Andrew G. Detwiler

THE SEVERE THUNDERSTORM ELECTRIFICATION AND PRECIPITATION STUDY

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

The North Dakota Thunderstorm Project: A Cooperative Study of High Plains Thunderstorms

Abstract The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification. Of primary interest were tracer experiment that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide. Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunder...

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...

Abundance of airborne heterotrophic protists in ground level air of South Dakota

Abstract The abundance of heterotrophic protists (as protozoan cysts) in the near-surface tropospheric air of Rapid City, SD, was determined using a filtration sampler capable of collecting particles in the 2.0 to 20.0 μm size range (sampling rate ca. 16 l min −1 ). The number of viable cysts collected on the filter was estimated after enrichment cultivation in soil extract medium. Samples were mainly collected in the spring, fall and winter of 1997 and abundances throughout the year ranged from below detection ( −3 ) to 1.08 cysts m −3 with an overall mean of 0.25 m −3 (SE=0.05). Not all of the isolates were identified to species but, over the study, 25 different morphotypes were found. Flagellates and naked amoebae were the commonest protozoa (as cysts) found; ciliates were rare. This reflects the abundance of these groups in soils, the likeliest source of most of the airborne protozoa in this study. The concentration of cysts generally increased as a function of the total particles in the atmosphere. The concentration of both particulates and cysts exhibited a wide range of variability, but highest values of both occurred on days with higher winds and lower relative humidity.

Precipitation development and electrification in Florida thunderstorm cells during Convection and Precipitation/Electrification Project

Precipitation development and electrification in Florida thunderstorms are observed using an instrumented aircraft and a multiparameter radar. A low concentration of raindrops initially develops in the updraft, and these raindrops begin to freeze when they are carried above the 0°C level. High concentrations of ice particles and downdrafts soon appear in the -5° to -10°C regions of the cloud, where the aircraft penetrated, as do electric fields in the range of tens of kilovolts per meter. In a cell with relatively weak updrafts, drops start to freeze at temperatures just below 0°C. Although significant electric fields are measured by the aircraft, no lightning is observed in this cell. In more vigorous cells, drops first begin to freeze at temperatures between -5°C and -10°C. The electric fields measured by the aircraft in these cells are similar in magnitude to those in the weaker cell, but lightning is observed in these more vigorous cells. The net charge in convective regions at altitudes just above the aircraft penetration levels, 6-7 km, appears to be negative.

Cloud Liquid Water Measurements on the Armored T-28: Intercomparison between Johnson–Williams Cloud Water Meter and CSIRO (King) Liquid Water Probe

Abstract Comparisons are made between liquid water concentration (LWC) readings obtained from a Johnson–Williams (J–W) cloud water meter and a King (Commonwealth Scientific and Industrial Research Organisation) liquid water probe, both mounted on the armored T-28 research aircraft during penetrations of springtime convective storms in Oklahoma and Colorado. The King probe readings are almost always higher, being up to twice those of the J–W instrument in clouds with narrower cloud droplet spectra. In clouds with broader droplet spectra, the ratio often climbs to three or greater. The King probe responds partially to drops larger than cloud droplet size, and also to some ice particles, so its reading can be higher than the cloud LWC present. However, this and earlier comparisons by others indicate that the primary reason for this discrepancy is that the J–W probe often underestimates the cloud LWC due to incomplete response to larger cloud droplets. Thus, published studies involving cloud LWC in convective...

Humidity Required for Ice Nucleation from the Vapor onto Silver Iodide and Lead Iodide Aerosols over the Temperature Range −6 to −67°C

Abstract The ice saturation ratio at which 1% of aged silver iodide and lead iodide aerosol particles nucleate ice from moist air is observed to depend on temperature. Between roughly −30 and −67°C the threshold for both aerosol types rises slowly with decreasing temperature in agreement with a simple classical nucleation theory. Between −6 and −30°C the threshold for the silver iodide aerosol rises more rapidly than predicted by simple theory while the threshold for lead iodide decreases.

Aircraft-Produced Ice Particles (APIPs): Additional Results and Further Insights

Abstract This paper presents new results from studies of aircraft-produced ice particles (APIPs) in supercooled fog and clouds. Nine aircraft, including a Beech King Air 200T cloud physics aircraft, a Piper Aztec, a Cessna 421-C, two North American T-28s, an Aero Commander, a Piper Navajo, a Beech Turbo Baron, and a second four-bladed King Air were involved in the tests. The instrumented King Air served as the monitoring aircraft for trails of ice particles created, or not created, when the other aircraft were flown through clouds at various temperatures and served as both the test and monitoring aircraft when it itself was tested. In some cases sulfur hexafluoride (SF6) gas was released by the test aircraft during its test run and was detected by the King Air during its monitoring passes to confirm the location of the test aircraft wake. Ambient temperatures for the tests ranged between −5° and −12°C. The results confirm earlier published results and provide further insights into the APIPs phenomenon. Th...

Contrail formation and propulsion efficiency

The contrail factor is the ratio of water vapor to enthalpy added by combustion to the exhaust plume from an aircraft engine. It is the key parameter determining the highest temperature at which contrails will form behind a particular engine on a particular aircraft. Cycle calculations are used to estimate contrail factors for a range of e ight environments at a range of power settings, for generic low-bypass and high-bypass turbofan gas-turbine engines. It is found that, contrary to assumptions made in current methods of contrail forecasting, the contrail factor is not constant for a given engine type. The contrail factor varies even forthe same engine at different power settings and e ight conditions. Results are shown for a range of conditions, including e ight levels between 25,000 and 50,000 ft in a standard atmosphere, e ight Mach numbers ranging from 0.4 to 0.9, and powersettings from idle (low) to military (high). The computed contrail factors range from 0.030 to 0.053 g¢ kg i 1±± C i 1 for the low-bypass engine and from 0.038 to 0.090 g¢ kg i 1± C i 1 for the high-bypass engine. Changes in contrail factor can be roughly related to changes in the threshold environmental temperature for contrail formation by the relationship that a 10% increase in contrail factor results in a 1 ± C higher threshold temperature in a typical upper tropospheric environment for threshold temperatures near i 50 ±± C. These calculations do not yield precise estimates of contrail factors for specie c engines, but demonstrate that contrail factors for a given engine are generally higher at lower power settings at a given Mach number, higher at higher Mach numbers at a given power setting, and higher at higher altitudes in general.

Magnitude of Error Factors in Estimates of Snow-Particle Masses from Images

Abstract Ice particles are captured, photographed, melted, and then photographed again. Mass is estimated from the size of the melted drop. Based on a sample of 640 particles, the standard error in estimating particle mass solely from the maximum dimension of the particle is found to be a factor of 4. The standard error in estimating mass concentration M in a cloud from a sample of n well-characterized particles recorded by an optical array probe is estimated to be approximately a factor of 100.6/n^1/2.