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Peter P. Dodge and Robert W. Burpee

Abstract

Characteristics of mesoscale rainbands and echoes in radar reflectivity data recorded during the field phase of the Genesis of Atlantic Lows Experiment (GALE) are presented. The primary sources of data were radar microfilm and manually digitized radar (MDR) reports from the operational National Weather Service (NWS) radars at Cape Hatteras (HAT) and Wilmington (ILM), North Carolina. The dataset also included cloud-to-ground lightning flashes that were recorded by the network operated by the State University of New York at Albany.

The analyses included rainbands of at least 90-km length with lifetimes of at least 2 h. Nearly all of the rainbands were within 400 km of synoptic-scale or coastal fronts. Warm-sector rainbands predominated. Rain-bands were clarified by the location of their initial detection relative to the land. coastal shelf, and Gulf Stream. Rainbands were initially identified more frequently over the Gulf Stream and ten often over the coastal shelf than the corresponding fractional arms monitored by the radars.

Statistical tests determined significant differences in the sample means of the MDR and lightning data between the Gulf Stream and land regions that were largely a consequence of many more hours with MDR and lightning over the Gulf Stream. Composites relative to the beginning and ending of the rainband cases indicated that differences between the Gulf Stream and land were small shortly after the initial detection of rainbands and large just before the final detection of rainbands. The largest Gulf Stream-land disparities occurred, average, during low-level cold and dry advection at HAT.

Trunk and Bosart reported a convective echo maximum over the Gulf Stream near HAT and discussed physical processes that can account for the convective maximum. Analysis of one idealized distribution of convecton, however, supports the likely role of sampling limitations of the NWS radar network in determining the location of the convective echo maximum near HAT.

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Mark D. Powell, Peter P. Dodge, and Michael L. Black

Abstract

Hurricane Hugo struck Charleston, South Carolina, on 22 September 1989 as the most intense hurricane to affect the United States since Camille in 1969. The northeastern eyewall, which contained the maximum winds measured by reconnaissance aircraft shortly before landfall, moved inland over a relatively unpopulated area and there were few fatalities. However, no observations were available to document the surface wind distribution in this part of the storm as it continued inland.

To improve specification of surface winds in Hugo, empirically adjusted aircraft winds were combined with coastal, offshore, and inland surface observations and were input to the Ooyama objective analysis algorithm. The wind analysis at landfall was then compared with subsequent analyses at 3 and 6 h after landfall. Reconstruction of the surface wind field at landfall suggests that the maximum (∼13 min mean) surface wind at the coast was 50 m s−1 in the Bulls Bay region, ∼40 km northeast of Charleston. Surface roughness over land caused wind speeds to drop off rapidly just inland of the coast to only 50% of values measured by reconnaissance aircraft at the same location relative to the storm over water. Despite relatively rapid increases in the central sea-level pressure and decreases in the mean circulation as Hugo progressed inland, hurricane-force wind gusts extended Hugo's damage pattern well past Charlotte, North Carolina, ∼330 km inland.

Accurate determination of surface wind distribution in land-falling hurricanes is dependent upon the spatial density and quality of surface wind measurements and techniques to adjust reconnaissance flight-level winds to the surface. Improvements should allow forecasters to prepare more-accurate warnings and advisories and allow more-thorough documentation of poststorm effects. Empirical adjustments to reconnaissance aircraft measurements may replace surface data voids if the vertical profile of the horizontal wind is known. Expanded use of the airborne stepped-frequency microwave radiometer for remote sensing of ocean surface winds could fill data voids without relying upon empirical methods or models. A larger network of offshore, coastal, and inland surface platforms at standard (10-m) elevations with improved sampling strategies is envisioned for better resolution of hurricane wind fields. A rapid-response automatic station network, deployed at prearranged coastal locations by local universities with meteorology and/or wind engineering programs, could further supplement the fixed platform network and avoid the logistical problems posed by sending outside teams into threatened areas.

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Shirley T. Murillo, Wen-Chau Lee, Michael M. Bell, Gary M. Barnes, Frank D. Marks Jr., and Peter P. Dodge

Abstract

A plausible primary circulation and circulation center of a tropical cyclone (TC) can be deduced from a coastal Doppler radar using the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex algorithm. The quality of the retrieved primary circulation is highly sensitive to the accuracy of the circulation center that can only be estimated from the degree of scattering of all possible centers obtained in GBVTD-simplex analyses from a single radar in real TCs. This study extends previous work to examine the uncertainties in the GBVTD-simplex-derived circulation centers and the GBVTD-derived primary circulations in Hurricane Danny (1997) sampled simultaneously from two Doppler radars [Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) in Mobile, Alabama, and Slidell, Louisiana] for 5 h.

It is found that the mean difference between the individually computed GBVTD-simplex-derived centers is 2.13 km, similar to the estimates in previous studies. This value can be improved to 1.59 km by imposing time continuity in the radius of maximum wind, maximum mean tangential wind, and the center position in successive volumes. These additional physical criteria, not considered in previous work, stabilized the GBVTD-simplex algorithm and paved the way for automating the center finding and wind retrieval procedures in the future.

Using the improved set of centers, Danny’s axisymmetric tangential wind structures retrieved from each radar showed general agreement with systematic differences (up to 6 m s−1) in certain periods. The consistency in the wavenumber-1 tangential winds was not as good as their axisymmetric counterparts. It is suspected that the systematic differences in the axisymmetric tangential winds were caused by the unresolved wavenumber-2 sine components rather than from the relatively small cross-beam mean wind components in Danny.

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Paul R. Harasti, Colin J. McAdie, Peter P. Dodge, Wen-Chau Lee, John Tuttle, Shirley T. Murillo, and Frank D. Marks Jr.

Abstract

The NOAA/NWS/NCEP/Tropical Prediction Center/National Hurricane Center has sought techniques that use single-Doppler radar data to estimate the tropical cyclone wind field. A cooperative effort with NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division and NCAR has resulted in significant progress in developing a method whereby radar display data are used as a proxy for a full-resolution base data and in improving and implementing existing wind retrieval and center-finding techniques. These techniques include the ground-based velocity track display (GBVTD), tracking radar echoes by correlation (TREC), GBVTD- simplex, and the principal component analysis (PCA) methods.

The GBVTD and TREC algorithms are successfully applied to the Weather Surveillance Radar-1988 Doppler (WSR-88D) display data of Hurricane Bret (1999) and Tropical Storm Barry (2001). GBVTD analyses utilized circulation center estimates provided by the GBVTD-simplex and PCA methods, whereas TREC analyses utilized wind center estimates provided by radar imagery and aircraft measurements. GBVTD results demonstrate that the use of the storm motion as a proxy for the mean wind is not always appropriate and that results are sensitive to the accuracy of the circulation center estimate. TREC results support a previous conjecture that the use of polar coordinates would produce improved wind retrievals for intense tropical cyclones. However, there is a notable effect in the results when different wind center estimates are used as the origin of coordinates. The overall conclusion is that GBVTD and TREC have the ability to retrieve the intensity of a tropical cyclone with an accuracy of ∼2 m s−1 or better if the wind intensity estimates from individual analyses are averaged together.

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Christopher W. Landsea, James L. Franklin, Colin J. McAdie, John L. Beven II, James M. Gross, Brian R. Jarvinen, Richard J. Pasch, Edward N. Rappaport, Jason P. Dunion, and Peter P. Dodge

Hurricane Andrew of 1992 caused unprecedented economic devastation along its path through the Bahamas, southeastern Florida, and Louisiana. Damage in the United States was estimated to be $26 billion (in 1992 dollars), making Andrew one of the most expensive natural disasters in U.S. history. This hurricane struck southeastern Florida with maximum 1-min surface winds estimated in a 1992 poststorm analysis at 125 kt (64 m s−1). This original assessment was primarily based on an adjustment of aircraft reconnaissance flight-level winds to the surface.

Based on recent advancements in the understanding of the eyewall wind structure of major hurricanes, the official intensity of Andrew was adjusted upward for five days during its track across the Atlantic Ocean and Gulf of Mexico by the National Hurricane Center Best Track Change Committee. In particular, Andrew is now assessed by the National Hurricane Center to be a Saffir–Simpson Hurricane Scale category-5 hurricane (the highest intensity category possible) at its landfall in southeastern Florida, with maximum 1-min winds of 145 kt (75 m s−1). This makes Andrew only the third category-5 hurricane to strike the United States since at least 1900. Implications for how this change impacts society's planning for such extreme events are discussed.

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