A study of the NOAA-15 AMSU-A brightness temperatures from 1998 through 2007

Abstract

Global mean brightness temperatures from the NOAA-15 Advanced Microwave Sounding Unit-A (AMSU-A) over the period from May 1998 through December 2007 are studied. A time series of global daily mean brightness temperatures are constructed. These time series show a clear pattern of annual variation of the measurements which can be modeled with a Fourier series. After the annual variation as represented by the Fourier series is removed from the time series, extraannual variations of the measurements are obtained for examination of information about climate trends. Linear regressions of the extraannual variations at individual channels are performed, and the results demonstrate that the slopes for channels below the tropopause are positive whereas those above it are negative. This is strong evidence that the troposphere is warming and that the stratosphere is cooling. It is interesting to note that channel 5 has a slope of 0.0092 Kelvin per annum (K/a; or 0.092 K/decade) which agrees well with some of currently known climate trends of the middle troposphere. Diurnal variations of the monthly mean brightness temperatures measured over the globe with three AMSU-A radiometers onboard NOAA-15, -16, and -17, respectively, are also studied to provide a simple approach for correction of the effect of drifts of local equator crossing time on the data. A sensitivity test of the climate trend detection demonstrates that the method developed in this study can accurately detect an arbitrary climate trend which is introduced into the time series.