Subsurface heat accumulation as an unseen driver of urban energy fluxes

Anthropogenic encroachments in urban environments significantly alter energy fluxes and elevate temperatures within cities. Despite extensive research on urban heat islands, limited attention has been paid to the heat exchange between the subsurface and the atmosphere as a bottom-up mechanism. In this work, the impacts of elevated subsurface temperatures, as we find them for example near underground infrastructures, on atmospheric energy fluxes in Berlin are analyzed, using the large eddy simulation urban climate model PALM-4U.

Our findings reveal pronounced differences in sensible heat flux (SHF), ground heat flux (GHF), surface temperature, and potential temperature, while horizontal wind speed shows minimal variation. There is a high intrinsic variability driven by different urban materials and structures. For example, a 5 K increase in soil temperature at 2.91 m depth (lowest soil layer in the model) results in a domain-averaged increase of 0.03 K in 2 m potential temperature over the course of a day. However, at specific locations, maximum differences of 0.86 K are observed (14:00 local time), already after two days of simulation. With increasing height, the influence of elevated soil temperatures diminishes.

These results highlight the importance of understanding soil-atmosphere interactions for urban climate studies. In future, the findings could inform strategies to mitigate urban heat islands - not by adding, but subtracting heat from the soil. One promising approach is shallow geothermal heat recycling, which could sustainably cool urban surfaces and support the development of climate-resilient cities.