Surface temperature spatial and temporal variations in North America from homogenized satellite SMMR-SSM/I microwave measurements and reanalysis for 1979-2008

We have developed procedures for deriving land surface temperature and homogenizing 30 years of daily microwave brightness temperatures from the NOAA/NASA Nimbus 7 scanning multichannel microwave radiometer (SMMR) and Defense Meteorological Satellite Program Special Sensor Microwave/Imager (DMSP SSM/I) Pathfinder EASE-Grid database. Processing includes normalization of variable acquisition overpass 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 degrees N), limited to snow-free periods, provides the first estimate of the trends of consistent mean daily summer land surface temperature over the last three decades. By comparison with near-surface air temperatures derived from the European Centre for Medium-Range Weather Forecasts 40 year Reanalysis (ERA-40), the National Centers for Environmental Prediction North American Regional Reanalysis (NARR), and the exhaustive ground-based meteorological measurements across Canada, we highlighted significant systematic biases in the satellite-derived surface temperatures for the 1983-1991 period, spanning from the second half of Nimbus 7 SMMR lifetime (1983-1987; mean bias of -0.87 degrees C compared to ERA-40) and the entire period of DMSP-F8 SSM/I lifetime (1987-1991; mean bias of -1.56 degrees C compared to ERA-40). The biased data were corrected with use of a relative offset derived from ERA-40 and DMSP-F11/F13 mean difference over the 1992-2002 period. The comparison of corrected data from the 9 day overlap between SMMR and DMSP-F8 SSM/I and from ground-based meteorological measurements over the 1983-1991 biased period shows the usefulness of the correction method. The mean difference between corrected satellite-derived surface temperature and in situ meteorological air temperature is +0.05 degrees C +/- 1.85 degrees C for the entire period (1979-2008). The NARR data appear, in general, to be too warm by about 1 degrees C. The satellite-derived homogenized database gives new observational evidence for global warming over North American regions (mean summer temperature trend of +0.018 degrees C/a throughout the studied period) with regional variable trends, in agreement with reanalysis and in situ measurement trends. However, over the Canadian Arctic tundra, the increase in observed land surface temperatures appears slightly less than the estimates based on near-surface air temperature.