U.S. Geological Survey simulations of hydrodynamics and morphodynamics during Hurricane Michael (2018)

The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST v3.8; Warner and others, 2010; Warner and others, 2019) modeling system was used to simulate ocean circulation, water levels, waves, and sediment transport that occurred during Hurricane Michael (2018). Simulations were performed with coupled and concurrent ocean and wave models simulated on a series of refined, cascading grids. They were first performed on an oceanic scale grid for the entire Gulf of America, and then further downscaled to focus on the resulting inundation, dune overtopping, and barrier island breaching during the storm along both Mexico Beach, FL, and at Cape San Blas, FL. The largest scale grid covered the entire US east coast and Gulf of America (5km resolution), and a subsequent grid (GOMSAB, 2km resolution) downscaled to cover Florida. Results on the GOMSAB grid were analyzed to investigate coastal ocean-scale dynamics and circulation, and then to further drive coastal scale grids to resolve the Florida Great Bend region (Mexi and Apala, each at 200m resolution). Finally, these results further drove grids to cover Mexico Beach (MexB, 10m alongshore and varied from 7 m to 3m in the cross-shore) and Cape San Blas (CSB, 5m resolution). Several simulations on the MexB and CSB grids were performed to investigate the impacts of infragravity waves on total water level at Mexico Beach and on effects of vegetation (included or excluded) on the breaching processes at Cape San Blas.

Surface atmospheric forcings were obtained from the Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones (COAMPS‐TC; Doyle and others, 2012), developed by the U.S. Naval Research Laboratory. COAMPS‐TC is a non‐hydrostatic, convection‐permitting model. The simulation was conducted by the US Navy with a 4‐km resolution storm‐following nest that was periodically re-initialized using a modified Rankine wind vortex. The track and intensity were nudged to fit to the Best Track from the National Hurricane Center (NHC; Beven and others, 2018). Results included surface winds, pressure, relative humidity, and air temperature.

Simulations were performed on the GOMSAB, Mexi, and Apala grids for a time period from 1200hr October 7, 2018 to 0000hr on October 12, 2018 UTC. These results were used to investigate the coastal current, waves, and inundation water levels. The MexB grid was simulated for a 14 hour period to cover the initial runup and the maximum inundation of Mexico Beach from 0600 to 2000 on October 10, 2018. This simulation was used to investigate the impacts of infragravity waves and coastal erosion at Mexico Beach. The CSB grid was simulated from October 10 at 0600 to October 11 at 0000. Four simulations on the CSB grid: 1) no morphology and no vegetation, 2) no morphology and yes vegetation, 3) yes morphology and no vegetation, and 4) yes morphology and yes vegetation. The first 2 provided an understanding of how the vegetation altered the bottom stress, and the last 2 allow that variation of bottom stress to evolve the morphology. The sediment on the seafloor was initialized with a uniform 10m thick distribution. These simulations were limited to one layer with a sand grain size 0.125mm, erosion rate of 2.0e-4 kg/m2/s, critical erosion stress of 0.14 N/m2, and a settling velocity of 8.7 mm/s.

References cited:

Beven, J., Berg, R., and Hagen, A., 2018, National Hurricane Center Tropical Cyclone Report AL 142018, Hurricane Michael 2018, https://www.nhc.noaa.gov/data/tcr/AL142018_Michael.pdf.

Doyle, J. D., Jin, Y., Hodur, R. M., Chen, S., Jin, H., Moskaitis, J., Reinecke, A., Black, P., Cummings, J., Hendricks, E., Holt, T., Liou, C.-S, Peng, M., Reynolds, C., Sashegyi, K., Schmidt, J., and Wang, S., 2012, Real‐time tropical cyclone prediction using COAMPS‐TC: Advances in Geosciences, Vol. 28, pp. 15–28, https://doi.org/10.1142/9789814405683_0002.

Warner, J.C., Armstrong, B., He, R., and Zambon, J.B., 2010, Development of a coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system: Ocean Modelling, v. 35, issue 3, p. 230-244, https://doi.org/10.1016/j.ocemod.2010.07.010.

Warner, J.C., Ganju, N.K., Sherwood, C.R., Kalra, T.S., Aretxabaleta, A., He, R., Zambon, J., and Kumar, N., 2019, Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System: U.S. Geological Survey Software Release, 23 April 2019, https://doi.org/10.5066/P9NQUAOW.

本研究采用耦合海-气-浪-泥沙输运（Coupled Ocean-Atmosphere-Wave-Sediment Transport, COAWST v3.8; Warner et al., 2010; Warner et al., 2019）模式系统，对2018年迈克尔飓风（Hurricane Michael）期间的海洋环流、水位、波浪及泥沙输运过程开展数值模拟。模拟采用耦合并行的海洋与波浪模式，在一系列多级嵌套加密网格上完成计算。首先在覆盖整个美洲湾的大洋尺度网格上开展模拟，随后进一步向下嵌套加密，聚焦佛罗里达州墨西哥海滩（Mexico Beach, FL）和圣布拉斯角（Cape San Blas, FL）沿岸风暴期间的漫滩、沙丘越流及障壁岛溃决过程。最大尺度网格覆盖美国东海岸及美洲湾（分辨率5km），后续嵌套的GOMSAB网格（分辨率2km）聚焦佛罗里达区域。对GOMSAB网格的模拟结果进行分析，以探究近海大洋尺度的动力过程与环流特征，进而进一步驱动海岸尺度网格，以解析佛罗里达大弯曲区域（Mexi与Apala网格，分辨率均为200m）。最终，基于上述结果进一步嵌套得到覆盖墨西哥海滩的MexB网格（沿岸分辨率10m，岸线垂直方向分辨率7m至3m不等）与圣布拉斯角CSB网格（分辨率5m）。针对MexB与CSB网格开展多组模拟，以探究亚重力波对墨西哥海滩总水位的影响，以及植被（启用/禁用）对圣布拉斯角溃决过程的影响。

地表大气强迫场数据来自美国海军研究实验室开发的热带气旋耦合海洋/大气中尺度预报系统（Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones, COAMPS-TC; Doyle et al., 2012）。COAMPS-TC是一款非静力、允许对流的数值模式。美国海军采用分辨率4km的风暴跟随嵌套网格开展模拟，并利用修正的朗肯风涡旋进行周期性再初始化。将模式的移动路径与强度同化，以匹配美国国家飓风中心（National Hurricane Center, NHC; Beven et al., 2018）发布的最佳路径数据集。模拟结果包含地表风速、气压、相对湿度与气温。

针对GOMSAB、Mexi与Apala网格的模拟时段为2018年10月7日1200时UTC至2018年10月12日0000时UTC，该结果用于探究沿岸流、波浪与漫滩水位。MexB网格的模拟时段为14小时，覆盖2018年10月10日0600时至2000时的初始岸线增水与墨西哥海滩最大漫滩过程，该模拟用于探究亚重力波对墨西哥海滩的影响与海岸侵蚀过程。CSB网格的模拟时段为2018年10月10日0600时至2018年10月11日0000时，共开展四组CSB网格模拟：1）无地形演变且无植被；2）无地形演变且启用植被；3）有地形演变且无植被；4）有地形演变且启用植被。前两组模拟用于理解植被如何改变底床应力，后两组则用于探究底床应力变化对地形演变的影响。海底沉积物初始分布为均匀10m厚度，该模拟仅采用单层沉积物，沙粒粒径为0.125mm，侵蚀速率为2.0×10^-4 kg/m²/s，临界侵蚀应力为0.14 N/m²，沉降速度为8.7 mm/s。

引用文献：
Beven, J., Berg, R., and Hagen, A., 2018, 美国国家飓风中心热带气旋报告AL 142018：2018年迈克尔飓风，https://www.nhc.noaa.gov/data/tcr/AL142018_Michael.pdf.
Doyle, J. D., Jin, Y., Hodur, R. M., Chen, S., Jin, H., Moskaitis, J., Reinecke, A., Black, P., Cummings, J., Hendricks, E., Holt, T., Liou, C.-S, Peng, M., Reynolds, C., Sashegyi, K., Schmidt, J., and Wang, S., 2012, 基于COAMPS-TC的实时热带气旋预报：地球科学进展，第28卷，第15-28页，https://doi.org/10.1142/9789814405683_0002.
Warner, J.C., Armstrong, B., He, R., and Zambon, J.B., 2010, 耦合海-气-浪-泥沙输运（COAWST）模式系统的开发：海洋建模，第35卷第3期，第230-244页，https://doi.org/10.1016/j.ocemod.2010.07.010.
Warner, J.C., Ganju, N.K., Sherwood, C.R., Kalra, T.S., Aretxabaleta, A., He, R., Zambon, J., and Kumar, N., 2019, 耦合海-气-浪-泥沙输运（COAWST）模式系统：美国地质调查局软件发布，2019年4月23日，https://doi.org/10.5066/P9NQUAOW.