Effects of the outer size on tropical cyclone track forecasts

The track forecast of a tropical cyclone (TC) located near the neutral point of a split subtropical high pressure system under the influence of a nearby trough is difficult and can lead to a large forecast spread among different global numerical weather prediction models. Three TCs with large forecast spread under such an environmental flow pattern have been identified: Hagupit (2008), Lupit (2009) and Nida (2009). Simulations using a bogus vortex of different radii of 15 m·s⁻¹ (R15) with the same environmental flow for each TC case are conducted to study the effect of TC size on the forecast track of the TC. With a nearby strong monsoon trough, a TC with a smaller R15 merges with the cyclonic circulation within the trough leading to a larger outer circulation. After removing the symmetric component of the TC from the total wind field, an extensive cyclonic asymmetric gyre is found and its circulation steers the TC to move more westward. The TC thus shifts northward within the neutral point. With a weak monsoon trough further away from the TC, a TC with larger R15 with a larger outer circulation interacts with the monsoon trough, causing the TC to move more westward. With the presence of a westerly trough, a TC with larger R15 and outer circulation interacts with the trough such that the TC moves more northeastward. The importance of a correct representation of TC size in a numerical weather prediction model in predicting the TC track is discussed.