A simulation study on pre-landfall erratic track of typhoon Haitang (2005) with GRAPES_TCM

A new typhoon model named as GRAPES_TCM is applied to study the pre-landfall erratic track of Typhoon Haitang (2005), which hit China twice in mid-July, by making landfall in Taiwan and Fujian Provinces respectively. This typhoon is quite unusual due to the fact that it loops near the east coast of Taiwan and turns sharply northward in Taiwan Strait, which has seldom occurred during the past 50 years and makes it quite difficult to predict correctly the typhoon's landfall time and position.

The GRAPES_TCM is developed based on Chinese Global/Regional Assimilation and PrEdiction System (GRAPES) with a horizontal resolution of 0.25 X 0.25 lat/lon degree and 31 vertical levels. It is put into real time operational test since 2004 and performs quite well for the track prediction of Typhoon Haitang with a 24-h average error of 121 km and 48-h 183 km. In particular, it predicts almost correctly the pre-landfall loop and sharp turn of Haitang and its asymmetric rainfall.

First of all, a sensitivity experiment is carried out with smoothed Taiwan terrain to see whether the terrain effect is important for the erratic track of Haitang. It is found that the basic erratic feature of Haitang's track is unchanged in the sensitivity experiment but it tends to have a larger loop and a weaker northward turn, demonstrating that Taiwan terrain may not be the key factor here.

The potential vorticity (PV) theory on tropical cyclone motion proposed by Chan et al(2002) is then used to diagnose the underlying mechanism for Haitang's erratic track. Results show that during the loop of Haitang, its PV exhibits a remarkable one-wave asymmetric structure with high PV center looping in the same direction of typhoon center. Calculation of the PV tendency shows that, in accordance with the PV theory on tropical cyclone motion, the typhoon center moves toward the area of maximum wave-number 1 (WN-1) PV tendency for most of the time. Among the terms contributing to WN-1 PV tendency, the horizontal advection of PV is most important with its magnitude nearly ten times larger than the others. The two major components of WN-1 horizontal advection of PV, symmetric advection of asymmetric PV (SAAPV) and asymmetric advection of symmetric PV (AASPV) are almost equal in magnitude with maximum WN-1 SAAPV area corresponding better to the future moving direction of typhoon, implying the relatively important role of asymmetric PV. Similar results are also obtained for Haitang's sharp turn in Taiwan Strait. The intrusion of mid-level warm-dry air and the existence of low-level south-westerly jet are considered to be the main reason for Haitang's WN-1 asymmetry.

The two experiments with and without Taiwan terrain are also compared to understand in detail the impact of terrain on Haitang's structure and track.