Stochastic plume ensembles for an unified shallow-deep mass flux cumulus
parameterization in the Community Earth System Model (CESM)
Abstract
Many biases in global climate models (GCMs) have been associated with
the poor representation of unresolved variability due to organised
convection in the tropics by the underlying cumulus parameterizations
(CP). Many researchers have suggested that the quasi-equilibrium
assumption (QEA) on which these CP’s are based is to blame. This is even
more problematic with the recent and future increases in grid
resolutions as the cloud large ensemble requirement for QEA breaks down.
The stochastic multi-cloud model (SMCM) of Khouider et al. 2010 was
proposed as a cheap alternative of overcoming this QEA dilemma by
emulating the variability of the three cloud types which characterize
tropical convection via a Markov jump birth-death process. The SMCM has
proven to be very successful in terms of the simulation of the main
modes of tropical variability when used as a simple alternative CP in a
GCM. Here, we propose to incorporate the SMCM directly into the
Zhang-McFarlane scheme (ZMS; Zhang and McFarlane 1995) to break the QEA
dead end by using instead a stochastic plume ensemble and generalise the
SMCM framework to cumulus schemes. The new stochastic ZMS (SZMS) uses a
random number of plumes that are launched for each one of the three
cloud types, shallow, congestus and deep, and that detrain at random
levels, according to the SMCM. The new approach somehow combines the
idea of Cohen and Craig (2006) of assuming a Poisson process for the
number of plumes and that of Gentine et al. (2013) of prescribing a
distribution of plume detrainment levels. Here we shall show the results
of our experiment, for the single column version of the Community Earth
System Model. Bibliography: Cohen BG, Craig GC. Fluctuations in an
equilibrium convective ensemble. Part II: Numerical experiments. J.
Atmos. Sci. 2006;63(8) Gentine P, Betts AK, Lintner BR, Findell KL, Van
Heerwaarden CC, D’andrea F. A probabilistic bulk model of coupled mixed
layer and convection. Part II: Shallow convection case. J. Atmos. Sci.
2013;70(6) Khouider B, Biello J, Majda AJ. A stochastic multicloud model
for tropical convection. Comm. Math. Sci. 2010;8(1) Zhang GJ, McFarlane
NA. Sensitivity of climate simulations to the parameterization of
cumulus convection in the Canadian Climate Centre general circulation
model. Atmos-ocean. 1995;33(3)