A Development Process of the Tropical Disturbance Observed in PALAU2010 for Understanding Tropical Cyclognesis

In this study, our interest is an organization process of a vortex embedded in tropical disturbance. This is aimed to understand the tropical cyclogenesis process. A tropical disturbance is observed in Pacific Area Long-term Atmospheric observation for Understanding climate change (PALAU) 2010 field campaign, which was organized by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). This tropical disturbance was accompanied with large scale cyclonic vortices, and persisted for more than three days over the western North Pacific. However, the perturbation did not reach tropical depression intensity. Tropical cyclogenesis process is still one of the big problem in meteorology, and the organization process of the core vortex of tropical cyclone is studied by many researchers recently. In order to know what are the key points or turning points in the tropical cyclogenesis process, understanding failures of that process is important. Especially, we will focus on the organization process of the core vortex of TC especially. In the case observed in PALAU2010, we can identify the cloud area which is circular shape from images of geostational meteorological satellite (MTSAT-1R, Japanese Meteorological Agency). This circular shape was accompanied with upper tropospheric trough. In the south part of this cloud area, positive vorticity areas at both low and middle troposphere can be identified from 19th June 2011 by using NCEP/FNL wind field. The disturbance continues moving westward till dissipate, and never become intensity of tropical depression in the end (23th June). This disturbance was observed by drop-sounding operated by JAMSTEC. From drop-sounding data, it is known that the potential temperature at lower troposphere in the vortex was warmer than surrounding area. Zehr (1992) studied both developing process and dissipating process of tropical disturbance. He found twice convective maximum in tropical disturbances until become TC. He also distinguished four type of dissipating cloud system based on progression of the tropical cyclogenesis. The subject of this study can be categorized in the Type-4 dissipation case. The Type-4 case experienced twice convective maximum, and also had the cyclonic vortex in the cloud system. However, the cloud system and vortices dissipated after the second convective maximum. In order to investigate the development process of the disturbance, we numerically simulated this tropical disturbance with the Weather Research and Forecast (WRF) model-the Advanced Research WRF (ARW) version 3.2.1 (Skamarock et al. 2008). The computational domains are multinested in two levels in a two-way interactive mode, with the finest grid spacing of 8 km. The model is initialized at 0000 Z 17 June 2010, 48 hours before observation, and is integrated for 4 days. NCEP/FNL data is used as initial condition and lateral boundary condition. The simulated disturbance has similar cloud pattern with that of observed by the geostationary meteorological satellite. In the simulated cloud system, a mid-tropospheric circulation is also simulated, which is seen in the wind field of NCEP/FNL. The mid-tropospheric cyclonic circulation can be found in the initial data of the simulation. The simulated circulation has Mesoscale Convective Vortex (MCV) like features; the vortex exist at the melting layer level in the stratiform cloud area mainly. 1 hour after the initiation of simulation, the synoptic scale trough existed below the MCV. In the trough, multiple meso-scale vorticies are diagnosed by the horizontal wind field at the surface. These multiple vortices merged each other, and become an intense vortex finally. This merging process is found tropical cyclogenesis case (Fuquing Zhang 2010). After low level vortices merger, the middle level circulation and the merged low level vortex seemed to be coupled, and the core vortex of the cloud system was organized. The vortex was elongated from lower troposphere to upper troposphere. The core vortex is like the initial vortex of TC, thus the initial vortex is organized successfully. However, the core vortex was not able to sustain for long time, and the simulated disturbance does not develop into tropical cyclone in the integration period. The important feature in present simulation is a dissipation process of the core vortex. The main reason for dissipation of the disturbance may be the collapse of the core vortex. The core vortex was tilting gradually, and torn in two vortices at different height finally. This is shown in the figure; where color shade is vorticity at the surface and contour is also vorticity but at 500 hPa height. When the core vortex was torn, it is known that the dry air was intruded from northern part of the disturbance from both NCEP/FNL and simulated relative humidity field. This dry intrusion occurred mainly at middle level. Thus, convection at the core vortex was suppressed, and was not able to sustain itself. Before the organization of the core vortex, both the middle and low level vortices were in high relative humidity area. In addition, there was vertical wind shear, which intense was approximately 8 m/s. These improper environments collapsed the core vortex, and leaded to dissipate the cloud system. From this simulation of dissipate case, it is suggested that the sustainable core vortex is important for tropical cyclogenesis. To sustain the core vortex the suitable environment may be necessary, which is weak vertical wind shear and/or moist air existence. In conclusion, the simulated cloud system which was not able to become tropical cyclone had a core vortex. The structure and organization process of the core vortex were similar to that of the initial vortex of tropical cyclone. However, the core vortex was not sustained for a long time, and collapsed finally. The reason for this collapsing may be dry air intrusion and vertical wind shear. From this simulation results, it is suggested that the suitable environments to sustain the core vortex for a long time is one of key point in the tropical cyclogenesis process.