Sensitivity of a modelled life cycle of a mesoscale convective system to soil conditions over West Africa

Abstract

Cloud-resolving real-case simulations initialized with European Centre for Medium-range Weather Forecasts (ECMWF) analysis data were performed to investigate the sensitivity of a mesoscale convective system (MCS) to soil properties. Four scenarios were investigated: original ECMWF soil moisture (MOI); ECMWF soil moisture reduced by 35% (CTRL), which better fits with the satellite observations; homogeneous soil type and soil moisture (HOM); and homogeneous soil type with a north-south oriented drier band of two degrees width (BAND).Initiation of convection was observed in all experiments. The initiation area was characterised by very low convective inhibition (CIN) and high convective available potential energy (CAPE). The simulations showed some evidence that convection was initiated in the vicinity of orography and along soil moisture inhomogeneities. Except for the MOI run, MCSs developed in all experiments. In CTRL, a weakening of the system was observed when approaching an area with reduced total water content and enhanced CIN. In BAND, convection developed almost simultaneously in the original initiation area and inside the dry band, where convergence, generated by thermally-induced circulation and supported by downward mixing of momentum from the African Easterly Jet, triggered convection. When the mature MCS approached the band from the east it weakened, caused by increasing CIN ahead of the band. Triggering of convection on this day was favoured by drier surfaces and/or soil moisture inhomogeneities, while a mature system weakened in the vicinity of a drier surface. Thus a positive feedback between soil moisture and convection existed for a mature system whereas a negative feedback was found for triggering of convection.

A modelling case‐study of soil moisture–atmosphere coupling

“…The sign of the local feedback can be different under other circumstances, as shown e.g. by Gantner and Kalthoff (2009) who obtain a positive local feedback for a mature system passing over a dry soil-moisture anomaly, in sensitivity simulations in a different modelling case-study. Many aspects can influence the feedback, from model parametrizations to specific features of the atmospheric conditions.…”

“…The sign of the soil moisture-precipitation feedback can depend also on the development phase of a convective system. This is shown by Gantner and Kalthoff (2009), who obtain in their simulations a negative feedback for triggering of convection and a positive feedback for a mature system.…”

Impact of soil moisture and convectively generated waves on the initiation of a West African mesoscale convective system

“…Although the soil moisture-precipitation feedback appears to be negative in the early stages of convection, when the system is in its mature stage the feedback can be positive (Gantner and Kalthoff, 2010). Taylor and Lebel (1998) use a dense network of rain-gauges to show a strong preference for increased rainfall at stations where it had rained in the previous 2-3 days.…”

“…Similarly, Wolters et al (2010) show, using a mesoscale weather model, that precipitation intensity is greater over wet soil. Convective systems become more intense over wet soil due to the increased availability of moisture and CAPE at the surface (Taylor and Clark, 2001;Gantner and Kalthoff, 2010).…”

New perspectives on land–atmosphere feedbacks from the African Monsoon Multidisciplinary Analysis

“…This suggested an important role for the surface-induced circulations discussed earlier in providing a favourable environment for convective initiation, alongside other mechanisms such as orographic forcing and cold pools. Further evidence showing the impact of surfaceinduced circulations on storm development came from numerical simulations Gantner and Kalthoff, 2010), observations from forest-crop transitions (Garcia-Carreras et al, 2010) and satellite cloud data around the Mali wetlands (Taylor, 2010). Once storms are initiated, our research suggests that various processes may govern the strength, and even the sign, of feedbacks between soil moisture and individual MCS.…”