A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part II: Water Budget

1989 ◽  
Vol 117 (4) ◽  
pp. 728-744 ◽  
Author(s):  
Michel Chong ◽  
Daniele Hauser
Keyword(s):  
West Africa ◽  
Water Budget ◽  
Squall Line

scholarly journals Interactions between Convection and a Moist Vortex Associated with an Extreme Rainfall Event over Southern West Africa

2019 ◽  
Vol 147 (7) ◽  
pp. 2309-2328 ◽  
Author(s):  
Marlon Maranan ◽  
Andreas H. Fink ◽  
Peter Knippertz ◽  
Sabastine D. Francis ◽  
Aristide B. Akpo ◽  
...  
Keyword(s):  
West Africa ◽  
Extreme Values ◽  
Vortex Formation ◽  
Daily Rainfall ◽  
Extreme Rainfall ◽  
Squall Line ◽  
Convective System ◽  
Extreme Rainfall Event ◽  
Guinea Coast ◽  
Free Atmosphere

Abstract An intense mesoscale convective system (MCS) in the Guinea Coast region caused one of the highest ever recorded daily rainfall amounts at the Nigerian station Abakaliki on 12 June 2016 (223.5 mm). This paper provides a detailed analysis of the meso- and synoptic-scale factors leading to this event, including some so far undocumented dynamical aspects for southern West Africa. The MCS formed over the Darfur Mountains due to diurnal heating, then moved southwestward along a mid- to lower-tropospheric trough, and developed into a classical West African squall line in a highly sheared environment with pronounced midlevel dryness. Strong moisture flux convergence over Nigeria prior to the MCS passage led to extreme values in precipitable water and was caused by the formation of a local, short-lived heat low. According to the pressure tendency equation, the latter resulted from tropospheric warming due to MCS-forced subsidence as well as surface insolation in the resulting almost cloud-free atmosphere. In this extremely moist environment, the MCS strongly intensified and initiated the formation of a lower-tropospheric vortex, which resulted in a deceleration of the MCS and high rainfall accumulation at Abakaliki. Following the vorticity equation, the vortex formation was realized through strong low-level vortex stretching and upper-level vertical vorticity advection related to the MCS, which became “dynamically large” compared to the Rossby radius of deformation. Eventually, moisture supply and lifting associated with the vortex are suggested to promote the longevity of the MCS during the subsequent westward movement along the Guinea Coast.

A Wave-Relative Framework Analysis of AEW–MCS Interactions Leading to Tropical Cyclogenesis

2020 ◽  
Vol 148 (11) ◽  
pp. 4657-4671
Author(s):  
Kelly M. Núñez Ocasio ◽  
Jenni L. Evans ◽  
George S. Young
Keyword(s):  
West Africa ◽  
Thermodynamic Characteristics ◽  
Convective Cloud ◽  
Type Systems ◽  
Squall Line ◽  
Tropical Cyclogenesis ◽  
Framework Analysis ◽  
Convective Systems ◽  
Cloud Clusters ◽  
Mesoscale Convective

AbstractAn African easterly wave (AEW) and associated mesoscale convective systems (MCSs) dataset has been created and used to evaluate the propagation of MCSs, AEWs, and, especially, the propagation of MCSs relative to the AEW with which they are associated (i.e., wave-relative framework). The thermodynamic characteristics of AEW–MCS systems are also analyzed. The analysis is done for both AEW–MCS systems that develop into tropical cyclones and those that do not to quantify significant differences. It is shown that developing AEWs over West Africa are associated with a larger number of convective cloud clusters (CCCs; squall-line-type systems) than nondeveloping AEWs. The MCSs of developing AEWs propagate at the same speed of the AEW trough in addition to being in phase with the trough, whereas convection associated with nondeveloping AEWs over West Africa moves faster than the trough and is positioned south of it. These differences become important for the intensification of the AEW vortex as this slower-moving convection (i.e., moving at the same speed of the AEW trough) spends more time supplying moisture and latent heat to the AEW vortex, supporting its further intensification. An analysis of the rainfall rate (MCS intensity), MCS area, and latent heating rate contribution reveals that there are statistically significant differences between developing AEWs and nondeveloping AEWs, especially over West Africa where the fraction of extremely large MCS areas associated with developing AEWs is larger than for nondeveloping AEWs.

scholarly journals Extratropical Dry-Air Intrusions into the West African Monsoon Midtroposphere: An Important Factor for the Convective Activity over the Sahel

2005 ◽  
Vol 62 (2) ◽  
pp. 390-407 ◽  
Author(s):  
Rémy Roca ◽  
Jean-Philippe Lafore ◽  
Catherine Piriou ◽  
Jean-Luc Redelsperger
Keyword(s):  
West Africa ◽  
Large Scale ◽  
Positive Association ◽  
Squall Line ◽  
Back Trajectory ◽  
Convective Activity ◽  
Monsoon Period ◽  
Dry Air ◽  
Convective Systems ◽  
Sahel Region

Abstract This paper investigates the relationship between large-scale dynamics, water vapor, and organized convection over West Africa. Making use of a simplified condensation hypothesis, a back-trajectory model fed by NCEP-analyzed winds is used to reconstruct the midtropospheric humidity field over Africa during July to August 1992. The approach documents both the moisture content and the origin of the air masses. Meteosat satellite infrared imagery is used to characterize the convective systems. A case study analysis reveals that very dry air patches (RH < 5%) are located in the immediate midtropospheric environment of a typical squall line. Such dry-air structures are shown to originate in the upper levels (200–250 hPa) on the anticyclonic side of the polar jet stream at 50°N. Focusing on the Sahel region, dry events are isolated using the time series of the 500-hPa relative humidity distribution during the monsoon period. These dry events are shown to be composed of extratropical air. Composite analysis of the convective activity indicator exhibits a strong positive association between dry intrusions and convection on the eastern side of the Sahelian region. Organized convective systems that are fast moving and long lasting are more likely over this region when a dry intrusion is present. This coincides with the well-established theory that midtropospheric dry air, when combined with sufficient wind shear, can maintain and intensify previously triggered deep convection through rain evaporation that feeds the cold pools, especially within squall lines. This paper suggests that the extratropical dry-air intrusions modulate the occurrence and duration of convective systems and, therefore, the mode of variability of rainfall over West Africa during the monsoon.

Sign in / Sign up

Export Citation Format

Share Document