Cloud-Shadow Effects on the Structure of the Convective Boundary Layer

Kurzfassung

The influence of shadow from a broken cloud layer at top of a convective boundary layer (CBL) on the turbulent flow structure in the layer is investigated. The study addresses fair weather shallow convection over dry land surfaces as exemplified by a high-resolution satellite observation. A three-dimensional Monte-Carlo radiative transfer calculation reveals the strong reduction of net solar flux in the shadow of such clouds with some enhancement between the clouds. The turbulent convection is studied using a simple conceptional model and a large eddy simulation. The models assume total shadow below clouds, instantaneous response of the vertical heat flux to shadow, and the kinematic association of clouds with updrafts in the upper CBL. The study shows that the convective motion is necessarily nonsteady in case of vertical shadow. With shadow the convective turbulent motions are of smaller scales compared to the reference case without shadow. Any asymmetry due to shadow for non-zero solar zenith angle has only very small impact on the turbulent motion field.