An algorithm for the estimation of upper tropospheric humidity from Kalpana observations: Methodology and validation

This study presents a methodology for estimating the upper tropospheric humidity (UTH) for a layer between 500 and 200 hPa from observations in the water vapor channel (5.6-7.2 mu m) of the Indian geostationary satellite, Kalpana. Radiative transfer simulations for different UTH conditions have been used to develop the relationship between water vapor channel radiances and UTH. A new technique has been described to include the normalized reference pressure in the algorithm, to account for latitudinal variation of temperature that is derived from a diverse radiosonde profiles data set and is a polynomial function of the latitude for different months. This has an advantage that the forecast or analysis profiles from the operational numerical weather prediction model are not required to compute the normalized reference pressure. The operationally retrieved UTH products have been extensively compared and validated for the period of 1 March to 1 May 2009, using Meteosat-7 UTH products over the Indian Ocean and the UTH computed from the radiosonde profiles. The results suggest that UTH estimates from Kalpana match very well with the Meteosat-7 UTH products having RMS difference of similar to 6%. Validation with the UTH computed from the radiosonde observed relative humidity shows that the RMS error of Kalpana UTH is 9.6% and the mean bias is -3.0%. Similar validation of Meteosat-7 UTH with the same set of radiosonde derived UTH shows an RMS error of 13.3% and the bias of -6.5%, which is higher in comparison to the Kalpana UTH.