Introduction

A tropical cyclone is a rapidly-rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon or tropical cyclone. Within Delft3D-FLOW, time-series for space varying wind velocity components and atmospheric pressure on a spiderweb grid can be used to simulate the impact of such an event. For more information see also the Delft3D-FLOW manual. 

Background

A fundamental part of cyclone modelling is the definition of relationships for the spiderweb. Often you only have information about the track and a maximum wind speed (Vmax). However, to generate a spiderweb you also need to have a maximum pressure drop (Pc), the radius of maximum wind (Rmax) and a wind profile. On top of that, constants are needed for the generation of the wind and pressure fields (e.g. the environmental pressure), inflow angle and the ratio between the maximum wind for 1 minute and 10 minutes.

1. Wind profile: parametric radial profiles of hurricane winds form the basis of current approaches to assess wind return periods and the development of engineering standards in hurricane-prone regions. Holland (2010) revised the Holland (1980) parametric approach by applying information readily available from hurricane archives or in hurricane warning information.
2. Vmax-Pc relation: A technique for relating central pressure and maximum winds in tropical cyclones. Default in Dashboard is Holland (2008).
3. Rmax relation: The RMW plays an important role in hurricane risk prediction. There are multiple relationships implemented in Delft Dashboard. Often the radius of maximum winds (RMW or Rmax) is related to the pressure drop and latitude. DDB supports the following relations: a constant RMW of 25nm, Gross et al. (2004, Vickery and Waldhera (2008) and Nederhoff et al. (2019).

Besides these relationships, the hurricane in a spiderweb is described with a limited amount of directional bins, radial bins, and a maximum radius. 

Usage of the toolbox

The Tropical Cyclone toolbox makes it possible to compute the wind field and pressure drop caused by a cyclone. All it takes are four steps

1. Determine the track of the cyclone
   1. 'Draw track' is used to draw a cyclone track with the mouse
      1. First select the time (start and time step) 
      2. Secondly, determine other settings like Rmax or Pdrop. Which settings to select depends on the 'Method selected in the Toolbox. Default is the Vmax, Pd and Rmax approach
   2. 'Load track' is used to load a previously saved track which was defined in Dashboard
   3.  It is also possible to load tracks of cyclones (*.cyc) from available databases using 'Import Track' from sources of the JTWC, NHC or UNISYS.
2. Define a number of settings related to the spiderweb under 'Spiderweb Parameters', such as the directional bins and radial bins that are used to define aspects of the spiderweb.
3. Click on 'Compute cyclone' to generate a spider web file (*. spw) containing wind field and pressure drop which can be imported in Delft3D-FLOW.
   
4. Run the Delft3D-FLOW simulation.

Figure: Example of the track of a tropical cyclone generated in Delft Dashboard for the coast of Bangladesh.