Soil moisture controls on convective initiation across the diverse landscapes and hydro-climates of Africa

A wealth of studies exist analysing the feedback between soil moisture and convective precipitation across a broad range of time and space scales, encompassing theoretical, numerical modelling and observational approaches. A critical step in this feedback is an understanding of how soil moisture, via its control on sensible and latent heat fluxes, influences the initiation of deep convective clouds. Knowledge of where soil moisture conditions favour triggering of new storms is also important for short-term weather forecasting. Whilst many analyses consider how soil moisture affects the vertical profiles of temperature and humidity (1-D perspective), other studies examine the role of spatially-varying soil moisture on convective initiation via surface-induced mesoscale circulations. Here we use a 20-year observational dataset of convective initiations across sub-Saharan Africa to draw more general conclusions about how soil moisture impacts convective initiation and subsequent rainfall across a diversity of hydro-climatic, topographic and wind conditions.

We use cloud-top temperature data from the geostationary Meteosat Second Generation (MSG) series of satellites to identify afternoon convective initiations for the period 2004-2023 and relate these to pre-storm observations of land surface state (land surface temperature from MSG, and surface soil moisture from the Advanced Scatterometer). Both datasets reveal a consistent Africa-wide picture of initiations favoured at the downwind end of elliptical dry soil structures, as found in previous analyses over the Sahel (Taylor et al, Nature Geoscience, 2011). The soil moisture signal weakens with stronger topographic variability, and in wetter climates and times of year, but outside of the Congo Basin and East African Highlands, the signal of initiation over locally dry soils is clear. Moreover, we show that the along-wind length scale of the dry soil feature increases with low-level wind speed. Our results, valid on scales of up to ~200km, fit with understanding of mesoscale circulations driven by soil moisture heterogeneity, and cannot be explained by 1-D consideration of thermodynamic profiles alone. We also show how the overall soil moisture-precipitation feedback from these events is influenced by wind conditions at storm steering level. In regions (including the Sahel) where winds at low and steering levels are in opposing directions, the feedback is strongly negative. Alternatively, when low and mid-level winds are aligned, the negative feedback weakens, and can become positive.