[A-AS03] Extreme Events and Mesoscale Weather: Observations and Modeling

[AAS03-07] スマートフォンカメラとフェーズドアレイレーダーを用いた冬季ドライマイクロバーストの詳細な時空間解析

*足立 透¹、新野 宏² (1.気象研究所台風・災害気象研究部、2.東京大学大気海洋研究所)

キーワード：ドライマイクロバースト、フェーズドアレイレーダー、スマートフォンカメラ

A dry microburst is a subcategory of downbursts characterized by a horizontal dimension of less than 4 km and the absence of significant precipitation on the ground. In contrast to vigorous documentation of dry microbursts observed in the United States, statistical studies in the 1990s reported that all the confirmed downbursts in Japan were wet. Although at least two cases of dry microbursts have been reported in Japan in the cold season since 2001, detailed studies on their time evolution, mechanism, and environment in Japan are strongly desired. This paper reports a fine-scale spatiotemporal structure of a wintertime dry microburst event observed in the Kanto plain of Japan on the afternoon of February 16, 2022. The environment was characterized by a dry layer (relative humidity, RH of ~40 %) at the lowest 1.5 km above the ground level (AGL) and a moist layer (RH of ~90 %) aloft at 2.8–4.0 km AGL. A quasi-linear convective system (QLCS) having a radar echo top (bottom) height of ~4 km (~1km) AGL developed in the northern Kanto plain and moved east-northeastward. A smartphone camera and an X-band phased array radar (PAR) were used to observe the QLCS from ~10–12 km south and ~12–17 km east-northeast, respectively. Around 1700 local standard time, when the QLCS was at the mature to dissipating stage, successive photo images at time intervals of 13–22 seconds captured distinct thermal-like structures lowering from the cloud base, suggesting signatures of descending hydrometeors. The PAR data at the time of the photography exhibited consistent radar echo structures with a reflectivity of 20–26 dBZ extending downward from the cloud base. Beneath the echo structures, diverging wind patterns, having a peak-to-peak radial velocity difference of ~12.4 m/s over a horizontal distance of ~1.2 km, emerged at ~0.3–0.4 km AGL. The Automated Meteorological Data Acquisition System data obtained beneath the QLCS showed the absence of measurable precipitation, showing that the observed phenomena were weak dry microbursts. A series of radar signatures, starting from the cloud base lowering and ending in the near-surface wind divergence, occurred in about three minutes, a time scale difficult to observe without a rapid volumetric weather radar such as PAR. Based on the Doppler velocity data observing the quasi-zenith angle, we estimate that the downdraft initiated at ~1.0–2.3 km in the middle of the low-level dry layer. Because the freezing level was at 0.7–0.8 km, we suggest that the downdraft was primarily driven by the sublimation of frozen hydrometeors, with the possible aid of the melting and evaporation processes in the near-surface region.
Acknowledgment: This work is supported by JSPS KAKENHI Grant JP21K03666.