Graupel Formation Research Articles

The Israeli Rainfall Enhancement Experiment A Physical Overview

This paper will attempt to give a summaryof the findings obtained during the period in which the two Israeli (I and II) cloud seeding experiments were conducted. It includes a synthesis of all the information obtained through extensive measurements of cloud and cloud system properties ranging from mesoscale characteristics of the cloud systems to the microphysical structure of the cloud elements that form these systems. It is suggested that the positive effects on rainfall, under seeding, obtained in the course of these two experiments can be attributed to the following: 1. The organized winter cloud systems responsible for most of the raim in Israel almost always form in the cold sector of the prevailimg low pressure systems. They consist of cumulus cloud elements which are typically continental in nature with a fairly high degree of colloidal stability. 2. The modal values of the cloud top temperature distributions on rain days are in the range of -15 to -22 C. Cloud temperatures are fairly uniformly distributed around 5 to 8 C. 3. The nature of the cloud droplet spectra in these clouds is such that ice crystal formation and its subsequent growth by riming seems to be the major rainforming process rather than that of the collision-coalescence mechanism. 4. On the average, the chain of events from initial ice crystal formation through graupel formation to the arrival of raindrops on the ground, can be described on a quantitative basis to suggest that seeding for "static" effects can increase the precipitation efficiency of the clouds in the range of top temperatures of -10 to -20 C and particularly in the range of -15 to -20 C. Statistical analyses of the rainfall data taking into account physical measurements lend the Israeli experiments the strongest support, namely that of physical plausibility. They suggest that: 1. The positive effects of seeding on all days can be stratified according to the daily mode of the cloud top temperature distributions to indicate a gradual and systematic increase of detectable seeding effects (and their statistical significance) in clouds converging from both ends of the cloud top temperature spectrum towards the most amenable range of clouds withbetween -15 to -21 C. Thus, confirming both our field studies and microphysical predictions. 2. The area of maximums eeding effect, at a distance of 35-50 km downwind from the line of seeding, which has consistently been found to exist, can be attributed to the known patterns of turbulent diffusion of the seeding material released at cloud base altitudes. While far from being complete the above studies and analyses provide a fair basis for understanding and accepting the statistical results and thus also indicate which criteria should be used to transfer this knowledge.

Statistical Analysis of Aerosol Effects on Simulated Mixed-Phase Clouds and Precipitation in the Alps

Abstract Increasing the aerosol number in warm-phase clouds is thought to decrease the rain formation rate, whereas the physical processes taking place in mixed-phase clouds are more uncertain. Increasing number concentrations of soluble aerosols may reduce the riming efficiency and therefore also decrease precipitation. On the other hand, the glaciation of a cloud by heterogeneous freezing of cloud droplets may enhance the formation of graupel and snow. Using a numerical weather prediction model with coupled aerosol microphysics, it is found, in a statistical framework with 270 clean and polluted 2D simulations of mixed-phase precipitation over an Alpine transect, that the presence of the ice phase determines the magnitude and the sign of the effect of an increasing aerosol number concentration on orographic precipitation. Immersion/condensation freezing is the only ice-nucleating process considered here. It is shown that this indirect aerosol effect is much less pronounced in cold simulations compared to a warmer subset and that cloud glaciation tends to compensate the loss of rain in polluted situations. Comparing the clean and polluted cases, a reduction of rain by 52%, on average (std dev = 25%), over the transect in the polluted cases is found. For frozen precipitation a much broader range of differences is found (mean = +4%, std dev = 60%). Furthermore, this study shows that in comparison with the clean cases more precipitation spills over to the leeward side of the major ridge in the polluted cases (median = +14.6%).

Numerical simulation of macro- and micro-structures of intense convective clouds with a spectral bin microphysics model

By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei (CCN), liquid droplets, ice crystals, snow and graupel particles, the spatial and temporal distributions of hydrometeors in a supercell observed by the (Severe Thunderstorm Electrification and Precipitation Study) STEPS triple-radar network are simulated and analyzed. The bin model is also employed to study the effect of CCN concentration on the evolution characteristics of the supercell. It is found that the CCN concentration not only affects the concentration and spectral distribution of water droplets, but also influences the characteristics of ice crystals and graupel particles. With a larger number of CCN, more water droplets and ice crystals are produced and the growth of graupel is restrained. With a small quantity of CCN the production of large size water droplets are promoted by initially small concentrations of water droplets and ice crystals, leading to earlier formation of small size graupel and restraining the recycling growth of graupel, and thus inhibiting the formation of large size graupel (or small size hail). It can be concluded that both the macroscopic airflow and microphysical processes influence the formation and growth of large size graupel (or small size hail). In regions with heavy pollution, a high concentration of CCN may restrain the formation of graupel and hail, and in extremely clean regions, excessively low concentrations of CCN may also limit the formation of large size graupel (hail).

The 11 August 2006 squall‐line system as observed from MIT Doppler radar during the AMMA SOP

AbstractOn the evening of 9 August 2006, a mesoscale convective system (MCS) having a north‐south oriented squall‐line organization formed over the border between Chad and Nigeria. It propagated westward, intensified over Nigeria on 10 August, and reached Niamey (Niger) at 0320 UTC on 11 August. Its passage over Niamey was accompanied by dust lifting and was well tracked by the Massachusetts Institute of Technology (MIT) Doppler radar. The three‐dimensional structure of the airflow and precipitation pattern is investigated from regular radar volume scans performed every ten minutes between 0200 and 0321 UTC. The 3D wind components are deduced from the multiple‐Doppler synthesis and continuity adjustment technique (MUSCAT) applied to a set of three volume scans obtained over a time period of one hour, which are equivalent to a three‐radar observation of the squall line when considering a reference frame moving with the system and the hypothesis of a stationary field.Results of the wind synthesis reveal several features commonly observed in tropical squall lines, such as the deep convective cells in front of the system, fed by the monsoon air and extending up to 15 km altitude, and the well‐marked stratiform rain region at the rear, associated with mesoscale vertical motions. Forward and trailing anvils are clearly identified as resulting from the outflow of air reaching the tropopause and transported to this level by the sloping convective updraughts occurring in a sheared environment. In the northern part, a deeper and stronger front‐to‐rear flow at mid‐levels is found to contribute to the rearward deflection of the leading line and to promote a broader (over 300 km) stratiform cloud region. Eddy vertical transports of the cross‐line momentum mainly accounts for the mid‐level flow acceleration due to a momentum redistribution from low to higher levels. The height distribution of hydrometeors and their associated production terms derived from a one‐dimensional microphysical retrieval model indicate the distinct roles of the convective and stratiform regions in the formation of graupel and rain, and the respective contributions of cold (riming) and warm (coalescence, melting) processes. Cooling from melting, and heating/cooling from condensation/evaporation processes yield a net decrease and increase of the potential temperature at low and mid‐to‐upper levels, respectively, with respect to an environmental thermodynamic profile taken three hours ahead of the analysis. Finally, the upper‐level rearward flow could convey the non‐negligible proportion of ice particles farther from the leading deep convection to the trailing stratiform region, thereby favouring the extent of this region. Copyright © 2009 Royal Meteorological Society

Polarimetric signatures and hydrometeor classification of West African squall lines

AbstractDuring the AMMA campaign, the Doppler and polarimetric radar Ronsard was deployed in Kopargo, Benin. In the present paper, three squall lines sampled during that period are described. It was the first time this type of system had been observed with a polarimetric radar in the West African region. The strongest case occurred on 28 July 2006, and the others on 12 September and 30 June. The polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems. Finally, the 3D wind field is retrieved for the 28 July event. The convective part of the squall lines is generally characterized by maxima of reflectivity, differential reflectivity (ZDR), and specific differential phase (KDP). The results of the hydrometeor classification show that the convective region is composed of strong (>30 mm h−1) to moderate rain (5–30 mm h−1) below the melting level, and graupel above. This zone seems to be characterized by relatively strong updraughts (5–10 m s−1), responsible both for the formation of graupel by riming and for the growth of drops due to coalescence. To the rear the systems are characterized by light rain (<5 mm h−1) below and snow or ice crystals above. These regions present downward flow below the melting layer, which prevents drop growth by condensation, consistent with the light rain observed. The stratiform part of these systems can be very non‐uniform, due to the presence of old cells dissipating, associated with moderate rain and thus stronger reflectivities. Copyright © 2010 Royal Meteorological Society

Relationships between Lightning Location and Polarimetric Radar Signatures in a Small Mesoscale Convective System

Abstract On 19 June 2004, the Thunderstorm Electrification and Lightning Experiment observed electrical, microphysical, and kinematic properties of a small mesoscale convective system (MCS). The primary observing systems were the Oklahoma Lightning Mapping Array, the KOUN S-band polarimetric radar, two mobile C-band Doppler radars, and balloonborne electric field meters. During its mature phase, this MCS had a normal tripolar charge structure (lightning involved a midlevel negative charge between an upper and a lower positive charge), and flash rates fluctuated between 80 and 100 flashes per minute. Most lightning was initiated within one of two altitude ranges (3–6 or 7–10 km MSL) and within the 35-dBZ contours of convective cells embedded within the convective line. The properties of two such cells were investigated in detail, with the first lasting approximately 40 min and producing only 12 flashes and the second lasting over an hour and producing 105 flashes. In both, lightning was initiated in or near regions containing graupel. The upper lightning initiation region (7–10 km MSL) was near 35–47.5-dBZ contours, with graupel inferred below and ice crystals inferred above. The lower lightning initiation region (3–6 km MSL) was in the upper part of melting or freezing layers, often near differential reflectivity columns extending above the 0°C isotherm, which is suggestive of graupel formation. Both lightning initiation regions are consistent with what is expected from the noninductive graupel–ice thunderstorm electrification mechanism, though inductive processes may also have contributed to initiations in the lower region.

Numerical simulation on the evolution of cloud particles in 3-D convective cloud

A 3-D convective cloud model with compressible non-hydrostatic dynamics and the spectral bin microphysics of a 2-D slab-symmetric model has been used to simulate an observed supercell storm occurring on 29 June, 2000 near Bird City, Kansas, USA. The main objective of this paper is to study the evolution of particles in this convective storm with bin spectral microphysics scheme. Graupels form and grow through two mechanisms, deposition and riming, with the riming process dominant on top of the inflow and in the upper portion of main updraft. Over the outflow and during the developing and mature stages of the storm, graupel particles mainly grow through deposition with dominant unimodal spectra. Most fall out after growing up. Reducing initial relative humidity disturbance (increasing initial potential temperature disturbance) has negative impact on the formation and growth of graupels over the inflow (outflow). This study shows that large graupel and hail could be suppressed by altering the deposition and coalescence process over the inflow and main updraft. At different locations of the convective cells and with different initial humidity and potential temperature disturbance, the graupel formation and growth mechanisms are different, so as to the feasible hail suppression locations and methods.

The effect of aerosol composition and concentration on the development and anvil properties of a continental deep convective cloud

AbstractA 3‐D cloud‐resolving model including an explicit aerosol module is used to examine the influence of a range of hygroscopic (CCN) and hydrophobic (IN) aerosol concentrations on the development of a mid‐latitude, continental deep convective cloud. The model reproduces a response to changes in CCN which is in agreement with previous studies of continental convection, i.e. lower precipitation rates and a later onset of deep convection for high CCN concentrations. However, the response is non‐linear and for low increments of CCN, coalescence and graupel formation becomes more efficient, which increases the total precipitation, i.e. a response similar to what has been obtained for oceanic deep convection. This result indicates the importance of using a range of CCN concentrations when examining aerosol influence on deep convection. The simulations show that an increased IN concentration has a substantial influence on the convective cloud development. Higher IN concentrations generally result in higher updraught velocities, a prolonged precipitation event and larger total precipitation amounts. The radiative forcing exerted by the cloud is determined by the anvil extent and ice crystal size. The anvil area is linked to the average updraught velocity which in turn is found to be correlated with the IN concentration. Homogeneously nucleated ice crystals dominate the total anvil ice mass formed, but heterogeneously formed ice crystals may significantly alter the homogeneous freezing process. For the simulated case, a key parameter for determining whether the number of homogeneously nucleated ice crystals will decrease or increase with increasing IN concentration is the initial updraught velocity. This dependence makes the results sensitive to the amount of heterogeneously formed ice crystals. Copyright © 2007 Royal Meteorological Society

T-dependent rate measurements of homogeneous ice nucleation in cloud droplets using a large atmospheric simulation chamber

The rates of photochemical reactions in the atmosphere depend on the optical properties and lifetimes of clouds. These are critically affected by the process of droplet freezing, because ice crystals can grow to large sizes at the expense of the metastable supercooled droplets, thereby initiating graupel formation and precipitation. The large evacuable and coolable aerosol chamber AIDA at Forschungszentrum Karlsruhe has been used to generate supercooled clouds under controlled conditions. Homogeneous freezing was detected below −35.5 °C, and nucleation rates J( T) were measured to about −37 °C. They vary between ∼10 6 cm −3 s −1 at the highest and ∼10 8 cm −3 s −1 at the lowest temperature, although the temperature dependence of the nucleation rate is not very well constrained by the measurements. The results agree within the combined error limits with recent literature data. Homogeneous ice nucleation, which sets a lower limit to cloud freezing temperatures when other nucleation mechanisms are inefficient in the atmosphere, is important in deep convective systems and in cirrus.

Microphysical modes of precipitation growth determined by S‐band vertically pointing radar in orographic precipitation during MAP

AbstractHigh‐resolution vertically pointing S‐band Doppler radar data obtained within orographic rain by the University of Washington Orographic Precipitation Radar (OPRA) at Locarno‐Monti, Switzerland during the Mesoscale Alpine Programme (MAP) Intensive Observing Periods (IOPs) 2b and 8 are examined to characterize the fine‐scale precipitation structure. The relative roles of collection of cloud water by raindrops versus collection of cloud water by ice particles (riming) in the same column are assessed with a one‐dimensional (1D) Kessler water‐continuity model using the observed data characteristics as input.The OPRA data reveal nearly ubiquitous small‐scale fallstreaks each <5 minutes in duration occurring within a wide range of precipitation intensities. Periods of higher rain rates >10 minutes in duration associated with convective cells evident in coarser‐resolution data manifested as increased frequency and intensity of fallstreaks in the OPRA data. A subset of fallstreaks had sufficient magnitude in reflectivity to indicate a clear connection from the snow region, across the melting layer and into the rain region. The small‐scale inhomogeneities in microphysical structure associated with fallstreaks are likely to be associated with convective overturning in the unstable IOP 2b case. In the more stable IOP 8 case, shear‐driven turbulence near the melting layer, evident in truck‐borne and airborne X‐band Doppler‐radar data shown in companion studies, is a likely contributor to fallstreak formation.The vertical profile of vertical air velocity (w) is critical to understanding the relationship between collection and riming, and their contributions to precipitation efficiency. Characteristics of the w profile within precipitating cloud were estimated in both rain and snow using observed radar parameters from the OPRA data. In rain, reflectivity‐weighted fall speed was derived from observed reflectivity and assumptions about the raindrop size distribution. The derived fall velocity was subtracted from the observed Doppler radial velocity to estimate w in rain regions to within ∼1.5 m s−1. In snow regions, a lower bound on peak updraught velocity was estimated from the observed maximum Doppler velocity. For the data examined from MAP IOP 2b, typical peak updraught velocities at Locarno‐Monti were between 2 and 5 m s−1.These estimated peak updraught velocities were used to construct plausible parabolic profiles of w that were in turn used as input to a 1D Kessler water‐continuity model. Model output shows local maxima in the riming rate and mixing ratio of ice within 2 km of the freezing level, indicating a favourable environment for graupel formation. The location of this layer in the model is consistent with the observed occurrence of graupel in National Center for Atmospheric Research S‐Pol radar data in the IOP 2b storm in a companion study. The modelled rates of collection and riming are comparable, and both make significant contributions to the growth by accretion of precipitation within the storm updraughts. Within the rain layer, liquid‐water coalescence (autoconversion and collection) in updraughts with peak velocities of 5 m s−1 can account for up to ∼40% of column‐integrated cloud water.Our analysis of the observational data and the 1D model results confirms and extends previous work indicating that a combination of liquid‐water coalescence and ice‐phase processes is needed to obtain high precipitation efficiencies >40% in orographic precipitation. Copyright © 2003 Royal Meteorological Society.

Idealized simulation of the Colorado hailstorm case: comparison of bulk and detailed microphysics

One of the purposes of the Fourth Cloud Modeling Workshop was to compare different microphysical treatments. In this paper, the results of a widely used bulk treatment and five versions of a detailed microphysical model are presented. Sensitivity analysis was made to investigate the effect of bulk parametrization, ice initiation technique, CCN concentration and collision efficiency of rimed ice crystal–drop collision. The results show that: (i) The mixing ratios of different species of hydrometeors calculated by bulk and one of the detailed models show some similarity. However, the processes of hail/graupel formation are different in the bulk and the detailed models. (ii) Using different ice initiation in the detailed models' different processes became important in the hail and graupel formation. (iii) In the case of higher CCN concentration, the mixing ratio of liquid water, hail and graupel were more sensitive to the value of collision efficiency of rimed ice crystal–drop collision. (iv) The Bergeron–Findeisen process does not work in the updraft core of a convective cloud. The vapor content was always over water saturation; moreover, the supersaturation gradually increased after the appearance of precipitation ice particles.

Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model

Abstract A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions ...

中国平凉での半乾燥地域に発達する雲の降水機構

Precipitation mechanisms were investigated in summer rainfall in the semi-arid region of Pingliang, China as part of the Heihe River Field Experiment (HEIFE). Evolution of precipitation particles was investigated using videosonde images. Radar, surface electric field and balloon ascent rates were also referred for the study. The most noteworthy result was finding two completely different rain systems in the area, one in locally developed thunderstorms and the other in rain systems associated with the Baiu front. In the electrically active local thunderstorms, rain is formed through graupel formation. Snowflakes are the primary precipitation particles in Baiu frontal cloud systems. The concept of generation cell involvement in rain formation may not be applicable in Baiu frontal systems.

Requisites of graupel formation in snow clouds over the sea of Japan

Snow and graupel formation mechanisms have been investigated to examine the modification potential of snow clouds over the Sea of Japan by seeding, with the help of observation and modeling. The observations showed the feature of microphysical structure of snow clouds over the Sea of Japan before and after precipitation formation. The feature is that there is much supercooled water with a small number concentration of ice particles before precipitation formation and that there is less supercooled water with a larger number concentration of ice particles after graupel formation. The seedability is expected at the early stage of snow clouds. The model result demonstrated that graupel formation is influenced by cloud top temperature, cloud thickness, updraft and ice crystal nucleation rate. The crucial physical factors leading to graupel formation were found to be liquid water content (LWC) and cloud droplet size. Graupel formation requires LWC more than 0.4 g/m 3 and droplet size larger than 10 μm. Another simulation of seeding suggested that there is some chance of snow cloud modification by alternation of microphysics of graupel formation due to seeding.

Ice particle concentrations and precipitation development in small continental cumuliform clouds

Maximum ice particle concentrations (IM) in modest (≦3.7 km deep) continental cumuliform clouds, with tops with temperatures between −6 and −25 °C, were found to be better correlated with the broadness of the droplet spectrum near cloud top (r = 0.78) than with cloud-top temperature (r = 0.58). Also, the broader the droplet spectrum the warmer was the cloud top at which ice first appeared. Stratification into three cloud-base-temperature (TB) categories, cool (0 °C ≦ TB −8 °C), cold (−8°C < TB < 0°C), and very cold (TB ≦ −8°C), produced correlations between IM and cloud-top temperature (TT) of 0.71, 0.88, and 0.89, respectively. The best-fit lines for these relationships shift to higher IM values as TB increases; this also reflects the effect on IM of the broadness of the droplet spectrum, since the size of the largest drops increases as TB increases. When the clouds contained drops with diameters ≧25 μm, ice particle formation was sudden and prolific. High concentrations of ice particles appeared coincident with, or very soon after, the formation of graupel; these high concentrations were observed in clouds at ambient temperatures between -11 and −28°C, including some clouds with TB <0°C. For clouds with similar droplet spectra and TT, the width of the cloud also affected IM; narrow clouds (<2 km wide) formed less ice than wider, multi-turreted clouds. Continental cumulus clouds have to be about 50% wider and about 5 degC colder at their tops than maritime cumulus to have the same chance of producing a radar echo. However, the difference in width to produce a radar echo disappears for clouds with widths >4 km, although continental clouds still need to be about 5 degC colder at cloud top than maritime clouds to produce a radar echo. Several extant theories for high ice particle concentrations in clouds are examined but none provides a satisfactory explanation for the observations.

Ice particle concentrations and precipitation development in small continental cumuliform clouds

AbstractMaximum ice particle concentrations (IM) in modest (≦3.7 km deep) continental cumuliform clouds, with tops with temperatures between −6 and −25 °C, were found to be better correlated with the broadness of the droplet spectrum near cloud top (r = 0.78) than with cloud‐top temperature (r = 0.58). Also, the broader the droplet spectrum the warmer was the cloud top at which ice first appeared.Stratification into three cloud‐base‐temperature (TB) categories, cool (0 °C ≦ TB −8 °C), cold (−8°C &lt; TB &lt; 0°C), and very cold (TB ≦ −8°C), produced correlations between IM and cloud‐top temperature (TT) of 0.71, 0.88, and 0.89, respectively. The best‐fit lines for these relationships shift to higher IM values as TB increases; this also reflects the effect on IM of the broadness of the droplet spectrum, since the size of the largest drops increases as TB increases. When the clouds contained drops with diameters ≧25 μm, ice particle formation was sudden and prolific. High concentrations of ice particles appeared coincident with, or very soon after, the formation of graupel; these high concentrations were observed in clouds at ambient temperatures between ‐11 and −28°C, including some clouds with TB &lt;0°C.For clouds with similar droplet spectra and TT, the width of the cloud also affected IM; narrow clouds (&lt;2 km wide) formed less ice than wider, multi‐turreted clouds.Continental cumulus clouds have to be about 50% wider and about 5 degC colder at their tops than maritime cumulus to have the same chance of producing a radar echo. However, the difference in width to produce a radar echo disappears for clouds with widths &gt;4 km, although continental clouds still need to be about 5 degC colder at cloud top than maritime clouds to produce a radar echo.Several extant theories for high ice particle concentrations in clouds are examined but none provides a satisfactory explanation for the observations.

ミクロネシア・ポナペ島上に発達した積乱雲の降水機構の研究

Eight specially developed radiosondes were sent into cumulonimbus clouds developed over Pohnpei, Micronesia. The radiosondes transmitted both the images and the electric charge of precipitation particles during their ascent through the clouds. Micronesian clouds contained numerous ice crystals. Interesting images of precipitation particles included spheroidal raindrops, frisbee-like snowflakes, tetrapod-like hailstones, and spatial crystals with wide planes near the tropopause. Precipitation mechanisms were identified through analysis of the precipitation particle distribution with regard to height. Three different cloud types were identified. Rainbands producing heavy precipitation grew frozen-particle-hailstones in a narrow layer just above the freezing level. Water accumulated heavily in this layer due to the interaction between the warm rain process and the drop freezing process. Two other cloud types seen were isolated cumulonimbus clouds and upper convective clouds. In the former, both warm rain and graupel formation processes worked independently, while graupel formation dominated in the latter cloud. Electric properties of clouds were also briefly described.

霰の形成と気象条件の関係

As the formation of graupel is considered to be quite important in the mechanism of snowfall, it was studied by thin-section observations to investigate its generation and growth. Graupel embryos were classified into two types and the internal structures of graupel into three types, respectively. Each type was closely related to the meteorological conditions. Namely, embryo types were related to the temperature at the cloud base and the thickness of mixed cloud, while internal structure types were classified by the wet-bulb potential temperature in clouds and the temperature at the cloud base. By knowing the meteorological conditions, embryo types and internal structure types of graupel may be predicted.

Sensitivity Studies on the Continentality of a Numerically Simulated Cumulonimbus

Abstract The cloud model of Tripoli and Cotton was used to simulate a cumulonimbus cloud observed during the Cooperative Convective Precipitation Experiment (CCOPE). We tested the sensitivity of the precipitation pathways in the model to the initial concentration of cloud droplets above cloud base Nc (which is related to the concentration of cloud condensation nuclei). The results showed that for large Nc, Manton and Cotton's autoconversion parameterization properly suppressed supercooled rain formation via the “warm-rain” process in a cold-based, continental cloud, forcing ice processes (e.g., riming, aggregation and deposition of vapor) to produce graupel. With lower droplet concentrations, rain formed first through warm-rain processes, then graupel formed through freezing. The value of Nc, which determined the transition from graupel formation by freezing rain to graupel formation by ice processes was found to be sensitive to the parameter acm, which represents the critical mean radius at which collisi...

Processes of Hydrometeor Development in Oklahoma Convective Clouds

This study employs in situ measurements to examine cloud conditions in which hydrometeors develop in mature Oklahoma convective clouds and to develop hypotheses as to how they formed. The measurements were made from penetrations on six days using a T-28 aircraft. Values of the maximum vertical velocity W in cells ranged from 5 to 35 m s−1, and the liquid water content (LWC) up to 6 gmminus;3;LWCs are usually less than adiabatic. Drops are found primarily in strong updrafts at T/>−8°c. Graupel are present in low concentrations in the strong updrafts and in moderate concentrations in the weak to intermediate updrafts. Planar and needle ice crystals and aggregates are present in copious concentrations in regions of low LWC and W. Strong evidence exists for production of secondary ice crystals (SICS) through a Hallett and Mossop type of mechanism involving cloud droplets >24μm in diameter. Particle growth calculations are used in conjunction with the measurements to infer the processes of formation of drops, graupel and hail, and secondary ice crystals. Most drops of diameters <500μm found at temperatures below 0°C are inferred to form through coalescence growth and most of diameters >500μm through shedding from growing and/or melting graupel and hail. Embryos of hailstones are found to develop to 1 cm in diameter most rapidly from millimetric size drops produced from shedding and from aggregates of planar ice-crystals. Most growth of particles to 1 cm hailstones occurs in the regions of intermediate values of LWC (1-2 gm−3) and W (5–15 m sminus;1) at temperatures higher than −20°C. In these regions, moderate concentrations of ice particles can develop over appreciable periods and depletion of the liquid water content due to collection by ice particles is minimal. The regions of high LWC and W are found to be the least conducive to SIC production. Initially, most SICs come from riming of aggregates in clouds which develop embedded within cloud layers and from frozen drops in clouds which develop in isolation. The SICs themselves are found to produce abundant SICs in regions of low LWC and W. Secondary ice crystal production is found to be more copious in embedded than in isolated clouds.