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Dasol Kim
,
Chang-Hoi Ho
,
Doo-Sun R. Park
,
Johnny C. L. Chan
, and
Youngsun Jung

Abstract

In this study, the variation of tropical cyclone (TC) rainfall area over the subtropical oceans is investigated using the Tropical Rainfall Measuring Mission precipitation data collected from 1998 to 2014, with a focus on its relationship with environmental conditions. In the subtropics, higher moving speed and larger vertical wind shear significantly contribute to an increase in TC rainfall area by making horizontal rainfall distribution more asymmetric, while sea surface temperature rarely affects the fluctuation of TC rainfall area. This relationship between TC rainfall area and environmental conditions in the subtropics is almost opposite to that in the tropics. It is suggested that, in the subtropics, unlike the tropics, dynamic environmental conditions are likely more crucial to varying TC rainfall area than thermodynamic environmental ones.

Full access
Woosuk Choi
,
Chang-Hoi Ho
,
Doo-Sun R. Park
,
Jinwon Kim
, and
Johnny C. L. Chan

Abstract

Prediction of tropical cyclone (TC) activity is essential to better prepare for and mitigate TC-induced disasters. Although many studies have attempted to predict TC activity on various time scales, very few have focused on near-future predictions. Here a decrease in seasonal TC activity over the North Atlantic (NA) for 2016–30 is shown using a track-pattern-based TC prediction model. The TC model is forced by long-term coupled simulations initialized using reanalysis data. Unfavorable conditions for TC development including strengthened vertical wind shear, enhanced low-level anticyclonic flow, and cooled sea surface temperature (SST) over the tropical NA are found in the simulations. Most of the environmental changes are attributable to cooling of the NA basinwide SST (NASST) and more frequent El Niño episodes in the near future. The consistent NASST warming trend in the projections from phase 5 of the Coupled Model Intercomparison Project (CMIP5) suggests that natural variability is more dominant than anthropogenic forcing over the NA in the near-future period.

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Kosuke Ito
,
Soichiro Hirano
,
Jae-Deok Lee
, and
Johnny C. L. Chan

Abstract

Recent idealized simulations have shown that a system of binary tropical cyclones (TCs) induces vertical wind shear (VWS) in each TC, which can subsequently modify the tracks of these TCs through asymmetric diabatic heating. This study investigates these three-dimensional effects in the western North Pacific using the best track and ERA5 reanalysis data. The TC motion was found to deviate systematically from the steering flow. The direction of deviation is clockwise and repelling with respect to the midpoint of the binary TCs with a separation distance of more than 1000 km. The large-scale upper-level anticyclonic and lower-level cyclonic circulations serve as the VWS for each TC in a manner consistent with the idealized simulations. The VWS of a TC tends to be directed to the rear-left quadrant from the direction of the counterpart TC, where the maxima of rainfall and diabatic heating are observed. The potential vorticity budget analysis shows that the actual TC motion is modulated by the diabatic heating asymmetry that offsets the counterclockwise and approaching motion owing to horizontal advection when the separation distance of the binary TCs is 1000–2000 km. With a small separation distance (<1000 km), horizontal advection becomes significant, but the impact of diabatic heating asymmetry is not negligible. The abovementioned features are robust, while there are some dependencies on the TC intensities, size, circulation, duration, and geographical location. This research sheds light on the motion of binary TCs that has not been previously explained by a two-dimensional barotropic framework.

Open access
Johnny C. L. Chan
,
K. S. Liu
,
Simon E. Ching
, and
Edwin S. T. Lai

Abstract

This study investigates the asymmetric distribution of convection associated with tropical cyclones making landfall along the south China coast using Geostationary Meteorological Satellite 5 (GMS-5) data and Doppler radar data. Four tropical cyclones (TCs; Maggie, Sam, York, and Cam) that made direct landfall over Hong Kong in 1999, are chosen as case studies to examine the changes in the distribution of convection prior to, during, and after landfall.

Based on the radar data, three of the four cases (Maggie, Sam, and York) generally showed enhanced convection to the western side of the TC center starting from ∼6 to 18 h before landfall, especially in the cases of Sam and York. For Cam, a dry continental air intrusion apparently destroyed much of the western and southern portion of its eyewall. From the satellite data, convection was generally more enhanced on the southward side. In other words, enhanced convection occurred to the west of the TC in the mid- to lower troposphere and was then advected to the southward side of the upper troposphere by the cyclonic flow and rising motion. These observations are consistent with previous observational and numerical studies.

The possible influences of storm motion and vertical wind shear on the observed convective asymmetries are also examined for the cases of Sam and York. Both storms showed similar evolution of convective distribution relative to storm motion as they approached the coastline. Such storm-motion-related convective distributions may partly explain the observed convective asymmetries. The environmental vertical wind shear may be another factor contributing to the large left–right convective asymmetry in the case of Sam.

Full access
Yu-Cheng Kao
,
Ben Jong-Dao Jou
,
Johnny C. L. Chan
, and
Wen-Chau Lee

Abstract

In this study, the structure and evolution of a coastal barrier jet (CBJ) along the east coast of Taiwan is documented using operational Doppler radars. The formation of the CBJ was controlled by the flow regime associated with the approaching Typhoon Haitang (2005). The CBJ persisted for 6 h and was approximately 140 km long and 25 km wide. The northern branch of the CBJ had stronger winds with maximum wind speed 49–52 m s−1, a greater vertical extent with jet core between 1.0 and 2.5 km in height, and a more persistent jet signal than the southern branch with maximum wind speed 43–46 m s−1 and jet core between 1.0 and 2.0 km. We investigated the terrain blocking effect leading to the CBJ formation using an idealized simulation. A vortex resembling Haitang is constructed based on circulation retrieved from generalized velocity track display (GVTD) technique. The result of a no-terrain simulation reveals wind speed 10–22 m s−1 lower than the observed Doppler velocity. The difference suggests the enhanced wind speed along the coast was most likely due to the terrain blocking effect.

Free access
Philip J. Klotzbach
,
Johnny C. L. Chan
,
Patrick J. Fitzpatrick
,
William M. Frank
,
Christopher W. Landsea
, and
John L. McBride

Abstract

Advances in knowledge in tropical meteorological research are discussed in the context of contributions made by Professor William M. Gray. Gray pioneered the compositing approach to observational tropical meteorology through assembling of global radiosonde datasets and tropical cyclone research flight data. In the 1970s, he made fundamental contributions to knowledge of convective–larger-scale interactions. Throughout his career, he wrote seminal papers on tropical cyclone structure, cyclogenesis, motion, and seasonal forecasts. His conceptual development of a seasonal genesis parameter also laid an important framework for both seasonal forecasting as well as climate change studies on tropical cyclones. His work was a blend of both observationally based studies and the development of theoretical concepts. This paper reviews the progress in knowledge in the areas where Dr. Gray provided his largest contributions and describes the scientific legacy of Gray’s contributions to tropical meteorology.

Open access
Yumi Choi
,
Kyung-Sook Yun
,
Kyung-Ja Ha
,
Kwang-Yul Kim
,
Soon-Jo Yoon
, and
Johnny C. L. Chan

Abstract

The effects of asymmetric sea surface temperature (SST) distribution on the tropical cyclone (TC) motion around East Asia have been examined using the Weather Research and Forecasting Model for the straight-moving Typhoon Ewiniar (2006) and recurving Typhoon Maemi (2003). The SST–TC motion relationships associated with the two different TCs and the physical mechanism of recurvature are investigated in the context of the potential vorticity tendency framework. A zonally asymmetric SST distribution alters the TC translating direction and speed, which is ascribable to the interaction between a TC and the environmental current associated with asymmetric SST forcing. A north–south SST gradient has an insignificant role in the TC motion. It is noted that the straight-moving (i.e., northward moving) TC deflects toward the region of warmer SST when SST is zonally asymmetric. A contribution of the horizontal advection including asymmetric flow induced by asymmetric forcing is dominant for the deflection. The recurving TC reveals northeastward acceleration and deceleration after the recurvature point in the western warming (WW) and eastern warming (EW) experiments, respectively. When it comes to a strong southerly vertical wind shear under the recurvature condition, diabatic heating can be a significant physical process associated with the downward motion over the region of upshear right. The enhanced (reduced) southwesterly flow effectively produces the acceleration (deceleration) of northeastward movement in WW (EW) after recurvature.

Full access
Thomas Knutson
,
Suzana J. Camargo
,
Johnny C. L. Chan
,
Kerry Emanuel
,
Chang-Hoi Ho
,
James Kossin
,
Mrutyunjay Mohapatra
,
Masaki Satoh
,
Masato Sugi
,
Kevin Walsh
, and
Liguang Wu
Full access
Thomas Knutson
,
Suzana J. Camargo
,
Johnny C. L. Chan
,
Kerry Emanuel
,
Chang-Hoi Ho
,
James Kossin
,
Mrutyunjay Mohapatra
,
Masaki Satoh
,
Masato Sugi
,
Kevin Walsh
, and
Liguang Wu

Abstract

An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.

Free access
Thomas Knutson
,
Suzana J. Camargo
,
Johnny C. L. Chan
,
Kerry Emanuel
,
Chang-Hoi Ho
,
James Kossin
,
Mrutyunjay Mohapatra
,
Masaki Satoh
,
Masato Sugi
,
Kevin Walsh
, and
Liguang Wu

Abstract

Model projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics that are projected to impact multiple regions. A 2°C anthropogenic global warming is projected to impact TC activity as follows. 1) The most confident TC-related projection is that sea level rise accompanying the warming will lead to higher storm inundation levels, assuming all other factors are unchanged. 2) For TC precipitation rates, there is at least medium-to-high confidence in an increase globally, with a median projected increase of 14%, or close to the rate of tropical water vapor increase with warming, at constant relative humidity. 3) For TC intensity, 10 of 11 authors had at least medium-to-high confidence that the global average will increase. The median projected increase in lifetime maximum surface wind speeds is about 5% (range: 1%–10%) in available higher-resolution studies. 4) For the global proportion (as opposed to frequency) of TCs that reach very intense (category 4–5) levels, there is at least medium-to-high confidence in an increase, with a median projected change of +13%. Author opinion was more mixed and confidence levels lower for the following projections: 5) a further poleward expansion of the latitude of maximum TC intensity in the western North Pacific; 6) a decrease of global TC frequency, as projected in most studies; 7) an increase in global very intense TC frequency (category 4–5), seen most prominently in higher-resolution models; and 8) a slowdown in TC translation speed.

Free access