The Saharan heat low and moisture transport pathways in the central Sahara—Multiaircraft observations and Africa‐LAM evaluation

S Engelstaedter,C Flamant,M C Todd,C J T Allen,D J Parker,R Washington

doi:10.1002/2015jd023123

Abstract

AbstractWe present a characterization of the Saharan heat low (SHL) based on dropsonde observations made on 22 June 2011 by two simultaneously flying aircraft during the Fennec project. The observations are used to identify moisture transport pathways and to validate the UK Met Office limited area model for northern Africa (Africa‐LAM). The observations capture the SHL, harmattan, and monsoon surge. The SHL has a northeast‐southwest orientated elongated shape centered over northern Mauritania. The SHL core is associated with a 950 hPa temperature minimum (36.4°C) in the morning caused by the monsoon surge and a maximum (42.6°C) in the afternoon. The monsoon surge east of the SHL core splits into two transport pathways: (a) curving around the SHL core in the north, especially pronounced in a morning near‐surface layer, and (b) northeastward transport within the ~2 km deep monsoon surge (afternoon observations only). In the morning the model forecasts the harmattan, monsoon surge, and the SHL geographic location and northeast‐southwest orientation well but the model represents the SHL flatter and more spatially extended and overestimates the convective boundary layer (CBL) by up to ~0.3 km. The simulated afternoon SHL location appears shifted westward by up to ~1°. The model overestimates the shallow afternoon monsoon surge CBL depth of ~1.8 km by >2 km resulting in southwestward transport of vertically mixed moisture above ~2.5 km contrasting observed northeastward‐only transport at lower levels. This moisture distribution model error is likely to have consequences for simulations of Saharan thermodynamics and dust emissions caused by convection‐driven cold pools.

Highlights

The Saharan heat low (SHL) has been identified as an important climatic system that interacts with the midlatitudes [Knippertz, 2007; Chauvin et al, 2010; Lavaysse et al, 2010a; Roehrig et al, 2011], the West African monsoon system [Sultan and Janicot, 2003; Parker et al, 2005; Peyrille et al, 2007], and the northern tropical Atlantic, [Grams et al, 2010; Evan et al, 2011] highlighting its role as a significant climatic driver on regional to continental scales [Lafore et al, 2011]
Bou Karam et al [2014] present a detailed analysis of a synoptic-scale 3 day dust event in August 2006 caused by a mesoscale convective system cold pool estimating an average total dust load of 1.5 Tg. These results indicate that SHL-associated moisture transport into the hyperarid central Sahara plays a critical role fuelling this major dust emission mechanism which could in part explain the co-location of the SHL
While the observed location of the SHL core identified in the level atmospheric thickness (LLAT) field does not change between morning and afternoon (Figures 5a and 5c), the level cyclonic circulation (LLCC) observed in the WSPD950 field (Figure 5e) and GPH950 field indicates that the SHL core at lower altitude may have moved westward between the morning and afternoon by about 1°

Summary

Introduction

The SHL has been identified as an important climatic system that interacts with the midlatitudes [Knippertz, 2007; Chauvin et al, 2010; Lavaysse et al., 2010a; Roehrig et al, 2011], the West African monsoon system [Sultan and Janicot, 2003; Parker et al, 2005; Peyrille et al, 2007], and the northern tropical Atlantic, [Grams et al, 2010; Evan et al, 2011] highlighting its role as a significant climatic driver on regional to continental scales [Lafore et al, 2011]. The seasonal and intraseasonal variabilities of the SHL seem to be linked to the onset, temporal evolution, and strength of the West African Monsoon, which affects the livelihood of millions of people in the Sahel [Sultan and Janicot, 2003; Lavaysse et al, 2010b; Roehrig et al, 2011]. Synoptic-scale oscillations in SHL intensity can alter the intraseasonal variability of Sahelian precipitation; during a strong (warm) phase of the SHL, there is an intensification of the SHL low-level cyclonic circulation, favoring increased deep convection in the central and eastern Sahel because of the anomalous surface southerlies and convection suppression in the western Sahel due to the anomalous surface northerlies [Lavaysse et al, 2010b]

Objectives

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Discussion

Conclusion