Global Atmospheric Temperature Monitoring with Satellite Microwave Measurements: Method and Results 1979–84

Abstract

A method for measuring global atmospheric temperature anomalies to a high level of precision from satellites is demonstrated. Global data from the Microwave Sounding Units (MSUs), flying on NOAA satellites since late 1978, have been analysed to determine the extent to which these data can reveal atmospheric temperature anomalies on bidaily and longer time scales for regional and larger space scales. The global sampling provided by the MSUs is an important asset, with most of the earth sampled bidaily from each of (typically) two instruments flying concurrently on separate satellites at different solar times. The primary source of tropospheric thermal information is from the MSU 53.74 GHz channel. This channel is primarily sensitive to thermal emission from molecular oxygen in the middle troposphere, with relatively little sensitivity to water vapor, the earth's surface, and cloud (especially cirrus) variations. The long-term stability of the oxygen mixing ratio in the atmosphere makes it an ideal tracer for climate monitoring purposes. Lower stratospheric temperature anomalies are derived from the MSU 57.95 GHz channel. Comparisons between monthly MSU temperature anomalies and corresponding thermometer-measured anomalies for the United States reveal a high (0.9) correlation, but hemispheric anomalies show much lower correlations. This results from some combination of poor thermometer sampling of remote regions and weak coupling of surface and deep-tropospheric temperature anomalies in tropical areas. Analysis of data from two of the MSUs (on NOAA-6 and NOAA-7), whose operational periods overlapped by two years, reveals that hemispheric temperature anomalies measured by the separate instruments are very similar (to about 0.01°C) on monthly time scales. Their combined time series of unfiltered two-day hemispheric averages show standard deviations of their mean of 0.15°?0.20°C and standard deviations of their average difference of 0.02°?0.03°C, indicating a signal-to-noise ratio of 40 for the Southern Hemisphere and 45 for the Northern Hemisphere. The intercomparison period also reveals no evidence of calibration drift between satellites at the 0.01°C level. This was substantiated by two 15-month comparisons of NOAA-6 with rawinsonde data from 45 stations in the eastern United States, which revealed 0.013°C net difference over five years. Monthly averaged comparisons between individual rawinsonde and NOAA-6 data from 1980 through 1982 reveal a monthly standard deviation of their difference of 0.04°C. The statistical and geophysical portions of this noise are found to be about equal in magnitude, 0.03°C. The single-satellite noise due to imperfect sampling for ten-day, 2.5° gridpoint temperatures was calculated by measuring the standard deviation of the difference between two satellites with ranges from 0.2°C in the tropics to 0.4°C in middle latitudes. The period of analysis (1979?84) reveals that Northern and Southern hemispheric tropospheric temperature anomalies (from the six-year mean) am positively correlated on multiseasonal time scales but negatively correlated on shorter time scales. The 1983 ENSO dominates the record, with early 1983 zonally averaged tropical temperatures up to 0.6°C warmer than the average of the remaining years. These natural variations are much larger than that expected of greenhouse enhancements, and so it is likely that a considerably longer period of satellite record must accumulate for any longer-term trends to be revealed.