## Daily total of shortwave radiation

Fig.1 Distribution of the daily dose of solar radiation incident on the surface of the Baltic Sea

The daily sum of solar radiation in the 300-4000 nm wavelength range (often termed “short-wave solar radiation” or “total solar radiation”), reaching the sea surface directly from the Sun or scattered in the atmosphere. Values are expressed in [MJ·m­-2]. The SatBałtyk system presents this parameter in the form of maps with a 1 km spatial resolution.

Methodology

The SolRad[1] model is used to determine the daily dose of short-wave solar radiation. It is calculated from values of the solar irradiance (radiation flux density) measured every 15 minutes. The input data for the model are obtained from the SEVIRI radiometer (HRV spectral channel) deployed on MSG series satellites, AVHRR, TOVS and SBUV/2 radiometers deployed on satellites of the TirosN/NOAA and Metop series, and from the UM or WRF meteorological models.

Validation

The statistical error, expressed as the standard deviation between values of the flux measured in situ and those calculated using the SolRad model:



where N – number of cases, Xi,C – measured value, Xi,M – calculated value (modelled).
The systematic error (the mean difference between the measured and calculated values):





was determined with respect to measurements made at the Hel station (Lat. 54°36’21.3”, Long. 18°47’53.4”) in 2013-2015. The values of the errors calculated for particular months are listed in Table 1.

Table 1. Validation results of the daily sums of irradiance determined by the SolRad model with respect to measurement data (all values in [MJm-2]); variable: Qd(Hel)-Qd(SolRad)

 Month Number of cases Average value Statistical error σε Systematic error <ε> January 56 -0.478 0.469 -0.299 February 52 -0.868 0.694 -0.192 March 60 -1.317 1.492 -0.171 April 54 -1.938 1.085 -0.142 May 54 -2.183 1.425 -0.128 June 58 -2.308 1.762 -0.132 July 62 -1.274 1.337 -0.076 August 55 -1.340 1.035 -0.082 September 55 -1.051 0.848 -0.096 October 61 -0.818 0.733 -0.176 November 60 -0.665 0.518 -0.359 December 55 -0.329 0.289 -0.276 Year 682 -1.211 1.231 -0.178