Boundary layer top detrainment and its impact on boundary layer growth in the Sahara

The Saharan boundary layer is one of the deepest on Earth, resulting in a unique vertical structure. The midday convective boundary layer is typically capped by a weak temperature inversion, and a deep, near-neutral residual layer which can reach up to ~500mb and persists until late in the afternoon. In this study we describe novel boundary layer processes that are linked to the distinctive vertical profile in the Sahara, and their impact on the diurnal development of the boundary layer.

A month of radiosonde observations from the remote Sahara are used to describe the typical vertical structure of the Saharan boundary layer. Radiosonde and heat-flux measurements were then used to initialize and drive the Met Office Large Eddy Model, a state-of-the-art model for large-eddy simulation and cloud-resolving modelling.

Despite the small temperature inversion and weak lapse rate above the inversion, the convective boundary layer does not erode the residual layer until late in the afternoon, consistent with past observations in the Sahara. This is shown to be linked to the detrainment of the warmest thermals at the convective boundary layer top, which weakens the entrainment heat fluxes, and slows down its warming and growth. This detrainment is unrelated to moist, diabatic processes, but is due to the negligible difference in temperature between the warmest parcels in the convective boundary layer, and the residual layer air above it, as well as the presence of weak mixing within the residual layer. As the boundary layer grows, overshooting thermals can also entrain free-tropospheric air into the residual layer, forming a second entrainment zone at the residual-layer top, and slowing down boundary layer growth further.

A single column model version of the Met Office Unified Model, representative of the capabilities of a typical numerical weather prediction model, is unable to reproduce this detailed evolution of the Saharan boundary layer, highlighting the difficulty of representing such processes in large-scale models.

The boundary layer processes described here are unique to the Sahara, or 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.