The forecast for hurricane Irma from September 5th, 8am. The red arrow indicates my location.
It seems that Irma will ruin our weekend even if we do not get hit directly. Obviously I am checking the weather forecast on a regular basis, which got me wondering how it is actually done.
I assume the models need wind speed, pressure, temperature and humidity at many grid points as initial data and I wonder how this is derived from satellite images or other sources for different heights. In particular, how do they determine pressure and humidity with the necessary space and time resolution?
I have another question about this wunderground page: What is the difference between computer models and ensemble models? My assumption is that (some) computer models use brute force numerical integration of the Navier Stokes equation(s), but I guess there must be some heuristic models as well, e.g. about rainfall etc. Are the ensemble models just different runs of the computer models with different settings or what? (*)
I am counting on CIP and friends to know this stuff or at least have some good links 8-)
And please let me know before Saturday, because afterwards power and/or internet might be out here ...
added later: (*) I see that xkcd answered one of my questions.
added even later: The above forecast turned out to be mostly correct and we were really lucky here on Paradise Island.
4 comments:
I don't know much about this in detail, but satellites use a whole variety of methods to get wind speed, temperature, pressure, etc, but they also use several other sources of information, like the world wide radiosonde network. For the past several days Hurricane Hunter aircraft have been flying through the hurricane to get detailed in situ measurements, from aircraft sensors, radar, and dropsondes. Satellites get temperature and moisture by measuring the infrared and microwave emissions at a variety of frequencies of different atmospheric opacities. Winds are tough, but some can be gotten by cloud tracking and surface winds from scatterometer measurements of capillary waves. This data, and much more, is ingested in the models and digested by elaborate mathematical techniques.
Detailed pressures and wind speeds come from the Hurricane Hunter aircraft at this point.
I like the Wunderground site and am a junkie there during hurricane season. https://www.tropicaltidbits.com/ has up to date model runs, and hurricane hunter data.
About ensembles. They are an attempt to deal with the problem of sensitive dependence on initial conditions as well as the fact that certain approximations are needed (parameterized). Members of the ensemble are initialized either with slightly different values of poorly known parameters or different parameterizations of some effects, or both. The different version are then run and results compared. If the members are all over the place it tells you something about uncertainties. If they converge, you have more confidence.
But I still think it would be a good time to visit almost anywhere else.
CIP,
thank you for the explanation(s).
I was particularly interested how they distinguish measurements for different heights.
e.g. windspeed at the same location would be different at e.g. 100m above sea level, 1000m and
10000m ...
I know wind is a tough problem because I briefly worked on it ten plus years ago. Weather radars are a potent tool but they are mostly on land, so they only work when storms approach. The hurricane hunters do have doppler radar though. Also, if you know pressures, it tells you something about the winds.
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