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.

中文翻译：

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全球范围内陆地上被动微波亮度温度的昼夜循环

卫星式无源微波辐射计可在较大的光谱范围内提供亮度温度（TB）测量，该光谱范围包括多个频道和通常两个极化：水平和垂直。这些TB被广泛用于检索一些大气和地表变量和参数，例如降水，土壤湿度，水蒸气，空气温度曲线和地表发射率。由于结核菌是用各种仪器在不同的微波频率下以不同的入射角，空间分辨率和辐射特性进行测量的，因此仅将它们与不同的微波传感器直接集成就不一定能提供一致性。但是，如果进行适当的协调，它们可以提供一个完整的数据集来估计昼夜周期。这项研究首先利用三次样条拟合，利用非太阳同步全球降水量测量（GPM）微波成像仪（GMI）观测值构建了陆地结核病的昼夜周期。GMI的获取时间每天都在变化，因此，通过合并几天的测量值，可以获得每个月的昼夜周期的形状（幅度和相位）。当使用其他每天固定获取时间的无源微波传感器（例如，特殊传感器微波成像仪/ Sounder和高级微波扫描辐射计2）对相互校准的TB进行修正和调整时，该昼夜模式将作为参考点全球范围内的每日昼夜周期。由于GMI不覆盖极地地区，因此拟议方法估计了全球范围内陆地结核病的昼夜周期一致。结果表明，所构建的结核病昼夜周期的形状和峰值与陆地表面温度的昼夜周期大致相似。还利用派生的TBs的昼夜循环探索了不同土地覆盖类型的昼间亮度温度范围。通常，对于草原，灌木丛和苔原土地覆盖类型，日平均结核病范围超过15 K，而在整个森林中则小于5K。此外，在不同的土地覆盖类型下，TB昼夜范围的季节性变化在南半球比北半球显示出更加一致的结果。然后，可以使用校准后的TB昼夜周期来一致地估算陆地表面发射率的昼夜周期。而且，