Entrainment rate estimates in the northeast Pacific stratocumulus cloud regime from ship-based data, geostationary satellite retrievals, and meteorological reanalysis

Description

Entrainment rates (W_e) in marine low clouds are estimated through a budget equation that relates W_e to the large-scale subsidence, cloud top height tendency, and advection. Cloud top height is derived from a Ka-band radar deployed during the MAGIC field campaign and the Geostationary Operational Environmental Satellite (GOES-15) during May to August of 2013. Meteorological reanalysis data are taken from the European Centre for Medium-Range Weather Forecasts ERA-Interim, with secondary use of NCEP/NCAR and MERRA reanalyses. W_e maps are computed using the hourly composited advection and subsidence, and the tendency, which is in turn estimated from the cloud height diurnal cycle. The tendency primarily explains the amplitude and phase of W_e diurnal cycle, whereas the contribution of the advective term is modest. W_e encompass typical values between 0.0 and 1.2 cm/s, with a broader diurnal cycle amplitude in coastal areas, reduced entrainment offshore, and overall minimum and maximum W_e that occur at 11:00 and 21:00 local time, respectively. This is, to the best of our knowledge, the first observationally based computation of entrainment rate that resolves the W_e diurnal evolution at regional scales. Future efforts will be devoted to compare this novel approach with aircraft and radar-based W_e over ARM sites.