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Roy M. Rasmussen, Andrew Crook, and Cathy Kessinger

Abstract

The formation and evolution of convective rain and snow bands prior to and during the crash of Continental Airlines flight 1713 on 15 November 1987 at Denver Stapleton Airport are discussed. Convective rain occurred during the early stages of the storm in association with the approach of an upper-level trough from the west. Snow bands were observed following the passage of a shallow Canadian cold front from the north. These bands formed above the cold front and moved from southeast to northwest at 7 m s−1 with a horizontal spacing of 10–30 km. The winds within the cloud layer were southeasterly from 5 to 10 m s−1, suggesting that the bands were advected by the mean, cloud-layer flow. The most likely mechanism producing these bands was a convective instability in the shear layer above the cold front.

As the upper-level trough moved to the east, the winds in the cloud layer shifted to northerly, causing the bands to move southward with the major axis of the band oriented north–south. The high snowfall rate just prior to the takeoff of flight 1713 occurred as a result of one of these north–south–oriented bands moving over Denver Stapleton Airport from the north during the latter stages of the storm.

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Gregory Thompson, Marcia K. Politovich, and Roy M. Rasmussen

Abstract

Recent advances in high-performance computing have enabled higher-resolution numerical weather models with increasingly complex data assimilation and more accurate physical parameterizations. With respect to aircraft and ground icing applications, a weather model’s cloud physics scheme is responsible for the direct forecasts of the water phase and amount and is a critical ingredient to forecasting future icing conditions. In this paper, numerical model results are compared with aircraft observations taken during icing research flights, and the general characteristics of liquid water content, median volume diameter, droplet concentration, and temperature within aircraft icing environments are evaluated. The comparison reveals very promising skill by the model in predicting these characteristics consistent with observations. The application of model results to create explicit forecasts of ice accretion rates for an example case of aircraft and ground icing is shown.

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Julie M. Thériault, Roy Rasmussen, Trevor Smith, Ruping Mo, Jason A. Milbrandt, Melinda M. Brugman, Paul Joe, George A. Isaac, Jocelyn Mailhot, and Bertrand Denis

Abstract

Accurate forecasting of precipitation phase and intensity was critical information for many of the Olympic venue managers during the Vancouver 2010 Olympic and Paralympic Winter Games. Precipitation forecasting was complicated because of the complex terrain and warm coastal weather conditions in the Whistler area of British Columbia, Canada. The goal of this study is to analyze the processes impacting precipitation phase and intensity during a winter weather storm associated with rain and snow over complex terrain. The storm occurred during the second day of the Olympics when the downhill ski event was scheduled. At 0000 UTC 14 February, 2 h after the onset of precipitation, a rapid cooling was observed at the surface instrumentation sites. Precipitation was reported for 8 h, which coincided with the creation of a nearly 0°C isothermal layer, as well as a shift of the valley flow from up valley to down valley. Widespread snow was reported on Whistler Mountain with periods of rain at the mountain base despite the expectation derived from synoptic-scale models (15-km grid spacing) that the strong warm advection would maintain temperatures above freezing. Various model predictions are compared with observations, and the processes influencing the temperature, wind, and precipitation types are discussed. Overall, this case study provided a well-observed scenario of winter storms associated with rain and snow over complex terrain.

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