Re: Vapor Pressure and Saturated Vapor Pressure

Hi Dennis and Jonathan,

I'm glad this has been raised because I have some similar and related questions about vapour pressure and relative humidity.

I was wondering if the ew(dewpt) and es(t) functions (Bolton and David, 1980) you've described below are suitable for an entire atmosphere calculation with very large temperature ranges and if not whether you have a function that is? I actually need to solve relhum(t,dewpt). I think this is via the relationship relhum=ew(dewpt)/es(t) is it not?

The quoted accuracy for ew(dewpt) in the function from Bolton & David is 0.03c or less for -35 < dewpt < 35c but is this likely to break down significantly beyond the thresholds quoted and would you recommend using it for an entire atmosphere conversion?
        
Solving ew(dept) from Bolton & David below is (sorry Excel and not FORTRAN syntax):

ew=e^((dewpt*19.48+440.8)/(dewpt+243.5))

Wheras a version of ew(dewpt) at: http://www.srh.noaa.gov/epz/?n=wxcalc is:

ew=6.11*10^((7.5*dewpt)/(237.7+dewpt))

These look fairly different but give very similar results. I suspect the one on the NWS site is a slight generalisation but I couldn't find a reference so I'm not sure. Does UCAR have a standard it uses for the relationship?

The Weather Calculator defines es(t) as having the same formula as ew(dewpt) with t substituded for dewpt. In other words, es=e(t) and ew=e(dewpt). Is this a theoretical relationship and if so why doesn't the es(t) function from Bolton and David (1980) below use the same principle? I'm also a little confused about the difference between the functions esat(t) and es(t) and I couldn't find a value for es0 in the Bolton and David es(t) function so I wasn't able to solve it. Can you give us the value or function for es0 Dennis?

Sorry for the long email and probably basic questions.

Simon.

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-----Original Message-----
From: ncl-talk-bounces@ucar.edu [mailto:ncl-talk-bounces@ucar.edu] On Behalf Of Dennis Shea
Sent: Thursday, April 14, 2011 12:25 PM
To: jrbuzan
Cc: ncl-talk@ucar.edu
Subject: Re: Vapor Pressure and Saturated Vapor Pressure

The following is fortran but writing and NCL functio to do this would be trivial.

         real function esat(t)
c NCLEND

c include 'lib_dev:[gudoc]edfvaxbox.for/list'
c g.s. stipanuk 1973 original version.
c reference stipanuk paper entitled:
c "algorithms for generating a skew-t, log p
c diagram and computing selected meteorological
c quantities."
c atmospheric sciences laboratory
c u.s. army electronics command
c white sands missile range, new mexico 88002
c 33 pages
c baker, schlatter 17-may-1982

c this function returns the saturation vapor pressure over
c water (mb) given the temperature (celsius).
c the algorithm is due to nordquist, w.s.,1973: "numerical approxima-
c tions of selected meteorlolgical parameters for cloud physics prob-
c lems," ecom-5475, atmospheric sciences laboratory, u.s. army
c electronics command, white sands missile range, new mexico 88002.

         tk = t+273.15
         p1 = 11.344-0.0303998*tk
         p2 = 3.49149-1302.8844/tk
         c1 = 23.832241-5.02808*alog10(tk)
         esat = 10.**(c1-1.3816e-7*10.**p1+8.1328e-3*10.**p2-2949.076/tk)
         return
         end

or

C NCLFORTSTART
         real function es(t)
c NCLEND

c this function returns the saturation vapor pressure es (mb) over
c liquid water given the temperature t (celsius). the formula appears
c in bolton, david, 1980: "the computation of equivalent potential
c temperature," monthly weather review, vol. 108, no. 7 (july),
c p. 1047, eq.(10). the quoted accuracy is 0.3% or better for
c -35 < t < 35c.

c include 'lib_dev:[gudoc]edfvaxbox.for/list'
c baker, schlatter 17-may-1982 original version.

c es0 = saturation vapor pressure over liquid water at 0c

         data es0/6.1121/
         es = es0*exp(17.67*t/(t+243.5))
         return
         end

---
If you have the dw point temperature you can solve for the vapor pressure using the following
C NCLFORTSTART
         real function dewpt(ew)
c NCLEND
c   this function yields the dew point dewpt (celsius), given the
c   water vapor pressure ew (millibars).
c   the empirical formula appears in bolton, david, 1980:
c   "the computation of equivalent potential temperature,"
c   monthly weather review, vol. 108, no. 7 (july), p. 1047, eq.(11).
c   the quoted accuracy is 0.03c or less for -35 < dewpt < 35c.
c       include 'lib_dev:[gudoc]edfvaxbox.for/list'
c       baker, schlatter  17-may-1982     original version.
         enl = alog(ew)
         dewpt = (243.5*enl-440.8)/(19.48-enl)
On 4/13/11 6:08 PM, jrbuzan wrote:
> I am attempting to calculate Vapor Pressure and Saturated Vapor 
> Pressure, but I have yet to find a function.
> My approach was to use the code from the Relhum function.  However, 
> when I search through the 'contributed.ncl' file, the only calculation 
> I can find for Relhum was with dew point temperature.
>
> Yet, the Relhum function calls Pressure, Temperature, and Mixing Ratio.
>
> Any help?
>
> -Jonathan
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