口頭発表

武樋蕗子 (東大先端研)
"Asymmetric Thermal Structure of a Tropical Cyclone Moving across the Oceanic Frontal Zone in the Southern East China Sea"
「東シナ海南部の水温前線帯を通過した台風の非軸対称温度構造」

Abstract:
For the transition of a tropical cyclone (TC) into an extratropical
cyclone (extratropical transition), many study have discussed about
structure changes of a TC that occur in a deep baroclinic zone
associated with an upper-tropospheric westerly jet. In contrast,
possible influence on structure changes of a TC that exerted by an
oceanic frontal zone has been overlooked, despite the importance of a
near-surface baroclinic zone anchored by an oceanic frontal zone (a zone
with strong sea surface temperature (SST) gradient) for the development
of extratropical cyclones, as pointed out recently. This study
investigates a particular TC, typhoon Songda (TY1102), which approached
an early-summer oceanic frontal zone in the southern East China Sea far
south of a westerly jet, with particular focus on the influence of its
frontal SST gradient on the asymmetric thermal structure of the TC
within the planetary boundary layer.

辻野 智紀 (名大HyARC)                                             
"Structure and Maintenance of Concentric Eyewalls in Simulated Typhoon Bolaven (2012)"
「2012 年台風 15 号の多重壁雲の構造と維持」
An intense TC occasionally has multiple eyewalls which are called as concentric eyewalls. Striking concentric eyewalls of hurricanes are studied by radar observations and numerical simulations. These previous studies indicated that eyewall replacement often occurs after concentric eyewalls are formed. In contrast to the replacement of eyewall, Typhoon Bolaven, which passed over the main Okinawa Island in 2012, had steady concentric eyewalls for very long time. It is the case that the eyewall replacement does not occur even if concentric eyewalls are formed. Structure of concentric eyewalls and process of eyewall replacement are not fully known. In this study, we investigate the structure of the concentric eyewalls and mechanism of maintenance in Typhoon Bolaven, using the Cloud Resolving Storm Simulator (CReSS) which is a three dimensional, nonhydrostatic model. To simulate the concentric eyewalls of Bolaven, we conducted numerical experiments with horizontal resolution of about 1 km. Simulated multiple eyewalls were located within about 150 km radius from Bolaven's center and they were steady for over one day. The simulated eyewalls has the moat regions, which is very dry with descending motion. These results almost agree with Doppler radar observations by Japan Meteorological Agency (JMA). In general an inner eyewall of TC gradually decays when, in planetary boundary layer, supply of vapor to the inner eyewall is decreased by an outer eyewall. To investigate tendency of the supply of water vapor and condensations to the inner eyewall, we analyzed water budget for the inner eyewall of simulated Bolaven. The result showed that the tendency of the supply was not changed, after the concentric eyewall formed clearly. The simulated outer eyewall had a structure tilted outward, relative to the inner eyewall. It suggests that upward flow in the outer eyewall is weakened due to the tilted structure. Thus, we suppose that the supply of water vapor to the outer eyewall is very weak and the supply to the inner eyewall is not changed because intensity of upward flow in an eyewall is related to a supplied amount of water vapor to the eyewall in PBL.

嶋田宇大(気象研)

An estimation of tropical cyclone intensity by using single Doppler radar data –a case study on Typhoon Bolaven (2012)

–単一ドップラーレーダーデータを利用した台風強度推定 -2012年台風第15号の事例-

The ground-based velocity track display (GBVTD) technique (Bell and Lee 2012), which is used in order to retrieve wind fields of a tropical cyclone (TC) from single ground-based Doppler velocity data, is applied to estimate the intensity (i.e., central pressure and maximum wind speed) of Typhoon Bolaven (2012) for 11 hours (from 1450 JST 26 August to 0145 JST 27 August 2012) at 5-min intervals. Assuming the gradient wind balance, we estimated the central pressures of Bolaven, using surface pressure observations.

The results show that the estimated central pressures and maximum wind speeds were almost equal to those of the JMA best track data. The radius of maximum wind (RMW) was located right outside the radius of the reflectivity maximum and the decrease of the radius of the strongest radar reflectivity seems to correspond to that of the RMW. This indicates that the GBVTD can reproduce the wind fields associated with the shrinking eyewalls. Most of RMSDs between the reconstructed Doppler velocity from the GBVDT-retrieved circulation and the original Doppler velocity were less than 2m/s. This result suggests that in this case the detected circulation center positions and the retrieved wind fields were reasonable and the techniques employed here are applicable to estimate TC intensity with sufficient accuracy.

 

単一ドップラーレーダーデータを利用して、2012年台風第15号の強度推定を行った。沖縄(糸数)現業ドップラーレーダーで台風の全貌を捉えることができた11時間(8月26日1450JSTから27日0145JSTまで)・5分毎のドップラー速度データに対し、GBVTDという手法(Bell and Lee, 2012)を適用し、台風周辺の風速場をリトリーブして、最大風速の推定を行うとともに、傾度風の仮定とアメダスの気圧データを使用して中心気圧の推定を行った。推定結果をベストトラックと比較した結果、両者ともベストトラックの値との良い一致を見た。また、レーダー反射強度の一番大きいところのすぐ外側に最大接線風速半径(RMW)が位置しており、レーダー反射強度分布にみられる眼の壁雲のわずかな収縮が、RMWにも現れていた。リトリーブした風速場からドップラー速度に逆変換したものと、元々のドップラー速度とのRMSDをとった結果、おおむね2m/s以下となった。このことから、この事例においては、本手法によるドップラー速度データの解析は妥当で、十分に精度よく台風の強度を推定できることが示唆された。


 武田一孝(東大AORI)

Numerical Study on the generation and development of tropical cyclone triggered by a cluster of cumulus clouds.

積雲対流群から生じる台風の発生・発達に関する数値的研究

Looking at the infrared image, it can be seen that some tropical disturbances reach tropical cyclone intensity after repeatedly rise and fall, others itself disappear. In this study, it was examined by idealized experiments that how a cluster of cumulus triggered by the potential temperature disturbances (cf. Yamasaki (1977, 1983)) reaches tropical cyclone intensity or doesn’t.

The model used in this study is Japan Meteorological Agency Non-Hydrostatic model (JMA-NHM).The calculation domain is 3000km x 3000km x 23.5km. To initiate a cloud cluster, spheroidal warm bubbles having potential temperature anomaly of 2K and horizontal and vertical radii of 20km and 1.2km, respectively are placed at regular grids of 40km interval within 200km from the center of the calculation domain.

 From the results of the experiments with varying stability of the stratification, it is revealed that as cold pool with downward flow is strong, new convection is likely to occur and therefore tropical cyclone is likely to generate. Because the value of GPI(Emanuel (2004)) on environmental field in this study is relatively small, the tropical cyclone in this study was also found to generate in different process from the spontaneous development(Nolan et al.(2007)).


柳瀬亘(東大AORI)

Parameter spaces of environmental fields responsible for cyclone development from tropics to extratropics

環境場のパラメタ空間を利用した熱帯〜温帯で発達する低気圧の統計

Cyclone development in the summer season is active in the tropics and extratropics and inactive in the subtropics. To understand this geographically bimodal distribution of cyclone development associated with tropical and extratropical cyclones quantitatively, the direct relationship between cyclone types and their environments in the JRA-25 reanalysis is assessed by using a parameter space of environmental variables (EPS). The number of cyclones is analyzed in terms of two different factors: the environmental conditions favorable for cyclone development, and the area size that satisfies the favorable condition.

  The EPS analysis is mainly conducted for two representative environmental parameters which are commonly used for cyclone analysis: potential intensity for tropical cyclones and baroclinicity for extratropical cyclones. The geographically bimodal distribution is attributed to the high sensitivity of the cyclone development to the change in the environmental fields from tropics to extratropics. In addition, the bimodal distribution is partly attributed to the rapid change in the environmental fields from tropics to extratropics. The EPS analysis also shows that other environmental parameters including relative humidity and vertical velocity may enhance the contrast between the tropics (extratropics) and subtropics, whereas they are not essential for determining cyclone types. 


金田幸恵(名大HyARC)

Impact of ice-phase microphysics on inner-core processes in simulated extremely intense tropical cyclones

非常に強い台風の内部コアプロセスに対する雲微物理過程の影響

Observations have been reported that most extremely intense tropical cyclones (TCs) such as Categories 4 and 5 underwent the rapid intensification (RI) and often possessed the secondary eyewall formation (SEF) and eyewall replacement cycles (ERC). Those distinguishing characteristics are closely related to the inner-core processes, yet the physical mechanisms of the RI, SEF and ERC and inner-core processes are not fully understood. To understand the factor controlling the RI, SEF and ERC, sensitivity experiments are conducted by using two 2-km mesh non-hydrostatic models, the JMANHM based on the Japan Metrological Agency operational mesoscale model (Saito et al. 2007) and Cloud Resolving Storm Simulator (CReSS: Tsuboki and Sakakibara 2002) developed in HyARC, Nagoya University. The initial and boundary conditions for an extremely intense TC are provided by the climate experiments by a 20-km mesh atmospheric general circulation model. The sensitivity experiments include 1-moment and 2-moment bulk-type microphysics with an ice-phase along with a second-order turbulence closure scheme (the MYNN scheme) and a 1.5-order turbulence closure scheme.

 All experiments by 2-km non-hydrostatic models undergo RI defined as a decrease of 42 hPa in less than 24 hours. The experiments having shallow inflow boundary layer with intense near-surface inflows are favorable for the intense and tall eyewall updraft during RI phase. The graupel-dominant experiments (CReSSs) tend to have smaller horizontal expansions, while snow-dominant experiments (JMANHMs) tend to have larger expansions. The sensitivity to the strength of near-surface inflows is also recognized in the horizontal expansion; the weaker near-surface inflow experiment shows a smaller horizontal expansion. Most experiments undergo SEF and ERC. Only the exception is a lesser snow experiment (CReSS with a 1-moment microphysics).

References

Saito, K., J. Ishida, K. Aranami, T. Hara, T. Segawa, M. Narita, and Y. Honda, 2007: Nonhydrostatic atmospheric models and operational development at JMA. J. Meteor. Soc. Japan, 85B, 271-304.

Tsuboki, K., and A. Sakakibara, 2002: Large–scale parallel computing of cloud resolving storm simulator. High Performance Computing, Springer, 243–259.



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