Mesoscale convective systems and nocturnal rainfall over the West African Sahel: role of the Inter-tropical front

Edward K Vizy,Kerry H Cook

doi:10.1007/s00382-017-3628-7

Abstract

A convection-permitting regional model simulation for August 2006 and observations are evaluated to better understand the diurnal cycle of precipitation over the Sahel. In particular, reasons for a nocturnal rainfall maximum over parts of the Sahel during the height of the West African monsoon are investigated. A relationship between mesoscale convective system (MCS) activity and inter-tropical front (ITF)/dryline dynamics is revealed. Over 90% of the Sahel nocturnal rainfall derives from propagating MCSs that have been associated with topography in earlier studies. In contrast, in this case study, 70–90% of the nocturnal rainfall over the southern Sahel (11°N–14°N) west of 15°E is associated with MCSs that originate less than 1000 km upstream (to the north and east) in the afternoon, in a region largely devoid of significant orography. This MCS development occurs in association with the Sahel ITF, combined with atmospheric pre-conditioning. Daytime surface heating generates turbulent mixing that promotes planetary boundary layer (PBL) growth accompanied by a low-level reversal in the meridional flow. This enhances wind convergence in the low-level moist layer within 2°–3° of latitude of the equatorward side of the ITF. MCSs tend to form when this vertical mixing extends to the level of free convection and is accompanied by a mid-tropospheric African easterly wave disturbance to the east. This synoptic disturbance enhances the vertical wind shear and atmospheric instability over the genesis location. These results are found to be robust across the region.

Highlights

Mesoscale convective systems (MCSs), including squall lines and African easterly wave (AEW) disturbances, are frequently observed over the West African Sahel, and past studies (Laurent et al 1998; Laing et al 1999; Mathon et al 2002; Mohr 2004) indicate that up to 95% of the boreal summer rainfall over this region is associated with mesoscale convective system (MCS) activity
The West African Sahel is of particular interest because evening (18Z–02Z) MCS activity and rainfall peak over much of the region during the height of the West African summer monsoon in August (Dai 2001; Mohr 2004; Zhang et al 2016a, b)
If the 925 hPa 0 m s−1 meridional wind boundary is used to approximate the daytime position of the intertropical front (ITF), we find that MCS genesis occurs in close proximity to the ITF gust frontal boundary for at least 6 of the cases (MCS1, MCS2, MCS3, MCS5, MCS9, and MCS13)

Summary

Introduction

Mesoscale convective systems (MCSs), including squall lines and African easterly wave (AEW) disturbances, are frequently observed over the West African Sahel, and past studies (Laurent et al 1998; Laing et al 1999; Mathon et al 2002; Mohr 2004) indicate that up to 95% of the boreal summer rainfall over this region is associated with MCS activity. This suggests that organized systems play an important role in establishing the diurnal cycle of precipitation including nocturnal rainfall. The WAWJ is strongest in the early morning hours (06Z) while the AEJ peaks later in the morning around 09Z and is weakest in the late afternoon when vertical mixing is strongest

Objectives

Methods

Findings

Conclusion