Estimation of sea surface temperature using passive microwave satellite imagery

Sea surface temperature is routinely estimated from space by infrared radiometers, but only where the ocean surface is cloud free. Thermal radiation at microwave frequencies however is much less likely to be blocked by cloud. Spaceborne passive microwave imagers can, in principle, use this radiation to estimate SST under a much wider range of atmospheric conditions than can infrared radiometers. It has been generally acknowledged that the range of microwave frequencies measured by the SSM/I sensor is not well suited to this purpose. In spite of this, we have devised a model for estimation of SST from SSM/I data, using band differences and derived geophysical parameters. We tested the model using data collected in May and June, 2001, in a region of the north-west Atlantic Ocean from the central Grand Banks across the Gulf Stream to the northern Sargasso Sea. Temperatures in the region varied from 2degreesC to 23degreesC. The model was calibrated with concurrent AVHRR and SSM/I imagery, and validated against an independent set of AVHRR-SSM/I image pairs. The model reproduced the AVHRR SST values with a mean difference of close to 0degreesC, and a standard deviation of +/-2.5degreesC. The strength of gradients and the direction of isotherms were reproduced very well by the model. These quantities were estimated consistently through cloud, in regions where AVHRR estimates were not possible. The model could not make an estimate of SST however in regions where cloud liquid water was present. Even though the model can provide a reliable estimate of SST from SSM/I in this region in most weather conditions, it has probably a bit too much scatter to be considered as a stand-alone system. SSM/I estimated SST fields however may provide useful input for interpolation systems working to create SST fields from sparse buoy data, or from time sequences of cloud filled AVHRR imagery.