Dynamics and Cloud Microphysies of the Rainbands in an Occluded Frontal System1

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle 98195
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Abstract

The dynamics and cloud microphysics of four rainbands in an occluded frontal system were examined. Aircraft, radar, raingage, and serial rawinsonde observations were obtained in addition to standard satellite and synoptic data. Two of the rainbands occurred in the leading portion of the frontal cloud shield and were oriented parallel to the warm front of the system. The other two bands occurred in the trailing portion of the cloud shield and had cold frontal orientations. Mesoscale pressure features were parallel to the rain-bands, except in mountainous areas. Computed air motions showed that the rainhands were supplied with moist air flowing into the rainband region from the south to south-southwest at low levels (below 800 mb). This air was swept abruptly upward in the rainbands just ahead of the cold air mass approaching from the west. Cumulus-scale convection in a layer between 4 and 5 km in clouds associated with these rainbands appeared to enhance the growth the ice particles. However, the ice crystal habits in these regions did not appear to be affected by the presence of the convection. As the ice particles settled below the convective layer, they grew first by vapor deposition and then, just above the melting layer, they began to grow by riming or aggregation. High ice particle concentrations were measured beneath the convective layer. Below the melting layer, very little precipitation growth took place in the rainbands, and in the two warm frontal bands, considerable evaporation of raindrops occurred below the melting layer.

Abstract

The dynamics and cloud microphysics of four rainbands in an occluded frontal system were examined. Aircraft, radar, raingage, and serial rawinsonde observations were obtained in addition to standard satellite and synoptic data. Two of the rainbands occurred in the leading portion of the frontal cloud shield and were oriented parallel to the warm front of the system. The other two bands occurred in the trailing portion of the cloud shield and had cold frontal orientations. Mesoscale pressure features were parallel to the rain-bands, except in mountainous areas. Computed air motions showed that the rainhands were supplied with moist air flowing into the rainband region from the south to south-southwest at low levels (below 800 mb). This air was swept abruptly upward in the rainbands just ahead of the cold air mass approaching from the west. Cumulus-scale convection in a layer between 4 and 5 km in clouds associated with these rainbands appeared to enhance the growth the ice particles. However, the ice crystal habits in these regions did not appear to be affected by the presence of the convection. As the ice particles settled below the convective layer, they grew first by vapor deposition and then, just above the melting layer, they began to grow by riming or aggregation. High ice particle concentrations were measured beneath the convective layer. Below the melting layer, very little precipitation growth took place in the rainbands, and in the two warm frontal bands, considerable evaporation of raindrops occurred below the melting layer.

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