TRACE A Trajectory intercomparison 2. Isentropic and kinematic methods

Kinematic and isentropic trajectories are compared quantitatively during a single 5-day period (October 13-18, 1992) when several flights for the Transport and Atmospheric Chemistry Near the Equator - Atlantic (TRACE A) experiment were conducted off the west coast of Africa. European Centre for Medium-Range Weather Forecasts (ECMWF) data are used to compute the 5-day backward trajectories arriving at locations over the South Atlantic Ocean and nearby parts of South America and southern Africa. Two versions of kinematic trajectories are examined. One version employs vertical motions supplied with the ECMWF data. These trajectories often differ greatly from those based on the isentropic assumption. The kinematic trajectories usually undergo considerably greater vertical displacements than their isentropic counterparts; however, most diabatic rates are consistent with those of synoptic-scale systems. Ratios of acetylene to carbon monoxide are related to backward trajectories at various locations along a TRACE A flight. A second version of kinematic trajectories employs vertical motions diagnosed from ECMWF horizontal wind components using the continuity equation. These vertical motions are stronger than those supplied with the ECMWF data, causing many of the trajectories to have larger vertical displacements and considerably different paths than the original kinematic versions. Many of these kinematic trajectories undergo diabatic rates that exceed generally accepted values on the synoptic scale. This occurs, in part, because the diagnosed vertical motions are inconsistent with the ECMWF data. The research indicates that the kinematic procedure yields realistic 5-day backward trajectories when the three-dimensional wind data are available from a numerical model or other dynamically consistent data set such as provided by ECMWF.