The Spiderweb Structure of Stratocumulus Clouds

Stratocumulus clouds have a distinctive structure composed of a combination of lumpy cellular structures and thin elongated regions, resembling canyons or slits. We refer to the elongated slits as “spiderweb” structure. Using very high resolution large-eddy simulations (LES) it is shown that the spiderweb structure is caused by cloud-top evaporative cooling and the resulting buoyancy reversal instability (BRI). Analysis of liquid water path and the cloud liquid water content shows that cloud-top evaporative cooling generates relatively shallow slits near the cloud top. Because most of the liquid water mass is concentrated near the cloud top, these regions of clear air have a large impact on the entire column LWP. When the evaporative cooling is suppressed in the LES, LWP exhibits cellular lumpy structure without the elongated low LWP regions. Even though the spiderweb signature on the LWP distribution is negligible, the cloud-top evaporative cooling process significantly affects integral boundary layer quantities, such as the vertically integrated turbulent kinetic energy, mean liquid water path, and the entrainment rate. In a pair of simulations driven only by cloud-top radiative cooling, evaporative cooling nearly doubles the entrainment rate.