Gfs Point Data Samples

[Evo]

Attached are some preliminary images covering the GFS from T+24 to T+96 at 18z for a couple of locations. Interesting to note that the GFS was pretty accurate, with the slightly odd exception of the dew point which was totally wrong really. In a couple of weeks, there will hopefully be something a bit better and at longer range to post up.

[johnholmes]

yes, not bad at all.

Just one point, well 2 really

GFS I think(!) gives speeds in knots

it also shows msl pressure whilst the airfield will give a slightly different pressure depending on its height above sea level

John

[Evo]

Hi John

The data source I'm using for the GFS gives kph, so the METAR is converted from knots to kph, so they match. I'd rather they were both in knots but they way it's just done now is the easiest! Also, I'll have to check about the pressure, I think you're probaby right - the airfield METAR will be giving the QNH and the GFS will be predicting the QFE.

[johnholmes]

the GFS will be predicting QFF which is msl pressure, QFE is airfield pressure level.

John

[Evo]

Thanks john, I think a little knowlege can be a dangerous thing!

I've found a formula to convert from QFE to QFF, do I need to convert QNH to QFE, then QFE to QFF or is there an easier way?

The formula I found is QFF = QFE / Exp (- ((g*Z)/(R*Tave)))

Where:

g = the gravitational constant, 9.80617 m/s2

Z = the airfield altitude in metres

R = the dry air gas constant, 287.04 m2/s2

Tave = the average temperature between sea level and the station altitude in degrees Kelvin. <--- I can see this being a problem.

If I've got this straight in my head, QNH is like QFF but QNH assumes the ISA standard atmosphere, so the above formula would convert QFE to QNH if the temperature used was adjusted from the observed temperature using the ISA tropospheric lapse rate of 0.0065 degrees K per metre. To find the QFF, I would need to know the true lapse rate to find Tave.

Am I on the right track or have I confused myself and made it more complicated than it needs to be? Would it instead be best to just allow a few hectopascals difference when analysing?

[johnholmes]

hi Evo

Been off line for a while.

I essence you are correct. Although I'm not sure why you are getting so involved. Unless the airfield is much above sea level then QFF, that given by GFS, in its 500/1000mb charts is perfectly acceptable.

I've copied from an ex colleague his definition, its almost certainly more lucid than mine would be,

• QFE:
  The pressure corrected to the official airfield elevation. An altimeter set to the particular airfield QFE reads zero when an aircraft is on the ground (strictly the height of the altimeter above the ground). In the circuit, the height indicated is the height above official airfield datum.
  

• QNH:
  The pressure 'reduced' to mean sea level, assuming ISA temperature profile from the station/airfield to MSL. An altimeter set to the airfield QNH reads the elevation of the airfield when on the ground.
  

• QFF:
  Barometric pressure 'reduced' to mean sea level, assuming an isothermal atmosphere from the airfield/station to MSL, using current (screen) temperatures. The difference between QFF and QNH can be considerable when atmospheric conditions are significantly different from ISA: i.e. at 'hot and high' airfields.
  

• QNE:
  When the ISA mean sea level standard pressure of 1013.2 hPa is set on an aircraft altimeter subscale, the height so indicated upon landing at an airfield is known as the QNE reading. More widely, this is also the PRESSURE ALTITUDE, which is alternatively defined as the height of any level in the international standard atmosphere (ISA-see above), above the level corresponding to a pressure of 1013.2 hPa.
  

Just check on the height amsl for any airfield you are going to use for checking purposes. Remember that 1mb is equivalent to about 30 ft.

hope this helps

John