Modeling Tropical Cyclones in EEMS
A tropical cyclone is a system of rapidly rotating wind around a low-pressure center that produces extreme winds, gusts, and heavy rainfall. In the Northwest Atlantic, a tropical cyclone is called a “hurricane” and in the Northwest Pacific, it is called a “typhoon”.
The extreme and rapid changes in atmospheric pressure and wind fields in a tropical cyclone may strongly influence hydrodynamics and waves in a water body, especially in nearshore areas. Storm surge induced by pressure depletion and strong winds tend to pile up water against the shore; subsequent coastal inundation and erosion can cause severe damage. Beyond the flooding issues due to cyclones, modeling their environmental impacts from erosion, spills, and contaminant transport is essential.
The upcoming release of EEMS will feature a new tropical cyclone module to enable you to efficiently model the hydrodynamic consequences of such cyclones. Numerically modeling the influences of a tropical cyclone requires inputs for both pressure and wind fields. Due to a cyclone’s rapid rotation and shifting nature, these fields need to be provided at high-frequency intervals, so the standard spatial interpolation is not appropriate. File input for these fields is not an efficient approach since it requires an external tool to generate the fields and requires considerable disk space to save the fields. EEMS10.4 will include new features to address these issues, which have not always been adequately addressed in other modeling software.
Cyclone Models in EFDC+
With this new module, EFDC+ will internally generate the cyclone fields during the simulation using very lightweight cyclone track data and a parametric cyclone model. Four existing models address tropical cyclones:
- Holland (1980)
- Hubbert et al. (1991)
- McConochie et al. (2004) and
- Willoughby et al. (2006).
All four of these approaches are available in the new EEMS Tropical Cyclone module. The Holland (1980) model uses symmetric pressure and gradient wind profiles without the inflow angle. Hubbert et al. (1991) is a variation on the Holland (1980) model but applies a constant inflow angle and bearing of the maximum wind speed relative to the cyclone’s direction of movement.
The McConochie et al. (2004) model is similar to Hubbert et al. (1991) but uses a distance-dependent inflow angle and a wind speed-dependent boundary layer coefficient. In Willoughby et al. (2006), the gradient wind speed profile is determined based on the tangential wind component in the eye and the one beyond the transition zone. You can choose which of these four parametric cyclone models you want to use when simulating a particular tropical cyclone. Refer to the EFDC+ Theory Guide for more details on these formulations after EEMS10.4 is released.
Pre-processing Data in EFDC_Explorer
EFDC_Explorer (EE) allows the user to import cyclone tracks in different formats. The pre-processing steps in EE include visualizing, editing, filling in the missing data, and other steps.
A number of online data sources are available to provide forecasts and archives of cyclone tracks. Some of the cyclone track formats supported by EEMS are:
– NOAA (National Oceanic and Atmospheric Administration of the United States)
Cyclone tracks are provided in HURDAT2 format with two datasets: the Atlantic hurricane database for 1851-2020 and the Northeast and North Central Pacific hurricane database for 1949-2020.
– JTWC (Joint Typhoon Warning Center of the U.S Navy)
The JTWC best track data source includes the following datasets: Southern Hemisphere (1945-2019), Northern Indian Ocean (1945-2019), and Western North Pacific Ocean (1945-2019).
– RSMC (Regional Specialized Meteorological Centre)
Six regional centers provide cyclone track forecasts and archives for different regions around the world as shown in Table 1 below.
Table 1 Tropical Cyclone Regional Specialised Meteorological Centres (RSMCs) in the world
RSMC Miami-Hurricane Center NOAA/NWS National Hurricane Center, USA. | The Caribbean Sea, Gulf of Mexico, North Atlantic and eastern North Pacific Oceans |
RSMC Tokyo-Typhoon Centre Japan Meteorological Agency | Western North Pacific Ocean and the South China Sea |
RSMC-tropical cyclones New Delhi India Meteorological Department | North Indian Ocean The Bay of Bengal and the Arabian Sea |
RSMC La Réunion-Tropical Cyclone Centre Météo-France | South-West Indian Ocean (including the Mozambique Channel) |
RSMC Nadi-Tropical Cyclone Centre Fiji Meteorological Service | South-West Pacific Ocean |
RSMC Honolulu-Hurricane Center NOAA/NWS, USA | Central North Pacific Ocean |
The Graphical User Interface (GUI) for the Tropical Cyclone module in EE can be activated from the Wind Forcing entry in the Hydrodynamic module, as shown in Figure 1. A dropdown list allows you to select the parametric cyclone model.
The imported raw cyclone tracks can be displayed and processed, as shown in Figure 2. You can clean the track by removing points outside a certain distance from the model’s center. The wind speed is assumed to be 10 m above the water surface. Ambient pressure is assumed to be a constant.
Once the cyclone track has been imported, you can visualize it in the two-dimensional horizontal plane (2DH). Figure 3 shows the pressure field and wind field of Hurricane Katrina, where the cyclone is indicated by low pressure and strong wind circulation. The cyclone track is shown in black. The current cyclone center (also known as the cyclone eye) is marked with a red cross.
Conclusion
The new Tropical Cyclone module in EEMS10.4 will greatly improve cyclone modeling capabilities. We will provide further information about this feature in future blogs, as well as sample models for you to download and run with EEMS in the free demo mode.
What’s next
The upcoming release of EEMS10.4 will also include a number of other exciting enhancements, including:
- Export hydrodynamic linkage files for WASP7 and WASP8
- NetCDF UGRID options
- High-frequency output for NetCDF
- Vegetation drag impact from macrophytes
Look out for the EEMS10.4 beta release in September.
