Articles | Volume 22, issue 6
Hydrol. Earth Syst. Sci., 22, 3197–3212, 2018
Hydrol. Earth Syst. Sci., 22, 3197–3212, 2018

Research article 07 Jun 2018

Research article | 07 Jun 2018

A simplified model of precipitation enhancement over a heterogeneous surface

Guido Cioni1,2,3 and Cathy Hohenegger1,3 Guido Cioni and Cathy Hohenegger
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2International Max-Planck Research School on Earth System Modelling, Hamburg, Germany
  • 3Hans-Ertel-Zentrum for Weather Research

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.

Short summary
Precipitating thunderstorms can occasionally be triggered by the propagation of a front associated with a circulation similar to a land–sea breeze but caused by the contrast of soil patches with different moisture contents. In this study, using an idealized framework, we quantify which fraction of surface rain comes from the contribution of the aforementioned front and which fraction comes from local evaporation. Results show that these two major sources contribute differently.