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The various precipitation types formed within winter storms (such as snow, wet snow and freezing rain) often lead to very hazardous weather conditions. These types of precipitation often occur during the passage of a warm front as a warm air mass ascends over a cold air mass. To address this issue further, we used a one-dimensional kinematic cloud model to simulate this gentle ascent (≤10 cm/s) of warm air. The initial temperature profile has an above 0°C inversion, a lower subfreezing layer, and precipitation falls from above the temperature inversion. The cloud model is coupled to a double-moment microphysics scheme that simulates the production of various types of winter precipitation. The results are compared with those from a previous study carried out in still air. Based on the temporal evolution of surface precipitation, snow reaches the surface significantly faster than in still air whereas other precipitation types including freezing rain and ice pellets have a shorter duration. Overall, even weak background vertical ascent has an important impact on the precipitation reaching the surface, the time of the elimination of the melting layer, and also the evolution of the lower subfreezing layer.

The mesoscale structure of the low-level flow and the character of the turbulence are investigated for a warm front as it approached the prominent terrain of the Pacific Northwest in the vicinity of Vancouver Island. Flight level and airborne Doppler radar measurements collected from a National Oceanic and Atmospheric Administration P-3 research aircraft on 9 December 1995 during the Coastal Observation and Simulation with Topography experiment and high-resolution numerical simulations, performed with the Naval Research Laboratory's nonhydrostatic Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) model, have been used to document the warm front as it approached the coast and the structure of cool air trapped along Vancouver Island. This air mass was capped by a flat, thin transition zone of enhanced static stability and vertical wind shear. The flow within the trapped air mass was oriented parallel to the terrain of Vancouver Island, but appears to have been more an outflow from the Strait of Juan de Fuca than a barrier jet. Model sensitivity experiments suggest that the cold air that exited from the strait and the steep topography of Vancouver Island acted in concert to impede the northward movement of the front and steepen it near the surface. A wind speed maximum near the exit of the Strait of Juan de Fuca was consistent with transcritical expansion fan behavior. Both the observations from the gust probe system on the P-3, and output from COAMPS, indicate that the turbulent kinetic energy at low levels was relatively weak in the warm sector, moderate in the frontal zone, and strongest in the trapped flow near Vancouver Island., Published in Monthly Weather Review, v129 p978-998, May 2001.

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The various precipitation types formed within winter storms (such as snow, wet snow and freezing rain) often lead to very hazardous weather conditions. These types of precipitation often occur during the passage of a warm front as a warm air mass ascends over a cold air mass. To address this issue further, we used a one-dimensional kinematic cloud model to simulate this gentle ascent (≤10 cm/s) of warm air. The initial temperature profile has an above 0°C inversion, a lower subfreezing layer, and precipitation falls from above the temperature inversion. The cloud model is coupled to a double-moment microphysics scheme that simulates the production of various types of winter precipitation. The results are compared with those from a previous study carried out in still air. Based on the temporal evolution of surface precipitation, snow reaches the surface significantly faster than in still air whereas other precipitation types including freezing rain and ice pellets have a shorter duration. Overall, even weak background vertical ascent has an important impact on the precipitation reaching the surface, the time of the elimination of the melting layer, and also the evolution of the lower subfreezing layer.