Understanding Tropical Cyclones

Understanding Winds Associated with Tropical Cyclones

• Tropical Depression
  Tropical low pressure from 100NM to 300NM in diameter, maintaining identify for 24 hours or more
• Tropical Disturbance
  Less than 33 knots
• Tropical Storm
  Between 34 and 63 knots
• Typhoon/Hurricane
  Greater than 64 knots 

Various agencies use different time-periods to calculate maximum winds. Both the U.S. National Hurricane Center (NHC) and Joint Typhoon Warning Center (JTWC) use 1-minute averages when measuring the maximum wind intensity of a storm. The Japanese Meteorological Agency (JMA) as example, uses 10-minute averages when measuring wind speeds.

A sample advisory regarding Typhoon Malakas last year showed maximum winds of 70 knots in the JMA bulletin but was nearly 15% higher at 80 knots in the JTWC advisory. The difference is more understandable when the different wind methodologies are known.

Understanding the "logic" of tropical cyclone tracks

Typhoons want to re-curve if given the chance. Once they do re-curve, they often want to do it quickly!

In fact, nature relies on this re-curvature process in order to dissipate the enormous amounts of energy that goes into heating the ocean surface to form and intensify the cyclone. As the overall ocean surface temperatures may increase in the years ahead, the process will be enhanced.

Therefore, when a tropical storm curves and moves into the mid-latitudes, encountering cold winds that cannot hold such enormous amounts of energy, the energy is released as wind and rain--and lots of both.

This can be seen in the following image as an Atlantic storm of moderate strength moves into the mid-latitudes, expands and becomes a major extra-tropical storm affecting many thousands of square miles.

Two tropical cyclones over the Atlantic on 07 October, 2016

Days later, one cyclone affects the U.S. East coast, then heads NE to Newfoundland

Within a few more days, the second storm is a major extra-tropical storm covering a huge area over the central ocean with 15 meter waves in the area.

Commercial Impact

 What can you do without being a cyclone expert?

• Avoid sole reliance on distance calculators. While they have their purpose, you need to know the likelihood of heavy weather along various routes because no one wants major deviations during a voyage which can ruin the budgeted profits. To expect a great circle, when situations as those shown above are possible, is to find disappointment after the voyage. Results and likelihood of heavy weather can be seen in the following image where the route and results vary by season, and the likelihood of encountering high winds and waves are tabularized.
• Be aware of how the above discussion about re-curving storms affects the safe and dangerous semi-circles of a storm as shown in the following image (for the northern hemisphere). Consult with StormGeo about the implications.
• Be aware of the aspects of ECO-steaming in areas of prolonged heavy weather that can be generated by slow moving typhoons. It is not always best policy to simply slow-down in heavy weather, especially if that heavy weather is not going anywhere and it may even preclude the ability to pass safely ahead of a particular storm.
• There are large parts of the South China Sea which are not navigable for commercial shipping as this image shows the main trade lanes through the central waters, with a secondary lane near Indonesia and the Philippines. The decision to add miles to avoid a storm which might be prevailing in an area can be costly, and StormGeo is well-equipped to help you understand the implications of these decisions. There are many other areas to be considered with similar restrictions.

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