The Effect of cloud shadows on coherent structures and secondary circulations in the convective boundary layer over homogeneous and heterogeneous terrain

Coherent flow structures and secondary circulations in the convective boundary layer (CBL) over homogeneously and heterogeneously heated terrain have been studied for many years with large-eddy simulations (LES). However, the effects of boundary-layer cloud shadows on the boundary-layer dynamics have been barely investigated so far. Cloud shadows introduce an additional, dynamically varying surface sensible heat flux heterogeneity, which may affect turbulence and flow structure in the CBL significantly, e.g. by inducing secondary circulations or by modifying existing circulation patterns. The effect of cloud shadows has been studied with a series of LES runs with a simple cloud detection method for homogeneous and idealized heterogeneous surfaces under different solar zenith angles.

For a homogeneously heated CBL and small solar zenith angles, cloud shadows strongly disturb the near-surface hexagonal flow structure typically observed in LES. In a negative feedback, shadows decrease the surface heating under evolving clouds, which in turn decreases the buoyancy responsible for the updraft below the cloud. For larger zenith angles shadows mainly shift to the downdraft areas between the clouds, and the resulting surface heat flux heterogeneities induce secondary circulations, which strengthen the updraft below the cloud (positive feedback).

A stripe-like 1D sinusoidal heat flux pattern has been used for the heterogeneously-heated simulations. Shadows of the developing clouds are modifying this pattern. The simulations show that the typically secondary circulations with updrafts above the stronger heated patches, which develop under cloud free conditions, are heavily affected by the cloud shadows. The impact can range from strengthening to a total breakdown of the secondary circulations. We will present the results of selected conditions in order to show responsible mechanisms and characteristic impacts of cloud shadows on the boundary-layer dynamics.