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.
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.
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.
POST (Physics Of Stratocumulus Top) was a study of low-level clouds layers that are the most prevalent cloud type globally. These martime stratocumulus clouds (Sc) act like a diffuse mirror reflecting solar radiation back to space, and in this way partially counteract global warming. The future coverage of the Sc is thus an important part of climate change predictions. POST addressed a phenomena called entrainment that has the potential of dissipating Sc. What is thought to happen during entrainment is that warmer and dryer air located above the clouds mixes with Sc cloud top causing Sc to evaporate. Much previous study has been applied to the entrainment process for Sc, but we are just now able to utilize advanced instrumentation to make significant headway in understanding and predicting this process. The Naval Post Graduate School Twin Otter research aircraft (Fig. 1) was used on numerous flights during POST to skim continuously through cloud top in sheets of unbroken Sc often found off the California Coast. Figure 2 shows a sketch of the typical flight path of the Twin Otter originating out of Marina near Monterey Bay. The zig-zag flight pattern matched the mean NW wind speed usually found off the coast (quasi-Lagrangian pattern). The aircraft carried a remarkable new temperature probe, the UFT (Ultra-Fast Temperature) probe that permitted for the first time high resolution temperature measurements needed to observe entrainment details. The UFT was accompanied by other high resolution measurements of cloud liquid water and a full complement of other variables thought to affect entrainment. For the first time the measurements were made close to a gust probe on the nose of the aircraft that permitted entrainment fluxes and rates to be measured at scales consistent with the size of the entrainment events. This new look at Sc cloud top has determined physical processes related to entrainment not previously fully understood, and processes that must be taken into account when predicting Sc evolution: Air from above cloud top enters the cloud with nearly neutral buoyancy; that is, the cloud and air temperature are almost identical; the air from above also evaporates cloud and causes the formation of the EIL (Entrainment Interface Layer) which is a layer between cloud top and the warm and dry air above that affects the entrainment process; and sharp changes in wind speed (wind shear) are found near cloud top that cause air turbulence that also affect the entrainment process. Given these and other findings from POST requires modification of our classical and current understanding of entrainment in order to make progress in Sc prediction. See www.eol.ucar.edu/projects/post for a complete description of POST.
