Greg Seroka

Glider observations and modeling of sediment transport in Hurricane Sandy

Regional sediment resuspension and transport are examined as Hurricane Sandy made landfall on the Mid-Atlantic Bight (MAB) in October 2012. A Teledyne-Webb Slocum glider, equipped with a Nortek Aquadopp current profiler, was deployed on the continental shelf ahead of the storm, and is used to validate sediment transport routines coupled to the Regional Ocean Modeling System (ROMS). The glider was deployed on 25 October, 5 days before Sandy made landfall in southern New Jersey (NJ) and flew along the 40 m isobath south of the Hudson Shelf Valley. We used optical and acoustic backscatter to compare with two modeled size classes along the glider track, 0.1 and 0.4 mm sand, respectively. Observations and modeling revealed full water column resuspension for both size classes for over 24 h during peak waves and currents, with transport oriented along-shelf toward the southwest. Regional model predictions showed over 3 cm of sediment eroded on the northern portion of the NJ shelf where waves and currents were the highest. As the storm passed and winds reversed from onshore to offshore on the southern portion of the domain waves and subsequently orbital velocities necessary for resuspension were reduced leading to over 3 cm of deposition across the entire shelf, just north of Delaware Bay. This study highlights the utility of gliders as a new asset in support of the development and verification of regional sediment resuspension and transport models, particularly during large tropical and extratropical cyclones when in situ data sets are not readily available.

Stratified coastal ocean interactions with tropical cyclones

Hurricane-intensity forecast improvements currently lag the progress achieved for hurricane tracks. Integrated ocean observations and simulations during hurricane Irene (2011) reveal that the wind-forced two-layer circulation of the stratified coastal ocean, and resultant shear-induced mixing, led to significant and rapid ahead-of-eye-centre cooling (at least 6 °C and up to 11 °C) over a wide swath of the continental shelf. Atmospheric simulations establish this cooling as the missing contribution required to reproduce Irenes accelerated intensity reduction. Historical buoys from 1985 to 2015 show that ahead-of-eye-centre cooling occurred beneath all 11 tropical cyclones that traversed the Mid-Atlantic Bight continental shelf during stratified summer conditions. A Yellow Sea buoy similarly revealed significant and rapid ahead-of-eye-centre cooling during Typhoon Muifa (2011). These findings establish that including realistic coastal baroclinic processes in forecasts of storm intensity and impacts will be increasingly critical to mid-latitude population centres as sea levels rise and tropical cyclone maximum intensities migrate poleward.

Hurricane Irene Sensitivity to Stratified Coastal Ocean Cooling

AbstractCold wakes left behind by tropical cyclones (TCs) have been documented since the 1940s. Many questions remain, however, regarding the details of the processes creating these cold wakes and their in-storm feedbacks onto tropical cyclone intensity. This largely reflects a paucity of measurements within the ocean, especially during storms. Moreover, the bulk of TC research efforts have investigated deep ocean processes—where tropical cyclones spend the vast majority of their lifetimes—and very little attention has been paid to coastal ocean processes despite their critical importance to shoreline populations. Using Hurricane Irene (2011) as a case study, the impact of the cooling of a stratified coastal ocean on storm intensity, size, and structure is quantified. Significant ahead-of-eye-center cooling (at least 6°C) of the Mid-Atlantic Bight occurred as a result of coastal baroclinic processes, and operational satellite SST products and existing coupled ocean–atmosphere hurricane models did not capt...

Rapid shelf‐wide cooling response of a stratified coastal ocean to hurricanes

Abstract Large uncertainty in the predicted intensity of tropical cyclones (TCs) persists compared to the steadily improving skill in the predicted TC tracks. This intensity uncertainty has its most significant implications in the coastal zone, where TC impacts to populated shorelines are greatest. Recent studies have demonstrated that rapid ahead‐of‐eye‐center cooling of a stratified coastal ocean can have a significant impact on hurricane intensity forecasts. Using observation‐validated, high‐resolution ocean modeling, the stratified coastal ocean cooling processes observed in two U.S. Mid‐Atlantic hurricanes were investigated: Hurricane Irene (2011)—with an inshore Mid‐Atlantic Bight (MAB) track during the late summer stratified coastal ocean season—and Tropical Storm Barry (2007)—with an offshore track during early summer. For both storms, the critical ahead‐of‐eye‐center depth‐averaged force balance across the entire MAB shelf included an onshore wind stress balanced by an offshore pressure gradient. This resulted in onshore surface currents opposing offshore bottom currents that enhanced surface to bottom current shear and turbulent mixing across the thermocline, resulting in the rapid cooling of the surface layer ahead‐of‐eye‐center. Because the same baroclinic and mixing processes occurred for two storms on opposite ends of the track and seasonal stratification envelope, the response appears robust. It will be critical to forecast these processes and their implications for a wide range of future storms using realistic 3‐D coupled atmosphere‐ocean models to lower the uncertainty in predictions of TC intensities and impacts and enable coastal populations to better respond to increasing rapid intensification threats in an era of rising sea levels.

Sea breeze, coastal upwelling modeling to support offshore wind energy planning and operations

In July 2014, BOEM issued the NJ Proposed Sale Notice of nearly 344,000 acres designated for offshore wind (OSW) energy development. The BOEM lease auction is expected to take place during the current year. The OSW developer(s) who win the lease(s) will submit their development application to the NJ Board of Public Utilities (NJ BPU). These applications must include a wind resource assessment and economic analysis. One major focus in the NJ BPU OSW rules is that applications “shall account for the coincidence between time of generation for the project and peak electricity demand.” Preliminary data analysis shows two mesoscale processes-coastal upwelling and sea breeze-may have a significant impact on wind generation during peak electricity demand. Tasked by NJ BPU, the Rutgers University Center for Ocean Observing Leadership (RUCOOL) is using the Weather Research and Forecasting (WRF) model to resolve these processes and quantify their impact on the wind resource. The WRF model set-up used is designed specifically for coastal/offshore regions, with three pertinent features for these regions. First, innovative satellite sea surface temperature (SST) composites at 2km resolution are used to resolve coastal upwelling. These composites integrate a) our own declouding algorithm set for the Mid Atlantic Bight to remove cloudy pixels from Advanced Very High Resolution Radiometer (AVHRR) SST scans, and b) coldest pixel composites of the resulting declouded AVHRR SST scans, rather than warmest pixel composites that would effectively remove coastal upwelling. Second, microscale grid spacing (<;1km) is used in WRF to resolve the sea breeze circulation, which can vary at meso- to microscales. Finally, validation of the WRF simulations is performed against coastal/offshore wind monitoring sites with atmospheric heights up to 200m, in order to ensure adequate model performance in coastal/offshore conditions. Three main results will be presented in this paper: (i) Coastal upwelling can produce high wind shear (~8 ms-1 across rotor blade dimensions). These significant shear values could potentially pose engineering challenges and should be considered in wind resource assessments. (ii) Lagrangian Coherent Structure (LCS) methodology can be used to identify key boundaries and fronts within the sea breeze circulation. While the onshore component of the sea breeze is well observed, very little is known about its unobserved offshore component, where OSW turbines will be installed. (iii) Power generation from a hypothetical 3000 MW OSW scenario off NJ was analyzed during three different sea breeze cases (one with strong upwelling, one with weak upwelling, and one without upwelling). Significant variability in power production occurred within each case and across the three sea breeze cases (net capacity factor ranged from 1 to 95%). WRF OSW potential power production data are being ingested by an electricity grid model to evaluate the impact of OSW energy penetration into the electrical power grid along with evaluating the economic portion of the applications. NJ is leading development of such an advanced joint atmospheric-economic modeling capability for determining the viability of OSW projects. Ongoing work includes development of a coupled atmosphere-ocean model (WRF-ROMS, Regional Ocean Modeling System), which will provide improved capabilities to diagnose coastal airsea processes (sea breeze and coastal upwelling) for OSW resource assessment (i.e. lowering uncertainty by including relevant mesoscale processes in simulations), and to more accurately predict these processes for operational forecasting during OSW construction and O&M phases.

Coastal ocean circulation during Hurricane Sandy

Hurricane Sandy (2012) was the second costliest tropical cyclone to impact the United States and resulted in numerous lives lost due to its high winds and catastrophic storm surges. Despite its impacts little research has been performed on the circulation on the continental shelf as Sandy made landfall. In this study integrated ocean observing assets and regional ocean modeling were used to investigate the coastal ocean response to Sandys large wind field. Sandys unique cross-shelf storm track, large size, and slow speed resulted in along-shelf wind stress over the coastal ocean for nearly 48 hours before the eye made landfall in southern New Jersey. Over the first inertial period (∼18 hours) this along-shelf wind stress drove onshore flow in the surface of the stratified continental shelf and initiated a two-layer downwelling circulation. During the remaining storm forcing period a bottom Ekman layer developed and the bottom Cold Pool was rapidly advected offshore ∼70 kilometers. This offshore advection removed the bottom Cold Pool from the majority of the shallow continental shelf and limited ahead-of-eye-center sea surface temperature (SST) cooling, which has been observed in previous storms on the MAB such as Hurricane Irene (2011). This cross-shelf advective process has not been observed previously on continental shelves during tropical cyclones and highlights the need for combined ocean observing systems and regional modeling in order to further understand the range of coastal ocean responses to tropical cyclones.

Analysis of the wind resource off New Jersey for offshore wind energy development

The state of New Jersey has the goal of producing 23% of its energy from renewable sources by 2021. Offshore wind is envisioned as being part of that renewable portfolio. To meet this goal New Jersey passed the nations first offshore wind renewable energy standard which requires that at least 1,100 megawatts (MW) by 2021. Currently NJ has 0 MW of offshore wind energy. In order to reduce the risk associated with installing these turbines, the Rutgers University Coastal Ocean Observation Laboratory has undertaken a two year study of the ocean winds and currents to provide insight to the wind farm developers to the best locations for siting the wind turbines. A 13 MHz HF radar network was installed to measure the surface currents every 2 km out to a range of 60 km from the coast. These surface current measurements were validated against surface wind measurements from available meteorological stations. The surface currents will then be used to validate the surface winds from a weather model that has been created for this program.

Sediment transport in Hurricane Sandy

Tropical and extra-tropical cyclones are episodic events that redistribute sediment, pollutants, nutrients and heat on continental shelves. The development and validation of ocean and sediment transport models is necessary to interpret short-term events on the long-term sediment record. Numerous studies have used pre- and post-storm surveys to validate three-dimensional sediment resuspension and transport models, but due to extreme sampling conditions in situ validation has been limited to a few point measurements. Over the last decade many of the acoustic and optical sensors used on tripods and benthic landers have been developed for, and included on, autonomous underwater vehicles. In this study we use a combination of the Regional Ocean Modeling System and a Teledyne-Webb Slocum glider as well as other regional ocean observing assets to characterize sediment transport on the New Jersey continental shelf during Hurricane Sandy in October of 2012.

Stratified coastal ocean processes in hurricanes and typhoons enhance ahead-of-eye cooling and reduce storm intensity

Integrated ocean observations from Hurricane Irene (2011) reveal widespread and significant ahead-of-eye cooling (at least 5°C and up to 11°C) as it crossed the seasonally stratified continental shelf of the Mid-Atlantic Bight of North America. Buoys and gliders deployed in the storm allow the detailed evolution of the surface temperature to be examined at select points, revealing 76%-94% of the total cooling occurs before eye passage. A range of ocean models were used to diagnose the processes responsible for the observed cooling. In Irene, 1D vertical mixing models generate only 17% of the total cooling ahead of eye, while deepwater 3-D models forced by Irenes nearly symmetrical offshore windfield produce an approximately 50-50 split in the cooling between the front and back side. A 3-D coastal ocean model (ROMS) generates a wind-forced two-layer circulation in the stratified MidAtlantic not present in the 1-D and 3-D deepwater models. The resultant shearinduced mixing more accurately reproduces both the magnitude and timing of the surface cooling with respect to eye passage. Atmospheric simulations establish that this cooling was the missing contribution required to reproduce Irenes accelerated reduction in intensity over the Mid Atlantic Bight. Historical buoys from 1985 to present show that ahead-of-eye cooling occurred beneath all 11 tropical cyclones that traversed along the Mid Atlantic Bight continental shelf during stratified summer conditions. The buoys also reveal that an average of about 75% of the cooling in these 11 hurricanes occurs ahead of eye, indicating a robust process in the Mid Atlantic. Similar to the Mid Atlantic Bight, the Yellow Sea have had 26 typhoons cross its shallow highly stratified waters in summer before making landfall in China or Korea. Typhoon Muifa (2011), whose intensity was also overpredicted, generated significant SST cooling (up to 7C) in the Yellow Sea, and a Yellow Sea buoy array similarly revealed 85% of the cooling was ahead of eye. These findings establish that including realistic 3D coastal ocean processes in forecasts of landfalling storm intensity and impacts will be increasingly critical to mid-latitude population centers as sea levels rise and tropical cyclone maximum intensities migrate poleward.

Coastal ocean mixing and advection during Hurricane Sandy and Arthur

The Mid Atlantic Bight continental shelf has one of the largest summer temperature gradients in the world, with near bottom temperatures below 8C and peak surface temperatures over 28C. This is largely due to the summer Cold Pool, remnant winter water that is generated on the northern MAB and transported southward along the continental shelf in spring and early summer. During tropical cyclones that impact the MAB continental shelf, such as Hurricane Irene in 2011, shear driven mixing of Cold Pool water across the thermocline has the potential to cool the oceans surface and reduce storm intensity. In this study we compare coastal ocean advection and mixing processes during Hurricane Sandy and Hurricane Arthur, an offshore tracking tropical cyclone in the summer of 2014, to demonstrate the range of potential storm impacts on the coastal ocean of the MAB. To perform this analysis we use data from advanced Slocum autonomous underwater glider deployments in each storm as well as the Regional Ocean Modeling System (ROMS).