Surface temperature spatial and temporal variations in NorthAmerica from homogenized satellite SMMR-SSM/I microwavemeasurements and reanalysis for 1979-2008

摘要

We have developed procedures for deriving land surface temperature andhomogenizing 30 years of daily microwave brightness temperatures from the NOAA/NASA Nimbus 7 scanning multichannel microwave radiometer (SMMR) and DefenseMeteorological Satellite Program Special Sensor Microwave/Imager (DMSP SSM/I)Pathfinder EASE-Grid database. Processing includes normalization of variable acquisitionoverpass time, removing the effects of changing satellite orbits, intercalibration of sensors,and filling gaps between missing data. The derived new database over North America(above 45°N), limited to snow-free periods, provides the first estimate of the trendsof consistent mean daily summer land surface temperature over the last three decades. Bycomparison with near-surface air temperatures derived from the European Centre forMedium-Range Weather Forecasts 40 year Reanalysis (ERA-40), the National Centersfor Environmental Prediction North American Regional Reanalysis (NARR), and theexhaustive ground-based meteorological measurements across Canada, we highlightedsignificant systematic biases in the satellite-derived surface temperatures for the1983-1991 period, spanning from the second half of Nimbus 7 SMMR lifetime(1983-1987; mean bias of –0.87°C compared to ERA-40) and the entire period ofDMSP-F8 SSM/I lifetime (1987-1991; mean bias of –1.56°C compared to ERA-40).The biased data were corrected with use of a relative offset derived from ERA-40and DMSP-F11/F13 mean difference over the 1992-2002 period. The comparison ofcorrected data from the 9 day overlap between SMMR and DMSP-F8 SSM/I andfrom ground-based meteorological measurements over the 1983-1991 biased periodshows the usefulness of the correction method. The mean difference between correctedsatellite-derived surface temperature and in situ meteorological air temperature is+0.05°C + 1.85°C for the entire period (1979-2008). The NARR data appear, in general,to be too warm by about 1°C. The satellite-derived homogenized database gives newobservational evidence for global warming over North American regions (mean summertemperature trend of +0.018°C/a throughout the studied period) with regional variabletrends, in agreement with reanalysis and in situ measurement trends. However, over theCanadian Arctic tundra, the increase in observed land surface temperatures appears slightlyless than the estimates based on near-surface air temperature.