by MAKG » Fri Mar 06, 2009 1:45 am
FWIW, I doubt folks care much, but global 4D models tend to be a bit coarse and inaccurate, due to the sheer data size.
The one I use (NCEP GFS) lives on a 1 deg x 1 deg grid. There are 0.5 deg grids available (e.g., NOGAPS, which has some obvious upper air "features", and ECMWF which gets bloody expensive and seems to overpredict water vapor), but the data size gets problematic quickly. One needs many pressure levels if one is going to model climbs and descents to low altitude (upper air is a bit easier, as long as one is only interested in altitudes below FL450).
The jet stream is VERY unsteady -- it's not just a constant wind, but it moves around daily. The good folks at KZOA plot transpacific routes based on it, daily. But you can't advect a simulation -- it took a supercomputer to make it, and it will take a supercomputer to evolve it correctly. I run into the same issue in my (very special) flight planner; I cheat (just like the poor Navy OPARS saps do), interpolating timepoints. This smears out the interesting features, like the (very) occasional 200 knot cells. It won't "feel" realistic.
FYI, 70 knots seems a bit on the slow side. It's not at all unusual to see 100+ knots somewhere on any given day, and 200 knots shows up every once in a great while.
The solution for FlightGear will be an analytic/probabilistic model. Not something I'm sure exists, and a whole lot of pain to create. Or maybe folks don't care too much and just want "something" there. That would be easier. I don't know what Jeppesen uses, but I'm skeptical that it can be global in scope and accurate at the same time. The conventional way to cheat is to restrict the domain substantially, such as for CONUS or EPAC.
And while the winds are the biggest factor, there are also issues of temperature. Sound speed is proportional to sqrt(T) where T is absolute temperature. Flying mach numbers (as is commonplace) in the stratosphere will depend upon this in the same proportion. And contrary to common assumptions, the stratosphere isn't -70 deg F everywhere, any more than the ground temperature at sea level is +59 deg F everywhere (ISA is great for generic planning, but sucks for any specific date or location).
That's probably more than you care about; the gist is that it's a difficult problem. I'd like to know how to solve it.