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Udai Shimada

Abstract

How environmental conditions vary among rapidly intensifying tropical cyclones (TCs) and which factors can help offset negative factors for intensification were examined using a dataset of geostationary satellites and environmental diagnostics. The dataset contains TCs in the western North Pacific from 1995 to 2020. A cluster analysis was performed to classify different morphologies of TC cloud patterns at the onset of rapid intensification (RI). Six clusters were identified, and each cluster had a distinct set of environmental conditions. Three clusters (clusters 1, 3, and 5) had some conditions unfavorable for RI. Cluster 1 TCs were exposed to moderate vertical (850–200 hPa) shear (∼6 m s−1). Relatively high sea surface temperature, a moist environment, and movement toward environments with weak vertical shear, high equivalent potential temperature, and high ocean heat content are potential factors that resist the effects of vertical shear. Cluster 3 TCs were characterized by a large 30-kt wind radius and moderate vertical shear (1 kt ≈ 0.51 m s−1). Large storm size and a moist environment caused by large-scale, strong, low-level convergence are possible factors for vortex resiliency against shear. Cluster 5 TCs were located in a very dry environment. Weak vertical shear and small storm size are factors that may offset the negative effects of dry air and ocean cooling. The results suggest that in the case of RI with negative conditions for intensification, other factors can offset the negative impacts of those conditions and that suitable combinations of environmental conditions and TC structural features are important for RI.

Open access
Udai Shimada and Takeshi Horinouchi

Abstract

Strong vertical wind shear produces asymmetries in the eyewall structure of a tropical cyclone (TC) and is generally a hostile environment for TC intensification. Typhoon Noul (2015), however, reintensified and formed a closed eyewall despite 200–850-hPa vertical shear in excess of 11 m s−1. Noul’s reintensification and eyewall formation in strong shear were examined by using Doppler radar and surface observations. The evolution of the azimuthal-mean structure showed that the tangential wind at 2-km altitude increased from 30 to 45 m s−1 in only 5 h. During the first half of the reintensification, the azimuthal-mean inflow penetrated into the ~40-km radius, well inside the radius of maximum wind (RMW), at least below 4-km altitude, and reflectivity inside the RMW increased. As for the asymmetric evolution, vigorous convection, dominated by an azimuthal wavenumber-1 asymmetry, occurred in the downshear-left quadrant when shear started to increase and then moved upshear. A mesovortex formed inside the convective asymmetry on the upshear side. The direction of vortex tilt between the 1- and 5-km altitudes rotated cyclonically from the downshear-left to the upshear-right quadrant as the vortex was vertically aligned. In conjunction with the alignment, the amplitude of the wavenumber-1 convective asymmetry decreased and a closed eyewall formed. These features are consistent with the theory that a vortex can be vertically aligned through upshear precession. The analysis results suggest that the vortex tilt, vigorous convection, and subsequent intensification were triggered by the increase in shear in a convectively favorable environment.

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Wataru Mashiko and Udai Shimada

Abstract

Very strong Typhoon Goni (2015) passed over the Yaeyama Islands in southwestern Japan during the rapid intensification stage on 23 August. Surface data collected by the dense network of weather stations as well as Doppler radar observations over the islands revealed a finescale structure in the inner core of the typhoon near the surface. Goni had a clear eye surrounded by a square-shaped eyewall with intense convection. The surface observations revealed that several vortices with a diameter of ~7–10 km accompanied by a pressure deficit were present inside the eye. From the Doppler velocity field, mesovortices with diameters of approximately 10 km were found at the apexes of the square-shaped eyewall. These mesovortices and the inner rainbands emanating outward from the apexes of the polygonal eyewall generally exhibited features typical of vortex Rossby waves. The mesovortices were accompanied by a pressure deficit at the surface and enhanced surface winds. The data also indicated the first observational evidence of near-surface mixing between the eye and eyewall through the mesovortices, that is, the transport of high equivalent potential temperature in the eye toward the eyewall. The radar data revealed that many radar-reflectivity filaments that had a pleated shape with lengths of a few kilometers extended perpendicularly from the inner edge of the eyewall at low levels. The filaments associated with wind perturbations at low levels caused significant wind gusts accompanied by sudden pressure drops and shifts in wind direction at the surface.

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Udai Shimada, Masahiro Sawada, and Hiroyuki Yamada

Abstract

A ground-based Doppler radar observed the rapid intensification (RI) of Typhoon Goni (2015) for 24 h immediately after it completed an eyewall replacement cycle. Goni’s RI processes were examined by using radar reflectivity and wind fields retrieved by the ground-based velocity track display (GBVTD) technique. The maximum wind at 2-km altitude increased by 30 m s−1 during the first 6 h of RI, and it further increased by 20 m s−1 during the subsequent 12 h. Around the onset of RI, relatively strong outflow (>2 m s−1) was present both inside and outside the radius of maximum wind (RMW) above the boundary layer (BL), suggesting the existence of supergradient flow in and just above the BL. Despite this outflow, angular momentum increased inside the RMW. The low-level RMW contracted rapidly from 50 to 33 km, causing the RMW to slope greatly outward with height. The radius of maximum reflectivity was a few kilometers inside the RMW. A budget analysis of absolute angular momentum showed that the outflow contributed to the contraction of the tangential wind field. During RI, eyewall convection was enhanced, and a well-defined eye appeared. The low-level outflow changed into inflow immediately outside the RMW. Then the tangential wind field and high inertial stability region expanded radially outward, followed by the formation of an outer reflectivity maximum at twice the RMW. The contraction speed of the low-level RMW slowed down.

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Udai Shimada, Masahiro Sawada, and Hiroyuki Yamada
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Udai Shimada, Masahiro Sawada, and Hiroyuki Yamada

Abstract

Intensities (central pressures) of 28 cases of 22 tropical cyclones (TCs) that approached Japan were estimated by using single ground-based Doppler radar observations, and the accuracy and utility of the estimation method were evaluated. The method uses the ground-based velocity track display (GBVTD) technique, which retrieves tangential winds, and the gradient wind balance equation. Before application of the method to the 28 cases, a preliminary experiment was performed with pseudo-Doppler velocities obtained by numerical simulation to confirm that the method could reasonably estimate central pressures. Compared with best track data from the Regional Specialized Meteorological Center (RSMC) Tokyo, the estimated intensities of the 28 cases had a root-mean-square error of 8.37 hPa and showed a bias of 1.51 hPa. This level of accuracy is comparable to or better than the accuracies of Dvorak and satellite microwave-derived estimates. Two distance metrics are defined: 1) the distance between the TC center and the radar location and 2) the distance between the TC center and the weather station whose sea level pressure was used as an anchor for pressure measurement. In general, the accuracy of the Doppler radar estimates was higher when the distance metrics were shorter, as well as when wind retrieval accuracy was better and radar coverage was denser. For TCs with a radius of maximum wind of 20–70 km, the estimated central pressures had a root-mean-square error of 5.55 hPa. These results confirm that Doppler radar intensity estimates have sufficient accuracy and utility for operational use.

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Udai Shimada, Kazumasa Aonashi, and Yoshiaki Miyamoto

Abstract

The relationship of tropical cyclone (TC) future intensity change to current intensity and current axisymmetricity deduced from hourly Global Satellite Mapping of Precipitation (GSMaP) data was investigated. Axisymmetricity is a metric that correlates positively with the magnitude of the axisymmetric component of the rainfall rate and negatively with the magnitude of the asymmetric component. The samples used were all of the TCs that existed in the western North Pacific basin during the years 2000–15. The results showed that, during the development stage, the intensification rate at the current time, and 6 and 12 h after the current time was strongly related to both the current intensity and axisymmetricity. On average, the higher the axisymmetricity, the larger the intensity change in the next 24 h for TCs with a current central pressure (maximum sustained wind) between 945 and 995 hPa (85 and 40 kt). The mean value of the axisymmetricity for TCs experiencing rapid intensification (RI) was much higher than that for non-RI TCs for current intensities of 960–990 hPa. The new observational evidence for the intensification process presented here is consistent with the findings of previous theoretical studies emphasizing the role of the axisymmetric component of diabatic heating.

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Wataru Yanase, Udai Shimada, and Nao Takamura

Abstract

Tropical cyclones that complete extratropical transition (ETTCs) in the western North Pacific are statistically analyzed to clarify the large-scale conditions for their reintensification. A dataset of ETTCs is grouped into intensifying, dissipating, and neutral classes based on the best track data documented by the Japan Meteorological Agency during the period 1979–2018. Intensifying ETTCs are most frequent in September–October, whereas dissipating ETTCs are most frequent in the later season, October–November. Intensifying ETTCs occur at higher latitudes than dissipating ETTCs, where the upper levels are characterized by high potential vorticity (PV) and a steep horizontal gradient of PV. The composite analysis demonstrates that intensifying ETTCs are associated with deep upper-level troughs to their northwest, intense ridge building to their northeast, and strong updrafts to their north associated with vorticity advection and warm-air advection. These results statistically support the findings of previous studies. Furthermore, an analysis using a time filter demonstrates the relationship between planetary-scale environments and synoptic-scale dynamics in the upper levels. The high PV to the northwest of ETTCs is attributed not only to eastward-moving troughs, but also to the environmental PV. The low PV to the northeast of ETTCs results from the negative PV formation associated with ridge building, which almost cancels the environmental PV. Thus, the environmental PV at relatively high latitudes enhances the intensity of positive PV to the northwest of ETTCs, and increases the upper limit of the magnitude of ridge building to the northeast.

Open access
Udai Shimada, Hisayuki Kubota, Hiroyuki Yamada, Esperanza O. Cayanan, and Flaviana D. Hilario

Abstract

The intensity and inner-core structure of an extremely intense tropical cyclone, Typhoon Haiyan (2013), were examined using real-time ground-based Doppler radar products from the Guiuan radar over the period of about 2.5 h immediately before the storm approached Guiuan in Eastern Samar, Philippines. Haiyan’s wind fields from 2- to 6-km altitude were retrieved by the ground-based velocity track display (GBVTD) technique from the Doppler velocity data. The GBVTD-retrieved maximum wind speed reached 101 m s−1 at 4-km altitude on the right side of the track. The relatively fast forward speed of Haiyan, about 11 m s−1, increased maximum wind speed on the right-hand side of the storm. Azimuthal mean tangential wind increased with height from 2 to 5 km, and a local maximum of 86 m s−1 occurred at 5-km altitude. The central pressure was estimated as 906 hPa with an uncertainty of ±4 hPa by using the GBVTD-retrieved tangential wind and by assuming gradient wind balance. The radius of maximum radar reflectivity was about 23 km from the center, a few kilometers inside the radius of maximum wind. The reflectivity structure was highly asymmetric at and above 3-km altitude, and was more symmetric below 3-km altitude in the presence of relatively weak vertical shear (~4 m s−1). The center of the eyewall ring was tilted slightly downshear with height.

Open access
Udai Shimada, Hiromi Owada, Munehiko Yamaguchi, Takeshi Iriguchi, Masahiro Sawada, Kazumasa Aonashi, Mark DeMaria, and Kate D. Musgrave

Abstract

The Statistical Hurricane Intensity Prediction Scheme (SHIPS) is a multiple regression model for forecasting tropical cyclone (TC) intensity [both central pressure (Pmin) and maximum wind speed (Vmax)]. To further improve the accuracy of the Japan Meteorological Agency version of SHIPS, five new predictors associated with TC rainfall and structural features were incorporated into the scheme. Four of the five predictors were primarily derived from the hourly Global Satellite Mapping of Precipitation (GSMaP) reanalysis product, which is a microwave satellite-derived rainfall dataset. The predictors include the axisymmetry of rainfall distribution around a TC multiplied by ocean heat content (OHC), rainfall areal coverage, the radius of maximum azimuthal mean rainfall, and total volumetric rain multiplied by OHC. The fifth predictor is the Rossby number. Among these predictors, the axisymmetry multiplied by OHC had the greatest impact on intensity change, particularly, at forecast times up to 42 h. The forecast results up to 5 days showed that the mean absolute error (MAE) of the Pmin forecast in SHIPS with the new predictors was improved by over 6% in the first half of the forecast period. The MAE of the Vmax forecast was also improved by nearly 4%. Regarding the Pmin forecast, the improvement was greatest (up to 13%) for steady-state TCs, including those initialized as tropical depressions, with slight improvement (2%–5%) for intensifying TCs. Finally, a real-time forecast experiment utilizing the hourly near-real-time GSMaP product demonstrated the improvement of the SHIPS forecasts, confirming feasibility for operational use.

Open access