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Recent Decrease in the Difference in Tropical cyclone Occurrence between the Atlantic and the Western North Pacific

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Advances in Atmospheric Sciences 2022年第9期39卷 1387-1397页

作者： johnny c.l.chan Kin Sik lIU Guy carpenter Asia-Pacific climate Impact centre School of Energy and EnvironmentCity University of Hong KongTat Chee Ave.KowloonHong KongChina

climatologically,among all ocean basins,the western North Pacific(WNP)has the largest annual number of tropical cyclones(Tcs)of around 26 while the Atlantic has around 13,yielding a difference of ***,the difference is-7 in 2020,with 30 Tcs in the Atlantic and 23 in the WNP,which is the most negative difference within the last 46 *** fact,during the last 26 years,the difference in Tc number is below 10 in ten years,with four years being *** a decreasing difference in Tc number can be attributed to the natural multidecadal variation of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation,as well as other external forcings such as anthropogenic aerosol forcing and increased greenhouse gases,with the additional impact from the la Niña *** result has significant implications on climate model projections of future Tc activity in the two ocean basins.

关键词： tropical cyclone frequency Atlantic Multidecadal Oscillation Interdecadal Pacific Oscillation climate change

Structural changes of a Tropical cyclone during landfall: β-plane Simulations

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Advances in Atmospheric Sciences 2010年第5期27卷 1143-1150页

作者： K.c.SZETO johnny c.l.chan School of Energy and Environment City University of Hong Kong

This study investigated the effects of landfall on the structure of a tropical cyclone （Tc）. Numerical simulations were performed using the Weather Research and Forecasting Model on a β-plane. Two landfall experiments, one with an east–west and another with a north–south oriented coastline, were performed. Similar to previous studies on an f-plane, large-scale flows in the low-to-middle troposphere were modified due to friction. A pair of counter rotating gyres was found, which was shown to be consistent with the slight deffection of the Tc relative to the control experiment without land. compared to previous f-plane simulations, because of the inherent asymmetries due to the β-gyres, the large-scale pattern of flows and convergences/divergences related to friction were found to depend on coastline orientations. On the other hand, regardless of the coastline orientation, convergences were found to be stronger to the left for both cases near landfall, as in previous f-plane simulations. Such a convergence pattern subsequently induced a change in convection and rainfall at the eyewall.

关键词： tropical cyclone landfall numerical simulations

Sensitivity of the Simulation of Tropical cyclone Size to Microphysics Schemes

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Advances in Atmospheric Sciences 2016年第9期33卷 1024-1035页

作者： Kelvin T.F.chan johnny c.l.chan School of Energy and Environment City University of Hong Kong

The sensitivity of the simulation of tropical cyclone （Tc） size to microphysics schemes is studied using the Advanced Hurricane Weather Research and Forecasting Model （WRF）. Six Tcs during the 2013 western North Pacific typhoon season and three mainstream microphysics schemes-Ferrier （FER）, WRF Single-Moment 5-class （WSM5） and WRF Single-Moment 6-class （WSM6）-are investigated. The results consistently show that the simulated Tc track is not sensitive to the choice of microphysics scheme in the early simulation, especially in the open ocean. However, the sensitivity is much greater for Tc intensity and inner-core size. The Tc intensity and size simulated using the WSM5 and WSM6 schemes are respectively higher and larger than those using the FER scheme in general, which likely results from more diabatic heating being generated outside the eyewall in rainbands. More diabatic heating in rainbands gives higher inflow in the lower troposphere and higher outflow in the upper troposphere, with higher upward motion outside the eyewall. The lower-tropospheric inflow would transport absolute angular momentum inward to spin up tangential wind predominantly near the eyewall, leading to the increment in Tc intensity and size （the inner-core size, especially）. In addition, the inclusion of graupel microphysics processes （as in WSM6） may not have a significant impact on the simulation of Tc track, intensity and size.

关键词： tropical cyclone microphysics size intensity track numerical modelling

Sensitivity of precipitation statistics to urban growth in a subtropical coastal megacity cluster

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Journal of Environmental Sciences 2017年第9期29卷 6-12页

作者： christopher claus Holst johnny c.l.chan chi-Yung Tam School of Energy and Enuironment City Uniuersity of liong KongHong KongChina Guy carpenter Asia-Pacific climate Impact centre City Uniuersity of Hong KongHong KongChina Earth System Science Programme The Chinese Uniuersity of Hong KongHong KongChina

This short paper presents an investigation on how human activities may or may not affect precipitation based on numerical simulations of precipitation in a benchmark case with modified lower boundary conditions, representing different stages of urban development in the model. The results indicate that certain degrees of urbanization affect the likelihood of heavy precipitation significantly, while less urbanized or smaller cities are much less prone to these effects. Such a result can be explained based on our previous work where the sensitivity of precipitation statistics to surface anthropogenic heat sources lies in the generation of buoyancy and turbulence in the planetary boundary layer and dissipation through triggering of convection. Thus only mega cities of sufficient size, and hence human-activity-related anthropogenic heat emission, can expect to experience such effects. In other words, as cities grow, their effects upon precipitation appear to grow as well.

关键词： Urban precipitation Micro climate sensitivity Urbanization

Importance of Air-Sea coupling in Simulating Tropical cyclone Intensity at landfall

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Advances in Atmospheric Sciences 2022年第10期39卷 1777-1786页

作者： charlie c.F.lOK johnny c.l.chan Ralf TOUMI School of Energy and Environment City University of Hong KongHong KongChina Department of Physics Imperial CollegeLondon SW72AZUK

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(Tcs)along the South china coast is upgraded by including ocean and wave models.A total of 642 Tcs have been re-simulated using the new system to produce a climatology of Tc intensity in the South china *** comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various Tc characteristics such as translational speed and *** particular,interaction with the ocean does not necessarily imply a weakening of the Tc,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the *** results suggest that to simulate the evolution of Tc structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.

关键词： tropical cyclone intensity tropical cyclone landfall seasonal prediction air-sea coupling

Development of Asymmetric convection in a Tropical cyclone under Environmental Uniform Flow and Vertical Wind Shear

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Advances in Atmospheric Sciences 2024年第12期41卷 2399-2418页

作者： Yubin lI Yuncong JIANG johnny c.l.chan Kevin K.W.cHEUNG School of Atmospheric Physics Nanjing University of Information Science&TechnologyNanjing 210044China Asia-Pacific Typhoon collaborative Research center Shanghai 201306China School of Energy and Environment City University of Hong KongHong Kong 999077China School of Emergency Management Nanjing University of Information Science&TechnologyNanjing 210044China

Idealized numerical simulations have been carried out to reveal the complexity in the development of asymmetric convection in a tropical cyclone(Tc)under the influence of an environment with either uniform flow,vertical wind shear(VWS),or *** show that rainwater is enhanced to the right of the motion in the outer rainband,but such enhancement occurs in the upshear-left area of the inner-core ***,due to the asymmetries introduced by environmental flow,wavenumber-1 temperature and height anomalies develop at a radius of~1000 km in the upper levels.A sub-vortex aside from the Tc center encompassing the wavenumber-1 warm center appears,and asymmetric horizontal winds emerge,which,in turn,changes the storm-scale(within 400 km)*** convection in the inner core closely follows the changing storm-scale VWS when its magnitude is larger than 2 m s^(-1) and is located downshear of the storm-scale VWS in all the experiments with environmental *** the outer rainbands,the maximum boundary layer convergence is mainly controlled by the direction of motion and is located in the rear-right *** results extend upon the findings of previous studies in three aspects:(1)The discovery of the roughly linear combination effect from the uniform flow and large-scale VWS;(2)The development of upper-level asymmetric winds on a 1000-km scale through the interaction between the Tc vortex and environmental flow,resulting in changes in the storm-scale VWS pattern within the Tc area;(3)The revelation that Tc asymmetric convection closely aligns with the direction-varying storm-scale VWS instead of the initially designated VWS.

关键词： tropical cyclone environmental flow vertical wind shear asymmetric convection inner core outer rainband

large tropical cyclone track forecast errors of global numerical weather prediction models in western North Pacific basin

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Tropical cyclone Research and Review 2021年第3期10卷 151-169页

作者： chi Kit Tang johnny c.l.chan Munehiko Yamaguchi School of Energy and Environment City University of Hong KongHong KongChina Meteorological Research Institute Japan Meteorological AgencyJapan

Although tropical cyclone(Tc)track forecast errors(TFEs)of operational warning centres have substantially decreased in recent decades,there are still many cases with large *** International Grand Global Ensemble(TIGGE)data are used to study the possible reasons for the large TFE cases and to compare the performance of different numerical weather prediction(NWP)***-four Tcs in the western North Pacific during the period 2007-2014 with TFEs(+24 to+120 h)larger than the 75 th percentile of the annual error distribution(with a total of 93 cases)are *** categories of situations are found to be associated with large *** include the interaction of the outer structure of the Tc with tropical weather systems,the intensity of the Tc,the extension of the subtropical high(SH)and the interaction with the westerly *** crucial factor of each category attributed to the large TFE is *** the TIGGE model predictions,the models of the European centre for Medium-Range Weather Forecasts and the UK Met Office generally have a smaller *** performance of different models in different situations is discussed.

关键词： consensus Numerical weather prediction Forecast error Tropical cyclones Track prediction TIGGE WGNE

Microphysical characteristics of Extreme-Rainfall convection over the Pearl River Delta Region, South china from Polarimetric Radar Data during the Pre-summer Rainy Season

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Advances in Atmospheric Sciences 2023年第5期40卷 874-886页

作者： Hao HUANG Kun ZHAO johnny clchan Dongming HU Key laboratory for Mesoscale Severe Weather/MOE and School of Atmospheric Sciences Nanjing UniversityNanjing 210023China State Key laboratory of Severe Weather and Joint center for Atmospheric Radar Research CMA/NJUBeijing 100081China School of Energy and Environment City University of Hong KongHong Kong 999077China Guangdong Meteorological Observatory Guangzhou 510062China

During the pre-summer rainy season,heavy rainfall occurs frequently in South *** on polarimetric radar observations,the microphysical characteristics and processes of convective features associated with extreme rainfall rates(ERcFs)are *** the regions with high ERcF occurrence frequency,sub-regional differences are found in the lightning flash rate(lFR)*** the region with higher lFRs,the ERcFs have larger volumes of high reflectivity factor above the freezing level,corresponding to more active riming *** addition,these ERcFs are more organized and display larger spatial coverage,which may be related to the stronger low-level wind shear and higher terrain in the *** the region with lower lFRs,the ERcFs have lower echo tops and lower-echo ***,no clear differences of the most unstable convective available potential energy(MUcAPE)exist in the ERcFs in the regions with different lFR *** of the lFRs,raindrop collisional coalescence is the main process for the growth of raindrops in the *** the ERcFs within the region with lower lFRs,the main mechanism for the rapid increase of liquid water content with decreasing altitude below 4 km is through the warm-rain processes converting cloud drops to ***,in those with higher lFRs,the liquid water content generally decreases with decreasing altitude.

关键词： microphysics extreme rainfall rate polarimetric radar lightning flash rate

A Simulation Study on Pre-landfall Erratic Track of Typhoon Haitang (2005)

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A Simulation Study on Pre-landfall Erratic Track of Typhoon ...