Calculating direct normal irradiance from sun photometer measurements
Juan Carlos Antuña-Marrero1, Victoria Eugenia Cachorro2, Frank García1, René Estevan1, Boris Barja1, and Ángel M. de Frutos21Grupo de Óptica Atmosférica de Camagüey, Centro Meteorológico Provincial de Camagüey, INSMET, Cuba 2Grupo de Óptica Atmosférica, Universidad de Valladolid, Spain
Received: 08 Mar 2016 – Accepted for review: 15 Apr 2016 – Discussion started: 20 Apr 2016
Abstract. An alternative method to modeling is proposed for determining direct normal irradiance from sun photometer measurements. Direct normal irradiance is calculated by integration of the sun photometer spectral irradiance measurements. Using pre and post calibration spectral coefficients and the extraterrestrial spectral irradiances at each filter effective wavelength, digital counts are converted into direct spectral irradiances data. Then the trapezoidal integration procedure is applied to calculate the direct normal beam. The error associated to the integration method is estimated between 6 and 2 % depending if the infrared filter is for the 1240 or 1640 nm respectively. Validation with collocated quasi-simultaneous manually measured and with modeled direct normal irradiance shows encouraging results. Although the results are not categorically conclusive, they evidence the capabilities of the proposed method for deriving direct normal irradiance under clear sky and also for cloudy conditions. Better results are expected for validations with simultaneous automatic direct normal irradiance measurements.
Antuña-Marrero, J. C., Cachorro, V. E., García, F., Estevan, R., Barja, B., and de Frutos, Á. M.: Calculating direct normal irradiance from sun photometer measurements, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-74, 2016.