FORECASTING OF COMPRESSIVE ICE CONDITIONS

摘要

Compressive conditions increase the difficulty of ice navigation. Channels are closed, ship speed is reduced, the ship may get stuck, and in severe case ice buildup and hull damage may occur. Inclusion of compression into ice forecasts would greatly improve the efficiency of winter navigation as difficult areas could be avoided and need of icebreaker assistance assessed better. It is well known to ice navigators that zones of compression may generate, relieve and change location in hours. The physical background is found in ice stress propagation which can experience a complete change of pattern after a slight alteration in wind forcing or ice conditions. The SAFEWIN project approach to compression forecasting is described. It is based on an extension of the Finnish Meteorological Institute operative ice model HELMI. Two day forecasts are provided with 3 hour interval and the compression is expressed as a simple numeral from 0 to 4. The forecasts exhibit similar sensitivity for forcing and ice conditions as the observed compression. This makes however the forecasts sensitive to errors in the model parameters, initial conditions and forecasted forcing fields. A natural solution is to include expected variation for all of these and derive an ensemble of predictions. The forecasts are then presented in terms of probability of occurrence and provided in several formats added to the routine ice forecasts. The forecasts are delivered trough a web site and via the icebreaker network IBNet. For selected ferries and other ships on regular routes experimental ship specific forecasts are possible. The forecasts and presentations are validated and enhanced further using icegoing data and feedback from the end users.