Forecast Isobar wind question

[James Marinero]

Here's a grib forecast for 01:00 13 Sept 2012, with 45 kts in the Gulf of Lyon.

Anyway, what I'm trying to understand is that only 10kts are forecast over the Alps, when the isobars are so closely packed. Is that a problem with the Grib display or something more fundamental? The angle of the wind to the isobars is almost 90 deg. What am I missing (not that I plan to sail in the Alps)? BTW the central pressure of the low is 1001 mb.

 

[John.edwards]

they look like only 1 mB isobar intervals so gradients look a bit deceptive. often see spacings of 4mB on forecast charts and these are obviously further apart

 

[James Marinero]

Fair point but...

It doesn't explain much stronger winds further south with similar pressure gradients. I don't think that the diff in latitude accounts for it either. 4 mb Cornwall-Brittany = F6 up the Channel was my guideline, so where's the wind at the Alps? Maybe it's friction...

 

[little_roundtop]

I have no doubt that Frank will be along to correct me, but I'm pretty sure that the basic NOAA GFS grib data takes no account of topography. So it's pretty good over open water but less accurate over land.

 

[flawed_logic]

My grib view show's something similar, it's not the isobars that's causing the high winds. Perhaps it's modelling a local variation to do with Katabatic effects?

 

[wotayottie]

Wind speed increases and direction changes by about 15 degrees between land and sea as a result of the effect on land of buildings and hills and mountains. To put it simply, the land is knobbly and these knobbles slow down the wind.

In the case of the Alps then I guess there will be a very big effect. The forecast wind speed will be at the same altitude thoughout so in the alps it will be at the very bottom of valleys.

And at sea level there will be the additional effect of cold winds rushing down the hills carrying high density air out to the sea - katabatic winds as suggested above.

 

[dt4134]

As Wotayottie says, the model takes a limited account of the surface, so is assuming less wind for the same pressure difference over land than it shows over the sea. Reality will be more complex.

The alps are going to affect the winds in all sorts of ways. Funneling down valleys, sheltered valleys, etc. So there will be windspeeds well in excess of those forecast in the alps, especially on the mountains.

Similarly at sea you will get the effects of funnelling and stronger higher level winds brought down to the surface by the mountains, so it could be much more than the forecast shows in some places.

 

[franksingleton]

Here's a grib forecast for 01:00 13 Sept 2012, with 45 kts in the Gulf of Lyon.

Anyway, what I'm trying to understand is that only 10kts are forecast over the Alps, when the isobars are so closely packed. Is that a problem with the Grib display or something more fundamental? The angle of the wind to the isobars is almost 90 deg. What am I missing (not that I plan to sail in the Alps)? BTW the central pressure of the low is 1001 mb.

The simple answer is that the surface winds produced by the models are just that. They are supposed to be about 10 m about the surface of the earth. The sea level isobars are fine over the sea or where the land is low. Over the Alps, Rockies, Himalaya etc the sea level isobars are a fiction calculated assuming some form of standard atmosphere. The movement of air over the Alps will be controlled by the Pressure patterns at those heights and, of course the topography. In comparing surface winds over such areas with “sea level” pressures is like comparing chalk and cheese.

Models do their calculations on a 3D grid. The vertical co-ordinates are percentage values of the ground level pressure. The lowest model level is 99.8% of the ground level pressure. That is around 20 metres above the surface at that point. In other words, the vertical co-ordinates are terrain following. If the ground is 1.5 km above sea level, then the lowest level of the model would be where the pressure is somewhere near 850 hPa. At a ground height of 3 km, the bottom level of the model would be at a pressure of around 700 hPa. Over the sea, of course, the bottom level would be around 1000 hPa. Very approximately, the bottom level of the models is about 20 m above the ground/sea.

Surface winds ie the 10 m wind is derived from algorithms that take roughness and the stability of the air into account.