Ground-based Retrieval of Entrainment Rates in Stratocumulus-topped Boundary Layers

A stratocumulus-topped boundary layer (STBL) is a shallow convecting layer in which both the updraft and the downdraft branches of the convective circulation are saturated in the upper part of the boundary layer. The convective circulation is vigorous, yet the cloud-top entrainment velocity is small, and therefore difficult to measure. Differences between parameterizations of the entrainment velocity currently used in global climate models can lead to STBLs that differ by as much as a factor of two in climatologically important properties such as liquid water path and boundary layer depth (Stevens 2002).

Wang and Albrecht (1986) developed a stratocumulus model with an internal circulation based on the mixed layer model studies of Schubert et al. (1979a) and the saturation point analyses of Betts (1983). The internal circulation is represented by updraft and downdraft branches. The properties of the updraft are modified at cloud top by entrainment and radiative fluxes that typically cool and dry the air, and at the ocean surface by turbulent fluxes that typically warm and moisten the air. These cloud-top and surface processes cause the updraft and downdraft properties to differ. Consequently, the updraft and downdraft cloud base heights may also differ. For a steady-state mixed layer with radiative cooling only at cloud top, and with no drizzle, Wang and Albrecht proposed that the difference between updraft and downdraft cloud base heights is a function of the entrainment velocity, the convective mass flux, the jumps in total water (water vapor plus liquid water) mixing ratio, moist static energy, and radiative flux across the inversion base, and several thermodynamic parameters. We are currently using large-eddy simulations of STBLs to test the proposed retrieval method.

It should be possible to use this method to retrieve the entrainment velocity in STBLs using DOE ARM measurements at the Eastern Northern Atlantic ARM site in the Azores. The convective mass flux can be obtained from vertical velocity retrievals from the vertically pointing Doppler cloud radar, updraft and downdraft cloud base heights from the ceilometer combined with the vertical velocity retrievals, the thermodynamic jumps from radiosonde profiles, and the radiative flux jump calculated from the radiosonde profiles of temperature and water vapor combined with a cloud liquid water profile estimated from cloud base and top heights and the liquid water path retrieved from a microwave radiometer, or from adiabatic ascent from the cloud base level.