Downward longwave irradiance LW_d

The surface energy flux density (downward vector irradiance [1]) of heat radiation emitted by the atmosphere within the 4-100 µm wavelength range reaching the sea surface. This is the so-called reverse radiation of the atmosphere. 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. 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

A semi-empirical model (Zapadka et al. 2008), improved within the framework of the   SatBałtyk project as the SBRS model, is used to calculate the downward flux of long-wave radiation. The input data for the model are obtained from the SEVIRI radiometer (HRV spectral channel, thermal channels) deployed on MSG series satellites and from the UM prognostic  meteorological model. Pyrgeometers (CGR3 and CGR4 – Kipp & Zonen) are used for measuring the instantaneous point values of the downward long-wave radiation flux.

Walidacja

The statistical error, expressed as the standard deviation of the differences between downward longwave irradiance in situ and those determined using the model for daily average values, was  σ=14.2 Wm^-2. The systematic error (the mean difference between measured and calculated values) was estimated at -1 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 LW_d,real and calculated LW_d,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