Abstract
Abstract. Soil moisture amount and distribution control evapotranspiration and thus impact the occurrence of convective precipitation. Many recent model studies demonstrate that changes in initial soil moisture content result in modified convective precipitation. However, to quantify the resulting precipitation changes, the chaotic behavior of the atmospheric system needs to be considered. Slight changes in the simulation setup, such as the chosen model domain, also result in modifications to the simulated precipitation field. This causes an uncertainty due to stochastic variability, which can be large compared to effects caused by soil moisture variations. By shifting the model domain, we estimate the uncertainty of the model results. Our novel uncertainty estimate includes 10 simulations with shifted model boundaries and is compared to the effects on precipitation caused by variations in soil moisture amount and local distribution. With this approach, the influence of soil moisture amount and distribution on convective precipitation is quantified. Deviations in simulated precipitation can only be attributed to soil moisture impacts if the systematic effects of soil moisture modifications are larger than the inherent simulation uncertainty at the convection-resolving scale.We performed seven experiments with modified soil moisture amount or distribution to address the effect of soil moisture on precipitation. Each of the experiments consists of 10 ensemble members using the deep convection-resolving COSMO model with a grid spacing of 2.8 km. Only in experiments with very strong modification in soil moisture do precipitation changes exceed the model spread in amplitude, location or structure. These changes are caused by a 50 % soil moisture increase in either the whole or part of the model domain or by drying the whole model domain. Increasing or decreasing soil moisture both predominantly results in reduced precipitation rates. Replacing the soil moisture with realistic fields from different days has an insignificant influence on precipitation. The findings of this study underline the need for uncertainty estimates in soil moisture studies based on convection-resolving models.
Highlights
Convective precipitation changes rapidly in space and time (Pedersen et al, 2010)
In the present case study, we carried out seven separate ensembles for different perturbations in soil moisture amount and soil moisture pattern
The soil moisture perturbations include both strong artificial changes by drying and wetting the model domain and realistic changes implemented by replacing the initial soil moisture field with real soil moisture patterns of a different day
Summary
Convective precipitation changes rapidly in space and time (Pedersen et al, 2010). The heterogeneity of convective precipitation and the interaction of different scales is a big challenge in atmospheric models on the global and regional scale. There is no distinct effect from soil moistening or drying on precipitation intensification, yet there exists a strong systematic influence of soil moisture changes on latent and sensible heat fluxes as well as on equivalent potential temperature, lifting condensation level and convective energy (Barthlott et al, 2011). Hohenegger and Schär (2007) investigated the error growth of random perturbation methods in cloud-resolving models using time-shifted model simulations and perturbed temperature fields in the initial conditions In their model study, using a model resolution of 2.2 km, a rapid error growth was found far away from the perturbed regions, but growth of uncertainties is limited by the large-scale atmospheric environment. We provide a description of changes in simulated precipitation resulting from a different amount or a changed pattern of soil moisture together with an assessment of the uncertainty in precipitation caused by random processes in the model.
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