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Masashi Nagata and Yoshi Ogura

Abstract

This paper presents results of simulations for a case of heavy precipitation that occurred on 23 July 1982 over western Japan. Special emphasis is placed on synoptic- and subsynoptic-scale processes that led to heavy precipitation and also its linkage with the low-level jet (LLJ).

The model result recaptures the major observed features of this event reasonably well. The sequence of events revealed by the model starts with the formation of a localized surface warm front caused by the deformation field that is associated with an eastward traveling, nondeepening meso-αscale low. It is followed by the initiation of both concentrated convective precipitation at the surface front and stratiform grid-scale precipitation along the sloping frontal surface. The simulations with different model physics reveal significant roles that diabatic heating processes play in the linkage between heavy precipitation and the LLJ. While condensation heating produces a cyclonic circulation with failing pressure manifested as a mesoscale trough over the front, evaporative cooling from stratiform raindrops generates a marked frontogenetic forcing and creates a cold pool beneath the sloping frontal surface. An anticyclonic outflow from the cold pool is accompanied by convergence on its southwestern flank, which further enhances and concentrates the convective activity and the mesoscale trough. The supergeostrophic LLJ is formed in this situation, where an air parcel crosses height contours into low pressure with large angles due to a combination of an alongfront flow in the southwestern part of the anticyclonic outflow anomaly induced by the evaporative cooling and a cross-front flow in the upper branch of the direct secondary circulation associated with the warm-frontogenetical processes.

Isentropic and isobaric ageostrophic motion diagnoses show that the inertial advective component, mainly arising from the horizontal displacement, is the dominant part in the ageostrophic wind in the entrance region of the LLJ, supporting the conclusion that the rapid parcel acceleration itself occurs almost adiabatically through the horizontal displacement crossing into low pressure.

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T. S. V. Vijaya Kumar, T. N. Krishnamurti, Michael Fiorino, and Masashi Nagata

Abstract

Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.

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Masashi Nagata, Lance Leslie, Yoshio Kurihara, Russell L. Elsberry, Masanori Yamasaki, Hirotaka Kamahori, Robert Abbey Jr., Kotaro Bessho, Javier Calvo, Johnny C. L. Chan, Peter Clark, Michel Desgagne, Song-You Hong, Detlev Majewski, Piero Malguzzi, John McGregor, Hiroshi Mino, Akihiko Murata, Jason Nachamkin, Michel Roch, and Clive Wilson

The Third Comparison of Mesoscale Prediction and Research Experiment (COMPARE) workshop was held in Tokyo, Japan, on 13–15 December 1999, cosponsored by the Japan Meteorological Agency (JMA), Japan Science and Technology Agency, and the World Meteorological Organization. The third case of COMPARE focuses on an event of explosive tropical cyclone [Typhoon Flo (9019)] development that occurred during the cooperative three field experiments, the Tropical Cyclone Motion experiment 1990, Special Experiment Concerning Recurvature and Unusual Motion, and TYPHOON-90, conducted in the western North Pacific in August and September 1990. Fourteen models from nine countries have participated in at least a part of a set of experiments using a combination of four initial conditions provided and three horizontal resolutions. The resultant forecasts were collected, processed, and verified with analyses and observational data at JMA. Archived datasets have been prepared to be distributed to participating members for use in further evaluation studies.

In the workshop, preliminary conclusions from the evaluation study were presented and discussed in the light of initiatives of the experiment and from the viewpoints of tropical cyclone experts. Initial conditions, depending on both large-scale analyses and vortex bogusing, have a large impact on tropical cyclone intensity predictions. Some models succeeded in predicting the explosive deepening of the target typhoon at least qualitatively in terms of the time evolution of central pressure. Horizontal grid spacing has a very large impact on tropical cyclone intensity prediction, while the impact of vertical resolution is less clear, with some models being very sensitive and others less so. The structure of and processes in the eyewall clouds with subsidence inside as well as boundary layer and moist physical processes are considered important in the explosive development of tropical cyclones. Follow-up research activities in this case were proposed to examine possible working hypotheses related to the explosive development.

New strategies for selection of future COMPARE cases were worked out, including seven suitability requirements to be met by candidate cases. The VORTEX95 case was withdrawn as a candidate, and two other possible cases were presented and discussed.

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