Journal of Climate, in press.
Benjamin R. Lintner, Pierre Gentine, Kirsten L. Findell,
Fabio D'Andrea, and Adam H. Sobel
A process-based, semi-analytic prototype model for understanding large-scale landatmosphere coupling is developed here. The metric for quantifying the coupling is the sensitivity of precipitation (P) to soil moisture (W), defined as dP/dW. For a range of prototype parameters typical of conditions found over tropical or summertime continents, the sensitivity measure exhibits a broad minimum at intermediate soil moisture values. This minimum is attributed to a tradeoff between evaporation (or evapotranspiration) E and large-scale moisture convergence across the range of soil moisture states. For low soil moisture (or water-limited) conditions, dP/dW is dominated by evaporative sensitivity dE/dW, reflecting high potential evaporation (E_p) arising from relatively warm surface conditions and a moisture-deficient atmospheric column under dry surface conditions. By contrast, under high soil moisture (or energy-limited) conditions, dE/dW becomes slightly negative as E_p decreases. However, because convergence and precipitation increase strongly with decreasing (drying) moisture advection, while soil moisture slowly saturates, dP/dW is large. Variation of key parameters is shown to impact the magnitude of dP/dW: e.g., increasing the timescale for convective adjustment lowers dP/dW at a given W, especially on the moist side of the profile where convergence dominates. While the prototype applicability for direct quantitative comparison to either observations or models is clearly limited, it nonetheless demonstrates how the complex interplay of surface turbulent and radiative fluxes and atmospheric column radiative fluxes, deep convection, and horizontal and vertical moisture transport influences the coupling of the land surface and atmosphere that may be expected to occur in either more realistic models or observations.