Non-Barotropic Processes in tropical cyclone motion. Part II: Individual Contributions from Diabatic Heating

This is the second of a series of three papers to study tropical cyclone (TC) motion through observational analyses by applying the diagnostic framework of Wu and Wang (1999) that relate potential vorticity (PV) tendency with TC motion. Part I (Chan 2000) presents results from a composite study of TCs moving in specific directions. In this paper, results from individual cases for the conditions at the mid troposphere are described.

According to Wu and Wang (1999), the PV tendency (PVT) is contributed by four physical processes: horizontal advection (HA), vertical advection, diabatic heating (DH) and friction. In the mid-troposphere, however, the vertical advection is relatively small and the friction is insignificant. Therefore, the HA and the DH are the main physical processes responsible for TC motion in the mid-troposphere. Wu and Wang's (1999) results show that the TC motion vector is approximately the resultant of the vectors pointing towards the maximum in HA and that in DH. The distribution of DH depends on the convection patterns around the TC center.

The data sets consist of UKMO analysis data (95-98) and high resolution GMS-5 IR imageries. TCs are categorized according to their speeds. The location of maximum DH is identified from the satellite imageries. Preliminary results suggest that the DH term plays a more important role in slow-moving TCs or those with erratic tracks. However, for fast-moving TCs, the PVT is mainly contributed by the HA term (i.e. steering).