A COMPARISON OF SEVERAL TECHNIQUES TO ASSIGN HEIGHTS TO CLOUD TRACERS

Satellite-derived cloud-motion vector (CMV) production has been troubl ed by inaccurate height assignment of cloud tracers, especially in thi n semitransparent clouds. This paper presents the results of an interc omparison of current operational height assignment techniques. Current ly, heights are assigned by one of three techniques when the appropria te spectral radiance measurements are available. The infrared window ( IRW) technique compares measured brightness temperatures to forecast t emperature profiles and thus infers opaque cloud levels. In semitransp arent or small subpixel clouds, the carbon dioxide (CO2) technique use s the ratio of radiances from different layers of the atmosphere to in fer the correct cloud height. In the water vapor (H2O) technique, radi ances influenced by upper-tropospheric moisture and IRW radiances are measured for several pixels viewing different cloud amounts, and their linear relationship is used to extrapolate the correct cloud height. The results presented in this paper suggest that the H2O technique is a viable alternative to the CO2 technique for inferring the heights of semitransparent cloud elements. This is important since future Nation al Environmental Satellite, Data, and Information Service (NESDIS) ope rations will have to rely on H2O-derived cloud-height assignments in t he wind field determinations with the next operational geostationary s atellite. On a given day, the heights from the two approaches compare to within 60-1 10 hPa rms; drier atmospheric conditions tend to reduce the effectiveness of the H2O technique. By inference one can conclude that the present height algorithms used operationally at NESDIS (with the CO2 technique) and at the European Satellite Operations Center (E SOC) (with their version of the H2O technique) are providing similar r esults. Sample wind fields produced with the ESOC and NESDIS algorithm s using Meteosat-4 data show good agreement.