Regional Specialized Meteorological Center Tokyo Research Articles

Comparison of Tropical Cyclone Wind Radius Estimates between the KMA, RSMC Tokyo, and JTWC

This study compared estimates of gale-force wind radii (R30 or R34) and storm-force wind radii (R50) of tropical cyclones (TC) by three agencies—the Korea Meteorological Administration (KMA), the Regional Specialized Meteorological Center (RSMC) Tokyo, and the Joint Typhoon Warning Center (JTWC)—in the western North Pacific during 2015–2018 and investigated the characteristics of these estimates. The results showed that the KMA’s R30 and R50 estimates were smaller (38% and 29%, respectively) than those of the RSMC Tokyo, and larger (11%) for R30 and smaller (12%) for R50 than those of the JTWC. The differences between these agencies seem to be largely determined by whether the agency estimates wind radii based only on a TC’s own winds or on TC winds combined with other mid-latitude synoptic systems to make TC warnings more comprehensive. The former is mainly the practice of the KMA and JTWC, whereas the latter is mainly the practice of the RSMC Tokyo. The factors considered for estimating wind radii also differ between the agencies: the KMA heavily relies on TC intensity—the higher the intensity, the larger the radius—while the RSMC Tokyo and JTWC rely less on TC intensity than the KMA but additionally consider the latitude and storm translation speed in their estimations. In particular, the TC translation speed considered by the RSMC Tokyo and JTWC explains why their estimated wind radii exhibit, on average, greater asymmetries (i.e., greater differences between the longest and shortest radii) than those estimated by the KMA. The findings of agency-dependent characteristics of TC wind radius data help to better determining and understanding the TC impact areas for TC risk reduction and management.

Prediction of Typhoon Track and Intensity Using a Generative Adversarial Network With Observational and Meteorological Data

To save lives and reduce damage from the destructive impacts of a typhoon, an accurate and fast forecast method is highly demanded. Particularly, predictions for short lead times, known as nowcasting, rely on fast forecasts allowing immediate emergency plannings in the affected areas. In this paper, we propose a generative adversarial network that operates on a single graphics processing unit, to predict both the track and intensity of typhoons for short lead times within fractions of a second. To investigate the effects of meteorological variables on typhoon forecasts, we conducted a parameter study for 6-h track predictions. The results of the study indicate that learning velocity, temperature, pressure, and humidity along with satellite images have positive effects on prediction accuracy. To address the limited access to observational data and facilitate predictions for 12-h intervals, we replaced satellite images with reanalysis data of the total cloud cover and vorticity fields. This replacement led to an increase in data from 76 to 757 typhoons, and it reduced the error of the 6-h track forecasts by 23.5%. The best combination of the parameter study yields track predictions in intervals of 6 and 12 h with the corresponding averaged absolute errors of 44.5 and 68.7 km. Typhoon intensities are predicted by extracting information from generated velocity fields with averaged hit rates of 87.3% and 83.2% for 6- and 12-h interval forecasts, respectively. For typhoons after 1994, tracks and intensities for 12-h intervals are compared to forecasts from the Joint Typhoon Warning Center and Regional Specialized Meteorological Center Tokyo.

An Analysis of Tropical Cyclone Activity over the Western North Pacific in 2011

ABSTRACT This paper uses China Meteorological Administration tropical cyclone (TC) best track data to analyze the characteristics of TC activity over the western North Pacific in 2011. Results derived from two other datasets, issued by the Regional Specialized Meteorological Center Tokyo and the Joint Typhoon Warning Center, were also compared. We show that the annual TC activity in 2011 was weaker than the climatology, and exhibited remarkable intraseasonal signals. The main TC genesis region migrated seasonally between the southwest (7° – 17°N, 120° – 140°E) and northeast (17°–25°N, 135°–165°E) quadrants. Moreover, three prevailing tracks were identified: westward tracks, recurving tracks between 125°–135°E, and northward tracks near 155°E. Special attention was paid to the intra-annual features of TC activity during 2011 and the potential modulators of this activity, including the tropical Indo-Pacific oceans, East Asian Monsoon (EAM), and Intraseasonal Oscillation (ISO). The seasonal migration of the main TC genesis region can be attributed to modulation by the EAM trough and ISO rather than the El Nino-Southern Oscillation. Both the EAM trough and ISO provided favorable conditions, including cyclonic vorticity in the low troposphere, limited vertical shear, and moisture convergence, in both of the main TC genesis quadrants, but unfavorable conditions outside the particular genesis quadrant. Moreover, both EAM and ISO generated a favorable circulation that steered TCs to move along the three prevailing tracks. These findings indicate that TC activity in 2011 was more significantly affected by both the EAM and ISO than by the ENSO.

The Long Term Variations of Tropical Cyclone Activity in the South China Sea and the Vicinity of Hong Kong

ABSTRACT This study examines the long-term variations of tropical cyclone (TC) frequency and intensity in the South China Sea (SCS) and the vicinity of Hong Kong from 1961 to 2010 based on the best track data of four main weather agencies in the western North Pacific, namely the Hong Kong Observatory, China Meteorological Administration, Regional Specialized Meteorological Center Tokyo and Joint Typhoon Warning Center of the United States. To account for the discrepancy in the best track data between agencies, the maximum sustained wind speeds are standardized to the 10-minute average before data analysis. Moreover, a sensitivity assessment based on three different data scenarios is also conducted to study the uncertainty in trend analysis due to the discrepancy in the datasets from various agencies. the results show that, likely modulated by the El Nino Southern Oscillation and the Pacific Decadal Oscillation, there exist strong inter-annual and inter-decadal variations in the TC frequency in the SCS and the vicinity of Hong Kong. For the long-term trend, all dataset/scenario combinations depict a decrease in the TC frequency in the SCS and the vicinity of Hong Kong during the study period, but the trend is not statistically significant at 5% level for most of the datasets. As for the TC intensity, the discrepancy between weather agencies remains very substantial even when the difference in the wind speed averaging period is accounted for. The large differences in the available datasets do not allow for a reliable detection of the long-term trend of the TC intensity in the SCS. The study of the TC impacts on Hong Kong reveals that there is no significant trend on the TC-induced extreme rainfall in Hong Kong. The extreme high winds associated with TCs within 500 km range of Hong Kong have no significant trend at Waglan Island (offshore island) while those of the urban station at Kai Tak have a significant decreasing trend.

The role of ocean‐atmosphere interaction in Typhoon Sinlaku (2008) using a regional coupled data assimilation system

AbstractFor improving analyses of tropical cyclone (TC) and sea surface temperature (SST) and thereby TC simulations, a regional mesoscale strongly coupled atmosphere‐ocean data assimilation system was developed with the local ensemble transform Kalman filter (LETKF) implemented with the Japan Meteorological Agency's nonhydrostatic model (NHM) coupled with a multilayer ocean model and the third‐generation ocean wave model. The NHM‐LETKF coupled data assimilation system was applied to Typhoon Sinlaku (2008) along with the original NHM‐LETKF system to investigate the sensitivity of Sinlaku to SST assimilation with the Level 2 Pre‐processed (L2P) standard product of satellite SST. SST calculated in the coupled‐assimilation experiment with the coupled data assimilation system and the satellite SST (CPL) showed a better correlation with Optimally Interpolated SST than SST used in the control experiment with the original NHM‐LETKF (CNTL) and SST calculated in the succession experiment with the coupled system without satellite SST (SUCC). The time series in the CPL experiment well captured the variation in the SST observed at the Kuroshio Extension Observation buoy site. In addition, TC‐induced sea surface cooling was analyzed more realistically in the CPL experiment than that in the CNTL and SUCC experiments. However, the central pressure analyzed in each three experiments was overestimated compared with the Regional Specialized Meteorological Center Tokyo best‐track central pressure, mainly due to the coarse horizontal resolution of 15 km. The 96 h TC simulations indicated that the CPL experiment provided more favorable initial and boundary conditions than the CNTL experiment to simulate TC tracks more accurately.

Impact of the subtropical high on the extratropical transition of tropical cyclones over the western North Pacific

Recent publications have investigated the interactions between the extratropical transitions (ETs) of tropical cyclones (TCs) and midlatitude circulations; however, studies of ET events have rarely considered the relationship between the storm and the nearby subtropical high. The TC best-track data provided by the Regional Specialized Meteorological Center-Tokyo Typhoon Center of the Japan Meteorology Agency are used in conjunction with the NCEP/NCAR reanalysis data to discuss the potential effects of the subtropical high on ETs over the western North Pacific basin. When the western Pacific subtropical high (WPSH) is weakened and withdrawn toward the east, more TCs follow recurving paths and the midlatitude trough activity is intensified. These changes lead to enhanced ET activity. By contrast, when the WPSH strengthens and extends westward, the number of TCs that follow direct westward paths increases and the midlatitude trough is relatively inactive. These conditions lead to reduced occurrences of ET cases. Abnormal activity of the WPSH should be considered as an important factor in determining ET activity.

Tropical cyclone track and intensity prediction with a structure adjustable balanced vortex

A new Tropical Cyclone (TC) initialization method with the structure adjustable bogus vortex was applied to the forecasts of track, central pressure, and wind intensity for the 417 TCs observed in the Western North Pacific during the 3-year period of 2005–2007. In the simulations the Final Analyses (FNL) with 1° × 1° resolution of National Center for Environmental Prediction (NCEP) were incorporated as initial conditions. The present method was shown to produce improved forecasts over those without the TC initialization and those made by Regional Specialized Meteorological Center Tokyo. The average track (central pressure, wind intensity) errors were as small as 78.0 km (11.4 hPa, 4.9 m s−1) and 139.9 km (12.4 hPa, 5.5 m s−1) for 24-h and 48-h forecasts, respectively. It was found that the forecast errors are almost independent on the size and intensity of the observed TCs because the size and intensity of the bogus vortex can be adjusted to fit the best track data. The results of this study indicate that a bogus method is useful in predicting simultaneously the track, central pressure, and intensity with accuracy using a dynamical forecast model.

Tropical Cyclone Initialization with a Spherical High-Order Filter and an Idealized Three-Dimensional Bogus Vortex

Abstract A tropical cyclone initialization method with an idealized three-dimensional bogus vortex of an analytic empirical formula is presented for the track and intensity prediction. The procedure in the new method consists of four steps: the separation of the disturbance from the analysis, determination of the tropical cyclone domain, generation of symmetric bogus vortex, and merging of it with the analysis data. When separating the disturbance field, an efficient spherical high-order filter with the double-Fourier series is used whose cutoff scale can be adjusted with ease to the horizontal scale of the tropical cyclone of interest. The tropical cyclone domain is determined from the streamfunction field instead of the velocities. The axisymmetric vortex to replace the poorly resolved tropical cyclone in the analysis is designed in terms of analytic empirical functions with a careful treatment of the upper-layer flows as well as the secondary circulations. The geopotential of the vortex is given in such a way that the negative anomaly in the lower layer is changed into positive anomaly above the prescribed pressure level, which depends on the intensity of the tropical cyclone. The geopotential is then used to calculate the tangential wind and temperature using the gradient wind balance and the hydrostatic balance, respectively. The inflow and outflow in the tropical cyclone are constructed to resemble closely the observed or simulated structures under the constraint of mass balance. The bogus vortex is merged with the disturbance field with the use of matching principle so that it is not affected except near the boundary of tropical cyclone domain. The humidity of the analysis is modified to be very close to the saturation in the lower layers near the tropical cyclone center. The balanced bogus vortex of the present study is completely specified on the basis of four parameters from the Regional Specialized Meteorological Center (RSMC) report and the additional two parameters, which are derived from the analysis data. The initialization method was applied to the track and the intensity (in terms of central pressure) prediction of the TCs observed in the western North Pacific Ocean and East China Sea in 2007 with the use of the Weather Research and Forecasting (WRF) model. No significant initial jump or abrupt change was seen in either momentum or surface pressure during the time integration, thus indicating a proper tropical cyclone initialization. Relative to the results without the tropical cyclone initialization and the forecast results of RSMC Tokyo, the present method presented a great improvement in both the track and intensity prediction.