A simulation of partial cloudiness in multilayered altocumuli

This paper seeks to gain understanding of what processes influence the cloud fraction of altocumulus clouds. Our starting point is a cloud system observed on 25 June 1996 during the CLEX-1 field experiment. This two-layered system comprised a partly cloudy cumuliform layer located about 500 meters below an overcast stratiform layer. This cloud system was documented with aircraft data and simulated in three dimensions with high resolution using the COAMPS-LES model. 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. Next, sensitivity studies were performed using the COAMPS-LES model. These gave 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 heated radiatively 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 small. However, the entire lower layer heats only if there is an upper cloud layer above. If the upper layer is removed, then the lower layer cloud top cools strongly to space, 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 dynamical interaction.