An observational case study of mesoscale atmospheric circulations induced by soil moisture

A sensitivity analysis of a moist convection case-study of the 2005 West African monsoon has been performed with the limited-area model PROMES. In the control simulation, soil moisture was initialized based on European Centre for Medium-Range Weather Forecasts (ECMWF) soil moisture. A relatively wet patch appears over an area that was affected later by the passage of a squall line. The control simulation is able to reproduce a squall line that crosses over this wet surface, which makes it possible to analyse the sensitivity of the modelled convective system to a soil moisture increase up to saturation in the cited patch. The wetter land surface in the sensitivity run generates a cooler and moister area in the near-surface atmospheric fields compared to the control run, and the surface wind differences between both runs show a divergent pattern. The humidity and temperature differences follow the diurnal cycle of the monsoon flow, as the cool and moist atmospheric perturbation remains over the wet patch during the afternoon, and moves towards the northeast during the night. There is a clear interaction between the soil moisture perturbation and an approximating African Easterly Wave. This interaction generates larger differences between the two runs on the second simulation day than on the first. The precipitation is reduced over the wet patch in the sensitivity run, but over a larger area a precipitation increase is obtained, reflecting a complex soil moisture-precipitation feedback.

Section: Introductionmentioning

confidence: 67%

A modelling case‐study of soil moisture–atmosphere coupling

Gaertner

Domínguez

Garvert³

2010

“…Soil moisture heterogeneity can cause horizontal gradients in near-surface temperature, which can generate mesoscale circulations analogous to a land-sea breeze (Segal and Arritt, 1992). These circulations can cause moisture convergence and a minimum in the entrainment-dilution of CBL air (Taylor et al, 2007;Adler et al, 2011;Garcia-Carreras et al, 2011), favouring deep convection during the afternoon (Emori, 1998). For these reasons, the optimum location for convection initiation appears to be on dry soil that is adjacent to strong gradients in soil moisture (Taylor et al, 2011).…”

Section: Convection and Land-surface Interaction Over West Africamentioning

confidence: 99%

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

Birch

Parker

O’Leary

et al. 2012

A mesoscale convective system (MCS) case study was observed over northeast Mali as part of the African Monsoon Multidisciplinary Analysis (AMMA) on 31 July 2006. Observations of this case suggest that the soil-moisture heterogeneity and atmospheric gravity waves emitted from a 'parent' MCS were important trigger mechanisms for this system. This study uses high-resolution Met Office Unified Model (MetUM) simulations to assess the importance of the synoptic circulation, land-surface and gravity waves in the initiation and development of the MCS. During the early afternoon shallow convection developed over a region of dry soil within a synoptic-scale convergence zone, which was caused by the confluence of the southerly monsoon flow with winds associated with the circulation around the Saharan heat low. Two pronounced waves were emitted from a nearby 'parent' storm and propagated towards the convergence zone. When the second wave reached the location of the shallow convection, deep convection was immediately initiated. Further convective cells developed later in the afternoon over dry soil, many adjacent to strong soil moisture gradients; these aggregated with the main storm, which later developed into the case study MCS. A comparison of model simulations with/without the soil-moisture heterogeneity and gravity waves shows that the synoptic-scale circulation and convergence zones, specified by the atmospheric analysis, were the most important factors for the successful simulation of the MCS. If the location of the initiation of the system is to be forecast accurately, the land-surface, that is, the soil moisture, must be represented adequately. In order to reproduce the timing of the secondary initiation of convection correctly the model must be able to capture gravity waves that are emitted by existing systems.

“…Finally, soil moisture patterns in the wake of the MCS may induce mesoscale atmospheric circulations (e.g. Taylor et al, 2007) that, in turn, may impact on the horizontal as well as the vertical distribution of dust in the lowest few kilometres by affecting the exchange processes between the PBL and the SAL. However, a detailed analysis of the influence of each of these mechanisms on the dust load and distribution is beyond the scope of this paper, essentially due to the lack of adequate observations in the vicinity of the MCS.…”

Section: The 13-14 June Mcs Over Beninmentioning

confidence: 99%

Multi‐platform observations of a springtime case of Bodélé and Sudan dust emission, transport and scavenging over West Africa

Flamant¹,

Lavaysse²,

Todd³

et al. 2009

ABSTRACT:The structure of the Saharan air layer over Niger and Benin during a major springtime dust event from the Bodélé region and Sudan is investigated using airborne lidar and dropsonde measurements. Aircraft operations were conducted on 13 and 14 June 2006, within the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. Complementary ground-based and satellite observations, as well as European Centre for Medium-range Weather Forecasts analyses are used to investigate the regional aspects of emission, transport and deposition in the period from 9 to 14 June 2006, to provide a framework for the interpretation of the airborne measurements. The study details the transport patterns of dust from eastern Saharan sources towards the southwest in the springtime, and highlights the role of the intertropical discontinuity and the Darfur mountains in injecting the aerosols from Bodélé and Sudan, respectively, over the monsoon flow, and in the African easterly jet (AEJ) region. It also illustrates the impact of the daily variability of the emissions in the source regions on the dust load advected westward by the AEJ and observed over Benin. The impact of a mesoscale convective system (MCS) on the dust load and vertical distribution observed over Benin and southern Niger was also investigated, nearly 12 hours after its passage over Benin. The only discernible impact on the dust distribution is observed to be associated with widespread subsidence in the wake of the MCS, over northern Benin and Niger. Wet scavenging related to convective or stratiform rain could not be observed, as processed air masses were replaced by fresh dust transported in the upper Saharan air layer by the AEJ. Over southern Benin, the dust distribution appeared to be mostly controlled by processes affecting the planetary boundary layer upstream, i.e. over Nigeria or Chad. Because springtime dust from remote eastern sources (such as observed in this case) is mainly transported into the AEJ, it could impact on the radiation budget in the AEJ region, thereby possibly modifying the West African weather at the synoptic scale. Copyright