Precision and Radiosonde Validation of Satellite Gridpoint Temperature Anomalies. Part II: A Tropospheric Retrieval and Trends during 1979–90

Abstract

TIROS-N satellite Microwave Sounding Unit (MSU) channel 2 data from different view angles across the MSU man swath are combined to remove the influence of the lower stratosphere and much of the upper troposphere on the measured brightness temperatures. The retrieval provides a sharper averaging kernel than the raw channel 2 weighting function, with a peak lowered from 50 kPa to 70 kPa and with only slightly more surface influence than raw channel 2. Monthly 2.5° gridpoint anomalies of this tropospheric retrieval compared between simultaneously operating satellites indicate close agreement, 0.15°C in the tropics to around 0.30°C over much of the higher latitudes. The agreement is not as close as with raw channel 2 anomalies because synoptic-scale temperature gradient information across the 2000-km swath of the MSU is lost in the retrieval procedure and because the retrieval involves the magnification of a small difference between two large numbers. Single gridpoint monthly anomaly correlations between the satellite measurements and the radiosonde calculations range from around 0.95 at high latitudes to below 0.8 in the tropical west Pacific, with standard errors of estimate of 0.16°C at Guam to around 0.50°C at high-latitude continental stations. Calculation of radiosonde temperature with a static weighting function instead of the radiative transfer equation degrades the standard errors by an average of less than 0.04°C. Of various standard tropospheric layers, the channel 2 retrieval anomalies correlate best with radiosonde 100–50- or 100–40-kPa-thickness anomalies. A comparison between global and hemispheric anomalies computed for raw channel 2 data versus the tropospheric retrieval show a correction in the 1979–90 time series for the volcano-induced stratospheric warming of 1982–83, which was independently observed by MSU channel 4. This correction leads to a slightly greater tropospheric warming trend in the 12-year time series (1979–90) for the tropospheric retrieval [0.039°C (±0.03°C) per decade] than for channel 2 alone [0.022°C (±0.02°C) per decade].