NSF Award
1020445
Marine stratocumulus, which are ubiquitous over regions of cool coastal upwelling and thought to be of considerable importance to Earth's climate system, were the focus the 2008 POST (Physics Of Stratocumulus Top) experiment. POST involved a combination of focused data collection by the Naval Postgraduate School's CIRPAS Twin Otter aircraft (operating off the coast of central California during July-August 2008) and supporting numerical modeling studies. The current effort will pursue analysis of POST in situ airborne measurements with a focus on calculating the entrainment velocity from data collected during multiple flights, and will employ the conditional sampling of "cloud holes" near stratocumulus top and the estimation of depleted total water flux in these suppressed regions. Entrainment velocities will also be independently estimated from measurements of subsidence and the growth of stratocumulus top. Previously published entrainment "rules" will be revisited with an eye toward their critical evaluation and potential improvement with attention to the role of shear and the unique physics within the entrainment interface layer (EIL) atop stratocumulus clouds. A separate but contemporaneous effort will expand a model-based exploration of drizzle formation in trade-wind cumuli observed during the 2004-05 RICO (Rain in Cumulus over the Ocean) project conducted over the Caribbean Sea. Here, sea-salt based ultra-giant nuclei (UGN) spectra over a range of surface wind speeds and sea states and accompanying in-cloud droplet spectra will support calculations of coalescence and condensational growth to estimate precipitation rates well above the bases of these cumulus clouds.<br/><br/>The intellectual merit of the POST-related effort centers on the challenge of demonstrating improved accuracy of POST entrainment rate measurements and follow-on improvements in cloud model parameterizations, while continued RICO modeling efforts will support development of a quantitative expression (vs. more binary switch) for the sensitivity of precipitation rates to concentrations of UGN in small cumulus common to the subtropics.<br/><br/>Broader impacts of the proposed work relate to improved understanding of the role of clouds in climate change scenarios, and through collaborations with a student at the University of Utah.
