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<h2>Subgrid-scale representation for cloud-resolving models</h2>
<i>Chin-Hoh Moeng</i><p></p>

With computing power rapidly increasing, more climate and weather forecasting
models will become cloud resolving. In other words, cloud-resolving models
(CRMs), which explicitly calculate the flow field associated with cloud systems,
will become a useful tool not only for research but also for meteorological
forecasts in the near future.  However, within CRMs, individual cloud elements
and turbulent motions remain subgrid-scale (SGS).  These SGS motions are clearly
important in transporting heat, moisture and other scalars so their effects
should be properly represented in CRMs.<p></p>

To investigate the representation of SGS effects in CRMs, CMMAP scientists
performed a very large domain large-eddy simulation (LES), arguably the
finest-grid simulation of a tropical deep convection system ever performed.
This simulation (also called Giga-LES because it consists of ~ 10^9 grid
points) uses a very fine grid cell to resolve a broad range of scales, including
mesoscale orgnization, gravity waves, individual clouds, and energy-containing
turbulent motions.  This simuluation is described by Khairoutdinov et al. (2009)
and also in one of the
<a href="/research/highlights.html">2008 CMMAP Highlights</a> entitled,
"High-Detail Simulation of Tropical Convection".  CMMAP scientists have used it
as a benchmark to study SGS effects in CRMs (Moeng et al. 2009; Bogenschutz et
al.  2010; Moeng et al. 2010).<p></p>

The Giga-LES shows that the convective motions are continuous across all scales
and the intensity of vertical-velocity fluctuations peaks at scales of several
kilometers, which happen to be on the range of a typical CRM grid spacing. It
also shows that a significant portion of heat and moisture transport is carried
by motions smaller than several kilometers, confirming the importance of SGS
transport in CRMs.  Moeng et al (2010) separate the benchmark flow field into
two components: large-scale (which is regarded as CRM resolvable scales) and
small-scale (which is regarded as CRM SGS) components. The scale separation
allows them to investigate the local relationship between the SGS fluxes and the
CRM resolvable field, which leads to the development of a mixed scheme of SGS
fluxes for CRMs.  They show that the mixed scheme yields a much accurate
representation (Fig. 1) of SGS fluxes of heat, moisture and momentum when
compared with an eddy viscosity/diffusivity K-scheme that is commonly used in
today's CRMs.<p></p>

<center><img src="/images/research/moeng1.jpg" border=0></center><br />

Fig. 1: Comparisions of the horizontal distributions of the SGS moisture fluxes
estimated by the conventional K scheme (left panel), estimated by the new mixed
scheme (right panel), with those retrieved from the Giga-LES shown at center
panel.<p></p>


<b>References</b><ul>
<li> Khairoutdinov, M.F., S.K. Krueger, C.-H. Moeng,  P.A. Bogenschutz, and D.A. Randall, 2009: Large-eddy simulation of maritime deep tropical convection. <i>J. Adv. Model. Earth Syst.</i>, <b>1</b>, 15. </li>
<li> Moeng, C.-H., M.A. LeMone, M.F. Khairoutdinov, S.K. Krueger, P.A. Bogenschutz, and D.A. Randall, 2009: The tropical marine boundary layer under a deep convection system: A large-eddy simulation study. <i>J. Adv. Model. Earth Syst.</i>, <b>1</b>, 16.</li>
<li> Bogenschutz, P. A., S. K. Krueger and M. Khairoutdinov, 2010: Assumed Probability Density Functions for Shallow and Deep Convection, <i>J. Adv. Model. Earth Syst.</i>, In Press</li>
<li> Moeng, C.-H., P.P. Sullivan, M.F. Khairoutdinov, and D.A. Randall, 2010: A
mixed scheme for subgrid-scale fluxes in cloud-resolving models. To appear in
JAS. </li></ul>


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