• Agustí-Panareda, A., , A. Beljaars, , C. Cardinali, , and I. Genkova, 2010: Impacts of assimilating AMMA soundings on ECMWF analyses and forecasts. Wea. Forecasting, 25, 11421160, doi:10.1175/2010WAF2222370.1.

    • Search Google Scholar
    • Export Citation
  • Birch, C. E., , D. J. Parker, , J. H. Marsham, , and G. M. Devine, 2012: The effect of orography and surface albedo on stratification in the summertime Saharan boundary layer: Dynamics and implications for dust transport. J. Geophys. Res., 117, D05105, doi:10.1029/2011JD015965.

    • Search Google Scholar
    • Export Citation
  • Brooks, I. M., , and A. M. Fowler, 2007: New measure of entrainment zone structure. Geophys. Res. Lett., 34, L16808, doi:10.1029/2007GL030958.

    • Search Google Scholar
    • Export Citation
  • Brooks, I. M., , S. Söderberg, , and M. Tjernström, 2003: The turbulence structure of the stable atmospheric boundary layer around a coastal headland: Aircraft observations and modelling results. Bound.-Layer Meteor., 107, 531559, doi:10.1023/A:1022822306571.

    • Search Google Scholar
    • Export Citation
  • Brown, A., , R. Beare, , J. Edwards, , A. Lock, , S. Keogh, , S. Milton, , and D. Walters, 2008: Upgrades to the boundary layer scheme in the Met Office Numerical Weather Prediction model. Bound.-Layer Meteor., 128, 117132, doi:10.1007/s10546-008-9275-0.

    • Search Google Scholar
    • Export Citation
  • Canut, G., , M. Lothon, , F. Saïd, , and F. Lohou, 2010: Observation of entrainment at the interface between monsoon flow and the Saharan air layer. Quart. J. Roy. Meteor. Soc., 136, 3446, doi:10.1002/qj.471.

    • Search Google Scholar
    • Export Citation
  • Conzemius, R. J., , and E. Fedorovich, 2006: Dynamics of sheared convective boundary layer entrainment. Part I: Methodological background and large-eddy simulations. J. Atmos. Sci., 63, 11511178, doi:10.1175/JAS3691.1.

    • Search Google Scholar
    • Export Citation
  • Couvreux, F., , F. Guichard, , A. Gounou, , D. Bouniol, , P. Peyrillé, , and M. Köhler, 2014: Modelling of the thermodynamical diurnal cycle in the lower atmosphere: A joint evaluation of four contrasted regimes in the tropics over land. Bound.-Layer Meteor., 150, 185214, doi:10.1007/s10546-013-9862-6.

    • Search Google Scholar
    • Export Citation
  • Cuesta, J., and et al. , 2008: Multiplatform observations of the seasonal evolution of the Saharan atmospheric boundary layer in Tamanrasset, Algeria, in the framework of the African Monsoon Multidisciplinary Analysis field campaign conducted in 2006. J. Geophys. Res., 113, D00C07, doi:10.1029/2007JD009417.

    • Search Google Scholar
    • Export Citation
  • Cuesta, J., , J. H. Marsham, , D. J. Parker, , and C. Flamant, 2009: Dynamical mechanisms controlling the vertical redistribution of dust and the thermodynamic structure of the west Saharan atmospheric boundary layer during summer. Atmos. Sci. Lett., 10, 3442, doi:10.1002/asl.207.

    • Search Google Scholar
    • Export Citation
  • Deardorff, J. W., , G. E. Willis, , and B. H. Stockton, 1980: Laboratory studies of the entrainment zone of a convectively mixed layer. J. Fluid Mech., 100, 4164, doi:10.1017/S0022112080001000.

    • Search Google Scholar
    • Export Citation
  • Fiedler, S., , K. Schepanski, , B. Heinold, , P. Knippertz, , and I. Tegen, 2013: Climatology of nocturnal low-level jets over North Africa and implications for simulating mineral dust emission. J. Geophys. Res. Atmos., 118, 6100–6121, doi:10.1002/jgrd.50394.

    • Search Google Scholar
    • Export Citation
  • Flamant, C., , J. P. Chaboureau, , D. J. Parker, , C. A. Taylor, , J. P. Cammas, , O. Bock, , F. Timouk, , and J. Pelon, 2007: 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. Quart. J. Roy. Meteor. Soc., 133, 11751189, doi:10.1002/qj.97.

    • Search Google Scholar
    • Export Citation
  • Fochesatto, G. J., , P. Drobinski, , C. Flamant, , D. Guedalia, , C. Sarrat, , P. H. Flamant, , and J. Pelon, 2001: Evidence of dynamical coupling between the residual layer and the developing convective boundary layer. Bound.-Layer Meteor., 99, 451464, doi:10.1023/A:1018935129006.

    • Search Google Scholar
    • Export Citation
  • Galperin, B., , S. Sukoriansky, , and P. S. Anderson, 2007: On the critical Richardson number in stably stratified turbulence. Atmos. Sci. Lett., 8, 6569, doi:10.1002/asl.153.

    • Search Google Scholar
    • Export Citation
  • Gamo, M., 1996: Thickness of the dry convection and large-scale subsidence above deserts. Bound.-Layer Meteor., 79, 265278, doi:10.1007/BF00119441.

    • Search Google Scholar
    • Export Citation
  • Garcia-Carreras, L., , J. H. Marsham, , D. J. Bain, , S. Milton, , A. Saci, , M. Salah-Ferroudj, , B. Ouchene, , and R. Washington, 2013: The impact of convective cold pool outflows on model biases in the Sahara. Geophys. Res. Lett., 40, 16471652, doi:10.1002/grl.50239.

    • Search Google Scholar
    • Export Citation
  • Gray, M. E. B., , J. Petch, , S. H. Derbyshire, , A. R. Brown, , A. P. Lock, , H. A. Swann, , and P. R. A. Brown, 2001: Version 2.3 of the Met Office large eddy model. Part II: Scientific documentation. APR Turbulence and Diffusion Rep. 276, Met Office, 52. [Available online at http://appconv.metoffice.com/LEM/docs.html.]

  • Haywood, J., , and O. Boucher, 2000: Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review. Rev. Geophys., 38, 513543, doi:10.1029/1999RG000078.

    • Search Google Scholar
    • Export Citation
  • Huang, Q., , J. H. Marsham, , D. J. Parker, , W. Tian, , and C. M. Grams, 2010: Simulations of the effects of surface heat flux anomalies on stratification, convective growth, and vertical transport within the Saharan boundary layer. J. Geophys. Res.,115, D05201, doi:10.1029/2009JD012689.

  • Jickells, T. D., and et al. , 2005: Global iron connections between desert dust, ocean biogeochemistry, and climate. Science, 308, 6771, doi:10.1126/science.1105959.

    • Search Google Scholar
    • Export Citation
  • Kaufman, Y. J., , I. Koren, , L. A. Remer, , D. Rosenfeld, , and Y. Rudich, 2005: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 20711 212, doi:10.1073/pnas.0505191102.

    • Search Google Scholar
    • Export Citation
  • Lavaysse, C., , C. Flamant, , S. Janicot, , D. J. Parker, , J. P. Lafore, , B. Sultan, , and J. Pelon, 2009: Seasonal evolution of the West African heat low: A climatological perspective. Climate Dyn., 33, 313–330, doi:10.1007/s00382-009-0553-4.

    • Search Google Scholar
    • Export Citation
  • Lock, A. P., , A. R. Brown, , M. R. Bush, , G. M. Martin, , and R. N. B. Smith, 2000: A new boundary layer mixing scheme. Part I: Scheme description and single-column model tests. Mon. Wea. Rev., 128, 31873199, doi:10.1175/1520-0493(2000)128<3187:ANBLMS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Marsham, J. H., , D. J. Parker, , C. M. Grams, , B. T. Johnson, , W. M. F. Grey, , and A. N. Ross, 2008: Observations of mesoscale and boundary-layer scale circulations affecting dust transport and uplift over the Sahara. Atmos. Chem. Phys., 8, 69796993, doi:10.5194/acp-8-6979-2008.

    • Search Google Scholar
    • Export Citation
  • Marsham, J. H., , P. Knippertz, , N. Dixon, , D. J. Parker, , and G. M. S. Lister, 2011: The importance of the representation of deep convection for modeled dust-generating winds over West Africa during summer. Geophys. Res. Lett., 38, L16803, doi:10.1029/2011GL048368.

    • Search Google Scholar
    • Export Citation
  • Marsham, J. H., , N. Dixon, , L. Garcia-Carreras, , G. M. S. Lister, , D. J. Parker, , P. Knippertz, , and C. Birch, 2013a: The role of moist convection in the West African monsoon system: Insights from continental-scale convection-permitting simulations. Geophys. Res. Lett., 40, 18431849, doi:10.1002/grl.50347.

    • Search Google Scholar
    • Export Citation
  • Marsham, J. H., , M. Hobby, , C. J. T. Allen, , J. R. Banks, , M. Bart, , B. J. Brooks, , and R. Washington, 2013b: Meteorology and dust in the central Sahara: Observations from Fennec supersite-1 during the June 2011 intensive observation period. J. Geophys. Res. Atmos., 118, 40694089, doi:10.1002/jgrd.50211.

    • Search Google Scholar
    • Export Citation
  • Matthews, A. J., , and R. A. Madden, 2000: Observed propagation and structure of the 33-h atmospheric Kelvin wave. J. Atmos. Sci., 57, 34883497, doi:10.1175/1520-0469(2000)057<3488:OPASOT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Messager, C., , D. Parker, , O. Reitebuch, , A. Agustí-Panareda, , C. M. Taylor, , and J. Cuesta, 2010: Structure and dynamics of the Saharan atmospheric boundary layer during the West African monsoon onset: Observations and analyses from the research flights of 14 and 17 July 2006. Quart. J. Roy. Meteor. Soc., 136,107124, doi:10.1002/qj.469.

    • Search Google Scholar
    • Export Citation
  • Moeng, C.-H., , and P. P. Sullivan, 1994: A comparison of shear- and buoyancy-driven planetary boundary layer flows. J. Atmos. Sci., 51, 9991022, doi:10.1175/1520-0469(1994)051<0999:ACOSAB>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Parker, D. J., and et al. , 2005: The diurnal cycle of the West African monsoon circulation. Quart. J. Roy. Meteor. Soc., 131, 28392860, doi:10.1256/qj.04.52.

    • Search Google Scholar
    • Export Citation
  • Pino, D., , J. Vila-Guerau de Arellano, , and P. G. Duynkerke, 2003: The contribution of shear to the evolution of a convective boundary layer. J. Atmos. Sci., 60, 19131926, doi:10.1175/1520-0469(2003)060<1913:TCOSTT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Prospero, J. M., , P. Ginoux, , O. Torres, , S. E. Nicholson, , and T. E. Gill, 2002: Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product. Rev. Geophys., 40, 1002, doi:10.1029/2000RG000095.

    • Search Google Scholar
    • Export Citation
  • Roehrig, R., , D. Bouniol, , F. Guichard, , F. Hourdin, , and J. L. Redelsperger, 2013: The present and future of the West African monsoon: A process-oriented assessment of CMIP5 simulations along the AMMA transect. J. Climate, 26, 6471–6505, doi:10.1175/JCLI-D-12-00505.1.

    • Search Google Scholar
    • Export Citation
  • Ryder, C. L., and et al. , 2013: Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign. Atmos. Chem. Phys., 13, 303325, doi:10.5194/acp-13-303-2013.

    • Search Google Scholar
    • Export Citation
  • Stein, T. H. M., , D. J. Parker, , J. Delanoë, , N. S. Dixon, , R. J. Hogan, , P. Knippertz, , R. I. Maidment, , and J. H. Marsham, 2011: The vertical cloud structure of the West African monsoon: A 4 year climatology using CloudSat and CALIPSO. J. Geophys. Res., 116, D22205, doi:10.1029/2011JD016029.

    • Search Google Scholar
    • Export Citation
  • Stull, R., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic, 666 pp.

  • Sullivan, P. P., , C.-H. Moeng, , B. Stevens, , D. H. Lenschow, , and S. H. Mayor, 1998: Structure of the entrainment zone capping the convective atmospheric boundary layer. J. Atmos. Sci., 55, 30423064, doi:10.1175/1520-0469(1998)055<3042:SOTEZC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Todd, M. C., and et al. , 2013: Meteorological and dust aerosol conditions over the western Saharan region observed at Fennec Supersite-2 during the intensive observation period in June 2011. J. Geophys. Res. Atmos., 118, 84268447, doi:10.1002/jgrd.50470.

    • Search Google Scholar
    • Export Citation
  • Walters, D. N., and et al. , 2014: The Met Office Unified Model Global Atmosphere 4.0 and JULES Global Land 4.0 configurations. Geosci. Model Dev., 7, 361386, doi:10.5194/gmd-7-361-2014.

    • Search Google Scholar
    • Export Citation
  • Washington, R., , M. Todd, , N. J. Middleton, , and A. S. Goudie, 2003: Dust-storm source areas determined by the total ozone monitoring spectrometer and surface observations. Ann. Assoc. Amer. Geogr., 93, 297313, doi:10.1111/1467-8306.9302003.

    • Search Google Scholar
    • Export Citation
  • Washington, R., and et al. , 2012: Fennec—The Saharan climate system. CLIVAR Exchanges, No. 60, International CLIVAR Project Office, Southampton, United Kingdom, 31–33. [Available online at http://www.clivar.org/sites/default/files/documents/Exchanges60.pdf.]

  • Weckwerth, T. M., , J. W. Wilson, , and R. M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon. Wea. Rev., 124, 769784, doi:10.1175/1520-0493(1996)124<0769:TVWTCB>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 131 131 21
PDF Downloads 117 117 17

The Turbulent Structure and Diurnal Growth of the Saharan Atmospheric Boundary Layer

View More View Less
  • 1 Department of Meteorology, and the Bert Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • | 2 Institute for Climate and Atmospheric Sciences, University of Leeds, Leeds, United Kingdom
  • | 3 Met Office, Exeter, United Kingdom
  • | 4 Institute for Climate and Atmospheric Sciences, University of Leeds, Leeds, United Kingdom
  • | 5 National Centre for Atmospheric Science, University of Leeds, Leeds, United Kingdom
© Get Permissions
Restricted access

Abstract

The turbulent structure and growth of the remote Saharan atmospheric boundary layer (ABL) is described with in situ radiosonde and aircraft measurements and a large-eddy simulation model. A month of radiosonde data from June 2011 provides a mean profile of the midday Saharan ABL, which is characterized by a well-mixed convective boundary layer, capped by a small temperature inversion (<1 K) and a deep, near-neutral residual layer. The boundary layer depth varies by up to 100% over horizontal distances of a few kilometers due to turbulent processes alone. The distinctive vertical structure also leads to unique boundary layer processes, such as detrainment of the warmest plumes across the weak temperature inversion, which slows down the warming and growth of the convective boundary layer. As the boundary layer grows, overshooting plumes can also entrain free-tropospheric air into the residual layer, forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer, thus slowing down boundary layer growth further. A single-column model is unable to accurately reproduce the evolution of the Saharan boundary layer, highlighting the difficulty of representing such processes in large-scale models. These boundary layer processes are special to the Sahara, and possibly hot, dry, desert environments in general, and have implications for the large-scale structure of the Saharan heat low. The growth of the boundary layer influences the vertical redistribution of moisture and dust, and the spatial coverage and duration of clouds, with large-scale dynamical and radiative implications.

Denotes Open Access content.

This article is licensed under a Creative Commons Attribution 4.0 license.

Corresponding author address: L. Garcia-Carreras, Department of Meteorology, Stockholm University, Stockholm 10691, Sweden. E-mail: luis.garciacarreras@misu.su.se

Abstract

The turbulent structure and growth of the remote Saharan atmospheric boundary layer (ABL) is described with in situ radiosonde and aircraft measurements and a large-eddy simulation model. A month of radiosonde data from June 2011 provides a mean profile of the midday Saharan ABL, which is characterized by a well-mixed convective boundary layer, capped by a small temperature inversion (<1 K) and a deep, near-neutral residual layer. The boundary layer depth varies by up to 100% over horizontal distances of a few kilometers due to turbulent processes alone. The distinctive vertical structure also leads to unique boundary layer processes, such as detrainment of the warmest plumes across the weak temperature inversion, which slows down the warming and growth of the convective boundary layer. As the boundary layer grows, overshooting plumes can also entrain free-tropospheric air into the residual layer, forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer, thus slowing down boundary layer growth further. A single-column model is unable to accurately reproduce the evolution of the Saharan boundary layer, highlighting the difficulty of representing such processes in large-scale models. These boundary layer processes are special to the Sahara, and possibly hot, dry, desert environments in general, and have implications for the large-scale structure of the Saharan heat low. The growth of the boundary layer influences the vertical redistribution of moisture and dust, and the spatial coverage and duration of clouds, with large-scale dynamical and radiative implications.

Denotes Open Access content.

This article is licensed under a Creative Commons Attribution 4.0 license.

Corresponding author address: L. Garcia-Carreras, Department of Meteorology, Stockholm University, Stockholm 10691, Sweden. E-mail: luis.garciacarreras@misu.su.se
Save