These back trajectories are designed to aid in the analysis of the Discoverer "transition periods" as defined by Tim Bates. Each filename in this dataset contains the date and time at which the back trajectories are started. For each transition, there are 5 series of trajectories (5 files) at -12, -06, +00, +06 and +12 hours. Each serier of trajectories has source altitudes of 100m, 500m and 2000m. Please see the general README file for all GASP backtrajectories. Five periods are identified by Tim Bates leading to four transitions. These are labelled as tr12, tr23, tr34, tr45 respectively. The beginning and end locations are considered also as tr01 and tr56. (This is as the Disco entered and left Hobart.) The start location of the transitions are day latitude longitude tr01 319 15 Nov 06Z 42.887 147.339 tr12 330 26 Nov 00Z 43.315 141.148 tr23 333 29 Nov 10Z 41.839 143.153 tr34 338 04 Dec 07Z 43.453 140.511 tr45 340 06 Dec 07Z 42.550 137.727 tr56 346 12 Dec 23Z 42.883 147.336 Backtrajectories are calculated with the NOAA HYSPLIT 3.2 code using the Australian Bureau of Meteorology (ABoM) GASP model. This model has approximately 5 degree horizontal resolution and 18 levels in the vertical. These data are then interopolated onto omega fields for use with HYSPLIT. For each series of trajectories we provide an ASCII file which contains the time, lat, lon, pressure, U, V, vertical velocity in pressure, temperature, relative humidity and surface pressure. These fields come from the GASP data set. Trajectories will fail if the track hits the surface, rises above approx 500 HPa or flows off the restricted domain of the GASP data. Also, the lowest interpolated GASP data is to the 0.991 omega level. This corresponds to roughly 110 meters height. Trajectories below this level are not accurate. Most likely they will be moving too quickly as they do not reflect the physics of the surface drag on the air flow. They still provide a good estimate of the air motion for the first 12-24 hours. Please contact steve siems (s.siems@sci.monash.edu.au) regarding questions.