William A. Cooper

Measurement of Condensed Water Content in Liquid and Ice Clouds Using an Airborne Counterflow Virtual Impactor

Abstract Condensed water content (CWC) measured using a counterflow virtual impactor (CVI) with a Lyman-α hygrometer downstream is compared with that measured by other airborne instruments (a hot-wire probe, a PMS FSSP, and a PMS 2D-C). Results indicate that the CVI system provides a reliable measurement of CWC in both liquid- and ice-phase clouds and that the CVI measures CWC contained in both large and small hydrometeors; this means that the condensed water present in both phases and virtually all hydrometeor sizes can be measured with a single device. Small ice contents of a few milligrams per cubic meter present in cirrus clouds can also be measured by the technique.

A Study of the Source of Entrained Air in Montana Cumuli

Abstract Data gathered by the University of Wyoming King Air, the Atmospheric Environmental Services Twin otter and an NCAR Queen Air were used in thermodynamic analyses to determine the sources of environmental air entrained into cumulus clouds. The measurements were made in clouds ranging from small cumuli a few kilometers deep to a large supercell system. Previous results have indicated that the source of entrained air in continental cumuli is generally above the flight level, often near cloud top. The results reported here, however, suggest that the source of entrained air is close to, or slightly above, the observation level of the aircraft, even when the aircraft descends through different levels in the cloud. The results are consistent with the idea that cumulus clouds consist of thermal-like elements from which the least buoyant mixed parcels are shed off and the most buoyant mixed parcels may continue with the general ascent. A schematic model of cumulus convection is presented and supported by m...

Effects of Variable Droplet Growth Histories on Droplet Size Distributions. Part I: Theory

Abstract A theoretical framework is developed that permits estimation of the effects of fluctuating supersaturation on the development of cloud droplet size spectra. The studies focus on the role of turbulent fluctuations in vertical wind and in the microphysical environments in which droplets grow, and represent the effects of droplets mixing together that have encountered different trajectories through the cloud. It is contended that the effects can be analyzed in terms of two contributions to the variance in supersaturation history, one dependent on the average microphysical environment (specifically, integral radius) of the near environment in which a droplet grows, and the other dependent on the correlation between the integral radius and the updraft along the droplet trajectory. Variations in the possible trajectories that all end at a given point (and so form the droplet spectrum there) are used to estimate the possible widths of droplet spectra, and methods of testing these predictions using exper...

Axis Ratios and Oscillations of Raindrops

Abstract Axis ratios were determined for about 3500 raindrop images recorded in summertime rainshowers by an instrumented aircraft. These ratios were used to determine the mean axis ratios and oscillation amplitudes of raindrops. A filtering algorithm using Fourier descriptors was applied to the raindrop images to reduce the quantization noise and the systematic errors, and simulations were used to estimate the standard errors of the measurement procedure. Drops with diameters <4 mm were observed to be slightly more spherical than would be expected for drops in equilibrium. Oscillation amplitudes were found to be typically ±10% in axis ratio for light to moderate rainfall rates, and such oscillations can account for the departures from equilibrium values. The effects of these axis ratios and oscillations on the differential radar reflectivity of rain are calculated and discussed.

Mountain Waves Entering the Stratosphere

Using the National Science Foundation (NSF)‐NCAR Gulfstream V and the NSF‐Wyoming King Air research aircraft during the Terrain-Induced Rotor Experiment (T-REX) in March‐April 2006, six cases of Sierra Nevada mountain waves were surveyed with 126 cross-mountain legs. The goal was to identify the influence of the tropopause on waves entering the stratosphere. During each flight leg, part of the variation in observed parameters was due to parameter layering, heaving up and down in the waves. Diagnosis of the combined wave-layering signal was aided with innovative use of new GPS altitude measurements. The ozone and water vapor layering correlated with layered Bernoulli function and cross-flow speed. GPS-corrected static pressure was used to compute the vertical energy flux, confirming, for the first time,

Performance of Some Airborne Thermometers in Clouds

Abstract The ability of airborne instruments to measure temperature in cloud is studied using theoretical analyses and experimental data. Theoretical predictions of the effects of sensor wetting are reviewed and modified, and are then compared to measurements. Two airborne immersion thermometers, the NCAR “reverse-flow” thermometer and the Rosemount 102 thermometer, are compared to each other and to a new radiometric thermometer. The comparisons show that out of cloud all three thermometers agree well with each other. However, there is clear evidence that the immersion thermometers become wet in some clouds and measure erroneously low temperatures as a result. The evidence, particularly from measurements in unmixed parcels, supports the validity of the measurements from the radiometric thermometer both inside and outside clouds. Supporting evidence that the immersion sensors are susceptible to wetting is provided from tests in a wind tunnel and from measurements using a conductivity sensor placed at the l...

Field Evidence Supporting Quantitative Predictions of Secondary Ice Production Rates

Abstract Field observations from three different areas in the United States are used to determine the rates of appearance of ice particles in cumulus clouds. Those rates are compared to predictions obtained using the laboratory studies of the Hallett-Mossop process for secondary ice production. The rates of appearance of ice show the trends predicted by laboratory studies of secondary ice production, and average rates of production are within about a factor of three of the predictions. The roles of graupel, large and small droplets, and temperature are consistent with the laboratory findings.

The Origin of Ice in Mountain Cap Clouds

Abstract Ice crystal development in relatively simple layer clouds was studied using airborne instrumentation. The patterns in the development of ice in those clouds suggest that the ice originates in association with the initial condensation process, near the upwind edge of the cloud. Since continued ice production does not occur beyond that region, the ice development can be attributed to nucleation. There is no evidence for secondary ice generation. Either condensation-freezing or contact nucleation could account for the observed nucleation process, but special properties are required for the nuclei in either case. Ice crystal concentrations show a clear temperature trend, as expected for a nucleation process.

The Stratosphere–Troposphere Analyses of Regional Transport 2008 Experiment

The Stratosphere–Troposphere Analyses of Regional Transport 2008 (START08) experiment investigated a number of important processes in the extratropical upper troposphere and lower stratosphere (UTLS) using the National Science Foundation (NSF)–NCAR Gulfstream V (GV) research aircraft. The main objective was to examine the chemical structure of the extratropical UTLS in relation to dynamical processes spanning a range of scales. The campaign was conducted during April–June 2008 from Broomfield, Colorado. A total of 18 research flights sampled an extensive geographical region of North America (25°–65°N, 80°–120°W) and a wide range of meteorological conditions. The airborne in situ instruments measured a comprehensive suite of chemical constituents and microphysical variables from the boundary layer to the lower stratosphere, with flights specifically designed to target key transport processes in the extratropical UTLS. The flights successfully investigated stratosphere–troposphere exchange (STE) processes, ...

Effects of Coincidence on Measurements with a Forward Scattering Spectrometer Probe

Abstract A PMS Forward Scattering Spectrometer Probe (FSSP) may fail to detect a droplet that enters the illuminated volume in coincidence with another droplet, or such a coincident pair may be assigned an erroneous size. This effect is shown to distort the droplet size spectra measured by the FSSP, and the effects can be quite important in clouds having droplet concentrations exceeding 500 cm−3. The most common coincidence error is one that arises when a droplet within the sample volume of the FSSP is rejected or sized incorrectly because of the coincident passage of another droplet outside that sample volume. Droplet spectra are measured to be too broad, and to contain too many large but too few small droplets, as a result of this effect. Some implications for past and future studies using this probe are discussed.