Convectivc Elements in the Marine Atmospheric Boundary Layer. Part II: Entrainment at the Capping Inversion

Abstract

Data from two aircraft flights in the marine atmospheric boundary layer are analyzed in an investigation of entrainment processes at the top of the well-mixed layer. Both days had strong wind shear across the inversion, which is reflected in the horizontal wind statistics. Conditional sampling is used to show that near the top of the mixed layer most updrafts are cool, moist and deficient in along-wind momentum (slow) with respect to their environment. About half of these updrafts are still positively buoyant. Downdrafts that are warm, dry and have an excess of along-wind momentum (fast) occupy the greatest area of any downdraft type. Most of them are positively buoyant. Also found near the top of the mixed layer are large numbers of warm/dry/fast updrafts and cool/moist/slow downdrafts, i.e., drafts that have overturned. Time series from this level reveal large masses of cool/moist/slow and warm/dry/fast air, usually containing both upward-moving and downward-moving elements. These observations are related to features seen in returns from clear convective boundary layers, using various remote sensing systems. Conditional sampling is also used to determine the net buoyant production or consumption of turbulence kinetic energy by each convective element. The results support the process partitioning method of entrainment closure but deviate from the assumption of linear buoyancy flux profiles for each proem.