Guirado C.,University of Valladolid |
Guirado C.,State Meteorological Agency of Spain AEMET |
Cuevas E.,State Meteorological Agency of Spain AEMET |
Cachorro V.E.,University of Valladolid |
And 14 more authors.
Atmospheric Chemistry and Physics | Year: 2014
More than 2 years of columnar atmospheric aerosol measurements (2006-2009) at the Tamanrasset site (22.79° N, 5.53° E, 1377 m a.s.l.), in the heart of the Sahara, are analysed. Aerosol Robotic Network (AERONET) level 2.0 data were used. The KCICLO (K is the name of a constant and ciclo means cycle in Spanish) method was applied to a part of the level 1.5 data series to improve the quality of the results. The annual variability of aerosol optical depth (AOD) and Ångström exponent (AE) has been found to be strongly linked to the convective boundary layer (CBL) thermodynamic features. The dry-cool season (autumn and winter) is characterized by a shallow CBL and very low mean turbidity (AOD ∼0.09 at 440 nm, AE ∼0.62). The wet-hot season (spring and summer) is dominated by high turbidity of coarse dust particles (AE ∼0.28, AOD ∼0.39 at 440 nm) and a deep CBL. The aerosol-type characterization shows desert mineral dust as the prevailing aerosol. Both pure Saharan dust and very clear sky conditions are observed depending on the season. However, several case studies indicate an anthropogenic fine mode contribution from the industrial areas in Libya and Algeria. The concentration weighted trajectory (CWT) source apportionment method was used to identify potential sources of air masses arriving at Tamanrasset at several heights for each season. Microphysical and optical properties and precipitable water vapour were also investigated. © Author(s) 2014.
Donat M.G.,University of New South Wales |
Peterson T.C.,National Oceanic and Atmospheric Administration |
Brunet M.,Rovira i Virgili University |
Brunet M.,University of East Anglia |
And 21 more authors.
International Journal of Climatology | Year: 2014
A workshop was held in Casablanca, Morocco, in March 2012, to enhance knowledge of climate extremes and their changes in the Arab region. This workshop initiated intensive data compilation activities of daily observational weather station data from the Arab region. After conducting careful control processes to ensure the quality and homogeneity of the data, climate indices for extreme temperatures and precipitation were calculated. This study examines the temporal changes in climate extremes in the Arab region with regard to long-term trends and natural variability related to ENSO and NAO. We find consistent warming trends since the middle of the 20th Century across the region. This is evident in the increased frequencies of warm days and warm nights, higher extreme temperature values, fewer cold days and cold nights and shorter cold spell durations. The warming trends seem to be particularly strong since the early 1970s. Changes in precipitation are generally less consistent and characterised by a higher spatial and temporal variability; the trends are generally less significant. However, in the western part of the Arab region, there is a tendency towards wetter conditions. In contrast, in the eastern part, there are more drying trends, although, these are of low significance. We also find some relationships between climate extremes in the Arab region and certain prominent modes of variability, in particular El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO). The relationships of the climate extremes with NAO are stronger, in general, than those with ENSO, and are particularly strong in the western part of the Arab region (closer to the Atlantic Ocean). The relationships with ENSO are found to be more significant towards the eastern part of the area of study. © 2013 Royal Meteorological Society.
Guirado C.,University of Valladolid |
Guirado C.,Meteorological State Agency of Spain AEMET |
Cuevas E.,Meteorological State Agency of Spain AEMET |
Cachorro V.,University of Valladolid |
And 10 more authors.
Optica Pura y Aplicada | Year: 2011
A Cimel sun photometer has been in operation at Tamanrasset station since late 2006. In this study, more than two years of aerosol measurements have been analyzed from October 2006 to January 2009. Two parameters, aerosol optical depth (AOD) and Ångström exponent (AE), have been used for this preliminar characterization. At this station, the mean AOD is 0.25±0.15 and the mean AE is 0.48±0.23. Both time series data show a clear seasonal cycle. A dry-cool season (fall and winter time), characterized by low AOD and high AE values, and a wet-hot season (in spring-summer), with strong and frequent mineral dust storms, giving high AOD and low AE values, are observed at Tamanrasset. Both, AOD and AE values show the behaviour of a station where desert mineral dust is the prevailing aerosol defining the characteristic of the site. However a significant number of episodes with AE values around 1 together with AOD greater than 0.2 have been found, what suggests the presence of pollution derived aerosols. © Sociedad Española de Óptica.
Hobby M.,University of Leeds |
Gascoyne M.,University of Leeds |
Marsham J.H.,University of Leeds |
Bart M.,University of Leeds |
And 16 more authors.
Journal of Atmospheric and Oceanic Technology | Year: 2013
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. © 2013 American Meteorological Society.
Evan A.T.,University Pierre and Marie Curie |
Evan A.T.,University of California at San Diego |
Flamant C.,University Pierre and Marie Curie |
Lavaysse C.,U.S. National Center for Atmospheric Research |
And 2 more authors.
Journal of Climate | Year: 2015
The 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. © 2015 American Meteorological Society.
Lavaysse C.,University Pierre and Marie Curie |
Eymard L.,University Pierre and Marie Curie |
Flamant C.,University Pierre and Marie Curie |
Karbou F.,Meteo - France |
And 2 more authors.
Atmospheric Science Letters | Year: 2013
The Saharan heat low (SHL) is defined as a stationary thermal depression below 700hPa located over the Sahara during the boreal summer season. This feature has a significant impact on the seasonal and intra-seasonal variability of rainfall over the Sahel. For various reasons, few observations are assimilated in operational numerical weather prediction models, especially surface sensitive observations. In this study we make use of the advanced microwave sounding unit (AMSU-A) brightness temperatures measured at 52.8GHz (noted A4 hereafter) and at 53.6GHz (noted A5 hereafter) to detect the heat low and to characterize its intensity at the seasonal and intra-seasonal scales. In terms of seasonal variability, AMSU-based estimates of SHL are found in good agreement with those based on the European Center Medium-range Weather Forecast ERA-I reanalyses for the period ranging from 2000 to 2011. However, differences exist between the two datasets at intraseasonal timescale. They may be related to mismatches between the numerical weather prediction model levels and the level of maximum of sensitivity of the AMSU observations. Particular meteorological situations can also explain some differences between the two types of products, as there is a lack of assimilated observations in this region. Finally, we show that the A4 brightness temperature is a suitable proxy to provide a good estimate of the location and the intensity of the SHL from daily to seasonal timescales. © 2013 Royal Meteorological Society.
Cuesta J.,Ecole Polytechnique - Palaiseau |
Cuesta J.,Institute Pierre Simon Laplace |
Lavaysse C.,Institute Pierre Simon Laplace |
Flamant C.,Institute Pierre Simon Laplace |
And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2010
The Hoggar Massif is a comparatively populated region in the Sahara, where water supply is a critical problem due to the lack of nearby sources and unaffordable water pumping. In the present paper, we analyse the influence of the West African monsoon (WAM) on precipitation over the Hoggar during summer. We investigate (1) two rainfall events during 23-27 July 2006, accounting for almost half of the precipitation of this year, and (2) the representativity of this period with regard to ERA-40 ECMWF re-analyses (1979 to 2001). By the end of July 2006, two consecutive northward bursts of the WAM flow reached the Hoggar and caused an increase in low-level humidity. In the afternoon of these days, clouds formed at the top of the convective boundary layer and rapidly grew to more than ∼9 km above mean sea level. Due to the comparatively moist sub-cloud layer (>45% mean relative humidity), considerable amounts of precipitation could reach the ground. The strong southerly WAM flow one day before and during the arrival of the moist air over the Hoggar is associated with both a south sector of an African easterly wave (AEW) and the cyclonic circulation around the south-eastern flank of the Saharan heat low (SHL). In addition, the northward excursions of the monsoon were supported by convective cold-pool outflows originating over Niger. The climatological analysis confirms the relation between precipitation over the Hoggar and AEW south sectors and also shows a conspicuous weakening of the SHL following rainfall. © 2009 Royal Meteorological Society.
Allen C.J.T.,University of Oxford |
Washington R.,University of Oxford |
Saci A.,Office National de la Meteorologie
Journal of Geophysical Research D: Atmospheres | Year: 2015
In boreal summer, the central Sahara is the dustiest place on Earth. Fennec supersite 1 is located within the dust maximum. With two field seasons completed, June 2011 (intensive observation period (IOP) 1) and June 2012 (IOP2), we now have an initial measure of their representativeness. Although the number of dust event hours in IOP2 is up to 169-h (60%) more than in IOP1, the relative importance of the different dust mechanisms is the same. In both years, emission by cold pool outflows from deep convection causes most dust, followed by dust advection, monsoon surges, low-level jet emission and lastly dry convective plumes. Given the dominance of cold pools, it is very important that they be incorporated in dust modeling efforts over the region. Because cold pools frequently occur at night and are associated with cloud cover, instruments that can monitor in these conditions are particularly valuable in this region. Sun photometer aerosol optical depth retrievals, for example, are only available for 23% (41%) of the time cold pool emission occurs at the supersite in IOP1 (IOP2). Deployment of instrumentation in remote regions being difficult and expensive, choosing the optimal instrument payload pays dividends. With this motivation, we evaluate the different dust detection instrumentation deployed at the supersite and develop an approach which can identify and characterize individual dust emission mechanisms with a high degree of success purely from routine meteorological observations and remote sensing. This identification is, however, much more challenging in the case of dust advection. Key Points Central Saharan dust storms of June 2011 and 2012 compared from observations Relative importance of dust mechanisms the same with cold pools dominating Dust detection field instrumentation evaluated and implications discussed ©2015. The Authors.