Determination of Cloud Base Height and Emissivity from Downwelling Angular Radiances

摘要

A technique has been devised which uses ground based radiance observations to estimate the base height and spectral emissivity of a homogeneous cloud layer. From the spectral emissivity, a broadband emissivity can also be deduced. An error sensitivity analysis was performed to simulate the effects of instrument errors. Model calculations at 10 micrometers and 11 micrometers showed that homogeneous cloud layers produce unique sets of ground radiance values over a range of zenith angles from o to 90 deg. By examining downwelling ground radiances at a very small zenith angle and a second zenith angle between 40 deg to 80 deg, one can distinguish between low emissivity low clouds ad high emissivity high clouds. This is possible because the cloud emissivity and the atmosphere transmissivity beneath a cloud are dependent on the viewing angle, thus affecting how much radiation reaches the ground; as a result, this relationship differs for every cloud height and emissivity. (Theses)