Software demo: {hum_calc} (DRAFT)

For example, the derived moisture variables included in the ICOADS monthly summaries—specific humidity (Q), relative humidity (R), and saturation specific humidity at sea surface temperature (Qs)—are calculated using the Fortran routines in PROFS_short (1981), which were selected from PROFS (1981), and called as described in Slutz et al. (1985), Supp. A, section 4.4 (see: icoads.noaa.gov/Release_1/suppA.html).

Other on-line resources for humidity calculations include implementations by Henry Kleta [Chair, Task Team on Instrument Standards (TT-IS) of the JCOMM Ship Observations Team (SOT)], of the WMO No. 8 (2010) algorithm to compute DPT:

http://www.indigo2.de/download/dewpoint-humidity.exe
http://www.indigo2.de/download/dewpoint-humidity.html
http://www.indigo2.de/download/dewpoint-humidity.jar

work done in conjunction with ongoing efforts to publish JCOMM Technical Report 63 (2014), which contains agreed testcases for testing externally developed DPT calculations, with the IDL Testbench and the output of the agreed testcases available in digital form here:

http://www.indigo2.de/download/dewpoint.pro
http://www.indigo2.de/download/dewpoint-idl.dat

 

Otherwise, the current situation in terms of the apparent lack of any other e.g. WMO-certified software for these types of calculations is a bit surprising and seems like an undesirable situation for the longer term. We note however the existence of this repository for the DPT algorithms associated with E-logbooks:

http://www.wmo.int/pages/prog/amp/mmop/JCOMM/OPA/SOT/dewpoint-algo.html

c Dew point temperature, given sea level pressure (SLP), air temperature (AT),
c wet bulb temperature (WBT), and the designated missing value (FMISS)
c
c An integer implementation of this method has been applied during dupelim for
c ICOADS processing since at least Release 2.0; original algorithm courtesy of
c Dennis Joseph NCAR
c
c Excerpt from Release 2.0 dupelim e-docs:
c         "Compute a missing dew point temperature if WBT and AT are extant; if
c          SLP is missing 1015.0 is used as SLP.  This rule is not applied if WBT
c          is greater than AT.  Constants ACON and BCON are set for computation of
c          DPT relative to water: ACON=7.5 and BCON=237.3.  The following Fortran
c          code is then used to attempt computation of DPT:"

C----------------------------------------------------------------------- 4Jun14
      FUNCTION DEWPT(SLP0,AT,WBT,FMISS)
C dew point temperature given sea level pressure, air and web bulb temperatures
      PARAMETER(ACON=7.5,BCON=237.3)
C
      DEWPT=FMISS
      IF(SLP0.EQ.FMISS)THEN
        SLP=1015.
      ELSE
        SLP=SLP0
      ENDIF
      ESW=6.1078*10.**(WBT*ACON/(WBT+BCON))
      E=ESW-(.00066*SLP)*(((.00115*WBT)+1)*(AT-WBT))
      IF(E.LT.0.)RETURN
      CCON=ALOG10(E/6.1078)
      DEWPT=BCON*CCON/(ACON-CCON)
      END

c Continuing excerpt from Release 2.0 e-docs:
c          "where the RETURN if vapor pressure (E) is less than zero leads to an
c           error diagnostic, and otherwise the resulting DPT is rounded to the
c           nearest 0.1°C.  DPTI is left unchanged (DPTI may be missing or extant
c           depending on input source; DPTI information does not exist in FM 13)."
c Notes: while the dew point temperature indicator (DPTI) is mentioned in those
c comments, it is not part of this implementation. Also, no error diagnostic
c is produced by this implementation in the event E<0, instead FMISS is returned.

C----------------------------------------------------------------------- 6Jun14
      function dewpoint(Tdry,Twet,Pressure)
! A Fortran implementation of:
!      http://www.indigo2.de/download/dewpoint.pro
! which provides for the:
! 'calculation of dewpoint with the formulas from
! WMO-No.8, 7th edition, 2008, Part I, Annex 4.B
! "Formulae for the computation of measures of humidity"'
! This implementation has been validated successfully against the JCOMM
! testcases and benchmark DPT output values in:
!      http://www.indigo2.de/download/dewpoint-idl.dat
! including when the result was -NaN.
! calculate f(p)
      f_p=1.0016+0.00000315*Pressure-0.074/Pressure
! calculate e'_w(t_w) 
      e_w=6.112*exp(17.62*Twet/(243.12+Twet))
! calculate e'
      e=f_p*e_w-0.000653*(1+0.000944*Twet)*Pressure*(Tdry-Twet)
! calculate dewpoint
      dewpoint=243.12*alog(e/(6.112*f_p))/(17.62-alog(e/(6.112*f_p)))
      end

JCOMM TR 63 (2014) documents a set of 252 numerical testcases defined by SOT TT-IS and the JCOMM Expert Team on Marine Climatology (ETMC), plus benchmark DPT output values, which are replicated in http://www.indigo2.de/download/dewpoint-idl.dat. These involve fixing SLP at four selected values, as shown in Figs. 1-4, respectively, and then varying both AT and WBT. In some cases the calculation of DPT fails numerically (i.e. produces -NaN, "not a number" in floating point arithmetic), in which case no value is plotted.

Figure 1. Dew point temperature difference °C between functions DEWPT and dewpoint (vertical axis) given 1013.3 mb pressure, air temperature (horizontal axis), and wet bulb temperature (colors).


Figure 2. As for Fig.1, 1000 mb pressure.


Figure 3. As for Fig.1, 980 mb pressure.


Figure 4. As for Fig.1, 1040 mb pressure.

References
JCOMM, 2014: Recommended Algorithms for the Computation of Marine Meteorological Variables. JCOMM Technical Report No. 63 (draft).

PROFS_short, 1981: icoads.noaa.gov/software/other/profs_short

PROFS (Schlatter, T.W., and D.V. Baker) 1981: Algorithms for thermodynamic calculations. NOAA/ERL PROFS Program Office, Boulder, CO, 34 pp. [icoads.noaa.gov/software/other/profs].

Slutz, R.J., S.J. Lubker, J.D. Hiscox, S.D. Woodruff, R.L. Jenne, D.H. Joseph, P.M. Steurer, and J.D. Elms, 1985: Comprehensive Ocean-Atmosphere Data Set; Release 1. NOAA Environmental Research Laboratories, Climate Research Program, Boulder, CO, 268 pp. (NTIS PB86-105723).

WMO, 2010: Guide to Meteorological Instruments and Methods of Observation, WMO-No 8 (7th edition, 2008, updated in 2010) [http://www.wmo.int/pages/prog/www/IMOP/CIMO-Guide.html].

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