apcal {sirad}

Calibrate Angstrom-Prescott model

Description

Function estimates Angstrom-Prescott model coefficients 'A' and 'B' based on reference data

Usage

apcal(lat, days, rad_mea,extraT=NULL, DL=NULL, SSD)

Arguments

lat

Latitude in decimal degrees.

days

Vector of class 'Date' of length n.

rad_mea

Vector of length n containing reference (e.g. measured) solar radiation [MJm-2].

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

DL

Optional. Vector of length n of day length [h]. If 'NULL' then it is calculated by the function. Providing day length speeds up the computation

SSD

Vector of length n containing sunshine duration [in hours].

Details

Function estimates Angstrom-Prescott model coefficients 'A' and 'B' based on reference (e.g. measured) solar radiation data. It performs a linear regression in which 'rad_mea' is dependent variable and a proporsion of 'SSD' and astronomical day length is an independent variable.

Value

Vector containing:

APa

Angstom-Prescott 'A' coefficient

APb

Angstom-Prescott 'B' coefficient

APr2

Coefficient of determination of performed linear regression

Author(s)

Jedrzej S. Bojanowski

References

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.

See Also

'ap' to use Angstrom-Prescott model

Examples

## Calibrate the model based on measured data
data(Metdata)
sunshine <- Metdata$meteo$SUNSHINE
days <- Metdata$meteo$DAY
lat <- Metdata$LATITUDE
rad_mea <- Metdata$meteo$RAD_MEA
apcal(lat=lat,days=days,rad_mea,extraT=NULL,DL=NULL,SSD=sunshine)