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Microphysical Processes Associated with Intense Frontal Rainbands and the Effect of Evaporation and Melting on Frontal Dynamics

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Previous studies have shown that a surface cold front often coincides with a heavy band of precipitation commonly designated as a narrow cold-frontal rainband. The maximum rainfall rate within this band can exceed 100–200 mm h−1. This study uses a nonhydrostatic two-dimensional cloud model with ice microphysics to investigate the precipitation processes within this type of rainband. Despite the relatively simple initialization and two-dimensionality, many aspects of these storms were well simulated. In these simulations, the intense but shallow updrafts produced large amounts of cloud water that were transformed primarily into rain and graupel within the zone of heavy precipitation and, to a lesser extent, into snow. The graupel and snow produced a zone of trailing stratiform precipitation. While the heavy rainfall could be represented in a warm rain model of the storm, an ice phase was needed in order to replicate the stratiform precipitation. Feedbacks of microphysical processes upon the dynamics of the flow were investigated. Sublimation and melting of frozen hydrometeors produced a pronounced cooling within the cold air mass, which slowly increased the depth and intensity of the cold air mass. This diabatic cooling within the cold air could potentially play a role in maintaining or even intensifying the circulations that lead to these rainbands. Previous studies of these types of fronts have instead concentrated on the role of melting in maintaining these structures through producing a stable layer across the cold air interface that could inhibit mixing.

Abstract

Previous studies have shown that a surface cold front often coincides with a heavy band of precipitation commonly designated as a narrow cold-frontal rainband. The maximum rainfall rate within this band can exceed 100–200 mm h−1. This study uses a nonhydrostatic two-dimensional cloud model with ice microphysics to investigate the precipitation processes within this type of rainband. Despite the relatively simple initialization and two-dimensionality, many aspects of these storms were well simulated. In these simulations, the intense but shallow updrafts produced large amounts of cloud water that were transformed primarily into rain and graupel within the zone of heavy precipitation and, to a lesser extent, into snow. The graupel and snow produced a zone of trailing stratiform precipitation. While the heavy rainfall could be represented in a warm rain model of the storm, an ice phase was needed in order to replicate the stratiform precipitation. Feedbacks of microphysical processes upon the dynamics of the flow were investigated. Sublimation and melting of frozen hydrometeors produced a pronounced cooling within the cold air mass, which slowly increased the depth and intensity of the cold air mass. This diabatic cooling within the cold air could potentially play a role in maintaining or even intensifying the circulations that lead to these rainbands. Previous studies of these types of fronts have instead concentrated on the role of melting in maintaining these structures through producing a stable layer across the cold air interface that could inhibit mixing.

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