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A Synoptic Climatology of Summertime Heat and Humidity in the Piedmont Region of North Carolina

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A Synoptic Climatology of Summertime Heat and Humidity in the Piedmont Region of North Carolina 674 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 45 A Synoptic Climatology of Summertime Heat and Humidity in the Piedmont Region of North Carolina FAN CHEN AND CHARLES E. KONRAD II Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina (Manuscript submitted 10 September 2004, in final form 14 August 2005) ABSTRACT The synoptic patterns and boundary layer conditions over a range of antecedent periods associated with the summertime hot events for the years 1951–93 are examined. A hot event is defined as a single day with the highest average daily temperature within a surrounding 5-day window. Among these events, four event subtypes were determined on the basis of extreme values of temperature and/or dewpoint. Composite synoptic maps and vertical profiles of atmospheric variables are produced to distinguish the hottest and moistest events. The hot events, including the extreme categories, are influenced by similar large-scale circulation features. The region is under the control of the Bermuda high, which is centered off the coast of Florida and in the Atlantic Ocean. An upstream 500-hPa ridge produces subsidence and adiabatic warming in the midlevels of the troposphere. Composite patterns of the hottest and moistest events indicate stronger upstream 500-hPa ridging and upper-level subsidence, which suggest greater suppression of local convection and reduction in the upward turbulence transfer of surface sensible heat and water vapor. The moistest events are tied to considerably greater antecedent precipitation, which suggests increased evapo- transpiration and accumulation of water vapor near the ground. The extreme hot and humid events are also associated with greater accumulated precipitation hours in the antecedent periods, especially on a 30-day scale. The hottest events also have less sky cover in the 30-day antecedent period, allowing more insolation and surface heating. The extreme events also have greater atmospheric thickness, lighter winds, and greater westerly component in the winds. Synoptic analysis shows that low-level thermal and moisture advection are not significant contributors to the heat and moisture in the extreme events of the Piedmont region. 1. Introduction varies regionally across the United States. However, Davis et al. (2004) indicate that climate-induced mor- Among the possible effects of climate changes is the tality calculated on a monthly scale during the 1990s higher frequency of extreme temperature and atmo- showed little variation across the United States. spheric moisture events (Houghton et al. 2001; Del- Heat waves have a typical duration of a week and worth et al. 1999). Heat waves of extreme high tem- often occur in connection with long-term drought, perature and humidity are one of the major natural which may extend for months or even years (Chang and hazards that cause severe human mortalities and eco- Wallace 1987). There are cases, however, in which heat nomic damages in the United States and worldwide waves develop in the absence of drought (Lyon and (Kalkstein 1991; De et al. 2004). It is also believed that Dole 1995). As precipitation deficits increase, daytime a human-induced climate change would increase sum- high temperatures increase because of the increase in mer mortality despite the development of acclimatiza- the sensible heat fluxes as a result of the depletion of tion (McMichael et al. 1996; Kalkstein and Greene soil moisture and the reduction of evapotranspiration 1997). Greene and Kalkstein (1996) show that the re- (ET) from vegetation. Huang and van den Dool (1993), lationship between mortality and high heat/humidity for example, showed that there is a negative relation- ship between precipitation and drought across the Mid- western United States. Namias (1982, 1991) has sug- gested that a positive feedback loop can be set up Corresponding author address: Dr. Charles E. Konrad II, De- partment of Geography, University of North Carolina at Chapel whereby long-term drought conditions promote and Hill, Chapel Hill, NC 27599-3220. maintain anticyclonic conditions that further inhibit E-mail: [email protected] precipitation. Research by Lyon and Dole (1995) and © 2006 American Meteorological Society Unauthenticated | Downloaded 09/26/21 10:36 AM UTC JAM2345 MAY 2006 C H E N A N D KONRAD 675 others indicates that there is a complex interplay be- tween local (e.g., decreased ET and a higher sensible heat flux) and remote forcings in the development and maintenance of heat-wave droughts. In the investiga- tion of two heat-wave droughts in the central United States, they found that remote forcings played a rela- tively greater role in the earlier stages of the event as local forcing became more significant later in the event. In one of the events, anomalously dry conditions during the prior spring contributed to the desiccation of veg- etation later in the summer and an increase in the sen- sible heat flux (i.e., hotter daytime temperatures). Several studies (Kunkel et al. 1996; Livezey and Tinker 1996; Palecki et al. 2001) have identified the synoptic conditions contributing to the buildup of ex- cessive heat and moisture in two deadly Midwest heat waves. They identified the 1) presence of an upper- tropospheric ridge that induces descending air and adiabatic warming; 2) advection of warm air; and 3) FIG. 1. Shaded relief map of the study area and locations of strong solar insolation over an urban heat island. It is stations. interesting, however, that these heat waves were not tied to dry conditions in the boundary layer (i.e., drought), but rather to abundant soil moisture in the 2. Study area upwind direction, which caused vigorous evapotranspi- Hot events in this study are based on the surface ration and moistening of the lower atmosphere. Fur- weather observations from the Raleigh–Durham Inter- thermore, a low-level capping inversion associated with national Airport (RDU), which is situated in a rapidly subsidence aloft effectively trapped the atmospheric urbanizing region between the cities of Raleigh and water vapor, allowing dewpoint temperatures to reach Durham in east-central North Carolina (Fig. 1). This record levels. In the southern United States, Henderson area lies in the eastern Piedmont, a region of gently (1995) showed that warm outbreaks are tied to a rolling terrain with elevations ranging between 60 and strengthened Bermuda high and increased subsidence, 250 m. The northeast–southwest-trending Blue Ridge which promote clearer skies and greater solar insola- Mountains are situated approximately 100 km north- tion. west of the area. The RDU site was chosen because it It is not clear if hot episodes in the southeastern has an excellent record of hourly observations of tem- United States correlate with drought or exceptionally perature and dewpoint. In a pilot study, the same analy- dry or moist boundary layer conditions. Moreover, the ses were also carried out for Charlotte, another Pied- associations between the synoptic environment and the mont city about 270 km southwest of Raleigh. Because occurrence of unusually warm conditions (hereinafter of the great similarities found in both synoptic patterns referred to as hot events) are unclear. More specifically, and boundary layer conditions, it was decided that the the role of the surface and boundary layer conditions in situations responsible for the hot events defined at mediating these relationships is not known. The objec- RDU are representative of those for the Piedmont re- tive of this study, therefore, is to identify relationships gion in general. With exception to the last several years between temperature/humidity, and synoptic/boundary of the study period, the local area around RDU was layer conditions in a large sample of hot events in the largely rural in character with patches of fields and Piedmont region of North Carolina. The following two early successional forests. Any urban heat island effects questions will be addressed: 1) What concurrent/ were therefore confined to those associated with the antecedent synoptic features and boundary layer con- sensible heating of the airport’s structures and runways. ditions best distinguish exceptionally hot and moist To test for urban heat island effects, comparisons were days from moderately warm summer days? On what made between the mean annual summer season tem- temporal scales do the greatest distinctions occur? 2) peratures at RDU relative to three cooperative ob- What variables most effectively distinguish hot, dry server sites in the broader region. The comparisons re- days from hot, moist days? vealed that the temperatures at RDU were slightly Unauthenticated | Downloaded 09/26/21 10:36 AM UTC 676 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 45 warmer than the cooperative observer sites on average (0.8°C); however, no secular trend was noted during the study period. In particular, the highest mean tempera- ture differences (1.7°C) were noted from the mid-1960s to the mid-1970s, while the lowest differences (0.4°C) were found in the late 1950s to early 1960s and the late 1980s and early 1990s. These differences may be related to several very localized movements of the weather sta- tion during the study period. 3. Data and methods FIG. 2. A sample temperature time series in which three hot Mean daily values of surface air temperature and events (gray circles) are defined. dewpoint at the National Weather Service (NWS) RDU station were calculated from the 3-hourly data from a quality-controlled dataset by Robinson (1998). 3) hot, humid (40)—both temperature and dewpoint in These data were extracted for the summer months the top quartile, and (June–August) of the period of 1951–93. The tempera- 4) hot, dry (40)—temperature in the top quartile and ture data alone were organized into time series and dewpoint in the lower quartile. used to identify the hot events for this study. A moving- windows approach was used to identify temperature Comparisons were made between the events in each maxima in the time series.
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