TRACE A TRAJECTORY INTERCOMPARISON .2. ISENTROPIC AND KINEMATIC METHODS

Kinematic and isentropic trajectories are compared quantitatively duri ng a single 5-day period (October 13-18, 1992) when several flights fo r the Transport and Atmospheric Chemistry Near the Equator-Atlantic (T RACE A) experiment were conducted off the west coast of Africa. Europe an 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 A frica. Two versions of kinematic trajectories are examined. One versio n employs vertical motions supplied with the ECMWF data. These traject ories often differ greatly from those based on the isentropic assumpti on. The kinematic trajectories usually undergo considerably greater ve rtical displacements than their isentropic counterparts; however, most diabatic rates are consistent with those of synoptic-scale systems. R atios of acetylene to carbon monoxide are related to backward trajecto ries at various locations along a TRACE A flight. A second version of kinematic trajectories employs vertical motions diagnosed from ECMWF h orizontal wind components using the continuity equation, These vertica l motions are stronger than those supplied with the ECMWF data, causin g many of the trajectories to have larger vertical displacements and c onsiderably different paths than the original kinematic versions. Many of these kinematic trajectories undergo diabatic rates that exceed ge nerally accepted values on the synoptic scale. This occurs, in part, b ecause the diagnosed vertical motions are inconsistent with the ECMWF data. The research indicates that the kinematic procedure yields reali stic 5-day backward trajectories when the three-dimensional wind data are available from a numerical model or other dynamically consistent d ata set such as provided by ECMWF.