Surface Flux Homogenization and its Impacts on Convection Across CONUS

Abstract In large scale Earth System Models (ESMs) used to study climate processes, surface heterogeneity that is sub-grid to the larger atmospheric grid is often represented by a number of land tiles, effectively providing a higher resolution land surface to a coarser resolution overlying atmosphere. ESMs, however, average the surface fluxes and other surface characteristics before they are communicated to the atmosphere, ignoring the affect that this variability can have on the atmosphere. In this study, we examine the impact of this flux averaging through 257 2-day summer WRF simulations over the Continental United States (CONUS) at 3km resolution, including runs where the surface fluxes and temperature are homogenized at 60 km prior to communication to the overlying atmosphere. Results show large increases (up to 200mm +) in precipitation in moisture limited regions of CONUS, a persistent increase in precipitation bias when compared to observations, and a near universal increase in evaporative fraction. Changes are most significant where moist areas (i.e. water bodies) are averaged with dry areas as the feedback between atmospheric moisture concentrations and the land are weakened when that moisture flux is more spatially distributed through homogenization. Results also show a significant decline in mesoscale flow activity within the atmospheric boundary layer, which in energy limited regions may cause the observed decreases in precipitation due to less frequent convective initiation. Overall, results indicate that flux averaging applied in large scale models can have unintended consequences by neglecting the heterogeneous imprint of the surface on the atmosphere.

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