Measuring Entrainment, Divergence, and Vorticity on the Mesoscale from Aircraft

Abstract

Three independent techniques for measuring entrainment at the top of the planetary boundary layer (PBL) from an aircraft are discussed: 1) measuring the terms in the budget of a scalar and solving for the entrainment term; 2) estimating the entrainment velocity as the negative of the ratio of a scalar flux at the top of the PBL to the jump in its mean value across the top; and 3) measuring the divergence within closed (circular) flight paths, integrating with height to obtain the mean vertical motion at the PBL top, and estimating the time rate of change of the PBL top to solve for the entrainment velocity. All of these techniques can be implemented using the same flight pattern. The first two techniques have been used with some success previously, but the divergence technique, as far as the authors know, has not been used for entrainment measurements. The closed flight track can also be used to measure vorticity, with somewhat greater accuracy than for divergence, since the vorticity is typically several times larger than the divergence. These techniques were implemented using the National Center for Atmosperic Research C-130 in the Aerosol Characterization Experiment (ACE-1) and the results for the divergence technique are discussed. It is shown that measuring divergence and vorticity with an aircraft is feasible but is at the edge of currently used air motion sensing technology.