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Surface Observations of Landfalling Hurricane Rainbands 454 MONTHLY WEATHER REVIEW VOLUME 133 Surface Observations of Landfalling Hurricane Rainbands G. D. SKWIRA National Weather Service, Lubbock, Texas J. L. SCHROEDER AND R. E. PETERSON Wind Science and Engineering Research Program, Texas Tech University, Lubbock, Texas (Manuscript received 29 October 2003, in final form 20 May 2004) ABSTRACT This study examines the rainband-scale fluctuations of various meteorological parameters for Hurricanes Bonnie (1998) and Dennis (1999). Hurricane rainbands, identified by Next Generation Weather Surveil- lance Radar-1988 Doppler (NEXRAD WSR-88D) data, are examined shortly after making landfall on the North Carolina coastline. Additionally, Wind Engineering Mobile Instrumented Tower Experiment (WEMITE) data are exploited to provide a unique look into the surface structure of the captured rainbands. The observed meteorological data suggest equivalent potential temperature minima and decreasing hurri- cane-relative inflow to be commonly associated with intensifying or mature landfalling hurricane rainbands. Available vertical thermodynamic profiles suggest the source of the lower equivalent potential temperature air to range anywhere from 750 to 869 m above the surface, assuming no entrainment. Additionally, no discernable trend in wind speed is found to accompany the rainband’s passage. 1. Introduction rection, higher wind speeds, and temperature de- creases. a. Background and objectives More recently, a number of studies (Barnes et al. Hurricane rainbands produce large amounts of rain- 1983; Willoughby et al. 1984; Barnes and Stossmeister fall and often lead to very costly and potentially fatal 1986; Powell 1990a,b; Barnes et al. 1991; and others) flooding. Additionally, since rainbands occupy such a have been completed on various hurricane rainbands large area within tropical cyclones, it is not surprising utilizing aircraft data on hurricanes located well out to that they may play an important role in the evolution sea. Many of these studies found decreasing hurricane- and intensity of the hurricane they are associated with relative inflow and decreasing equivalent potential tem- (Barnes and Stossmeister 1986). Hence, a better under- perature in conjunction with the hurricane rainbands. standing of rainbands will lead to a more complete un- Additionally, some of the studies found hurricane rain- derstanding of hurricanes as a whole. The objective of bands to be favored regions for enhanced wind speeds. this paper is to quantify the surface kinematic and ther- Overall, the studies suggested that the magnitude of the modynamic fields within landfalling hurricane rain- boundary layer modification (interruption in inflow, bands in order to gain a more complete understanding equivalent potential temperature decrease, and wind of rainbands as they impact the landmass they encroach speed increase) was directly related back to the amount upon. of convective activity, with more convectively active rainbands producing greater boundary layer modifica- b. Historical approach tion. Moreover, it has been suggested that this bound- ary layer modification, if spatially and temporally large Wexler (1947) and Ligda (1955) identified a number enough, could well have a direct impact of the overall of hurricane rainbands that tracked over Florida and intensity of the parent hurricane. determined, from available surface observations, that The purpose of this study is twofold. First, this study they contained regions of heavy rain, veering wind di- expands upon the previous knowledge by examining surface data across various rainbands in an effort to determine whether the more recent results uncovered Corresponding author address: G. D. Skwira, National Weather with aircraft data also apply to the observation made at Service, 2579 S. Loop Suite 100, Lubbock, TX 79423-1400. the surface. Second, this study examines the rainbands E-mail: [email protected] as they make landfall. The hope is to better quantify the © 2005 American Meteorological Society Unauthenticated | Downloaded 09/25/21 07:38 AM UTC MWR2866 FEBRUARY 2005 SKWIRA ET AL. 455 rainband properties where the greatest human impact is experienced, near the surface over land. c. Rainband definition Rainbands were identified through radar reflectivity data. Areas of relatively high base-level reflectivity (32.5ϩ dBZ), bordered by lower reflectivity on both sides, with a length substantially greater than its width, were chosen. This criterion was chosen to pull out the heavier rain while excluding the lighter stratiform pre- cipitation within the rainbands. In addition to the above stipulations, only rainbands that passed over the Wind Engineering Mobile Instrumented Tower Experiment (WEMITE) towers were chosen for further analysis to ensure the availability of high-resolution surface data. FIG. 1. Plot of the WEMITE 1 and the KMHX radar locations The reflectivity criterion was modeled after the 25-dBZ for Hurricane Bonnie. Also displayed is the best-fit track for Hur- cutoff employed by Barnes et al. (1983). The arbitrary ricane Bonnie [day/time (UTC)]. criterion, for this study, was raised from 25 to 32.5 dBZ to provide a better differentiation between rainbands. riod in which WEMITE data were collected. Data were Using the lower 25-dBZ criterion would have included not collected during the landfall of Tropical Storm most of the precipitation occurring with the hurricanes Dennis, but were gathered during Hurricane Dennis’ near the WEMITE towers; therefore, the lower cutoff initial pass off the North Carolina coastline. would have precluded the differentiation of individual rainbands. b. Radar Level II radar data were obtained from the National 2. Data sources Climatic Data Center (NCDC; available online at http://www.ncdc.noaa.gov/). More specifically, radar a. Wind Engineering Mobile Instrumented data from the Morehead City, North Carolina Tower Experiment (KMHX), Next Generation Weather Surveillance Ra- WEMITE is a project developed by the Texas Tech dar-1988 Doppler (NEXRAD WSR-88D) were ac- University Wind Science and Engineering program quired from 1001 UTC 26 August to 0633 UTC 27 Au- (Schroeder and Smith 2003). WEMITE was initiated in gust 1998 and from 0338 UTC 30 August to 2122 UTC 1998 and at the time consisted of one mobile tower, 30 August 1999 for Hurricanes Bonnie and Dennis, re- approximately 10 m tall, instrumented to collect tem- spectively. KMHX was the nearest functioning perature, pressure, and relative humidity information at NEXRAD with respect to the WEMITE tower location the 1.2-m level, as well as wind speed and direction for both hurricanes. The KMHX radar data were the information at multiple heights (10.7-, 6.1-, and 3.0-m primary source for identifying rainbands as they passed levels). The top anemometer height was lowered to 9.1 over the WEMITE towers. m after the 1998 season. In 1999 WEMITE also gained a second 10-m mobile tower capable of sampling me- teorological parameters similar to those of the original tower; the second tower measures wind speed at four heights (2.1, 4.0, 6.1, and 10.1 m). The weather data were acquired at 5 Hz in 1998 and at 10 Hz since then (J. L. Schroeder 2003, personal communication). The goal of WEMITE was to place the tower(s) in the vicinity of landfalling hurricanes with the objective of gathering high-resolution temperature, pressure, hu- midity, and wind data within the hurricane boundary layer. At the time of this paper, 17 tropical storms and/ or hurricanes have been intercepted. Data gathered from Hurricanes Bonnie (1998) and Dennis (1999) are employed in this paper. Figures 1 and 2 depict the lo- cation of the WEMITE tower(s) during the passage of FIG. 2. Plot of the WEMITE 1, WEMITE 2, and the KMHX Hurricanes Bonnie and Dennis, respectively, along with radar locations for Hurricane Dennis. Also displayed is the best- the storm’s best-fit track. Table 1 displays the time pe- fit track for Hurricane Dennis [day/time (UTC)]. Unauthenticated | Downloaded 09/25/21 07:38 AM UTC 456 MONTHLY WEATHER REVIEW VOLUME 133 TABLE 1. Period of data collection for WEMITE platforms during Hurricanes Bonnie and Dennis. Storm Platform Start time End time Bonnie WEMITE 1 0915 UTC 26 Aug 1998 2015 UTC 27 Aug 1998 Dennis WEMITE 1 0448 UTC 30 Aug 1999 0247 UTC 31 Aug 1999 Dennis WEMITE 2 0123 UTC 30 Aug 1999 0123 UTC 31 Aug 1999 c. Dropsondes and radiosondes WEMITE 2 tower. Five rainbands fitting the outlined criteria were selected from Dennis. The selected rain- Dropsonde data obtained from the National Hurri- bands occurred as Dennis made an initial pass off the cane Center (NHC; available online at http:// North Carolina coastline. www.nhc.noaa.gov/) and rawinsonde data obtined from the Forecast Systems Laboratory (FSL; available online 4. WEMITE data manipulation and processing at http://raob.fls.noaa.gov/) were analyzed when present near rainbands that passed over the WEMITE a. Quality control towers. The sondes were used to define the vertical The first step with the WEMITE data was to “clean structure of the atmosphere inside and outside of the it up,” since some of temperature, pressure, and rela- rainband and to quantify the available instability. tive humidity data contained “spikes.” The spikes con- sisted of large instantaneous changes in the parameters. 3. Study area Although significantly deviant, the spikes generally did not persist for more than2s(5–10 data points). When Hurricane Bonnie (1998) and Hurricane Dennis spikes were found, the values of good data on either (1999) were chosen for this rainband study primarily side of the spike were averaged and inserted in place of because high-resolution surface data were collected for the spikes. For the purpose of this process, a spike was both storms through WEMITE. The high-resolution defined as an instantaneous change (0.1- or 0.2-s surface data within the hurricane environment, coupled change, depending on the sampling rate) in tempera- with nearby radar data, provided a great opportunity to ture greater than 0.25°C, change in relative humidity quantify the low-level meteorological characteristics of greater than 1.5%, or a change in barometric pressure the landfalling rainbands.
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