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Marcia K. Politovich

Abstract

Cloud droplets may take a variety of paths to reach some location in a cloud and therefore may encounter varying growth conditions. This paper examines the effect of these variations on broadening of the droplet size distribution in cumuli. Diffusional growth of a collection of droplets is studied in detail using measurements obtained during the Cooperative Convective Precipitation Experiment (CCOPE) from the University of Wyoming's King Air research aircraft. A technique is described by which these measurements may be converted to the frame of reference of a moving droplet.

Entrainment processes that are associated with variations in vertical velocity appear to have the effect of intensifying fluctuations in integral radius, which in turn produce variations in the supersaturation encountered by the droplets and thus opportunities for variations in growth. Contributions to spectral broadening arise from turbulent structures in the cloud, producing variations in integral radius as well as in the correlation between integral radius and vertical velocity. A relation among these variations and the resultant spectral broadening is developed and applied to measurements in growing cumuli. In unmixed updraft, the resulting spectral broadening is small. Calculations based upon measurements obtained in mixed parcels produce reasonable qualitative and quantitative agreement with the observed dispersions. The relation of recent studies on spectral broadening to this work is explored.

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Marcia K. Politovich

Abstract

The characteristic of aircraft icing environments containing large supercooled droplets are described. Substantial loss in rate of climb capability can result from less than 10 minutes duration in conditions where fewer than 0.1–1 cm−3 of droplets 30–400 μm in diameter are present. These conditions are found to have a greater effect than those where the liquid water was confined to smaller (generally less than about 30 μm diameter) droplets. Measurements from research aircraft flying in regions containing these large droplets, located in the Sierra Nevada in California, near Amarillo, Texas, and in northern Arizona are presented. Temperatures ranged from −5.5° to −9.4°C in 13 regions. The sizes of the droplets responsible for performance loss varied with each encounter but ranged from tens to hundreds of micrometers, and these were accompanied by few to no ice crystals. Two case studies are examined in further detail, including the weather conditions present at the time of the encounters.

The meteorological situations leading to formation of these large droplets provide suitable environments for coalescence growth, or for prolonged depositional growth, and include weak atmospheric instability, warm (temperatures greater than about −15°C) cloud tops, and sufficient moisture.

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Marcia K. Politovich and Gabor Vali

Abstract

The relatively simple orographic clouds forming in winter over Elk Mountain, Wyoming provided useful opportunities for field studies of cloud formation and of ice crystal development. In this paper, the observations of cloud droplet populations spanning a range of five consecutive years are summarized.

Date are presented which describe the climatology of the cloud droplet spectra. Selected cases are described in detail to illuminate the process at work and to allow comparisons with theoretical predictions.

Droplet concentrations are mostly around 300 cm−3 in accordance with the weak updrafts of the clouds and with the mid-continental, unpolluted cloud condensation nucleus concentrations prevailing in the region. In general, the data are in agreement with one-dimensional microphysical model calculations.

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Paul Schultz and Marcia K. Politovich

Abstract

An automated procedure is developed for detecting and forecasting atmospheric conditions conductive to aircraft icing over the continental United States. The procedure uses gridded output from the Nested-Grid Model, and is based on the manual techniques currently in use at the National Aviation Weather Advisory Unit in Kansas City, Missouri.

Verification of the procedure suggests forecasting performance on par with the human forecasters. Unfortunately, efforts at more-rigorous performance analysis are hindered by the inadequacies of the verification database, which consists of pilots’ subjective reports of airframe ice buildup. In general, no-ice conditions are not reported.

The physics of aircraft icing are reviewed, and the current manual techniques are discussed. The automated procedure provides an infrastructure for implementing incremental improvements in the algorithm as observations and numerical models improve.

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William A. Cooper and Marcia K. Politovich

Abstract

No abstract available.

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Marcia K. Politovich and Ben C. Bernstein

Abstract

During the 1990 Winter Icing and Storms Project (WISP), a shallow cold front passed through northeastern Colorado, followed by a secondary cold front. A broad high pressure area behind the initial front set up a Denver cyclone circulation within a well-mixed boundary layer, which was capped by a stable, nearly saturated layer of air left in place by the initial cold front. As the secondary cold front passed through the WISP domain, these layers of air were lifted. The lifted boundary layer formed only broken cloud, but the lifted moist layer formed a stratiform cloud that contained high liquid water contents. Cloud characteristics were measured in situ with a research aircraft, and remotely by ground-based radars, microwave radiometers, and a lidar ceilometer. Moderate to severe icing conditions were reported by aircraft flying in the area during the event and also affected the flight of the research aircraft through an increase in drag on the airframe. Liquid water was depleted in portions of the lower stratiform cloud as ice crystals, produced in midlevel clouds embedded in westerly flow, fell into the lower cloud, and quickly rimed to form showers of graupel at the ground. After these midlevel clouds passed over the area, liquid production resumed. Supercooled liquid cloud persisted for 36 h as cloud formed within the surface cold air mass behind the secondary cold front as it entered the Denver area and was lifted over the local terrain.

The evolution of weather events is discussed using a variety of datasets, including radar, surface mesonet, balloon-borne soundings, research aircraft, satellite imagery, microwave radiometers, and standard National Weather Service observations. By combining information from these varied sources, processes governing the production and depletion of supercooled liquid from the synoptic to the microscale are examined. The storm is also discussed in terms of its potential for causing moderate to severe aircraft icing. The effect of accreted ice on the research aircraft is described, as are implications of the meteorology for detection and forecasting inflight icing.

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Peter V. Hobbs and Marcia K. Politovich

Abstract

The seeding of clouds in Miles City, Montana with AgI pyrotechnics at cloud tops generally produced large increases in ice particle concentrations, decreases in liquid water contents, and increases in precipitation particles lower down in the clouds. Similar, but even more pronounced, changes were observed when clouds were seeded with dry ice. Seeding with AgI-NH4I-acetone solution at cloud base generally did not produce observable changes in cloud microstructures at higher levels but in one case changes attributable to this type of seeding were observed.

Comparisons of the structure of seeded clouds at Miles City with unseeded clouds suggests that the seeding of small and embedded cumulus clouds with dry ice to produce ice panicle concentrations of ∼1–10 ℓ−1 may offer the best potential for enhancing precipitation.

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Jeffrey L. Stith and Marcia K. Politovich

Abstract

Sulfur hexafluoride was released at the base of a small nonprecipitating warm cumulus to study cloud mixing and entrainment processes. The tracer gas traveled to the top of the cloud where, during a 2.5 min period, it had mixed to produce a dilute mixture containing 30%, 19% and 51% of air from the original tracer region, an adjacent region of the same cloud, and the environment surrounding the cloud, respectively. The droplet size distributions measured at the top of the cloud represented a mixture of larger droplets that had been growing from the base and smaller, recently activated droplets. The observations suggest that the source region for the small droplets was near cloud top. The large droplet concentration was conserved during the mixing process. These observations are compared with predictions from some recent models for cloud entrainment and droplet evolution.

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Marcia K. Politovich and William A. Cooper

Abstract

The quasi-steady supersaturation in cumulus clouds from the Cooperative Convective Precipitation Experiment of 1981 are calculated from measurements of vertical wind, cloud droplet size, temperature and pressure. Mean values and spectral characteristics of the supersaturation are presented. The supersaturation values in 147 cloud regions averaged near 0%, with standard deviations in the (10-m average) supersaturation values that ranged from 0.1% in unmixed regions to about 0.4% in regions about 80% diluted by entrainment. The Eulerian variance spectra for vertical wind, integral radius and supersaturation were determined, and a technique is described for estimating the Lagrangian spectra. The Lagrangian characteristic times for supersaturation were estimated to be about 50 s for mixed regions but substantially larger for some regions with strong updrafts. It is argued that the observed variability in supersaturation is comparable to that needed to account for typical dispersions in droplet size spectra.

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Marcia K. Politovich and Tiffany A. O. Bernstein

Abstract

The in-flight icing environment in northeastern Colorado is described through remote and in situ measurements. Four field efforts in the winters of 1990 through 1994 were conducted in the area, and comprehensive datasets from research aircraft, ground-based multichannel microwave radiometers, and in-flight pilot reports were collected. Many aspects of the icing environment are examined: synoptic and mesoscale weather related to icing conditions, frequency of liquid-bearing clouds and potential icing conditions, microphysical characteristics of the clouds, and icing type and severity. Most clouds with supercooled liquid water sampled during these field projects were associated with cold fronts or cold surges or with widespread cyclonic or anticyclonic circulations that produce upslope cloudiness. The cloud characteristics were typically winter continental, with small droplets and low liquid water contents (both as in situ–measured cloud liquid water content and remotely sensed integrated amounts). Integrated liquid water values from ground-based microwave radiometers were typically low (usually less than 0.1 mm) but were higher over instruments situated nearer to the mountains; little diurnal trend was noted for any site. The aircraft database is described in detail in terms of three icing-critical cloud parameters (liquid water content, droplet size, and temperature). Most icing conditions reported by pilots flying in the area were light to moderate in severity and were rime in type. Comparisons of the data with measurements from other projects and locations are made.

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