Ben C. Bernstein

An Analysis of Freezing Rain, Freezing Drizzle, and Ice Pellets across the United States and Canada: 1976–90

Abstract A comprehensive analysis of freezing rain, freezing drizzle, and ice pellets was conducted using data from surface observations across the United States and Canada. This study complements other studies of freezing precipitation in the United States and Canada, and provides additional information about the temporal characteristics of the distribution. In particular, it was found that during this period 1) spatial variability in the annual frequency of freezing precipitation and ice pellets is large across the United States and Canada, and these precipitation types occur most frequently across the central and eastern portions of the United States and Canada, much of Alaska, and the northern shores of Canada; 2) freezing precipitation and ice pellets occur most often from December to March, except in northern Canada and Alaska where it occurs during the warm season, as well; 3) freezing rain and freezing drizzle appear to be influenced by the diurnal solar cycle; 4) freezing precipitation is often s...

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...

Regional and Local Influences on Freezing Drizzle, Freezing Rain,and Ice Pellet Events

Regional and local influences on frequency and type of freezing precipitation (freezing drizzle, freezing rain, and ice pellets) are investigated via in-depth climatologies of six continental United States (CONUS) sounding sites. For each site, wind roses of precipitation type occurrences are compared with those for nearby stations and the aggregate values for the CONUS. Synoptic scenarios and sounding structures are identified for prolonged events of each precipitation type and probable formation mechanisms are discussed. Station location relative to topographic features smaller than 1 km in height, water bodies ranging in size from oceans to small bays, and dominant wintertime storm tracks are shown to play a major role in the determination of the frequency and type of freezing precipitation at each site. Results help to explain the regional maxima and minima of freezing precipitation across the CONUS, as well as the dominance of certain precipitation types and formation mechanisms in different portions thereof. Understanding these differences is necessary for proper development of techniques used to diagnose and forecast surface precipitation type and the occurrence of hazardous aircraft icing conditions associated with freezing precipitation aloft.

Ice Fog in Arctic During FRAM–Ice Fog Project: Aviation and Nowcasting Applications

Ice fog and frost occur commonly (at least 26% of the time) in the northern latitudes and Arctic regions during winter at temperatures usually less than about –15°C. Ice fog is strongly related to frost formation—a major aviation hazard in the northern latitudes. In fact, it may be considered a more dangerous event than snow because of the stronger aircraft surface adhesion compared to snow particles. In the winter of 2010/11, the Fog Remote Sensing and Modeling–Ice Fog (FRAM-IF) project was organized near Yellowknife International Airport, Northwest Territories, Canada, with the main goals of advancing understanding of ice fog microphysical and visibility characteristics, and improving its prediction using forecast models and remotesensing retrievals. Approximately 40 different sensors were used to measure visibility, precipitation, ice particle spectra, vertical thermodynamic profiles, and ceiling height. Fog coverage and visibility parameters were estimated using both Geostationary Operational Environm...

Aviation Applications for Satellite-Based Observations of Cloud Properties, Convection Initiation, In-Flight Icing, Turbulence, and Volcanic Ash

Abstract Advanced Satellite Aviation Weather Products (ASAP) was jointly initiated by the NASA Applied Sciences Program and the NASA Aviation Safety and Security Program in 2002. The initiative provides a valuable bridge for transitioning new and existing satellite information and products into Federal Aviation Administration (FAA) Aviation Weather Research Program (AWRP) efforts to increase the safety and efficiency of the airspace system. The ASAP project addresses hazards such as convective weather, turbulence (clear air and cloud induced), icing, and volcanic ash, and is particularly applicable in extending the monitoring of weather over data-sparse areas, such as the oceans and other observationally remote locations. ASAP research is conducted by scientists from NASA, the FAA AWRPs Product Development Teams (PDT), NOAA, and the academic research community. In this paper we provide a summary of activities since the inception of ASAP that emphasize the use of current-generation satellite technologies ...

The Relationship between Aircraft Icing and Synoptic-Scale Weather Conditions

Abstract More than 2700 aircraft icing pilot reports are compared to analyses of operationally available data for 37 cases of winter weather. Statistical results regarding the number of occurrences of icing reports with airmass origin, location relative to fronts, troughs and low pressure centers, precipitation type, cloud cover, lightning/thunder, fog, radar reflectivity, and synoptic-scale forcing mechanisms are developed. Statistics are created for several combinations of icing severity and type, including a category for some of the worst icing encountered by aircraft (clear or mixed icing of moderate or greater severity), then normalized by the areal extent of the weather features. Results indicate that the locations most conducive to icing conditions were arctic, West Coast, and East Coast air masses; 250–600 km ahead of active and stationary warm fronts; in areas of freezing drizzle, freezing rain, and ice pellets when precipitation was occurring; and in areas with obscured and overcast sky conditio...

Production and depletion of supercooled liquid water in a Colorado Winter Storm

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 th...

An Inferred Climatology of Icing Conditions Aloft, Including Supercooled Large Drops. Part I: Canada and the Continental United States

Because of a lack of regular, direct measurements, little information is available about the frequency and spatial and temporal distribution of icing conditions aloft, including supercooled large drops (SLD). Research aircraft provide in situ observations of these conditions, but the sample set is small and can be biased. Other techniques must be used to create a more unbiased climatology. The presence and absence of icing and SLD aloft can be inferred using surface weather observations in conjunction with vertical profiles of temperature and moisture. In this study, such a climatology was created using 14 yr of coincident, 12-hourly Canadian and continental U.S. surface weather reports and balloonborne soundings. The conditions were found to be most common along the Pacific Coast from Alaska to Oregon, and in a large swath from the Canadian Maritimes to the Midwest. Prime locations migrated seasonally. Most SLD events appeared to occur below 4 km, were less than 1 km deep, and were formed via the collision–coalescence process.

Snowfall associated with a terrain-generated convergence zone during the Winter Icing and Storms Project

Abstract The Longmont anticyclone, a region of low-level anticyclonic turning and convergence during episodes of northerly winds along the Front Range of the Rocky Mountains, is documented for a snow event that occurred during the Winter Icing and Storms Project. The complex terrain in this region, especially the barrier to the west and the sloping Cheyenne Ridge to the north, is critical for the formation of this mesoscale feature. Upward motions related to this persistent convergent region downstream of the Cheyenne Ridge can strongly influence local snowfall distributions. The particular event studied in this manuscript was weakly forced on the synoptic scale. Through close examination of Doppler radar, special sounding and surface mesonetwork data, the effects of the Longmont anticyclone on snowfall were determined. The results of the analyses suggest that the convergence triggered convective snowbands in a region of delayed postfrontal cold advection at low levels. A series of mesoscale model simulat...