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Numerical Simulations of an Observed Narrow Cold-Frontal Rainband

Chaing ChenMesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland, and Science Systems and Applications, Inc., Lanham, Maryland

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Craig H. BishopDepartment of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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George S. LaiMesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland, and Science Systems and Applications, Inc., Lanham, Maryland

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Wei-Kuo TaoMesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

A cold-frontal rainband, which occurred during the afternoon of 28 December 1988, is numerically simulated using the Penn State–NCAR three-dimensional MM5 modeling system. This case is characterized by a line of severe convection that has a gravity current–like structure along the leading edge of a strong surface cold front. The authors test whether this gravity current–like structure is associated with the cold-air outflow boundary generated by the evaporation of hydrometeors from the postfrontal precipitation system. It is found that the neglect of moist processes in the model did not significantly affect the gravity current–like structure of the front. PBL (planetary boundary layer) frictional processes, which produce cross-frontal low-level wind shear (uz), play an important role. The role of this cross-frontal low-level wind shear is to generate low-level convergence and to steepen the frontal slope. Thus, the observed intense NCFR (narrow cold-frontal rainband) is closely related to frictionally induced PBL processes.

Corresponding author address: Dr. Chaing Chen, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Code 912, Greenbelt, MD 20771.

Email: chen@betsy.gsfc.nasa.gov

Abstract

A cold-frontal rainband, which occurred during the afternoon of 28 December 1988, is numerically simulated using the Penn State–NCAR three-dimensional MM5 modeling system. This case is characterized by a line of severe convection that has a gravity current–like structure along the leading edge of a strong surface cold front. The authors test whether this gravity current–like structure is associated with the cold-air outflow boundary generated by the evaporation of hydrometeors from the postfrontal precipitation system. It is found that the neglect of moist processes in the model did not significantly affect the gravity current–like structure of the front. PBL (planetary boundary layer) frictional processes, which produce cross-frontal low-level wind shear (uz), play an important role. The role of this cross-frontal low-level wind shear is to generate low-level convergence and to steepen the frontal slope. Thus, the observed intense NCFR (narrow cold-frontal rainband) is closely related to frictionally induced PBL processes.

Corresponding author address: Dr. Chaing Chen, Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Code 912, Greenbelt, MD 20771.

Email: chen@betsy.gsfc.nasa.gov

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