Abstract
Satellite-borne passive microwave radiometers provide brightness temperature (TB) measurements in a large spectral range which includes a number of frequency channels and generally two polarizations: horizontal and vertical. These TBs are widely used to retrieve several atmospheric and surface variables and parameters such as precipitation, soil moisture, water vapor, air temperature profile, and land surface emissivity. Since TBs are measured at different microwave frequencies with various instruments and at various incidence angles, spatial resolutions, and radiometric characteristics, a mere direct integration of them from different microwave sensors would not necessarily provide consistency. However, when appropriately harmonized, they can provide a complete dataset to estimate the diurnal cycle. This study first constructs the diurnal cycle of land TBs using the non-sun-synchronous Global Precipitation Measurement (GPM) Microwave Imager (GMI) observations by utilizing a cubic spline fit. The acquisition times of GMI vary from day to day and, therefore, the shape (amplitude and phase) of the diurnal cycle for each month is obtained by merging several days of measurements. This diurnal pattern is used as a point of reference when intercalibrated TBs from other passive microwave sensors with daily fixed acquisition times (e.g., Special Sensor Microwave Imager/Sounder, and Advanced Microwave Scanning Radiometer 2) are used to modify and tune the monthly diurnal cycle to daily diurnal cycle at a global scale. Since the GMI does not cover polar regions, the proposed method estimates a consistent diurnal cycle of land TBs at global scale. Results show that the shape and peak of the constructed TB diurnal cycle is approximately similar to the diurnal cycle of land surface temperature. The diurnal brightness temperature range for different land cover types has also been explored using the derived diurnal cycle of TBs. In general, a large diurnal TB range of more than 15 K has been observed for the grassland, shrubland, and tundra land cover types, whereas it is less than 5K over forests. Furthermore, seasonal variations in the diurnal TB range for different land cover types show a more consistent result over the Southern Hemisphere than over the Northern Hemisphere. The calibrated TB diurnal cycle may then be used to consistently estimate the diurnal cycle of land surface emissivity. Moreover, since changes in land surface emissivity are related to moisture change and freeze–thaw (FT) transitions in high-latitude regions, the results of this study enhance temporal detection of FT state, particularly during the transition times when multiple FT changes may occur within a day.
Highlights
Satellite-borne passive microwave (PMW) radiometers provide relatively higher temporal resolution brightness temperature (TB) measurements in comparison to active microwave systems
The objective of this study is twofold: (1) to generate half-hourly TB diurnal cycle from combination of the intercalibrated GPM Microwave Imager (GMI), Advanced Microwave Scanning Radiometer-2 (AMSR2), and SSMIS measurements for common channels, and (2) to depict spatially consistent maps of diurnal brightness temperature range (DTR) and assess its variability for different vegetation types at the global scale
A spline fit was primarily applied to monthly GMI data between
Summary
Satellite-borne passive microwave (PMW) radiometers provide relatively higher temporal resolution brightness temperature (TB) measurements in comparison to active microwave systems. TBs from PMW sensors have conventionally been used to retrieve several atmospheric and surface parameters such as soil moisture, vegetation structure, precipitation, snow cover, atmospheric water vapor content, land surface temperature, and land surface emissivity [1,2,3,4,5,6,7,8,9,10,11]. Active and passive microwave instruments onboard remote sensing satellites have been proven to be promising tools in the detection of FT states at the global scale [12,13,14,15,16,17]. FT detection from direct TBs are influenced and tainted by atmospheric water vapor and the presence of clouds especially for higher frequencies, and they may not be purely reflective of the surface state. The use of PMW remotely sensed land surface emissivity in FT detection is physically consistent because land surface emissivity estimates are usually free from atmospheric effects and are sensitive to surface characteristics [16]
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