[Wrf-users] Relative Humidity

[Hein Zelle]

Ehsan Beigi wrote:

> Does anyone know the formula for calculating relative humidify from
> specific humidity, temperature and sea level pressure? RH = f(Q,T,PSL)?

We use the following (wrf postprocessor uses slightly different
constants, I believe):

! define relative humidity matching the algorithm used for hindcasts
let pq0 = 379.90516
let a2 = 17.2693882
let a3 = 273.16
let a4 = 35.86
let /title="relative humidity" /units="fraction" f_rh2 = q2 / ( (pq0 / psfc) * exp(a2 * (t2 - a3) / (t2 - a4)) )


where 
      q2 = Q2 variable from wrf
      t2 = T2 variable from wrf
      psfc = surface pressure from WRF

This calculates 2 m (approximately) relative humidity.  If you want a
3D variable, replace t2, qsurf and psfc by the appropriate 3D variables.
Note that this algorithm occasionally results in relative humidity
values greater than 1 (up to 1.2).  In daily use, we limit these to 1.0.


Kind regards,
     Hein Zelle


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Dr. Hein Zelle Advisor Meteorology & Oceanography

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