact area per unit volume and may form weak layers within the snowpack. In this paper, characteristics of snow layers that include graupel are described by angle of repose, shear frame index (SFI), grain-size distribution, density and temperature. The mean value of the angle of repose was found to be 45°, independent of the grain-size. This suggests that a graupel layer near 0°C will not be formed on a slope exceeding 45°. SFI data for three different graupel layers were measured over 1–3 days for each layer, and were correlated with density. An exponential relationship between shear strength and density was found. The results indicate that the SFI of graupel layers is approximately one-third that of dry new snow with the same density.

Effective Ice Particle Densities Derived from Aircraft Data

In this study, aircraft data are used to derive effective ice particle densities. This density is defined as the ice particle mass divided by the volume of an equivalent diameter sphere. Measured ice particle size distributions and total ice water contents are used to derive effective ice densities for ice particle populations (ρe) as a function of particle size [ρe(D)]. The density values are critical for modeling and remote sensing applications. The method uses particle size distributions (PSDs) measured by several particle spectrometers to compute the total particle volume per unit volume of air, assuming that the particles are spheres. Simultaneous direct measurements of ice water content from a counterflow virtual impactor (CVI) yield values for the number of grams of ice per unit volume of air, enabling the overall effective ice density for a population to be calculated. The measured PSD together with the CVI measurements are used to derive mass–dimension relationships. The methods are applied to measurements acquired in two field programs. More than 1200 population densities were derived from the Atmospheric Radiation Measurement (ARM) program and more than 5500 for the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) in southern Florida during July 2002. The population densities are represented in terms of two properties of particle size distributions: the spectral slope and the median mass diameter. The datasets have been divided into populations associated with predominantly synoptically generated ice cloud regions, convectively generated ice cloud regions, regions with moderately to heavily rimed and graupel particles, and those within the melting layer. Average particle density relationships are derived for each regime. Values of ρe are generally higher in synoptically than convectively generated cloud layers, and rimed particles are denser than unrimed ones. Values of ρe also decrease systematically downward within the ice clouds except in the melting layer, where they increase downward.

A Case Study of the Origin of Hail in a Multicell Thunderstorm Using In Situ Aircraft and Polarimetric Radar Data

An armored T-28 research aircraft made direct observations of the hydrometeors present at approximately the 238C temperature level in the inflow region of a multicell thunderstorm. During the penetration, both the Colorado State University (CSU)‐University of Chicago and Illinois State Water Survey (CHILL) 11-cm-wavelength dualpolarization research radar and the Denver, Colorado, Front Range Airport (KFTG) Weather Surveillance Radar1988 Doppler (WSR-88D) were scanning this storm. Polarimetric radar indications of hail (high reflectivity and low differential reflectivity) appeared near the surface in the echo core adjacent to the aircraft track approximately 6 min after the T-28’s inflow transit. Radial velocity data from the KFTG radar were combined with those recorded at CSU‐CHILL to synthesize the airflow fields in the storm around the time of the T-28 penetration. Hail trajectories were initiated from a location at which the T-28 encountered a burst of approximately 1-cmdiameter, low-density graupel particles within the general storm inflow region. Forward-time trajectory calculations indicated that these graupel particles subsequently grew slightly into small hailstones and ended up within a few kilometers of the near-surface polarimetric radar hail-signature location. Trajectories computed backward in time imply that these hail embryos originated aloft in the forward portion of the echo complex. These are the first quantitative, direct in situ observations of recirculating precipitation becoming embryos for hail development.

Extra large particle images at 12 km in a hurricane eyewall: Evidence of high‐altitude supercooled water?

The conventional wisdom about hurricanes suggests that updrafts are weak and supercooled water is scarce in the eyewall, and almost non‐existent at temperatures colder than about −5°C [Black and Hallett, 1986]. However, there is evidence that some hurricanes are different. Questions about the existence of high‐altitude supercooled cloud water cannot be answered with only the instruments aboard the typical propeller‐driven aircraft. During the summer of 1998, the NASA DC‐8 aircraft made penetrations of the intensifying eyewall of Hurricane Bonnie at 12 km MSL, collecting the first truly high‐altitude 2‐D particle imagery in a hurricane. The similarity of the splash images in Hurricane Bonnie to those from raindrops obtained at higher temperatures in other hurricanes suggests that the large images obtained by the DC‐8 were soft, low density graupel, rather than hard, high‐density graupel particles or frozen raindrops. This implies that these particles grew to several millimeters in diameter at altitude, rather than simply advecting from lower, warmer altitudes. This growth in turn requires the presence of deeply supercooled cloud droplets. Thermal emission from supercooled water aloft increases the microwave brightness temperatures, giving a misleading impression that there is much less ice aloft than actually exists. The extra attenuation from the occasional presence of large graupel at these altitudes reduces the ability of microwave sensors to see precipitation at lower altitudes. Both of these effects impede efforts to accurately quantify condensate mass remotely from radiometric data such as that provided by the TRMM satellite.

Rime and graupel: description and characterization as revealed by low-temperature scanning electron microscopy.

Snow crystals, which form by vapor deposition, occasionally come in contact with supercooled cloud droplets during their formation and descent. When this occurs, the droplets adhere and freeze to the snow crystals in a process known as accretion. During the early stages of accretion, discrete snow crystals exhibiting frozen cloud droplets are referred to as rime. If this process continues, the snow crystal may become completely engulfed in frozen cloud droplets. The resulting particle is known as graupel. Light microscopic investigations have studied rime and graupel for nearly 100 years. However, the limiting resolution and depth of field associated with the light microscope have prevented detailed descriptions of the microscopic cloud droplets and the three-dimensional topography of the rime and graupel particles. This study uses low-temperature scanning electron microscopy to characterize the frozen precipitates that are commonly known as rime and graupel. Rime, consisting of frozen cloud droplets, is observed on all types of snow crystals including needles, columns, plates, and dendrites. The droplets, which vary in size from 10 to 100 microm, frequently accumulate along one face of a single snow crystal, but are found more randomly distributed on aggregations consisting of two or more snow crystals (snowflakes). The early stages of riming are characterized by the presence of frozen cloud droplets that appear as a layer of flattened hemispheres on the surface of the snow crystal. As this process continues, the cloud droplets appear more sinuous and elongate as they contact and freeze to the rimed crystals. The advanced stages of this process result in graupel, a particle 1 to 3 mm across, composed of hundreds of frozen cloud droplets interspersed with considerable air spaces; the original snow crystal is no longer discernible. This study increases our knowledge about the process and characteristics of riming and suggests that the initial appearance of the flattened hemispheres may result from impact of the leading face of the snow crystal with cloud droplets. The elongated and sinuous configurations of frozen cloud droplets that are encountered on the more advanced stages suggest that aerodynamic forces propel cloud droplets to the trailing face of the descending crystal where they make contact and freeze.

The Precipitation Process in Convective Cells Embedded in Deep Snow Bands over the Sea of Japan

Convective cells in two deep snow bands were examined over the Sea of Japan with an instrumented aircraft and dual-Doppler radars. A combination of the relatively warm air stream (westerly) at lower levels secluded near the center of the low and the polar air stream (west-southwesterly) at middle and upper levels produced a deep unstable stratification to the southwestern quadrant of a well-developed low, where snow bands A1 and B1 were respectively observed at around 1600 and 1700 JST on 28 Jan. 1993. Radar data showed that both snow bands were in quasi-steady state within a dual Doppler radar observation area. However, the two snow bands showed quite different development; the maximum re-flectivities in bands A1 and B1 were 15 dBZ and 30 dBZ, respectively. Aircraft data showed that the accretional growth of snow particles was the primary precipitation formation mechanism in the convective cells embedded in both snow bands. From aircraft and radar data, it appears that vertical wind shear within the cloud layer influenced the precipitation process. In the cell embedded in snow band A1, the updraft at middle and upper levels tilted downshear due to a strong wind shear and precipitation embryos initiated in the updraft were advected downshear from the main supercooled cloud water region. Therefore, these embryos did not grow to large graupel particles. By comparison, in the cell embedded in snow band B1, the updraft tilted upshear at lower levels and stood almost erect at middle and upper levels due to weak wind shear. Therefore, precipitation embryos initiated in the updraft grew into 5 mm graupel particles as they descended through the main supercooled cloud water region. In addition, re-circulation of graupel particles, which had descended in a vicinity of the updraft core, seemed to play an important role to efficiently produce larger graupel particles.

Air motions and precipitation growth in Alpine storms

AbstractThis study documents two strong Mesoscale Alpine Programme (MAP) storms. Each occurred ahead of a strong baroclinic trough. Even though their 500 and 850 hPa flow patterns were strikingly similar upstream of the Alps, their flow at lower levels was different and resulted in different forms of orographic enhancement of the precipitation associated with baroclinic systems by the Alpine terrain. During Intensive Observing Period (IOP) 2b, the low‐level flow and thermodynamic conditions over the Alps north‐west of the Lago Maggiore region constituted an unstable atmosphere with Froude number Fr>1 (i.e. unblocked or flow‐over regime), while in IOP 8 the flow was stable with Fr<1 (i.e. blocked or flow‐around regime in the region immediately upstream of the slopes on the western side of Lago Maggiore). For IOP 2b (IOP 8) the wind field and precipitation patterns north‐west of the Lago Maggiore were very similar to the high‐ (low‐)Fr autumn climatology. Thus the two cases represent fundamentally distinct regimes of orographic modification of the baroclinic precipitation.During IOP 8 (blocked case), the strong stability and weak wind speed at low levels prevented the airflow from rising over the slopes north‐west of the Lago Maggiore region and forced it to turn away from the mountains. The precipitation over the mountains was produced as the strong flow above 900 hPa was forced over the blocked low‐level air as well as the terrain. In contrast, during IOP 2b (unblocked case) the flow was strong at all levels with low static stability; therefore the low‐level air rose easily over the abruptly rising terrain. Since the lower‐level air rose together with the upper‐level air, it could transport upward moisture unavailable in the blocked case; this moisture condensed to add significantly to the precipitation production in the unblocked case. In addition to the high Fr, the air was potentially unstable; the slight instability of the airstream impinging upon the upslopes favoured the development of convective cells over the lower slopes which were embedded in the stratiform background lifting, thus further enhancing the formation of cloud and precipitation on the lower windward slopes.Examination of all the cases observed by polarimetric radar in MAP confirmed the microphysical processes seen in IOPs 2b and 8, and suggests fundamental microphysical differences between the unstable unblocked, and stable blocked cases. In both cases precipitation forms by a simple stratiform process in the form of dry snow aloft, becoming wet snow at the melting layer and falling out as rain below. However, in the unstable unblocked cases rainfall is enhanced by the participation of the low‐level flow in the orographic lifting; when the low‐level air rises easily over the first high peaks of terrain, raindrops grow rapidly by coalescence at low levels, and graupel forms just above the 0°C level. The coalescence‐produced drops and melted graupel particles contribute to heavy rain on the lower Alpine slopes. The orographic enhancement in the stable blocked cases is fundamentally limited since these low‐altitude precipitation processes cannot add significantly to the background stratiform precipitation. Copyright © 2003 Royal Meteorological Society.

An overview of microphysical properties of Arctic clouds observed in May and July 1998 during FIRE ACE

Microphysical data were collected by the NCAR C‐130 research aircraft during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment Arctic Cloud Experiment (FIRE ACE). Boundary layer clouds 100 to 400 m thick were observed on 11 of the 16 missions. The all‐water clouds varied from being adiabatic and homogeneous with monomodal drop spectra to subadiabatic and inhomogeneous with bimodal drop spectra and drizzle. The subadiabatic clouds were observed to be actively mixing near cloud top. The adiabatic clouds provided a test of the performance of the liquid water content (LWC) probes but only in low LWC conditions. A mixed‐phase boundary layer cloud displayed striking variability in the hydrometeor fields on a horizontal scale of 10 km and a vertical scale of 100 m. Cloud Particle Imager (CPI) data showed separate regions with small supercooled cloud drops, supercooled drizzle (at −25°C) and graupel particles. A deep stratus cloud with its base at 2 km (+2°C) and top at 6 km (−25°C) contained drizzle near cloud top and (lower in the cloud) very high (2500 to 4000 L−1) concentrations of ice particles in conditions that did not meet all the Hallett‐Mossop criteria. CPI data showed that an Arctic cirrus cloud was composed of very high (∼100,000 L−1) concentrations of small ice particles interspersed with single, large (mostly bullet rosette) crystals. The data showed that the cirrus cloud was inhomogeneous on scales down to tens of meters. The average ice particle concentrations measured in the cirrus by the FSSP and CPI probes were several hundred to a few thousand per liter, much higher than commonly found in the literature.