Improved multisensor approach to satellite‐retrieved near‐surface specific humidity observations

Abstract

A multisensor microwave retrieval of near‐surface 10 m specific humidity (Qa) using satellite observations from the advanced microwave sounding unit‐A (AMSU‐A), the Special Sensor Microwave Temperature Sounder‐2, and the Special Sensor Microwave Imager is improved upon in this study. Refinements to the regression formula, training data set, collocation procedure, and height adjustment to 10 m were used to improve retrievals from two different sensor combinations. Independent validation with the International Comprehensive Ocean‐Atmosphere Data Set (ICOADS) indicates a lower overall bias of ∼0.3 g/kg and a smaller root‐mean‐square difference of ∼0.4 g/kg than with several previously published single‐sensor Qa retrievals. A significant regional Qa wet bias of ∼3 g/kg in the summer over the North Pacific was found for all satellite retrievals, and a correction was developed using an inversion index defined using sea surface temperature and AMSU‐A lower tropospheric temperature observations. An assessment of ICOADS ship and buoy validation data indicated uncertainties related to height adjustments of these in situ observations to be 0.2–0.4 g/kg, while hygrometer differences and solar heating effects had smaller uncertainties of less than 0.05 g/kg. Validation of the updated multisensor retrievals with ICOADS over an 8‐year period from 1999 to 2006 revealed a reduced magnitude of the regional biases when compared to previously published retrievals. Regional Qa differences, particularly in the subtropical high regions, are shown to play a significant role in determination of surface latent heat flux.