Understanding the Role of Mid- and Low-Level Circulations in the Development of Typhoon Nuri

A number of studies have shown that a mid-level vorticity develops first in developing tropical cyclones. However, whether the appearance of this feature is vital for cyclogenesis as well as the mechanisms through which it develops is a matter of debate and current research. In particular, it is important to understand how it relates to the near-surface vorticity and associated warm core when the spin up occurs.

The THORPEX Pacific Asian Regional Campaign (TPARC) and the associated Tropical Cyclone Structure experiment (TCS08) were conducted in the western Pacific during August and September 2008 to study tropical cyclones there. One of the studied cases, Nuri, was sampled during four successive days (16-19 August 2008) as the storm developed from a tropical wave to typhoon stage. Observations from the campaign, as well as the FNL analysis and the CFSR and ERA-Interim reanalyses were used to analyze some dynamic and thermodynamic relations in order to understand the role of the mid-level vortex on cyclogenesis and cyclone development during the development of typhoon Nuri.

Storm-relative circulation centers at mid- and low-levels were displaced from each other as Nuri was in the early stages of development. As Nuri intensified, the distance between circulation centers decreased until they were collocated with each other. The mid-level storm-relative center was located from the low-level center 90 degrees to the left of the shear vector and new cyclonic areas in the storm seemed to form downwind of the shear vector, in the overlapping region between both centers. The three datasets showed an increase in the horizontal mass flux into Nuri with intensification, with maximum values occurring in the layer 1000-900 hPa. Although there were differences among datasets, they all show that Nuri evolved from a top-heavy to a bottom-heavy vertical mass flux profile with intensification. Strong correlations between the averaged absolute vorticity at mid-levels, the saturation fraction and the instability index during the evolution of Nuri are indicated, particularly in the ERA-Interim reanalysis, as a result of a better representation of Nuri's thermodynamic environment.