Dennis J. Musil

Microphysical Characteristics of a Well-Developed Weak Echo Region in a High Plains Supercell Thunderstorm

Abstract 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...

Computer Modeling of Hailstone Growth in Feeder Clouds

Abstract The growth of hailstones is studied using a one-dimensional non-steady-state model of a growing feeder cloud. Conditions within the mature cloud are assumed to be adiabatic. Vertical velocity and liquid water content profiles change at a predetermined rate, beginning with small values and gradually increasing to values that can be expected in a mature thunderstorm. Hailstone growth equations used in the model are developed in terms of the well-known dry- and wet-growth regimes and include terms that account for growth in a mixture of supercooled cloud droplets and ice crystals. Hailstone embryos in the form of liquid cloud droplets are introduced into the model at various heights and times, and the subsequent trajectories are analyzed. Different versions of the model that incorporate changes in updraft magnitude and shape are tested. Results show that the model produces hailstones, radii ranging from near zero to 1.3 cm in 40 min or less, from 20 to 50 μ radius embryos introduced under a variety ...

Observations of mixed-phase precipitation within a CaPE thunderstorm

Abstract Various procedures for inferring hydrometeor characteristics from polarimetric radar data have indicated that regions with echoes exhibiting relatively high linear depolarization ratios along with relatively low differential reflectivity contain wet graupel or hail. Such particles could be found either in a melting zone below the 0°C level in a cloud or in a region of wet growth where the rate of supercooled cloud water accretion overwhelms the rate at which the latent heat associated with complete freezing can be dissipated. In subtropical clouds such as those studied in the Convection and Precipitation/Electrification (CaPE) project in Florida, at the −5°C level or higher, neither condition is obtained. Yet similar polarimetric radar signatures were nevertheless observed at such levels during CaPE. Examination of in situ observations by the T-28 aircraft in the Florida clouds, along with results from previous laboratory and theoretical studies, suggests that the signature regions were character...

Case Study of a Hailstorm in Colorado. Part II: Particle Growth Processes at Mid-Levels Deduced from in-situ Measurements

Abstract The microphysical, thermodynamic and vertical air motion characteristics of a hailstorm which occurred in northeast Colorado were investigated through penetrations by the T-28 research aircraft. On the basis of these measurements, the structure and composition of the regions of the strongest updrafts, the peripheral updraft and downdraft regions, and the quiescent regions of the storm were estimated. Several general aspects of the hydrometeor development processes operating within the storm were deduced from these measurements. The particle habits and sizes at any location within the storm depended primarily upon their position relative to the main updraft core. A region which extended around the forward portion of the storm was found to be an environment in which ice particles could grow rapidly through aggregation so that they were subsequently in a position to serve as graupel and hail embryos. Particles grew most rapidly along the boundaries of the WER, because the depletion of the liquid wat...

Some Interior Observations of Southeastern Montana Hailstorms

Abstract This study investigates some of the characteristics or the interior regions of several hailstorms penetrated by the armored T-28 aircraft during the 1981 CCOPE field project. The vertical wind data were analyzed to identify updraft and downdraft regions to facilitate the discussion of measurements made within the identified draft regions, such as draft sizes and speeds, cloud liquid water concentrations (LWC), turbulent and hail. Updrafts were more numerous than downdraft regions, tended to have greater horizontal extent and higher speeds, and were somewhat more turbulent. Significant correlations existed between peak vertical wind speeds and peak values of LWC or turbulence, as well as (for updrafts) between peak speed and draft size. Values of the LWC were generally quite low compared to adiabatic values, with the exception of two large severe hailstorms that may have had adiabatic updraft cores. The general characteristics of the hail observations, such as number and mass concentrations, maxim...

The 1 August 1981 MOPE Storm: Observations and Modeling Results

Abstract Observations made by the T-28 armored research aircraft, radar, and other data sources were used to study an eastern Moutana hailstorm that developed on 1 August 1981 during the Cooperative Convective Precipitation Experiment season. The storm featured a wide, persistent, vertically oriented updraft with speeds exceeding 35 m s−1. Hailstones of over 5 cm diameter were collected at the ground, while the T-28 encountered hail up to about 2.5 cm diameter. Them was no evidence of feeder cells or a weak echo region. The IAS two-dimensional, time-dependent “bulk water” model was run on this case, using a sounding from this day. Some areas of agreement between the simulation and observations include the maximum updraft speed, cloud top height, presence of a rounded cloud dome, and maximum radar reflectivity. The simulation failed to properly model the width and orientation of the updraft, as well as its long lifetime. The IAS hail category model was also run on this case. This model features 20 categori...

Interior characteristics at mid-levels of thunderstorms in the southeastern United States

Data characterizing the updraft structures and hydrometers observed in moderate thunderstorms by the armored T-28 aircraft during the 1986 Cooperative Huntsville Meteorological Experiment (COHMEX) are presented. Updraft regions in the storms were relatively narrow, but usually very turbulent. The vertical winds were generally weak, the peak updraft speeds usually being <5 m s−1; the absolute maximum was 19 m s−1. Cloud liquid-water concentrations measured between the −7.5°C and +5.5°C levels were typically <25% of adiabetic values. Size spectra for the precipitation particles appear truncated, with very high concentrations of particles with sizes between 1 and 5 mm and few larger than 8 mm. hail occured in less than 25% of the penetrations, and particles larger than about 1 cm were very infrequent, despite reflectivities often between 50 and 60 dBz. The large numbers of precipitation-size particles usually found in the updraft regions that appeared to be well-mixed indicate that a coalescence mechanism was active in the COHMEX storms. The depth warm cloud, the generally weak updrafts, and the presence of large cloud droplets all support a coalescence mechanism. Ice processes also must play a role in precipitation development because most COHMEX storms were characterized by extremely deep convection. However, the high conceentrations of growth centers observed at T-28 penetration levels suggest that a natural “beneficial competition” process limits the growth of large particles.

Characteristics of Radar-Identified Big Drop Zones in Swiss Hailstorms

Abstract The characteristics of the so-called “radar-identified big drop zones” (rBDZ) have been investigated. The study employs radar observations of several thunderstorms and simultaneous microphysical and vertical wind measurements with a penetrating T-28 aircraft. The comparison of aircraft-measured vertical wind and radar data revealed good coincidence between rBDZs and updraft regions, indicating that this part of the seeding hypothesis upon which Grossversuch IV was based is reasonable. The microphysical observations of rBDZs, however, show large concentrations of ice particles and practically no supercooled raindrops indicating that the latter do not play a significant role in the development of hail in the Swiss storms. There is no reason to believe that directing seeding material into such regions where natural ice already exists in great abundance will have any significant effect an the hail process.

Structure of an Evolving Hailstorm Part IV: Internal Structure from Penetrating Aircraft

Abstract Precipitation particle sizes were measured using a continuous hydrometeor sampler (foil impactor) during penetrations of hailstorms with an armored T-28 aircraft. Data have been analyzed from three penetrations of a storm near Raymer, Colorado, on 9 July 1973 at altitudes between 5.5 and 7.2 km MSL, which correspond to temperatures between about −2°C and −12°C. Other results pertinent to the Raymer storm are discussed in Parts I,II,III and elsewhere in this issue. Most of the particles were identified as ice particles or ones containing both ice and water; however, significant amounts of liquid particles were found in the updrafts of developing cells at temperatures as cold as −12°C. Particles larger than 5 mm in diameter were typically found along the edges of the updrafts, with the precipitation concentrations being strongly dependent on these larger particles. The downdrafts were composed of ice particles. Several particle size distributions from one of the penetrations were examined. The dist...

Spatial and temporal variations of the interior characteristics of Swiss thunderstorms

Abstract The characteristics of mid-level high reflectivity zones (HRZ) in Swiss thunderstorms were investigated by the armored T-28 research aircraft as part of Grossversuch IV. Updrafts, substantial amounts of supercooled cloud water, and high concentrations of ice particles were located in the HRZs, while in the surrounding regions these quantities were generally lower. A multi-response permutation procedure (MRPP) analysis was applied to vertical wind and cloud water observations in the storms. The results showed that the updrafts were significantly stronger in the HRZs than in the surrounding areas, while there was substantially more cloud water in the HRZ than in most of the surrounding regions. Further MRPP tests showed no significant variation in the vertical winds and cloud water from one portion of an HRZ to another or between the HRZs and adjacent echo overhang regions. The evolution of an HRZ was examined and it appears that values of vertical velocity, cloud liquid, and concentrations of large particles reach a maximum during the midstage of its development, followed by decline as the HRZ ages. The associated hail growth mechanism appears to involve complex interactions among vertical winds, cloud liquid, and the ice particles present. There were sufficient ice particles and supercooled cloud droplets in the HRZ for a riming process to occur. The presence of large cloud droplets suggests the possibility of a coalescence mechanism as part of the precipitation process.