Influence of Irregular Coastlines on a Tornadic Mesovortex in the Pearl River Delta during the Monsoon Season. Part Ⅰ:Pre-storm Environment and Storm Evolution

The Pearl River Delta(PRD),a tornado hotspot,forms a distinct trumpet-shaped coastline that concaves toward the South China Sea.During the summer monsoon season,low-level southwesterlies over the PRD’s sea surface tend to be turned toward the west coast,constituting a convergent wind field along with the landward-side southwesterlies,which influences regional convective weather.This two-part study explores the roles of this unique land–sea contrast of the trumpet-shaped coastline in the formation of a tornadic mesovortex within monsoonal flows in this region.Part I primarily presents observational analyses of pre-storm environments and storm evolutions.The rotating storm developed in a lowshear environment(not ideal for a supercell)under the interactions of three air masses under the influence of the land–sea contrast,monsoon,and storm cold outflows.This intersection zone(or“triple point”)is typically characterized by local enhancements of ambient vertical vorticity and convergence.Based on a rapid-scan X-band phased-array radar,finger-like echoes were recognized shortly after the gust front intruded on the triple point.Developed over the triple point,they rapidly wrapped up with a well-defined low-level mesovortex.It is thus presumed that the triple point may have played roles in the mesovortex genesis,which will be demonstrated in Part II with multiple sensitivity numerical simulations.The findings also suggest that when storms pass over the boundary intersection zone in the PRD,the expected possibility of a rotating storm occurring is relatively high,even in a low-shear environment.Improved knowledge of such environments provides additional guidance to assess the regional tornado risk.

Influence of Irregular Coastlines on a Tornadic Mesovortex in the Pearl River Delta during Monsoon Season.Part II:Numerical Experiments

As demonstrated in the first part of this study(Part I),wind-shift boundaries routinely form along the west coast of the Pearl River Delta due to the land-sea contrast of a“trumpet”shape coastline in the summer monsoon season.Through multiple numerical simulations,this article(Part II)aims to examine the roles of the trumpet-shaped coastline in the mesovortex genesis during the 1 June 2020 tornadic event.The modeling reproduced two mesovortices that are in close proximity in time and space to the realistic mesovortices.In addition to the modeled mesovortex over the triple point where strong ambient vertical vorticity was located,another mesovortex originated from an enhanced discrete vortex along an airmass boundary via shear instability.On the fine-scale storm morphology,finger-like echoes preceding hook echoes were also reproduced around the triple point.Results from sensitivity experiments suggest that the unique topography plays an essential role in modifying the vorticity budget during the mesovortex formation.While there is a high likelihood of an upcoming storm evolving into a rotating storm over the triple point,the simulation's accuracy is sensitive to the local environmental details and storm dynamics.The strengths of cold pool surges from upstream storms may influence the stretching of low-level vertically oriented vortex and thus the wrap-up of finger-like echoes.These findings suggest that the trumpet-shaped coastline is an important component of mesovortex production during the active monsoon season.It is hoped that this study will increase the situational awareness for forecasters regarding regional non-mesocyclone tornadic environments.

Radar-based Characteristics and Formation Environment of Supercells in the Landfalling Typhoon Mujigae in 2015

This study presents the radar-based characteristics and formation environment of supercells spawned by the tornadic landfalling Typhoon Mujigae(2015)in October 2015.More than 100 supercells were identified within a 24-hour period around the time of the typhoon’s landfall,of which three were tornadic with a rotational intensity clearly stronger than those of non-tornadic supercells.The identified supercells were concentrated within a relatively small area in the northeast quadrant beyond 140 km from the typhoon center.These supercells were found more likely to form over flat topography and were difficult to maintain in mountainous regions.During the study period,more supercells formed offshore than onshore.The mesocyclones of the identified supercells were characterized by a small diameter generally less than 5 km and a shallow depth generally less than 4 km above ground level.An environmental analysis revealed that the northeast quadrant had the most favorable conditions for the genesis of supercell in this typhoon case.The nondimensional supercell composite parameter(SCP)and entraining-SCP(E-SCP)were effective in separating supercell from non-supercell environment.Even though the atmosphere in the typhoon’s northeast quadrant was characterized by an E-SCP/SCP value supportive of supercell organization,orography was an impeditive factor for the supercell development.These findings support the use of traditional parameters obtained from midlatitude supercells to assess the supercell potential in a tropical cyclone envelope.

Forecasting tropical cyclone tornadoes and impacts:Report from IWTC-X

This report synthesizes global tropical cyclone(TC)tornado research and operational practices to date.Tornadoes are one of the secondary(and lesser researched)hazards contributing to the devastation TCs leave in their wake.While gale-force winds and storm surge produce the majority of damage and fatalities globally,TC tornadoes also pose a fatal threat,complicating evacuation plans and protective actions as the storm moves inland.Climatological studies characterize TC-spawned tornadoes as usually weak and short-lived,primarily originating from miniature supercells in the outer rainbands.These tornadic features pose challenges to forecasting and radar detection.Additionally,TC tornadoes can pose a threat to communities 12 h prior to and beyond 48 h after a TC makes landfall.Research,both basic and operational,has increased globally over the last few years in efforts to move from a climatological to ingredientsbased approach to detect and forecast TC tornadoes.While the United States has led the charge,given the increased exposure to tornadoes year round,other nations such as China,Japan,and Australia have increased their efforts to record and detect TC tornadoes.Despite these advancements,more work needs to be done globally to understand the TC environment conducive for tornadic activity.Recommendations for future forecasting and research for TC tornadoes include i)develop a comprehensive global tornado database to improve research and forecasting efforts;ii)apply innovative technology to detect tornadoes;and iii)conduct field campaigns to thoroughly sample TC tornado environments,particularly along coastlines.

Climatology of tropical cyclone tornadoes in China from 2006 to 2018

We surveyed the occurrence of tropical cyclone(TC) tornadoes in China from 2006 to 2018. There were 64 cataloged TC tornadoes, with an average of five per year. About one-third of the landfalling TCs in China were tornadic. Consistent with previous studies, TC tornadoes preferentially formed in the afternoon shortly before and within about 36 h after landfall of the TCs. These tornadoes mainly occurred in coastal areas with relatively flat terrains. The maximum number of TC tornadoes occurred in Jiangsu and Guangdong provinces. Most of the TC tornadoes were spawned within 500 km of the TC center. Two notable characteristics were found:(1) TC tornadoes in China mainly occurred in the northeast quadrant(Earth-relative coordinates) rather than the right-front quadrant(TC motion-relative coordinates) of the parent TC circulation;and(2) most tornadoes were produced by TCs with a relatively weak intensity(tropical depressions/storms), in contrast with the United States where most tornadoes are associated with stronger TCs. Further analyses showed that TC tornadoes in China tend to be spawned in an environment with large low-level storm relative helicity and large convective available potential energy taking entrainment effects into account. TC tornadoes were particularly active in 2018, with 24 reported tornadoes accounting for 37.5% of the total surveyed samples. The first recorded tornado outbreak in the modern history of China occurred in the envelope of TC Yagi(2018), in which 11 tornadoes were reported in association with significant midlevel intrusions of dry air and the interaction of Yagi with an approaching midlatitude midlevel trough.

Wind estimation around the shipwreck of Oriental Star based on field damage surveys and radar observations

Based on observational analyses and on-site ground and aerial damage surveys, this work aims to reveal the weather phenomena--especially the wind situation--when Oriental Star capsized in the Yangtze River on June 1, 2015. Results demonstrate that the cruise ship capsized when it encountered strong winds at speeds of at least 31 m s ^-1 near the apex of a bow echo embedded in a squall line. As suggested by the fallen trees within a 2-kin radius around the wreck location, such strong winds were likely caused by microburst straight-line wind and/or embedded small vortices, rather than tomadoes.