Use of MODIS and synthetic aperture radar wind speed imagery to describe the morphology of open cell convection

Abstract

Moderate resolution imaging spectroradiometer (MODIS) imagery, meteorological analyses, and numerical weather prediction model forecasts for a 28–29 October 2006 case study are employed to argue that squall-and-lull patterns observed within a corresponding synthetic aperture radar (SAR) derived wind speed image are the result of open cell convection. In addition, the suite of data is exploited to advance a new conceptual model of open cell convection. MODIS imagery reveals that the open cell convection reported in this paper is composed primarily of cumulus humulus and cumulus congestus clouds, with cloud shadow measurements yielding a factor-of-three variation in cloud top height between the highest clouds (those on the leading or downwind edge of the cells) and the more shallow clouds along the sides and rear of the cells. The MODIS imagery reveals that the tallest clouds typically contain ice, which suggests that precipitation-driven downdrafts may cause the wind squalls seen in SAR-derived wind speed imagery. Individual squall-and-lull features have an elliptical boundary with an arc-shaped squall along the downwind edge of the feature, with wind speed decreasing gradually across the feature to its upwind edge.