Cumulus transport of chemical tracers 1. Cloud-resolving model simulations

Convective transport in the tropics and middle latitudes is a critical mech anism leading to the vertical and, eventually, the horizontal redistributio n of chemical tracers throughout the lower atmosphere. Accordingly, a bette r understanding and accurate representation of convective transport are ess ential in simulating tracer distributions and long-range dispersion in chem ical transport models. We employ the UCLA cloud-resolving model (CRM) to si mulate a 5-day period during the GATE phase III experiment, during which se veral mesoscale convective systems were observed. The dynamical structures and thermodynamic balances of the simulated versions of these events compar e favorably with observations. The distributions of six tagged tracers orig inating at different levels in the atmosphere are calculated and analyzed. This represents the first in-depth analysis of regional dispersion over a m ulti-day period utilizing multiple tracers in a high-resolution model. It i s found that the dominant process affecting tracer transport by individual clouds involves convective updrafts and related downdrafts. Entrainment, an vil cloud outflow, and detrainment at various levels ail have important eff ects in different regions of the troposphere. In addition to the convective -scale transport, mesoscale subsidence induced by deep convection is capabl e of moving chemical tracers downward as much as several hundred hPa during a single deep convective event, which can substantially modify tracer dist ributions in the middle and lower troposphere. On the basis of our analysis we conclude that deep convection profoundly affects the distribution of tr opospheric chemical tracers in at least three fundamental ways: (1) by quic kly and directly transporting tracers from the boundary layer to the upper troposphere, (2) by accelerating downward transport of chemical tracers fro m the upper troposphere to the surface, both locally in downdrafts and over broader areas via subsidence, and (3) by inducing the exchange of air betw een the upper troposphere and the lower stratosphere.