Attributes of mesoscale convective systems at the land‐ocean transition in Senegal during NASA African Monsoon Multidisciplinary Analyses 2006

Abstract

[1] In this study we investigate the development of a mesoscale convective system (MCS) as it moved from West Africa to the Atlantic Ocean on 31 August 2006. We document surface and atmospheric conditions preceding and following the MCS, particularly near the coast. These analyses are used to evaluate how thermodynamic and microphysical gradients influence storms as they move from continental to maritime environments. To achieve these goals, we employ observations from NASA African Monsoon Multidisciplinary Analyses (NAMMA) from the NASA S band polarimetric Doppler radar, a meteorological flux tower, upper-air soundings, and rain gauges. We show that the MCS maintained a convective leading edge and trailing stratiform region as it propagated from land to ocean. The initial strength and organization of the MCS were associated with favorable antecedent conditions in the continental lower atmosphere, including high specific humidity (18 g kg ), temperatures (300 K), and wind shear. While transitioning, the convective and stratiform regions became weaker and disorganized. Such storm changes were linked to less favorable thermodynamic, dynamic, and microphysical conditions over ocean. To address whether storms in different life-cycle phases exhibited similar features, a composite analysis of major NAMMA events was performed. This analysis revealed an even stronger shift to lower reflectivity values over ocean. These findings support the hypothesis that favorable thermodynamic conditions over the coast are a prerequisite to ensuring that MCSs do not dissipate at the continental-maritime transition, particularly due to strong gradients that can weaken West African storms moving from land to ocean.

Radar characteristics of continental, coastal, and maritime convection observed during AMMA/NAMMA

“…More vertically developed storms display a greater propensity for mixed-phase processes (DeMott and Rutledge, 1998;Nesbitt et al, 2006), enhancing the stratiform region and leading to larger precipitation contribution over the course of the study from deep convection observed over the continent (Figure 9(e)-(f)). DeLonge et al (2010) showed that MCSs transitioning from land to ocean exhibit signs of disorganization, resulting in less intense convective characteristics over the ocean. Figure 4 indicates that storms at the coast experienced a higher likelihood to enter a region with higher CAPE, which would theoretically produce stronger updrafts and significant lofting of precipitationsized particles.…”

“…Land-to-ocean transitioning MCSs (coastal site) have been shown to diminish in strength (e.g. DeLonge et al, 2010) due to less favourable thermodynamic (e.g. lower specific humidity) and dynamic (e.g.…”

“…Mesoscale convective systems (MCSs) are primary contributors of rainfall totals in West Africa (Le Barbé and Lebel, 1997;D'Amato and Lebel, 1998;Laurent et al, 1998;Mathon et al, 2002;Redelsperger et al, 2002;Fink et al, 2006), despite their small contribution to convective system population numbers (Mohr et al, 1999). Highly organized continental MCSs often take the form of squall lines (SLs) (Hamilton et al, 1945;Eldridge, 1957;Bolton, 1984;Houze et al, 1989), while systems in the adjacent east Atlantic region are often more stratiform in nature Houze, 2003, 2006;Fuentes et al, 2008), with a transitional region for systems exiting the West African coast (Sall and Sauvageot, 2005;DeLonge et al, 2010).…”

Regional comparison of West African convective characteristics: a TRMM‐based climatology

“…This instability leads to the formation and maintenance of (often intense) MCSs (Aspliden et al, 1976;Payne and McGarry, 1977;Houze and Betts, 1981;Barnes and Sieckman, 1984;Rowell and Milford, 1993;Hodges and Thorncroft, 1997;Mohr and Thorncroft, 2006;Nicholls and Mohr, 2010). These westward-moving systems generally exhibit a linear (squall line) morphology over the continent (Hamilton et al, 1945;Eldridge, 1957;Bolton, 1984), a non-squall (amorphous) morphology over the eastern Atlantic Houze, 2003, 2006;Fuentes et al, 2008), and a transition stage upon exiting the coast (Sall and Sauvageot, 2005;DeLonge et al, 2010).…”

Observations of Coastally Transitioning West African Mesoscale Convective Systems during NAMMA

“…During the summer of 2006, the African Monsoon Multidisciplinary Analysis (AMMA) international field campaign and NASA-AMMA (NAMMA) collaborated to provide an observation network of various platforms stretching from Nigeria to the Cape Verde Islands [30,31]. Several studies have used the NAMMA data to examine the AEWs and MCSs moving off the African coast (e.g., [32][33][34][35]). In addition, one of the three main goals of AMMA-NAMMA was to determine a relationship between AEWs and MCSs that could provide insight into TC genesis.…”