Upward longwave irradiance LW_u

The energy flux density  (upward vector irradiance [1]) of thermal radiation emitted by the sea surface and thermal radiation reflected from this surface within the 4-100 µm wavelength range. 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

Instantaneous values of the upward longwave irradiance flux are determined on the basis of the Stefan-Boltzmann law and on the assumption that the sea radiates like a grey body. Input data are satellite maps of sea surface temperature SST, compiled on the basis of the thermal channels on the AVHRR radiometer deployed on NOAA- and MetOp-type satellites. The mean daily map is plotted from all the instantaneous maps available for a given 24-hour period (SBRS model). If there are no flux values on the mean map, owing to the long-term presence of clouds above a given pixel, the final mean product may contain pixels without any assigned value. A CGR3 pyrgeometer (Kipp & Zonen) is used for measuring the instantaneous point values of the upward long-wave radiation flux.

Validation

The statistical error, expressed as the standard deviation of the differences between upward longwave irradiance in situ and those determined using the model for daily average values, was  σ=-7.1 Wm^-2. The systematic error (the mean difference between measured and calculated values) was estimated at 0.4 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 upward longwave irradiance LW_u,real and calculated LW_u,model on the basis of the SBRS model[2].

[1]Dera J., 2003, Fizyka Morza, PWN, Warszawa, 541 SBRS - Surface Radiation Budget for SatBałtyk