IMPORTANCE AND PREDICTABILITY OF METEOTSUNAMIS IN THE GULF OF MEXICO

INTRODUCTION Meteorological tsunamis or meteo-tsunamis are sea level oscillations with observed

periods between minutes and hours. They are triggered by atmospheric disturbances linked to fronts such as squalls, atmospheric gravity waves, and fast (order 1 hr) pressure changes. Unlike seismic or landslide tsunamis, the occurrence and impact of meteorologically generated tsunamis is rather local. However, their effects can be as severe as those caused by tsunamis and become catastrophic (Rabinovich et al., 2009). Meteotsunamis have been reported worldwide under different names (e.g. Rissaga in Ciutadella (Spain), Marrobbio in Sicily (Italy)). Meteotsunamis have been described extensively by Monserrat et al. (2006) and Rabinovich et al. (2009). A recent special issue of Natural Hazards (74(1), 2014) has been dedicated to meteotsunamis investigations.

Atmospheric disturbances are typically transmitted to the ocean’s surface through the inverse barometer effect. The maximum energy transfer from the atmosphere to the ocean occurs when the pace of the atmospheric disturbance matches the long wave celerity Ca ( Ca=[gH]½, where g is gravity’s acceleration and H represents the total water depth) matches Ca. This situation is known as Proudman resonance (Proudman, 1929) and maximizes the energy transfer from the atmosphere to the ocean. In this case, atmospheric forcing is bound to the ocean’s surface wave. The amplitude of the meteotsunami generated under these circumstances depends on the intensity of the atmospheric perturbation and on the time or distance in which the meteotsunami was is bound to the atmospheric pressure disturbance. If the water depths change and the celerity of the atmospheric pressure disturbance remains unmodified, the formerly bound meteotsunami becomes a free wave and does not absorb more energy from the atmosphere. Although Proudman resonance is the best-known process that drives meteotsunamis, there are other resonance processes. The two most relevant are the Greenspan (1956) and shelf (Monserrat et al., 2006) resonance. Greenspan resonance occurs when the alongshore component of the atmospheric disturbance velocity equals the phase speed of one of the edge wave modes. Shelf resonance takes place when the atmospheric disturbance and the associated ocean wave have periods and/or wavelengths equal to the resonant scales of the shelf region. Because meteotsunamis are shallow water waves (wavelength 20 times the local depth), their amplitude can further increase by refraction, shoaling, diffraction, and harbor resonance.

One of the best known examples of meteotsunamis affecting the east coast of the United States is the event of Daytona Beach of July 4th 1992. It produced a water surge of 3 m that caused at least 75 minor injuries to individuals and damage to dozen vehicles near the beach. This extreme wave was generated by a southward-propagating squall line (Churchill et al., 1995; Sallenger et al., 1995). A comparable event occurred on March 25th 1995, when a 3-m high wave hit the west coast of Florida. This ocean wave was generated by a train of atmospheric gravity waves (Paxton and Sobien, 1998). A third example on the east coast of the United States occurred in Boothbay, Maine, on October 28th 2008. Apart from the meteotsunami described by Paxton and Sobien (1998), other meteotsunamis have not been reported in the Gulf of Mexico. However, a preliminary analysis of tidal gauge measurements along the coast between Alabama

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and West Florida during the year 2014, has shown that at least 4 meteotsunamis occurred in between March and November. On October 14th and March 28th the maximum observed meteotsunamis exceeded 0.8 m (above predicted tide) at Panama City Beach, northern Gulf of Mexico. On November 17th a maximum elevation of 0.5 m was measured at Clearwater Beach, eastern Gulf of Mexico. These meteotsunamis were generated under from storm conditions, with squall lines oriented perpendicular to the coastline and propagating parallel to the coast. Figure 1a shows the ‘surge’ (observed minus predicted water level) time series during the passage of the main meteotsunami events identified in 2014 in the northern and eastern Gulf of Mexico. Figure 1b presents radar reflectivity maps corresponding to each of the events. The high reflectivity region indicates the presence of a squall line, characterized by convective atmospheric motions associated with heavy precipitation and intense thunderstorms.

Figure 1. a) Time series of the high-pass filtered surge and atmospheric pressure during the main meteotsunamis observed in 2014 in the eastern Gulf of Mexico, and b) reflectivity maps associated with the main meteotsunamis detected in 2014.

This preliminary analysis suggests that meteotsunamis in the Gulf of Mexico might be more frequent than previously thought. Several questions then arise from the observations. For instance, what are the atmospheric conditions that generate these meteotsunamis? In most of these events, the passage of the squall line was accompanied by a drastic change in the speed and direction of the wind. Most of the meteotsunami studies attribute the formation of these ocean long waves to atmospheric pressure jumps. However, the relative contribution of the sudden changes in wind shear stress have has not been addressed yet in detail. Another question that arises from this analysis is whether how much the amplitude of the meteotsunamis depends on the coastline shape, and on the bathymetric slope, or and the water depth. Is Proudman resonance the main resonance effect producing these meteotsunamis or are shelf and Greenspan resonance effects equally relevant? Along the Gulf of Mexico, are there areas more prone to meteotsunami formation and if so, why? Can we use radar reflectivity data as a first predictor of the

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meteotsunami severity? Can we use coupled ocean-atmospheric numerical models to simulate and predict the occurrence of these long waves? What is the resolution required in the models to correctly resolve the formation of the atmospheric waves producing the ocean waves? What are the atmospheric conditions leading to the persistence of these atmospheric gravity waves? Clearly, there is a need to understand the conditions (propagation speed, persistence, and amplitude of the atmospheric disturbances) for triggering meteotsunamis in the Gulf of Mexico. All of these questions are addressed as part of this proposal’s objectives. Although meteotsunamis can contribute to extreme surge levels and to increased hazards to coastal communities, their generation mechanisms and their forecast feasibility are still obscure and need further research.

OBJECTIVES The overall objective of this investigation is to determine the ubiquity of meteotsunamis,

as well as their causes and their predictability along the United States’ Gulf of Mexico seaboard of the United States. In order to address the overall objective, we propose the following specific objectives with associated hypotheses. 1) Specific Objective 1. Use historical data to a) determine the appearance of meteotsunamis in the United States’ coast of the Gulf of Mexico; and b) identify triggering atmospheric forcing conditions.Hypothesis. The occurrence of meteotsunamis in the Gulf of Mexico might be more usual than originally thought, will be associated with squalls of atmospheric pressure changes of at least 2 hPa, and drastic wind and air temperature shifts. No less than 4 meteotsunamis were identified along the coast of Florida in 2014. At least one of those also affected the coasts of Alabama, Mississippi and Louisiana. Therefore, meteotsunamis might be more usual than previously thought. Up to now, Mmeteotsunamis are considered atypical phenomena, especially in the Gulf of Mexico, where only the occurrence of one meteotsunami has been historically reported (Paxton and Sobien, 1998). It is likely that we will change conventional knowledge on this phenomenon.The occurrence of meteotsunamis in the Gulf of Mexico might be more usual than originally thought, associated with squalls of atmospheric pressure changes of at least 2 hPa, and drastic wind and air temperature shifts. No less than 4 meteotsunamis were identified along the coast of Florida in 2014. At least one of those also affected the coasts of Alabama, Mississippi and Louisiana.2) Specific Objective 2. Use field measurements and tidal gauge data to elucidate the spatial structure of meteotsunamis along the Florida coast of the Gulf of Mexico.Hypothesis. The spatial structure of individual meteotsunamis is dependsent onf the atmospheric forcing characteristics, the bathymetry and the coastal shape. The mMeteotsunamis will be disrupted by points, capes, and inlets. The current coverage of NOAA tidal gauges along the Gulf of Mexico (most of them located inside estuaries) is not sufficient to follow the formation and propagation of individual meteotsunamis. 3) Specific Objective 3. Implement coupled ocean-atmospheric numerical experiments to a) study the dynamics associated with meteotsunamis in the Gulf of Mexico; and b) investigate the feasibility to achieve accurate predictions of this phenomenon.

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Hypothesis. Although Proudman resonance has been proposed as the main responsible of the amplification of these long waves, Greenspan resonance and shelf resonance will also be relevant.

BACKGROUND The study of “Meteorological tsunamis” is relatively new, beginning systematically in the

mid-1990s. Historically, this kind ofthese extreme non-seismic destructive events were described as “events of unknown origin”. Although meteotsunamis can produce extreme surge levels and increased hazards to coastal communities, their generation mechanisms and their forecast feasibility are still unclear and are ripe for investigation. The project “Towards a meteotsunami warning system along the U.S. coastline (TMEWS)” (October 2011 to June 2013, PI: Ivica Vilivic, Funding Agency:funded by NOAA, National Weather Service, Department of Commerce) catalogued and analyzed and catalogued the main meteotsunami events over the East US East coast. This analysis excluded the Gulf of Mexico region and only considered those meteotsunamis occurring along the US East Coast after the year 2007. One of the main conclusions of the project was that meteotsunamis occur more often than expected. Our own exploration of sea level data since 1996 at Naples station, SW Florida, has indicated that meteotsunamis are ubiquitous in the northern and eastern Gulf of Mexico: ten 10 events of high frequency surges (period < 3h, surge height > 0.8 m) were identified at that station. Out of the 10 events identified, 6 were associated with the passage of tropical storms or hurricanes between the months of June and October (Figure 2a). But the other 4 events were associated with 4 extreme surges during winter months.

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Figure 2. a) Storm surge events exceeding 0.8 m at Naples (FL) tidal gauge. The red boxes indicate those events not associated with Tropical storms; b, c, and d) Atmospheric radar reflectivity maps associated with the major winter surges observed at Naples.

Figures 2b, c, and d show radar reflectivity conditions during 3 of the 4 surge events. In all cases, the surge was associated with a high reflectivity band oriented perpendicular but propagating parallel to the coastline. As indicated in the Introduction, 2 meteotsunamis exceeding 1 m were identified in the Eastern Gulf of Mexico during the year 2014. These observations suggested that meteotsunamis might be more ubiquitous than previously thought. The verification of this hypothesis is related to Objective 1. Such high reflectivity is associated with strong precipitation and downdrafts that causeing enhanced surface wind speeds.

The meteotsunami on 28th March 2014 at Panama City Beach (FL) provides justification for the main objectives of this proposal. During this meteotsunami, the maximum surge (water level above predicted) reached 1.2 m, as a consequence of the combined contribution of a long period (3 days), Gaussian- shaped surge (0.25 m high) and a fast relatively short period (1 hour period) solitary wave. A total of ten 10 records of sea surface elevation, wind velocity, air temperature, and atmospheric pressure were obtained from the US National Oceanic and Atmospheric Administration (NOAA) at coastal stations between Louisiana and the west coast of Florida (Fig. 3). The high frequency surge (black lines) and barometric pressure (blue lines) time

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series illustrated the nature of these oscillations (Fig.3b). The red dot represents the peak of high frequency surge or the meteotsunami.

Three pulses in the time series of Figure 3 illustrated the effects of this specific March 2014 storm. A main meteotsunami (Wave 1) was generated in the coastal area between Dauphin Island (Alabama) and Panama City Beach (stations 3 and 5 in Fig. 3). Its generation was associated with an atmospheric pressure drop of 5.5 hPa propagating eastward at ~19-20 m/s, which is equivalent to the celerity of a long wave propagating over water depths of 40 m ( the depth of the shelfobservable between the Mississippi Delta and Pensacola, Florida).

Moreover, the effect of wind shifts and temperature changes needs to be considered further. Marked wind shifts and temperature inversions accompanied the passage of the atmospheric pressure drops that originated the main three meteotsunamis. For example, at Shell Beach (Station 2 of Fig. 3) a 3ºC drop in air temperature and an increase of 15 m/s of wind speed followed the atmospheric pressure drop which that forced Wave 2. These atmospheric pressure drops were followed by smaller amplitude gravity waves (which did not produce meteotsunami waves), but the associated temperature variations (< 0.3 ºC) and wind speed changes (up to 5 m/s) were much weaker. Wind direction shifts associated with these gravity waves were <5º. From the present analysis it seems that air temperature and wind speed shifts could be related with to atmospheric conditions that produce stable and long lasting atmospheric gravity waves. Moreover, sudden wind stress changes, also travelling with the squall line, could contribute to the intensification of the meteotsunami. Verification of this hypothesis is part of Objective 1.

The atmospheric pressure drop that triggered Wave 1 was originated near Dauphin Island and its shape remained unchanged between 15:00 and 18:00 h of March 28th. However, the atmospheric pressure jump damped noticeably from Panama City Beach to Apalachicola. At Dauphin Island the meteotsunami amplitude was a few centimeters but it increased as it propagated toward Panama City Beach, where it attained its maximum elevation (0.85 m). Records also showed that the atmospheric pressure drop and the meteotsunami traveled together. However, the atmospheric pressure pulse was not visible by the time of meteotsunami arrival at Apalachicola (station 7 in Fig. 3), where it had diminished amplitude and a delay of 2.5 h relative to Panama City Beach. Panama City (another station different from Panama City Beach) and Apalachicola tidal gauges are located inside estuaries and the propagation from the open ocean to the inner part of the estuaries might contribute to the damping and delay of the observed meteotsunami waves. This Wave 1 was only apparent in Panama City Beach but current present measurements are were unable to provide information on the location of maximum amplitude of the wave or its spatial extent. Clearly, there is a compelling need to determine the spatial structure of meteotsunamis in this region. Because most of the tidal gauges are located inside estuaries, to follow the development of the meteotsunamis it is important essential to deploy more pressure sensors outside, in the coastal ocean to follow the development and propagation of meteotsunamis. Moreover, between Apalachicola (station 7) and Clearwater Beach (station 9) there is only one tidal gauge (Cedar Key), which and this is not enoughinsufficient to track the development propagation or attenuation of these ocean waves. This is the main reason toThat is why we propose as part of Objective 2 the deploymentto deploy of pressure sensors with a higherimproved spatial resolution along the coast as part of Objective 2.

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Figure 3. High frequency storm surge and atmospheric pressure disturbance time series at the tidal gauges along the Alabama and West Florida coasts. a) Location of the tidal gauges (the maximum observed high frequency surge height and the arrival times are indicated in the text; three main meteotsunami waves weare detected associated withfrom the one storm analyzed. The area of affectionaffected of by each wave has been indicated with arrows of different color in the map, b) Time series of theHigh-pass filtered (period < 3h) storm surge (black lines) and atmospheric pressure (blue lines) measured at each of the considered stations considered..

Many meteotsunami studies (e.g. summarized in Pattiarachi et al. 2014, Pellika et al. 2014) consider Proudman resonance as the primary mechanism by which ocean waves forced by atmospheric pressure are amplified in the ocean. However, Greenspan resonance has been less documented or considered common. The relevance of Greenspan resonance depends on the beach slope, the shape of the atmospheric pressure disturbance, the pressure anomaly intensity, and the moving speed of the atmospheric system. As recently explained by Seo and Liu (2014), the resurgent edge waves caused by a given atmospheric pressure disturbance propagating in the along shore direction, are possibly found in the range 0.7 ≤ V ≤ 1, where V is the normalized speed of the pressure disturbance. Such speed is normalized with the wavelength and period of the atmospheric system, and the bed slope ofn the inner shelf. Analysis of the Panama City Beach meteotsunami of March 2014 showed that the length of the atmospheric pressure disturbance was 70 km. This was computed with the period of the pressure drop (1 hour) and the celerity at which it was moving (19 m/s). Given that the bed slope in Panama City Beach is 0.0013, the resulting non-dimensional or normalized propagation velocity V for the atmospheric pulse producing the Panama City Beach meteotsunami ranged between 0.6 and 0.75. These values fell in the range proposed by Sea and Liu (2014). Although Proudman resonance has

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beenwas identified as the main resonancet process in othermany observed meteotsunamis (e.g. the Daytona Beach meteotsunami in 1996), Greenspan resonance was found to be an important contributor for the March 2014 meteotsunami (Olabarrieta et al., to be submitted to Geophysical Research Letters). Greenspan resonance is expected to be relevant especially if the squall line propagates in the alongshore direction in areas with relatively constant beach slopes. This hypothesis will be tested as part of Objective 3.

Surface weather maps for March 28th and 29th 2014 (Figure 4) showed the presence of a cold front connecting two main low pressure systems., the first oOne low was initially located in the boundary between Arkansas and Oklahoma and another the other one was in Ohio. South of the cold front, a high precipitation area was also identified at the coast between Louisiana and Western Florida coast, a high precipitation area was also identified. This precipitation area was related to the formation of a surface trough or prefrontal squall line (orange dashed line). The squall line emanated for from athe region of strong heavy rainfall and thunderstorms, and expanded radially (southward) from March 28th to the 29th. Prefrontal squall lines are in general associated with the formation of a cold air wedge ahead of the cold front. Radar mosaics showed high reflectivity in regions of sharp changes in atmospheric pressure. Figure 4.b shows reflectivity maps corresponding to the meteotsunami peaks as observed in Pensacola, Panama City Beach, Clearwater Beach and Naples. All the maps clearly show squall lines in orange/red. The location of the squall line coincided with the position and time of the meteotsunami.

A priori, the area of maximum radar reflectivity seems to reveal the location of the atmospheric pressure drop. Figure 5 shows the time series of atmospheric pressure and radar reflectivity at Shell Beach and Pensacola. The radar reflectivity time series were derived from the 5- minute resolution reflectivity maps at the location of different tidal gauges. In every analyzed station analyzed, the pressure drop occurred exactly at the maximum reflectivity zone. In other words, Rreflectivity gradients were thus correlated with atmospheric pressure gradients. A tight reflectivity gradient implied a strong updraft/downdraft interface and greaterwith its associated threat for an active and potentially damaging squall line. Although the presence of the squalls was usually identified with radar data, it was not clear whether a direct relation between the reflectivity and the atmospheric pressure existed for all the meteotsunami events. Further analysis is needed includingfrom other meteotsunami events is needed to ascertain whether the reflectivity data could be directly used to inferrepresent the characteristics of the atmospheric pressure drops and serve as an accurate predictors of meteotsunami occurrence. The analysis is of this relation is proposed also as part of Objective 3.

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Figure 4. a) The sSurface weather maps of thefor March 28th and 29th ,of March and b) National reflectivity mosaic maps derived obtained for from the NOAA climate service.

Figure 5. Time series of reflectivity and atmospheric pressure at Shell beachDauphin Island and

Pensacola.

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One of the most challenging aspects of simulating the generation of a meteotsunami is to accurately characterize the atmospheric forcing that feeds it. Most of numerical models that simulate meteotsunamis force a long wave propagation model with simplified atmospheric forcing. The COAWST modeling (Warner et al, 2010) system can simultaneously represent atmospheric conditions using WRF (Weather Research Forecast model) and long wave generation with ROMS (Regional Ocean Modeling System). This modeling system has been used to model other types of meteotsunamis (e.g., Renault et al., 2011). However, the model skill for simulating meteotsunamis has not been evaluated yet, neither has the resolution requirements for both atmospheric and oceanic numerical modules. Although similar high convection events, have been successfully captured by the Japanese Climate Simulator, the closure models to simulate the high conductivity region in the atmospheric model need analysis, too. Objective 3 will also deal with the numerical simulation of historical meteotsunamis and will determine the model skill for simulating these long waves.

APPROACH TO ADDRESS OBJECTIVES

Objective 1 (determine the appearance of Gulf of Mexico meteotsunamis and identify triggering atmospheric forcing conditions) will be addressed with tide and meteorological archived data from the National Oceanic and Atmospheric Administration (NOAA). Objectives 2 and 3 (related to the spatial structure, dynamics and predictability of meteotsunamis) will be addressed combining field data collection, data analysis, coupled atmosphere-ocean numerical model implementation, and analysis of numerical model output.

Archived data sources and analysis

Time series consisting of 6-minute water level (1-minute when available), atmospheric pressure, and wind velocity are readily available from NOAA’s website tidesandcurrents.noaa. gov . All records available will be examined for every station in the US coast of the Gulf of Mexico. For instance, data from stations in Florida and Texas go back to 1996; stations in Alabama, Mississippi and Louisiana date back to mid to late 2000s. Most of these stations, however are in semi-enclosed bodies of water and may provide attenuated signals from meteotsunamis. Extreme surge levels will be determined by subtracting predicted tides from verified water levels, applying a high-pass filter (e.g. Kaiser-Bessel filter, Emery and Thomson, 2001) to the surge signal, and identifying positive pulses with more than one standard deviation from the mean. For each extreme surge level we will determine whether there was any sudden variation of atmospheric pressure, air temperature or wind speed and direction. This analysis will allow us to address objective 1.

Atmospheric reflectivity Nexrad data from the NOAA’s National Climatic Data Center (which can be readily viewed with the map viewer http://gis.ncdc.noaa.gov/map/viewer/#app= cdo&cfg =radar&theme=radar&display=nexrad) will be used to determine whether these events were related to the passage of squall lines. If so, together with the atmospheric pressure measurements at six minute temporal resolution, Nexrad 5-min (300 s) data will serve to determinebe used to calculate the velocity of propagation of the squall lines and the direction with respect to the coast

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line. Because the spatial resolution of the Nexrad data is x m, the uncertainty in the velocity calculation will be of x m/300 s or < y m/s (????). The velocity of propagation of squalls is typically between 10 and 30 m/s. Furthermore, Tthe 5-minute resolution reflectivity data will be used to reconstruct reflectivity time series at the tidal gauge locations and analyze determine the correlation relation between the reflectivity signal and the atmospheric pressure variations during the passage of the squall. Relationships will be determined with correlation, coeherence and regression estimates during the periods of squall activity, All historical water level and atmospheric data are freely available at NOAA websites. GOES IRW satellite data will also be used to analyze quantify the cloud coverage during the formation of the meteotsunami generations. This Remotely sensed cloud coverage will be compared to the coverage computed by from the coupled ocean-atmospheric model., to identify if similar cloud structures are formed and also possible sources of discrepancyto determine the quality of the atmospheric forcing simulation. Discrepancies between atmospheric model and remotely sensed data will be resolved by tuning model parameters (further explained in the “Numerical Simulations” section) to ensure adequate forcing. This analysis will address Objectives 1 and 3.

Outcome for archived data miningfrom archived data sources and analysis: identification of major meteotsunamis occurring in the Gulf of Mexico since 1996, and their associated atmospheric forcing characteristics.

Field experiments

The currentPresent spatial coverage of tide gauge and weather stations maintained by the National Ocean Service of the National Oceanographic and Atmospheric AdministrationNOAA throughout the Gulf of Mexico is not enough to follow the formation and follow the propagation of the observed meteotsunamis observed. Most of these gauges are located inside estuaries and where the meteotsunami signal might seems to be damped. In order to resolve the spatial structure of meteotsunamis in the northern and eastern Gulf of Mexico we will supplement existing measurements with the deployment of additional sensors deployment. Preliminary assessment of meteotsunami impacts in northwest Florida (Panama City Beach) and west Florida (Naples) indicates that the time of greatest incidence is fall and late winter, and that. Analyses theiralso show that the period of these waves is of tens of minutes. Furthermore, the present spatial resolution of water level measurements in those critical areas is 100 to 200 km. Such spatial resolution is insufficient to characterize the propagation and amplification properties of the waves. To address resolve the spatial resolution problemextent of the amplified waves, we will deploy a series of pressure sensors and current profilers in the vicinity of the existing NOAA sensors within the areas historically impacted.

Specifically, we will deploy 3 pressure sensors up-coast (in the wave propagation sense) of the Panama City Beach station at spacing of 10 km and over depths of 3-5 m. We will deploy 3 other pressure sensors down-coast of the same station and with the sameequivalent spacing. Pressure sensors and locating pingers will be mounted on weighted tripods that will be deployable from a 24’ boat. Tripods will be located and retrieved by divers. Similar methodology has been used extensively and successfully by Valle-Levinson (e.g. Valle-Levinson

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et al., 2001; Waterhouse et al., 2009; Valle-Levinson, 2012). All Ppressure sensors will be purchased by this project and collect data every minute for 90 days.Pressure sensors will collect data every minute for 90 days. Pingers are already in our possession. A Teledyne RD Instruments Sentinel V acoustic Doppler current profiler (ADCP, to be purchased) will be deployed on a tripod in the vicinity of NOAA’s station at a depth of 10 m. It will record profiles every minute for the same ~9045 days as the pressure sensors. recording profiles every second. The ADCP itself will provide additional pressure measurements. The same strategy (pressure sensors & ADCP deployments) will be used in the vicinity of the Naples station. Fall and late winter deployments will seek to resolve along-shelf and cross-shelf structures of the meteotsunamis, including, for the first time, the structure of the a velocity profile associated with these waves. A total of 12 pressure sensors and 2 ADCPs (all to be purchased) will be deployed 2 times per year in years 1 and 2 of this proposal.

Outcome: the observations will provide critical information to improve understanding on the propagation and spatial structure and propagation of these waves, and for the validation of models and forecast systems to be developed.

Numerical simulations

Simulations will explore two types of general scenarios: I) realistic scenarios related to meteotsunamis identified in the tidal gauge data analysis; and II) process-oriented scenarios. In these,The latter will consider different coastal and shelf configurations, will be considered together with different idealized atmospheric forcing scenarios. Also, the pProcess-oriented scenarios will identify the characteristics (propagation direction, celerity and amplitude) of the atmospheric forcing producing the most energetic meteotsunamis, the relative importance of the wind stress variations, and also the importance relative contributions fromof Proudman and Greenspan resonance.

The COAWST modeling system (Warner et al, 2010) will be used to simulate the generation and propagation of historically observed meteotsunamis. Since the model can simultaneously represent atmospheric conditions using WRF and long wave generation with ROMS, the system represents the most advanced numerical models for representing the formation and propagation of meteotsunamis. This modeling system has been used to model other types of meteotsunamis (e.g., Renault et al., 2011), but the model skill was not quantified in detail. The model skill for reproducing the observed meteotsunamis will be evaluated by comparing observed surge levels with measurements. Moreover, this same task will determine the atmospheric forcing (spatial and temporal) and the bathymetric resolution required to characterize these extreme events correctly.

The first two years of this project will focus on the simulation of the two of the main eventsmeteotsunamis identified in 2014 in the Eastern Gulf of Mexico. As done byFollowing Horvack et al. (2104) for the BoothBay meteotsunamimeteotsunamis in Maine, the first step will consist on performing some numerical experiments with WRF to assess the model sensitivity to:

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1. Initial and boundary conditions: small uncertainties in initial and lateral boundary conditions may have a large influence on mesoscale simulation results. We will consider 3 different sources of initial and boundary conditions characterized by different resolution and degree of data assimilation: North-American Regional Reanalysis (NARR, grid spacing ~ 0.333 deg), ERA-Interim (T255, grid spacing ~ 0.75 deg), ECMWF operational analysis, ECMWF_OA (T799, grid spacing ~ 0.22 deg).

2. Boundary layer and convective parametrizations: meteotsunamis in the Gulf of Mexico are associated with atmospheric convective activity. These parameterizations could also influence wave-CISK, the interaction between convection and internal gravity-waves. This has been proposed as one of the possible mechanisms for generating atmospheric conditions favoring meteotsunamis (Horvack et al., 2104).

3. Microphysics parametrization: in high-resolution (1 Km) modeling domains, when convection parametrization is switched off, the microphysics scheme controls the explicit precipitation (both convective and stratiform) and could affect the convection degree.

[4.] Nesting strategy: The numerical simulation of the atmospheric conditions triggering meteotsunamis requires high spatial and temporal resolution simulations. This is usually achieved with a multi nesting strategy, which can be one way or two-way depending on the model or model setup (some models such as WRF have both options). In the one-way nesting setup the results from higher resolution domains do not affect the results from lower resolution domains. In two way nesting the results from the higher resolution domains are feedbacked to lower resolution domains. This feedback might be relevant especially for some of the proposed meteotsunami generation mechanisms (e.g. trapping of internal gravity waves or their interaction with the convection). As explained by Horvack et al., 2104, if internal gravity waves of wavelengths smaller than 10 dx (where dx represents the grid horizontal resolution) do exist in the atmosphere of the higher resolution domains, they may be aliased in the lower resolution. AlTthough two-way nesting most often should lead to more accurate results, most implementations of the operational numerical weather prediction models use one-way nesting. Therefore, the analysis of results using the one-way nesting strategy is important for assessmentwill allow assessment of the potentials for numerical prediction of atmospheric conditions related to meteotsunamis.

Model results will be verified with the atmospheric pressure, wind, and temperature measurements from the NOAA stations. Data from GOES IRW data will also be used to verify the model performance on simulating cloud formation. Once the assessment and sensitivity of the atmospheric model is concluded, the ocean module (ROMS) will be activated. The ocean model is three-dimensional, free surface, terrain-following and solves finite-difference approximations of the Reynolds-Averaged Navier-Stokes equations. It uses the hydrostatic and Boussinesq approximations (Chassignet et al., 2000; Haidvogel et al., 2000) with a split-explicit time stepping algorithm (Shchepetkin and McWilliams, 2005; Haidvogel et al., 2008). In this study, the model will be implemented in a depth-integrated mode, disregarding baroclinic flows. The bathymetry will be derived from the highest resolution Digital Elevation Model (DEM) from NOAA. These DEMs integrate bathymetry and topography along the coast of the Gulf of Mexico. A numerical grid will be created of with constant and uniform spatial resolution will be

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created (100 m). The meteotsunami generation and its evolution will be verified by comparing the numerical sea surface elevations with the NOAA tideal gauge and our complementary measurements.

Apart from the real meteotsunami simulations, processes-oriented simulations will consider coastal regions with different water depths and coastal slopes. The aAtmospheric forcing will consist ofn a Gaussian shape meteotsunami, and the period and the celerity will vary between 0.5 to 2.5 hours and 10 to 40 m/s respectively. With these simulations we will identify the conditions, for a given costal configuration, that produce the highest meteotsunami amplification. Depending on the coastal slope we will be able to ascertain whether the Proudman or Greenspan is the main resonancet process. The results of the numerical simulations will be compared with the results of the analytical models presented by Hibiya and Kaijura (1982) and Seo and Liu (2014).

The Both realistic and process-oriented numerical simulations will be run at the Hyper Gator, a high computational cluster at the University of Florida. in the Hyper Gator, a high computational cluster.

Outcome: A functioning model that provides advances understanding onf the dynamics of the meteotsunamis occurring in the analyzed areaGulf of Mexico. Graphs showing maximum expected meteotsunami amplitudes for different types of idealized atmospheric pressure jumps.

TIMELINEYear 1. We will compile and analyze dataData compilation and analysis of the meteotsunami events in the period 1996 to -present day (some of the tidal gauges did not start to measure untilstarted measuring later than 1996). We will analyze Analysis of the meteorological conditions that triggeringed the meteotsunamis and examine the relation between the radar reflectivity signal and the atmospheric pressure dropjumps. We will also dDeployment of pressure sensors and ADCPs in late winter. We will prepare Mmanuscripts on i) the characteristics of the main historical meteotsunamis in the Gulf of Mexico and their meteorological conditions, and ii) report results from of pressure sensor and ADCP deployments.Year 2. We will carry out nNumerical simulations of the main largest historical meteotsunamis identified in Year 1, deploy sensors in late fall and late winter, and effect data analysis of the pressure sensors and ADCPs deployed in year 1. We will write Mmanuscripts on i) nNumerical model results, sensitivity analysis to the atmospherically initial conditions, boundary conditions, grid resolution, and closure models, and ii) analysis of other deployments of pressure sensors and ADCP data analysis. Year 3. We will deploy sensors in late autumn and continue with the analysis of data. We will also combine numerical model results and analytical approaches to determine Analysis of the relative contribution of resonance and propagation processes affecting the amplitude of the main meteotsunamis occurring in the Gulf of Mexico. We willCombine numerical modelling, and analytical derivations. aAnalyze the possible meteotsunami impacts from theon flooding risks, sediment transport and tracer transport perspective. Analyze Moreover, we will determine the

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probability of the meteotsunami occurrence probability in the different regions of the Gulf of Mexico. Finalize the pressure gauge data collection, post processing and analysis. We will prepare mManuscripts on i) the analysis of the different processesing affecting the meteotsunami impact in the Gulf of Mexico and the occurrence probability, and b) results from data analysis and collection.

PROJECT MANAGEMENTThe project will be managed and executed by the PIs M. Olabarrieta, A. Valle-Levinson,

and M. Bourassa. A. Valle-Levinson will supervise the data compilation and organization for their analysis and also the deployment of the pressure gauges, post processing, and analysis. M. Olabarrieta and M. Bourassa will work on the coupled Ocean-Atmosphere numerical simulations. The interaction between Prof. Bourassa and Dr. Olabarrieta and Prof. Valle-Levinson will be ideal for this analysis since because of the need of expertise in both atmosphere and ocean wave dynamics. This combined expertise will help expertise is needed to understand simulate and numerically simulateunderstand the occurrence development of this kind ofthese waves. While M. Olabarrieta will focus on the ocean processes, and will determine the relevance of the different resonance and wave propagation processes., M. Bourassa will focus on the atmospheric conditions that produce stable and persistent atmospheric gravity waves. Moreover, the contribution of Prof Valle-Levinson is essential forwill concentrate on the data acquisition and analysis purposes. M. Olabarrieta is a recently hired faculty member at the University of Florida and will be in charge of supervising and running the numerical scenarios. The work will be done with the help of a graduate student, supported by this project and co-supervised by the PIs, in the analysis of data and the execution and analysis of numerical experiments. An Uundergraduate students per year will also be involved, during non-summer semesters, through the University Scholars Program at the (University of Florida,) and through the Undergraduate Honors Center at (Florida State University). Undergraduate students will help in the compilation and organizations of NOAA files containing water level, atmospheric forcing variables and radar reflectivity data. This project will therefore involve three PIs, one graduate student, and three undergraduate students. In order to ensure appropriate progress, the PIs will have biweekly remote meetings.

BROADER IMPACTSThis proposal seeks to study a phenomenon that can affect the entire coast of the Gulf of

Mexico. Findings generated from the this study will help us understanding the which areof the atmospheric conditions that produceing meteotsunamis, which and of the resonance processes that magnifying the amplitude of these waves. In addition, the project and also will explore the capabilities of the most developedstate-of-the-art numerical models to simulate the formation of these kind of wavesgenesis of meteotsunamis (“metsugenesis”). Although meteotsunamis have been thought to be rare waves, there is evidence that, especially in the northern and eastern coast of the Gulf of Mexico, they occur more often than expected. This region is especially vulnerable to storm surges, given the low terrain elevation. Better understanding of the generation mechanisms of these waves”metsugenesis” will help to reduce risk to population by improving storms surge predictions and risk analysis in the Gulf of Mexico coast. It Findings of our study

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maywill also be valuable in the determination ofhelp guide the optimal location of future tidal sensors in the areas prone to meteotsunamis. Moreover, the period of these ocean waves is between 1-2 hours, and the water velocities they produce are strongshould be large (O 1 m/s). Is therefore that tThe propagation of these waves could therefore affect the sediment and tracer transport processes along the coast. It is also worth mentioning that, since there is evidence ofAlso, as the frequency and intensity of extreme storms is expected to increase with climate variabilitychange, affecting the frequency and intensity of extreme storms, the probability of meteotsunami occurrence could also be affected by those larger scale variationsincrease., is therefore that the present study could open new scientific questions and research directions, related with climate variabilityThese effects could open new research avenues associated with climate change.

Most of the data that will be used in this project is already available from NOAA. We do propose to acquire new measurements, that we believe will be very helpfulshould help to correctly understand the meteotsunami propagation mechanisms and dynamics. However, the occurrence of meteotsunamis is not deterministic and therefore the probability of big meteotsunamis in the area during the duration of the project is not 100% guaranteed. Most of tThe objectives and outcomes of the proposal are notDO NOT dependent on the occurrence of meteotsunamis during the project, and can be elaborated achieved with the existing historical data. Even if there is no occurrence of meteotsunamis, the new data to be collected will be the first in their kind in the inner shelf of this part of the Gulf of Mexico and should indeed produce innovative findings.

In addition to disseminating findings in peer-reviewed journal, we will disseminate our results to coastal managers along the US Florida Coast (water management district and estuarine research reserves) most affected by the occurrence of meteotsunamis. We will also disseminate our results in meteorological prediction agencies. We will disseminate findings also through newsletters xxx.

This project provides the opportunity for extensive outreach activities as it studies a phenomenon that can be seen and tracked by the general public. We will be able to have the public doing science with us because the incidence of meteotsunamis on the Gulf of Mexico coast can actually be confirmed via the internet. We will involve the community to participate in this investigation by sensitizing them to rapid changes in weather, which are part of life in coastal Gulf of Mexico. We will instruct them to follow squall lines and the concomitant rapid changes in storm surge. The public will be able to track weather systems, through the nexrad website, and to monitor storm surges, through NOAA’s website. We will ask the public to report the potential development of a meteotsunamis and its actual incidence at one of the real-time sensors. Because of general public involvement, we will use social media to disseminate these reports through a twitter account. People who report a meteotsunamis correctly will be recognized in the project’s facebook account, which will be linked to the tweeter account. In order to train people to recognize possible triggers of meteotsunamis and their actual incidence, we will organize open forums at the 3 cities in Florida that we have identified as being most

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affected by meteotsunamis: Panama City, Clearwater Beach, and Naples. We will work with the major’s office at each one of those cities to organize and present the forums to the general public. In addition, we will work with the public school system of each city to deliver training sessions at no less than two local high schools. At those presentations we will talk about the relevance and threat of meteotsunamis in the area, and will illustrate the websites to obtain data related to them. In addition, we will invite the participants to participate in our research by following meteostsunami development in the Gulf of Mexico. Participation at the forums and in the reporting of meteotsunamis will be incentivized by small tokens of appreciation. We will raffle t-shirts to forum participants, and we will give them to the people who recognize their incidence first. We will offer honorary distinctions on facebook to those who first report them correctly. We will keep statistics on total number of reports, as well as the ratio of correct to incorrect reports. This community-based scientific activity will increase appreciation for science, and its approach, by the general public.

In addition to disseminating findings in the social networks and through peer-reviewed journals, we will disseminate our results to coastal managers along the US Florida Coast (emergency managers, water management district and estuarine research reserves) most affected by the occurrence of meteotsunamis. We will also disseminate our results into meteorological prediction agenciesthe National Weather Service and local weather forecast activities. We will disseminate findings also through newsletters xxx.

The pProject will support one graduate student, Cristian Rojas, at the University of Florida who will be co-advised by M. Olabarrieta and A. Valle- Levinson. Bourassa will be the external committee member. Cristian Rojas will be in charge of compiling and analyzing data, together with Valle-Levinson, and of implementing and analyzing numerical simulations in conjunction with Olabarrieta and Bourassa. The project will also support a series of individual undergraduate honors students to help with the historical data analysis. We will actively attempt to recruit a members from an underrepresented group, through the minority Mentorship Program.

Materials and relevant results derived from this project will also be incorporated in classes taught by the PIs at the University of Florida (Physical Oceanography, Data Analysis, Linear Waves, Littoral Processes, and Fluid Mechanics) and at Florida State University (Air/Sea Interaction). The involvement of Dr. Olabarrieta and Prof. Valle-Levinson, and by grad student Cristian Rojas, enhances the representation participation of underrepresented groups in science.

RESULTS OF PRIOR NSF SUPPORT

NFS Award: OCE-1332718 to A. Valle-Levinson and M. Olabarrieta Amount: $ 399,426Period: 08/15/2013-07/31/2016

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During the project entitled “Semidiurnal perturbations to storm surges on the U.S. eastern seaboard” we have analyzed the wind, atmospheric pressure and sea level measurements along the US east coast associated with the passage of all the Tropical Storms (TS) and hurricanes occurring between 1996 and 2014. Intellectual merit: The analysis has shown that the surge-tide interaction (which affects the maximum peak of the total water levels and the arrival time of the peak at a given point in the coast) occurred in many hurricanes (Arthur, 2014; Sandy, 2012; Irene, 2011; Earl, 2010; Frances, 2004; Dennis 1999; Floyd, 1999; Bonnie, 1998; Fran, 1996). The result of this tide-surge interaction is a phase-lag between the predicted and the observed astronomic tide during the passage of the storm. This phase-lag is what produces the appearance of the semidiurnal perturbations in the surge signal. The tide-surge interaction phenomena is relevant in the South Atlantic Bight, where the astronomic tide shows a quasi-standing behavior. We have identified that the strength of these semidiurnal perturbations is affected by the wind speed in the along-shelf direction. Currently we are numerically simulating each of these hurricanes, verifying the model with the observed water levels, and exploring which are the physical mechanisms responsible for these tide surge interactions.Broader impacts: The current study has revealed that the effects of the tide-surge interaction are relevant in many extreme storm conditions and that can affect to the surge over the whole South Atlantic Bight. The results have been presented in two international conferences and in two international invited talks, one at Hohai University (Nanjing, China) and the second at La Rochelle University (La Rochelle, France). A manuscript describing the findings of the data analysis has recently been submitted to the Journal of Geophysical Research: Oceans. A second manuscript describing the results derived from the numerical simulations is under elaboration.

NFS Award: 0917685 to A. S. R. Smith and M.A. Bourassa Amount: $ 81,045Period: 09/1/2009—10/31/2010

The goal of the project was to quality assess UNOLS marine meteorological data, and make these data, metadata, and quality assessments publicly available.Intellectual Merit: These high quality observations have been used to validate satellite retrievals of wind, 10m air temperature, and 10m humidity (May and Bourassa, 2011).Broader Impacts: The meteorology observations are quality assessed and publicly available at http://samos.coaps.fsu.edu/html/. These data are well suited for validation of input data for calculating surface turbulent heat fluxes in the proposed work. These data have been used in graduate student research and to study sub-grid scale variability. Recent applications have included validation of satellite retrievals of 10m air temperature and humidity.Please include your prior NSF support

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