Flux heterogeneity and boundary layer circulations

In work package 1 it will be quantified to what extend Monin-Obukhov similarity-theory can be applied over heterogeneous terrain. For this, high-resolution large-eddy simulations (LES) with grid spacing of 10m down to 1m will be used. This allows to explicitly resolve the surface layer instead of parameterizing it. Unlike previous studies, a land-surface model (LSM) will be employed together with the LES. In an extensive preceding study, feedback-processes in LES-LSM studies have been analyzed. Furthermore, it will be accounted for flux heterogeneities induced by cloud shadows.

The influence of the soil-vegetation continuum on land-surface-atmosphere feedback

Work package 2 addresses the consistent simulation of water and energy fluxes at the land surface focusing on agricultural landscapes and their interaction with the evolution and the thermodynamic structure of the atmospheric boundary layer (ABL). About 50% of the land surface in Germany and Europe is determined by agricultural activities, which highlights the importance of its appropriate representation in land surface models. The performance of the land surface model of ICON-LEM, ICON-NWP (ICON in numerical weather prediction mode), and TERRA will be evaluated and improvements with respect to the simulation of soil moisture and surface flux partitioning will be suggested.

The accuracy of the Monin-Obukhov similarity-theory (MOST) for the simulation of land-surface fluxes will be studied. The corresponding stability coefficients used in TERRA will be extracted and compared with recent improved stability functions that enhance the applicability of MOST to the very stable nocturnal ABL and the strongly unstable ABL. The performance of TERRA will be studied by extensive comparisons of stability parameters and soil moisture extracted from the ICON-LEM runs with surface and soil measurements from HOPE, TR 32 , TERENO (all around JOYCE) and FOR 1695 (Kraichgrau and Swabian Jura).

In this work package, the coupling between surface fluxes due to heterogeneities and their extensions into the ABL are studied. The role of LSA (land surface atmosphere) feedback over heterogeneous landscapes with respect to the simulation of clouds and precipitation will be investigated. The performance of turbulence parameterization are also evaluated. Figure 1 shows the time series of a water vapor differential absorption lidar (DIAL) measurements (10 s, 67.5 m) during the HOPE campaign on 20 April 2013 (11:30 – 13:30 UTC).

The turbulent higher order moments of moisture from HOPE on 20 April 2013 were compared with ICON-LEM output at a horizontal resolution of 156 m. A close agreement was found in the variance profiles, but the third-order moments still show significant differences especially in the entrainment zone. This needs to be well accounted for in order to achieve proper representation of clouds in the model output. Studies in this work  package are performed in strong cooperation with work package 1 and with studies on ABL clouds in project S2. Moreover, findings also feed into the upscaling methodology developed in work package 6.

Catchment-scale circulations

As we know, topography and related land-use patterns will result in catchment-scale circulation patterns, for example, valley winds, cold pools, fog and so on. These circulations are not only impacted by the aforementioned effects but also by soil moisture variations which are caused by groundwater flow and convergence along river corridors. In work package 3, we will try to answer how catchment-scale heterogeneity and induced circulations impact boundary-layer, cloud, and precipitation development.

In order to understand how catchment-scale soil moisture variation impacts boundary-layer development, we conducted serial simulations with heterogeneous “river-like” soil moisture patterns (soil moisture is high in the domain center and low in the upper and lower edges). Results show that a smaller domain seems to have a higher boundary-layer height when compared with larger domains, even though the land surface available energy, sensible heat, and latent heat flux are identical among different domains.

Impact of ABL Heterogeneity/ Troposphere Coupling on Convection

A key role in the process chain from land-surface variations, Atmospheric Boundary Layer (ABL) heterogeneity and convection is played by ABL-troposphere coupling - a process that occurs on different scales. In our WP we will provide a better understanding of this coupling, which is important for a better estimation of the occurrence and strength of convection and its influence on heavy precipitation events. This includes the quantification of ABL tropospheric coupling and the connection between coupling and regions of convection initiation and intensity.

Lightning is a proxy of convection and coupling (Fig.1). Based on the land surface features and the hotspots of convection we aim at studying three areas: A1 (flat terrain), A2 (isolated orography) and A3 (complex orography). Suitable days for simulation have been selected using the criteria of low wind speed and considerable number of flashes over the respective areas.

Surface fluxes are one quantification measure of the surface heterogeneity (Fig.2). The possible cause of the fluxes pattern can be seen from the standardized multiple regression coefficients between surface sensible Heat flux (H) and the following parameters Leaf Area Index (LAI); orography; Soil Moisture (SM); net Shortwave radiation (SW).

Effect of the land surface on regional climate and its role in extreme event evolution

An interesting way to study the impact of the land surface on convection in models are land cover change experiments. They allow to investigate the sensitivity of the simulated boundary layer and the development of convective events to the external land surface parameters. In order to see a clear effect, we choose in work package 5  a special kind of “extreme case” land use change experiment, which nonetheless has some realistic background: An afforestation of the domain, which means that we cover the entire Germany domain with mixed forest which would have been the natural forest cover in Central Europe in the absence of human influence.

We simulate diurnal cycles of selected days with considerable convective rainfall with ICON-LEM, both with the land surface description for today’s conditions, as well as with potential afforestation. Results so far show an increase in latent heat flux, a decrease in sensible heat flux, and intensification of convective rainfall under afforestation conditions.

Detecting and Parameterizing Effects of Land Heterogeneity

Land heterogeneity influences cloud and precipitation initiation and evolution, on the regional and on the climate scale. This is due to its strong local influence on partitioning of fluxes of carbon, water and energy. While computation challenges still exist to explicitly represent this heterogeneity in regional and climate models with finer grid resolution, its effects on boundary layer evolution needs to be better understood and parameterized for coarser grid resolutions currently being used. This heterogeneity is ubiquitous even when grids are moved to resolutions less than 1 km, such these parameterizations will always be essential.

Work package 6 investigates the effects of the non-resolved land surface heterogeneity in the ICON-NWP and ICON-GCM simulations over multiple observation sites. We hypothesize the existence of a dynamic linkage between the turbulent mixing length scales in the PBL schemes and the land surface heterogeneity, which could implicitly improve the land surface heterogeneity effects on the larger NWP/GCM scales. Using the ICON modeling platform over multiple observation sites, a benchmark database will be generated and used to explore and develop the existence of such linkages.

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