Influence of high altitude clouds on upper tropospheric radiance measurements

Altitude profiles of atmospheric window radiance measured with upward-looking sensors frequently show a rapid decrease in radiance with increasing height over a narrow altitude region in the upper troposphere. This region of rapid decrease is termed a radiometric knee in the altitude profile. The top of this knee defines a radiometric tropopause with a latitudinal height dependence similar to that of the usually defined barometric tropopause. Atmospheric window (10-12-microm) radiance at these altitudes can be associated with the presence of ice particulates. Comparison of the measurements with predicted altitude profiles of atmospheric radiance from the LOWTRAN 7 atmospheric model code shows that a well-defined knee occurs when there is a cloud layer (liquid or ice) such as a subvisual cirrus cloud present. The rate and magnitude of the radiance decrease depend on the optical depth and, therefore, the water content of the layer. Atmospheric background radiance values for near horizontal (large zenith angle) viewing with upward-looking sensors can be as much as a factor of 100 lower above the knee than below it. Comparisons between calculated and observed radiance profiles were used to estimate the vertical extent, total optical depth, and water content of the clouds.