Marcia K. Politovich

Aircraft Icing Caused by Large Supercooled Droplets

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 met...

Icing Conditions Encountered by a Research Aircraft

Abstract The characteristics of clouds which have led to airframe icing on an instrumented Beechcraft Super King Air are summarized. The icing encounters occurred at altitudes from 0–8000 m MSL, in summer and winter, in stratiform and cumuliform clouds, and at temperatures from 0 to −30°C. The characteristics of icing encounters in different areas and in different seasons are compared. The fraction of measurements exceeding various threshold values of liquid water content, average liquid water content over a given distance, volume-median droplet diameter, droplet concentration, ice crystal concentration, and potential ice accumulation are given. The effects of these cloud characteristics on aircraft performance were measured by comparing the rate of climb of the aircraft with ice to the rate of climb for the clean aircraft under the same conditions. Most icing encounters led to a reduction in the rate of climb that increased linearly with the path integral of the supercooled liquid water content. The volu...

Current Icing Potential: Algorithm Description and Comparison with Aircraft Observations

Abstract The “current icing potential” (CIP) algorithm combines satellite, radar, surface, lightning, and pilot-report observations with model output to create a detailed three-dimensional hourly diagnosis of the potential for the existence of icing and supercooled large droplets. It uses a physically based situational approach that is derived from basic and applied cloud physics, combined with forecaster and onboard flight experience from field programs. Both fuzzy logic and decision-tree logic are applied in this context. CIP determines the locations of clouds and precipitation and then estimates the potential for the presence of supercooled liquid water and supercooled large droplets within a given airspace. First developed in the winter of 1997/98, CIP became an operational National Weather Service and Federal Aviation Administration product in 2002, providing real-time diagnoses that allow users to make route-specific decisions to avoid potentially hazardous icing. The CIP algorithm, its individual c...

Winter icing and storms project (WISP)

Abstract Field studies in support of the Winter Icing and Storms Project (WISP) were conducted in the Colorado Front Range area from 1 February to 31 March 1990(WISP90) and from 15 January to 5 April 1991 (WISP91). The main goals of the project are to study the processes leading to the formation and depletion of supercooled liquid water in winter storms and to improve forecasts of aircraft icing. During the two field seasons, 2 research aircraft, 4 Doppler radars, 49 Mesonet stations, 7 CLASS sounding systems, 3 microwave radiometers, and a number of other facilities were deployed in the Front Range area. A comprehensive dataset was obtained on 8 anticyclonic storms, 16 cyclonic storms, and 9 frontal passages. This paper describes the objectives of the experiment, the facilities employed, the goals and results of a forecasting exercise, and applied research aspects of WISP. Research highlights are presented for several studies under way to illustrate the types of analysis being pursued. The examples chose...

Conditions Associated with Large-Drop Regions

Abstract In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops. This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and ...

Variability of the Supersaturation in Cumulus Clouds

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 s...

A Study of the Broadening of Droplet Size Distributions in Cumuli

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 Wyomings 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 c...

Retrieval of atmospheric liquid and ice characteristics using dual-wavelength radar observations

Dual-wavelength (K/sub /spl alpha//- and X-band) radar measurements have shown promise in estimating the amount of liquid water in a cloud. By taking advantage of the attenuation by liquid water of the K/sub /spl alpha//-band signal as compared to X-band, the range-differentiated difference in reflectivity can be used to estimate the spatial distribution of cloud liquid water. One limitation is that the method is based on the assumption that all particles in the radar beams act as Rayleigh scatterers, that is, their diameters are small compared to the radar wavelengths. In natural clouds in wintertime conditions, this often may not be the case. This paper presents simulations of the response of these two wavelengths to conditions measured in several geographic locations. The simulations are used to build simplified relations between radar reflectivity and total mass and size distribution functions of liquid droplets and ice particles. Using these relations, it may be possible to estimate the sizes of the droplets, as well as total mass contents and size distributions of ice particles that may also be present in the sampled volume. Results of radar-based retrieval methods applied to measurements in a winter stratiform cloud are discussed, and compared with a previous result. A technique is described for detecting regions of non-Rayleigh scattering and for subsequently estimating liquid water content (LWC). Additional examples of dual-wavelength measurements in regions containing cloud droplets, small ice particles, and larger snowflakes are discussed.

Toward the improvement of aircraft-icing forecasts for the continental United States

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.

Surface weather features associated with freezing precipitation and severe in-flight aircraft icing

Surface observations of freezing precipitation and pilot reports of severe in-flight aircraft icing for the continental United States are compared to the location of surface weather features, including airmasses of different origin and position relative to fronts, low-pressure centers and troughs. Statistics are calculated to determine where freezing precipitation and severe aircraft icing occur most often, and are produced most efficiently (number of occurrences per unit area). Airmasses found along the east and west coasts of the US are the most efficient producers of freezing precipitation and pilot reports of severe icing, respectively, while the areas ahead of surface warm fronts are the most efficient producers of both of these phenomena. The weather conditions typically found in these locations, and others that are of interest for freezing precipitation and aircraft icing, are discussed.