Abstract
Abstract. Soil moisture heterogeneities influence the onset of convection and subsequent evolution of precipitating systems through the triggering of mesoscale circulations. However, local evaporation also plays a role in determining precipitation amounts. Here we aim at disentangling the effect of advection and evaporation on precipitation over the course of a diurnal cycle by formulating a simple conceptual model. The derivation of the model is inspired by the results of simulations performed with a high-resolution (250 m) large eddy simulation model over a surface with varying degrees of heterogeneity. A key element of the conceptual model is the representation of precipitation as a weighted sum of advection and evaporation, each weighed by its own efficiency. The model is then used to isolate the main parameters that control precipitation variations over a spatially drier patch. It is found that these changes surprisingly do not depend on soil moisture itself but instead purely on parameters that describe the atmospheric initial state. The likelihood for enhanced precipitation over drier soils is discussed based on these parameters. Additional experiments are used to test the validity of the model.
Highlights
Will a wetter soil lead to more or less precipitation? This apparently simple question inspired many studies over the course of the last 50 years
As a result of the circulation, and as found in past studies, convection preferentially develops over the dry patch and in particular at the edge of the front associated with the mesoscale circulation
The heterogeneity in the land surface was prescribed by dividing the domain into two patches with different initial values of soil moisture
Summary
Will a wetter soil lead to more or less precipitation? This apparently simple question inspired many studies over the course of the last 50 years. The presence of heterogeneity in surface soil moisture induces thermally driven mesoscale circulations (Segal and Arritt, 1992) which transport moist air from spatially wetter patches to spatially drier patches, acting against the initial perturbation of soil moisture, and which can affect the distribution of precipitation. Avissar and Liu (1996) found that the land-surface wetness heterogeneity (i.e., spatial gradients of soil moisture) controls the transition from a randomly scattered state of convection to a more organized one where clouds form ahead of the front associated with the mesoscale circulation. The presence of such circulations tends to enhance the precipitation amount. Further analyses have shown that this basic response can be modified by many environmental factors
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