Dynamics of Interactions between Wave Packets and Explosive Cyclogenesis over Western North Pacific

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During the cool season, the western North Pacific is frequented by strong cyclones. Some of these deepen explosively to generate dangerous winds and huge waves. There are indications that these cyclones are less predictable than less intense storms. Western Pacific cyclones bring hazardous weather to the region; at the same time, they excite Rossby wave trains that propagate rapidly eastward to affect the U.S. and even Europe within a few days. Recent studies have shown that analysis and forecast errors over the western Pacific propagate quickly downstream to affect weather forecasts for the U.S. Recent statistical analyses by the investigator showed that cyclone development over the western and central Pacific is strongly affected by pre-existing upper-tropospheric wave packets over Asia - the frequency of occurrence of explosive cyclogenesis is significantly higher three days after wave packets having the appropriate configuration are found either over Russia or across southern Asia. Moreover, preliminary modeling studies suggest that explosive cyclogenesis that occurs under the influence of these wave packets may be much more predictable than that which occurs in the absence of wave packets - in some cases, western Pacific explosive cyclogenesis can be robustly forecasted up to eight days before it occurs. The main goal of this project is to explore and to understand the dynamics of the interactions between upper-level wave packets and surface cyclogenesis. In addition, factors affecting the predictability of these events will be explored, and the impacts of wave packets on U.S. weather will be examined. Cases in which cyclogenesis did not occur despite the presence of significant upstream dynamical forcings will be compared to cases in which cyclogenesis can be robustly simulated, in order to understand the conditions that inhibit or favor rapid cyclogenesis. A suite of modeling and diagnostic studies will be employed. The main modeling tool is the NCAR Community Atmospheric Model (CAM). For each selected case, ensemble data assimilation, using the technique of the ensemble Kalman filter, will be conducted using CAM. This ensemble of analyses will be used as initial conditions to obtain an ensemble of forecasts. With this forecast ensemble, the predictability of these events can be assessed. Ensemble-based sensitivity analyses will expose the important factors that affect the evolution of these events. The technique of potential vorticity diagnostics and inversion will be used to conduct feature-based sensitivity study to examine dynamical factors that may have significant impacts on the evolution of these events over the medium range. This study addresses a major issue for forecasting - explosive cyclogenesis generates high impact weather, and frequently these events are not well forecasted. The results may assist forecasters in interpreting model forecasts of these events by stratifying events into different types and quantifying the differences in predictability of each type. Detailed studies of the dynamics of these events will provide researchers with new information about how to improve the prediction of these events by clarifying the different roles played by diabatic processes, dynamics, and uncertainties in initial conditions. Western Pacific cyclones frequently excite Rossby wave trains that propagate eastward to produce high impact weather events, such as blizzards and floods, over the U.S. The broader impact of this study is in the significant societal benefits that can derive from improved forecasts of high impact weather.