Soil Moisture Control of Precipitation Reevaporation over a Heterogeneous Land Surface

Abstract

Soil moisture heterogeneity can induce mesoscale circulations due to differential heating between dry and wet surfaces, which can, in turn, trigger precipitation. In this work, we conduct cloud-permitting simulations over a 100 km × 25 km idealized land surface, with the domain split equally between a wet and dry region, each with homogeneous soil moisture. In contrast to previous studies that prescribed initial atmospheric profiles, each simulation is run with fixed soil moisture for 100 days to allow the atmosphere to equilibrate to the given land surface rather than prescribing the initial atmospheric profile. It is then run for one additional day, allowing the soil moisture to freely vary. Soil moisture controls the resulting precipitation over the dry region through three different mechanisms: as the dry domain gets drier, (1) the mesoscale circulation strengthens, increasing water vapor convergence over the dry domain, (2) surface evaporation declines over the dry domain, decreasing water vapor convergence over the dry domain and (3) precipitation efficiency declines due to increased re-evaporation, meaning proportionally less water vapor over the dry domain becomes surface precipitation. We find that the third mechanism dominates when soil moisture is small in the dry domain: drier soils ultimately lead to less precipitation in the dry domain due to its impact on precipitation efficiency. This work highlights an important new mechanism by which soil moisture controls precipitation, through its impact on precipitation re-evaporation and efficiency.

Anomalously Darker Land Surfaces Become Wetter Due To Mesoscale Circulations

“…Full details of the derivation and justifications for relevant approximations are provided in Supporting Information S1 (Figure S4 in Supporting Information S1), with the main steps briefly summarized here. Specifically, we combine the atmospheric and soil moisture budgets (Equations S5 and S11 in Supporting Information S1), a "bucket" model for evaporation (Equation S7 in Supporting Information S1; Manabe, 1969;Seneviratne et al, 2010;Cioni and Hohenegger, 2018;Cheng et al, 2021), a simple mesoscale circulation model (Equation S8 in Supporting Information S1; Cioni and Hohenegger, 2018;Cheng et al, 2021), and a simple seepage model (Equation S9 in Supporting Information S1; Laio et al, 2001); assume anomalies in net radiation are dominated by anomalies in shortwave radiation, and that effects of anomalous cloud albedo are negligible; and approximate the land surface as a zero heat capacity surface with low permeability at the base of the surface soil layer. d), (f), and (h), the comparison is with simulations in which cold pools were artificially suppressed.…”

Assessing the Atmospheric Response to Subgrid Surface Heterogeneity in the Single‐Column Community Earth System Model, Version 2 (CESM2)

“…Berg and Stull (2005) suggest that the formation of boundary layer cumulus clouds can be tied directly to a joint probability density function (PDF) of virtual potential temperature and water vapor mixing ratio over a heterogeneous surface. Others have highlighted the ability of horizontal gradients in surface fluxes, temperature, and soil moisture to generate secondary circulations that redistribute energy and moisture (Avissar & Schmidt, 1998; Bou‐Zeid et al., 2020; Cheng et al., 2021; Doran et al., 1995; Ookouchi et al., 1984). Those circulations play a key role in cloud and rainfall development (Avissar & Liu, 1996; Cheng et al., 2021; Graf et al., 2021; Taylor et al., 2011).…”

“…Others have highlighted the ability of horizontal gradients in surface fluxes, temperature, and soil moisture to generate secondary circulations that redistribute energy and moisture (Avissar & Schmidt, 1998; Bou‐Zeid et al., 2020; Cheng et al., 2021; Doran et al., 1995; Ookouchi et al., 1984). Those circulations play a key role in cloud and rainfall development (Avissar & Liu, 1996; Cheng et al., 2021; Graf et al., 2021; Taylor et al., 2011).…”

Assessing the Atmospheric Response to Subgrid Surface Heterogeneity in CESM2

“…Berg and Stull (2005) suggest that the formation of boundary layer cumulus clouds can be tied directly to a joint probability density function (PDF) of virtual potential temperature and water vapor mixing ratio over a heterogeneous surface. Others have highlighted the ability of horizontal gradients in surface fluxes, temperature, and soil moisture to generate secondary circulations that redistribute energy and moisture (Ookouchi et al, 1984;Doran et al, 1995;Avissar & Schmidt, 1998;Bou-Zeid et al, 2020;Cheng et al, 2021). Those circulations play a key role in cloud and rainfall development (Cheng et al, 2021;Graf et al, 2021;Taylor et al, 2011;Avissar & Liu, 1996).…”

“…Others have highlighted the ability of horizontal gradients in surface fluxes, temperature, and soil moisture to generate secondary circulations that redistribute energy and moisture (Ookouchi et al, 1984;Doran et al, 1995;Avissar & Schmidt, 1998;Bou-Zeid et al, 2020;Cheng et al, 2021). Those circulations play a key role in cloud and rainfall development (Cheng et al, 2021;Graf et al, 2021;Taylor et al, 2011;Avissar & Liu, 1996).…”