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The global distribution of DCS and seasonal trends in the number of DCS (NDCS) are examined in this section. First, the Southern and Northern Hemispheres (SH and NH, respectively) were each divided into zones: low latitude (LL: 0#cod#x000b0;-30#cod#x000b0;), midlatitude (ML: 30#cod#x000b0;-60#cod#x000b0;) and high latitude (HL: 60#cod#x000b0;-90#cod#x000b0;). Seasons were defined as follows:
(1)December, January and February for winter in the NH and summer in the SH;
(2) March, April and May for spring in the NH and autumn in the SH;
(3) June, July and August for summer in the NH and winter in the SH;
(4) September, October and November for autumn in the NH and spring in the SH.
Figure 2. Total NDCS per month in different zonal bands for (a) the NH, (b) the SH, (c) the NH over land, (d) the SH over land, (e) the NH over the ocean, and (f) the SH over the ocean. The dotted line represents the NDCS over all zonal bands; the dashed line is for 0#cod#x000b0;-30#cod#x000b0;; the dash-dot line is for 30#cod#x000b0;-60#cod#x000b0;; and the solid line is for 60#cod#x000b0;-90#cod#x000b0;.
The mean latitude and longitude of all the DCC in each DCS was used to represent the location of the DCS; they were calculated over 5#cod#x000b0;#cod#215; 5#cod#x000b0; grid boxes. The NDCS was calculated as the sum of the DCS in each grid box per season over the four-year period. Figure 2 shows the monthly time series of NDCS in different zonal bands. In the NH, the NDCS changes significantly with season in all zonal bands; the maximum NDCS occurs in summer and the minimum in winter (Fig. 2a). In the LL zone of the SH, the maximum NDCS also occurs in summer and the minimum in winter; in the HL zone in the SH, the reverse is seen (Fig. 2b). The NDCS remains generally the same year-round in the ML zone of the SH. Globally, for any single month over the four-year period, the maximum NDCS (4221) occurred in August 2007 and the minimum NDCS (561) occurred in April 2010.
Total number of DCS in different zonal regions for different seasons over the period 2007-10. #cod#160; LL/NH ML/NH HL/NH Total/NH LL/SH ML/SH HL/SH Total/SH Spring NH/Autumn SH 7971 6063 1107 15 141 9414 4454 1917 15 785 Summer NH/Winter SH 13 006 9367 2968 25 341 4222 4076 5705 14 003 Autumn NH/Spring SH 11 524 5954 2543 20 021 6759 4145 3709 14 613 Winter NH/Summer SH 4296 3563 1711 9570 9479 3977 801 14 257 Convective instability caused by radiative heating is the main contributing factor toward the generation of DCS in the LL zone of both hemispheres, which explains why the NDCS in this zonal band is large in summer and small in winter. Figures 2c-f demonstrate how different underlying surfaces can lead to large differences in the seasonal variation of the NDCS in the ML and HL zones of both hemispheres. The seasonal variations of the NDCS over land and over the ocean in the LL zone are the same. The much larger NDCS over the ocean than over land is because many tropical cyclones are generated over the ocean in the LL zone. The seasonal variation of the NDCS over land in the NH (Fig. 2c, dash-dot line) is much more significant than over the oceans (Fig. 2e, dash-dot line) because cloud development over land is more complex. Oceans dominate in the ML zone of the SH so cloud formation is a simpler process over this surface and little seasonal variation is seen in the NDCS (Fig. 2f). The largest NDCS in the HL zone occurs in the SH where the sub-polar low is located. DCS here are formed from the convergence of cold air from the polar region and warm air from the subtropical region.
The NDCS for different zonal regions and seasons is summarized in Table 1; the global distribution of NDCS for each season is shown in Fig. 3. The global distribution of NDCS are consistent with previous deep-cloud-related studies (Sassen et al., 2009; Savtchenko, 2009), which indicate that the representation according to CloudSat/CALIPSO's observations is still reasonable despite the limited spatial and temporal resolution. The maximum NDCS (39 343) occurs in the NH summer/SH winter and the minimum NDCS (23 827) occurs in the NH winter/SH summer. The highest density of DCS occurs in central Africa, northern South America, Indonesia and Tibet, with the maximum values of NDCS in a single grid box within each of these regions over the four-year period being 86, 112, 87 and 154, respectively. Note the significant increase in the NDCS over the Antarctic polar region during NH summer/SH winter (Fig. 3b).