Is there useful predictability of Sahelian convective storms from soil moisture?

Abstract:

The Sahel consistently shows up as a region with strong feedbacks between rainfall and soil moisture (SM). Evapotranspiration in the Sahel is usually strongly constrained by SM, whilst atmospheric circulations and convection are very sensitive to space-time variations in surface fluxes. Mesoscale Convective Systems (MCSs) are the dominant contributor to both total and also extreme rainfall in the region. Understanding how SM influences MCS life cycles is therefore key to understanding the land feedback on rainfall, at least on time scales out to several days. We know that SM gradients affect initiation of MCSs through development of mesoscale circulations. New research shows how mature MCSs also exhibit surprisingly strong SM sensitivity. Systems intensify as they propagate over drier soils on scales of the order of 200 km and upwards. At these scales, perturbed horizontal temperature gradients in the Planetary Boundary Layer (PBL) provide favourable conditions for MCS intensification by focusing moisture convergence within the West African Monsoon, and through enhanced wind shear, which helps to organise convection.

The MCS life cycle is strongly controlled by diurnal forcing. New systems tend to initiate in the afternoon, typically reaching their maximum vertical extent around 1800LT, expanding during the evening before dissipating, often in the morning hours. This storm lifecycle, in combination with the diurnal cycle of surface fluxes, provides a window for short-term (hours) predictability of MCS activity from SM. During the morning-midday period of minimum MCS activity, the SM pattern can be considered essentially static, and PBL properties map closely on to surface patterns. In late afternoon and early evening, MCS activity is strongly concentrated in the vicinity of drier soils. The SM-induced PBL structures gradually weaken overnight, but evidence of suppressed MCS activity over extensive recently-wetted surfaces is still clear. The influence of SM on MCS activity on longer time scales is more challenging to identify as every day, new MCSs propagate hundreds of kilometres across the Sahel, creating new surface structures. Here we assess the extent to which the identified negative SM-MCS feedback on the event time scale affects predictability of convection several days ahead.