Intra-seasonal Oscillation and Typhoon Activity during PALAU Intensive Observation around the Tropical Western Pacific

To understand the formation mechanism of typhoon and its relations to intra-seasonal oscillations over the tropical western Pacific, an intensive observational campaign called PALAU2013 (Pacific Area Long-term Atmospheric observation for Understanding climate change in 2013) was carried out around the Palau Islands from May to July in 2013. During the intensive observation period of PALAU2013, R/V Mirai was stayed at 12N 135E, and conducted atmospheric and oceanic observations using such as a Doppler radar, radiosondes, and CTDs. To capture the monsoon activity with a wide range, we constructed intensified sounding network from Philippines, Palau, and Yap to Guam. Three Doppler radars (one is on R/V Mirai, and the others are at Palau) were utilized to obtain the internal structure of cloud systems. Dual Doppler analysis is available near the west coast of the Palau Islands, and dual-polarization parameters can be also captured to the direction of hydrometeor video-sonde (HYVIS) launching. We also launched CFH (Cryogenic Frost-point Hygrometer) and ECC ozone sondes to grasp the condition of the TTL (Tropical Tropopause Layer) during the campaign. Quasi-real-time forecasts were executed by using numerical models (NICAM and CReSS), and they were well performed.

Three events of an early stage of typhoon were captured by PALAU2013 observational network in June. All of the initial disturbances corresponded to a kind of easterly waves with vortical structures, and after passing through the Palau Islands, they developed into typhoons on the Philippine Sea. As these typhoons caused strong surface westerly winds in the formative stage, they represented a close relation to the monsoon onset and intensification of the MJO activity over the tropical western Pacific.

One of meso-scale convective systems (MCSs) propagated westward over the range of the radars on 15 June 2013, and developed into typhoon LEEPI. We launched several HYVISs into the MCS to catch the vertical profiles of ice particle's properties (type, size, and number concentration). The first HYVIS, which observed the transition region just behind the convection, captured super-cooled water droplets and several types of snow crystals (column, column-with-plate, and plate) in different height level respectively. The second and the third HYVISs were launched into the stratiform region, and showed that the identified height of each particle type gradually descended. These indicated that the ice particles were presumed to be mainly formed in the convective region, and transported into the stratiform region with gravitational sedimentation. These features were in good agreement with polarimetric parameters detected by the radar measurements.