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J. Egger, G. Meyers, and P. B. Wright

Abstract

Maps of regression coefficients of sea level pressure, surface wind and cloudiness on a Southern Oscillation Index are presented for Northern Hemisphere fall and winter. To cheek the significance of the fields obtained, an attempt is made to relate the wind fields to the pressure fields by assuming a balance of the pressure gradient force, the frictional force and the Coriolis force. It turns out that the most conspicuous features of both the wind and cloudiness fields can be derived from the pressure field. It is found that the Southern Oscillation Index is positively correlated with surface easterlies and downward motion near the dateline. Significant fluctuations of the meridional wind are found over the Coral Sea and in the vicinity of the ITCZ.

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Peter B. Wright, John M. Wallace, Todd P. Mitchell, and Clara Deser

Abstract

Relationships among the atmospheric phenomena associated with the Southern Oscillation and El Niño are investigated, using the Comprehensive Ocean-Atmosphere Data Set (COADS) of marine surface observations from ships of opportunity and the World Monthly Surface [Land] Station Climatology (WMSSC) for the period 1950-79. Annual mean (April–March) sea level pressure at Darwin, Australia is used as an index of the Southern Oscillation. Results are based on simple linear correlation techniques stratified by season as in the Rasmusson and Carpenter (1982) composite.

Correlations on the order of +0.9 are observed between Darwin pressure, sea surface temperature (SST) and rainfall in the equatorial central Pacific, and zonal wind in the equatorial western Pacific. Relations among these variables are strongest from July through November, when the month to month autocorrelation is also at its strongest. Sea surface temperature along the Peruvian coast and pressure in the eastern Pacific are also most strongly coupled to the Southern Oscillation during thew months, which correspond to the cool season in that region.

The amplitude of the tropical pressure and central Pacific SST anomalies associated with the Southern Oscillation appear to be just as large and the relationships between them just as coherent during positive excursions of the Southern Oscillation (cold episodes) as during negative excursions (warm episodes).

Lead/lag relationships among climatic variables associated with the Southern Oscillation and El Niño events along the South American coast are also examined in the context of the same seasonal stratification. Our results are generally consistent with the traditional view that the Southern Oscillation is, to first order, a standing oscillation with geographically fixed nodes and antinodes.

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C. D. Bosma, D. B. Wright, P. Nguyen, J. P. Kossin, D. C. Herndon, and J. M. Shepherd
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Christopher D. Bosma, Daniel B. Wright, Phu Nguyen, James P. Kossin, Derrick C. Herndon, and J. Marshall Shepherd

Abstract

Recent tropical cyclones (TCs) have highlighted the hazards that TC rainfall poses to human life and property. These hazards are not adequately conveyed by the commonly used Saffir–Simpson scale. Additionally, while recurrence intervals (or, their inverse, annual exceedance probabilities) are sometimes used in the popular media to convey the magnitude and likelihood of extreme rainfall and floods, these concepts are often misunderstood by the public and have important statistical limitations. We introduce an alternative metric—the extreme rain multiplier (ERM), which expresses TC rainfall as a multiple of the climatologically derived 2-yr rainfall value. ERM allows individuals to connect (“anchor,” in cognitive psychology terms) the magnitude of a TC rainfall event to the magnitude of rain events that are more typically experienced in their area. A retrospective analysis of ERM values for TCs from 1948 to 2017 demonstrates the utility of the metric as a hazard quantification and communication tool. Hurricane Harvey (2017) had the highest ERM value during this period, underlining the storm’s extreme nature. ERM correctly identifies damaging historical TC rainfall events that would have been classified as “weak” using wind-based metrics. The analysis also reveals that the distribution of ERM maxima is similar throughout the eastern and southern United States, allowing for both the accurate identification of locally extreme rainfall events and the development of regional-scale (rather than local-scale) recurrence interval estimates for extreme TC rainfall. Last, an analysis of precipitation forecast data for Hurricane Florence (2018) demonstrates ERM’s ability to characterize Florence’s extreme rainfall hazard in the days preceding landfall.

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C. W. Wright, E. J. Walsh, D. Vandemark, W. B. Krabill, A. W. Garcia, S. H. Houston, M. D. Powell, P. G. Black, and F. D. Marks

Abstract

The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane's inner core over open water. The NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane research aircraft at 1.5-km height acquired the open-ocean data on 24 August 1998 when Bonnie, a large hurricane with 1-min sustained surface winds of nearly 50 m s−1, was about 400 km east of Abaco Island, Bahamas. The NOAA aircraft spent more than five hours within 180 km of the eye and made five eye penetrations. Grayscale coded images of Hurricane Bonnie wave topography include individual waves as high as 19 m peak to trough. The dominant waves generally propagated at significant angles to the downwind direction. At some positions, three different wave fields of comparable energy crossed each other. Partitioning the SRA directional wave spectra enabled determination of the characteristics of the various components of the hurricane wave field and mapping of their spatial variation. A simple model was developed to predict the dominant wave propagation direction.

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C. W. Wright, E. J. Walsh, W. B. Krabill, W. A. Shaffer, S. R. Baig, M. Peng, L. J. Pietrafesa, A. W. Garcia, F. D. Marks Jr., P. G. Black, J. Sonntag, and B. D. Beckley

Abstract

Over the years, hurricane track forecasts and storm surge models, as well the digital terrain and bathymetry data they depend on, have improved significantly. Strides have also been made in the knowledge of the detailed variation of the surface wind field driving the surge. The area of least improvement has been in obtaining data on the temporal/spatial evolution of the mound of water that the hurricane wind and waves push against the shore to evaluate the performance of the numerical models. Tide gauges in the vicinity of the landfall are frequently destroyed by the surge. Survey crews dispatched after the event provide no temporal information and only indirect indications of the maximum water level over land. The landfall of Hurricane Bonnie on 26 August 1998, with a surge less than 2 m, provided an excellent opportunity to demonstrate the potential benefits of direct airborne measurement of the temporal/spatial evolution of the water level over a large area. Despite a 160-m variation in aircraft altitude, an 11.5-m variation in the elevation of the mean sea surface relative to the ellipsoid over the flight track, and the tidal variation over the 5-h data acquisition interval, a survey-quality global positioning system (GPS) aircraft trajectory allowed the NASA scanning radar altimeter carried by a NOAA hurricane research aircraft to demonstrate that an airborne wide-swath radar altimeter could produce targeted measurements of storm surge that would provide an absolute standard for assessing the accuracy of numerical storm surge models.

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D. R. Jackson, A. Gadian, N. P. Hindley, L. Hoffmann, J. Hughes, J. King, T. Moffat-Griffin, A. C. Moss, A. N. Ross, S. B. Vosper, C. J. Wright, and N. J. Mitchell

Abstract

Gravity waves (GWs) play an important role in many atmospheric processes. However, the observation-based understanding of GWs is limited, and representing them in numerical models is difficult. Recent studies show that small islands can be intense sources of GWs, with climatologically significant effects on the atmospheric circulation. South Georgia, in the South Atlantic, is a notable source of such “small island” waves. GWs are usually too small scale to be resolved by current models, so their effects are represented approximately using resolved model fields (parameterization). However, the small-island waves are not well represented by such parameterizations, and the explicit representation of GWs in very-high-resolution models is still in its infancy. Steep islands such as South Georgia are also known to generate low-level wakes, affecting the flow hundreds of kilometers downwind. These wakes are also poorly represented in models.

We present results from the South Georgia Wave Experiment (SG-WEX) for 5 July 2015. Analysis of GWs from satellite observations is augmented by radiosonde observations made from South Georgia. Simulations were also made using high-resolution configurations of the Met Office Unified Model (UM). Comparison with observations indicates that the UM performs well for this case, with realistic representation of GW patterns and low-level wakes. Examination of a longer simulation period suggests that the wakes generally are well represented by the model. The realism of these simulations suggests they can be used to develop parameterizations for use at coarser model resolutions.

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E. J. Walsh, C. W. Wright, D. Vandemark, W. B. Krabill, A. W. Garcia, S. H. Houston, S. T. Murillo, M. D. Powell, P. G. Black, and F. D. Marks Jr.

Abstract

The NASA Scanning Radar Altimeter (SRA) flew aboard one of the NOAA WP-3D hurricane research aircraft to document the sea surface directional wave spectrum in the region between Charleston, South Carolina, and Cape Hatteras, North Carolina, as Hurricane Bonnie was making landfall near Wilmington, North Carolina, on 26 August 1998. Two days earlier, the SRA had documented the hurricane wave field spatial variation in open water when Bonnie was 400 km east of Abaco Island, Bahamas. Bonnie was similar in size during the two flights. The maximum wind speed was lower during the landfall flight (39 m s−1) than it had been during the first flight (46 m s−1). Also, Bonnie was moving faster prior to landfall (9.5 m s−1) than when it was encountered in the open ocean (5 m s−1). The open ocean wave height spatial variation indicated that Hurricane Bonnie would have produced waves of 10 m height on the shore northeast of Wilmington had it not been for the continental shelf. The gradual shoaling distributed the wave energy dissipation process across the shelf so that the wavelength and wave height were reduced gradually as the shore was approached. The wave height 5 km from shore was about 4 m.

Despite the dramatic differences in wave height caused by shoaling and the differences in the wind field and forward speed of the hurricane, there was a remarkable agreement in the wave propagation directions for the various wave components on the two days. This suggests that, in spite of its complexity, the directional wave field in the vicinity of a hurricane may be well behaved and lend itself to be modeled by a few parameters, such as the maximum wind speed, the radii of the maximum and gale force winds, and the recent movement of the storm.

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W. L. Smith, H. E. Rvercomb, H. B. Howell, H. M. Woolf, R. O. Knuteson, R G. Decker, M. J. Lynch, E. R. Westwater, R. G. Strauch, K. P. Moran, B. Stankov, M. J. Falls, J. Jordan, M. Jacobsen, W. F. Dabberdt, R. McBeth, G. Albright, C. Paneitz, G. Wright, P. T. May, and M. T. Decker

During the week 29 October–4 November 1988, a Ground-based Atmospheric Profiling Experiment (GAPEX) was conducted at Denver Stapleton International Airport. The objective of GAPEX was to acquire and analyze atomspheric-temperature and moisture-profile data from state-of-the-art remote sensors. The sensors included a six-spectral-channel, passive Microwave Profiler (MWP), a passive, infrared High-Resolution Interferometer Sounder (HIS) that provides more than 1500 spectral channels, and an active Radio Acoustic Sounding System (RASS). A Cross-Chain Loran Atmospheric Sounding System (CLASS) was used to provide research-quality in situ thermodynamic observations to verify the accuracy and resolution characteristics of each of the three remote sensors. The first results of the project are presented here to inform the meteorological community of the progress achieved during the GAPEX field phase. These results also serve to demonstrate the excellent prospects for an accurate, continuous thermodynamic profiling system to complement NOAA's forthcoming operational wind profiler.

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G. de Boer, C. Diehl, J. Jacob, A. Houston, S. W. Smith, P. Chilson, D. G. Schmale III, J. Intrieri, J. Pinto, J. Elston, D. Brus, O. Kemppinen, A. Clark, D. Lawrence, S. C. C. Bailey, M.P. Sama, A. Frazier, C. Crick, V. Natalie, E. Pillar-Little, P. Klein, S. Waugh, J. K. Lundquist, L. Barbieri, S. T. Kral, A. A. Jensen, C. Dixon, S. Borenstein, D. Hesselius, K. Human, P. Hall, B. Argrow, T. Thornberry, R. Wright, and J. T. Kelly
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