Observations of an 11 September Sahelian Squall Line and Saharan Air Layer Outbreak during NAMMA-06

J W Smith,A Gaye,S Salack,T Fall,D Robertson,S Chan,B Klotz,D Grant,T L Battle,A Diop,A E Reynolds,M S Delonge,A S Pratt

doi:10.1155/2012/153256

Abstract

The 2006 NASA-African Monsoon Multidisciplinary Analyses (NAMMA-06) field campaign examined a compact, low-level vortex embedded in the trough of an AEW between 9–12 September. The vortex triggered a squall line (SL) in southeastern Senegal in the early morning of 11 September and became Tropical Depression 8 on 12 September. During this period, there was a Saharan Air Layer (SAL) outbreak in northwestern Senegal and adjacent Atlantic Ocean waters in the proximity of the SL. Increases in aerosol optical thicknesses in Mbour, Senegal, high dewpoint depressions observed in the Kawsara and Dakar rawinsondes, and model back-trajectories suggest the SAL exists. The close proximity of this and SL suggests interaction through dust entrainment and precipitation invigoration.

Highlights

Propagating squall lines (SLs) are frequent phenomena in the West African Sahel during the Northern Hemisphere summer [1, 2]
The development of SLs involves a combination of atmospheric conditions including conditional instability, low-level convergence, and vertical wind shear between relatively cool, moist, monsoon lower tropospheric air and warmer, drier Harmattan winds driven by the 650 hPa AEJ [3, 5]
The southern two-thirds of the SL maintained its strength. This is noted by its NPOL reflectivity and the robust lightning flashes observed by WWLLN south of 14◦N

Summary

Introduction

Propagating squall lines (SLs) are frequent phenomena in the West African Sahel during the Northern Hemisphere summer [1, 2]. This SL formed and propagated ahead of the trough region of an African Easterly Wave (AEWs) [3, 4]. The development of SLs involves a combination of atmospheric conditions including conditional instability, low-level convergence, and vertical wind shear between relatively cool, moist, monsoon lower tropospheric air and warmer, drier Harmattan winds driven by the 650 hPa AEJ [3, 5]. The trough of the AEW highlighted as “Wave 7” of the 2006 NASA-African Monsoon Multidisciplinary Analyses (NAMMA-06) [15] took on these coupled dynamical features

Objectives

Discussion

Conclusion