My back trajectory run today looks different than the same trajectory I ran yesterday using the GDAS current7days archive. How often is the GDAS archive current7days file updated?

You can look at the forecast hour column in the endpoints (text) file for both the trajectories. That is the column after year, month, date, hour, minute. Even though the GDAS is is called an “analysis” file it is a series of analysis (0-hour) and 3-hour forecasts. The most recent run, in the current7days file, adds the 6 and 9-hour forecasts, to be overwritten by the next run. If the forecast hours shown in the endpoints file are all 3 or less that is the “best” trajectory. There is about an 8-h delay, such that at 23 UTC the data will go through 15 UTC (+3-h forecast). One way to notice this when you run the trajectory at the “Model Setup Section” web page where you enter the start time etc, it says at the top the GDAS1 contains data from XXXXX to YYYYY. The YYYYY is the latest analysis date/time.

Barbara Stunder
NOAA Air Resources Laboratory