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Takio Murakami and Tetsuo Nakazawa

Abstract

During the period from 15 February to 20 June 1979, the northward migration of the zones of low outgoing longwave radiation was most clearly defined over the convectively active continental regions of equatorial Africa, the maritime continent, and equatorial South America. Regionally, the withdrawal of the Southern Hemisphere summer monsoon over Indonesia and Australia was followed by the first establishment of the summer monsoon over the Malaysian Peninsula during the midtransition around 15 April, which is about two months earlier than the monsoon onset over central India.

In investigating the wind changes during the transition period, smoothed time-series data were constructed by adding three components of winds; viz., the annual mean winds, which are approximately symmetric about the equator; and the yearly and half-yearly wind harmonics that are asymmetric about the equator with a distinct seasonal character. The 200 mb wind patterns during the midtransition exhibit new symmetric character with pairs of anticyclones straddling the equator over Africa, the maritime continent, and South America, respectively. Of particular interest is the first establishment of the Northern Hemisphere summer monsoon anticyclone near southern Indochina as a response to symmetric equatorial heat sources over the Sumatra-Borneo-New Guinea region during the midtransition. The yearly component of the winds. which merely reflects the seasonal variations of the Hadley circulation and its associated zonal winds, contributes little to the formation of the Indochina anticyclone. The half-yearly harmonics are primarily responsible for the establishment of the anticyclone, which propagates northward through Indochina, finally reaching the southern periphery of the Tibetan Plateau by midsummer.

This study also confirmed that the character of long-period (30–50 day) oscillations changes drastically during the transition period. Prior to about 15 April, the 30–50 day modes are prominent over the Southern Hemisphere tropics (5–15°S). They propagate eastward along the equator over the Indian Ocean. After 15 April, the 30–50 day perturbations exhibit the Northern Hemisphere summer character of systematic eastward propagation and large amplitude over the global tropics between the equator and 20°N.

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Yoshi-Yuki Hayashi and Tetsuo Nakazawa

Abstract

A numerical experiment with an atmospheric general model (GCM) indicate that moist convection in the equatorial region is spontaneously organized into a form of “supercluster” which is an area of precipitation with a spatial extent of about 2000 km and an eastward propagation speed of about 1 5 m s−1.

In this article, the existence of superclusters in the real atmosphere is shown through a comparison between satellite observations and the GCM results. It is argued that eastward motion of convective activity occurs not only as the well-known property of the 30–60 day oscillation, but as a property of synoptic-scale disturbances at the equator.

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Kotaro Bessho, Tetsuo Nakazawa, Shuji Nishimura, and Koji Kato

Abstract

The temperature profiles of organized cloud clusters developing or not developing (nondeveloping) into tropical storms (TSs; maximum surface wind >34 kt) over the western North Pacific in 2004 were investigated using Advanced Microwave Sounding Unit (AMSU) observations in combination with the independently created early stage Dvorak analysis. Typical temperature profiles of the developing and nondeveloping cloud clusters were compared. From this comparison, positive upper-troposphere temperature anomalies were found in both cluster types; however, the spatial extent of the temperature anomalies for the developing cloud clusters was larger than those of the nondeveloping cloud clusters. Statistical analysis was performed on the temperature anomalies near the center of all clusters retrieved from AMSU observational data. Findings indicate that the area-average temperature anomalies increased along with the intensity of the clusters indicated by the Dvorak T-number classification. Using time series analysis of upper-level temperature anomalies associated with these cloud clusters, a definition of warm core structures showing the temperature anomaly greater than a threshold (WCT) was created. WCT exists when the area averaged temperature anomaly exceeds 0.9 K. Using this definition, almost 70% of the cloud clusters that had WCTs later became TSs, while 85% of those that did not have WCTs eventually dissipated without being classified as a TS. For the WCT clusters that developed into TSs, the lead time from the detection of their AMSU-based WCT to their classification as TSs was 27.7 h. These results indicate that there is a good possibility that the detection and forecasting of tropical cyclone formation, particularly those storms that later may become classified as TSs, will be improved using temperature anomalies derived from AMSU data.

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Munehiko Yamaguchi, Takeshi Iriguchi, Tetsuo Nakazawa, and Chun-Chieh Wu

Abstract

An Observing System Experiment (OSE) has been performed to investigate the effectiveness of dropwindsonde observations and a sensitivity analysis technique on a typhoon track forecast. Using dropwindsonde observations for Typhoon Conson at 1200 UTC 8 June 2004, which are derived from Dropwindsonde Observation for Typhoon Surveillance near the Taiwan Region (DOTSTAR), four numerical experiments are conducted, which are different only in terms of the number of dropwindsonde observations used in a data assimilation system: (i) no observation is assimilated; (ii) all observations are assimilated; (iii) observations within a sensitive region as revealed by a singular vector method at the Japan Meteorological Agency (JMA) are assimilated; and (iv) observations outside the sensitive region are assimilated. In the comparison of the four track forecasts, Conson’s northeastward movement is expressed in the second and third simulations while in the first and fourth experiments Conson stays at almost the same position as its initial position. Through the OSE, it is found that DOTSTAR observations had a positive impact on the track forecast for Conson, and that observations within the sensitive region are enough to predict the northeastward movement of Conson, indicating that the JMA singular vector method would be useful for the sampling strategy of targeted observations like DOTSTAR.

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Kun-Hsuan Chou, Chun-Chieh Wu, Po-Hsiung Lin, Sim D. Aberson, Martin Weissmann, Florian Harnisch, and Tetsuo Nakazawa

Abstract

The typhoon surveillance program Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) has been conducted since 2003 to obtain dropwindsonde observations around tropical cyclones near Taiwan. In addition, an international field project The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in which dropwindsonde observations were obtained by both surveillance and reconnaissance flights was conducted in summer 2008 in the same region. In this study, the impact of the dropwindsonde data on track forecasts is investigated for DOTSTAR (2003–09) and T-PARC (2008) experiments. Two operational global models from NCEP and ECMWF are used to evaluate the impact of dropwindsonde data. In addition, the impact on the two-model mean is assessed.

The impact of dropwindsonde data on track forecasts is different in the NCEP and ECMWF model systems. Using the NCEP system, the assimilation of dropwindsonde data leads to improvements in 1- to 5-day track forecasts in about 60% of the cases. The differences between track forecasts with and without the dropwindsonde data are generally larger for cases in which the data improved the forecasts than in cases in which the forecasts were degraded. Overall, the mean 1- to 5-day track forecast error is reduced by about 10%–20% for both DOTSTAR and T-PARC cases in the NCEP system. In the ECMWF system, the impact is not as beneficial as in the NCEP system, likely because of more extensive use of satellite data and more complex data assimilation used in the former, leading to better performance even without dropwindsonde data. The stronger impacts of the dropwindsonde data are revealed for the 3- to 5-day forecast in the two-model mean of the NCEP and ECMWF systems than for each individual model.

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Martin Weissmann, Florian Harnisch, Chun-Chieh Wu, Po-Hsiung Lin, Yoichiro Ohta, Koji Yamashita, Yeon-Hee Kim, Eun-Hee Jeon, Tetsuo Nakazawa, and Sim Aberson

Abstract

A unique dataset of targeted dropsonde observations was collected during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in the autumn of 2008. The campaign was supplemented by an enhancement of the operational Dropsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) program. For the first time, up to four different aircraft were available for typhoon observations and over 1500 additional soundings were collected.

This study investigates the influence of assimilating additional observations during the two major typhoon events of T-PARC on the typhoon track forecast by the global models of the European Centre for Medium-Range Weather Forecasts (ECMWF), the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction (NCEP), and the limited-area Weather Research and Forecasting (WRF) model. Additionally, the influence of T-PARC observations on ECMWF midlatitude forecasts is investigated.

All models show an improving tendency of typhoon track forecasts, but the degree of improvement varied from about 20% to 40% in NCEP and WRF to a comparably low influence in ECMWF and JMA. This is likely related to lower track forecast errors without dropsondes in the latter two models, presumably caused by a more extensive use of satellite data and four-dimensional variational data assimilation (4D-Var) of ECMWF and JMA compared to three-dimensional variational data assimilation (3D-Var) of NCEP and WRF. The different behavior of the models emphasizes that the benefit gained strongly depends on the quality of the first-guess field and the assimilation system.

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Chun-Chieh Wu, Jan-Huey Chen, Sharanya J. Majumdar, Melinda S. Peng, Carolyn A. Reynolds, Sim D. Aberson, Roberto Buizza, Munehiko Yamaguchi, Shin-Gan Chen, Tetsuo Nakazawa, and Kun-Hsuan Chou

Abstract

This study compares six different guidance products for targeted observations over the northwest Pacific Ocean for 84 cases of 2-day forecasts in 2006 and highlights the unique dynamical features affecting the tropical cyclone (TC) tracks in this basin. The six products include three types of guidance based on total-energy singular vectors (TESVs) from different global models, the ensemble transform Kalman filter (ETKF) based on a multimodel ensemble, the deep-layer mean (DLM) wind variance, and the adjoint-derived sensitivity steering vector (ADSSV). The similarities among the six products are evaluated using two objective statistical techniques to show the diversity of the sensitivity regions in large, synoptic-scale domains and in smaller domains local to the TC. It is shown that the three TESVs are relatively similar to one another in both the large and the small domains while the comparisons of the DLM wind variance with other methods show rather low similarities. The ETKF and the ADSSV usually show high similarity because their optimal sensitivity usually lies close to the TC. The ADSSV, relative to the ETKF, reveals more similar sensitivity patterns to those associated with TESVs. Three special cases are also selected to highlight the similarities and differences among the six guidance products and to interpret the dynamical systems affecting the TC motion in the northwestern Pacific. Among the three storms studied, Typhoon Chanchu was associated with the subtropical high, Typhoon Shanshan was associated with the midlatitude trough, and Typhoon Durian was associated with the subtropical jet. The adjoint methods are found to be more capable of capturing the signal of the dynamic system that may affect the TC movement or evolution than are the ensemble methods.

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