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Fig2.2-9: Seasonal forecast charts for Tropical Storm, Hurricane, Typhoons frequencies are available on Open Charts by selecting Long Seasonal option in the menu (See Fig2.2-5)  and then may be displayed by clicking on the desired icon.

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Giant swell (above 6 meters in height) struck the coasts of Ecuador and Peru causing significant damage to boats and infrastructure.  The swell originated from a storm lying off California several days earlier and was sufficiently powerful to propagate some 5000km while while maintaining a significant amount of energy.  Long period swell was mainly the reason for the damage caused.

Fig2.2-17B: Snapshot of propagation of long period (21-25sec) swell from Californian coastal area towards Ecuador and Peru coasts.  The significant wave heights are about 1m to 2m but amplification can be expected in shelving coastal waters and swell of some  6m were observed causing destruction.

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With sufficiently strong winds, the drag on the low-level air flow is modelled using the Charnock parameter which is used to specify an aerodynamical roughness length scale.  In the IFS, this roughness length scale tends to increase with 10m wind speed.  However, observational evidence suggests that for exceptionally strong winds the coupling between the ocean surface and the wind becomes less efficient at transferring momentum.  So, for exceptionally strong (mean) 10m winds (33m/s) the roughness length scale should be capped.  This limitation avoids the effect of too much drag on the lower atmosphere and enables more realistic (stronger) winds to be forecast in the vicinity of relatively intense tropical cyclones.  It should also be noted that tropical cyclone development with strengthened winds has only a limited effect upon the size and character of waves developed by the Wave model.  The model change to limit the roughness length scale at very high wind speeds was introduced in cycle 47r1 in June 2020.  It is important not to confuse to the aerodynamic roughness length scale with the actual roughness of the sea.  The aerodynamic roughness length scale is a parameter concerning the amount of momentum that is exchanged between the atmosphere and the ocean surface via the surface stress.   The actual roughness of the sea is concerned with wave heights, wave steepness and how much wave breaking is happening.

When considering forecast wave parameters in the vicinity of typhoons, hurricanes etc., it should be remembered that IFS still has difficulties in producing some intense tropical cyclones and their subsequent motion.

Additional Sources of Information

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