Thursday, 6 June 2002: 9:45 AM
On the Entrainment Process in Stratocumulus Clouds
Entrainment in stratocumulus clouds Sc is thought to be a key process in the breakup of the large maritime Sc sheets and in their evolution into trade-wind Cu. How this process takes place in the shallow and undulating Sc-top interface layer is poorly understood, which likely affects our inability to adequately parameterize the process in models. While aircraft have traversed this interface on numerous occasions in prior studies, the response time of the typical aircraft probes has been insufficiently rapid making it difficult to unravel the details of the entrainment process. The DYCOMS-II study of West Coast Sc provided the first opportunity to fly co-located on a research aircraft (NCAR C-130) the following three new microphysics probes with greatly enhanced response times: Fast Forward Scattering Spectrometer Probe (FFSSP), Ultra-Fast Temperature sensor (UFT), and PVM (Particle Volume Monitor) measuring respectively droplet spectra and concentration, temperature, and liquid water content and effective radius at rates up to and exceeding 1000 Hz which corresponds to a 10-cm in cloud path length at typical aircraft speed. We describe the high-resolution Sc interface measurements made with the three fast probes, and relate the measurements to an improved conceptual model for the entrainment process, to the roles played by the competing processes of evaporative and radiative cooling, to gradients at cloud top and within the interface, and to cloud-top entrainment instability (CTEI). Initial data evaluation suggests some new and surprising results including strong cloud detrainment into the interface layer, nearly buoyancy-neutral entrainment of interface air into cloud top, and fine-scale and small temperature variability potentially associated with cloud top radiative cooling.