Retrieving cloud top structure from infrared satellite data

Abstract

A new retrieval method to detect steep temperature gradients between the convective overshoots of cumulonimbus clouds and the surrounding cirrus has been applied to determine gradients and their orientation in the image plane of infrared data of the advanced very high resolution radiometer (AVHRR). These orientations are used to derive cloud elevations which are brightened by the Sun or are in shadow, which strongly affects the visible signal. The impact of temperature gradients on the visible radiances is illustrated by two examples that indicate deep convective overshoots. Both examples show that the illuminated side of the overshoots can exceed the cloud top reflectivity by 50%, while the shadows account for <50% of the cloud top reflectance. The shadows usually extend several pixels beyond the base of the overshoots. Here we show that statistical analyses of cloud optical depth are affected by the cloud top structure, based on 1 month of high‐resolution AVHRR satellite data. The contribution of shadow side pixels with steep temperature gradients (≥16 K) can exceed 30% for small optical depths (<3). The contribution of illuminated sides of cloud top structures and cloud sides with steep temperature gradients can exceed 70% for large optical depths (>32).