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

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 freetropospheric 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.

The Saharan heat low and moisture transport pathways in the central Sahara—Multiaircraft observations and Africa‐LAM evaluation

“…This result is consistent with the observational and modeling study of Garcia‐Carreras et al . [], who have shown that entrainment into the Saharan CBL, which is capped by a near‐neutral residual layer, is much weaker than (around one quarter of) that of classical convective boundary layers capped by a stable inversion and that models therefore overestimate the rate of boundary layer growth in the morning, for the Sahara.…”

“…The Sahara is the world's largest and most active dust source region [Prospero et al, 2002;Washington et al, 2003], and dust is semipermanent in the overlying atmosphere during Northern Hemisphere summer [Israelevich et al, 2003] with dry convection mixing the dust within one of the deepest atmospheric boundary layers on the planet (up to 6 km) [Gamo, 1996;Cuesta et al, 2008;Garcia-Carreras et al, 2014]. Satellite-derived observations of dust aerosol distributions and reanalysis data sets show that the SHL develops contemporaneously with increasing dust concentrations to reach a simultaneous peak in June [Engelstaedter et al, 2006].…”

Weakening and moistening of the summertime Saharan heat low through convective cold pools from the Atlas Mountains

“…Improved climate projections for the WAM are only achievable if all aspects are taken into account. This also includes a proper representation of the complex Saharan boundary layer, which ultimately distributes moisture and dust on the continental scale [ Garcia‐Carreras et al , ].…”

“…What remains after the development of the daytime convective boundary layer is the additional moisture from rain evaporation, which usually gets mixed vertically much deeper than the original cold pool. The Saharan PBL can reach up to 550 hPa [ Garcia‐Carreras et al , ]. Accordingly, the mixing ratio gets reduced, but the vertically integrated amount, which is relevant for LW down , remains unchanged.…”

Five Years Lidar Research on Board the Facility for Airborne Atmospheric Measurements (FAAM)

“…The lidar measurements have been very successful at mapping the turbulent structure of the Saharan atmospheric boundary layer [5]. …”