Precision Lower Stratospheric Temperature Monitoring with the MSU: Technique, Validation, and Results 1979–1991

Abstract

Microwave Sounding Unit channel 4 data from the TIROS-N series of NOAA satellites are intercalibrated to provide a continuous global record of deep-layer averaged lower stratospheric temperatures during 1979?1991. A 13-year record of temperature anomalies is time averaged into pentads and months on a 2.5° grid. The monthly gridpoint anomalies are validated with ten years of radiosonde data during 1979?88. The calibration stability of each satellite's measurements is evaluated during satellite overlap periods, the longest of which reveal no measurable instrumental drift at the level of 0.01°C yr?1. Intercomparisons between NOAA-6 and NOAA-7 anomalies indicate monthly gridpoint precision of 0.05°C in the tropics to around 0.10°C in the extratropies, and signal-to-noise ratios generally over 500, while global monthly precision is 0.01° to 0.02°C. These precision and stability statistics are much better than have been previously reported by other investigators for MSU channel 4. Pentad precision is about 0.10°C in the tropics to around 0.25°C at high latitudes and signal-to- noise ratios generally over 250 in the tropics and high latitude but 100?200 in the middle latitudes. Radiosonde comparisons to the monthly gridpoint anomalies have correlations ranging from 0.90 in the tropics (when the interannual variability is smallest) to as high as 0.99 at high-latitude stations. The corresponding standard error of estimate is generally around 0.3°C. A significant difference in decadal trends is found between the satellite and radiosonde systems, with a step change of 0.217°C (sondes cooler) compared to the satellite measurements. Investigations of the possible sources of the discrepancy lead us to suspect that the gradual transition from on-site calibration of sondes with thermometers to factory calibration of sondes around 1982 might have caused a change in the calibration, although this conclusion must be viewed as tentative. The largest globally averaged temperature variations during 1979?91 occur after the El Chichón (1982) and Pinatubo (1991) volcanic eruptions. These warm events are superimposed upon a net downward trend in temperatures during the period. This cooling trend has more of a step function than linear character, with the step occurring during the El Chichón warm event. It is strongest in polar regions and the Northern Hemisphere middle latitudes. These characteristics are qualitatively consistent with radiative adjustments expected to occur with observed ozone depictions.