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Fig. 5. The same as Fig. 4, except for turbulent kinetic energy (contoured at
0.5 m
2
s
−
2
).
smoothly deepens, which leads to cumulus development at 1100 LT as seen
in Fig. 4(a). A similar development can still be identified for the 1-km
grid case. Note that the TKE values in Figs. 5(a) and 5(b) are significantly
larger than those in the 100 m run (see Fig. 2(b)); this is due to the enhanced
eddy viscosity, which takes into account the effects of nonlocal mixing in
the sensitivity runs. On the other hand, although an increase of boundary-
layer activity is seen for the 4-km grid case, the increase is much slower; thus,
the cumulus development does not occur before noon and is significantly
retarded. Considering that both boundary-layer and cumulus convection
play a critical role in enhancing dust emission and transport,
6
the slow
boundary-layer development in the coarsest grid case seems to be a reason
for the significant difference in the dust transport from the finer-grid cases.
These results indicate that the grid spacing of 4 km, which is well in the
range of explicit representation of deep convection in mesoscale systems,
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is not sucient to resolve shallow and deep convection under the present
fair weather condition. In particular, the scales of updrafts for shallow
convection are typically smaller than 4 km. The 4 km grid spacing is actually
arguable, because there is no robust, satisfactory solution for convection
and turbulence parameterization in the simulations.
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The 4-km mesh, on
the other hand, is obviously not coarse for regional-scale (let alone global-
scale) simulations of convective dust transport, and hence most studies on
atmospheric transport aim at performing simulations with grid spacings
of 1-10 km; therefore, a proper parameterization for activating convection
that induces dust emission and transport is necessary in this range of grid
spacing.
One possibility for the better representation of the processes is to
include a cumulus parameterization in the 4-km simulation. We further
