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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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Maofeng Liu, Gabriel A. Vecchi, James A. Smith, and Hiroyuki Murakami

Abstract

Landfalling–tropical cyclone (TC) rainfall is an important element of inland flood hazards in the eastern United States. The projection of landfalling-TC rainfall under anthropogenic warming provides insight into future flood risks. This study examines the frequency of landfalling TCs and associated rainfall using the GFDL Forecast-Oriented Low Ocean Resolution (FLOR) climate model through comparisons with observed TC track and rainfall over the July–November 1979–2005 seasons. The projection of landfalling-TC frequency and rainfall under the representative concentration pathway (RCP) 4.5 scenario for the late twenty-first century is explored, including an assessment of the impacts of extratropical transition (ET). In most regions of the southeastern United States, competition between increased storm rain rate and decreased storm frequency dominates the change of annual TC rainfall, and rainfall from ET and non-ET storms. In the northeastern United States, a prominent feature is the striking increase of ET-storm frequency but with tropical characteristics (i.e., prior to the ET phase), a key element of increased rainfall. The storm-centered rainfall composite analyses show the greatest increase at a radius of a few hundred kilometers from the storm centers. Over both ocean and land, the increase of rainfall within 500 km from the storm center exceeds the Clausius–Clapeyron scaling for TC-phase storms. Similar results are found in the front-left quadrant of ET-phase storms. Future work involving explorations of multiple models (e.g., higher atmospheric resolution version of the FLOR model) for TC-rainfall projection is expected to add more robustness to projection results.

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Maofeng Liu, Gabriel A. Vecchi, James A. Smith, and Hiroyuki Murakami

Abstract

This study explores the simulations and twenty-first-century projections of extratropical transition (ET) of tropical cyclones (TCs) in the North Atlantic, with a newly developed global climate model: the Forecast-Oriented Low Ocean Resolution (FLOR) version of the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model version 2.5 (CM2.5). FLOR exhibits good skill in simulating present-day ET properties (e.g., cyclone phase space parameters). A version of FLOR in which sea surface temperature (SST) biases are artificially corrected through flux-adjustment (FLOR-FA) shows much improved simulation of ET activity (e.g., annual ET number). This result is largely attributable to better simulation of basinwide TC activity, which is strongly dependent on larger-scale climate simulation. FLOR-FA is also used to explore changes of ET activity in the twenty-first century under the representative concentration pathway (RCP) 4.5 scenario. A contrasting pattern is found in which regional TC density increases in the eastern North Atlantic and decreases in the western North Atlantic, probably due to changes in the TC genesis location. The increasing TC frequency in the eastern Atlantic is dominated by increased ET cases. The increased density of TCs undergoing ET in the eastern subtropics of the Atlantic shows two propagation paths: one moves northwest toward the northeast coast of the United States and the other moves northeast toward western Europe, implying increased TC-related risks in these regions. A more TC-favorable future climate, evident in the projected changes of SST and vertical wind shear, is hypothesized to favor the increased ET occurrence in the eastern North Atlantic.

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Dan Li, Ting Sun, Maofeng Liu, Linlin Wang, and Zhiqiu Gao

Abstract

The interaction between urban heat islands (UHIs) and heat waves (HWs) is studied using measurements collected at two towers in the Beijing, China, metropolitan area and an analytical model. Measurements show that 1) the positive interaction between UHIs and HWs not only exists at the surface but also persists to higher levels (up to ~70 m) and 2) the urban wind speed is enhanced by HWs during daytime but reduced during nighttime as compared with its rural counterpart. A steady-state advection–diffusion model coupled to the surface energy balance equation is then employed to understand the implication of changes in wind speed on UHIs, which reveals that the observed changes in wind speed positively contribute to the interaction between UHIs and HWs in both daytime and nighttime. The vertical structure of the positive interaction between UHIs and HWs is thus likely an outcome resulting from a combination of changes in the surface energy balance and wind profile.

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