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Maofeng Liu and James A. Smith

Abstract

Hurricane Irene produced catastrophic rainfall and flooding in portions of the eastern United States from 27 to 29 August 2011. Like a number of tropical cyclones that have produced extreme flooding in the northeastern United States, Hurricane Irene was undergoing extratropical transition during the period of most intense rainfall. In this study the rainfall distribution of landfalling tropical cyclones is examined, principally through analyses of radar rainfall fields and high-resolution simulations using the Weather Research and Forecasting (WRF) Model. In addition to extratropical transition, the changing storm environment at landfall and orographic precipitation mechanisms can be important players in controlling the distribution of extreme rainfall. Rainfall distribution from landfalling tropical cyclones is examined from a Lagrangian perspective, focusing on times of landfall and extratropical transition, as well as interactions of the storm circulation with mountainous terrain. WRF simulations capture important features of rainfall distribution, including the pronounced change in rainfall distribution during extratropical transition. Synoptic-scale analyses show that a deep baroclinic zone developed and strengthened in the left-front quadrant of Irene, controlling rainfall distribution over the regions experiencing most severe flooding. Numerical experiments were performed with WRF to examine the role of mountainous terrain in altering rainfall distribution. Analyses of Hurricane Irene are placed in a larger context through analyses of Hurricane Hannah (2008) and Hurricane Sandy (2012).

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Long Yang, Maofeng Liu, James A. Smith, and Fuqiang Tian

Abstract

The August 1975 flood in central China was one of the most destructive floods in history. Catastrophic flooding was the product of extreme rainfall from Typhoon Nina over a 3-day period from 5 to 7 August 1975. Despite the prominence of the August 1975 flood, relatively little is known about the evolution of rainfall responsible for the flood. Details of extreme rainfall and flooding for the August 1975 event in central China are examined based on empirical analyses of rainfall and streamflow measurements and based on downscaling simulations using the Weather Research and Forecasting (WRF) Model, driven by Twentieth Century Reanalysis (20CR) fields. Key hydrometeorological features of the flood event are placed in a climatological context through hydroclimatological analyses of 20CR fields. Results point to the complex evolution of rainfall over the 3-day period with distinctive periods of storm structure controlling rainfall distribution in the flood region. Blocking plays a central role in controlling anomalous storm motion of Typhoon Nina and extreme duration of heavy rainfall. Interaction of Typhoon Nina with a second tropical depression played a central role in creating a zone of anomalously large water vapor transport, a central feature of heavy rainfall during the critical storm period on 7 August. Analyses based on the quasigeostrophic omega equation identified the predominant role of warm air advection for synoptic-scale vertical motion. Back-trajectory analyses using a Lagrangian parcel tracking algorithm are used to assess and quantify water vapor transport for the flood. The analytical framework developed in this study is designed to improve hydrometeorological approaches for flood-control design.

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