Tropical Storm Modelling

Aktis Hydraulics regularly provides design wind wave and current conditions in cyclone areas. These conditions are usually extremes but, in some areas, cyclones occur frequently enough to have a significant effect on the operational statistics. We have a range of tools and databases available to deal with project phases between site selection and detailed design.

Aktis is practiced in making the balance between the quality of the available data and the level of the sophistication of the modelling, considering not only the modelling uncertainties but also the statistical ones. Sophisticated numerical modelling delivers great results if it is coupled with an understanding of the frequency of the storms modelled as well as the sensitivity to the storm track.

With the WRF model

Cyclones behave differently close to the coast where they are dampened by land roughness and wind is channeled between hills. Accurate wave conditions and water levels can only be hindcast if the wind has been well hindcast. WRF is an atmospheric model that can represent the effects of mountains or land / sea temperature differences on the wind and get the effects of land roughness on the surface wind speed. It also gives pressure, cloud cover, precipitation and a number of other variables that could be of interest. Its advantage is that it can model the interaction between a tropical cyclone and the external conditions such as land masses or temperature. It has a high computational cost and requires good input data but it also has high added value for the accuracy of the wind fields.

We have run the WRF model for cyclones in Myanmar, South Korea and Hong Kong.

3D animation of hurricane Katrina computed with WRF.

The ground overlay shows the temperature and the streamlines present the wind flow in the hurricane. During the top view the cloud cover is also shown.

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With Parametric models

Parametric models such as the Cooper or Holland models represent the pressure and wind fields around a cyclone based on some key information of cyclone (location of the eye, maximum wind speed and radius of maximum wind speed). Contrary to the WRF model, these do not include the effect of land masses, but they are easy to run and reliable on open sea. They represent a good alternative when we have access to a large number of cyclone best tracks and are interested in open sea wind fields. Their great advantage is that they use very little computer time. They are therefore an ideal tool for preliminary studies, for the selection of critical storms and for use in montecarlo simulation.

The use of these parametric models is common at Aktis Hydraulics and we have performed such studies for multiple locations in the South and East China Seas.

With Monte Carlo simulations

In some cases, we are interested in cyclonic conditions at a location that is on the edge of an active cyclone area. This means that the amount of cyclonic data available may be very limited. In those situations, Aktis Hydraulics uses a probabilistic approach to create a synthetic population of cyclones and derive extreme statistics based on the parameters of these synthetic cyclones. This method requires a good experience in cyclonic studies and a trained pragmatic eye one which processes are driving the cyclones in the area of interest.

Aktis has successfully applied this method in a number of countries: Mozambique, Papua New Guinea or Myanmar for example.