Sensitivity of Three-Dimensional Trajectories to the Spatial and Temporal Densities of the Wind Field

Abstract

Initialization and forecast fields from the National Weather Service's (NWS) Nested Grid Model (NOM) were archived on the 90 km calculational grid at 2-hour intervals out to 12 hours twice per day, for the 3-month period of January?March 1987. The resulting time series of meteorological data were used to determine the sensitivity of calculated trajectories to changes in temporal and spatial density of meteorological data during a wide range of synoptic conditions. Trajectories were started from 63 evenly spaced locations, twice per day, for a duration of 4 days each over the 74-day period. The 9324 separate trajectories were computed using the meteorological data at 90, 180, and 360 km grid spacing and at 2-, 4-, 6-, and 12-hour time intervals. Calculated trajectories were compared with the base ?truth? case of 2-hour data on the 90 km grid. Trajectories were most sensitive to changes in temporal resolution when the grid resolution was 90 and 180 km. Trajectories computed on the coarser 360 km grid had substantially larger deviations from the base case and were no longer sensitive to changes in temporal resolution. Relative horizontal transport deviations ranged from 5?25% of the travel distance at 96 hours depending upon the spatial and temporal resolution. Results suggest that it rawinsonde observations are the primary source of meteorological data (400 km spacing every 12 hours), then the greatest improvement in trajectory accuracy can be achieved by enhancing the temporal frequency of observations to 6-hour intervals. Results were not different when trajectories were categorized by cyclonic or anticyclonic conditions. However, horizontal deviations during cyclonic conditions were as much as 30% larger than those during anticyclonic conditions. This was attributed primarily to stronger wind speeds in cyclonic systems.