The vertical structure of the Saharan atmospheric boundary layer: insights from a large eddy model and observations

The Saharan atmosphere is a key component of the climate system. Routine observations are scarce, and mostly from the periphery of the desert, leading to large disagreements between analyses, significant biases in operational models, and a limited understanding of the processes that drive the Saharan heat low. The Saharan boundary layer plays a key role in the vertical redistribution of heat, moisture and dust, as well as the formation of Saharan clouds, which in turn alter the radiative budget and large-scale state of the region. The Fennec field campaign aimed to improve our understanding of the remote Saharan region with a suite of airborne and ground observations.

In this study we use radiosonde and aircraft observations, as well as a large-eddy model simulation driven by data from the Fennec campaign, to describe the characteristics of a typical Saharan boundary layer, focusing on the turbulent and vertical mixing between its various layers and its diurnal evolution.

The Saharan boundary layer is characterized by a well-mixed convective boundary layer, capped by a small temperature inversion (<1 K) at 800 mb on average, and a deep, near-neutral residual layer which typically reaches 550 mb. Turbulent processes alone lead to a large variability in boundary layer depth and properties over distances of a few kilometres in both model and observations. While the strong surface heating leads to vigorous up and downdraughts in the convective boundary layer (±5 m s^-1), potential temperatures are confined to ±0.5 K of the boundary-layer mean. The residual layer has a turbulent state which is either dominated by shear or indeterminate, and significant vertical velocities are also measured (±2 m s^-1). These are due to a combination of externally-imposed turbulence and waves from overshooting thermals, as well as shear-induced turbulence at the residual layer top, which produce positive momentum fluxes and turbulent mixing within the RL. The combination of a small temperature inversion and mixing within the residual layer leads to detrainment of boundary layer thermals, which reduces the entrainment fluxes at the boundary layer top and slows down convective development during the day. The results presented here highlight how the particular structure of the Saharan boundary layer, and in particular the small temperature inversion and deep near-neutral residual layer, have a key role in its dynamics and diurnal evolution.