Role of boundary layer and cumulus convection on dust emission and transport over a midlatitude desert area

Abstract

A series of high‐resolution simulations explicitly resolving diurnally varying convective motion is performed for investigating the dynamical processes of dust emission and transport induced by boundary layer and cumulus convection under fair‐weather conditions in a midlatitude desert area. The simulation set examines the sensitivity of the dust dynamics to vertical wind shear, upper level wind speed, and moist convection. The simulated results qualitatively capture the diurnal variation of the dust layer observed by lidar in a desert area in northern China. Dry convection plays a primary role in vertically mixing dust within the boundary layer. If cumulus convection also comes into play, mass concentration of dust increases not only within the boundary layer but also in the free troposphere. A coupled effect of dry and moist convection is important because convection is more enhanced with the coupled effect than without moist processes and in addition transports upper level higher momentum down to the surface, intensifying surface winds and hence dust emission. A wind speed exceeding the threshold for surface dust emission is necessary at the upper levels of the daytime boundary layer for the surface wind enhancement. Although the amount of dust lifted is much smaller during a single diurnal cycle of fair weather than is associated with a single case of synoptic disturbances, the total amount of dust emission due to fair‐weather processes may not be neglected in a longer timescale. This suggests that a proper parameterization for dry and moist convection in fair weather needs to be employed in large‐scale simulations.