Climatology of convective density currents in the southern foothills of the Atlas Mountains

[1] Density currents fed by evaporationally cooled air are an important dust storm–generating feature and can constitute a source of moisture in arid regions. Recently, the existence of such systems has been demonstrated for the area between the High Atlas Mountains and the Sahara desert in southern Morocco on the basis of case studies. Here, a climatological analysis is presented that uses data from the dense climate station network of the IMPETUS project (An Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa) for the 5 year period 2002–2006. Objective criteria mainly based upon abrupt changes in wind and dew point temperature are defined to identify possible density current situations. The preselected events are then subjectively evaluated with the help of satellite imagery and surface observations to exclude causes for air mass changes other than moist convective cold pool formation. On average, 11 ± 4 density currents per year are detected with the main season lasting from April to September. Density currents occur mainly in the afternoon and evening due to the diurnal cycle of moist convection. Mean changes at the leading edge are increases in 2 m dew point temperature and wind speed by 5.4°C and 8.2 m s−1, respectively, and a decrease in 2 m air temperature of 2.3°C. The High Atlas and Jebel Saghro are found to be the most important source regions, while only a few systems originate over the Saharan lowlands. Labilization of the atmosphere due to upper-level troughs over northwest Africa and an enhanced moisture content favor density current formation. In addition, detailed case studies representative of different density current types are presented.

[1]  D. Fryrear,et al.  Sedimentary characteristics of a haboob dust storm , 2002 .

[2]  N. Middleton,et al.  Saharan dust storms: nature and consequences , 2001 .

[3]  Morris L. Weisman,et al.  “A Theory for Strong Long-Lived Squall Lines” Revisited , 2004 .

[4]  M. Kerschgens,et al.  Downscaling of current and future rainfall climatologies for southern Morocco. Part I: Downscaling method and current climatology , 2007 .

[5]  Christian M. Grams,et al.  Uplift of Saharan dust south of the intertropical discontinuity , 2008 .

[6]  S. B. Idso,et al.  An American Haboob , 1972 .

[7]  Jacques Pelon,et al.  Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter‐tropical discontinuity region of the West African Monsoon , 2007 .

[8]  Axel Lauer,et al.  © Author(s) 2006. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics Analysis and quantification of the diversities of aerosol life cycles , 2022 .

[9]  D. Goossens,et al.  Ten years of aeolian dust dynamics in a desert region (Negev desert, Israel): analysis of airborne dust concentration, dust accumulation and the high-magnitude dust events , 2001 .

[10]  K. Hamilton.  Gravity Currents in the Environment and the Laboratory , 1998 .

[11]  Michael Garstang,et al.  Saharan dust in the Amazon Basin , 1992 .

[12]  M. Majewski,et al.  African and Asian Dust: From Desert Soils to Coral Reefs , 2003 .

[13]  P. Knippertz,et al.  Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa , 2003 .

[14]  M. Lawrence.  The relationship between relative humidity and the dewpoint temperature in moist air - A simple conversion and applications , 2005 .

[15]  T. Lawson,et al.  HABOOB STRUCTURE AT KHARTOUM , 1971 .

[16]  P. Knippertz.  NOTES AND CORRESPONDENCE Tropical-Extratropical Interactions Causing Precipitation in Northwest Africa: Statistical Analysis and Seasonal Variations , 2003 .

[17]  M. Wendisch,et al.  Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview , 2009 .

[18]  Timothy D. Jickells,et al.  Air‐borne dust fluxes to a deep water sediment trap in the Sargasso Sea , 1998 .

[19]  Richard T. Barber,et al.  African dust and the demise of Caribbean Coral Reefs , 2000 .

[20]  Nick Middleton,et al.  Saharan dust: sources and trajectories , 2001 .

[21]  P. Knippertz,et al.  High-resolution simulations of convective cold pools over the northwestern Sahara , 2009 .

[22]  C. de Jong,et al.  Snowmelt and sublimation: field experiments and modelling in the High Atlas Mountains of Morocco , 2004 .

[23]  P. Knippertz.  Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa: Statistical Analysis and Seasonal Variations , 2003 .

[24]  A. Ansmann,et al.  Vertical profiling of convective dust plumes in southern Morocco during SAMUM , 2009 .

[25]  J. Reid,et al.  Haboob dust storms of the southern Arabian Peninsula , 2008 .

[26]  J. Heintzenberg.  The SAMUM-1 experiment over Southern Morocco: overview and introduction , 2009 .

[27]  Michael Manga,et al.  Gravity Currents in the Environment and the Laboratory, by John E. Simpson , 1999 .

[28]  Downscaling of current and future rainfall climatologies for southern Morocco. Part II: Climate change signals , 2007 .

[29]  N. Renno,et al.  The role of convective plumes and vortices on the global aerosol budget , 2004 .

[30]  Q. Min,et al.  Evidence of mineral dust altering cloud microphysics and precipitation , 2008 .

[31]  E. Williams,et al.  The electrification of dust-lofting gust fronts (‘haboobs’) in the Sahel , 2009 .

[32]  Konrad Kandler,et al.  Dust mobilization due to density currents in the Atlas region: Observations from the Saharan Mineral Dust Experiment 2006 field campaign , 2007 .

[33]  Masson-Delmotte,et al.  The Physical Science Basis , 2007 .

[34]  D. Membery.  a Gravity-Wave Haboob? , 1985 .

[35]  P. Kållberg,et al.  The ‘yellow snowepisode’ of northern Fennoscandia, march 1991—A case study of long-distance transport of soil, pollen and stable organic compounds , 1994 .

[36]  Roger K. Smith,et al.  On the Movement and Low-Level Structure of Cold Fronts , 1988 .

[37]  R. Betts,et al.  Changes in Atmospheric Constituents and in Radiative Forcing. Chapter 2 , 2007 .

[38]  M. Freeman.  Duststorms of the Anglo-Egyptian Sudan , 1952 .

[39]  M. Legrand,et al.  Determination of the wind speed threshold for the emission of desert dust using satellite remote sensing in the thermal infrared , 1999 .

[40]  R. Rotunno,et al.  A Theory for Strong, Long-Lived Squall Lines , 1988 .