Characteristics of upper-tropospheric outflow-layer clouds of Typhoon Francisco (2013) observed by hydrometeor videosonde

Abstract

Ice crystals in upper-tropospheric outflow layers of typhoons are an important factor in determining typhoon intensity and track through radiative cooling of the secondary circulation. Quantitative representation of ice crystal distribution is fundamental for accurate reproduction of typhoon intensity and track in cloud-resolving numerical models. This study used hydrometeor videosondes (HYVISs) released from Okinawa Island (Japan) to conduct microphysical observations of ice crystals in the outflow layer of Typhoon Francisco (2013). During the approach of the typhoon center, seven HYVISs were released between 02:53 JST October 23, 2013 and 13:33 JST October 24, 2013 at distances of between 230 and 550 km from the typhoon center (r = 230 and 550 km). The observation site was located at the downshear left flank, and at least three HYVISs were released within 100 km of active convection in the eyewall from which outflow-layer clouds extended. During the observations, the central pressure of the typhoon increased slightly from 945 to 960 hPa. Ice particles were observed at heights of 7–14 km. The numbers of ice particles tended to decrease with distance from the typhoon center. The maximum number concentrations of ice crystals were 180 and 20 L −1 at r = 230 and 550 km, respectively. Ice particles with diameter of a few tens of micrometers were found dominant. Large particles with diameter > 100 μm were restricted to just outside the eyewall and the inner-core precipitation region. Ice supersaturation layers with thickness > 2 km were present from r = 230 to 360 km, i.e., reasonably close to the typhoon center. In addition, supercooled liquid droplets were observed in areas with relative humidity >95% in two profiles (r = 260 and 290 km). Conversely, there was no ice supersaturation layer in the outermost profile at r = 550 km. Saw-like radar echo patterns were common along the bottom of outflow-layer clouds, below which a considerably drier layer was evident. Saturation ratio minima relative to ice within the dry layer were 0.01–0.34 (for relative humidities of 1%–31%). It was considered that ice particles formed in the upper levels of rainbands near the typhoon center, and that they were sublimated in both the dry layer below the outflow layer and the ice subsaturation area in the outermost region of the typhoon, which resulted in decreased number concentrations of ice particles. • Upper-tropospheric outflow-layer clouds of Typhoon Francisco (2013) were observed. • Microphysical quantities of ice particles were measured throughout the outflow layer. • Dissipation process of ice particles was clarified. • Supercooled water droplets existed in the outflow-layer clouds of the typhoon. • A saw-like radar echo pattern was found along the bottom of the outflow-layer clouds.