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Lynn K. Shay
,
Hans C. Graber
,
Duncan B. Ross
, and
Rickey D. Chapman

Abstract

The quality and vertical correlation scales of high-frequency (HF) radar-derived ocean surface current measurements from an ocean surface current radar (OSCR) are assessed by comparing surface to subsurface current observations from 11 June to 8 July 1993 at directional discus buoys DW and DE, each instrumented with a three-axis ultrasonic current meter at the 13.8- and 9.5-m depths, respectively. A dual-station OSCR mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 30 km × 44 km domain inshore of the Gulf Stream using the HF (25.4 MHz) mode. Over a 27-day experimental period, surface current observations were acquired 97% of the time extending to the maximum theoretical range of 44 km. Linear regression analyses indicated a bias of 2–4 cm s−1 and slopes of O(1). While there were periods when the daily averaged complex correlation coefficients were highly correlated (>0.8), periods of low correlation (<0.3) are explained in terms of vertical phase differences and a decoupling between surface and subsurface records.

Surface and subsurface current time series at the two mooring sites were decomposed into the tidal, mean (>48 h), near-inertial (20.7 h), and high-frequency (4.5 h) bands. Tidal analyses, based on the semidiurnal (K 2, M 2, L 2, S 2) and diurnal (K 1, O 1, P 1, Q 1) constituents, indicated maximum amplitudes of 5 cm s−1 at DW, whereas these amplitudes increased offshore to a maximum of 13 cm s−1 at DE. Net differences between the surface and subsurface tidal currents ranged between 2 and 5 cm s−1 with the largest difference of 7.7 cm s−1 for the K 1, constituent at DE. The tidal currents were removed from the surface and subsurface current time series and low-pass filtered at 48 h, bandpass filtered between 18 and 23 h, and high-pass filtered at 8 h. The mean current components were highly correlated (>0.9) over most of the record with small phase differences. Intrusions of the mean flow at 3–5-day intervals were correlated with bursts of near-inertial motions having amplitudes of 20 cm s−1 at DE and 15 cm s−1 at DW. The frequency of these motions was shifted 5%–10% above and below fduring these episodes of mean flow intrusions. The higher-frequency surface motions with amplitudes of 5–8 cm s−1 oscillated at periods of 4.3–4.7 h but were directly out of phase with the subsurface currents, which caused the correlations to decrease below 0.3. Thus, temporal decorrelations appeared to be a result of high-frequency motions in the internal wave band between the inertial and Nyquist (1.5 cph) frequencies.

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Edgar An
,
Manhar R. Dhanak
,
Lynn K. Shay
,
Samuel Smith
, and
John Van Leer

Abstract

Bathymetry, current, temperature, and depth (CTD) measurements using a small, mobile, autonomous underwater vehicle (AUV) platform are described. Autonomous surveys of a shallow water column off the east coast of Florida during December 1997 were carried out using a 2.13-m long, 0.53-m maximum diameter Ocean Explorer series AUV, equipped with a 1200-kHz acoustic Doppler current profiler (ADCP) and a CDT package. At a speed of 1–2 m s−1, this AUV can perform preprogrammed missions over a period of several hours, collecting in situ oceanographic data and storing it on an onboard datalogger. The vehicle may also carry side-scan sonar or a custom small-scale turbulence measurement package or other instruments for subsidiary measurements. The versatility of the AUV allows measurement of oceanographic data over a substantial region, the motion of the platform being largely decoupled from that of any surface mother ship.

In the missions of 5 and 11 December 1997, “lawn mower pattern” AUV surveys were conducted over 1 km2 regions on the east coast of Florida, north of Fort Lauderdale, at depths of 7 and 3 m, respectively, in a water column where the depth ranged from 10 to 32 m. During 5 December, the region was subjected to a cold front from the northwest. Local wind measurements show presence of up to 10 m s−1 winds at temperatures of up to 10°–15°C below normal for the time of the year. The fixed ADCP indicates occurrence of significant internal wave activity in the region. The data collected using the mobile AUV are utilized to develop a map of the bottom topography and examine current, temperature, and density variations in the context of the background information from a fixed bottom–mounted ADCP and Coastal-Marine Automated Network buoys. The work described here is a significant step in the development of an autonomous oceanographic sampling network, illustrating the versatility of an AUV platform. The data collected during the missions described will form part of a bank for information on the impact of a cold front on shallow subtropical waters. The authors expect to repeat the missions during other such fronts.

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James M. Kaihatu
,
Robert A. Handler
,
George O. Marmorino
, and
Lynn K. Shay

Abstract

Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to three-dimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis “converges” in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.

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Joshua B. Wadler
,
David S. Nolan
,
Jun A. Zhang
, and
Lynn K. Shay

Abstract

The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (TCs) (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye–eyewall mixing below 2-km altitude. Additionally, a layer of higher θ e between 1- and 2-km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θ e air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θ e deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θ e by 10–14 K, showing that air–sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θ e midtropospheric air toward the inflow layer. Here, the low-θ e air entrains into the updraft in less than 5 min with a mean θ e deficit of 8.2 K. In general, θ e recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.

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Joshua B. Wadler
,
Jun A. Zhang
,
Benjamin Jaimes
, and
Lynn K. Shay

Abstract

Using a combination of NOAA P-3 aircraft tail Doppler radar, NOAA and NASA dropsondes, and buoy- and drifter-based sea surface temperature data, different types of downdrafts and their influence on boundary layer (BL) thermodynamics are examined in Hurricane Earl (2010) during periods prior to rapid intensification [RI; a 30-kt (15.4 m s−1) increase in intensity over 24 h] and during RI. Before RI, the BL was generally warm and moist. The largest hindrances for intensification are convectively driven downdrafts inside the radius of maximum winds (RMW) and upshear-right quadrant, and vortex-tilt-induced downdrafts outside the RMW in the upshear-left quadrant. Possible mechanisms for overcoming the low entropy (θ e ) air induced by these downdrafts are BL recovery through air–sea enthalpy fluxes and turbulent mixing by atmospheric eddies. During RI, convective downdrafts of varying strengths in the upshear-left quadrant had differing effects on the low-level entropy and surface heat fluxes. Interestingly, the stronger downdrafts corresponded with maximums in 10-m θ e . It is hypothesized that the large amount of evaporation in a strong (>2 m s−1) downdraft underneath a precipitation core can lead to high amounts of near-surface specific humidity. By contrast, weaker downdrafts corresponded with minimums in 10-m θ e, likely because they contained lower evaporation rates. Since weak and dry downdrafts require more surface fluxes to recover the low entropy air than strong and moist downdrafts, they are greater hindrances to storm intensification. This study emphasizes how different types of downdrafts are tied to hurricane intensity change through their modification of BL thermodynamics.

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S. Daniel Jacob
,
Lynn K. Shay
,
Arthur J. Mariano
, and
Peter G. Black

Abstract

Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core eddy in the region with velocities of O(1) m s−1. Based upon conservation of heat and mass, the three-dimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m−2 and a heat flux of 1200 W m−2 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m−2 corresponding to a less than 7 m s−1 wind speed.

Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme predicts a narrow region of cooling compared to the the stress-induced mixing scheme and observed SST decreases. After the storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.

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Joshua B. Wadler
,
Jun A. Zhang
,
Robert F. Rogers
,
Benjamin Jaimes
, and
Lynn K. Shay

Abstract

The spatial and temporal variation in multiscale structures during the rapid intensification of Hurricane Michael (2018) are explored using a coupled atmospheric–oceanic dataset obtained from NOAA WP-3D and G-IV aircraft missions. During Michael’s early life cycle, the importance of ocean structure is studied to explore how the storm intensified despite experiencing moderate vertical shear. Michael maintained a fairly symmetric precipitation distribution and resisted lateral mixing of dry environmental air into the circulation upshear. The storm also interacted with an oceanic eddy field leading to cross-storm sea surface temperature (SST) gradients of ~2.5°C. This led to the highest enthalpy fluxes occurring left of shear, favoring the sustainment of updrafts into the upshear quadrants and a quick recovery from low-entropy downdraft air. Later in the life cycle, Michael interacted with more uniform and higher SSTs that were greater than 28°C, while vertical shear imposed asymmetries in Michael’s secondary circulation and distribution of entropy. Midlevel (~4–8 km) outflow downshear, a feature characteristic of hurricanes in shear, transported high-entropy air from the eyewall region outward. This outflow created a cap that reduced entrainment across the boundary layer top, protecting it from dry midtropospheric air out to large radii (i.e., >100 km), and allowing for rapid energy increases from air–sea enthalpy fluxes. Upshear, low-level (~0.5–2 km) outflow transported high-entropy air outward, which aided boundary layer recovery from low-entropy downdraft air. This study underscores the importance of simultaneously measuring atmospheric and oceanographic parameters to understand tropical cyclone structure during rapid intensification.

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Mark DeMaria
,
Michelle Mainelli
,
Lynn K. Shay
,
John A. Knaff
, and
John Kaplan

Abstract

Modifications to the Atlantic and east Pacific versions of the operational Statistical Hurricane Intensity Prediction Scheme (SHIPS) for each year from 1997 to 2003 are described. Major changes include the addition of a method to account for the storm decay over land in 2000, the extension of the forecasts from 3 to 5 days in 2001, and the use of an operational global model for the evaluation of the atmospheric predictors instead of a simple dry-adiabatic model beginning in 2001.

A verification of the SHIPS operational intensity forecasts is presented. Results show that the 1997–2003 SHIPS forecasts had statistically significant skill (relative to climatology and persistence) out to 72 h in the Atlantic, and at 48 and 72 h in the east Pacific. The inclusion of the land effects reduced the intensity errors by up to 15% in the Atlantic, and up to 3% in the east Pacific, primarily for the shorter-range forecasts. The inclusion of land effects did not significantly degrade the forecasts at any time period. Results also showed that the 4–5-day forecasts that began in 2001 did not have skill in the Atlantic, but had some skill in the east Pacific.

An experimental version of SHIPS that included satellite observations was tested during the 2002 and 2003 seasons. New predictors included brightness temperature information from Geostationary Operational Environmental Satellite (GOES) channel 4 (10.7 μm) imagery, and oceanic heat content (OHC) estimates inferred from satellite altimetry observations. The OHC estimates were only available for the Atlantic basin. The GOES data significantly improved the east Pacific forecasts by up to 7% at 12–72 h. The combination of GOES and satellite altimetry improved the Atlantic forecasts by up to 3.5% through 72 h for those storms west of 50°W.

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Brian K. Haus
,
Lynn K. Shay
,
Paul A. Work
,
George Voulgaris
,
Rafael J. Ramos
, and
Jorge Martinez-Pedraja

Abstract

Wave-height observations derived from single-site high-frequency (HF) radar backscattered Doppler spectra are generally recognized to be less accurate than overlapping radar techniques but can provide significantly larger sampling regions. The larger available wave-sampling region may have important implications for observing system design. Comparison of HF radar–derived wave heights with acoustic Doppler profiler and buoy data revealed that the scale separation between the Bragg scattering waves and the peak energy-containing waves may contribute to errors in the single-site estimates in light-to-moderate winds. A wave-height correction factor was developed that explicitly considers this scale separation and eliminates the trend of increasing errors with increasing wind speed.

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Lynn K. Shay
,
Thomas M. Cook
,
Zachariah R. Hallock
,
Brian K. Haus
,
Hans C. Graber
, and
Jorge Martinez

Abstract

As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September–October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km × 44 km domain. Comparisons to subsurface current measurements at 1–2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and −3 cm s−1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8.

Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M 2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S 2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K 1, O 1) tidal constituents were considerably weaker with amplitudes of 1–5 cm. From 19 days of HF-derived surface currents, the M 2 and S 2 tidal current amplitudes had a maximum of about 50 and 8 cm s−1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s−1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M 2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M 2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M 2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.

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