Water surface temperature estimates using active and passive microwave remote sensing: preliminary results from an outdoor wind-wave tank

Accurate remote sensing measurements of sea surface temperature (SST) are a necessity for weather and climate operational and research activities. Infrared radiometers yield SST to about 0.5 degC accuracy but cannot be used in cloudy areas. Passive microwave techniques are useful in cloudy areas, but are limited to an accuracy of about 1.5 to 2 degC by the relatively large variation of microwave emissivity with surface conditions, such as wind speed. To improve the accuracy of passive microwave estimates of SST, active microwave (radar) observations can be added to estimate emissivity more precisely. To assess this approach the authors performed preliminary active and passive measurements in an outdoor wind-wave tank during the summer of 1993. C-band (5.2 GHz) and X-band (9.4 GHz) radar measurements were combined with C-band radiometer observations of the water surface in a 4 m diameter circular pool. Air flow over the surface was produced by a bank of fans and environmental conditions were measured, including air and water surface temperature, water surface roughness, air flow velocity, etc. Preliminary results showed a strong correlation of the rise in both C- and X-band radar backscatter, C-band in particular, with the rise in radiometer brightness temperature as air flow speed was increased. These results suggest that active and passive microwave observations can be used together to produce more accurate estimates of SST and possibly other ocean surface parameters.