Western North Pacific Basin Research Articles

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

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 TFEs. The 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) models. Forty-four TCs in the western North Pacific during the period 2007–2014 with TFEs (+24 to +120 h) larger than the 75th percentile of the annual error distribution (with a total of 93 cases) are identified.Four categories of situations are found to be associated with large TFEs. These 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 trough. The crucial factor of each category attributed to the large TFE is discussed.Among the TIGGE model predictions, the models of the European Centre for Medium-Range Weather Forecasts and the UK Met Office generally have a smaller TFE. The performance of different models in different situations is discussed.

Landfalling tropical cyclone characteristics and their multi‐timescale variability connected to monsoon and easterly formation environments over the western North Pacific

AbstractThis study analyses the impacts of tropical cyclones (TCs) that formed in monsoon and easterly environments on the major coastlines of the western North Pacific (WNP) basin during 1981–2009. The TC formation processes, defined as the development of tropical cloud cluster to TC, associated with monsoon environments (monsoon trough, monsoon confluence, and north of monsoon trough) are categorized into monsoon‐type TCs (monsoon‐TCs). Similarly, those associated with easterly flow environments (easterly flow west and southwest of subtropical high) are categorized into easterly‐type TCs (easterly‐TCs). Monsoon‐TCs form farther westward and at lower latitudes than easterly‐TCs, contributing to higher landfall proportion on the coastal countries for monsoon‐TCs. Monsoon‐TCs have a higher probability of affecting southern China, Taiwan and Vietnam, while easterly‐TCs tend to affect eastern China, southern Japan and the Philippines. Monsoon‐TCs have more widely dispersed rainfall and slower translation speed during landfall than easterly‐TCs. These characteristics are consistent with stronger environmental moisture transport and weaker steering flow in monsoon environments. Landfalling TC intensity and size are not different between easterly‐ and monsoon‐TCs. Both easterly‐ and monsoon‐TCs have interannual (1–4 years) and interdecadal (8–11 years) variability, which are related to variability of the large‐scale monsoon trough. El Niño–Southern Oscillation is significantly correlated with the interannual variability of easterly‐ and monsoon‐TCs, and changes in the monsoon‐TC landfall proportion and easterly‐TC landfall intensity. The interdecadal variability mainly affects the background vorticity and cyclonic circulation, leading to changes in the formation number of easterly‐ and monsoon‐TCs. In summary, this study provides evidence for connections between multiscale variability of the large‐scale monsoon and easterly patterns, TC formation environments, and TC impacts on the WNP coasts.

Predicting Tropical Cyclogenesis Using a Deep Learning Method From Gridded Satellite and ERA5 Reanalysis Data in the Western North Pacific Basin

This article proposes a deep learning model to predict tropical cyclogenesis (TCG) from gridded satellite and ERA5 reanalysis data in the western North Pacific basin. The proposed model contains two modules. First, convolutional neural network (CNN)-based deep features are extracted for each predictor, and then, the extracted features are fused with two fully connected layers to differentiate and investigate the relationship between predictors and TCG. The experimental data of this study are composed of 3232 developing tropical cluster clouds and 6657 nondeveloping ones; 90% of the collected data are utilized to train the model, and the rest are used to evaluate the trained model. Totally, nine predictors have been considered for the study, and the results show that the brightness temperature (IR), relative vorticity (Vo), and geopotential height (Z) perform better than the other predictors. A combined model with six predictors [IR, Z, RH (relative humidity), Vo, WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10 m</sub> (wind speed at the height of ten meters above the surface of the Earth), and mslp (mean sea-level pressure)] achieves the best TCG predicting performance, i.e., 97.1% of developing tropical cyclones are detected at a probability threshold of 0.13 with a false alarm rate of 20.3%. The experimental results demonstrate that the proposed method is superior to the existing methods and also indicate that the fusion of satellite and reanalysis data is a promising method to predict TCG.

Development of objective forecast guidance on tropical cyclone rapid intensity change

AbstractForecasting tropical cyclone (TC) intensity and rapid intensification (RI) are major challenges for numerical weather prediction models. In the present study, a model post‐processing technique, namely the TC intensity guidance on rapid intensification (TINT‐RI), has been developed for the western North Pacific basin (South China Sea excluded). It combines logistic regression and the naïve Bayes classifier to provide RI forecasts up to the next 48 hr. Predictors describing physical and environmental conditions are taken from the outputs of the European Centre for Medium‐Range Weather Forecasts model and the National Oceanic and Atmospheric Administration tropical cyclone heat potential field. Several observed TC characteristics such as persistence (change of past 12 hr intensity) are also included in the algorithm. To further reduce the false alarm ratio, the TINT‐RI makes reference to the intensity forecasts from a previously developed statistical‐dynamical TC intensity forecast model and the change of model vertical wind shear. Verification for TCs with RI reveals that the TINT‐RI is significantly more skilful than the direct model outputs from major global numerical weather prediction models and climatology. A case study on extreme RI of Super Typhoon Hato (1713) during its substantial intensification over the Luzon Strait before entering the northern part of the South China Sea is presented to discuss the performance of the TINT‐RI. Potential ways to improve the TINT‐RI such as using sea surface temperature and its anomaly as the oceanic predictor are discussed.

Impact of tibetan plateau snow cover on tropical cyclogenesis via the Madden–Julian oscillation during the following boreal summer

This study investigates the role of the interannual variation of boreal winter (December–February) snow cover over the Tibetan Plateau (TPSC) in modulating the relationship between the Madden Julian Oscillation (MJO) and tropical cyclogenesis over the western North Pacific (WNP) during the following boreal summer (June–October). During the boreal summer following a high snow cover anomaly (SCA), MJO-associated convection tends to be confined to the west of 140 °E, coinciding with more frequent tropical cyclone (TC) activity in this region. By contrast, there tends to be stronger MJO-associated convection extending farther east during years following low SCA. The MJO-associated convection extends to ~ 160 °E over the WNP basin with a peak around 140 °E. Thus, more TCs form farther east in the WNP in these years. We use composite analyses of large-scale environmental factors to demonstrate that low-level relative vorticity is one of the most important factors in controlling TCs in response to the change of MJO-associated convection between years with high SCA and low SCA. Meanwhile, eddy kinetic energy variations associated with the MJO are consistent with changes in tropical cyclogenesis over the WNP basin between years with high and low SCA. The Asian summer monsoon plays an important role in linking prior winter TPSC to MJO activity in the following summer. During high-SCA years, increased snow cover results in positive soil moisture anomalies and cooling over the Tibetan Plateau, thus weakening the following summer Asian monsoon and weakening associated water vapor advection, while the opposite chain of events occurs in low-SCA years. These changes in high-SCA years are favorable for MJO propagation and are unfavorable for MJO propagation in low-SCA years. Meanwhile, the interannual variation of SCA exerts a significant impact on the upper-tropospheric circulation and thus changes in vertical wind shear. During high-SCA years, there are anomalous upper-level easterlies in the Indian Ocean and upper-level westerlies in the Pacific Ocean, which result in anomalous easterly wind shear in the Indian Ocean and anomalous westerly shear in the Pacific Ocean. The opposite shear pattern arises in low-SCA years. In addition to changes in wind shear, the background vertical motion associated with changes in TPSC may also be partly responsible for changes in MJO activity.

Typhoons of western North Pacific basin under warming climate and implications for future wind hazard of East Asia

How global warming will affect the typhoon activities and therefore impact future wind hazard is of importance to human being’s preparedness for climate change. Herein a statistical dynamics synthetic tropical cyclone (TC) model, combining with the environmental parameters produced by five global climate models, is proposed for exploring quantitatively the typhoon wind hazard in the period 2071–2100 under the scenario of the least constrained emission. Compared with the current climate of the period 1981–2010, the future annual TC occurrence number will decrease for most climate models, but the amplitude varies from different models. The shift of TC tracks to the northeast may mitigate typhoon risk in the southeast coast of China while increase typhoon hazards in northern East Asia. Additionally, the proportion of super typhoon increases, which turns to enhance the extreme wind speeds in most coastal areas of East Asia. In general, the future environmental conditions of warming climate will contribute to forming more extreme typhoons in the western North Pacific basin and the typhoon wind hazard tends to increase in East Asia.

Dominant Influence of ENSO-Like and Global Sea Surface Temperature Patterns on Changes in Prevailing Boreal Summer Tropical Cyclone Tracks over the Western North Pacific

AbstractA conventional empirical orthogonal function (EOF) analysis is performed on summertime (May–October) western North Pacific (WNP) tropical cyclone (TC) track density anomalies during 1970–2012. The first leading EOF mode is characterized by a consistent spatial distribution across the WNP basin, which is closely related to an El Niño–Southern Oscillation (ENSO)-like pattern that prevails on both interannual and interdecadal time scales. The second EOF mode is represented by a tripole pattern with consistent changes in westward and recurving tracks but with an opposite change for west-northwestward TC tracks. This second EOF pattern is dominated by consistent global sea surface temperature anomaly (SSTA) patterns on interannual and interdecadal time scales, along with a long-term increasing global temperature trend. Observed WNP TC tracks have three distinct interdecadal epochs (1970–86, 1987–97, and 1998–2012) based on EOF analyses. The interdecadal change is largely determined by the changing impact of ENSO-like and consistent global SSTA patterns. When global SSTAs are cool (warm) during 1970–86 (1998–2012), these SSTAs exert a dominant impact and generate a tripole track pattern that is similar to the positive (negative) second EOF mode. In contrast, a predominately El Niño–like SSTA pattern during 1987–97 contributed to increasing TC occurrences across most of the WNP during this 11-yr period. These findings are consistent with long-term trends in TC tracks, with a tripole track pattern observed as global SSTs increase. This study reveals the potential large-scale physical mechanisms driving the changes of WNP TC tracks in association with climate change.

Validation of the fast intensity model for typhoon and its application to the estimation of typhoon wind hazard for the southeast coast of China

Tropical cyclone intensity model is one of the most important components in the tropical cyclone hazard assessment. In this study, the tropical cyclone fast intensity model, which uses features of the surrounding large-scale environment to predict the evolution of the tropical cyclone intensity, is used to estimate the typhoon wind hazard in the southeast coast of China. The fast tropical cyclone intensity model that consists of an intensity dynamic equation and a moisture dynamic equation is first validated for the western North Pacific basin. The coefficients in the moisture dynamic equation are calibrated by minimizing the error between the model and the observation from the best track dataset. The model is then integrated with a typhoon track model and a parametric radial wind profile to estimate the 50-year and 100-year recurrence-interval surface wind speeds at selected sites. Three different models for the radius of the maximum wind are considered in this analysis. Accordingly, surface wind records are utilized to estimate 50-year and 100-year recurrence-interval winds. The comparison shows a good agreement between the results of the model and that of the surface observations, which validates the adequacy of the model for engineering applications in the southeast coast of China.

A Climatology of Indirect Tropical Cyclone Interactions in the North Atlantic and Western North Pacific Basins

AbstractWhile it is understood that a recurving tropical cyclone (TC) that interacts with the midlatitude flow can cause large changes to the midlatitude flow pattern, it is much less understood if, and how, such events could impact a downstream tropical cyclone. Here, an indirect TC interaction is defined as one in which a primary TC perturbs the downstream midlatitude waveguide within one synoptic-scale wavelength of a secondary TC. In this study, a climatology and composite analysis using ERA-Interim reanalysis data is completed for all indirect interactions occurring between two tropical and/or subtropical cyclones in the North Atlantic and western North Pacific basins between 1989 and 2018. In all, 26 cases are identified in the North Atlantic and 56 cases are identified in the western North Pacific. The composite-mean interaction between a primary TC and upstream trough amplifies the immediate downstream ridge, increasing the tropospheric-deep vertical wind shear on its poleward and, in the western North Pacific, eastern, and equatorward flanks. An amplified downstream trough is detectable farther downstream in the western North Pacific 1–2 days after interaction onset; however, the same is not true in the North Atlantic, in which some cases exhibit anticyclonic Rossby wave breaking of the immediate downstream ridge. Secondary TCs that weaken following the indirect-interaction events are primarily located along the gradient between the downstream ridge and trough (North Atlantic) or at high latitudes (western North Pacific); those that strengthen are primarily located equatorward of the downstream ridge, particularly in the western North Pacific.

Possible Influence of Tropical Indian Ocean Sea Surface Temperature on the Proportion of Rapidly Intensifying Western North Pacific Tropical Cyclones during the Extended Boreal Summer

AbstractThis study examines the possible impact of tropical Indian Ocean (TIO) sea surface temperature anomalies (SSTAs) on the proportion of rapidly intensifying tropical cyclones (PRITC) over the western North Pacific (WNP) during the extended boreal summer (July–November). There is a robust interannual association (r= 0.46) between TIO SSTAs and WNP PRITC during 1979–2018. Composite analyses between years with warm and cold TIO SSTAs confirm a significant impact of TIO SSTA on WNP PRITC, with PRITC over the WNP basin being 50% during years with warm TIO SSTAs and 37% during years with cold TIO SSTAs. Tropical cyclone heat potential appears to be one of the most important factors in modulating the interannual change of PRITC over the WNP with a secondary role from midlevel moisture changes. Interannual changes in these large-scale factors respond to SSTA differences characterized by a tropics-wide warming, implying a possible global warming amplification on WNP PRITC. The possible footprint of global warming amplification of the TIO is deduced from 1) a significant correlation between TIO SSTAs and global mean SST (GMSST) and a significant linear increasing trend of GMSST and TIO SSTAs, and 2) an accompanying small difference of PRITC (~8%) between years with detrended warm and cold TIO SSTAs compared to the difference of PRITC (~13%) between years with nondetrended warm and cold TIO SSTAs. Global warming may contribute to increased TCHP, which is favorable for rapid intensification, but increased vertical wind shear is unfavorable for TC genesis, thus amplifying WNP PRITC.

Continued Increases in the Intensity of Strong Tropical Cyclones

AbstractIn a 2008 paper, using satellite-derived wind speed estimates from tropical cyclones over the 25-yr period 1981–2006, we showed the strongest tropical cyclones getting stronger. We related the increasing intensity to rising ocean temperatures consistent with theory. Oceans have continued to warm since that paper was published, so the intensity of the strongest cyclones should have continued upward as well. Here I show that this is the case, with increases in the upper-quantile intensities of global tropical cyclones amounting to between 3.5% and 4.5% in the period 2007–19 relative to the earlier base period (1981–2006). All basins individually show upward intensity trends for at least one upper quantile considered, with the North Atlantic and western North Pacific basins showing the steepest and most consistent trends across the quantiles.

Assimilation of Himawari-8 Rapid-Scan Atmospheric Motion Vectors on Tropical Cyclone in HWRF System

This study investigates the assimilation impact of rapid-scan (RS) atmospheric motion vectors (AMVs) derived from the geostationary satellite Himawari-8 on tropical cyclone (TC) forecasts. Forecast experiments for three TCs in 2016 in the western North Pacific basin are performed using the National Centers for Environmental Prediction (NCEP) operational Hurricane Weather Research and Forecasting Model (HWRF). An ensemble-variational hybrid data assimilation system is used as an initialization. The results show that the assimilation of RS-AMVs can improve the track forecast skill, while the weak bias or slow intensification bias increases at the shorter forecast lead time. A vortex initialization in HWRF has a substantial impact on TC structure, but it has neutral impacts on the track and intensity forecasts. A thinning of AMVs mitigates the weak bias caused by RS-AMV assimilation, resulting in the reduction of intensity error. However, it degrades the track forecast skill for a longer lead time. A decomposition of the TC steering flows demonstrated that the change in TC-induced flow was a primary factor for reducing the track forecast error, and the change in environmental flow has less impact on the track forecast. The investigation of the structural change from the assimilation of RS-AMV revealed that the following two factors are likely related to the intensity forecast degradation: (1) an increase of inertial stability outside the radius of maximum wind (RMW), which weakens the boundary layer inflow; and (2) a drying around and outside the RMW.

Interdecadal variability of tropical cyclone genesis frequency in western North Pacific and South Pacific ocean basins

Variability of tropical cyclone (TC) genesis frequency in the western North Pacific and South Pacific ocean basins in the interdecadal scale is studied. It is demonstrated that the TC genesis frequency in these ocean basins experienced an abrupt decrease near the end of the 20th century. The decreased occurrence of TC genesis in the two recent decades is mainly located in the low-latitude regions at the eastern side of the two ocean basins. It is also shown that a significant part of the decreased TC genesis occurred during October to December, i.e. the post-peak season in the western North Pacific ocean basin and the pre-peak season in the South Pacific ocean basin. The interdecadal trend of variation in the TC genesis frequency in these adjacent two ocean basins seems to be mainly due to a common mechanism, i.e. the variation in the atmospheric vorticity. In contrast to the decrease in the total TC genesis, the intense typhoon occurrence frequency experienced an interdecadal increase during the same period. This trend for intense typhoons is particularly clear in the western North Pacific ocean basin. The zonal distribution of the increased number in the intense typhoon occurrence shows a similar pattern to the increased value of the sea surface temperature. It is then suggested that the variation in the intense typhoon occurrence frequency in the western North Pacific ocean basin is related to a change in the La Niña-like sea surface warming pattern.

Influence of varying time scale flows on tropical cyclone track changes

AbstractIt is often difficult to predict tropical cyclone (TC) tracks for the case of binary or even multiple TCs. To investigate this issue, two TCs over the western North Pacific basin in September 2016, Meranti and Malakas, were explored in this study. The two TCs formed close to each other, and their tracks were similar at an early stage but then deflected northward over different regions. The results indicate that the Meranti track change may be due to the asymmetric high‐frequency flows induced by topography, while the Malakas track change is attributed to the formation and strengthening of low‐frequency southerlies. It was found that the formation and evolution of low‐frequency monsoon circulation is the main cause for the emergence of the low‐frequency flows, and the process is accompanied by the enhancement and movement of low‐frequency positive vorticity. Therefore, based on the diagnostic results using the low‐frequency vorticity budget equation, it is shown that the low‐frequency vorticity advection and convergence induced by the low‐frequency flows play varying roles in the development of low‐frequency positive vorticity. More noteworthy is that the high‐frequency positive vorticity of TCs can trigger the enhancement of low‐frequency positive vorticity through nonlinear interactions, which have rarely been discussed in previous studies. It can be concluded that the interactions between high‐ and low‐frequency flows and vorticity may affect the development and evolution of the TCs at different time scales, finally leading to TC track deflection.

Third assessment on impacts of climate change on tropical cyclones in the Typhoon Committee Region – Part I: Observed changes, detection and attribution

Published findings on climate change impacts on tropical cyclones (TCs) in the ESCAP/WMO Typhoon Committee Region are assessed. We focus on observed TC changes in the western North Pacific (WNP) basin, including frequency, intensity, precipitation, track pattern, and storm surge. Results from an updated survey of impacts of past TC activity on various Members of the Typhoon Committee are also reported. Existing TC datasets continue to show substantial interdecadal variations in basin-wide TC frequency and intensity in the WNP. There has been encouraging progress in improving the consensus between different datasets concerning intensity trends. A statistically significant northwestward shift in WNP TC tracks since the 1980s has been documented. There is low-to-medium confidence in a detectable poleward shift since the 1940s in the average latitude where TCs reach their peak intensity in the WNP. A worsening of storm inundation levels is believed to be occurring due to sea level rise-due in part to anthropogenic influence-assuming all other factors equal. However, we are not aware that any TC climate change signal has been convincingly detected in WNP sea level extremes data. We also consider detection and attribution of observed changes based on an alternative Type II error avoidance perspective.

Tropical cyclone track prediction using a large-area WRF model at the Hong Kong Observatory

Tropical cyclone (TC) track predictions of the 10-km resolution WRF (provisionally named "AAMC-WRF") of the Hong Kong Observatory (HKO), spanning (20⁰S - 60⁰N, 45⁰E − 160⁰E) is studied for a 1-year period from April 2018 to Mar 2019. Real-time predictions, up to 4 times a day and T+48 h ahead, are verified against operational analysis positions of HKO for storms over the South China Sea (SCS) and Western North Pacific (WNP); and of the New Delhi Regional Specialised Meteorological Centre (RSMC) for storms over the North Indian Ocean basin (NIO; including the Bay of Bengal). Out of 21 named TCs over SCS and WNP, mean positional errors of the AAMC-WRF are 33 km (T+0), 63 km (T+24), and 107 km (T+48) based on 209, 178 and 142 forecasts. The AAMC-WRF outperformed Meso-NHM, also run in real-time at HKO, with mean error reduction up to 34 km or 24%. Mean positional errors for 13 NIO storms are 38 km (T+0), 69 km (T+24) and 107 km (T+48) based on 183, 131 and 85 forecasts. This is the first study in which TC predictions of a regional model are simultaneously examined over the SCS, WNP and NIO basins through real-time experiments.

Riverine Flooding and Landfalling Tropical Cyclones Over China

AbstractRiverine flooding associated with landfalling tropical cyclones (TCs) in the western North Pacific basin is responsible for some of the most severe socioeconomic losses in East Asian countries. However, little is known about the spatial and temporal patterns of TC flooding and its climate controls, which constrain the predictive understandings of flood risk in this highly populated region. We provide a climatological characterization of TC flooding over China based on an exceptional network of stream gauging stations across the entire country. The most extreme floods in central and northeastern China are associated with TCs despite infrequent TC visits in these regions. Temporal variations in TC flooding demonstrate a mixture of controls tied to surface temperature anomalies in the northern hemisphere. The established links between TC flooding and climate controls present a potentially predictive tool of TC flood risk over China and other East Asian countries under future climate conditions.

A Look at the Relationship between the Large-Scale Tropospheric Static Stability and the Tropical Cyclone Maximum Intensity

AbstractVarious modeling and observational studies have suggested that tropical cyclone (TC) intensity tends to increase in the future due to projected warmer sea surface temperature (SST). This study examines the effects of the tropospheric stratification that could potentially offset the direct increase of TC intensity associated with the warmer SST. Using reanalysis datasets and TC records in the northwestern Pacific and the North Atlantic basins, it is shown that there exists a consistently negative correlation between the annually averaged TC intensity and the basinwide average of the tropospheric static stability. This negative correlation is more robust in the northwestern Pacific basin when using the TC lifetime maximum intensity but is somewhat less significant in the North Atlantic basin. Further separation of the troposphere into a lower (1000–500 hPa) and an upper layer (500–200 hPa) reveals that it is the upper-tropospheric static stability that plays a more dominant role in governing the TC intensity variability. The negating effects of a stable troposphere on TC intensity as found in this study suggest a partial offset of the projected increase in the TC potential intensity due to the future warmer SST. Thus, the tropospheric static stability is one of the key large-scale factors that need to be properly taken into account in studies of long-term TC intensity change.

The Diurnal Cycle of Clouds in Tropical Cyclones over the Western North Pacific Basin

The Diurnal Cycle of Clouds in Tropical Cyclones over the Western North Pacific Basin

Classification of Tropical Cyclone Intensity in the Western North Pacific Basin by Applying a Deep Convolutional Neural Network to Geostationary Satellite Imagery

Classification of Tropical Cyclone Intensity in the Western North Pacific Basin by Applying a Deep Convolutional Neural Network to Geostationary Satellite Imagery