Upper tropospheric humidity from SAPHIR on-board Megha-Tropiques

Abstract

Upper tropospheric humidity (UTH) has been derived using a ‘brightness temperature (T_b) transformation’ method from the humidity sounder channels of SAPHIR payload on - board Megha - Tropiques (MT). These channels are very close to the water vapour absorption peak at 183.31 GHz. The channel at 183.31  0.2 GHz enables retrieval of humidity up to the hig h est altitude possible wit h the present nadir - looking microwave humidity sounders. Megha - Tropiques satllite has an equatorially inclined orbit, which e n sures frequent spatial and temporal coverage of the global tropical belt. Transformation coeff i cients for the first three channels for all the incidence angles have been derived and are used to convert brightness temperatures to weighted average upper tropospheric humidity having weighting function peaks at different pressure levels. The methodology has been validated by comparing the SAPHIR - derived UTH with that derived from radiosonde observations. Inter - comparison of the derived UTH has been done with layer averaged humidity product from SAPHIR measurements and with UTH product using infrared measurements from Kalpana satellite ( MOSDAC). UTH over the tropical belt for six months has been studied taking the advantage of the humidity product with high spatial and temporal resolution. The transformation coefficients and methodology to identify the cloud - free pixels to derive UTH from the three channels for all the possible incidence angles are presented here, so that the users can directly derive UTH from the brightness temperature data.