Abstract
Abstract. The magnitude and sign of soil moisture–precipitation coupling (SMPC) is investigated using a probability-based approach and 10 years of daily microwave satellite data across North Africa at a 1∘ horizontal scale. Specifically, the co-existence and co-variability of spatial (i.e. using soil moisture gradients) and temporal (i.e. using soil moisture anomaly) soil moisture effects on afternoon rainfall is explored. The analysis shows that in the semi-arid environment of the Sahel, the negative spatial and the negative temporal coupling relationships do not only co-exist, but are also dependent on one another. Hence, if afternoon rain falls over temporally drier soils, it is likely to be surrounded by a wetter environment. Two regions are identified as SMPC “hot spots”. These are the south-western part of the domain (7–15∘ N, 10∘ W–7∘ E), with the most robust negative SMPC signal, and the South Sudanese region (5–13∘ N, 24–34∘ E). The sign and significance of the coupling in the latter region is found to be largely modulated by the presence of wetlands and is susceptible to the number of long-lived propagating convective systems. The presence of wetlands and an irrigated land area is found to account for about 30 % of strong and significant spatial SMPC in the North African domain. This study provides the first insight into regional variability of SMPC in North Africa, and supports the potential relevance of mechanisms associated with enhanced sensible heat flux and mesoscale variability in surface soil moisture for deep convection development.
Highlights
Soil moisture can affect the state of the lower atmosphere through its impact on evapotranspiration and surface energy flux partitioning (e.g. Eltahir, 1998; Klüpfel et al, 2011)
We start our assessment by investigating the spatial soil moisture–precipitation coupling relationship
In agreement with the global-scale studies of T12 and G15, we find a dominantly negative spatial soil moisture–precipitation coupling (SMPC) in the North African domain on the 5◦ × 5◦ grid, i.e. a strong preference for convective rainfall events to occur over spatially drier soils (Fig. 4a)
Summary
Soil moisture can affect the state of the lower atmosphere through its impact on evapotranspiration and surface energy flux partitioning (e.g. Eltahir, 1998; Klüpfel et al, 2011). Observational evidence of the SMPC largely relies on the measurements of recent field campaigns (like African HAPEX and AMMA: Goutorbe et al, 1994; Redelsperger et al, 2006), and is often limited to a short spatio-temporal scale. Such observational analyses present unique evidence of environmental conditions preceding convection development Lothon et al, 2011) and can be further used as a testing ground to evaluate and improve the physical parametrizations of models (e.g. Couvreux et al, 2013) Both observational and modelling studies agree reasonably well on the effect of soil moisture availability and heterogeneity on the lower atmospheric stability Both observational and modelling studies agree reasonably well on the effect of soil moisture availability and heterogeneity on the lower atmospheric stability (e.g. Kohler et al, 2010) and convective initiation at the mesoscale (e.g. Taylor, 2010; Birch et al, 2013)
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