Tadayasu Ohigashi

Vertical distribution of precipitation particles in Baiu frontal stratiform intense rainfall around Okinawa Island, Japan

The vertical distribution of precipitation particles in an intensely precipitating stratiform cloud associated with the Baiu front around Okinawa Island was observed. X-band polarimetric radar, disdrometer, and hydrometeor videosonde data were used to examine the precipitation processes. The cloud top was approximately 12 km above sea level, as convection was depressed while stratiform regions developed near Okinawa Island. In the rain region below 3 km, the mean median volume diameter of the raindrop size distribution (DSD) estimated from the radar variables was 1.55 mm, and the mean normalized intercept parameter was 104.12 mm−1 m−3 with a mean radar reflectivity of 40.5 dBZe. The DSD indicates that the stratiform precipitation was characterized by higher number concentrations of smaller drops than observed previously in convective cells in a Baiu frontal convective precipitation region around Okinawa Island. The DSD also suggests the presence of larger raindrops than in convective cells embedded in a Baiu frontal stratiform precipitation region around Okinawa Island. In the ice region at 5–6 km, just above the melting layer and 6 km below the cloud top, the differential reflectivity and specific differential phase showed positive values, and videosonde measurements revealed that the number concentration of column-, plate-, and capped-column-like crystals (maximum dimensions of ≥0.1 mm) was 112 L−1. The high number concentration of these crystals contributed to the intense stratiform rainfall associated with the Baiu front.

A Hydrometeor Classification Method for X-Band Polarimetric Radar: Construction and Validation Focusing on Solid Hydrometeors under Moist Environments

AbstractA fuzzy-logic-based hydrometeor classification (HC) method for X-band polarimetric radar (X-pol), which is suitable for observation of solid hydrometeors under moist environments producing little or no hail, is constructed and validated. This HC method identifies the most likely hydrometeor at each radar sampling volume from eight categories: 1) drizzle, 2) rain, 3) wet snow aggregates, 4) dry snow aggregates, 5) ice crystals, 6) dry graupel, 7) wet graupel, and 8) rain–hail mixture. Membership functions are defined on the basis of previous studies. The HC method uses radar reflectivity Zh, differential reflectivity Zdr, specific differential phase Kdp, and correlation coefficient ρhv as its main inputs, and temperature with some consideration of relative humidity as supplemental information. The method is validated against ground and in situ observations of solid hydrometeors (dry graupel, dry snow aggregates, and ice crystals) under a moist environment. Observational data from a ground-based ima...

Relationship between cloud‐to‐ground lightning polarity and the space‐time distribution of solid hydrometeors in isolated summer thunderclouds observed by X‐band polarimetric radar

To understand the charge distribution in thunderclouds associated with solid hydrometeors, the relationship between cloud-to-ground (CG) lightning polarity and the space-time distribution of solid hydrometeors for isolated summer thunderclouds in Japan is examined using X-band polarimetric radar. Hydrometeor classification was conducted to examine the space-time distribution of dry snow, ice crystal, dry graupel (DG), and wet graupel in thunderclouds. Two thunderstorm cases were selected for analysis: 26 July 2010, which generated few positive CG flashes, and 25 August 2010, which generated positive CG flashes in the reflectivity cores. In both cases, negative CG flashes were observed in all reflectivity cores when a large volume of DG was identified above the height of −10°C level. This is consistent with previous studies showing that graupel particles have a negative charge below temperatures of −10°C. Reflectivity cores with positive CG flashes had a large volume of DG up to high altitudes (around or above the −45°C level). Further, reflectivity cores that sustained large DG volumes at high altitudes had a relatively large number of positive CG flashes. The top height of the DG volume reached lower altitudes for reflectivity cores without positive CG flashes compared with those with positive CG flashes. These results suggest that the persistence of graupel particles in reflectivity cores at high altitudes, implying the existence of strong updraft, is a necessary condition for positive CG flashes in summer thunderclouds. This effect would likely be caused by the positively charged graupel particles under high rime accretion rates.