Abstract
Simultaneous infrared (10–12 μm) and visible (0.694 μm) 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 (∼200 m) dense clouds forming between 0 and −5C and with high infrared absorption coefficients (∼8 km−1), and thicker, low-density clouds, forming between −10 and −30C and with average infrared absorption coefficients of less than 1 km−1. The former type were undoubtedly liquid water clouds, whereas the latter were probably ice or mixed-phase clouds. Both types were often semi-transparent to infrared radiation.
Lidar backscatter coefficients for the higher clouds were typically between 0.1 and 3 km−1.
The experimental backscatter-to-extinction ratio at 0.694 μm and the ratio of the backscatter at 0.694 μm to the absorption at 10–12 μm are compared with theoretical computations. In certain cases, changes in these ratios could be identified with changes in the cloud microstructure.
Calculated infrared cooling rates in the clouds varied from 0.1 to 1.5C hr−1 depending on cloud density.
