Scientific utilities

The McIDAS-X library provides a set of scientific utilities for computing meteorological parameters such as potential temperature, equivalent potential temperature, and mixing ratio.

The table below provides an alphabetical listing of the scientific utilities provided in McIDAS-X.

C function	Fortran function	Description
not available	lab	computes potential and equivalent potential temperature and mixing ratio, given the temperature, dew point, pressure, and station elevation
McAdvectParm	mcadvectparm	advects a gridded parameter
McBeta	mcbeta	computes beta parameter
McCape	mccape	computes Convective Available Potential Energy
McCoriolis	mccorfor	computes the coriolis parameter (f=2ΩsinΦ)
McDewpt	mcdewpt	computes dewpoint
McDirec	mcdirec	computes meteorological direction of wind for u- and v-components
McDiver	mcdiver	computes divergence
McDivergeParm	mcdivergeparm	compute divergence of gridded parameter
McHeatIndex	mcheatindex	computes the heat index, given the temperature and dewpoint
McHelic	mchelic	computes helicity
McHypsoP	mchypsop	integrates hypsometric equation to compute a pressure at a given height
McHypsoZ	mchypsoz	integrates hypsometric equation to compute a new height at a given pressure
McLatentHeat	mclatvap	latent heat of vaporization
McLiftCLevel	mclcl	parcel temperature and pressure at LCL
McMixing	mcrmix	mixing ratio
McPFromTheta	mcpfromtheta	returns pressure when given theta and temperature
McRelativeHumidity	mcrelativehumidity	computes the relative humidity, given the temperature and dewpoint
McSatVapor	mcsatvap	saturation vapor pressure over water
McSatVaporIce	mcsatvapi	saturation vapor pressure over ice
McSheard	mcsheard	computes shear deformation
McSndAnl	mcsndanl	computes various stability parameters, given a sounding
McSpeed	mcspeed	computes wind speed from u- and v-components
McStationPres	mcstnpres	station pressure, given altimeter and station elevation
McStretd	mcstretd	computes the stretching deformation
McTDFromMixing	mctamr	computes dew point from mixing ratio
McTempAtThetae	mctasa	temperature along saturated adiabats
McTFromTheta	mctfromtheta	returns temperature when given theta and pressure
McTheta	mctheta	potential temperature
McThetae	mcthetae	equivalent potential temperature
McThetaw	mcthetaw	wet bulb potential temperature
McUandV	mcuandv	computes wind u- and v-components
McVirtTemp	mcvirttemp	virtual temperature
McVort	mcvort	computes vorticity
McWetBulb	mcwetbulb	wet bulb temperature
McWindChill	mcwindchill	computes the wind chill, given the temperature and wind speed (old formula)
McWindChill2001	mcwindchill2001	computes the wind chill, given the temperature and wind speed (uses the 2001 algorithm)
not available	rmix	determines the mixing ratio, given the temperature and pressure

For more information about these scientific functions, see the online man pages provided with the McIDAS-X software.

Computing isentropic surfaces

Given the temperature, dew point and pressure, the lab function computes the following:

Potential temperature
Equivalent potential temperature
Mixing ratio

To compute these meteorological parameters using the pressure at the station instead of the reported pressure, which has been corrected to sea level, the lab function will automatically adjust for the pressure differences due to elevation.

The following example computes potential and equivalent potential temperatures, and mixing ratio with adjustments made to the pressure based on the height.

     double precision derived(3)   ! derived parameters
     double precision dewpt        ! dewpt in K
     double precision elev         ! elevation in meters
     double precision miss         ! missing data code
     double precision press        ! pressure in mb
     double precision temp         ! temperature in K

     temp = 292.d0
     dewpt= 289.d0
     press=1018.29
     miss=1.e35
     elev=214

C  the adjustment based on elevation is done

   call lab(temp,dewpt,press,elev,1,miss,derived,3)

C  this will return theta-e in derived(1), theta in derived(1)
C  and mixing ratio in derived(3)

C  the values returned will be:
C    derived(1) = 323.115
C    derived(2) = 292.612
C    derived(3) = 11.521

C  the adjustment based on elevation is not done

   call lab(temp,dewpt,press,elev,0,miss,derived,3)

C  the values returned will be:
C    derived(1) = 319.961
C    derived(2) = 290.491
C    derived(3) = 11.226

Computing Heat Index

Given the temperature and dew point, the functions McHeatIndex and mcheatindex will compute the heat index. The value returned will be given in the same units as the input temperature. Below is an example of the function mcheatindex.

 integer ok
    double precision temperature
    double precision dewpoint
    double precision heatindex

    temperature = 30.d0
    dewpoint    = 20.d0

    ok = mcheatindex (temperature, dewpoint, 'C', heatindex)

    if (ok .lt. 0)then
       call sdest('error calculating the heat index',0)
       return
    endif

c--- upon successful completion the value of heatindex will
c---    be 32.5 degrees Celsius

Computing Relative Humidity

Given the temperature and dew point, the functions McRelativeHumidity and mcrelativehumidity will compute the relative humidity. The value returned will be given in the dimensionless units of percentage. Below is an example of the function mcrelativehumidity.

integer ok
    double precision temperature
    double precision dewpoint
    double precision rh

 temperature = 30.d0
    dewpoint    = 20.d0

 ok = mcrelativehumidity (temperature, dewpoint, 'C', rh)

    if (ok .lt. 0)then
       call sdest('error calculating the relative humidity',0)
       return
    endif

c--- upon successful completion the value of rh will be 55.1

Computing Wind Chill

Given the temperature and wind speed, the functions McWindChill and mcwindchill will compute the wind chill. The value returned will be given in the same units as the input temperature. Below is an example of the mcwindchill function.

 integer ok
    double precision temperature
    double precision windspeed
    double precision windchill

 temperature = 20.d0
    windspeed   = 10.d0

 ok = mcwindchill (temperature, windspeed, 'F', 'MPS',
        &                   windchill)

 if (ok .lt. 0)then
        call sdest('error calculating the wind chill',0)
        return
    endif

c---upon successful completion the value of windchill will
c---be -15 degrees Fahrenheit

Computing mixing ratio

Given a temperature, in Kelvin, and the pressure, in mb, the rmix function will return the mixing ratio. The returned value, ws, is defined as the mass of water vapor per mass of dry air. It is dimensionless, with units of g/kg. The sample code below computes the saturation mixing ratio at 0° Celsius for all pressures 100, 150, 200, 250, ... 950, 1000 mb, resulting in 40.44, 26.39, 19.59, 15.57, ... 4.02, 3.82 g/kg.

     integer i
     real temperature
     real pressure
     real mix
     temperature=273.15

     do 100 i=1000,100,-50
       pressure=float(i)
       mix=rmix(temperature,pressure)

100    continue

Computing stability parameters

Given vertical profiles of pressure, temperature, dew point, wind speed and direction, the sndanl function computes these stability indices:

Parcel dew point, potential temperature, equivalent potential temperature and mixing ratio
Precipitable water
Convective temperature and forecast maximum temperature (for data from 1200 UTC only)
Lifted index, total-totals index, K-index and sweat index
Equilibrium pressure level

Below is an example of the sndanl function.

    parameter (NLEV = 9)
    real press(NLEV)
    real temp(NLEV)
    real dewpt(NLEV)
    real dir(NLEV)
    real spd(NLEV)
    real stabil(12)

c---    get sounding data

    :

c---    assume the arrays; press, temp, dewpt, dir and spd
c---    have been initialized with the following values

c---    press(mb)  temp(K)  dewpt(K)  dir(DEG)  spd(MPS)
c---    988.0      300.0    299.8     250   1.5
c---    925.0      303.0    280.0     270   3.8
c---    850.0      297.7    269.0     285   5.6
c---    700.0      277.5    276.1     290   12.3
c---    500.0      262.1    252.1     265   13.3
c---    400.0      253.6    240.5     280   29.8
c---    300.0      237.6    226.0     280   33.4
c---    250.0      229.7    215.7     285   41.6
c---    200.0      216.3    206.3     285   45.7

c---    perform the sounding analysis for 00Z

       call sndanl (0, NLEV, press, temp, dewpt, dir, spd, stabil)

c---    upon return, the values stored in stabil will be as follows:

c---    stabil(1)   282.4   parcel dewpoint (k)
c---    stabil(2)   301.1   potential temperature (k)
c---    stabil(3)   324.1   equivalent potential temperature (k)
c---    stabil(4)     8.3   mixing ratio (g/kg)
c---    stabil(5)    23.7   precipitable water (mm)
c---    stabil(6)    32.7   convective temperature (c)
c---    stabil(7)    26.9   forecast maximum temperature (c)
c---    stabil(8)     2.7   lifted index (k)
c---    stabil(9)    33.4   total totals
c---    stabil(10)  445.5   equivalent pressure (mb)
c---    stabil(11)   20.6   k index
c---    stabil(12)   47.6   sweat index

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