STRUCTURE AND OPTICAL PROPERTIES OF SOME MIDDLE-LEVEL CLOUDS.

Abstract

Simultaneous infrared (10-12 mu m) and visible (0. 694 mu m) optical properties of some altocumulus and altostratus clouds have been determined using infrared radiometry and lidar. Broadly, two types of middle-level clouds were observed; rather thin ( similar 200 m) dense clouds forming between 0 and minus 5C and with high infrared absorption coefficients ( similar 8 km** minus **1), and thicker, low-density clouds, forming between minus 10 and minus 30C and with average infrared absorption coefficients of less than 1 km** minus **1. Both types were often semi-transparent to infrared radiation. Lidar backscatter coefficients for the higher clouds were typically between 0. 1 and 3 km** minus **1. The experimental backscatter-to-extinction ratio at 0. 694 mu m and the ratio of the backscatter at 0. 694 mu m to the absorption at 10-12 mu m are compared with theoretical computations. Calculated infrared cooling rates in the clouds varied from 0. 1 to 1. 5C hr** minus **1 depending on cloud density.