Geoscience Reference
In-Depth Information
performed a 4-km simulation that included either the Kuo or the
Kain-Fritsch cumulus parameterization built in the ARPS model,
15
and
found that the boundary-layer development was not improved. This
result is reasonable, since those parameterizations are not suitable for
parameterizing both shallow and deep convections. Another possibility is
to improve the TKE prediction by enhancing the production terms in
the TKE closure equation; however, this requires a detailed analysis of
a turbulence simulation data set. In the following, we examine a simple
representation for facilitating the development of both boundary-layer and
cumulus convection and hence enhancing dust production.
3.3
. Subgrid-scale updraft acceleration
The TKE diagnosis in the previous subsection indicated that the diurnal
evolution of turbulence intensity is slower in the 4-km simulation than in
the finer-grid cases. In order to enhance the boundary-layer development
in the 4-km case, therefore, the subgrid-scale turbulent effects need to
be intensified, which will contribute to the acceleration of convective
updrafts and downdrafts. Since turbulence intensity is increased especially
in the updraft regions, updraft enhancement is considered to be critical in
reproducing reasonable development of the PBL.
In order to develop a representation of convection effects in cloud-
resolving simulations, we apply the idea of Deng
et al.
23
who parameterize
shallow convection in mesoscale models. They hypothesized that cloud-
forming rising parcels were positively correlated with vertical velocity
perturbations and proposed that an eddy vertical velocity should be added
to resolved vertical motion that is related to activating cloud formation
in a cloud microphysics parameterization; note that this enhanced updraft
speed is not used anywhere except in this cloud activation computation.
Such an adjustment to updraft velocity was also considered by Lohmann
et al.
24
in activating cloud droplet nucleation. A point stressed here is
that incorporating the turbulent effects into updraft speeds is critical in
obtaining better results in simulations with ∆
x
of a couple of kilometers.
Motivated by these studies, we assume that updraft is enhanced if
there is a significant amount of SGS turbulent intensity. This assumption
for updraft enhancement is based on the fact that updrafts are generally
stronger than downdrafts in a single cell of shallow and deep convective
clouds in which precipitation is not strong enough to produce cold air and
intense downdraft. The enhancement of updrafts is conducted by adding
