Asymmetric Recovery from Wet versus Dry Soil Moisture Anomalies

Abstract

The energy and moisture state in the soil and new-surface atmosphere evolve due to fluxes that are themselves a function of these states. Because the soil moisture and temperature are negatively correlated (dry warm or cool moist), physical mechanisms that tend to restore each state individually (soil moisture control of evaporation and temperature dependence of saturation specific humidity, for example) act as anomaly enhancing (positive) feedback mechanisms for the other state. Model and observational studies in the literature indicate that dry hydrologic anomalies tend to persist longer than moist ones, suggesting that thee feedback mechanisms are stronger when the soil layer is dry. A conceptual land-atmosphere model, consisting of heat and moisture balance for a soil layer and a turbulently mixed atmospheric boundary layer, is subjected to stochastic forcing. The covariability structure evolves through the state-dependent turbulent and radiative fluxes in the land-atmosphere system and is not prescribed a priori. The model is analyzed in terms of recovery times from anomalous to corner soil moisture. Longer recovery times are found to be more probable for dry anomalies than for moist anomalies, when evaporation efficiency is formulated to switch between soil and atmospheric control.