HIROMB is an abbreviation for High Resolution Operational Model for the Baltic. As the name suggests, it is a circulation model with high resolution intended to be used for the Baltic Sea, but it is used for other regions as well.

Output variables from HIROMB include:

  • sea levels
  • currents
  • salinity
  • temperature
  • ice concentration
  • level ice thickness
  • deformed ice thickness
  • total ice thickness
  • number of ridges per kilometer
  • mean height of ice ridges
  • ice drift velocity
  • ice convergence or divergence
  • Turbulent Kinetic Energy (TKE)
  • dissipation rate of TKE
  • turbulent diffusivity


HIROMB has been used for operational forecasting at SMHI since 1995, with increasingly higher resolution vertically as well as horizontally. It is a so-called "nested" model, which means it can have higher resolution in a smaller region of interest. These model grids are two-way coupled, which means they exchange information with each other along the boundary of the smaller grid.

The forecasting area extends out to the English Channel in the west and to the northern North Sea in the northwest, where the horizontal resolution is 3 nm (nautical miles); see figure 1. This grid is nested to a higher-resolution grid (1 nm resolution) which covers the whole Baltic Sea, the Danish Straits, Kattegat and Skagerrak. Thus, its western boundary is along a longitude near Skagen in Denmark.

HIROMB model domain covering the Baltic and North Sea
Figure 1. Domain of the operational 3-nm grid. The colour code shows the number of vertical levels in the model. Maximum depth (50 levels) is obtained in the Norwegian trench south of Norway.
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Further, there are some local, high-resolution one-way coupled set-ups of HIROMB, which obtain boundary information from the 1-nm grid but gives no information back. The ones currently run in real time include one local model for the small area Brofjorden on the Swedish west coast (60 m resolution), run by SMHI. Another one is a regional model which covers the Gulf of Finland and the Gulf of Riga. It has a horizontal resolution of 0.5 nm and is run by the Marine Science Institute in Estonia. Other regions of the Baltic Sea have also been tested, e.g. the south-eastern Baltic Sea (the Coastal Research and Planning Institute at Klaipeda University as well as the Maritime Institute in Gdansk) and Lake Vanern in Sweden (370 m resolution).

HIROMB model domain covering the Arctic Ocean
Figure 2. Domain of the High-Latitude setup of HIROMB, used to make daily 60-hour forecasts.
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Finally, there is a High-Latitude setup of HIROMB with 6 nm resolution, which makes daily 60-hour forecasts. It covers the north-eastern part of the North Atlantic and part of the Arctic. The purpose is to support research expeditions as well as oil combating by supplying forecasts of ocean currents.