shortwavetopo: Downscales shortwave radiation accounting for topographic...
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shortwavetopo

R Documentation

Downscales shortwave radiation accounting for topographic effects

Description

shortwavetopo is used to downscale components of the flux density of shortwave radiation received at the surface of the Earth using a high-resolution digital elevation dataset, ignoring canopy effects.

Usage

shortwavetopo(
  dni,
  dif,
  julian,
  localtime,
  lat = NA,
  long = NA,
  dtm = array(0, dim = c(1, 1)),
  slope = NA,
  aspect = NA,
  svf = 1,
  alb = 0.23,
  albr = 0.23,
  ha = 0,
  reso = 100,
  merid = round(long/15, 0) * 15,
  dst = 0,
  shadow = TRUE,
  component = "sw",
  difani = TRUE
)

Arguments

`dni`	a single numeric value, raster object, two-dimensional array or matrix of coarse-resolution direct radiation perpendicular to the solar beam (MJ m^-2 hr^-1).
`dif`	a single numeric value, raster object, two-dimensional array or matrix of diffuse radiation horizontal ot the surface (MJ m^-2 hr^-1).
`julian`	a single integer representing the Julian as returned by `julday()`.
`localtime`	a single numeric value representing local time (decimal hour, 24 hour clock).
`lat`	a single numeric value representing the mean latitude of the location for which downscaled radiation is required (decimal degrees, -ve south of equator).
`long`	a single numeric value representing the mean longitude of the location for which downscaled radiation is required (decimal degrees, -ve west of Greenwich meridian).
`dtm`	an optional raster object, two-dimensional array or matrix of elevations (m), orientated as if derived using `is_raster()`. I.e. `⁠[1, 1]⁠` is the NW corner.
`slope`	a single value, raster object, two-dimensional array or matrix of slopes (º). If an array or matrix, then orientated as if derived using `is_raster()`. I.e. `⁠[1, 1]⁠` is the NW corner.
`aspect`	a single value, raster object, two-dimensional array or matrix of aspects (º). If an array or matrix, then orientated as if derived using `is_raster()`. I.e. `⁠[1, 1]⁠` is the NW corner.
`svf`	an optional single value, raster object, two-dimensional array or matrix of values representing the proportion of isotropic radiation received by a partially obscured surface relative to the full hemisphere as returned by `skyviewtopo()`.
`alb`	an optional single value, raster object, two-dimensional array or matrix of surface albedo(s) (range 0 - 1) derived using `albedo()` or `albedo_adjust()`.
`albr`	an optional single value, raster object, two-dimensional array or matrix of values of albedo(s) of adjacent surfaces (range 0 - 1) as returned by `albedo_reflected()`.
`ha`	an optional raster object, two-dimensional array or matrix of values representing the mean slope to the horizon (decimal degrees) of surrounding surfaces from which radiation is reflected for each cell of `dtm` as returned by `mean_slope()`.
`reso`	a single numeric value representing the spatial resolution of `dtm` (m).
`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).
`shadow`	an optional logical value indicating whether topographic shading should be considered (False = No, True = Yes).
`component`	an optional character string of the component of radiation to be returned. One of "sw" (net shortwave radiation, i.e. accounting for albedo), "sw2" (total incoming shortwave radiation), "dir" (direct), "dif" (diffuse), "iso" (isotropic diffuse), "ani" (anistopic diffuse), "ref" (reflected).
`difani`	an optinional logical indicating whether to treat a proportion of the diffuse radiation as anistropic (see details).

Details

If slope is unspecified, and dtm is a raster, slope and aspect are calculated from the raster. If slope is unspecified, and dtm is not a raster, the slope and aspect are set to zero. If lat is unspecified, and dtm is a raster with a coordinate reference system defined, lat and long are calculated from the raster. If lat is unspecified, and dtm is not a raster, or a raster without a coordinate reference system defined, an error is returned. If dtm is specified, then the projection system used must be such that units of x, y and z are identical. Use projectRaster() to convert the projection to a Universal Transverse Mercator type projection system. If dtm is a raster object, a raster object is returned. If dtm is a raster object, a raster object is returned. If dni or dif are raster objects, two-dimensional arrays or matrices, then it is assumed that they have been derived from coarse-resolution data by interpolation, and have the same extent as dtm. If no value for ha is provided, the mean slope to the horizon is assumed to be 0. If no value for svv is provided, then the entire hemisphere is assumed to be in view. If no value for svf is provided, then the entire hemisphere is assumed to be in view. If values of alb and albr are not specified, then a default value of 0.23, typical of well-watered grass is assumed. If single values of alb and albr are given, then the entire area is assumed to have the same albedo. If dtm is specified, then the projection system used must be such that the units of x, y and z are identical. Use projectRaster() to convert the projection to a Universal Transverse Mercator type projection system. If no value for dtm is provided, radiation is downscaled by deriving values on the inclined surfaces specified in slope and aspect and topographic shadowing is ignored. If single values are provided for slope and aspect single values of components of shortwave radiation for an inclined surface are returned. Only single values of lat and long are taken as inputs. Under partially cloudy conditions, a proportion of diffuse radiation is typically anistropic (directional). If difani is TRUE (the default), then the assumption is made that hourly direct radiation transmission can define the portions of the diffuse radiation to be treated as anistropic and isotropic. If difani is FALSE, all diffuse radiation is treated as isotropic. If dtm covers a large extent, the dtm is best divided into blocks and seperate calculations performed on each block. Since horizon angles, topographic shading and sky view correction factors may be influenced by locations beyond the extent of dtm, it is best to ensure dtm covers a larger extent than that for which radiation values are needed, and to ensure sub-divided blocks overlap in extent. Calculations are faster if values for all inputs are provided.

Value

If component is "sw", a raster object or two-dimensional array of numeric values representing net shortwave radiation (MJ m^-2 hr^-1).

If component is "sw2", a raster object or two-dimensional array of numeric values representing total incoming shortwave radiation (MJ m^-2 hr^-1).

If component is "dir", a raster object or two-dimensional array of numeric values representing direct shortwave radiation (MJ m^-2 hr^-1).

If component is "dif", a raster object or two-dimensional array of numeric values representing diffuse shortwave radiation (MJ m^-2 hr^-1).

If component is "iso", a raster object or two-dimensional array of numeric values representing isotropic diffuse shortwave radiation (MJ m^-2 hr^-1).

If component is unspecified, then the default "sw" is returned.

Examples

library(raster)
# =================================
# Extract data for 2010-05-24 11:00
# =================================
dni <-dnirad[,,3444]
dif <-difrad[,,3444]
# ===========================
# Resample to 100m resolution
# ===========================
dnir <- raster(dni, xmn = -5.40, xmx = -5.00, ymn = 49.90, ymx = 50.15)
difr <- raster(dif, xmn = -5.40, xmx = -5.00, ymn = 49.90, ymx = 50.15)
crs(dnir) <- '+init=epsg:4326'
crs(difr) <- '+init=epsg:4326'
dnir <- projectRaster(dnir, crs = "+init=epsg:27700")
difr <- projectRaster(difr, crs = "+init=epsg:27700")
dni <- resample(dnir, dtm100m)
dif <- resample(difr, dtm100m)
sv <- skyviewtopo(dtm100m)
jd <- julday(2010, 5, 24)
ha <- mean_slope(dtm100m)
# ================================================================
# Calculate and plot net shortwave radiation for 2010-05-24 11:00
# ================================================================
netshort100m <- shortwavetopo(dni, dif, jd, 11, dtm = dtm100m,
                              svf = sv, ha = ha)
plot(mask(netshort100m, dtm100m),
     main = "Net shortwave radiation")

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