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.