Colorado State University
General Circulation Model
A Prognostic Cumulus Parameterization
We tested the sensitivity of the CSU GCM to two different approaches of the
Arakawa-Schubert cumulus parameterization - the quasi-equilibrium (Arakawa and
Schubert, 1974) and the prognostic CKE (Randall and Pan, 1993) approaches. To
bring the global-mean planetary albedo back to the observed, we introduced
"fractional coverage" of the anvil clouds (Pan, 1995). Simulated January
precipitation is compared with the observations by Legates and Willmott (1990).
Simulation using the quasi-equilibrium closure.
Arakawa, A and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble
with the large-scale environment, Part I. J. Atmos. Sci., 31, 674-701.
Simulation using the prognostic CKE. The global-mean is reduced by more than
7% mostly in the tropics associated with a dramatic increase of anvil incidence
in that region. The relaxation of the quasi-equilibrium assumption allows an
"adjustment time" for cumulus convection and hence more persistent cumulus
activity. The large anvil incidence leads to a large planetary albedo and
reveals the problem in representing interactions among the parameterized
physical processes.
Randall, D. A. and D.-M. Pan, 1993: Implementation of the Arakawa-Schubert
cumulus parameterization with a prognostic closure. Monograph on Cumulus
parameterization, 137-144.
To bring the global-mean planetary albedo back to the observed, we
introduced "fractional coverage" of the anvil clouds. This simple anvil
parameterization produces the results in the figure above. Comparing the three
simulations with the observations shown below, we see the improvement in the
precipitation distribution. The maxima over the Amazon and southern Africa
appear to be obvious. The fractional anvil allows more incoming solar radiation
in the tropics. The land surface is more sensitive to the solar radiation than
oceans. The intensified surface heat fluxes activate more vigorous cumulus
convection over land.
Pan, D.-M., 1995: Development and application of a prognostic cumulus
parameterization. Ph. D. dissertation. Colorado State University, Ft. Collins.,
207 pp.
Observed Precipitation.
Legates and Willmott(1990).
Dzong-Ming Pan
pan@halo.ps.uci.edu