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Ben C. Bernstein

Abstract

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.

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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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Ben C. Bernstein and Christine Le Bot

Abstract

Because of a lack of regular, direct measurements, limited information is available about the frequency and the 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. Surface observations of freezing fog and freezing precipitation provide additional insight, but cannot be used alone to assess the presence of icing aloft. Climatologies based solely on such observations can underestimate their presence in areas where subfreezing temperatures are uncommon. Other techniques can be used in an effort to reduce some of these biases and limitations. Expanding upon results for North America reported in Part I, the frequencies of icing and SLD over Europe and Asia are inferred here using 1) surface weather observations in conjunction with vertical profiles of temperature and moisture and 2) model reanalyses of temperature and moisture. Icing maxima and minima are found to migrate seasonally, both geographically and in the vertical, and to vary in their intensity. They are linked to the location of common storm tracks and other forcing, such as that associated with sloped terrain. After establishing reasonable consistency between the methods over data rich regions, the model analyses are used to examine icing frequencies over the remainder of the globe.

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Ben C. Bernstein and Richard H. Johnson

Abstract

Doppler and conventional radar, surface mesonetwork, and sounding data are used to investigate heat bursts that accompanied a mesoscale convective system (MCS) that traversed the OK PRE-STORM (Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central) mesonetwork on 23–24 June 1985. The MCS formed along a dryline in western Kansas. As the system matured, an area of stratiform precipitation developed behind a line of convective towers that moved toward the southeast. Southwesterly flow at upper levels caused the stratiform precipitation to extend northeastward, allowing it to become isolated from the convective line. A broad surface mesohigh was observed beneath the core of the stratiform region. This feature was flanked by sharp pressure gradients and mesoscale low pressure areas to the northwest, north, and northeast. It was within these mesolows that the heat bursts occurred.

Time series of surface mesonetwork data show that heat bursts were characterized by sudden dramatic rises in temperature and falls in dewpoint. Strong, gusty winds and modest falls in θe also accompanied the bursts. Dual-Doppler radar data reveal an anvil-like structure in the precipitation field above a surface mesonet station that received heat bursts. Light stratiform precipitation was falling just to the west of the station, and a strong reflectivity gradient existed at the edge of the precipitation. Strong mesoscale inflow entered the anvil cloud region at midlevels, descended along the base of the anvil, and reached the surface in the area of the strong reflectivity gradient. Downdrafts exceeding 4 m s−1 were observed there. It is proposed that this lateral inflow jet warmed dry adiabatically as it descended, deformed a surface stable layer, and caused dramatic warming and drying at the surface.

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Douglas A. Wesley, Roy M. Rasmussen, and Ben C. Bernstein

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 simulations predicted the behavior of low-level northerly flow along the Front Range and demonstrated the role of the terrain during the development of the Longmont anticyclone. The results of these simulations were compared to the case study results.

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Ben C. Bernstein, Cory A. Wolff, and Frank McDonough

Abstract

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.

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Ben C. Bernstein, Roy M. Rasmussen, Frank McDonough, and Cory Wolff

Abstract

Using observations from research aircraft flights over the Great Lakes region, synoptic and mesoscale environments that appear to drive a relationship between liquid water content, drop concentration, and drop size are investigated. In particular, conditions that fell within “small drop” and “large drop” regimes are related to cloud and stability profiles, providing insight regarding whether the clouds are tied to the local boundary layer. These findings are supported by analysis of flight data from other parts of North America and used to provide context for several icing incidents and accidents where large-drop icing was noted as a contributing factor. The relationships described for drop size discrimination in continental environments provide clues that can be applied for both human- and model-generated icing forecasts, as well as automated icing algorithms.

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Ben C. Bernstein, Tiffany A. Omeron, Frank McDonough, and Marcia K. Politovich

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 conditions when precipitation was not occurring. Icing conditions were also associated with overrunning conditions and troughs analyzed on upper-air charts. Conditions in some of these locations were conducive to the formation of large supercooled water droplets, which have recently been shown to be related to hazardous icing conditions.

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John V. Cortinas Jr., Ben C. Bernstein, Christopher C. Robbins, and J. Walter Strapp

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 short lived; 5) most freezing rain and freezing drizzle are not mixed with other precipitation types, whereas most reports of ice pellets included other types of precipitation; 6) freezing precipitation and ice pellets occur most frequently with a surface (2 m) temperature slightly less than 0°C; and 7) following most freezing rain events, the surface temperature remains at or below freezing for up to 10 h, and for up to 25 h for freezing drizzle.

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Roy M. Rasmussen, Ben C. Bernstein, Masataka Murakami, Greg Stossmeister, Jon Reisner, and Boba Stankov

Abstract

The mesoscale and microscale structure and evolution of a shallow, upslope cloud is described using observations obtained during the Winter Icing and Storms Project (WISP) and model stimulations. The upslope cloud formed within a shallow arctic air mass that moved into the region east of the Rocky Mountains between 12 and 16 February and contained significant amounts of supercooled liquid water for nearly 30 h. Two distinct layers were evident in the cloud. The lower layer was near neutral stability (boundary layer air) and contained easterly upslope flow. The upper layer (frontal transition zone) was thermodynamically stable and contained southerly flow. Overlying the upslope cloud was a dry, southwesterly flow of 20–25 m s −1, resulting in strong wind shear near cloud top. Within 10 km of the Rocky Mountain barrier, easterly low-level flow was lifted up and over the mountains. The above-described kinematic and thermodynamic structure produced three distinct mechanisms leading to the production of supercooled liquid water: 1) upslope flow over the gently rising terrain leading into the Colorado Front Range, up the slopes of the Rocky Mountains and over local ridges, 2)upglide flow within a frontal transition zone, and 3) turbulent mixing in the boundary layer. Supercooled liquid water was also produced by 1) upward motion at the leading edge of three cold surges and 2) vertical motion produced by low-level convergence in the surface wind field. Large cloud droplets were present near the top of this cloud (approximately 50-µm diameter), which grew by a direct coalescence process into freezing drizzle in regions of the storm where the liquid water content was greater than 0.25 g m −3 and vertical velocity was at 10 cm s −1

Ice crystal concentrations greater than 1 L−1 were observed in the lower cloud layer containing boundary layer air when the top of the boundary layer air when the top of the boundary layer was colder than −12°C. The upper half of the cloud was ice-free despite temperatures as low as −15°C, resulting in long-lived supercooled liquid water in this region of the cloud.

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