Monday, 23 January 2012
Integration of Satellite Image and In-Situ Observations for Analysis of Meso-Gamma Scale Eddies in the Marine Stratocumulus Photographed by Pilots near the California Coast
Hall E (New Orleans Convention Center )
Poster PDF (1.8 MB)
Wind, instrument tower, and other meteorological observations from local independent air pollution and emergency-release monitoring entities are combined and overlaid on GOES satellite imagery to track and analyze eddies in marine stratocumulus clouds that originally were photographed by commercial pilots flying California coastal routes. While satellite images of similar eddies have appeared in the literature for years, it is believed that these are the first actual photographs of such eddies, likened to von Karman vortices, to appear in publication. Two eddy photographs exist, both taken during summer, 2006. One eddy was photographed near Santa Cruz Island, and the other just offshore from Grover Beach, California, both marked by a cyclonic cloud vortex with a striking cloud-free “eye” feature roughly 3 km in diameter. The satellite imagery enables tracking the movement and evolution of these features, and also allows for estimates of their dimensions. Relevant meteorological observations for the Santa Cruz Island eddy were not located, but in-situ observations from the Diablo Canyon Nuclear Power Plant, California Polytechnic State University (Cal Poly) pier, and the San Luis Obispo County Air Pollution Control District made possible a more detailed examination of the Grover Beach eddy, allowing for speculation on a formation mechanism consistent with a hypothesis from the literature on low Froude number, continuously stratified flow. Both eddies occurred in the lee of high, inversion-penetrating terrain, with strong inversion conditions topping a shallow, cool marine boundary layer. Tower and surface wind measurements of the Grover Beach eddy overlaid on the satellite imagery directly address the question posed by Young and Zawislak (2006) in their study of island wake vortex streets as to whether such features occur only at cloud level and within the inversion, or extend through the marine boundary layer down to the surface. Fortuitous movement of the eddy directly over both the instrument tower at the power plant and a wind monitor at Grover Beach demonstrate that the cyclonic circulation indeed penetrated through the marine boundary layer down to the surface. Additionally, the tower data indicated a 2oC temperature increase and then decrease with the eddy passage, suggesting cross-inversion mixing of warm, dry air into the marine boundary layer, producing a cloud-free, warm-core “eye.” Satellite-tracked eddy motions also suggest cross-inversion mixing: the eddy first moved toward the southwest, consistent with the warm, northeasterly inversion-layer winds, but ultimately turned toward the east, apparently embedded in the cool, westerly marine boundary layer flow rounding San Luis point. Observed characteristics of eddy formation were consistent with the hypothesis of Smolarkiewicz and Rotunno (1989) on low Froude number, continuously stratified flow, that very stable air near the top of inversion-penetrating terrain is accelerated downward in the lee, producing “backward rolling” (negative) horizontal vorticity from shear with the slower air above. The strong lee subsidence of inversion-layer warm, dry air side-by-side with non-vertically moving air over the open ocean simultaneously tilts the “backward rolling” horizontal vortex tube into a vertical orientation at the periphery of the high terrain, creating the warm-core eddy and its cloud-free “eye.” While the supporting meteorological and satellite data point toward this hypothesis, more complete observations, or a detailed model simulation would be needed to actually confirm the formation mechanism. Attempting to analyze and understand the very small scale meteorological features in this case brings to light a variety of issues of increasing importance to modern meteorology and modeling of boundary layer flows at the coastal margin: the accuracy and timing of satellite position data in relation to very high time resolution in-situ measurements when dealing with features on the order of kilometers, accuracy and precision of mapping data bases, the ability of an analysis and/or a model to resolve the sharp vertical changes present in coastal temperature inversions and their interaction with details of the local terrain, appropriate iso-surfaces on which to represent data for interpretation purposes at such small scales, and predictability of flow features and cross-inversion mixing regimes relevant to pollution transport issues.
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