# Diurnal Cycle of Passive Microwave Brightness Temperatures over Land at a Global Scale
## 5 Summary and Conclusions  
In this paper, a procedure for retrieving and estimating high temporal resolution TB diurnal cycles using multi-satellite PMW observations was introduced. A one-year data record of intercalibrated land TBs from the GMI, AMSR2, and three SSMIS sensors from nearly common frequencies with both horizontal and vertical polarizations was utilized for 2016 in this study. A spline fit was primarily applied to monthly GMI data between 68\({}^{\circ}\)N and 68\({}^{\circ}\)S latitudes and then the estimated half-hourly TB diurnal cycle was enhanced by AMSR2 and SSMIS daily values in order to obtain a more precise diurnal cycle. For the areas above 68\({}^{\circ}\)N and below 68\({}^{\circ}\)S, a novel method was introduced to smooth the SSMIS and AMSR2 data based on GMI normalized diurnal cycle on each of the ten land cover types. This is the first time that we are able to construct a TB diurnal cycle with the existence of time varying GMI TB observations. Evaluation of the constructed diurnal cycle of TBs was performed by qualitative comparison at different frequencies and polarizations for distinct land cover types. It was found that the present method showed promising results in capturing the diurnal variations of PMW TBs for different land cover types. Furthermore, the interpolated half-hourly TB data were used to calculate the DTR parameters. An almost consistent variability was found among the different land cover types. A larger DTR of more than 15 K was observed over the grassland, shrubland, and tundra, whereas a smaller DTR of less than 5 K was evident over the forests. However, seasonal variations in DTR for different land cover types showed a more consistent result over the Southern Hemisphere than over the Northern Hemisphere. Analysis of global DTR was also performed to find correlation among DTR, latitude, season, land cover, and PMW frequency channels. The half-hourly PMW TBs derived in this study could be further used to retrieve high temporal resolution land surface emissivity diurnal cycle in order to characterize surface FT conditions. This would enhance temporal detection of FT which is more helpful during the transition times when multiple FT changes may occur within a day. As accurate detection of FT changes, especially in the high-latitude regions, is crucial for several applications such as for the terrestrial water cycle, net primary productivity, methane cycle, and surface energy budget, the proposed method would enable the detection of finer temporal resolution FT states. Nevertheless, there are uncertainties in the present interpolation method due to factors such as footprint, frequency, and incidence angle differences among the sensors. In addition, there are differences in local solar time for the same pixel on different revisit days of the radiometers due to their intrinsic scanning characteristics. These characteristics produce discrepancies to some extent that need further investigation. This study can  
Figure 10: Global DTR (K) map (**a**) from initial GMI TB values, and (**b**) expanded to the polar region from GMI final interpolated TB values for June 2016 at 23 GHz vertical polarization. further benefit from comparisons with a global network of ground-based observations to obtain a more comprehensive validation. Conceptualization, Z.S. and H.N. ; formal analysis, Z.S. and H.N. ; writing--original draft preparation, Z.S., and S.P. ; writing--review and editing, S.P., H.N., R.B. and R.K.; supervision, H.N. and R.K. All authors have read and agreed to the published version of the manuscript. This research was supported by the National Oceanic and Atmospheric Administration--Cooperative Science Center for Earth System Sciences and Remote Sensing Technologies (NOAA-CESSRST) under the Cooperative Agreement Grant NA16 SEC4810008, by the NASA under Grant NNH15 ZDA001N, and by the Center for Remote Sensing and Earth System Sciences at the New York City College of Technology.