Scale dependent impact of dynamic vegetation heterogeneity on heat and moisture fluxes at the blending height

• Applicant: Dr. Kirsten Warrach-Sagi
• Subject Area: Atmospheric Science
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514721519

Atmospheric models employ an interface between the land surface model and the atmospheric boundary layer (ABL) flux parameterization. Via a surface layer scheme, momentum, heat and moisture fluxes are exchanged between the surface and the lowest atmospheric model layer. In this approach a blending height is introduced where the surface fluxes above a heterogeneous land surface are considered as homogeneous at the grid scale. At this height assumed within the lowest atmospheric model layer, the transition to the ABL parameterization takes place. However, for convection permitting (CP) model simulations (grid scale < 3 km) over heterogeneous vegetation, the lower model layers can be below the blending height leading to errors in the simulated fluxes. A major challenge in atmospheric modelling is to parameterize the interface between heterogeneous dynamic vegetation and ABL in unstable, stable and neutral conditions with advection from different wind directions. Accordingly, our objectives are the identification of the blending height in dependence of vegetation heterogeneity and states and of atmospheric framing conditions and the quantification of the impact of vegetation heterogeneity on the energy fluxes at the blending height. The results will be used to derive representative, scale-dependent fluxes at this level for land-atmosphere (L-A) feedback studies and turbulence parameterizations. WRF-NoahMP-Gecros model simulations from the CP to large eddy simulation scales will be compared with observations at LAFO and MOL-RAO sites to identify the blending height and effective vegetation roughness parameters for CP simulations in dependence of atmospheric framing conditions. The simulations will be embedded in the Multi Model Experiment (MME) via the Cross Cutting Working Group (CCWG)-MME. The impact of heterogeneity on the strength of L-A feedback will be investigated and the understanding of exchange processes between the surface and the atmosphere as well as within the ABL will be advanced. The synergy of these model results and 3D observational data will be used to study the scale dependent impact of dynamic vegetation heterogeneity on energy fluxes at the blending height. This project addresses the LAFI main objectives 2, 3, 4, S and E. It participates in CCWG-MME and CCWG-DL. Simulations are performed in collaboration with the projects P6, P8 and P9. P2 will provide leaf area index and vegetation cover fractions for model initialization. LAFI observations from P1-P5 will be used for model evaluation.

• DFG Programme: Research Units
• Subproject of: FOR 5639: Land-Atmosphere Feedback Initiative (LAFI)
• Co-Investigator: Dr. Hans-Stefan Bauer