Upward shortwave irradiance SW_u

The energy flux density (upward vector irradiance [1]) of solar radiation reflected from the water surface and emerging from under the surface within the 300-4000 nm wavelength range. It is often termed the sea albedo. 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, and on-line, instantaneous point values of this flux recorded at the measurement stations on the Baltic Beta drilling rig.

Methodology

For calculating the upward shortwave irradiance flux we use a semi-empirical model based on instantaneous solar radiation values at the sea surface and at the top of the atmosphere, instantaneous values of the atmosphere’s transmittance[1], and angular values linked with the Sun’s current position. Maps of mean daily values are compiled on the basis of maps of instantaneous values measured every 15 minutes. For measuring the instantaneous point values of the upward short-wave radiation flux we use a pyranometer (CMP6 – Kipp & Zonen).

Validation

The statistical error, expressed as the standard deviation of the differences between upward shortwave irradiance in situ and those determined using the model for daily average values, was  σ=4 Wm^-2. The systematic error (the mean difference between measured and calculated values) was estimated at 0.8 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_u,real and calculated SW_u,model on the basis of the SBRS model[2].

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

[2] SBRS - Surface Radiation Budget for SatBałtyk