An Unusual Aerial Photograph of an Eddy Circulation in Marine Stratocumulus Clouds

Abstract

n aerial photograph of a cyclonic, von Kármán?like vortex in the marine stratocumulus clouds off the California coast, taken by a commercial pilot near Grover Beach, is presented. It is believed that this is the first photograph of such an eddy, taken from an airplane, to appear in publication.The eddy occurred with a strong inversion above a shallow marine boundary layer, in the lee of high, inversion-penetrating terrain. Tower and surface wind measurements plotted on satellite imagery demonstrate that the Grover Beach eddy was not just a cloud-level feature, but extended through the marine atmospheric boundary layer (MABL) to the surface. Evolution of the flow during the formation of the eddy appears similar to idealized numerical simulations of blocked MABL flow from the literature. The tower measurements sampled the northern part of the eddy circulation during its formation just offshore. The 2°?3°C temperature increases and then decreases during and after the eddy passage may be indicative of warmer air, from sheltered locations to the southeast, and/or downslope flow, being advected by and included into the eddy circulation. Satellite data compared with sequences of wind reversals at two different levels of the meteorological tower suggest that the eddy is tilted with height, at least during its formation stage. Formation mechanisms are discussed, but the subsynoptic observations are inadequate to resolve basic questions about the flow; ultimately a high-resolution model simulation is needed.