Advances in understanding mineral dust and boundary layer processes over the Sahara from Fennec aircraft observations

Abstract

International audienceThe Fennec climate program aims to improve understanding of the Saharan climate system through a synergy of observations and modelling. We present a description of the Fennec airborne observations during 2011 and 2012 over the remote Sahara (Mauritania and Mali) and the advances in the understanding of mineral dust and boundary layer processes they have provided. Aircraft instrumentation aboard the UK FAAM BAe146 and French SAFIRE Falcon 20 is described, with specific focus on instrumentation specially developed and relevant to Saharan meteorology and dust. Flight locations, aims and associated meteorology are described. Examples and applications of aircraft measurements from the Fennec flights are presented, highlighting new scientific results delivered using a synergy of different instruments and aircraft. These include: (1) the first airborne measurement of dust particles sized up to 300 microns and associated dust fluxes in the Saharan atmospheric boundary layer (SABL), (2) dust uplift from the breakdown of the nocturnal low-level jet before becoming visible in SEVIRI satellite imagery, (3) vertical profiles of the unique vertical structure of turbulent fluxes in the SABL, (4) in-situ observations of processes in SABL clouds showing dust acting as CCN and IN at −15 °C, (5) dual-aircraft observations of the SABL dynamics, thermodynamics and composition in the Saharan heat low region (SHL), (6) airborne observations of a dust storm associated with a cold-pool (haboob) issued from deep convection over the Atlas, (7) the first airborne chemical composition measurements of dust in the SHL region with differing composition, sources (determined using Lagrangian backward trajectory calculations) and absorption properties between 2011 and 2012, (8) coincident ozone and dust surface area measurements suggest coarser particles provide a route for ozone depletion, (9) discrepancies between airborne coarse mode size distributions and AERONET sunphotometer retrievals under light dust loadings. These results provide insights into boundary layer and dust processes in the SHL region – a region of substantial global climatic importance

The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

“…A distinction can be made between the cooler, drier, less dusty Saharan "maritime" phase from around 8 to 12 June to a hotter, moister, dustier "heat low" phase from around 13 to 30 June (Fig. 2a), during which time both synoptic-scale monsoon surges and mesoscale convective cold-pool events transported both water vapour and dust into the heart of the SHL (see Ryder et al, 2015;Todd et al, 2013 for full details). For comparison, profiles of water vapour from ERA-I reanalysis are shown in Fig.…”

“…We use observations from ground-based instruments deployed at the Fennec supersite at BBM and various aircraft flights (see Ryder et al, 2015 for an overview) complemented with fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis (ERA-I) (Dee et al, 2011) and Modern-Era Retrospective analysis for Research and Application (MERRA) (Rienecker et al, 2011) reanalysis, where direct observations are inadequate.…”

A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

“…After the monsoon onset, the midtropospheric circulation over West Africa is dominated by the African easterly jet (AEJ), which is caused by the strong meridional temperature and moisture gradient at low levels (Cook, 1999;Wu et al, 2009). The AEJ is maintained by the anticyclonic circulation associated with the monsoonal subsidence, which characterises the mid-upper troposphere over the Sahara (Chen, 2005;Thorncroft and Blackburn, 1999) and above the shallow dry convection in the SHL (GarciaCarreras et al, 2015;Ryder et al, 2015). Barotropic and baroclinic instabilities associated with the jet create an environment favourable to the generation of African easterly waves (AEWs) (Thorncroft and Hoskins, 1994a, b;Wu et al, 2012), synoptic-scale disturbances characterised by a 2-6-day period in the Sahel.…”

Long-term profiling of mineral dust and pollution aerosol with multiwavelength polarization Raman lidar at the Central Asian site of Dushanbe, Tajikistan: case studies

“…More observations, especially in western and Central Asia, are needed to describe global and regional dust transport and to estimate the effect of this dust on air quality (Chin et al, 2007), and climate, via direct and various indirect radiative effects. Extended investigations into Saharan dust close to its source regions (e.g., SAMUM-1, 2, Saharan Mineral Dust Experiment; Fennec climate programm; and SHADOW, Study of SaHAran Dust Over West Africa) (Heintzenberg, 2009;Ansmann et al, 2011a;Ryder et al, 2015;Veselovskii et al, 2016) as well as regarding dust longrange transport across the Atlantic ocean (e.g., SALTRACE, Saharan Aerosol Long-range Transport and Aerosol-CloudInteraction Experiment; Weinzierl et al, 2017) have been conducted to obtain novel data to reduce uncertainties in the above-mentioned estimations. However, the global dust belt, which reaches from the Sahara over the Arabian deserts to the Taklamakan and Gobi deserts, contains a lot more arid and semi-arid regions which act as sources for atmospheric mineral dust in the northern hemisphere (Darmenova et al, 2009;Ridley et al, 2016).…”