Water Vapor–Forced Greenhouse Warming over the Sahara Desert and the Recent Recovery from the Sahelian Drought

International audienceThe Sahel region of West Africa experiences decadal swings between periods of drought and abundant rainfall, and a large body of work asserts that these variations in the West African monsoon are a response to changes in the temperatures of the tropical Atlantic and Indian Oceans. However, here it is shown that when forced by SST alone, most state-of-the-art climate models do not reproduce a statistically significant upward trend in Sahelian precipitation over the last 30 years and that those models with a significant upward trend in rainfall seem to achieve this result for disparate reasons. Here the role of the Saharan heat low (SHL) in the recovery from the Sahelian drought of the 1980s is examined. Using observations and reanalyses, it is demonstrated that there has been an upward trend in SHL temperature that is coincident with the drought recovery. A heat and moisture budget analysis of the SHL suggests that the rise in temperature is due to greenhouse warming by water vapor, but that changes in water vapor are strongly dependent upon the temperature of the SHL: a process termed the Saharan water vapor–temperature (SWAT) feedback. It is shown that the structure of the drought recovery is consistent with a warming SHL and is evidence of a fundamental, but not exclusive, role for the SHL in the recent increase in Sahelian monsoon rainfall

An overview of observations from Fennec supersites: Dust uplift, transport and impacts

International audienceThe summertime Sahara is the world's largest dust source. We present the first detailed in-situ meteorological observations from the central Sahara, made during June 2011 at Fennec supersite 1 (SS1, in the central Sahara) and SS2 (towards the western edge of the Saharan Heat Low, SHL). AODs at SS1 varied from approximately 0.2 to 4, with significant local dust uplift. AODs were much lower at SS2, generally below 1.0, but reaching 2.5. Most dust was in the second half of June, when the SHL was shifted westwards, leading to stronger low-level jets (LLJs) and cold-pool outflows from moist convection at the SSs. At SS1, this meant that most dust was observed in moist periods, when the site was close to the leading edge of the monsoon. The deep boundary layer in late June allowed dust to be mixed upwards to approximately 5 km. The impact of dust on peak downward shortwave radiation amounted to 100 to 200 W/m2 per AOD. The more complete data at SS1 show that there the variations in daily mean downward shortwave can be explained by variations in dust (correlation -0.96), or clouds (0.80), since clouds and dust are correlated (0.7). A more complex analysis is therefore required to separate the impacts of clouds and dust on the energy balance in the SHL. Wind-speed and nephelometer data provide a quantification of the roles of different meteorological mechanisms to dust uplift at SS1. Here, approximately 55% of uplift occurred at night, largely from cold-pool outflows which caused approximately 50% of the total uplift. Both large outflows ("haboobs") and more microburst-like events were observed, with haboobs dominating uplift and AODs. Approximately 15% of uplift occurred between 12 and 18 UTC and approximately 30% as momentum from the nocturnal low-level jet (LLJ) was mixed to the surface between 06 and 12 UTC. The same mechanisms were observed at SS2, but there, before the SHL moved west, the LLJ was often weakened by the Atlantic Inflow and throughout June haboobs were both much rarer and weaker that at SS1. As a result, wind-speeds, uplift frequencies and uplift intensities are all lower at SS2 and increased AODs were typically from dust advected from sources upstream, mostly from enhanced northeasterly LLJ events, rather than from local dust uplift. Global models are known to represent LLJs poorly, and cold-pool outflows are often almost entirely missing. The data show that this is expected to lead to major biases in all global dust models. The results show that much dust uplift occurs under clouds, in moist air, or at night, where satellite retrievals are either impossible or subject to increased errors. Thus all satellite climatologies are subject to important sampling biases

Fennec - The Saharan Climate System: Project Overview

International audienceThe central Sahara forms an important part of the global climate system.During the northern summer months, the Saharan Heat Low (SHL), caused by intense solar heating, develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. Dry convection through more than 5000m of the atmosphere, is routine in what is thought to be the deepest such layer on the planet. The SHL also co-locates with the largest loadings of dust anywhere in the Earth's atmosphere, making for a complex yet crucial component of WAM. Much of what is known about the SHL derives from numerical models rather than observations although it is widely accepted that such models show significant systematic errors over the Sahara desert manifested as differences in radiation reaching and leaving the surface, surface temperature, winds, dust and in representation of the boundary layer. In an effort to address the observational deficit in the region, as well as to improve model performance, the Fennec project is a large scale, multi-platform, extended duration observational campaign in the Saharan Heat Low (SHL) region. During the summers of 2011and 2012 a major campaign set about addressing the data deficiency of this important region. The campaign, which involved many more people than are indicated by the authorship of this abstract, featured the use of the instrumented BAe-146 and Falcon aircraft as well as supersite ground-stations both Algeria and Mauritania. The purpose of this overview is to describe project aims, institutional involvement and key elements of the observational campaign, particularly as it relates to the effort to understand dust over the region

The fennec automatic weather station (AWS) network: Monitoring the Saharan climate system

The Fennec automatic weather station (AWS) network consists of eight stations installed across the Sahara, with four in remote locations in the central desert, where no previous meteorological observations have existed. The AWS measures temperature, humidity, pressure, wind speed, wind direction, shortwave and longwave radiation (upwelling and downwelling), ground heat flux, and ground temperature. Data are recorded every 3 min 20 s, that is, at 3 times the temporal resolution of the World Meteorological Organization's standard 10-min reporting for winds and wind gusts. Variations in wind speeds on shorter time scales are recorded through the use of second- and third-order moments of 1-Hz data. Using the Iridium Router- Based Unrestricted Digital Internetworking Connectivity Solutions (RUDICS) service, data are transmitted in near-real time (1-h lag) to the United Kingdom, where calibrations are applied and data are uploaded to the Global Telecommunications System (GTS), for assimilation into forecast models. This paper describes the instrumentation used and the data available from the network. Particular focus is given to the engineering applied to the task of making measurements in this remote region and challenging climate. The communications protocol developed to operate over the Iridium RUDICS satellite service is described. Transmitting the second moment of the wind speed distribution is shown to improve estimates of the dust-generating potential of observed winds, especially for winds close to the threshold speed for dust emission of the wind speed distribution. Sources of error are discussed and some preliminary results are presented, demonstrating the system's potential to record key features of this region