Assigning Heights to Cloud Motion Vectors

Satellite derived cloud motion vector (CMV) production has been troubled by inaccurate height assignment of cloud tracers, especially in thin semi-transparent clouds. This paper presents the results of an intercomparison of operational height assignment techniques. Heights are assigned by one of three techniques when the appropriate spectral radiance measurements are available: the infrared window (IRW) technique, the carbon dioxide (CO2) ratio technique, and the water vapor (H2O) intercept technique. The results presented in Nieman et al. (1993) suggest that the H2O technique is a viable alternative to the CO2 technique for inferring the heights of semi-transparent cloud elements. For the several days studied, the heights from the two approaches compare to within 60 to 110 hPa rms; drier atmospheric conditions tend to reduce the effectiveness of the H2O intercept technique. Problems associated with determinations of cloud free radiances in both the CO2 and H2O techniques have been largely overcome. Difficulties remain when more than one layer of cloud is present; progress in assigning heights to cirrus over lower opaque clouds is being made.