Downward shortwave irradiance SW_d

The energy flux density (downward vector irradiance [1]) of direct solar radiation and that scattered in the atmosphere within the 300-4000  nm wavelength range, reaching the sea surface. Values are expressed in [Wm^-2]. The SatBałtyk System provides maps (spatial resolution 1 km) of mean daily values of this flux (as a component of the radiation budget), daily doses are expressed in Jm^-2day^-1 and, every 15 min, maps of instantaneous values under the heading Solar radiation. In addition, the on-line maps show instantaneous point values of this flux recorded at the measurement stations in Słupsk, Gać, Hel and on the Baltic Beta drilling rig.

Methodology

The SolRad[2] model is used to determine the magnitude of the downward short-wave radiation flux. It is calculated from the solar irradiance (radiation flux density) measured every 15 minutes; these values are used to calculate daily sums (doses) of radiant energy. 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. Pyranometers (CMP6, CMP11, CMP21 – Kipp & Zonen) are used for measuring the instantaneous point values of the upward short-wave radiation flux.

Validation

The statistical error, expressed as the standard deviation of the differences between downward shortwave irradiance in situ and those determined using the model for daily average values, was  σ=11 Wm^-2. The systematic error (the mean difference between measured and calculated values) was estimated at -2 Wm^-2. The empirical measurements used for validating the model were obtained from  Baltic Beta drilling rig localted on the Baltic Sea (φ=55°28’50.9’’N, λ=18°10’44.7’’E).

Fig. Comparison of mean daily values of measured downward shortwave irradiance SW_d,real and calculated SW_d,model on the basis of the SBRS model[3].

[1] Dera J., 2003, Fizyka Morza, PWN, Warszawa, 541

[2] SolRad - Solar radiation

^[3]SBRS -Surface Radiation Budget for SatBałtyk