Integrating Subsurface Dynamics into PALM-4U Urban Simulations: Sensitivity Response to Perturbations and Implemented Modules

Abstract. In this study we investigated the short-term atmospheric response of the large eddy simulation urban microscale model PALM-4U to disturbances of model parameters in Berlin. By forcing the model's dynamics with output from the WRF model, we tested the effects of simulating simple cloud physics to atmospheric dynamics in an environment with a dominant subsurface urban heat island. To do so we compare a clear sky to an overcast day when (i) raising only deep soil layer (2.91 m) temperature by 5 K and (ii) when changing the entire soil temperature profile to a more temporally advanced (i.e., more balanced state).

In accordance with our expectations we found that higher soil temperatures in all simulations contributed to higher sensible heat fluxes, more so during clear sky conditions. Ground heat fluxes on the other hand decreased with higher soil temperatures.

The more balanced soil temperature profile produced higher radiative cooling under clear sky, which counter intuitively led to lower sensible heat fluxes and surface temperatures, despite warmer soil. With clouds, in contrast, a more balanced soil temperature yielded higher surface heat fluxes, as expected. The largest simulated surface temperature differences exceeded ±3 K, with sensible heat flux differences of up to ±40 W/m², predominantly in green spaces. Our findings emphasize that the coupling between soil and atmosphere is not only highly variable in space and time, but both, soil temperature profiles and differences in radiative energy balance, substantially contribute to the urban surface energy balance and fluxes. Due to the different inertial scales in soil- and atmospheric energy fluxes, and the generally low availability of spatial soil temperature field data, the accurate study of Soil-Atmosphere-Interactions in urban environments remains challenging.