best wind predictor sites

[dgadee]

I was pointed towards:

http://www.yr.no

a couple of weeks ago, and have found it more reliable than xcweather which I have previously used.

 

[wazza]

www.yr.no is a good quick reference. Here on the Swedish west coast using my iPhone they tell you the nearest 'place', I don't know if that'd work for UK waters but you can certainly type in your nearest town to get a decent idea..

 

[franksingleton]

I was pointed towards:

http://www.yr.no

a couple of weeks ago, and have found it more reliable than xcweather which I have previously used.

There are two useful parts of yr.no but not that particular page. Presentations of that form are strictly for the birds and not for thinking sailors. They have their uses but not for our purposes.

Look at http://om.yr.no/verdata/grib/ for some useful GRIB products. Also, http://www.yr.no/kart/#lat=64.2929&lon=12.2307&zoom=6&laga=vind&proj=3575. These are HIRLAM products. They start with the ECMWF 15 kn output. Add more data – or analyse with more detail. They may even now add radar data; I know that they were trying to follow the UK in that respect. If you want short term detail, they are as good as possible just now.

 

[AntarcticPilot]

My 4 to 5 grid lengths derives from a statement in a paper that I could provide if you wish. It referred to defining weather features. Basically, it says that a grid can only fully resolve weather features of size around 4 or 5 grid lengths. For that reason, weather features less than that size have to be damped out in order to avoid computational instability. That was a failing of early models.

By extension, weather determined by topography smaller than 4 or 5 grid lengths would have to be damped out. When I give talks, I usually show http://weather.mailasail.com/w/uploads/Franks-Weather/iom-25km-grid.jpg and http://weather.mailasail.com/w/uploads/Franks-Weather/iom-octagon.png. They show what the Isle of Man would be treated by a 25 km grid. IoM is roughly 30 x 10 km. A 2 km grid would give a reasonable description of its shape.

The top of the models (UK and GFS certainly, ECMWF, I am pretty sure is 80 km. The levels are not equally distributed. There is more definition near the surface and near the tropopause than elsewhere. ECMWF uses 91 levels.

I cannot explain the rationale except to say that there are rough relationships between horizontal and vertical grid lengths and time steps. ECMWF uses a horizontal grid of ~ 15 km and 91 levels. The UK uses ~ 25 km grid and 70 levels. The GFS uses ~ 27 km and 67 levels. Back in the 1970s, the Met Office operational model used ~ 100 km grid and 10 levels.

I do not have the theory for all this at my fingertips. I can only say that, over the past 60+ years a great deal has been learned through experience. These organisations and the other major players would not go to these degrees of complexity were it not worthwhile.

One point that I hope has come over, and I may be labouring it, is that it is quite easy to get hold of a model, say the WRF, bang in some data and get a forecast. The difficult part that these people do not realise, or ignore, is that it is greater problem to derive a good analysis to initialise their models. Hence my analogy with medical diagnosis/prognosis.

Thanks, Frank - that makes sense of the horizontal sampling. However, the vertical sampling must also have an effect in the visibility of topological features, especially given the low relief of most of the UK. Some of this will be offset because the levels in the models are pressure levels, not absolute height, though I can't visualize it very well. But a feature of low relief that affects the surface winds might not be visible to the model because the vertical sampling doesn't distinguish relatively subtle topography. For example, I understand that channeling of katabatic winds in Antarctica requires special modelling; that the usual models don't capture it.

I may well have this entirely wrong!

 

[franksingleton]

Thanks, Frank - that makes sense of the horizontal sampling. However, the vertical sampling must also have an effect in the visibility of topological features, especially given the low relief of most of the UK. Some of this will be offset because the levels in the models are pressure levels, not absolute height, though I can't visualize it very well. But a feature of low relief that affects the surface winds might not be visible to the model because the vertical sampling doesn't distinguish relatively subtle topography. For example, I understand that channeling of katabatic winds in Antarctica requires special modelling; that the usual models don't capture it.

I may well have this entirely wrong!

Early models used to use pressure as the vertical co-ordinate. That is pressure levels were something like 1000, 900, 800 etc hPa. Clearly that was not very sensible although necessary at the time given the limited computing power and the necessary crudity of the models.

Now they use what are called sigma co-ordinates. The bottom layer of the model is at a height where the pressure is 0.998 of the surface pressure. That is around 20 metres above ground or sea level at a grid point.

Higher levels are all fractions of the surface pressure eg 0.9, 0.8, 0.7 of the surface pressure. That means that models are terrain following. See http://www.met.tamu.edu/class/metr452/models/2001/sigma.png. There are many more levels than the indicative ones I list here.

Winds at the nominal 10 metre height are derived from the values at 0.998 hPa using algorithms that include effects of surface roughness and stability. The effects of topography, eg straits, large headlands etc can only be defined by the horizontal grid of the model.

The vertical spacing of levels is closer near the surface and around the jet-stream/tropopause than elsewhere because there are more marked changes here.

 

[AntarcticPilot]

Early models used to use pressure as the vertical co-ordinate. That is pressure levels were something like 1000, 900, 800 etc hPa. Clearly that was not very sensible although necessary at the time given the limited computing power and the necessary crudity of the models.

Now they use what are called sigma co-ordinates. The bottom layer of the model is at a height where the pressure is 0.998 of the surface pressure. That is around 20 metres above ground or sea level at a grid point.

Higher levels are all fractions of the surface pressure eg 0.9, 0.8, 0.7 of the surface pressure. That means that models are terrain following. See http://www.met.tamu.edu/class/metr452/models/2001/sigma.png. There are many more levels than the indicative ones I list here.

Winds at the nominal 10 metre height are derived from the values at 0.998 hPa using algorithms that include effects of surface roughness and stability. The effects of topography, eg straits, large headlands etc can only be defined by the horizontal grid of the model.

The vertical spacing of levels is closer near the surface and around the jet-stream/tropopause than elsewhere because there are more marked changes here.

Thanks, Frank. I was aware of the many different types of pressure-related coordinates, and had a suspicion that they might use a terrain following one; thanks for clearing that up for me. My favourite coordinate was Potential Vorticity; we actually put that in the standard as an example of a particular type of coordinate. I had to take the definition on trust from Met Office experts!

I should say that my interest stemmed from the widespread use of parametric coordinates in the Z dimension in scientific modelling of the atmosphere and oceans. By that, I mean the use of pressure (or pressure related) coordinates in atmospheric modelling, and similar coordinates in oceanography. Sadly, the existing standard for such coordinates explicitly excluded this type of standard, so an additional standard was required so that parametric coordinates could be used in other standards compliant work.

PS, there was a commercial imperative for the standards work, as well. Aircraft don't fly on absolute height levels, they fly on a pressure altitude system, and flight levels (depending on where you are in the world) are defined according to a standard atmosphere, NOT according to absolute height. So, the new standard was required to cover this as well.

 

[franksingleton]

PS, there was a commercial imperative for the standards work, as well. Aircraft don't fly on absolute height levels, they fly on a pressure altitude system, and flight levels (depending on where you are in the world) are defined according to a standard atmosphere, NOT according to absolute height. So, the new standard was required to cover this as well.

Meteorologists are, essentially, practical people. The basic elements that they are trying to predict are wind, temperature and water content of the air. For numerical weather prediction a 3-D grid is needed. The X and Y co-ordinates are givens. Pressure is the most convenient in the vertical.


As you say. Aircraft fly at constant pressure levels. Which is one reason why upper air charts are heights of a pressure level. The other reason is the purely practical one that one geostrophic scale can be used at all heights to relate height contour spacing to wind speed and direction. Unless near the ground, it is irrelevant to know what exact height an aircraft is flying at. It is important that that the calibration of altimeters is consistent throughout the world.

 

[franksingleton]

..................


Remember that no model can do better than represent weather and topography on a scale of about 5 grid lengths. The Solent is well below the resolution of a 10 km model. To cope with the Solent sea breeze, for example needs a model with a grid of 1 to 2 km. That requires a very detailed analysis, including weather radar imagery.

Re the last para of # 28, we returned to the UK on 4 August. We had been expecting SSW winds around F touching 5. On the day before we left, the GFS increased force to F 4-5 touching 6. That was confirmed by UK Met Office, Météo France and Jersey Met.

That was how it turned out giving one of our fastest sails for that passage. It was a true widow. Had we left a day later, we might still have been there. We could probably have left a day earlier.