Semidiurnal Perturbations to Storm Surges on the U.S. Eastern Seaboard

半日摄动改变了飓风桑迪（2012年）和飓风艾琳（2011年）的非潮汐潮，影响了整个美国东海岸。在南大西洋浅海区，扰动幅值高达浪涌总幅值的50-60%。一般来说，当从观测水位中减去预测水位时，半日扰动变得明显。这种涨潮相互作用的一个响应是改变潮汐低潮和高潮的时间，预计潮汐低潮和高潮的时间将不同于几乎渐进的潮汐到几乎不变的潮汐。该项目旨在促进对触发风暴潮半日扰动和决定扰动大小的过程的理解。大气强迫可以通过直接的表面应力和通过诱导额外的底部应力和潮汐应力来干扰浪涌。每种应力的相对贡献需要进一步了解。本研究的具体目标是确定a)在其他热带风暴和温带风暴下是否出现半日扰动，b)与扰动相关的动力因子的相对大小，以及c)产生这些扰动的大气和潮汐强迫阈值。具体目标将通过美国东海岸潮汐和气象站的观测以及现实和面向过程的数值模拟来解决。智力价值：近几十年来，世界各地的沿海地区都受到风暴的影响，造成严重的洪水、建筑物破坏和海滩侵蚀。风暴潮的相互作用可以在与涨潮无关的时间产生最高的与风暴有关的水位，但仍处于半日周期。因此，有必要了解由风暴引起的半日扰动的行为，以便改进风暴潮的预报和警报系统。这个项目将试图回答几个关键问题：在美国东部移动的每一次风暴中都会发生半日扰动吗？在东北风期间是否也会发生半日扰动？与这些扰动相关的动力学是什么？是否存在使这些扰动消失的大气强迫阈值？扰动是否取决于潮汐振幅？半日扰动的峰值是否在潮汐周期的一个阶段持续发展？显然，有必要了解触发浪涌半日摄动的适当条件。这项研究的发现将有助于理解高频（周期小于惯性周期）扰动出现的原因，即改变风暴潮的半日差。更广泛的影响：该项目将研究一种可能影响整个美国东海岸的现象。由于在南大西洋湾，半日摄动的幅度可达总浪涌信号的50-60%，因此必须包括潮汐-浪涌相互作用。这些相互作用也将影响最大浪涌的时间，相对于高潮。因此，与风暴潮到达有关的概率密度函数将受到半日残差的影响。更好地了解这些扰动将有助于完善风暴潮预测和对整个美国东部沿海地区的风险分析。该项目将支持一名研究生，一名本科生，通过学期的研究经验，一名高中生通过暑期实习。这名高中生将在佛罗里达州盖恩斯维尔的少数族裔高中中招募。从这个项目中得出的结果将呈现给K-12的观众，并将被纳入佛罗里达大学pi教授的课程中。这两个ppi的参与加强了代表性不足的群体对科学的参与。

Semidiurnal perturbations modified the non-tidal surges of hurricanes Sandy (2012) and Irene (2011) which affected the entire U.S. eastern seaboard. In the South Atlantic Bight, the amplitude of the perturbations reached up to 50-60% of the total surge amplitude. In general, semidiurnal perturbations become evident when predicted water levels are subtracted from observed water levels. A response of such tide-surge interaction is to modify the timing of tidal low and high water, which is expected to be different from nearly progressive to nearly standing tides. This project seeks to advance understanding of the processes that trigger semidiurnal perturbations on storm surges and that determine the size of the perturbations. Atmospheric forcing can disturb the surge through direct surface stress and by inducing additional bottom stress and tidal stress. The relative contribution of each stress needs improved understanding. Specific objectives of this investigation are to determine a) whether semidiurnal perturbations appear under other tropical storms and under extratropical storms, b) the relative size of the dynamic agents associated with the perturbations, and c) the atmospheric and tidal forcing thresholds that produce them. Specific objectives will be addressed with observations from tidal and meteorological stations along the U.S. eastern coast, and with realistic and process-oriented numerical simulations.Intellectual Merit: In recent decades, coastal areas worldwide have been affected by storms that cause severe flooding, damage to buildings, and beach erosion. Storm-tide interactions can produce the highest storm-related levels at times not linked to high tide but still at semidiurnal periodicities. It is therefore necessary to understand the behavior of semidiurnal perturbations triggered by storms in order to improve forecasts and warning systems of storm surges. This project will try to answer several key questions: Do semidiurnal perturbations occur in every storm that moves along the eastern US? Do semidiurnal perturbations also occur during nor'easters? What are the dynamics associated with these perturbations? Are there atmospheric forcing thresholds below which these perturbations vanish? Do perturbations depend on the tidal amplitude? Does the peak in semidiurnal perturbations develop consistently in one phase of the tidal cycle? Clearly, there is a need to understand appropriate conditions for triggering semidiurnal perturbations on the surge. Findings generated from the study will help understand the causes for the appearance of high frequency (periods smaller than the inertial period) perturbations, semidiurnal residuals, which modify storm surges.Broader Impacts: This project will study a phenomenon that can affect the entire U.S. eastern seaboard. Because semidiurnal perturbations can reach amplitudes of up to 50-60% of the total surge signal in the South Atlantic Bight, it is imperative to include tide-surge interactions. These interactions will also affect the timing of maximum surge, relative to high tide. Probability density functions related to arrival of storm surges will thus be affected by semidiurnal residuals. Better understanding of these perturbations will help to refine storm surge predictions and risk analysis for the entire eastern U.S. seaboard. The project will support one graduate student, one undergraduate, through semester-long research experiences, and one high school student through summer internships. The high school student will be recruited among minority groups among high schools in Gainesville, Florida. Results derived from this project will be presented to K-12 audiences and will be incorporated in classes taught by the PIs at the University of Florida. The involvement of both PIs enhances the participation of underrepresented groups in science.