The effects of entrainment and mixing on droplet populations in trade wind cumuli
Abstract
A recently-developed modeling framework, within which droplets can have different supersaturation histories as they move through a simulated cloud as a result of entrainment and mixing, is used to investigate the evolution of droplet populations in trade wind cumuli using observations collected during the Rain in Cumulus over the Ocean (RICO) field campaign. The modeling framework consists of a three-dimensional cloud model coupled with a Lagrangian microphysical model, and includes the activation of entrained CCN and representations of homogenous mixing, inhomogeneous mixing, or a combination of both. New observations at multiple levels within the observed clouds on a single day are used to evaluate the main trends in the cloud characteristics and the drop size distributions to compare with the model predictions. The predicted droplet size distributions are in general agreement with those observed, but the mean droplet diameter is often too large, likely as a result of the underestimation of entrainment in the three-dimensional cloud simulation. The model predictions are generally closer to the observations when the mixing of entrained air is represented as half inhomogeneous and half homogeneous, while pure homogeneous or inhomogeneous mixing alone produces results that represent relative extremes. The intensity of the entrainment events was found to be important in determining the width of the droplet size distribution because of the large influence of entrained CCN, which when activated, decrease the mean droplet diameter of the droplet size distribution, and broadens it toward smaller sizes. The timing of the entrainment events appears to have less importance to the broadening of the droplet size distribution. It was found in the RICO observations that the total cloud droplet number concentrations could increase with height, and this behavior was reproduced in the modeling results, occurring more often with stronger entrainment events, as well as greater number concentrations of entrained CCN. Activation of entrained CCN or CCN from evaporated drops is a likely explanation for the increase in total droplet number concentration with height observed during the RICO field campaign and in past studies of warm maritime cumuli.
Degree
Ph.D.
Advisors
Lasher-Trapp, Purdue University.
Subject Area
Atmospheric sciences
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