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airmass sources at a particular field site.
What would be the most accurate way to determine back trajectories (BT) for
airmass sources at a particular field site. The sampling time
at the field site was ~24 hours. Should I run a 24 hr BT for every hour during
sampling? Would I then combine all of these BT's to get an accurate BT
for the airmass collected over the 24 hr sampling period? Also what height would
you suggest to end the BT at, 10 m or 950 h Pa?
You have stumbled across a difficult question. It's not always a
straightforward thing to determine the best way to get at the question
of source regions using trajectory models, particularly in the absence
of additional information.
The best scenario is where the 24hr period is mostly unchanging -- like
under a high pressure system, or perhaps with easterly offshore winds.
If the airmass isn't moving much and if you are interested in source
regions that are not terribly distant (within several hundred km or so)
chances are that trajectories calculated over the period are not going
to change much so it doesn't matter if you calculate only one or many
trajectories over the period, and it probably doesn't matter much about
the height variable either.
The worst case occurs when the opposite is true. Frankly, if you are
collecting samples across a frontal boundary, I'm not sure if you can
say much about the source region. In the absence of additional
information, I would probably discard the sample. Unfortunately, these
cases are frequently among the most interesting.
When we collect precipitation samples for chemical analysis, we also
keep a record of hourly rainfall. For a 24hr case we would run
trajectories corresponding to the heaviest periods of precipitation,
plus, perhaps, the onset and cessation of precipitation. If the
trajectories are similar over the period, we can be fairly certain about
source regions. If there is a lot of sheer, you really need additional
information to support further analysis.
The bottom line: If 24hr sampling results in insufficient information
to pursue additional analysis, you should move to a shorter sampling
period. If you cannot move to a shorter sampling period, you need to
collect additional information to support your analysis. Sometimes a
compromise is possible. An shorter sampling period may be useful to
establish some baseline information about your sources -- and then you
can go to 24hr or longer periods once you establish a notion of what is
happening out there. You should also have some apriori knowledge about
the sources you expect to track -- such as an emissions inventory.
With regard to 10m or 950 h Pa, I'm not sure it matters a whole lot. Do
a sensitivity test on the model and see what difference it makes. My
guess is not much. For precipitation trajectories, we tend to shoot for
just above the top of the boundary layer -- typically ~850mb.
So, sort out your assumptions up front and make sure you have stated
them when you present your results. There is no "correct" way to do
this. You may also wish to look into cluster analysis for a
semi-objective way of looking at your trajectory results.
Rick Artz