How does the spatial scale of surface flux variability affect MCS properties?

Understanding drivers and controls on Mesoscale Convective Systems (MCSs) is critical for predicting rainfall extremes and its impacts across time scales, from nowcasting to climate change. For MCSs over land, heterogeneity in surface fluxes across length scales presents a primary influence on storms. In West Africa, for example, MCS initiation is enhanced by ~20km scale gradients in soil moisture [1]; mature MCS cores are favoured over ~200km scale dry soil anomalies [2]; and the regional circulation responds to ~2000km scale soil moisture gradients [3], with this response explaining an observed intensification in MCSs over the last 30 years [4].

To better understand how MCSs respond to this spectrum of surface flux gradients, here we present a novel sensitivity experiment framework in which a convection-permitting Control simulation is reinitialised daily from a soil moisture field where we have modified the spectrum of surface variability using wavelet filtering. We conduct two scale experiments: one in which all sub-1000km scale soil moisture variability is suppressed; and one in which we return sub-mesoscale variability. The Control simulation is run at 1.5km over West Africa for 40 days using the Met Office Unified Model and features realistic land-surface and radiation schemes and a full suite of moisture tracers. Combining results from this simulation and outputs from 2-day long sensitivity experiments gives 200 days of CP data, enabling investigation of the impact of land-surface heterogeneity on MCSs in unprecedented detail.

We hereby elucidate the chain of mechanisms through which variability in mesoscale soil moisture anomalies propagates through surface fluxes to planetary boundary layer (PBL) fields and the regional circulation, and crucially, the effect on MCS lifecycles and intensities. We find a substantial reduction in MCSs when all sub-1000km soil moisture variability is suppressed, with numbers recovering when <100km scale variability is reintroduced. Precursor PBL fields found at MCS core locations in all experiments are consistent with those about relatively dry mesoscale soil moisture anomalies. However, the relative control of soil moisture and insolation on heat fluxes is modified in the sensitivity experiments, indicating that PBL conditions preferential for mature MCS cores are achieved via different controls and at different frequencies, affecting storm populations.

References

• Taylor et al, Frequency of Sahelian storm initiation enhanced over mesoscale soil-moisture patterns. Nature Geoscience 4, 430-433 (2011)
• Klein and Taylor, Dry soils can intensify mesoscale convective systems. Proceedings of the National Academy of Sciences, 202007998, (2020)
• Cook, Generation of the African Easterly Jet and Its Role in Determining West African Precipitation, Journal of Climate 12(5), 1165–1184 (1999)
• Taylor et al, Frequency of extreme Sahelian storms tripled since 1982 in satellite observations. Nature 544, 475-478, (2017).