Collective Impacts of Orography and Soil Moisture on the Soil Moisture‐Precipitation Feedback

• Published in: Geophysical research letters 2017-11, Vol.44 (22), p.11,682-11,691
• Main Authors: Imamovic, Adel, Schlemmer, Linda, Schär, Christoph
• Format: Article
• Language: English
• Subjects: Atmospheric precipitations; Climate models; Computer simulation; Convection; Convection modes; convection‐resolving modeling; Convective precipitation; Feedback; Height; Heterogeneity; Moisture distribution; Mountain regions; Mountainous areas; Mountains; Negative feedback; Orography; Patchiness; Perturbations; Precipitation; Rain; Rainfall; Regions; Soil; Soil investigations; Soil moisture; Soil moisture distribution; soil moisture‐precipitation feedback; Soil strength; Weather forecasting
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL075657

Ensembles of convection‐resolving simulations with a simplified land surface are conducted to dissect the isolated and combined impacts of soil moisture and orography on deep‐convective precipitation under weak synoptic forcing. In particular, the deep‐convective precipitation response to a uniform and a nonuniform soil moisture perturbation is investigated both in settings with and without orography. In the case of horizontally uniform perturbations, we find a consistently positive soil moisture‐precipitation feedback, irrespective of the presence of low orography. On the other hand, a negative feedback emerges with localized perturbations: a dry soil heterogeneity substantially enhances rain amounts that scale linearly with the dryness of the soil, while a moist heterogeneity suppresses rain amounts. If the heterogeneity is located in a mountainous region, the relative importance of soil moisture heterogeneity decreases with increasing mountain height: A mountain 500 m in height is sufficient to neutralize the local soil moisture‐precipitation feedback. Plain Language Summary Mountains or regions with patchy soil moisture favor precipitation. While the driving mechanisms have been extensively studied in isolation, only a few investigations have focused on their combined impact. We use kilometer‐scale simulations to systematically compare and quantify their relative importance for deep‐convective rain. We find the strength of the soil moisture‐precipitation feedback to strongly depend on the mountain: once the mountain exceeds a critical, relatively moderate height the relative importance of the local soil moisture distribution becomes less important. Our findings suggest that numerical weather prediction of rainfall is less sensitive to soil moisture patchiness in mountainous areas, as opposed to flat regions. Furthermore, the high‐resolution simulations allow to identify potential misrepresentations of the soil moisture‐precipitation feedback in climate models that are incapable of treating deep convection explicitly. Key Points We simulated the soil moisture‐precipitation feedback in the presence of a mountain For shallow mountains the feedback turns negative if soil moisture heterogeneity is present The strength of the negative feedback is strongly reduced for mountains of moderate height