Abstract
AbstractOn the evening of 9 August 2006, a mesoscale convective system (MCS) having a north‐south oriented squall‐line organization formed over the border between Chad and Nigeria. It propagated westward, intensified over Nigeria on 10 August, and reached Niamey (Niger) at 0320 UTC on 11 August. Its passage over Niamey was accompanied by dust lifting and was well tracked by the Massachusetts Institute of Technology (MIT) Doppler radar. The three‐dimensional structure of the airflow and precipitation pattern is investigated from regular radar volume scans performed every ten minutes between 0200 and 0321 UTC. The 3D wind components are deduced from the multiple‐Doppler synthesis and continuity adjustment technique (MUSCAT) applied to a set of three volume scans obtained over a time period of one hour, which are equivalent to a three‐radar observation of the squall line when considering a reference frame moving with the system and the hypothesis of a stationary field.Results of the wind synthesis reveal several features commonly observed in tropical squall lines, such as the deep convective cells in front of the system, fed by the monsoon air and extending up to 15 km altitude, and the well‐marked stratiform rain region at the rear, associated with mesoscale vertical motions. Forward and trailing anvils are clearly identified as resulting from the outflow of air reaching the tropopause and transported to this level by the sloping convective updraughts occurring in a sheared environment. In the northern part, a deeper and stronger front‐to‐rear flow at mid‐levels is found to contribute to the rearward deflection of the leading line and to promote a broader (over 300 km) stratiform cloud region. Eddy vertical transports of the cross‐line momentum mainly accounts for the mid‐level flow acceleration due to a momentum redistribution from low to higher levels. The height distribution of hydrometeors and their associated production terms derived from a one‐dimensional microphysical retrieval model indicate the distinct roles of the convective and stratiform regions in the formation of graupel and rain, and the respective contributions of cold (riming) and warm (coalescence, melting) processes. Cooling from melting, and heating/cooling from condensation/evaporation processes yield a net decrease and increase of the potential temperature at low and mid‐to‐upper levels, respectively, with respect to an environmental thermodynamic profile taken three hours ahead of the analysis. Finally, the upper‐level rearward flow could convey the non‐negligible proportion of ice particles farther from the leading deep convection to the trailing stratiform region, thereby favouring the extent of this region. Copyright © 2009 Royal Meteorological Society
Highlights
The documentation of the mesoscale convective systems (MCSs) that are associated with the West African monsoon (WAM) was one of the major components of the special observing period (SOP) of the African Monsoon Multidisciplinary Analysis (AMMA) project (Redelsperger et al, 2006)
AMMA was dedicated to improve our knowledge and understanding of the WAM and its variability at daily-to-interannual time-scales and from sub-mesoscale to global scale including mesoscale and regional scale. These MCSs are closely related to synoptic-scale African easterly waves (AEW) and/or the low-level monsoon trough called the intertropical convergence zone
A mesoscale updraught within the trailing anvil cloud of the stratiform region and a mesoscale downdraught below it are found to be fed by mid-tropospheric convergence (Gamache and Houze, 1982)
Summary
The documentation of the mesoscale convective systems (MCSs) that are associated with the West African monsoon (WAM) was one of the major components of the special observing period (SOP) of the African Monsoon Multidisciplinary Analysis (AMMA) project (Redelsperger et al, 2006). MCSs in the Tropics and mid-latitudes have been described extensively in previous studies and Houze (2004) provided a complete review of their various internal structures and dynamics along with interaction with larger-scale motions, they all have regions of both convective and stratiform precipitation. Convective-scale downdraughts fed by mid-tropospheric air participate in the formation of a cold rear-to-front flow normal to the squall line (Zipser, 1969). Thermodynamic and dynamic processes are invoked to explain the development and evolution of the rear inflow This descending flow is attributed to the cooling associated with sublimation, melting and evaporation of precipitation particles (Smull and Houze, 1987; Lafore and Moncrieff, 1989) while it could be reinforced in the presence of a mid-level mesoscale vortex generated at the ends of the convective line (Skamarock et al, 1994). In a companion paper (Risi et al, 2009) the wind field analysed here is used to constrain a 2D transport model including simplified microphysical parametrizations, in order to evaluate the relative contribution of dynamics and microphysics to the evolution of the isotopic composition of the rain over the Sahelian region
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