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A5 - The role of soil moisture and surface- and subsurface water flows on predictability of convection

Principal investigators: Prof. Dr. Harald Kunstmann, Dr. Kirstin Kober

Other researcher: Dr. Joel Arnault (Postdoc)

Moist convection is an atmospheric process whose initiation depends both on the synoptic-scale weather situation and local forcing. In weather situations characterized by weak synoptic-scale forcing, local characteristics (e.g. land surface variability) are more likely to be significant in the initiation of convective precipitation. Current state of the art numerical weather prediction (NWP) models still have a limited representation of terrestrial hydrological processes, particularly with respect to soil moisture and lateral terrestrial water flows. In the same time these NWP models are known for their limited forecast quality during weak synoptic-scale forcing conditions, which could be related to a larger contribution of unresolved land-atmosphere coupling processes in such weather situations.

In this project we will investigate which improvements in convective precipitation predictability can be achieved by a more sophisticated treatment of terrestrial hydrological processes in NWP models.

To reach this objective we will simulate a panel of joint case-studies in Germany and West Africa using the Weather Research and Forecasting (WRF) model, a hydrologically enhanced version of WRF, namely WRF-Hydro, and the Consortium of Small-scale Modeling (COSMO) model. We will then develop methods to quantify the physical processes at stake in soil moisture – precipitation feedback mechanisms, especially for the cases where more complex descriptions of surface and subsurface lateral water flows improve precipitation predictability.

A further focus will be set on the description of uncertainties by adopting and applying a stochastic boundary layer parameterization. This parameterization scheme aims to represent subgrid-scale variability caused by specific processes important for convective initiation. Such a parameterization is developed by project A6 for the COSMO model and will be transferred and implemented in the WRF/WRF-Hydro modeling system. The ability of this approach to account for the land-atmosphere exchange variability originating from surface and subsurface lateral water flows will be assessed.

Ensembles of soil moisture fields produced by WRF-Hydro will be shared with project B3, in order to investigate the sensitivity of cloud microphysical and boundary layer processes to a physically-enhanced description of soil moisture initial and boundary condition in the COSMO model. We will finally assess the role of lateral water flows at the surface and subsurface for improved soil moisture initialization in weather forecasting.