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18
x=100m
x=4km
x=4km w/SUA
16
14
12
10
8
6
4
2
0
6
9
12
15
18
Time (h)
Fig. 7. Temporal variation of the maximum updraft in the domain for the cases of
100 m grid, 4-km grid, and 4-km grid with subgrid-scale updraft acceleration.
The temporal variation of the maximum upward velocity in the
computational domain is compared in Fig. 7. Although the velocity in the
4-km grid run with SUA is weaker than that in the 100-m grid case, adding
the forcing term overall improves the velocity variation as compared with
the 4-km run without the parameterization. Furthermore, the distribution
of the frequency of surface wind speed (not shown) indicated that the
feature seen in the 100-m grid case was better captured in the case of
4-km grid with SUA than without the parameterization.
With the boundary-layer development and associated surface wind
variation being better represented with the SUA parameterization, the
convective dust transport therefore is better reproduced as shown in Fig. 8
(compare with Fig. 4(e)). The diurnal evolution of vertical dust transport
with SUA compares well with the control case as shown in Fig. 3: the initial
development and gradual increase of dust content is better represented with
the parameterization than without it. The column dust content at the end
of the simulation period is 0.72 g m 2 ; this value also compares well with
the estimate by the control simulation.
In this way, the simple formulation of Eq. (1) seems to be a plausible
candidate for parameterizing both shallow and deep convection under
the present meteorological setting and for representing the associated
convective dust transport. However, a proper choice of τ remains to be
determined. We consider that τ should be determined from a characteristic
timescale for convective motion with a scale of
(1 km). A typical
velocity for this motion is on the order of 1 m s 1 , and thus the equivalent
timescale becomes
O
1000 s. Therefore, the present choice of τ =10min
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