ap: Angstrom-Prescott solar radiation model
In sirad: Functions for Calculating Daily Solar Radiation and Evapotranspiration

Description Usage Arguments Details Value Note Author(s) References See Also Examples

Angstrom-Prescott model is used to calculate daily global irradiance for a horizontal surface based on sunshine duration.

1	ap(days, lat, lon, extraT=NULL, A=NA, B=NA, SSD)

`days`	Vector of class 'Date' of length n.
`lat`	Latitude in decimal degrees.
`lon`	Longitude in decimal degrees.
`extraT`	Optional. Vector of length n of extraterrestrial solar radiation [MJm-2]. If 'NULL' then it is calculated by the function. Providing extraterrestrial solar radiation speeds up the computation
`A`	Angstrom-Prescott model 'A' coefficient. If 'NA' then A is derived from the map of interpolated coefficients estimated from Meteosat solar radiation data. See details.
`B`	Angstrom-Prescott model 'B' coefficient. If 'NA' then B is derived from the map of interpolated coefficients estimated from Meteosat solar radiation data. See details.
`SSD`	Vector of length n containing sunshine duration [in hours].

Model proposed by Angstrom (1924) and modified by Prescott (1940) assumed linear relationship between: (1) a proportion of bright sunshine hours and astronomical day length and (2) proportion of incoming daily global solar radiation and daily extra-terrestrial radiation. This linear relationship is described by empirical model coefficients: A - intercept, B - slope. Both astronomical day length and daily extra-terrestrial radiation are calculated within this function based on location and time. Model coefficients A and B (if not provided) are derived from interpolated Meteosat-based coefficients from Bojanowski et al. 2013.

Vector of length n of daily solar radiation [MJm-2].

SSD input can contain NA's, but length of vectors 'SSD' and 'days' has to be the identical.

Jedrzej S. Bojanowski

Bojanowski, J.S., Vrieling, A., Skidmore, A.K., 2013. Calibration of solar radiation models for Europe using Meteosat Second Generation and weather station data. Agricultural and Forest Meteorology 176:1-9.
Angstrom, A., 1924. Solar and terrestrial radiation. Quarterly Journal of the Royal Meteorological Society, 50:121-125.
Prescott, J.A., 1940. Evaporation from a water surface in relation to solar radiation. Transactions of the Royal Society of South Australia, 64:114-118.

'apcal' to calibrate the model

require(zoo)
#A <- 0.21
#B <- 0.57
sunshine <- Metdata$meteo$SUNSHINE
days <- Metdata$meteo$DAY
lat <- Metdata$LATITUDE
lon <- Metdata$LONGITUDE
plot(zoo(ap(days,lat,lon, extraT=NULL,A=NA,B=NA,sunshine),order.by=days))