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Climatological Analyses of Thunderstorms and Flash Floods in the Baltimore Metropolitan Region

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Climatological Analyses of Thunderstorms and Flash Floods in the Baltimore Metropolitan Region 88 JOURNAL OF HYDROMETEOROLOGY VOLUME 8 Climatological Analyses of Thunderstorms and Flash Floods in the Baltimore Metropolitan Region ALEXANDROS A. NTELEKOS AND JAMES A. SMITH Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey WITOLD F. KRAJEWSKI IIHR-Hydroscience and Engineering, University of Iowa, Iowa City, Iowa (Manuscript received 9 February 2006, in final form 22 June 2006) ABSTRACT The climatology of thunderstorms and flash floods in the Baltimore, Maryland, metropolitan region is examined through analyses of cloud-to-ground (CG) lightning observations from the National Lightning Detection Network (NLDN) and discharge observations from 11 U.S. Geological Survey (USGS) stream gauging stations. A point process framework is used for analyses of CG lightning strikes and the occurrences of flash floods. Analyses of lightning strikes as a space–time point process focus on the mean intensity function, from which the seasonal, diurnal, and spatial variation in mean lightning frequency are examined. Important elements of the spatial variation of mean lightning frequency are 1) initiation of thunderstorms along the Blue Ridge, 2) large variability of lightning frequency around the urban cores of Baltimore and Washington D.C., and 3) decreased lightning frequency over the Chesapeake Bay and Atlantic Ocean. Lightning frequency has a sharp seasonal maximum around mid-July, and the diurnal cycle of lightning frequency peaks between 2100 and 2200 UTC with a frequency that is more than an order of magnitude larger than the minimum frequency at 1200 UTC. The seasonal and diurnal variation of flash flood occur- rence in urban streams of Baltimore mimics the seasonal and diurnal variation of lightning. The peak of the diurnal frequency of flash floods in Moores Run, a 9.1-km2 urban watershed in Baltimore City, occurs at 2200 UTC. Analyses of the lightning and flood peak data also show a close link between the occurrence of major thunderstorms systems and flash flooding on a regional scale. 1. Introduction ments, which is tied to expansion of the drainage net- work through the storm drain system and increasing Flash flooding in urban drainage basins is an increas- impervious area, is characterized by increased sensitiv- ingly important hazard in terms of loss of life and prop- ity to short-duration rainfall rates (Smith et al. 2002, erty damage (i.e., Carpenter et al. 1999; Smith et al. 2005a,b). The altered hydrologic response associated 2002; Ogden et al. 2000). In this paper we examine the with urbanization leads to the hypothesis that warm- linkage between organized thunderstorm systems and season thunderstorm systems should become an in- flash floods in urban drainage basins through analyses creasingly important component of the climatology of of cloud-to-ground (CG) lightning observations and flash flooding. discharge data from the Baltimore, Maryland, metro- Alteration of the physical environment due to urban- politan region. ization also affects the climatology of precipitation in The questions that motivate this study arise from the urban environments (Landsberg 1956; Manley 1958; alteration of the physical environment through urban- Changnon et al. 1971; Diem and Mote 2005; Shepherd ization. The altered flood hydrology of urban catch- and Burian 2003; Moelders and Olson 2004). Observa- tional and numerical modeling studies suggest that the urban heat island (UHI), frictional effects associated Corresponding author address: James A. Smith, Princeton Uni- versity, Department of Civil and Environmental Engineering, with building canopy, and urban aerosols can affect the Princeton, NJ 08544. initiation and evolution of thunderstorm systems E-mail: [email protected] (Shepherd and Burian 2003; Bornstein and Lin 2000; DOI: 10.1175/JHM558.1 © 2007 American Meteorological Society Unauthenticated | Downloaded 09/29/21 09:15 AM UTC JHM558 FEBRUARY 2007 NTELEKOS ET AL. 89 Ϫ2 TABLE 1. USGS/BES stations used in this study along with the available data and the flood level, zo (cms km ), for each station. Station ID Station name Available data zo 01583580 Baisman Run at Broadmoor, MD Jan 2000–Dec 2004 0.09 01589330 Dead Run at Franklintown, MD Jan 2000–Oct 2004 1.02 01589100 East Branch Herbert Run at Arbutus, MD Jan 2000–Aug 2000 1.02 01589197 Gwynns Falls near Delight, MD Jan 2000–Dec 2004 0.41 01589238 Gwynns Falls Tributary at McDonogh, MD Jan 2000–Aug 2003 0.15 01583980 Minebank Run at Loch Raven, MD Jan 2000–Aug 2003 0.51 01585230 Moores Run at Radecke Avenue at Baltimore, MD Jan 1998–Aug 2003 1.76 01585225 Moores Run tributary near Todd Avenue at Baltimore, MD Jan 1998–Aug 2003 2.30 01583570 Pond Branch at Oregon Ridge Jan 2000–Nov 2004 0.11 01589340 Rognel Hgts Storm Sewer Outfall at Baltimore, MD Jan 2000–Sep 2004 1.20 01585090 Whitemarsh Run Near Fullerton, MD Jan 2000–Jun 2003 0.99 Rozoff et al. 2003; Bentley and Stallins 2005; Orville et network records the date, time, latitude, and longitude al. 2001; Baik et al. 2001; Lowry 1998; Peterson 2003; Li (decimal degrees, three decimal points accuracy), po- et al. 2004; Hafner and Kidder 1999; Van den Heever larity (kA), and multiplicity (number of strokes per and Cotton 2004). Summer convective precipitation flash) of CG lightning strikes over the conterminous anomalies over urban environments have been exam- United States. Upon detection, only CG lightning ined over several cities in the United States and world- strikes that have an effective peak current magnitude wide [see Shepherd (2005) for a recent discussion and greater than 5 kA are recorded. The network has been summary]. Although there have been numerous studies extensively used for climatological analyses of lightning of precipitation anomalies in urban environments, and thunderstorms over the United States (Carey and there have been few studies that examine thunderstorm Rutledge 2003; Orville and Huffines 2001; Orville and and flash flood climatologies for urban regions [see Un- Silver 1997). The NLDN has an overall detection effi- derwood and Schultz (2004) and Holle and Bennett ciency of about 80%–90% and a location accuracy of (1997) for studies dealing with aspects of the issue]. approximately 1 km (Cummins et al. 1998; Idone et al. We use a point process framework (Karr 1991; 1998a,b). Error characteristics of the NLDN sensors Diggle 1983; Guttorp 1995; Smith and Karr 1985) for vary with geographic location, peak current magnitude, analyses of CG lightning strikes and the occurrences of season of the year, and density of the detectors (for flash floods. Analyses of lightning strikes as a space– additional details, see discussion in the references listed time point process focus on the mean intensity function, above). For this study, a record of 14 yr (1991–2004) of from which we examine the seasonal, diurnal, and spa- lightning data from the NLDN were used. tial variation in mean lightning frequency. Cloud-to- The Baltimore Ecosystem Study (BES) is a Long- ground lightning data from 1991 to 2004 from the Na- Term Ecological Research (LTER) project that focuses tional Lightning Detection Network (NLDN) are used on the impacts of urbanization on riparian ecosystem to study the spatial and temporal frequency of thunder- form and function (see Groffman et al. 2003). A major storm systems. Discharge data during the period 2000– observational component of the BES has been the de- 04 from 11 U.S. Geological Survey (USGS) stream velopment of a dense network of stream gauging sta- gauging stations are used to analyze flash flood occur- tions in small urban watersheds by the USGS. We use rences over the Baltimore metropolitan region. discharge observations for 11 of these stations (see The structure of the paper is as follows. In section 2 Table 1). The time resolution of discharge observations the study area and data are introduced. The point pro- ranges from 1 to 15 min. The discharge data used in this cess modeling framework used for statistical analysis of study cover the 5 year period from 2000 to 2003 or 2004, lightning and flash flood occurrences is presented in with the exception of the Moores Run station at section 3. Results follow in section 4 and conclusions Radecke Avenue (hereafter abbreviated as “Moores are summarized in section 5. Run”) and its tributary, for both of which the observing period is 1998–2003. 2. Data and study region We concentrate our study on a region centered over the Baltimore metropolitan area (Fig. 1). The urban The NLDN (Cummins et al. 1998; Orville et al. 2002) environment of Baltimore and Washington D.C., the consists of 106 CG lightning sensors operated and mountainous terrain of the Blue Ridge and Valley and maintained by Vaisala, Inc., in Tucson, Arizona. The Ridge province to the west, and the ocean–land bound- Unauthenticated | Downloaded 09/29/21 09:15 AM UTC 90 JOURNAL OF HYDROMETEOROLOGY VOLUME 8 FIG. 1. Study area (large white box) and subareas of focus (smaller white boxes). ary to the east make this region a complex setting for {Tk, Yk} is a space–time point process defined on examining warm-season thunderstorm systems. [0, 1] ϫ D, where the unit interval represents time dur- For analyses of the diurnal cycle of thunderstorms ing the day and the domain of study is the two- and investigation of urbanization effects on thunder- dimensional region D. We choose the beginning and storm rainfall, we focus on three subareas. These areas ending time of the day, that is, times 0 and 1, to be 1200 are depicted in Fig. 1 with the three intermediate size UTC. squares of the same latitude marked as “Piedmont,” The CG lightning sample consists of observations “Urban,” and “Ocean.” The urban area covers the Bal- from every day of the year and from multiple years. Let timore and Washington D.C., metropolitan areas.
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