Physics of stratocumulus top (POST) project results and analysis of the thermodynamic and microphysical processes within the entrainment interface layer

The Physics Of Stratocumulus Top (POST) project took place during the summer of 2008 in the vicinity of Monterey Bay, California. Atmospheric thermodynamic and cloud microphysical measurements were taken via aircraft over a one month period. The goal of this study is to understand the relationship among (a) cloud microphysical properties near and within the entrainment interface layer (EIL) (b) thermodynamic properties of the EIL, boundary layer and free troposphere, e.g. jumps in equivalent potential temperature and total water, and cloud-top entrainment instability parameter, and (c) larger-scale meteorological properties such as wind shear and subsidence rate. In addition, GOES satellite images are used to help determine the evolution of the cloud layer during each research flight, e.g. whether the layer is thickening, thinning or stable. Preliminary results show that the EIL thickness varies widely, from less than 100 m to more than 300 m. In addition, the position of the EIL relative to cloud top also shows significant variation. Initial analysis shows that cloud drop size distributions tend not to shift to either smaller or larger sizes within the EIL, although in some cases a broadening at the small size range is observed. No obvious relationship between the EIL characteristics and wind shear in the vicinity of the boundary layer top has thus far been identified.