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91
Examining the relative roles convective parameterization precipitation and
cloud material feedback
Christopher J. Anderson
3010 Agronomy Hall, Iowa State University, Ames, 50011-1010, cjames@iastate.edu
1. Introduction
Mesoscale convective systems (MCSs) produce unique
regional climate signals in the central United States that
include nocturnal precipitation maximum, eastward
nocturnal propagation of precipitation, and mixing line
(Anderson and Arritt 2007, Markowski and Stensrud 1998)
but are notoriously difficult to simulate with grid spacing
>1km (Bryan et al. 2003, Correia et al. 2008). The
challenge arises in simulating an upscale transition of
dynamical forcing. MCSs begin as independent convective
storms that self-organize into larger dynamical scale
circulation by means of low-level outflow, gravity waves,
and mid-level virtual warming (Bryan et al. 2003, Cotton et
al. 1989, Pandya et al. 2000). The scale and configuration of
latent heat release and evaporative cooling are very
important in the evolution of these processes and are
represented within a regional climate model (RCM) by the
convective and moist physics parameterizations. In this
paper, the role of precipitation and cloud material generated
by convective parameterization is examined in simulations
of a two-month period in which MCSs are prevalent
(Anderson and Arritt 1998).
2. Experimental Design
A suite of RCM experiments were used to systematically
study sensitivity of climate characteristics of MCSs to the
partitioning of water vapor flux within the convective
parameterization into precipitation and cloud material. The
simulation period was 1993 June 1 – July 31. More than 35
large MCSs occurred during this period, which equates to
about one occurrence per two days (Anderson and Arritt
1998).
The simulations were performed with the Weather Research
and Forecast (WRF; Skamarock et al. 2001) model, using
the Kain-Fritsch (KF) convective parameterization (Kain
2004). Initial and boundary conditions were provided by the
NCEP-DOE Renalysis-II (R2; Kanamitsu 2002).
The fundamental experiment was designed to compare two
versions of the KF scheme. A control simulation was made
in which the RCM used the default KF scheme and was
compared to a test simulation in which the KF scheme was
altered to produce cloud material at the expense of
precipitation. Comparison of results from the default and
modified KF scheme provides insight into the importance of
producing widespread mid-level cloudiness and latent
heating, and low-level evaporative cooling relative to local
heating and cooling profiles from the convective
parameterization. A number of experiments were performed
by varying the moisture physics parameterization, trigger
function in the KF scheme, and model grid point spacing.
3. Results
The diabatic theta change, KF scheme theta change, and KF
scheme water species change, where change refers to the
one time-step update to the RCM theta and water species,
were accumulated over all time steps and over a 10 O x10 O
region centered on the observed maximum of two-month
precipitation. The accumulated values are converted to
daily average values and shown in profile in Figure 1.
(a)
(b)
Figure 1. Sixty-one day average of daily diabatic
theta change, KF theta change, and KF water species
change for (a) default KF scheme and (b) altered KF
scheme. ‘figure’.
Large differences are evident in the average daily change
of diabatic theta, KF theta, and KF water species. In the
context of MCSs, the notable differences are a large
increase in positive diabatic change 3000-7500 m AGL
and larger magnitude of negative diabatic change below
7500 m AGL. The positive KF theta change is nearly
identical in both simulations; whereas, the negative KF
theta change is slightly larger in magnitude below 3000 m
and much, much smaller in magnitude near the surface.
The increase in mid-level positive diabatic theta change
occurs in the transition zone from large values of KF cloud
water and KF cloud ice change. This result shows that the
microphysics parameterization is engaged by altering the
KF scheme to produce cloud material at the expense of
precipitation. In particular, the diabatic heating appears to
occur within a vertical level in which cloud water would
freeze to form cloud ice. Likewise, the increase in
magnitude of negative diabatic theta change at mid- and
low-level altitude is evidence of the activities of the
microphyiscs parameterization. In this case, melting of
frozen water and evaporation of liquid water create
cooling.