Events

 
Colloquium

Implementing a stochastic shallow convectionscheme ICON - benefits and challenges

Tuesday, 18 May 2021, 15:15-16:15
online

The Icosahedral Model (ICON) of the German Weather Service (Deutscher Wetterdienst, DWD) is used for numerical weather prediction at global and regional scales. In the limited area mode, resolution is typically on the order of a few kilometers horizontal grid spacing. Deep convective transport is partially resolved at these scales, but shallow convection remains poorly represented without a parameterization.

A stochastic shallow convection scheme was developed in collaboration with the Max Planck Institute for Meteorology, and is now being implemented in ICON with a view towards operational use. The scheme is scale-adaptive and renders resolution-dependent tuning of the convection parameterization unnecessary. Mass flux limiters essential for the stable operation of the unaltered convection scheme can be removed when the stochastic perturbations are introduced.

Alongside the original, explicit stochastic scheme an approximation using stochastic differential equations (SDE) has been developed. The advantage of the SDE version is a lower computational and memory cost, and the ability to save and restart the model‘s stochastic cloud state easily.

Equivalence of the two versions can be demonstrated by running one version interactively, the other passively (“piggy-backing”). While the SDE approximation is computationally more efficient, the explicit version of the scheme can be easily extended to keep track of additional properties of the shallow cloud ensemble. For example, the convective updraft core fraction can be calculated for use in the diagnostic subgrid cloud scheme. Or knowledge of individual clouds’ depth can be used to derive a more realistic lateral detrainment profile than is currently in use.

However, challenges remain to reach competitive forecast performance with the scheme. At its core, the stochastic convection parameterization relies on a traditional mass flux parameterization, and use in the stochastic context reveals some weaknesses of this parameterization.

This event is part of the eventgroup Meteorology Colloquium Karlsruhe
Speaker
Dr. Maike Ahlgrimm

Deutscher Wetterdienst, Offenbach, Deutschland
Organizer
IMK-TRO
Institute of Meteorology and Climate Research
KIT
Wolfgang-Gaede-Str. 1
76131 Karlsruhe
Tel: 0721 608 43356
Mail: imk-tro does-not-exist.kit edu
https://www.imk-tro.kit.edu
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