Retrieving cloud top structure from infrared satellite data

A new retrieval method to detect steep temperature gradients between the co nvective overshoots of cumulonimbus clouds and the surrounding cirrus has b een 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. Th e impact of temperature gradients on the visible radiances is illustrated b y two examples that indicate deep convective overshoots. Both examples show that the illuminated side of the overshoots can exceed the cloud top refle ctivity by 50%, while the shadows account for < 50% of the cloud top reflec tance. The shadows usually extend several pixels beyond the base of the ove rshoots. Here we show that statistical analyses of cloud optical depth are affected by the cloud top structure, based on 1 month of high-resolution AV HRR satellite data. The contribution of shadow side pixels with steep tempe rature gradients (greater than or equal to 16 K) can exceed 30% for small o ptical depths (< 3). The contribution of illuminated sides of cloud top str uctures and cloud sides with steep temperature gradients can exceed 70% for large optical depths (> 32).