TRACE A TRAJECTORY INTERCOMPARISON .1. EFFECTS OF DIFFERENT INPUT ANALYSES

We address the problem of air mass trajectory uncertainty through an i ntercomparison of trajectories computed from operational meteorologica l analyses from the region and time period of the NASA/GTE/TRACE A exp eriment. This paper examines the trajectory uncertainty that results f rom the input meteorological analyses, We first compare the National M eteorological Center (NMC) and European Centre for Medium-Range Foreca sts (ECMWF) meteorological analyses to an independent set of observati ons, the dropsondes released from the NASA DC-8 over the South Atlanti c during TRACE A, We also compare the gridded wind and temperature fie lds with selected rawinsonde data that entered the analyses. These com parisons show that the ECMWF fields are marginally better than the one s from NMC, particularly in the tropical regions of the southern hemis phere. The NMC analyses are marginally better in the midlatitude weste rlies in some cases. In general, slightly more confidence can be place d in trajectories computed with ECMWF data over the TRACE A region, ba sed on our comparisons of the analyses with observations. Second, we c ompute 5-day back trajectories with three different models from a grid of points over the South Atlantic and adjacent portions of South Amer ica and Africa as well as on the track of TRACE A flight 15 over the S outh Atlantic. When using the Goddard Space Flight Center isentropic m odel, horizontal separations of greater than 1000 km occur for about 5 0% of the points when trajectories run with the ECMWF and NMC analyses are compared. Greater sensitivity to the input analysis differences i s noted when trajectories are computed with the FSU kinematic model (s eparations exceed 1000 km for 75% of the points). The problem of meteo rological uncertainty should be addressed with two approaches. There a re large differences between both sets of analyses and the TRACE A sou ndings; this is also likely to be the case in other remote regions. Th erefore we recommend that a test set of trajectories be computed with both sets of input data to quantify the uncertainty due to analysis di fferences. In addition, clusters of trajectories about the points of i nterest should be run to assess the uncertainty due to wind shear. The se recommendations are applicable to any region of the globe with spar se observations. The companion paper [Fuelberg et al., this issue, par t 2] addresses uncertainties due to trajectory technique.