Entrainment rates during ACE-2 Lagrangian experiments calculated from aircraft measurements

Abstract

Exchange rates of aerosol particles and vapor species between layers in the atmosphere allow us to estimate the lifetime of particles in the lower troposphere. This work analyzes data obtained during the ACE-2 campaign to calculate exchange using two independent methods, divergence and flux. The net entrainment rates obtained from the divergence method are based on spatially-integrated horizontal winds as well as on the average boundary layer height change. The flux method is based on an eddy correlation approach, but relies in this case on a point measurement of concentration change across an atmospheric interface. The thermodynamic structure in these three experiments included well-mixed layers in addition to overlying, more-stratified buffer layers, between which we have studied the net entrainment of air between adjacent layers. The range of entrainment rate magnitudes reported from both methods was from 0.000 m s^-1 to 0.050 m s^-1(with the exception of a few outlying values). Since both methods have significant uncertainties, we believe the best estimates are the average net entrainment rates for both methods, which were 0.007, 0.007, and 0.006 m s^-1 at the subsidence inversion, for Lagrangians 1, 2, and 3, respectively. The uncertainties were high for both methods, involving a factor of two uncertainty for entrainment rates below 0.020 m s^-1. This high uncertainty suggests that continued use of multiple independent methods for measuring entrainment, preferably with the aid of improved instrumentation for fast measurement of conserved tracers and well-designed sampling strategies, is essential for improving models of the sources and sinks for chemical evolution. In some cases, net entrainment rates calculated from the 2 methods were comparable, but in others the spatial inhomogeneity and sampling limitations led to significant discrepancies in the predicted rates.