Microphysical structure of a mesoscale convective system observed by hydrometeor videosondes (HYVISs) and a polarimetric radar over the western tropical Pacific Ocean

To clarify the microphysical structure of mecoscale convective systems (MCSs) over the tropical ocean, we conducted an intensive field experiment in Republic of Palau in June 2013 (PALAU2013). An MCS accompanied leading convective and trailing stratiform regions with meridional length of 200 km and zonal one of 150 km propagated westward over the observation area on 15 June 2013. Direct observations of ice particles using hydrometeor videosondes (HYVISs) were conducted within the observation ranges of a polarimetric radar. Four HYVISs were intermittently launched into the MCS. Compared properties of ice particle (type and shape) obtained by HYVISs with polarimetric parameters (reflectivity: Zh, differential reflectivity: ZDR, specific differential phase: KDP), we presume a three-dimensional distribution of ice particles both in the convective and stratiform regions of the MCS.

Several frozen drops are confirmed above the melting level by the first HYVIS launched into the convective region. On the other hand, needle, dendrite, and dense rimed ice crystals cannot be detected. Polarimetric parameters (Zh = 15~20 dBZ, ZDR = -0.5~0 dB and KDP = 0.0~1.0 deg./km) are observed between the melting level (approximately 5 km) and a height of 8 km in the convective region. The region corresponds to the existence of frozen drops and column crystals. We consider that the negative (but near 0) ZDR and positive KDP values should be attributed to the contributions of relatively large frozen drops and small column crystals, respectively.

Aggregates formed by column crystals are confirmed by the second HYVIS observed in the stratiform region. The existence of these aggregates is suggested by polarimetric parameters (Zh = 15~30 dBZ, ZDR = 0 dB and KDP = 0.0~1.0 deg./km) from 50 to 70 km behind the convective region between the melting level and a height of 6.5 km. Above the region, the existence of column and plate crystals is suggested by the polarimetric parameters (Zh = 10~15 dBZ, ZDR = 0~1 dB, KDP = 0.5~2.0 deg./km) between heights of 6.5 km and 8 km. Since Zh is less than 10 dBZ above a height of 8 km, the size of ice crystals is expected to increase by depositional growth with their sedimentation in the region. Melting of aggregates below the melting level intensifies the surface rainfall and its amount in the region is greater than that of the another stratiform region.

Several observational facts shown in the present study have already been shown by Houze (1989); i.e., drops form in the convective region, ice crystals advect from the convective to stratiform regions with gravitational sedimentation, and heavier surface precipitation in the stratiform region is attributed to deposition and aggregation processes above the melting level. However, the present study shows less existence of dense rimed ice crystals and graupels in the convective region, needle and dendrite crystals cannot be confirmed both in the convective and stratiform regions. Also, the width and depth of the area where aggregations are detected in the stratiform region can be confirmed by 3-dimensional distribution of polarimetric parameters. We emphasize that it is quite useful to understand the microphysical processes in a MCS over the tropical ocean using HYVISs and a polarimetric radar simultaneously.