# Diurnal Cycle of Passive Microwave Brightness Temperatures over Land at a Global Scale
## 1 Introduction  
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]. The change in dielectric properties can dramatically affect the TB signal when water transits from the liquid to the solid phase which makes PMW remote sensing unique for characterizing the surface freeze/thaw (FT) processes. About one-third of the globe, especially high-latitude regions, is subject to seasonal freezing and thawing, but ground-based observations of FT conditions in boreal regions are generally sparse and inconsistent. 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. Atmospheric condition, topography, and surface type also influence FT detection. However, 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]. Hence, estimation of diurnal variations of TBs over land is vital for the exact or adequate retrieval of the solar insolation, atmospheric states, and land surface characteristics (e.g., vegetation cover, soil moisture, land cover type, etc.). Higher temporal resolution TBs are also crucial for FT detection during the period when transition between freeze and thawing states may occur more frequently, even within a day. Apart from that, there are two important dates for FT cycles: (1) the onset of thaw date, and (2) the end date of daily FT cycles at the surface. Both dates may be related to biogeochemical processes, especially carbon fluxes in boreal forests. The latter corresponds to the switch from source to sink in evergreen boreal forest environments as illustrated via comparison with eddy flux tower data and xylem sap flow records [18]. In general, ground-based observations of TBs are sparse and inadequate, especially at the global scale. They are, therefore, not suitable to study large scale spatial variability. However, satellite remote sensing provides an effective way to measure TBs uniformly at the global scale. PMW sensors provide TB observations at different times from non-sun-synchronous satellites and twice daily from sun-synchronous satellites. These PMW sensors onboard different satellites measure TBs in a large spectral range including a number of channels and usually for two polarizations: horizontal and vertical. However, since these PMW sensors have different incidence angles, footprints, and radiometric characteristics, their measured TBs would not necessarily be consistent and would need to be inter-calibrated [19; 20]. Only after suitable inter-calibration, they can provide physically consistent datasets to estimate a high temporal TB diurnal cycle. These inter-calibrated TBs can also be utilized for the retrieval of several geophysical parameters such as precipitation, soil moisture, land surface emissivity, total precipitable water, and cloud liquid water at the global scale [21]. There are very few studies that estimate the diurnal cycle of PMW TBs e.g., [6; 7; 22]. Two of these few studies, [PERSON] et al. [6; 7], utilized TBs from only sun-synchronous satellites such as the Special Sensor Microwave Imager (SSM/I) and the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), and did not take non-sun-synchronous satellites into consideration.