How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa

Abstract. This study presents the first quantitative estimate of the mineral dust emission associated with atmospheric depressions and mobile cyclones in North Africa. Atmospheric depressions are automatically tracked at 925 hPa based on ERA-Interim data from the European Centre for Medium-Range Weather Forecasts for 1989–2008. A set of filter criteria is applied to identify mobile cyclones, i.e. migrating and long-lived cyclones. The shorter term cyclone is used as a synonym for mobile cyclones. Dust emission is calculated with a dust emission model driven by 10 m winds and soil moisture from ERA-Interim. Emission peaks during winter and spring with spatial averages of 250–380 g m−2 per month. Comparison of the dust source activation frequency from the model against SEVIRI satellite observation shows a good agreement in the Bodele Depression but differences in the north and west of North Africa. Depressions are abundant, particularly in summer when the Saharan heat low is situated over West Africa and during spring in the lee of the Atlas Mountains. Up to 90% (55% annually and spatially averaged) of dust emission occurs within 10 degrees of these depressions, with embedded mechanisms such as nocturnal low-level jets playing a role. Cyclones are rarer and occur primarily north of 20° N in spring in agreement with previous studies and over summertime West Africa consistent with near-surface signatures of African Easterly Waves. Dust emission within 10 degrees of cyclones peaks over Libya with up to 25% in spring. Despite the overall small contribution of 4% annually and spatially averaged, cyclones coincide with particularly intense dust emission events exceeding the climatological mean by a factor of four to eight. Soil moisture weakens dust emission during cyclone passage by about 10%.

[1]  G. Bergametti,et al.  Parametrization of the increase of the aeolian erosion threshold wind friction velocity due to soil moisture for arid and semi-arid areas , 1999 .

[2]  L. Bengtsson,et al.  A Comparison of Extratropical Cyclones in Recent Reanalyses ERA-Interim, NASA MERRA, NCEP CFSR, and JRA-25 , 2011 .

[3]  C. Anagnostopoulou,et al.  A 40 year objective climatology of surface cyclones in the Mediterranean region: spatial and temporal distribution , 2001 .

[4]  Iliana Genkova,et al.  Impacts of Assimilating AMMA Soundings on ECMWF Analyses and Forecasts , 2010 .

[5]  J. Cuesta,et al.  Dust emission and transport associated with a Saharan depression: February 2007 case , 2010 .

[6]  Chao Luo,et al.  The Role of Easterly Waves on African Desert Dust Transport , 2003 .

[7]  Peter Knippertz,et al.  The importance of the representation of deep convection for modeled dust‐generating winds over West Africa during summer , 2011 .

[8]  N. Mahowald,et al.  Temporal variability of dust mobilization and concentration in source regions , 2004 .

[9]  M. Klose,et al.  Sahel dust zone and synoptic background , 2010 .

[10]  Robert W. Burpee.  The Origin and Structure of Easterly Waves in the Lower Troposphere of North Africa , 1971 .

[11]  Andreas H. Fink,et al.  Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak , 2006 .

[12]  Kevin I. Hodges,et al.  African Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity , 2001 .

[13]  D. Stensrud,et al.  Sensitivities of an intense Mediterranean cyclone: Analysis and validation , 2004 .

[14]  Yi Wang,et al.  Meteorological and dust aerosol conditions over the western Saharan region observed at Fennec Supersite‐2 during the intensive observation period in June 2011 , 2013 .

[15]  V. Masson,et al.  Satellite climatology of African dust transport in the Mediterranean atmosphere , 1998 .

[16]  U. Lohmann,et al.  The global aerosol-climate model ECHAM-HAM, version 2: sensitivity to improvements in process representations , 2012 .

[17]  V. Freudenthaler,et al.  Regional modelling of Saharan dust and biomass-burning smoke , 2011 .

[18]  M. Heimann,et al.  Impact of vegetation and preferential source areas on global dust aerosol: Results from a model study , 2002 .

[19]  B. Marticorena,et al.  Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme , 1995 .

[20]  Andreas Macke,et al.  Meteorological processes forcing Saharan dust emission inferred from MSG-SEVIRI observations of subdaily dust source activation and numerical models , 2009 .

[21]  A. Culf.  An application of simple models to Sahelian convective boundary-layer growth , 1992 .

[22]  Helen Brindley,et al.  A critical evaluation of the ability of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) thermal infrared red-green-blue rendering to identify dust events: theoretical analysis , 2012 .

[23]  C. Flamant,et al.  Dry cyclogenesis and dust mobilization in the intertropical discontinuity of the West African Monsoon: A case study , 2009 .

[24]  Richard Washington,et al.  A new high‐resolution central and western Saharan summertime dust source map from automated satellite dust plume tracking , 2013 .

[25]  C. Ramis,et al.  A deep cyclone of African origin over the Western Mediterranean: diagnosis and numerical simulation , 2002 .

[26]  Isabel F. Trigo,et al.  Climatology of Cyclogenesis Mechanisms in the Mediterranean , 2002 .

[27]  Jacques Pelon,et al.  Seasonal evolution of the West African heat low: a climatological perspective , 2009 .

[28]  Kerstin Schepanski,et al.  Comparison of satellite based observations of Saharan dust source areas , 2012 .

[29]  B. Weinzierl,et al.  A model study of Saharan dust emissions and distributions during the SAMUM-1 campaign , 2010 .

[30]  K. Schepanski,et al.  A new Saharan dust source activation frequency map derived from MSG‐SEVIRI IR‐channels , 2007 .

[31]  P. Knippertz,et al.  The central west Saharan dust hot spot and its relation to African easterly waves and extratropical disturbances , 2010 .

[32]  C. Thorncroft,et al.  Analysis of convection and its association with African easterly waves , 2006 .

[33]  Wim Cornelis,et al.  The effect of surface moisture on the entrainment of dune sand by wind: an evaluation of selected models , 2003 .

[34]  Soon-Chang Yoon,et al.  Dust cycle: An emerging core theme in Earth system science , 2011 .

[35]  G. Mann,et al.  A review of natural aerosol interactions and feedbacks within the Earth system , 2010 .

[36]  Kerstin Schepanski,et al.  Comparing two years of Saharan dust source activation obtained by regional modelling and satellite observations , 2012 .

[37]  C. Thorncroft,et al.  West African storm tracks and their relationship to Atlantic tropical cyclones , 2007 .

[38]  A. Jansà,et al.  Towards a systematic climatology of sensitivities of Mediterranean high impact weather: a contribution based on intense cyclones , 2007 .

[39]  K. Schepanski,et al.  The role of deep convection and nocturnal low-level jets for dust emission in summertime West Africa: Estimates from convection-permitting simulations , 2013, Journal of geophysical research. Atmospheres : JGR.

[40]  G. Kiladis,et al.  Intermittent African Easterly Wave Activity in a Dry Atmospheric Model: Influence of the Extratropics , 2011 .

[41]  J. Kok,et al.  The physics of wind-blown sand and dust , 2012, Reports on progress in physics. Physical Society.

[42]  J. Thepaut,et al.  The ERA‐Interim reanalysis: configuration and performance of the data assimilation system , 2011 .

[43]  Michael Schulz,et al.  Global dust model intercomparison in AeroCom phase I , 2011 .

[44]  R. Kahana,et al.  Synoptic climatology of major floods in the Negev Desert, Israel , 2002 .

[45]  P. Alpert,et al.  The Sharav Cyclone: Observations and some theoretical considerations , 1989 .

[46]  Peter Knippertz,et al.  Mineral dust aerosols over the Sahara: Meteorological controls on emission and transport and implications for modeling , 2012 .

[47]  M. Bosilovich,et al.  Evaluation of the Reanalysis Products from GSFC, NCEP, and ECMWF Using Flux Tower Observations , 2012 .

[48]  P. Knippertz,et al.  The role of moist convection in the West African monsoon system: Insights from continental‐scale convection‐permitting simulations , 2013 .

[49]  A. Hannachi,et al.  Climatology and classification of Spring Saharan cyclone tracks , 2011 .

[50]  C. Schwierz,et al.  Surface Cyclones in the ERA-40 Dataset (1958–2001). Part I: Novel Identification Method and Global Climatology , 2006 .

[51]  C. Thorncroft,et al.  Three-Dimensional Structure and Dynamics of African Easterly Waves. Part III: Genesis , 2008 .

[52]  R. Trigo,et al.  Objective climatology of cyclones in the Mediterranean region: a consensus view among methods with different system identification and tracking criteria , 1999 .

[53]  P. Alpert,et al.  Climatological analysis of Mediterranean cyclones using ECMWF data , 1990 .

[54]  K. Schepanski,et al.  Soudano‐Saharan depressions and their importance for precipitation and dust: a new perspective on a classical synoptic concept , 2011 .

[55]  K. Schepanski,et al.  Climatology of nocturnal low-level jets over North Africa and implications for modeling mineral dust emission , 2013, Journal of geophysical research. Atmospheres : JGR.