R/MurphyKoop.R

In NCAR/Ranadu: Functions for use with RAF aircraft data files

TZERO = StandardConstant("Tzero")
#' @title MurphyKoop
#' @description Returns the water vapor pressure corresponding to a supplied temperature.
#' @details Calculates the equilibrium water vapor pressure according to the 
#' Murphy-Koop equation
#' @aliases murphykoop
#' @author William Cooper
#' @export MurphyKoop
#' @param DP A numeric vector containing the dew point in deg. C
#' @param P An optional numeric vector representing the total pressure in hPa. Set zero 
#' to suppress the 'enhancement factor' correction for total pressure. Use the pressure to
#' find the equilibrium water vapor in the presence of that total pressure of air.
#' @return Water vapor pressure [hPa] in equilibrium with a plane water surface at dew point DP
#' @examples 
#' e <- MurphyKoop (-12.)
#' e <- MurphyKoop (10., 800.)
#' EW2 <- MurphyKoop (RAFdata$DPXC, RAFdata$PSXC)

MurphyKoop <- function (DP, P=0) {
# returns vapor pressure via Murphy-Koop equations.
# Supply DP=dewpoint (deg C) and optionally P=pressure (hPa),
# the latter for the enhancement-factor correction.
  b0 <- 9.550426
  b1 <- -5723.265
  b2 <- 3.53068
  b3 <- -0.00728332
  b7 <- 54.842763
  b8 <- -6763.22
  b9 <- -4.210
  b10 <- 0.000367
  b11 <- 0.0415
  TR2 <- 218.8
  b12 <- 53.878
  b13 <- -1331.22
  b14 <- -9.44523
  b15 <- 0.014025
  tk <- DP + TZERO
  ess <- exp (b7 + b8 / tk + b9 * log(tk) + b10 * tk + tanh (b11 * (tk - TR2))
             * (b12 + b13 / tk + b14 * log(tk) + b15 * tk))
  ess <- ess/100.
  f=1.e-5 * P * (4.923 - 0.0325 * tk + 5.84e-5 * tk**2)+1.
  ess <- f * ess
  return (as.vector (ess))
}

#' @title MurphyKoopIce
#' @description Returns the water vapor pressure
#' @details Calculates the water vapor pressure in equilibrium over a plane ice surface according to the Murphy-Koop equation
#' @aliases murphykoopice
#' @author William Cooper
#' @export MurphyKoopIce
#' @param FP A numeric representing the frost point in deg. C
#' @param P An optional numeric representing total pressure in hPa, set zero to 
#' suppress 'enhancement factor' correction for total pressure
#' @return Water vapor pressure in equilibrium with a plane ice surface at frost point FP
#' @examples 
#' e <- MurphyKoopIce (-50.)
#' e <- MurphyKoopIce (-60., 200.)
MurphyKoopIce <- function (FP, P=0) {
  b0 <- 9.550426
  b1 <- -5723.265
  b2 <- 3.53068
  b3 <- -0.00728332
  b7 <- 54.842763
  b8 <- -6763.22
  b9 <- -4.210
  b10 <- 0.000367
  b11 <- 0.0415
  TR2 <- 218.8
  b12 <- 53.878
  b13 <- -1331.22
  b14 <- -9.44523
  b15 <- 0.014025
  tk <- FP + TZERO
  ess = 6.111536 * exp (b1 * (TZERO - tk) / (TZERO * tk)
                   + b2 * log (tk / TZERO) + b3 * (tk - TZERO))
  f <- 1.e-5 * P * (4.923 - 0.0325 * tk + 5.84e-5  * tk**2) + 1.
  return (as.vector (f * ess))
}