An internal tropical cyclone tracker for numerical models

An internal tropical cyclone tracker has been developed to track all tropical cyclones within the model domain during a model forecast run. There are several ways to define a tropical cyclone center, for example at a low-level vorticity maximum, sea-level pressure minimum, or average position of the vorticity maximum and sea-level pressure minimum. For this tracker, we define a tropical cyclone center at the low-level circulation center near a sea-level pressure local minimum in order to extract cyclone wind structure based upon the center. The tracking method searches for the circulation center using the tropical cyclone characteristics near the center that: (1) the circulation is always circularly or clam and (2) the low-level relative vorticity maximum is always located close to the sea-level pressure minimum. Therefore, the tracker does not need initial guesses in searching and it can detect cyclogenesis. The advantages of an internal tracker over an offline external tracker are: (1) it can track tropical cyclones more often up to every time step; (2) it can extract tropical cyclone wind structure directly from the full resolution model forecast without interpolation; and (3) it is a useful tool to monitor model forecast even before the forecast run is completed. Once a tropical cyclone center is located, the maximum wind speed of the tropical cyclone is determined by searching for the maximum wind speed of the bottom model level near the center, and the 34 and 50-knot wind radii of the tropical cyclone at 4 quadrants are extracted by searching for the azimuthally averaged wind speed inward and outward in each quadrant.