674 JOURNAL OF APPLIED AND VOLUME 45

A Synoptic Climatology of Summertime Heat and 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 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 . The moistest events are tied to considerably greater antecedent , 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 , 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 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 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

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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- 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. In particular, the tempera- category and the events in the remainder of the sample ture mean for each day of time series was compared (e.g., the 40 hottest events were compared with the with the daily temperature means for the 2 days prior to remaining 594 events.) These comparisons reveal the and following the given day. If the temperature mean combination of ingredients that most distinguish a was greater, the day was defined as a hot event. Each given event type. event was therefore defined by the occurrence of a The daily surface weather data were supplemented daily mean temperature within a surrounding 5-day-or- by hourly precipitation data obtained from the National greater time window (e.g., Fig. 2). This definition en- Climatic Data Center (NCDC). Additionally, hourly sures that any two hot events are at least 3 days apart. weather conditions were obtained from NCDC Surface In most cases, the 3-day-or-greater separation period Airways Data CD-ROMs (EarthInfo 2001). To identify ensures that adjacent events are not connected with the the synoptic features associated with hot events, grid- same synoptic-scale circulation feature. However, ex- ded (2.5° latitude ϫ 2.5° longitude mesh), twice-daily tended periods of hot weather (e.g., heat wave) typi- synoptic fields were extracted from the National Cen- cally have durations on the order of a week (Chang and ters for Environmental Prediction–National Center for Wallace 1987), and therefore could conceivably be rep- Atmospheric Research (NCEP–NCAR) 40-year re- resented by two consecutive hot events in the tempera- analysis dataset (Kalnay et al. 1996). These fields ture time series. It should be noted that a hot event is (Table 1) were spatially interpolated onto a 1332 km ϫ not necessarily hot in an absolute measure, but is the 1332 km floating grid at a 222-km interval centered warmest day within a 5-day period or greater. As a over the eastern Piedmont of North Carolina. Using the result, these events are evenly distributed across the 0000 and 1200 UTC gridded synoptic fields, a temporal summer months, rather than clustering in the hottest interpolation was undertaken to estimate field values month of July. Using these criteria, 634 events were for the 1500 LST, which is typically the map time im- identified during the study period. mediately prior to the hottest time of the day. Synoptic Among these hot events, we have focused on the fields were extracted for the 6-day period prior to each extreme ones in terms of temperature and humidity or event. An inverse distance technique was used to carry combinations of both variables. Four extreme-event out all spatial and temporal interpolations. types are defined, with the number of events in paren- Composite (i.e., mean) synoptic fields were created theses in each category, as indicated below: for each category and then compared. These compos- 1) hottest (63)—temperature at or above the 90th per- ites highlight synoptic features common to each cat- centile, egory. Twice-daily radiosonde data from the station at 2) moistest (63)—dewpoint at or above the 90th per- Greensboro (GSO, WBAN 13723), North Carolina, centile, were used to represent the vertical profiles of tempera-

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TABLE 1. Synoptic fields and local weather variables examined.

NCEP–NCAR field Unit Surface weather Unit 500-hPa height m Surface temperature °C 1000-hPa height m Surface dewpoint °C 1000–500-hPa thickness m Sky cover % 200-hPa divergence 1 ϫ 10Ϫ6 sϪ1 Total rainfall hours h 850-hPa u component m sϪ1 Total precipitation cm 850-hPa ␷ component m sϪ1 Surface u component m sϪ1 Surface ␷ component m sϪ1 Surface direction ° 850-hPa moisture advection g kgϪ1 (12 h)Ϫ1 Surface moisture advection g kgϪ1 (12 h)Ϫ1 850-hPa thermal advection °C (12 h)Ϫ1 Surface thermal advection °C (12 h)Ϫ1 850-hPa relative humidity % ture, humidity, and wind associated with each event in play a significantly greater westerly flow, suggesting the sample. Greensboro is situated about 96 km west of that the adiabatically warmed air descending from the the Raleigh–Durham observing site (Fig. 1); it is the east slopes of the Blue Ridge Mountains may be ad- nearest site in the radiosonde network. It should be vected to the study area. The hottest events are also noted that mesoscale variations in the vertical profiles significantly higher in average dewpoint than the rest of exist between the two sites, which cannot be readily the sample. The moistest events have a significantly estimated in this work. Composite vertical profiles were greater southerly component that may be tied to mois- also produced. The earliest available sounding data ture advection from the Gulf of Mexico. No significant were in 1954 (i.e., no data for the events during the first secular trends are noted in the frequencies of the hot- year of the study period). To assess antecedent condi- test and moistest events during the study period (Fig. tions in the boundary layer, temperature, dewpoint, 3). A marked peak is observed in the frequencies of and sky cover were averaged for a range of time periods these extreme events between 1977 and 1981. More- (i.e., 2–30 days) prior to each event. Additionally, rain- over, their frequencies exhibit decadal-scale variations fall hour and precipitation totals were determined for with higher frequencies observed several years before each antecedent time period. Comparisons were made and during each decadal change. between the means of the variables in order to identify the variables that most effectively distinguish the b. Circulation patterns boundary layer character between each extreme-event Composites of the entire event sample reveal that type and the remainder of the sample. North Carolina is typically situated under the down- stream limb of a 500-hPa ridge in a region of north- 4. Results westerly flow aloft (Fig. 4). This pattern is similar to the summer season composite of 500-hPa flow, except the a. General characteristics of hot events amplitude of the upstream ridge in the hot events is Table 2 summarizes the surface weather variables as- higher. The 1000-hPa-height composite indicates an an- sociated with each event type. The hottest events dis- ticyclone (i.e., the Bermuda high) centered south of

TABLE 2. Mean values of temperature, dewpoint, and wind for hot events. Boldface values indicate extreme-event means that are significantly different from the remainder of the sample. The U, V vectors represent wind components from due west and south respectively.

Temperature (°C) Dewpoint (°C) Wind speed (m sϪ1) U (m sϪ1) V (m sϪ1) Hottest 29.7 21.6 2.9 2.0 1.6 Moistest 27.7 23.1 2.8 1.6 1.9 Hot, dry 28.6 19.3 2.8 1.7 1.2 Hot, humid 28.6 23.1 2.7 1.7 1.6 Overall 26.2 20.3 2.9 1.3 1.5

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FIG. 3. Histogram depicting the annual frequencies of the hottest (solid) and moistest (transparent) events.

North Carolina and off the coast of Florida. This pat- est departures are observed to the north in Pennsylva- tern is associated with southwesterly surface flow nia (Fig. 6). The lower-tropospheric heights are also (Table 2); however, the winds aloft, which are not sub- higher than those found in the remainder of the sample. ject to the influences of surface friction (i.e., geo- The surface winds in association with the moistest strophic winds), are more westerly and may be tied to events show that both the u and v components (i.e., downsloping flow off of the Blue Ridge Mountains to southwesterly surface wind) are significantly greater the west. The composite 200-hPa divergence map indi- than the rest of the sample (Table 2). Calculated com- cates upper-level convergence and midlevel subsidence posites of 850-hPa moisture advection reveal little in aloft over the region. This subsidence provides adia- the way of moisture or dry air advection. Similar to the batic warming in the atmospheric column above the hottest events, stronger 200-hPa convergence is present Piedmont. and the greatest departures are observed in the western Composite difference maps were developed to illus- Atlantic Ocean. trate differences between the synoptic patterns of the The mean difference fields for the hot, moist and hot, extreme events and the remainder of the sample. Dif- dry events closely resemble the differences highlighted ferences were calculated by subtracting the mean value in the hottest events. To distinguish these two event of events outside a given event type from that of the types, the mean values of the synoptic fields in the hot, event type of interest. For the hottest events (Fig. 5), dry events were subtracted from those in the hot, humid composite 500-hPa height is substantially higher events (Fig. 7). The composite differences reveal that throughout the eastern United States, with the greatest the hot, moist events display stronger upper-level sub- differences identified over the Midwest (i.e., toward the sidence and greater 1000- and 500-hPa heights than the axis of the upstream ridge). This indicates that the 500- hot, dry events, with the greatest differences observed hPa flow associated with the hottest events is slightly off the southeast coast. more northwesterly relative to the cooler events. Com- To assess the potential influence of downsloping posite 1000-hPa heights are also higher in the hottest winds off the Blue Ridge Mountains, the mean 1000– events with the greatest increases found to the north- 850-hPa wind was calculated for a 6-day period prior to west (i.e., a slight northwestward extension of the sub- the occurrence of each event. The southwestern termi- tropical high). Stronger upper-level convergence (i.e., nus of the Blue Ridge Mountains is situated roughly at negative values of 200-hPa divergence) is present over a 252° (i.e., 72° west of south) heading in the study area the region with the maximum difference observed off (see Fig. 1); therefore, it is assumed that winds north of the coast of New Jersey. This is consistent with higher- this heading are tied to downsloping flow. Over 60% of amplitude ridging to the west (i.e., higher anticyclonic the hottest events are associated with downsloping vorticity advection) and suggests higher values of adia- flow, which is a significantly greater frequency than the batic warming in the air column above the event. remainder of the sample (Fig. 8). Slightly more than For the moistest events, the composites indicate 40% of the moistest events are tied to downsloping higher 500-hPa heights, however, in this case, the great- flow. This is slightly more than the rest of the sample,

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FIG. 5. Same as Fig. 4, but for the hottest events in the sample. FIG. 4. Composite (a) 500-hPa and (b) 1000-hPa height and Dashed lines indicate the mean differences between the hottest 200-hPa divergence (°C) fields for all hot events at 1500 LST. events and the remainder of the sample.

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FIG. 7. Same as Fig. 4, but for the hot, humid events. Dashed FIG. 6. Same as Fig. 4, but for the moistest events. lines indicate mean differences between the hot, humid and the hot, dry subsamples (i.e., hot, humid minus hot, dry).

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A low-level temperature inversion is commonly present in the morning sounding of the hot events (Table 3), which can be tied at least partially to rela- tively clear conditions (i.e., radiation inversion) that characterize hot periods. The moistest events show a relatively low mean inversion height. The hot, humid events have the lowest mean inversion height of the event types, while hot, dry events have the highest. There is a very significant difference (i.e., 348 m) in the mean height of the inversions between the two event types. Most distinctive, however, is the low mean inver- sion height of the hot, moist events (i.e., also 348 m lower than the overall hot event mean). This implies FIG. 8. Proportion of events that can be tied to a downsloping that the moist air is trapped near the ground. wind during the 6-day period prior to each event. A downsloping wind is defined by the occurrence of a mean 1000–850-hPa wind that has a direction exceeding 252°, the angle defined by a straight d. Antecedent surface weather line passing from RDU to the southwestern terminus of the Blue The mean values for five surface weather variables Ridge Mountains west of the region. Dashed line with squares and triangles represent the hottest and moistest events, respectively; were calculated for a range of time periods before each the solid line with dots represents all hot events. event type and compared with the means for events associated with the remainder of the sample. Student’s t test statistics were calculated to assess the relative but the difference is not statistically significant. Also significance of these differences (Table 4). In the hot- noteworthy is the fact that the percentage of hot events test and moistest events, the most significant differ- tied to downsloping winds increases markedly during ences are found in the temperature and dewpoint vari- the 2-day period leading up to the event. In the overall ables, respectively. The high t scores (i.e., greater dif- sample, for example, roughly 23% of the events are tied ference of means) simply reflect the effects of temporal to downsloping 2 days before the hot event while over autocorrelation; however, it is interesting to note that 41% are tied to downsloping on the day of the event. the mean temperature and dewpoint in the 30 days This increase is also noted in the hottest and the moist- prior to the event are also significantly warmer and est events. moister in these respective event types. Also notewor- thy is the fact that the relative differences in the sky c. Sounding patterns cover, rain hours, and total precipitation variables are greater for the longer antecedent periods (i.e., 21–30 Composite soundings of the hottest events are com- days). In other words, the hottest events are most pared with the remainder of the sample. The hottest strongly tied to an extended antecedent period charac- events display significantly greater (p Ͻ 0.05) isobaric terized by less , rain hours, and total rainfall. The heights and temperature throughout the troposphere in moistest events are relatively less distinguishable from comparison with other events. They also have signifi- the remainder of the sample; however, these events cantly lower mean wind speeds at higher altitudes show significant associations with cloudiness and pre- (above 650 hPa) and higher mixing ratios in the lower cipitation in the 14–21-day antecedent period. troposphere (below 850 hPa). The moistest events are Because of the effect of temporal autocorrelation, also significantly warmer throughout the troposphere, with lower mean wind speeds in the upper troposphere (above 500 hPa) and higher mixing ratios in the lower– TABLE 3. Mean pressures and heights for the lowest inversion in middle troposphere (below 500 hPa). Hot, dry events the events in which a sounding was available and an inversion was have significantly higher temperature, especially in the present. lowest levels, while there is no significant difference in Pressure Height No. of events mixing ratio with other hot events throughout the pro- (hPa) (m) available file. The most distinguishing aspect for hot, moist Hottest 964 505 52/63 events is their greater mixing ratio in the lower levels of Moistest 971 415 57/63 the troposphere (Ͻ700 hPa). Relative to the hot, dry Hot, dry 945 694 33/40 events, the hot, moist events are significantly warmer Hot, humid 979 346 36/40 and with lighter wind speeds throughout troposphere. All hot events 946 694 546/634

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TABLE 4. A summary of the t scores that assess the difference in component. This suggests the influence of adiabatically means of the surface weather variables between the extreme warmed air from downslope winds upstream of the events and the remainder of the sample; t scores are shown across study region. This relationship is the strongest in the 2 a range of antecedent periods (days prior) spanning 2–30 days. Italicized values are statistically significant at the 0.05 level. Bold- days leading up to the event; however, it is also present face values highlight the highest t scores for each weather variable. across the 6-day antecedent period (Fig. 8). The hottest events are also strongly associated with lower values of Days Sky Rain Total relative humidity at 850 hPa, but higher surface dew- prior Temperature Dewpoint cover hours precipitation points. The low may be tied to middle- Hottest events vs the remainder of the sample tropospheric subsidence and upper-level convergence, 2 15.25 5.03 Ϫ5.48 Ϫ3.05 Ϫ2.5 the latter of which also effectively distinguishes the hot- 7 11.36 4.41 Ϫ5.32 Ϫ3.25 Ϫ2.92 test events. Relatively clear skies and dry conditions in Ϫ Ϫ Ϫ 14 8.65 3.38 5.36 3.6 3.5 the weeks leading up to the event are also quite signifi- 21 7.51 2.89 Ϫ5.68 Ϫ4.56 Ϫ4.23 30 6.45 2.76 Ϫ5.57 Ϫ4.51 Ϫ5.09 cant. Warm advection at 850 hPa, while significantly greater in the hottest events, displays lower t scores Moistest events vs the remainder of the sample (i.e., is a less significant distinguisher of the hottest 2 6.57 9.52 0.12 Ϫ1 0.02 events). Moreover, its mean values in the overall hot Ϫ 7 6.12 8.94 1.31 1.09 0.81 event sample are quite low, suggesting that it is not an 14 5.61 8.44 2.53 0.79 1.76 21 5.67 7.98 2.02 0.44 2.32 important contributor to the heat. The surface thermal 30 5.71 7.31 1.4 Ϫ0.34 2.05 and moisture advection field means are slightly nega- Hot, dry vs hot, humid tive, indicating a slight tendency for hot air and mois- ture to be advected out of the region. 2 Ϫ1.04 Ϫ9.85 Ϫ1.38 0.29 0.29 The moistest events are most effectively distin- 7 Ϫ2.31 Ϫ8.95 Ϫ3.58 Ϫ1.96 Ϫ3.9 14 Ϫ2.66 Ϫ7.6 Ϫ3.32 Ϫ2.59 Ϫ4.17 guished by lower-tropospheric thickness and tempera- 21 Ϫ2.95 Ϫ6.68 Ϫ2.66 Ϫ2.42 Ϫ4.55 ture as well, which is consistent with the fact that hot 30 Ϫ3.47 Ϫ6.21 Ϫ2.21 Ϫ1.26 Ϫ5.34 events tend to have a higher dewpoint. Among the four extreme-event types, the moistest events display the most negative value of 200-hPa divergence, suggesting hot, dry and hot, moist events are most distinguished stronger midlevel subsidence, which would promote a from each other by the 2-day antecedent dewpoint tem- stronger lower-tropospheric capping inversion. Addi- peratures. More interesting is the increasingly negative tionally, these events display significantly more cloudi- t score associated with temperatures over longer ante- ness and higher precipitation totals during the 2–3- cedent time periods. This indicates that the hot, dry week period leading up to the event. events are tied to cooler conditions relative to the hot, Precipitation totals over the past 30 days most effec- moist events. The strength of this association increases tively distinguish hot, dry events from the hot, moist over a longer time period. Moreover, the hot, dry events (Table 6). The hot, moist events are tied to 4.67 events are characterized by much lower precipitation cm more inches of rain on average during the anteced- totals. The strength of this distinction is greatest over ent period relative to the hot, dry events. Moreover, the the antecedent 30-day period, suggesting the contribu- precipitation hours in the hot, humid events are almost tion of sensible heating results from dry soils in the double that of the hot, dry events during the 2 weeks month leading up to the event. prior to the event. Mean temperature over the past 30 days is 1.2°C higher in the hot, humid events. This is especially noteworthy given that there is apparently less e. Summary of variables that distinguish each event sensible heating taking place (i.e., the higher values of type precipitation should lead to more evapotranspiration Table 5 presents a summary of the synoptic and sur- and a higher latent heat flux). face weather variables that most effectively discrimi- nate each event type. Lower-tropospheric thickness 5. Discussion and conclusions most effectively distinguishes the hottest events; how- ever, this is not surprising given that it is dependent on In this study, a large sample of hot events were de- lower-tropospheric temperature. In terms of indepen- fined in the Piedmont region of North Carolina and dent variables, wind direction shows the strongest con- described climatologically in terms of their connections nection with the hottest events. In particular, the hot- with the present as well as antecedent surface and at- test events display a significantly greater westerly wind mospheric conditions. In particular, the environments

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TABLE 5. Summary of the means of the synoptic fields and surface weather variables and associated t scores that assess the relative difference between (a) the hottest events and (b) the moistest events and the remainder of the sample. The highest-value t scores identified over the antecedent periods identified are listed, and all listed values are statistically significant at the 0.05 level.

(a) Variable Day Hottest Remainder t score 1000–500-hPa thickness (m) 0 5764 5720 19.3 Surface wind direction (°) Ϫ1 228 187 7.90 850-hPa relative humidity (%) 0 59.4 66.1 Ϫ7.39 Surface dewpoint (°C) 0 21.5 20.1 6.94 Sky cover (%) 0–21 50.5 57.7 Ϫ5.68 200-hPa divergence (1 ϫ 10Ϫ6 sϪ1) Ϫ3 Ϫ1.77 Ϫ0.77 Ϫ5.21 Total precipitation (cm) 0–30 6.73 10.26 Ϫ5.09 No. of rain hours 0–21 13.3 21.8 Ϫ4.56 850-hPa thermal advection [°C (12 h)Ϫ1] Ϫ1 0.26 0.02 2.73 Surface moisture advection [°C (12 h)Ϫ1]0Ϫ0.13 Ϫ0.03 Ϫ2.68 Surface thermal advection [°C (12 h)Ϫ1]0Ϫ0.32 Ϫ0.20 Ϫ2.13 850-hPa moisture advection [°C (12 h)Ϫ1] Ϫ5 Ϫ0.27 Ϫ0.04 Ϫ2.00 (b) Variable Day Moistest Remainder t score 1000–500-hPa thickness (m) Ϫ1 5750 5714 13.38 Surface temperature (°C) 0 27.6 26.1 8.87 200-hPa divergence (1 ϫ 10Ϫ6 sϪ1) Ϫ2 Ϫ2.01 Ϫ1.05 Ϫ4.77 850-hPa thermal advection [°C (12 h)Ϫ1]0Ϫ0.24 Ϫ0.52 3.03 Sky cover (%) 1–14 59.3 56.7 2.53 Total precipitation (cm) 1–21 8.2 6.83 2.32 850-hPa relative humidity (%) Ϫ5 69.9 67.1 2.33 Surface wind direction (°) Ϫ1 201.93 189.8 2.13 associated with the most extreme hot and moist events into the Gulf of Mexico. This pattern is similar to that were distinguished from the events constituting the re- observed by Henderson (1995) in warm outbreaks iden- mainder of the sample. Additionally, the environments tified across the southern United States. The south- of hot, dry events were distinguished from those asso- westerly to westerly low-level flow associated with this ciated with hot, moist events. Composite analyses re- circulation is adiabatically warmed because of its de- vealed that the synoptic circulations were generally scent off the lee of the Blue Ridge Mountains to the similar across the different event types. Most events west. Composite 200-hPa divergence fields showed per- occurred immediately downstream of a 500-hPa ridge, sistent convergence, suggesting subsidence and adia- and in some cases, beneath this ridge. At the surface, a batic warming in the midlevels of the troposphere. quasi-stationary Bermuda high was well established Sounding composites at 1200 UTC indicated a low-level over the western Atlantic Ocean with an arm extending inversion, which could be attributed to the combined effects of adiabatic warming aloft and the remnants of

TABLE 6. Same as Table 5, except comparisons are made radiatively cooled air at the surface. The relatively between hot, dry and hot, humid event samples. stable environment characterizing the events encour- aged relatively clear skies and strong solar insolation. Hot, Hot, t Additionally, the more stable conditions inhibited deep Variable Day dry humid score convective mixing, thus inhibiting the ventilation of the Ϫ Total precipitation (cm) 1–30 5.46 10.13 5.34 heat and moisture in the daytime boundary layer. 1000–500-hPa thickness 0 5748 5765 Ϫ4.93 (m) The mean wind in the hot event sample veered from Sky cover (%) 0–7 41.2 53.7 Ϫ3.58 southwesterly at the surface to west-northwest in the Surface temperature (°C) 0–30 24.1 25.3 Ϫ3.47 upper levels, suggesting the occurrence of warm advec- 850-hPa thermal advection Ϫ5 Ϫ0.76 Ϫ0.08 3.29 tion; however, the composites showed no significant Ϫ1 [°C (12 h) ] thermal advection immediately before or during the Surface thermal advection Ϫ1 Ϫ0.24 Ϫ0.07 3.27 [°C (12 h)Ϫ1] event. In most events, this lack of warm advection can Surface wind direction (°) Ϫ3 146 192 3.18 be tied to the relative absence of a north–south thermal Precipitation hours 0–14 7.3 13.4 Ϫ2.59 gradient. This stands in contrast with the July 1995 Mid- 850-hPa relative humidity Ϫ6 65.85 69.92 2.54 western U.S. heat wave, where a southwesterly lower- (%) tropospheric flow produced warm air and moisture ad-

Unauthenticated | Downloaded 09/26/21 10:36 AM UTC 684 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 45 vection that contributed to the high heat indices 30 days is also 1.2°C higher in the hot, humid events, (Livezey and Tinker 1996). which further encourages evapotranspiration. Note that The hottest events in the sample are most distin- these higher temperatures result from the relatively guished by a strong westerly wind component in the higher positive skewness in the temperature distribu- lower troposphere. It is estimated that 65% of the tion of the hot, humid events. There were no significant events displayed a sufficiently strong westerly wind differences identified in moisture advection between component to cross the Blue Ridge Mountains and the hot, dry and hot, moist event samples. This suggests warm adiabatically on the leeside. Our analysis suggests that the moisture source is the increased evapotranspi- that downslope adiabatic warming is a contributor to ration resulting from wet soils and active vegetation the hot conditions in the Piedmont region of North because of the higher precipitation totals in the weeks Carolina. Upper-level divergence also effectively dis- leading up to the event. This pattern was observed in tinguishes these events; it contributes to middle- the Midwestern U.S. heat wave of 1995; however, the tropospheric subsidence and adiabatic warming. How- exceptionally moist conditions occurred upstream, ever, it is unclear how much of this adiabatically which provided water vapor that was then advected warmed air is actually mixed down to the surface and into the region (Livezey and Tinker 1996). can therefore contribute to the surface heating. The Although the relationships between summer weather hottest events were also distinguished by a low sky and health have not been explicitly examined in the cover (i.e., strong solar insolation), little precipitation, Piedmont region of North Carolina, excessive heat and and relatively fewer precipitation hours during the an- humidity are known to affect mortality and morbidity. tecedent period leading up to the event. This finding The antecedent atmospheric conditions associated with hot, moist events in this work (i.e., those associated implies a high Bowen ratio promoted by the high levels with dangerously high heat indices) provide valuable of sensible warming resulting from the relatively dry prognostic information in the advance issuance of heat- soils. The relatively dry antecedent conditions are simi- wave warnings. In particular, this work suggests that the lar to those observed in the heat-wave droughts of 1980 occurrence of positive monthly precipitation anomalies and 1988 in the Southern Great Plains and Midwestern can set the stage for a potentially hot and moist event. United States, respectively (Lyon and Dole 1995). If a northwesterly circulation develops aloft concur- The moistest events in the sample are most distin- rently with these positive precipitation anomalies, the guished by high levels of convergence in the upper tro- probability of exceptionally hot and moist conditions is posphere, which promote middle-tropospheric subsi- greatly increased. Future work should be aimed at the dence and lower-tropospheric stability. These events development of predictive relationships between ante- showed the lowest mean inversion height (at 1200 cedent atmospheric patterns and temperature/dewpoint UTC), which can be tied to stronger subsidence aloft. conditions. The stability may act to inhibit the ventilation of mois- Additionally, this work suggests a connection be- ture out of the daytime boundary layer as discussed tween hot events and adiabatic warming via downslop- above. The moistest events also show a significantly ing westerly flow immediately east of the Appalachian higher coverage of clouds and precipitation totals dur- Mountains. It is unclear, however, how much this effect ing the period leading up to the event, which implies contributes to the hot temperatures. Confounding in- moister soils and more active vegetation growth. As a fluences, such as adiabatic warming resulting from result, evapotranspiration is relatively greater, thereby synoptic-scale subsidence, make this determination dif- providing more water vapor to the boundary layer. This ficult. Moreover, the spatial scale of this effect down- atmospheric pattern was identified in the very humid stream of a mountain range is unclear. Detailed analy- heat waves that occurred over the Midwestern United ses of circulation trajectories are needed to address States during the 1990s (Kunkel et al. 1996; Livezey and these questions. Tinker 1996; Palecki et al. 2001). Comparisons of the hot, moist and hot, dry events Acknowledgments. We acknowledge the efforts of revealed the importance of antecedent precipitation. two anonymous reviewers in providing suggestions that The hot, moist events were associated with much higher helped to improve this article. precipitation totals (4.57 cm) during the antecedent 30- day period as compared with the hot, dry events. The REFERENCES increased precipitation provides wetter soils and in- Chang, F. C., and J. M. Wallace, 1987: Meteorological conditions creased vegetation growth, which in turn increases rates during heat waves and droughts in the United States Great of evapotranspiration. Mean temperature over the past Plains. Mon. Wea. Rev., 115, 1253–1269.

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Davis, R. E., P. C. Knappenberger, P. J. Michaels, and W. M. relationships in large U.S. cities and the possible impacts of a Novicoff, 2004: Seasonality of climate-human mortality rela- climate change. Environ. Health Perspect., 105, 84–93. tionships in US cities and impacts of climate change. Climate Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- Res., 26, 61–76. analysis Project. Bull. Amer. Meteor. Soc., 77, 437–471. De, U. S., M. Khole, and M. M. Dandekar, 2004: Natural hazards Kunkel, K. E., S. A. Changnon, B. C. Reinke, and R. W. Arritt, associated with meteorological extreme events. Nat. Hazards, 1996: The July 1995 heat wave in the Midwest: A climatic 31, 487–497. perspective and critical weather factors. Bull. Amer. Meteor. Delworth, T. L., J. D. Mahlman, and T. R. Knutson, 1999: Soc., 77, 1507–1518. Livezey, R. E., and R. Tinker, 1996: Some meteorological, clima- Changes in heat index associated with CO2-induced global warming. Climate Change, 43, 369–386. tological and microclimatological considerations of the se- EarthInfo, 2001: NCDC Surface Airways, East: 3. EarthInfo, vere U.S. heat wave of mid-July 1995. Bull. Amer. Meteor. Boulder, CO, CD-ROM. Soc., 77, 2043–2054. Greene, J. S., and L. S. Kalkstein, 1996: Quantitative analysis of Lyon, B., and R. M. Dole, 1995: A diagnostic comparison of the J. Climate, summer air masses in the eastern United States and an ap- 1980 and 1988 U.S. summer heat wave-droughts. 1658–1675. plication to human mortality. Climate Res., 7, 43–53. 8, McMichael, A. J., A. Haines, and R. Slooff, Eds., 1996: Climate Henderson, K. G., 1995: Circulation patterns associated with ex- change and human health. World Health Organization, treme temperature events in the southern United States. Pre- World Meteorological Organization, United Nations Envi- prints, Ninth Conf. on Applied Climatology, Dallas, TX, ronmental Program, 297 pp. Amer. Meteor. Soc., 344–345. Namias, J., 1982: Anatomy of Great Plains protracted heat waves Huang, J., and H. M. van den Dool, 1993: Monthly precipitation- (especially the 1980 United States summer drought). Mon. temperature relations and temperature prediction over the Wea. Rev., 110, 824–838. United States. J. Climate, 6, 1111–1132. ——, 1991: Spring and summer 1988 drought over the contiguous Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der United States—Causes and prediction. J. Climate, 4, 54–65. Linden, X. Dai, K. Maskell, and C. A. Johnson, Eds., 2001: Palecki, M. A., S. A. Changnon, and K. E. Kunkel, 2001: The na- Climate Change 2001: The Scientific Basis. Cambridge Uni- ture and impacts of the July 1999 heat wave in the Midwest- versity Press, 881 pp. ern United States: Learning from the lessons of 1995. Bull. Kalkstein, L. S., 1991: A new approach to evaluate the impact of Amer. Meteor. Soc., 82, 1353–1367. climate upon human mortality. Environ. Health Perspect., 96, Robinson, P. J., 1998: Monthly variations of dewpoint tempera- 145–150. ture in the coterminous United States. Int. J. Climatol., 18, ——, and J. S. Greene, 1997: An evaluation of climate/mortality 1539–1556.

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