Modeling Impacts of Subsurface Thermal Anomalies on Atmospheric Energy Fluxes in Berlin, Germany Using PALM-4U

Anthropogenic encroachments in urban environments lead to altered energy fluxes and elevated temperatures. While there is much attention on surface and canopy heat islands, the heat exchange between subsurface and atmosphere as a bottom-up scheme is largely overlooked the role of subsurface heat accumulation and its upward heat transfer through the surface–atmosphere interface as a component of bottom-up energy dynamics remains largely overlooked. So far, subsurface temperature anomalies are not adequately considered in urban climate studies. Hence, in this study the impacts of elevated subsurface temperatures on atmospheric energy fluxes in Berlin are investigated. The analysis is performed with the large eddy simulation urban climate model PALM-4U. We developed different set-ups with cyclic lateral boundary conditions (LBC) and external dynamic forcing from the mesoscale model WRF. For each setup, we subtract a heat scenario in which the soil temperature in the deepest layer (2.91 m below ground) is increased by 5 K from a reference scenario.
Under cyclic LBC we detect marginal air potential temperature differences averaged over the domain but significant differences in variance and distribution. ... mehrThose differences have a high diurnal variation and are height dependent. In contrast, using WRF as forcing, there are pronounced differences in surface sensible heat flux (maximum positive and maximum negative: 13.1~W/m² at 21:00 local time, -7.0~W/m² at 14:00), ground heat flux (4.1~W/m² at 20:00, -3.6~W/m² at 14:00), and 2~m air potential temperature (0.64~K at 17:00, -0.87~K at 14:00). Hence, additional subsurface heat does not lead to uniform warming, but rather to an energy redistribution, ultimately resulting in an overall increase in total energy. With increasing height, differences due to altered soil temperatures become negligible.