Evaluating the Impact of Surface Flux Homogenization in the Coupling of a High Resolution Surface to a Coarse Resolution Atmosphere

Tackling the issue of how to appropriately represent heterogeneity and scale in Earth System Models (ESMs), particularly within the realm of land-atmosphere interactions, has been a prominent goal in the community since ESMs were first conceived. Many ESMs approach this issue by using tiling schemes to produce an effectively higher resolution land; each tile produces its own surface fluxes which are then averaged to one value before being passed to the coarser resolution atmosphere. This averaging process has the potential to introduce a number of errors into the system, especially in locations where there is a strong gradient in surface or atmospheric characteristics. Spatial heterogeneities can cause circulations, roll-like structures, lake breezes, sea-breeze analogues and general increase in the kinetic energy of the boundary layer. These mesoscale and sub-mesoscale motions can then lead to important broader atmospheric impact including changes to initiation of deep convection, which are neglected when fluxes are homogenized. This presentation will explore how surface heterogeneity causes changes in smaller mesoscale motion (~1 km – 60 km) and convection in the modeling context where these phenomena are often neglected.

In this work, we leverage a series of continental WRF simulations to examine the impact that the scale of surface heterogeneity, or rather scaling of this averaging applied often in tiling schemes, has on atmospheric motion and additional, related impacts to clouds, rainfall and convection. Simulations consist of daily WRF runs at 3km resolution over the Continental United States (CONUS) for three summers (JJA, 2021-23). In addition, “homogenized” runs, where the surface fluxes from a 3km land model are homogenized to 60km prior to passing them to the 3km resolution atmosphere, to simulate flux averaging applied in ESMs, are completed over the same period. This allows us to examine the effect of surface homogenization on the atmosphere, independent of the resolution of either the underlying land model or the scale of atmospheric dynamics resolved.

Results show significant changes in mesoscale motion, convection and precipitation as a result of flux homogenization. In some regions of North America, modeled precipitation nearly doubled as a result of flux averaging. A significant decrease in mesoscale motion is also reported, which in particular appears to affect convective initiation in energy limited regions. Observed impacts were most significant in regions with high moisture heterogeneity, such as coastlines in arid regions. Results overall illustrate the importance of developing more sophisticated methods than simple averaging for communicating surface fluxes to the atmosphere, particularly in modeling frameworks where ocean and land are averaged together (i.e. models where coastline resolution is finer than the atmospheric resolution).