A study of AMSU‐A measurement of brightness temperatures over the ocean

Abstract

Brightness temperatures observed by the NOAA 15, 16, 17, and 18 Advanced Microwave Sounding Unit‐A (AMSU‐A) over the tropical ocean region 20°S–20°N are investigated for the months of January, April, July, and October 2006. Monthly mean angular distributions of the data from each month show a very stable pattern. The two sets of data measured by the NOAA 16 and 18 satellites agree well with each other. It is found that the observed brightness temperatures from the descending passes at local equator crossing time (LECT) 0200 are higher than those of the ascending passes at LECT 1400 in the window channels (i.e., channels 1–3 and 15) even though the sea surface temperatures (SSTs) at ∼1400 LECT are usually warmer than those at ∼0200 LECT. The cause of such surprising observations is attributed to the differences in atmospheric transmittance and ocean surface emissivities. As a function of SST, the ocean surface emissivity increases as the SST decreases and vice versa. It is found that the transmittances at the descending (nighttime) passes are smaller than those at the ascending (daytime) ones and that smaller transmittances enhance the upwelling atmospheric contribution to the brightness temperatures. Angular distributions were simulated with a radiative transfer model of one layer of effective atmosphere with optical depth α and temperatures T0 and TS at the top and bottom of the layer, respectively. A least squares fitting method is used to match the model calculations with the AMSU‐A measurements. The simulated results agree well with all AMSU‐A measurements. The NOAA 16 data can be reproduced with the best fit parameters obtained from the NOAA 18 data. Such a close resemblance and stable pattern of angular distributions from the two satellites are potentially useful for postlaunch calibration of future microwave radiometers. The data from the four satellites with different equator crossing times are used to study the diurnal variation of the ocean surface brightness temperatures, which show patterns of daytime cooling and nighttime warming.