What causes partial cloudiness to form in multilayered midlevel clouds? A simulated case study

[1] This paper seeks to gain understanding of what processes influence the cloud fraction of altocumulus and altostratocumulus clouds. Our starting point is a midlevel cloud system observed on 25 June 1996 during the first Complex Layered Cloud Experiment. The two-layered system comprised a partly cloudy cumuliform layer located several hundred meters below an overcast stratiform layer. The cloud system was documented with aircraft data and was simulated in three dimensions with high resolution using large-eddy simulation (LES). Given observed initial atmospheric profiles, the model successfully simulates the qualitative structure of the layered system, although the modeled cumuliform layer has less cloud fraction, liquid water, and turbulence than the observations. Sensitivity studies were performed using an LES model. These give evidence that partial cloudiness in our simulations is caused primarily by conditional instability and the consequent vertical motions of air parcels within the layer. This mechanism is complicated by the presence of radiative heating and cooling. When the lower cloud layer is radiatively heated throughout its entire depth, it becomes conditionally unstable and partly cloudy. This is true even if the layer is initially overcast and absolutely stable, and even if the radiative heating rates are moderate. However, the entire lower layer heats only if there is a cloud layer above it. If the upper layer is removed, then the lower layer cloud top cools strongly to space in the longwave, and the lower layer remains overcast. Through this radiative effect, the presence or absence of an upper layer influences the cloud fraction of a lower layer, even though the two layers are spatially separated and have little microphysical interaction or turbulent exchange of heat and moisture.