difprop: Calculates the diffuse fraction from incoming shortwave...
In ilyamaclean/microclima: microclimate modelling with R

difprop

R Documentation

Calculates the diffuse fraction from incoming shortwave radiation

Description

difprop calculates proportion of incoming shortwave radiation that is diffuse radiation using the method of Skartveit et al. (1998) Solar Energy, 63: 173-183.

Usage

difprop(
  rad,
  julian,
  localtime,
  lat,
  long,
  hourly = FALSE,
  watts = TRUE,
  merid = round(long/15, 0) * 15,
  dst = 0,
  corr = 1
)

Arguments

`rad`	a vector of incoming shortwave radiation values (either MJ m^-2 hr^-1 or W m^-2)
`julian`	the Julian day as returned by `julday()`
`localtime`	a single numeric value representing local time (decimal hour, 24 hour clock)
`lat`	a single numeric value representing the latitude of the location for which partitioned radiation is required (decimal degrees, -ve south of equator).
`long`	a single numeric value representing the longitude of the location for which partitioned radiation is required (decimal degrees, -ve west of Greenwich meridian).
`hourly`	specifies whether values of `rad` are hourly (see details).
`watts`	a logical value indicating whether the units of `rad` are W m^-2 (TRUE) or MJ m^-2 hr^-1 (FALSE).
`merid`	an optional numeric value representing the longitude (decimal degrees) of the local time zone meridian (0 for GMT). Default is `round(long / 15, 0) * 15`
`dst`	an optional numeric value representing the time difference from the timezone meridian (hours, e.g. +1 for BST if `merid` = 0).
`corr`	an optional numeric value representing a correction to account for over- or under-estimated diffuse proportions. Values > 1 will apportion a greater ammount of total radiation as diffuse than originally calculated by the formula.

Details

The method assumes the environment is snow free. Both overall cloud cover and heterogeneity in cloud cover affect the diffuse fraction. Breaks in an extensive cloud deck may primarily enhance the beam irradiance, whereas scattered clouds may enhance the diffuse irradiance and leave the beam irradiance unaffected. In consequence, if hourly data are available, an index is applied to detect the presence of such variable/inhomogeneous clouds, based on variability in radiation for each hour in question and values in the preceding and deciding hour. If hourly data are unavailable, an average variability is determined from radiation intensity.

Value

a vector of diffuse fractions (either MJ m^-2 hr^-1 or W m^-2).

Examples

rad <- c(5:42) / 0.036 # typical values of radiation in W/m^2
jd <- julday(2017, 6, 21) # julian day
dfr <- difprop(rad, jd, 12, 50, -5)
plot(dfr ~ rad, type = "l", lwd = 2, xlab = "Incoming shortwave radiation",
     ylab = "Diffuse fraction")

ilyamaclean/microclima documentation built on Oct. 31, 2023, 11:41 p.m.