A simplified model of precipitation enhancement over a heterogeneous surface

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.

Relation between Convective Rainfall Properties and Antecedent Soil Moisture Heterogeneity Conditions in North Africa

“…In addition, as was already mentioned earlier, soil moisture heterogeneity impact on AR amount and possibly AR size will rather occur via the contribution of added moisture due to convergence to the total moisture amount rather than via increased strength of updrafts [46]. Given this complexity of factors and non-linear relationships involved in the rainfall evolution, quantification of soil moisture impact on rainfall amount has proven to be notoriously difficult in the past [81][82][83][84] and will likely remain so.…”

“…Yet, accurate surface flux estimates at high temporal resolution would be required to draw firm conclusions. Finally, in contrast to rainfall triggering, precipitation volume and perhaps also area are expected to be more closely correlated with the total available moisture amount, than with the strength of turbulent updrafts [23,46,71]. This together with the complexity of factors involved in precipitation development would explain the lack of correlation for the other AR parameters.…”

“…A recent analysis showed that deep convection forms 1-4 h faster in the presence of surface heterogeneity, with the timing of the initiation being correlated with the magnitude of the sensible heat flux [26]. Furthermore, the strength of the spatial soil moisture gradient was found to be positively correlated with the amount of rainfall under benign atmospheric conditions [46]. Hence, the expectation is that a stronger soil moisture gradient with drier soil may lead to an earlier precipitation onset and more rain.…”

Assessing the Atmospheric Response to Subgrid Surface Heterogeneity in CESM2

“…Critically, the current SCAM parameterization lacks the ability to explicitly represent secondary circulations, which Simon et al (2022) suggest are critical to the atmospheric response present in these WRF-LES experiments. In other observational and LES experiments as well, the atmospheric differences that arise over heterogeneous surfaces stem from these mesoscale secondary circulations that transport heat and moisture between parts of the domain (Cioni & Hohenegger, 2018;Avissar & Schmidt, 1998;Doran et al, 1995;Ookouchi et al, 1984). Such circulations are currently outside the scope of what CLUBB is designed to capture in its statistical representation of SGS heterogeneity, and the parameterization implemented here was not intended to add that phenomenon to the model.…”

Numerical Study of the Impact of Complex Terrain and Soil Moisture on Convective Initiation

“…Under weak synoptic backgrounds, terrain and land surface characteristics, such as soil moisture and vegetation, play a more important role in determining the development of convection [14]. The spatial heterogeneous soil moisture affects CI by changing the surface energy partitioning and triggering mesoscale circulation [15][16][17][18][19]. Taylor et al [20] analyzed the relationship between afternoon convective precipitation and soil moisture on continents based on observations and pointed out that afternoon convective precipitation tends to occur in areas with relatively dry soil, especially in semi-arid areas.…”