B3 - Relative impact of surface and aerosol heterogeneities on the initiation of deep convection

Principal investigators: Dr. Christian Barthlott, Prof. Dr. Corinna Hoose, Dr. Christian Keil

Other researcher: Florian Baur (PhD)

Former researcher: Linda Schneider (PhD)

Despite the improvements in the parameterizations of physical processes and higher grid spacings of numerical weather prediction (NWP) models over the past decades, quantitative precipitation forecasting still remains a challenge for state-of-the-art numerical models. In particular, the forecast of deep moist convection in situations with weak synoptic forcing over both flat and orographically structured terrain is still inadequate for many applications. With prevailing weak synoptic-scale forcing, land-atmosphere interactions are assumed to be decisive for cloud formation and subsequent convective precipitation. Surface heterogeneities ranging from large scales (100 km) to kilometric scales can cause atmospheric circulations which themselves modify the boundary layer structure and cloud formation. For example, soil moisture influences heat and moisture fluxes into the planetary boundary layer and uncertainties in the initial state can introduce large forecast errors. Furthermore, the feedback between soil moisture and precipitation is still unclear, especially over complex terrain. Complex orography influences cloud formation and structure through lifting, flow deviation and blocking, or the generation of slope and valley winds caused by the heating of elevated terrain. Finally the aerosol background can vary on scales of 10s-100s of kilometers due to local sources and long-range transport. The heterogeneous land surface and the aerosol background can therefore be a source of uncertainty for climate and weather prediction models.

In this project, we investigate the relative contribution of orographic features, land surface heterogeneities and heterogeneities in the aerosol field on cloud formation, cloud features, and subsequent precipitation. To achieve this, numerical simulations with the Consortium for Small-scale Modeling (COSMO) model at a convection-permitting horizontal grid spacing will be performed for several cases with different synoptic conditions. By using an advanced two-moment microphysical parameterization, aerosol effects on clouds and precipitation are modeled by taking into account aerosol assumptions for cloud condensation nuclei (CCN). This way, we will identify which inhomogeneity has the strongest influence on convective processes over both flat and complex terrain and to what extent the synoptic forcing modifies the sensitivity of clouds and precipitation. The area of investigation will be Germany where processes over both flat and orographically structured terrain can be studied.

All parts of the project will use the same numerical model but for different target areas and with different horizontal grid spacing (LMU: operational COSMO-DE grid spacing; KIT: 500m) to answer the important question of whether the sensitivities to the above mentioned parameters also depend on the model resolution. Furthermore, probability density functions of cloud-related parameters gained from the simulations with different horizontal resolution will be used to quantify the subgrid-scale variability.

The overall goals of this project are:

• to determine the link between land-surface and aerosol heterogeneities and the resulting cloud size distribution to better understand the mechanisms for convection initiation, cloud formation and subsequent precipitation over both flat and complex terrain, and
• to identify which process is important at a specific meteorological situation and why.