Dry soils can intensify mesoscale convective systems

Abstract

Soil moisture can feed back on rainfall through the impact of surface fluxes on the environment in which convection develops. The vast majority of previous research has focused on the initiation of convection, but in many regions of the world, the majority of rain comes from remotely triggered mesoscale convective systems (MCSs). Here we conduct a systematic observational analysis of soil moisture feedbacks on propagating MCSs anywhere in the world and show a strong positive impact of drier soils on convection within mature MCSs. From thousands of storms captured in satellite imagery over the Sahel, we find that convective cores within MCSs are favored on the downstream side of dry patches ≥200 km across. The effect is particularly strong during the afternoon–evening transition when convection reaches its diurnal peak in intensity and frequency, with dry soils accounting for an additional one in five convective cores. Dry soil patterns intensify MCSs through a combination of convergence, increased instability, and wind shear, all factors that strengthen organized convection. These favorable conditions tend to occur in the vicinity of a surface-induced anomalous displacement of the Sahelian dry line/intertropical discontinuity, suggesting a strong link between dry line dynamics and soil moisture state. Our results have important implications for nowcasting of severe weather in the Sahel and potentially in other MCS hotspot regions of the world.

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

“…Decreases in relative humidity may to some extent counteract the aerosol increase. For the LDS, Kniffka et al (2019) have already demonstrated the high sensitivity of rainfall to incoming solar radiation, supporting the idea of a direct aerosol effect. Such work does not exist for the FRS and SRS, but the rainfall classification by Maranan et al (2018) suggests a predominance of long-lived, organized systems in the former and more locally triggered convection in the latter, possibly explaining the larger aerosol signal in SRS.…”

“…3 shows the mean vertical profile of cloud cover at 06:00 UTC (corresponds to local time in the study region), the analysis time closest to the diurnal maximum of low clouds (van der Linden et al, 2015). These clouds are crucial for the radiative energy budget and, thus, for vertical mixing and the triggering of convection in the afternoon (Kalthoff et al, 2018;Kniffka et al, 2019). We anticipate this factor to influence the significance of aerosol changes to the surface energy balance when incoming radiation is strongly reduced by cloud cover.…”

“…In contrast, the radiative effect appears to be significant, particularly as the very high relative humidity in the moist, deep monsoon layer leads to wet growth of aerosol particles (Deetz et al, 2018a;Haslett et al, 2019b). Combined with the high sensitivity of rainfall to destabilization by local radiative heating found by Kniffka et al (2019), this creates potential for a significant direct effect. Consistently, sensitivity experiments have shown that changing anthropogenic emissions along the Guinea coast has the potential to shift the entire monsoonal rainband and regional circulation.…”

Meteorological OSSEs for New Zenith Total Delay Observations: Impact Assessment for the Hydroterra Geosynchronous Satellite on the October 2019 Genoa Event

“…Due to the lack of observations in that area, the Hydroterra ZTD observations could be very valuable for improving the forecast capabilities, especially when coupled with the Hydroterra soil moisture observations, because soil moisture plays a fundamental role in the dynamics of MCSs in this region [93]. In fact, the MCSs which form over land (e.g., in the Sahel where they are responsible for the majority of annual rainfall [94]) are known to be controlled by the surface properties [95]. The added value of the Hydroterra soil moisture observation in the hydrological framework have been discussed in [29].…”

Synoptic timescale linkage between midlatitude winter troughs Sahara temperature patterns and northern Congo rainfall: A building block of regional climate variability

“…It is, however, still possible that shear is strongly contributing to some rainfall events. Targeted moisture budget and stability assessments including sub-daily data and atmospheric model experiments can be expected to provide insights into the role of the low-level and mid-level fluctuations revealed in the statistical analyses, with a different balance of factors likely for different rainfall types (Maranan et al, 2018), including stronger role for lowlevel temperature gradients a few hours before rainfall types involving MCSs (consistent with T18 and Klein and Taylor, 2020).…”