# Diurnal Cycle of Passive Microwave Brightness Temperatures over Land at a Global Scale
## 4 Results and Discussion
### Spatial Distributions of Global Land Diurnal Brightness Temperature Range  
The diurnal cycle of TBs enables the computation of magnitudes and time of occurrence of daily maximum and minimum TBs. The difference in daily maximum and minimum TB estimates is termed as diurnal brightness temperature range (DTR). Figure 8 illustrates the global mean monthly DTR derived from the estimated half-hourly TBs for  
Figure 6: Diurnal variations of interpolated TB out of smoothed SSMIS F16, F17, F18, and AMSR2 for sample locations with different land covers for 13 March 2016 at 18.7 GHz horizontal polarization for **(a)** 71.5\({}^{\circ}\)S and 12.9\({}^{\circ}\)E with ice land cover, **(b)** 73.7\({}^{\circ}\)N and 122\({}^{\circ}\)W withundra and mossy bog land cover, **(c)** 71\({}^{\circ}\)N and 114\({}^{\circ}\)E with deciduous woodland land cover, and **(d)** 77\({}^{\circ}\)N and 106.4\({}^{\circ}\)E with non-vegetated desert land cover. Time in each subplot is in local solar hours. two PMW bands of 37 GHz and 89 GHz at vertical polarization and for two contrasting months of February and August of 2016. In general, DTR shows large spatial variation during both months. In the Northern Hemisphere, the largest magnitudes of DTR (>15 K) are evident in Mexico, the Sahara in North Africa, Western Asia, and parts of Siberia in Russia. In the Southern Hemisphere, the Kalahari Desert in the Southern Africa, Madagascar, shrublands of the Andes Mountain chain in South America, and Australian deserts are regions with the most noticeable DTR. Right on the equator, the horn of Africa is another region with high DTR values. To recap, DTR is consistently large over the three land cover types: wooded and non-wooded grassland, boreal and xeromorphic shrubland, and tundra and mossy bog. Since in the  
Figure 8: Spatial distributions of monthly diurnal brightness temperature range (DTR) (K) at **(a,b)** 37 GHz vertical polarization, and **(c,d)** 89 GHz vertical polarization for two contrasting seasons of **(a,c)** February 2016 and **(b,d)** August 2016. Figure 7: Comparison of constructed TB diurnal cycle for different PMW channels in two contrasting seasons at **(a)** 45.9\({}^{\circ}\)N and 103\({}^{\circ}\)W with boreal and xeromorphic shrubland land cover for 31 August 2016; and **(b)** 3.6\({}^{\circ}\)N and 142.7\({}^{\circ}\)W with wooded and non-wooded grassland land cover for 5 February 2016. Time in each subplot is in local hours. fall and spring seasons, the emissivity values of forest canopy, frozen ground, and wet snow were relatively close to each other, the mean TBs show similar variations [33]. Lower DTR (<5 K) can be found in all sorts of forests: tropical rainforests, deciduous forests, and even evergreen broad-leaved and needle-leaved forests. Generally, DTR characteristics exhibit noticeable differences with both latitude and land cover. It is most likely that the vegetation type has a systematic effect on solar reflection and surface emissions [34; 35]. The mean monthly DTR averaged over different land cover types for both hemispheres was shown in Figure 9. These results correspond to two contrasting seasons and four different PMW bands at vertical polarization. In general, the calculated values for the Southern Hemisphere display an orderly relation among DTR, land cover type and season. It is evident that the DTR is higher during July (e.g., Southern Hemispheric winter) than in January (e.g., Southern Hemispheric summer) for all land cover types. In the Northern Hemisphere, each PMW band shows slightly distinct behavior. The only land cover which presents a consistent behavior is the non-vegetated desert where DTR is higher during July in comparison to January. In most of the desert regions, as the frequency increases and the penetration depth becomes smaller, the radiometric measurements display larger diurnal variations. Moreover, in arid regions, the effect of the atmosphere is more considerable at higher frequencies [36; 37; 38].