Reintensification and Eyewall Formation in Strong Shear : A Case Study of Typhoon Noul (2015)

Strong vertical wind shear produces asymmetries in the eyewall structure of a tropical cyclone (TC) and is generally a hostile environment for TC intensification. Typhoon Noul (2015), however, reintensified and formed a closed eyewall despite 200-850-hPa vertical shear in excess of 11 m s(-1). Noul's reintensification and eyewall formation in strong shear were examined by using Doppler radar and surface observations. The evolution of the azimuthal-mean structure showed that the tangential wind at 2-km altitude increased from 30 to 45 m s(-1) in only 5 h. During the first half of the reintensification, the azimuthal-mean inflow penetrated into the similar to 40-km radius, well inside the radius of maximum wind (RMW), at least below 4-km altitude, and reflectivity inside the RMW increased. As for the asymmetric evolution, vigorous convection, dominated by an azimuthal wavenumber-1 asymmetry, occurred in the downshear-left quadrant when shear started to increase and then moved upshear. A mesovortex formed inside the convective asymmetry on the upshear side. The direction of vortex tilt between the 1- and 5-km altitudes rotated cyclonically from the downshear-left to the upshear-right quadrant as the vortex was vertically aligned. In conjunction with the alignment, the amplitude of the wavenumber-1 convective asymmetry decreased and a closed eyewall formed. These features are consistent with the theory that a vortex can be vertically aligned through upshear precession. The analysis results suggest that the vortex tilt, vigorous convection, and subsequent intensification were triggered by the increase in shear in a convectively favorable environment

Convective Bursts With Gravity Waves in Tropical Cyclones : Case Study With the Himawari-8 Satellite and Idealized Numerical Study

Convective bursts occur frequently in tropical cyclones and help their intensification by diabatic heating, but their quantitative importance has not been established. By using the high-frequency observation of infrared brightness temperature with Himawari-8, a latest-generation geostationary meteorological satellite, convective bursts in Typhoon Lan (2017) were studied. Aided with a series of numerical simulations, it was revealed that the anvil edges of many bursts are associated with finite-amplitude gravity waves consistent with internal bores, creating warm anomalies by subsidence ahead of the edges. As the edges spread, they are thinned, and their propagation speeds are often decreased. In many such instances, gravity waves, now linear, are separated from the edges to propagate away, spreading convective heating. It is proposed that by quantifying these processes with geostationary satellites, diabatic heating by convective bursts can be estimated to diagnose their impacts on tropical-cyclone intensification. Plain Language Summary Convective bursts (CBs) are intense long-lasting cumulonimbus that occur frequently in tropical cyclones (TCs). Many studies suggested that condensation heating associated with CBs is an important factor in TC intensification but to what extent they are important has not been established. The latest-generation (also known as the third-generation) geostationary meteorological satellites such as Himawari-8 provided opportunities to observe TCs at frequencies much higher than before. We studied the anvil clouds (outflow clouds near the tropopause) of CBs in Typhoon Lan (2017) by using infrared images from Himawari-8. We also conducted numerical simulations to help interpretation. It was found that the anvil of a CB frequently spreads as an internal gravity wave in such a way that the anvil edge is preceded by the subsidence and temperature increase due to an internal bore (finite-amplitude gravity wave similar to tidal bore). Since the anvil spreads circularly, its edge is thinned gradually. It was found that its expansion speed is often slowed down, and the gravity wave is separated to propagate further ahead, disseminating convective heating. It is proposed that geostationary-satellite observations can be used to estimate convective heating associated with each CB. It will help establish the impacts of CBs in TC intensification

Multiple Dynamics of Precipitation Concentrated on the North Side of Typhoon Hagibis (2019) during Extratropical Transition

Torrential rain in Typhoon Hagibis caused a devastating disaster in Japan in October 2019. The precipitation was concentrated in the northern half of Hagibis during extratropical transition (ET). To elucidate the mechanisms of this asymmetric precipitation, synoptic- and mesoscale processes were mainly analyzed using the Japan Meteorological Agency Non-Hydrostatic Model. The present study demonstrates that the asymmetric processes were different depending on the ET stages. When Hagibis was close to the baroclinic zone at middle latitudes on around 12 October (the frontal stage), heavy precipitation in the northeastern part of Hagibis was attributed to warm frontogenesis and quasigeostrophic ascent, as reported in many previous studies. In contrast, when Hagibis was moderately distant from the baroclinic zone on around 11 October (the prefrontal stage), heavy precipitation in the northern part occurred in a slantwise northward ascending motion in the outer region. This slantwise motion developed in a region with strong westerly vertical shear, which was enhanced between Hagibis and a westerly jet stream. Based on the analyses of potential vorticity and absolute angular momentum, this region was characterized by reduced moist symmetric stability in the lower and middle troposphere accompanied by inertial instability in the upper troposphere and conditional instability in the lower troposphere. These results provide additional insights into the time evolution of asymmetric processes during ET in the absence of a distinct upper-tropospheric trough, particularly, the slantwise motion in the prefrontal stage

Stationary and Transient Asymmetric Features in Tropical Cyclone Eye with Wavenumber-1 Instability : Case Study for Typhoon Haishen (2020) with Atmospheric Motion Vectors from 30-Second Imaging

Dynamics of low-level flows in the eye of Typhoon Haishen (2020) in its late phase of intensification are investigated with a special rapid-scan observation of the Himawari-8 geosynchronous satellite conducted every 30 s. This is accomplished by deriving storm-relative atmospheric motion vectors at an unprecedentedly high spatiotemporal resolution by tracking clouds across five consecutive visible-light reflectivity. The overall low-level circulation center was situated several kilometers away from the storm center defined in terms of the inner edge of the lower part of eyewall clouds. The shift direction is rearward of the storm translation, consistently with a numerical study of the tropical cyclone (TC) boundary layer. Over the analysis period of 10 h, azimuthal-mean tangential wind around this center was increased at each radius within the eye, and the rotational angular velocity was nearly homogenized. The instantaneous low-level circulation center is found to orbit around the overall circulation center at distances around 5 km. Its orbital angular speed was close to the maximum angular speed of azimuthal-mean tangential winds. This rotating transient disturbance is found to transport angular momentum inward, which explains the tangential wind increase and the angular velocity homogenization in the eye. These features are consistent with an algebraically growing wavenumber-1 barotropic instability, whose impact on TC structures has not been explored. This instability enhances wavenumber-1 asymmetry in ring-shaped vorticity, which can be induced by various processes such as translation, environmental shear, and exponential barotropic instability. Therefore, it may appear broadly in TCs to affect wind distribution in their eyes