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#' @export
#'
Ld<-function(Ta=20, RH=0.5, n=0.5){
# Longwave radiation downward to surface. Units: W/m2
# Derived from Konzelmann et al 1994. PARAMETERIZATION OF GLOBAL AND LONGWAVE INCOMING RADIATION FOR THE GREENLAND ICE-SHEET.
# Global and Planetary Change. 9: 143-164
# Based on estimating the actual sky emissivity, since if you know the air temperature
# you can estimate incoming infrared radiation using the standard e*theta*T^4 relationship
# RH = relative humidity (fraction from 0 to 1)
# n=fractional cloud cover (0=clear, 1=cloud)
# ecs=emissivity clear sky
# WVP=water vapour pressure (kPa)
# AT=air temperature (Kelvin)
AT <- Ta + 273.15
WVPs<-611*exp(17.27*(AT-273.15)/(AT-36)) # Pascals
# saturated vapour pressure at AT
WVP<-RH*WVPs
ecs<-0.23 + 0.443*(WVP/AT)^(1/8) # emissivity clear sky
ecl<-0.976 # emissivity of cloud
etotal<-ecs*(1-n^2) + ecl*n^2
Ld<-etotal*StephBoltz()*AT^4
Ld
}
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