AWT Weather Routing/Routeing: Applied Weather Technology

Weather Model Enhancement

By Herm Wojdylak

Global Numerical Weather Prediction (NWP) models are the foundation for weather forecasting, especially in the medium and long range period required for optimum ship routing. On a global scale, the model output is quite representative of the prevailing weather patterns, but raw NWP model data can be misleading since each global model has its strengths weaknesses in handling certain weather situations. For this reason, AWT modifies the raw ‘gridded’ model data to better reflect what is likely to happen before the data are used in ship routing or provided to customers as gridded weather data, images, or text forecasts.

One weather phenomenon that is generally poorly handled by global models is the tropical cyclone. Model analyses and forecasts for tropical cyclones often depict the systems as much weaker than they actually are, and the model's predicted track and speed of movement are frequently quite poor. To ensure accuracy, AWT uses the warnings issued by agencies specializing in forecasting tropical systems, such the National Hurricane Center (NHC), Joint Typhoon Warning Center (JTWC), Japan Meteorological Agency (JMA), etc. The location of the storm center, central pressure, maximum wind, and wind radii data contained in these warnings are used by AWT to modify the gridded model pressure and wind data to better reflect the pressure pattern and wind distribution around the storm. As an example, consider the two charts below for Typhoon NIDA, located just west of Guam on November 25, 2009. One chart shows the raw NCEP GFS model pressure and wind analysis, and the other shows the pressure and wind analysis after the model data were modified by AWT using the latest warning information for the typhoon. The raw model data shows a low pressure system in the area, but the low is quite weak and the winds are quite light – not very reflective of a typhoon in the area. However, typhoon NIDA had winds in excess of 100 knots near the center at this time. The chart showing the AWT modified data accurately depicts a deep low and very strong winds in the area in order to provide a safer, more efficient route.

Another phenomenon that global models frequently mis-forecast is a strong cold outbreak off a continent into coastal waters. Model winds can be 10 to 30 percent lower than the winds actually experienced in these cases. For this reason, during the late fall and winter months, AWT modifies the model winds in the Yellow Sea, East China Sea, Taiwan Strait, and northern South China Sea during these strong cold outbreaks.

The modified winds fields are then used to drive AWT's global Wavewatch III ocean wave model to produce more realistic wave and swell data than could be obtained using the raw model winds. This results in enhanced safety, better wind and wave forecasts, better route optimizations, more accurate ETA predictions, and better weather depictions for customers using AWT pressure, wind, and wave data.