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Robert J. Chant
,
Wayne R. Geyer
,
Robert Houghton
,
Elias Hunter
, and
James Lerczak

Abstract

A series of dye releases in the Hudson River estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales. Dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring–neap cycle. Diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer. On flood the dye extended vertically through the bottom mixed layer, and its concentration decreased abruptly near the base of the pycnocline, usually at a height corresponding to a velocity maximum. Boundary layer growth is consistent with a one-dimensional, stress-driven entrainment model. A model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u 2 */N , where u * and N are the bottom friction velocity and buoyancy frequency above the boundary layer. The model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0.06N u 2 * or, based on the structure of the boundary layer equal to 0.1N BL u 2 *, where N BL is the buoyancy frequency across the flood-tide boundary layer. Estimates of shear production and buoyancy flux indicate that the flux Richardson number in the flood-tide boundary layer is 0.1–0.18, consistent with the model indicating that the flux Richardson number is between 0.1 and 0.14. During ebb, the boundary layer was more stratified, and its vertical extent was not as sharply delineated as in the flood. During neap tide the rate of mixing during ebb was significantly weaker than on flood, owing to reduced bottom stress and stabilization by stratification. As tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood. Tidal straining modestly increased the entrainment rate during the flood, and it restratified the boundary layer and inhibited mixing during the ebb.

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William E. Shenk
,
Herbert E. Hunter
,
Frederick V. Menkello
,
Robert Holub
, and
Vincent V. Salomonson

Abstract

An objective statistical procedure has been developed using satellite infrared window radiation measurements to estimate the central pressure (Pc ), the deviation of the central pressure from the climatological normal (ΔP), the intensity (I = ∇2 P), and the deepening or filling rate (dP/dt) of extratropical cyclones. The independent variables for 40 cylones over the North Atlantic and Pacific Oceans were the Nimbus 2 High Resolution Infrared Radiometer (HRIR) measurements at 79 locations surrounding the center of each cyclone, the date, and the geographical location of the center. Optimum empirical orthogonal functions were used to reduce the dimensionality and establish the regression relationship between the cyclone parameters and the radiation measurements for 30 of the cyclones. The remaining 10 cyclones were used to test the accuracy of the regression relationship. When the test cyclones were well represented by the cyclones in the sample employed to establish the relationship, a standard error of estimate for Pc of 6 mb was achieved for the test cyclones with slightly lower percentage accuracies for ΔP and I. An a priori decision could be made for each test cyclone regarding the probable success of parameter estimation. This was dependent on how well the test cyclone was represented by the orthogonal functions derived by the cyclones used to establish the regression equation.

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Harold L. Crutcher
,
James C. Hunter
,
Robert A. Sanders
, and
Saul Price

A numerical index of vorticity at 700 mb is obtained from synoptic maps and shown to be highly correlated with the observed heights of cumulus cloud-tops on the Washington-Bermuda airways route. The usefulness of this relationship in forecasting is supported by tests made on independent data.

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New York City's Vulnerability to Coastal Flooding

Storm Surge Modeling of Past Cyclones

Brian A. Colle
,
Frank Buonaiuto
,
Malcolm J. Bowman
,
Robert E. Wilson
,
Roger Flood
,
Robert Hunter
,
Alexander Mintz
, and
Douglas Hill

New York City, New York (NYC), is extremely vulnerable to coastal flooding; thus, verification and improvements in storm surge models are needed in order to protect both life and property. This paper highlights the Stony Brook Storm Surge (SBSS) modeling system. It utilizes surface winds and sea level pressures from the fifth-generation Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) or the Weather Research and Forecasting (WRF) model to drive the Advanced Circulation Model for Coastal Ocean Hydrodynamics (ADCIRC). For this study, the MM5 is utilized at 12-km grid spacing and ADCIRC is run on an unstructured grid down to ~10-m resolution in areas around Long Island and NYC.

This paper describes the SBSS and its performance across the NYC region during the 11–12 December 1992 nor'easter and Tropical Storm Floyd on 16–17 September 1999. During the 1992 event, east-northeasterly surface winds of 15–25 m s−1 (30–50 kts) persisted for nearly 24 h, while hurricane-force winds (35–40 m s−1) occurred for a few hours just south of western Long Island. This created a 1.0–1.5-m storm surge around NYC and western Long Island Sound over three tidal cycles. ADCIRC successfully simulated the peak water levels to within ~10%, and it realistically simulated some of the flooding across lower Manhattan. The surface winds for Tropical Storm Floyd were only 5–10 m s−1 weaker than the 1992 event, but no coastal flooding occurred during Floyd, because the storm approached during a low tide. Additional Floyd simulations were completed by shifting the storm's landfall to the spring high tide the previous week, and by doubling the wind speed to mimic a category-1 hurricane. A combination of the spring high tide and a category-1 hurricane scenario during Floyd would have resulted in moderate flooding at several locations around NYC.

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John A. Knox
,
Jared A. Rackley
,
Alan W. Black
,
Vittorio A. Gensini
,
Michael Butler
,
Corey Dunn
,
Taylor Gallo
,
Melyssa R. Hunter
,
Lauren Lindsey
,
Minh Phan
,
Robert Scroggs
, and
Synne Brustad

Using publicly available information gleaned from over 1700 found-and-returned objects on the “Pictures and Documents found after the 27 April 2011 Tornadoes” Facebook page, the authors have created a database of 934 objects lofted by at least 15 different tornadoes during the 27 April 2011 Super Outbreak in the southeast United States. Analysis of the takeoff and landing points of these objects using GIS and high-resolution numerical trajectory modeling techniques extends previous work on this subject that used less specific information for much smaller sets of tracked tornado debris. It was found that objects traveled as far as 353 km, exceeding the previous record for the longest documented tornado debris trajectory. While the majority of debris trajectories were 10° to the left of the average tornado track vector, the longest trajectories exhibited a previously undocumented tendency toward the right of the average tornado track vector. Based on results from a high-resolution trajectory model, a relationship between this tendency and the altitude of lofting of debris is hypothesized, with the debris reaching the highest altitudes taking the rightmost trajectories. The paper concludes with a discussion of the pros and cons of using social media information for meteorological research.

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