Highresolution simulation

The results of the control simulation are demonstrated as a reference for the sensitivity experiments.

Figure 2 shows the diurnal variation of the atmosphere in terms of stability and turbulence intensity. Virtual potential temperature and turbulent kinetic energy (TKE) are horizontally averaged over a computational area. The initial atmosphere has a strongly stable surface layer capped with a neutral layer (which is a residual of the previous daytime PBL); this initial stable layer is eroded by surface heating and disappears before 0900 LT. The turbulent intensity gradually increases from the surface, which leads to the formation of convective mixed layer. At 1000 LT, the top of the mixed layer reaches a height above 4 km and then deep convective clouds develop above the mixed layer. In the afternoon, the PBL convection remains to be strong, and deep convective clouds sporadically develop.

The diurnal variation of dust transport associated with the shallow and the deep convection is indicated in Fig. 3. Surface emission starts at 1000 LT when deep convective activity is intensified. The emitted dust is immediately raised within the PBL; the development of the dust layer agrees well with that of the layer with high TKE, which suggests that at

Fig. 2. Time—height section of horizontally averaged (a) virtual potential temperature (contoured at 1-K interval) and (b) turbulent kinetic energy (contoured at 0.05-m2 s~2 interval). A cloud region defined as having cloud water plus ice mixing ratio of 0.025— 0.05 g kg-1 (lightly shaded) and greater than 0.05 gkg-1 (darkly shaded) is indicated in (a).

06 09 12 15 18

06 09 12 15 18

Fig. 3. Time—height section of area-averaged dust concentration (contoured at 0.001, 0.01, 0.05, and 0.1 mgm-3) and vertical flux of dust (shaded, in mgm-2 s~x). Cloud regions are depicted by dotted lines as a contour of cloud mixing ratio being 0.025 g kg-1.

this stage the turbulent motion induces the transport of dust within the PBL depth. After 1100 LT the dust is further transported above the PBL owing to deep convective activity. In the afternoon, intense vertical flux of dust continues in the PBL, due to active turbulence associated with the PBL convection, and the dust content gradually increases. The transport into the free troposphere is also seen. Although the area-mean dust content is at most 0.2 mgm-3, the maximum dust content during the simulation period amounts to 30.1 gm-3 at 1325 LT. This large difference between the mean and the maximum values suggests that a short-period variability due to turbulent and convective motion is critical in determining the peak values of atmospheric tracers.

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