Abstract
Abstract. Microwave remote sensing can be used to monitor the time evolution of some key parameters over land, such as land surface temperature or surface water extent. Observations are made with instruments, such as the Scanning Microwave Multichannel Radiometer (SMMR) before 1987, the Special Sensor Microwave/Imager (SSM/I) and the subsequent Special Sensor Microwave Imager/Sounder (SSMIS) from 1987 and still operating, and the more recent Global Precipitation Measurement Microwave Imager (GMI). As these instruments differ on some of their characteristics and use different calibration schemes, they need to be inter-calibrated before long-time-series products can be derived from the observations. Here an inter-calibration method is designed to remove major inconsistencies between the SMMR and other microwave radiometers for the 18 and 37 GHz channels over continental surfaces. Because of a small overlap in observations and a ∼6 h difference in overpassing times between SMMR and SSM/I, GMI was chosen as a reference despite the lack of a common observing period. The diurnal cycles from 3 years of GMI brightness temperatures are first calculated and then used to evaluate SMMR differences. Based on a statistical analysis of the differences, a simple linear correction is implemented to calibrate SMMR on GMI. This correction is shown to also reduce the biases between SMMR and SSM/I, and can then be applied to SMMR observations to make them more coherent with existing data records of microwave brightness temperatures over continental surfaces.
Highlights
Since 1978, passive microwave satellite imagers have provided Earth observations at multiple frequencies, over ocean and land, for atmospheric or surface applications such as cloud and precipitation monitoring, surface temperature estimation, ocean wind speed measurement, or sea ice concentration retrievals (Ulaby et al, 1986)
The inter-calibration performed by the Climate Monitoring Satellite Application Facility (CM SAF) is based on the double-difference technique between the Sensor Microwave Imager (SSM/I) F08 and Scanning Multichannel Microwave Radiometer (SMMR) brightness temperatures (Tbs), using radiative transfer simulations from reanalysis to account for the changes in frequencies, bandwidths, and Earth incidence angles (EIAs)
The variability associated to each Global Precipitation Measurement Microwave Imager (GMI) average is computed and displayed
Summary
Since 1978, passive microwave satellite imagers have provided Earth observations at multiple frequencies, over ocean and land, for atmospheric or surface applications such as cloud and precipitation monitoring, surface temperature estimation, ocean wind speed measurement, or sea ice concentration retrievals (Ulaby et al, 1986). Dai and Che (2009) tested a modeling of the diurnal variation of the surface temperature to inter-calibrate instruments with different overpassing times over land Another inter-calibration method consists in using matchups with a reference instrument that has a different orbit type, making it possible to provide quasi-direct comparisons over a large range of latitudes, even for satellites with different overpassing times. We suggest to use GMI as a reference instrument, assuming that the environmental conditions have not changed drastically from the SMMR to the GMI era, to allow the comparison of a large set of observations averaged over time This strategy does not allow a detailed intercalibration to be performed, but it makes it possible to correct for major biases that so far have hampered the use of SMMR over land for the generation of climate records of geophysical parameters.
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