Maps of latent and sensible heat fluxes, as well as downward fluxes of short- and long-wave radiation over the Baltic are plotted from the WRF model.

Fluxes of latent and sensible heat and of downward short- and long-wave radiation at the surface, are obtained with the aid of a numerical weather model (representative of a 4×4 km area).

The exchange of energy between the atmosphere and substrate (surface of land or sea) is extremely important for processes taking place both in the lower layers of the atmosphere and in the surface layers of the land or sea. This exchange occurs through the mediation of (i) latent heat, associated with the evaporation or condensation of water; (ii) sensible heat, associated with the temperatures of the air and substrate; (iii) short-wave radiation, arriving through the atmosphere from the Sun and partially reflected back into the atmosphere; (iv) long-wave radiation, passing through the atmosphere towards the land/sea surface and emitted from the land/sea surface back into the atmosphere. Determining these components is an important aspect of weather models. The model used in the SatBałtyk system is WRF (Weather Research and Forecasting), one of the most commonly used mesoscale weather models in the world (appropriate for analysing phenomena at a scale of several tens to several hundred kilometres, which corresponds to the dimensions of the Baltic Sea).

The WRF model in the SatBałtyk system covers the Baltic Sea area and uses results from the global weather model as a source of information about neighbouring areas. The details of the WRF model and its configuration in the SatBałtyk system can be found here.

The SO SatBałtyk service gives the distributions of the various components of the heat flux and the fluxes of downward radiation over the Baltic are given in watts per square metre [W/m²], 24 times in 24 h, in the form of maps with a 4 km resolution.

Methodology of determining heat and radiation fluxes at a surface

The fluxes of heat and radiation at the lower boundary of the atmosphere depend not only on the properties of the atmosphere, but also on those of the substrate. In land areas, the properties of the substrate – its temperature, humidity, snow cover, etc. –  are determined using a weather model containing a separate module solving the equations of heat and water transport in the upper soil layer. In sea areas, the only information regarding the substrate is the sea surface temperature (SST), which is given as a boundary condition. The SatBałtyk system uses SSTs obtained from remote-sensing data. It is assumed that the long-wave radiation emerging from the sea surface depends solely on SST. It is also assumed that the sea surface has a constant albedo for short-wave (solar) radiation. It is for this reason that upward radiation fluxes are not available as products of the SatBałtyk system. They can, however, be calculated in a simple way from other available variables, although, obviously, they are included in the energy balance of the atmosphere’s lower layer.

Downward radiation fluxes are determined from equations of radiation transport through the several layers of the atmosphere, with absorption, scattering and reflection by clouds and other constituents of the atmosphere being taken into account (the so-called radiation transfer model). The sensible heat flux is determined from the wind speed and the temperature difference between the water/land and the air. The latent heat flux is linked directly with the intensity of evaporation/condensation at the surface, which, in turn, is proportional to the wind speed and difference in humidity between the air and the substrate.

Validation (assessment of accuracy) The accuracy of the surface flux maps is estimated from an analysis of the differences between measured and modelled values. The results of the model’s validation can be found here.

Odnośniki do parametrów w Systemie SatBałtyk:

Latent heat (WRF model) (for registered users)

Sensible heat (WRF model) (for registered users)

Downward shortwave radiation (WRF model) (for registered users)

Downward longwave radiation (WRF model) (for registered users)