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Jiayi Pan and David A. Jay

Abstract

The utility of the acoustic Doppler current profiler (ADCP) for sampling small time and space scales of coastal environments can be enhanced by mounting a high-frequency (1200 kHz) ADCP on an oscillating towed body. This approach requires both an external reference to convert the measured shears to velocities in the earth coordinates and a method to determine the towed body velocities. During the River Influence on the Shelf Ecosystems (RISE) project cruise, a high-frequency (1200 kHz) and narrowbeam ADCP with mode 12 sampling was mounted on a TRIAXUS oscillating towfish, which steers a 3D path behind the ship. This deployment approach extended the vertical range of the ADCP and allowed it to sample near-surface waters outside the ship’s wake. The measurements from a ship-mounted 1200-kHz narrowbeam ADCP are used as references for TRIAXUS ADCP data, and a method of overlapping bins is employed to recover the entire vertical range of the TRIAXUS ADCP. The TRIAXUS vehicle horizontal velocities are obtained by removing the derived ocean current velocity from the TRIAXUS ADCP measurements. The results show that the method is practical.

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Edward D. Zaron and David A. Jay

Abstract

Hourly sea level is examined at 25 open-ocean stations in the Pacific Ocean with records longer than 30 yr. A search for trends finds that the amplitude of the dominant semidiurnal tide M2 is increasing at 12 of the 13 sites where a statistically significant trend can be identified. It is also found that nontidal variance in the neighborhood of M2 is decreasing at all 12 of the sites where a significant increase in M2 tide is occurring. The trend in amplitude of the dominant diurnal tide K1 is significant at six stations, and it is both increasing (four stations) and decreasing (two stations). The trend in semidiurnal-band variance suggests the hypothesis that increases in M2 could be caused by improvements in time keeping or data processing that would reduce the apparent phase variability of the tide. This possibility is examined and found to be the likely explanation for tidal trends at four stations. Local changes are found to explain the trend at two stations, Johnston Island and Mokuoloe, as diagnosed from correlated changes in nonlinear overtides and site history. Changes in M2 at the equatorial station of Kanton are significant and unexplained by either timing errors or local morphodynamics; although, strong nonlinear overtides are present. Changing tides at five stations in the western Pacific, Malakal, Yap, Saipan, Kapingamarangi, and Pohnpei cannot be explained by the above and suggest a region where a connection between climate and tides may yet be identified.

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Jieshuo Xie, Jiayi Pan, and David A. Jay

Abstract

Interaction of barotropic tides with subsurface topography is vital to ocean mixing. Yet the behavior of large-amplitude, nonlinear, internal solitary waves (ISWs) that can cause strong mixing remains poorly understood, especially that of higher-mode and multimodal internal waves. Therefore, a 2.5-dimensional, nonhydrostatic model with adjustable vertical resolution was developed to investigate effects of upper-ocean stratification on tidally induced multimodal internal waves and to show how they are generated by the subcritical ridge where only upward-propagating internal wave beams (IWBs) are present. The effects of the stratification on properties and characteristics of the excited IWBs and on the energy partition of the radiated mode-1 and mode-2 internal waves were investigated based on the model results. Higher modes of internal waves can also be effectively generated in the IWBs by the subcritical topography, and the contribution to IWBs from higher modes increases with the upper-ocean stratification. Mode-2 ISWs can be excited from the IWBs if both the tidal Froude number and the contribution to IWBs from mode-2 waves are sufficiently high (U 0 is the tidal current speed, and c 2 is the phase speed of mode-2 waves). In a moderately stratified upper ocean, both mode-1 and mode-2 ISWs can be produced, but for weak (strong) stratification, only mode-1 (mode-2) ISWs are generated. Further, it is found that the distance between two successive mode-1 or mode-2 ISW trains increases linearly with the upper-ocean stratification. The ratio of the kinetic energy to the available potential energy for the mode-2 ISWs increases with the upper-ocean stratification in a strongly stratified ocean.

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Pascal Matte, David A. Jay, and Edward D. Zaron

Abstract

One of the most challenging areas in tidal analysis is the study of nonstationary signals with a tidal component, as they confront both current analysis methods and dynamical understanding. A new analysis tool has been developed, NS_TIDE, adapted to the study of nonstationary signals, in this case, river tides. It builds the nonstationary forcing directly into the tidal basis functions. It is implemented by modification of T_TIDE; however, certain concepts, particularly the meaning of a constituent and the Rayleigh criterion, are redefined to account for the smearing effects on the tidal spectral lines by nontidal energy. An error estimation procedure is included that constructs a covariance matrix of the regression coefficients, based on either an uncorrelated or a correlated noise model. The output of NS_TIDE consists of time series of subtidal water levels [mean water level (MWL)] and tidal properties (amplitudes and phases), expressed in terms of external forcing functions. The method was tested using records from a station on the Columbia River, 172 km from the ocean entrance, where the tides are strongly altered by river flow. NS_TIDE hindcast explains 96.4% of the signal variance with a root-mean-square error of 0.165 m obtained from 288 parameters, far better than traditional harmonic analysis (38.5%, 0.604 m, and 127 parameters). While keeping the benefits of harmonic analysis, its advantages compared to existing tidal analysis methods include its capacity to distinguish frequencies within tidal bands without losing resolution in the time domain or data at the endpoints of the time series.

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Neil F. Laird, L. Jay Miller, and David A. R. Kristovich

Abstract

This article presents a detailed examination of the kinematic structure and evolution of the 5 December 1997 winter mesoscale vortex in the vicinity of Lake Michigan using the synthetic dual-Doppler (SDD) technique. When such a mesoscale event propagates a distance large enough that the viewing angle from a single-Doppler radar changes by about 30° and the circulation is sufficiently steady during this time period, then the SDD method can reveal reliable details about the circulation. One such detail of the observed vortex was a pattern of convergence and divergence associated with radial bands, where heavier snowfall was located. Another was the steadiness and vertical coherence of the derived vorticity and convergence patterns within the cyclonic circulation.

On 5 December 1997, the observed reflectivity field remained remarkably steady for nearly 2.5 h as the vortex moved southeastward allowing for the application of the SDD technique. The reflectivity field exhibited a pronounced asymmetric convective structure with at least three well-defined, inward-spiraling radial snowbands, and a distinct weak-reflectivity region or “eye” near the center of cyclonic circulation. The SDD results showed the vortex circulation was composed of a combination of rotation on the meso-β scale and convergence on the meso-γ scale associated with the embedded radial snowbands. Vertical profiles of derived meso-β-scale, area-mean convergence and vorticity suggest that this winter vortex was likely a warm-core system, similar to both tropical cyclones and polar lows.

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Adam T. Devlin, Edward D. Zaron, David A. Jay, Stefan A. Talke, and Jiayi Pan

Abstract

An analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern South China Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. The model perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff.

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Jay Lawrimore, Thomas R. Karl, Mike Squires, David A. Robinson, and Kenneth E. Kunkel

Abstract

The 100 most severe snowstorms within each of six climate regions east of the Rocky Mountains were analyzed to understand how the frequency of severe snowstorms is associated with seasonal averages of other variables that may be more readily predicted and projected. In particular, temperature, precipitation, and El Niño/La Niña anomalies from 1901 to 2013 were studied. In the southern United States, anomalously cold seasonal temperatures were found to be more closely linked to severe snowstorm development than in the northern United States. The conditional probability of occurrence of one or more severe snowstorms in seasons that are colder than average is 80% or greater in regions of the southern United States, which was found to be statistically significant, while it is as low as 35% when seasonal temperatures are warmer than average. This compares with unconditional probabilities of 55%–60%. For seasons that are wetter (drier) than average, severe snowstorm frequency is significantly greater (less) in the Northern Plains region. An analysis of the seasonal timing of severe snowstorm occurrence found they are not occurring as late in the season in recent decades in the warmest climate regions when compared to the previous 75 years. Since 1977, the median date of occurrence in the last half of the cold season is six or more days earlier in the Southeast, South, and Ohio Valley regions than earlier in the twentieth century. ENSO conditions also were found to have a strong influence on the occurrence of the top 100 snowstorms in the Northeast and Southeast regions.

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Michael F. Squires, Jay H. Lawrimore, Richard R. Heim Jr., David A. Robinson, Mathieu R. Gerbush, and Thomas W. Estilow

This paper describes a new snowfall index that quantifies the impact of snowstorms within six climate regions in the United States. The regional snowfall index (RSI) is based on the spatial extent of snowfall accumulation, the amount of snowfall, and the juxtaposition of these elements with population. Including population information provides a measure of the societal susceptibility for each region. The RSI is an evolution of the Northeast snowfall impact scale (NESIS), which NOAA's National Climatic Data Center began producing operationally in 2006. While NESIS was developed for storms that had a major impact in the Northeast, it includes all snowfall during the lifetime of a storm across the United States and as such can be thought of as a quasi-national index that is calibrated to Northeast snowstorms. By contrast, the RSI is a regional index calibrated to specific regions using only the snow that falls within that region. This paper describes the methodology used to compute the RSI, which requires region-specific parameters and thresholds, and its application within six climate regions in the eastern two-thirds of the nation. The process used to select the region-specific parameters and thresholds is explained. The new index has been calculated for over 580 snowstorms that occurred between 1900 and 2013 providing a century-scale historical perspective for these snowstorms. The RSI is computed for category 1 or greater storms in near–real time, usually a day after the storm has ended.

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Neil F. Laird, David A. R. Kristovich, Robert M. Rauber, Harry T. Ochs III, and L. Jay Miller

Abstract

This study examines complex flow patterns associated with the Cape Canaveral sea breeze and sea-breeze front using dual-Doppler radar, sounding, and surface data collected on 26 July 1991 during the Convection and Precipitation/Electrification Experiment. This case focuses on (a) the structure of the sea breeze, an associated trailing convergence line, river-induced convergence zones, and thunderstorm outflow boundaries, and (b) the development of convection where these features interacted.

Variations in the direction of the sea breeze in the vicinity of irregular coastlines, such as Cape Canaveral, can lead to persistent zones of convergence within the sea-breeze air. The findings show that these zones of convergence, in turn, can locally increase the depth of the sea-breeze air and create circulations at the top of the sea breeze, which can support the development of convection. The observational study is the first to document the development and evolution of the trailing convergence line over Cape Canaveral and show that its presence can be instrumental in thunderstorm initiation.

Small inland water bodies, such as the Indian River, can have a strong influence on the location where thunderstorms first develop as the sea breeze propagates inland. Divergence over the small, relatively cooler Indian River during daytime was sufficient to maintain a quasi-stationary convergence zone that, when approached and disrupted by the sea-breeze front, triggered thunderstorms. The intersection point between the sea-breeze front and the river-induced convergence zone identified the location where successive thunderstorms developed during the day.

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Jay H. Lawrimore, David Wuertz, Anna Wilson, Scott Stevens, Matthew Menne, Bryant Korzeniewski, Michael A. Palecki, Ronald D. Leeper, and Thomas Trunk

Abstract

The National Oceanic and Atmospheric Administration (NOAA) has operated a network of Fischer & Porter gauges providing hourly and subhourly precipitation observations as part of the U.S. Cooperative Observer Program since the middle of the twentieth century. A transition from punched paper recording to digital recording was completed by NOAA’s National Weather Service in 2013. Subsequently, NOAA’s National Centers for Environmental Information (NCEI) upgraded its quality assurance and data stewardship processes to accommodate the new digital record, better assure the quality of the data, and improve the timeliness by which hourly precipitation observations are made available to the user community. Automated methods for removing noise, detecting diurnal variations, and identifying malfunctioning gauges are described along with quality control algorithms that are applied on hourly and daily time scales. The quality of the hourly observations during the digital era is verified by comparison with hourly observations from the U.S. Climate Reference Network and summary of the day precipitation totals from the Global Historical Climatology Network dataset.

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