Thermimage (version 2.1)

Ld: Estimates downward facing longwave radiation (W/m2)

Description

Estimates downward incoming longwave radiation (W/m2) using relationship derived from Konzelmann et al. 1994.

Usage

Ld(Ta = 20, RH = 0.5, n = 0.5)

Arguments

Ta

Local air temperature (degrees Celsius), ~ 2 m above ground

RH

Local relative humidity (fractional value from 0 to 1)

n

Fractional cloud cover (fractional value from 0 to 1)

Value

• A value, vector of length one, corresponding to the incoming longwave radiation, units: W/m2.

Details

By estimating the sky emissivity, from information on humidity and cloud cover, the incoming infrared radiation can be estimated using the Stephan-Boltzmann relationship: emissivity*Stephan Boltzmann constant * T^4. The effective atmospheric emissivity is determined from known cloud emissivity (0.97) and empirically determined clear sky emissivities.

References

Konzelmann et al 1994. Parameterization of gloabl and longwave incoming radiation for the Greenland ice-sheet. Global and Planetary Change. 9: 143-164.

See Also

Lw

Examples

## The function is currently defined as
function (Ta = 20, RH = 0.5, n = 0.5) 
{
    AT <- Ta + 273.15
    WVPs <- 611 * exp(17.27 * (AT - 273.15)/(AT - 36))
    WVP <- RH * WVPs
    ecs <- 0.23 + 0.443 * (WVP/AT)^(1/8)
    ecl <- 0.976
    etotal <- ecs * (1 - n^2) + ecl * n^2
    Ld <- etotal * StephBoltz() * AT^4
    Ld
  }

# Returns a value in W/m2 of the estimated incoming longwave radiation  
# Example calculation:

Ta<-30
RH<-0.5
n<-0
Ld(Ta, RH, n)