Surface Fronts, Troughs, and Baroclinic Zones in the Great Lakes Region

AUGUST 2011 P A Y E R E T A L . 555

MELISSA PAYER Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

NEIL F. LAIRD Department of Geoscience, Hobart and William Smith Colleges, Geneva, New York

RICHARD J. MALIAWCO JR. Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois

ERIC G. HOFFMAN Department of Atmospheric Science and Chemistry, Plymouth State University, Plymouth, New Hampshire

(Manuscript received 3 September 2010, in ﬁnal form 8 January 2011)

ABSTRACT

The temporal frequency and spatial distributions of fronts and troughs in the Great Lakes region from a 6-yr period (January 2000–December 2005) are presented. Frontal frequencies indicated that cold fronts were the most common, followed by stationary, warm, and occluded fronts, in that order. The variation in the annual frequency of all front types was small throughout the period. Troughs were present more frequently than any front type and exhibited greater variability of annual frequency. An investigation of the relation of fronts and troughs to surface baroclinic zones found that approximately 54% of analyzed fronts were associated with a moderate potential temperature gradient of at least 3.58C (100 km)21. Nearly 10% of all fronts were associated with a strong baroclinic zone having a potential temperature gradient of at least 7.08C (100 km)21. Moderate or strong baroclinic zones were associated with about 34% of the analyzed troughs in the Great Lakes region and this relationship was further examined for three different trough classiﬁcations (i.e., synoptic, subsynoptic, and mesoscale). This study is one of very few to present the climatological variability of surface fronts of all types across a region and the results provide useful insights into the current operational determination of analyzed surface fronts and troughs by quantifying their association with surface baroclinic zones in the Great Lakes region.

1. Introduction frontal frequencies and associated characteristics for speciﬁc regions and a single type of front, most com- A small number of studies have examined the fre- monly cold and coastal fronts (e.g., Schultz 2004; Appel quency of fronts in North America (e.g., Chiang 1961; et al. 2005; Shafer and Steenburgh 2008). The relation of Morgan et al. 1975; Cousins 2006). Morgan et al. (1975) frontal analyses to surface temperature gradients, wind completed an analysis of frontal frequency across the shifts, and other meteorological variables has been widely contiguous United States for the period from January discussed. Uccellini et al. (1992) revealed that there is a 1961 through December 1970 using the Daily Weather wide variety of opinions among meteorologists concerning Map Series produced by the National Weather Service what constitutes a front and what meteorological variables (NWS). Additional studies have focused on determining should be considered when analyzing fronts. Sanders and Doswell (1995) suggested that surface analyses should fo- cus on thermal boundaries. Sanders (1999) proposed that Corresponding author address: Neil F. Laird, Dept. of Geo- science, Hobart and William Smith Colleges, 300 Pulteney St., surface frontal analyses should be conﬁrmed with potential Geneva, NY 14456. temperature gradients to ensure the presence of zones E-mail: [email protected] of intense temperature contrast. Sanders and Hoffman

DOI: 10.1175/WAF-D-10-05018.1

Ó 2011 American Meteorological Society 556 WEATHER AND FORECASTING VOLUME 26

(2002), Sanders (2005), and Hoffman (2008) investigated 2. Data and methods the climatology of surface baroclinic zones over the contiguous United States and found that strong gradients of U.S. surface analyses (0000, 0600, 1200, and 1800 UTC) surface potential temperature most frequently occurred conducted by the National Centers for Environmental over the Atlantic Ocean and Gulf of Mexico coastlines, the Prediction (NCEP) were used as the dataset for de- Pacific Ocean coastline, and the eastern slopes of the termining the frequency and evolution of surface fronts North American Rocky Mountains. Additionally, Sanders and troughs in the Great Lakes region for the 6-yr period and Hoffman (2002) and Sanders (2005) performed direct of January 2000 through December 2005. Analyses were comparisons of operationally analyzed fronts and surface collected from the National Climatic Data Center (in- baroclinic zones for a 3-month time period in the winter of formation online at http://nomads.ncdc.noaa.gov/ncep/ 1999/2000 and a 2-month time period in early (February– NCEP). Only a few analyses were unavailable from the March) 2002, respectively. They found many analyzed archived dataset (1.2%; 105 of 8768). The region used for fronts were not associated with moderate or intense baro- this study is denoted in Fig. 1. Fronts (cold, warm, sta- clinic zones, and this discrepancy was particularly large for tionary, and occluded) and troughs that intersected at cold fronts. least one of the Great Lakes were recorded along with the The passage of fronts and troughs across the Great identifying information of type, date and time of the first Lakes is a critical factor in influencing the weather that and last appearance in the region, and the lakes, states, affects millions of people living in the region. Fronts often and provinces along the track of the analyzed front or play a role in the initiation and evolution of severe thun- trough. Each frontal segment in the study region was re- derstorms, winter storms, and lake-effect snowstorms, corded, even if multiple segments of different front types which may potentially affect transportation, utilities, were associated with a single boundary (Fig. 1). municipal decisions, and recreational activities. Frontal Two-meter potential temperature data available from movement can also have important implications on air the 32-km-resolution North American Regional Rean- trajectories, pollution transport, and precipitation dis- alysis (NARR) were collected at 6-h intervals (0000, tributions. For example, Grover and Sousounis (2002) 0600, 1200, and 1800 UTC). The relationship of NCEP- examined the contributing role of fronts to an increasing analyzed surface frontal positions to the presence and precipitation trend from 1935 to 1995 in the Great Lakes strength of baroclinic zones was subjectively determined Basin. Warm, stationary, and occluded fronts were found following criteria established by Sanders and Hoffman to contribute the most to the 15% basin-wide precipitation (2002) and Sanders (2005). Fronts were categorized as increase that was dominated by an increase in the preci- being associated with regions of potential temperature pitation per day and an increase in the frequency of pre- gradient having strong, moderate, or weak intensity. cipitation days. Moderate and strong baroclinic zones were defined as The current study presents the temporal frequency having gradients of at least 3.58 and 78C (100 km)21, and spatial distribution of fronts and troughs in the respectively. A baroclinic zone was considered to be Great Lakes region for a 6-yr period and is one of very associated with a front during time periods when the few studies to present the climatological variability of central axis of the defining gradient was within 200 km surface fronts of all types across a region. In addition, of the surface front and present along at least 50% of the the work of Sanders and Hoffman (2002) and Hoffman frontal length. In addition, the orientations of the front (2008) is significantly extended by investigating the re- and baroclinic zone needed to be within 458 of each lation of fronts and troughs to surface baroclinic zones other. As an example, at 0000 UTC 19 May 2000, cold using a substantially longer time period. This relation- and stationary frontal segments were analyzed along ship provides insights into the current operational de- the same boundary south and east of the Great Lakes termination of analyzed surface fronts and information (Fig. 1). The cold frontal segment was associated with a regarding the assortment of spatial scales in surface moderate baroclinic zone while the stationary frontal troughs in the Great Lakes region. The results should segment was associated with a strong baroclinic zone. not be interpreted as a judgment on operationally ana- Due to the use of NCEP surface frontal analyses, lyzed fronts and troughs; rather, the results should help many may be hesitant to have confidence in the frontal better quantify the relationship of fronts to surface frequency and baroclinic zone results due to the po- baroclinic zones and the frequency of fronts and troughs tential variability introduced from analyses of different in the Great Lakes region. The data and methods used NCEP analysts. As an example, frontal analyses com- in this study are described in section 2. The results and pleted by various skilled meteorologists who attended conclusions are provided in sections 3 and 4, respec- a 1991 surface analysis workshop indicated that there tively. was large variability in determining the presence and AUGUST 2011 P A Y E R E T A L . 557

FIG. 1. Great Lakes region showing surface potential temperature (every 4 K, dashed) and frontal analysis at 0000 UTC 19 May 2000. Light and dark shading represents potential temperature gradients of $3.58 and $7.08C (100 km)21, respectively. The dark outline denotes the Great Lakes states and portions of Canadian providences included in the study region. positioning of fronts using observations of an identical analysts that completed few analyses (i.e., ,10). The weather situation (see Fig. 2; Uccellini et al. 1992). To limited variation for each front type, as represented by investigate the potential impacts on the current study, the interquartile range in Fig. 2b, implies that NCEP the number of fronts identified in the Great Lakes re- surface analyses can be used as a foundational dataset gion by each NCEP analyst was determined and nor- when examining the frequencies of fronts and their malized to account for the total number of surface maps connection to surface baroclinic zones. Although the that each individual had analyzed during the 6-yr period variability was larger for analyzed troughs in the region, studied. Any potential biases that certain analysts may the range of variation remained reasonable for an anal- have had for identifying fronts of a specific type or ysis of trough type, trough frequency, and the relation of troughs were examined. The goal was not to determine troughs to surface baroclinic zones. The definitions pro- the validity of each surface analysis but, rather, to de- vided in the Unified Surface Analysis Manual (NWS 2006) termine if there was consistency in the approaches for for a trough and different types of fronts are included in identifying fronts and troughs among the large number the appendix. of analysts completing surface analyses. Thirty-three NCEP analysts completed surface anal- 3. Results yses from 2000 through 2005 that contained a front or a. Front and trough climatology trough in the Great Lakes region. The number of maps analyzed by each person varied considerably (Fig. 2a). A total of 2173 fronts and 1075 troughs in the Great For example, one individual analyzed nearly 1500 sur- Lakes region were identified during the 6-yr period face maps while several other individuals analyzed less studied. Frontal frequencies indicated that cold fronts than 10. Limited variability in the frequency of desig- were the most common (with a mean of 146.3 yr21), nating a particular front type suggests that the analysts followed by stationary (87.0 yr21), warm (82.8 yr21), used and adhered to consistent procedures and guide- and occluded (46.0 yr21) fronts (Fig. 3). The variation lines for identifying fronts (Fig. 2b), such as those de- in the annual frequency of all front types was small scribed in the Unified Surface Analysis Manual (NWS throughout the period. Warm fronts had the largest 2006). The outliers shown in Fig. 2b are a result of variation in annual frequency, with a mean and standard 558 WEATHER AND FORECASTING VOLUME 26

FIG. 3. Number of fronts and troughs in the Great Lakes region from January 2000–December 2005. Labeling as in Fig. 2.

Troughs were present in the Great Lakes region more frequently than any front type and the interannual frequency of troughs contained a larger amount of vari- FIG. 2. (a) Number of surface maps having Great Lakes fronts or ability (Fig. 3). On average, 179.2 6 61.8 troughs per year troughs analyzed by each NCEP analyst during the period from were observed in the region. Troughs were divided into January 2000 through December 2005. (b) Percentage of analyses for each analyst having a specific front type or trough in the Great three categories (i.e., synoptic, subsynoptic, and meso- Lakes region. Labeling represents cold front (C), warm front (W), scale) in order to further investigate the large amount of occluded front (OC), stationary front (S), and trough (T). Box plots annual variability and the potential differences in their show the medians of the distribution (central solid line), inter- association with surface baroclinic zones. Synoptic quartile range (box), outlier data points (circles representing values troughs extended beyond the boundaries of the Great that are between 1.5 and 3.0 times the interquartile range outside the box), extreme data points (asterisks representing values that Lakes or passed through the region in association with are greater than 3 times the interquartile range outside the box), the evolution of an extratropical cyclone. A subsynoptic and box-plot whiskers representing the minimum and maximum trough was defined as a quasi-stationary trough that values not marked as extreme or outlier data points. extended across most or all of the Great Lakes and had characteristics similar to lake-aggregate troughs described by Petterssen and Calabrese (1959) and Sousounis (1997). deviation of 82.8 6 16.6, and stationary fronts varied the Finally, mesoscale troughs were defined as quasi-stationary least, with a mean and standard deviation of 87.0 6 4.2. troughs that were confined to a lake and usually oriented Morgan et al. (1975) also presented annual frontal fre- along the long lake axis. Synoptic troughs occurred most quencies in the Great Lakes region. While the ordering frequently, accounting for 73.1% of all troughs and the of front types based on frequency was similar, their frequencies of subsynoptic and mesoscale troughs were frequencies were substantially smaller than the current nearly equal at 13.7% and 13.2%, respectively. results. They found that cold fronts were the most com- The frequency of fronts that intersected each of the mon (with a mean of about 60 yr21), followed by sta- Great Lakes was determined (Fig. 4a). These numbers tionary (42 yr21), warm (30 yr21), and occluded (18 yr21) differ from the results shown in Fig. 3 since many Great fronts, respectively. The difference between the results of Lakes fronts often intersected more than a single lake. the current study and those of Morgan et al. (1975) may be Results showed that cold fronts were the most common related to their use of daily weather maps rather than the over each of the lakes. On average over an individual 6-hourly surface analyses used in the current study. lake, cold fronts occur 84.8 6 11.5 yr21, followed by AUGUST 2011 P A Y E R E T A L . 559

larger than the number of cold fronts in the region (Fig. 3), the number of cold fronts was larger than the number of troughs over each lake. This result occurs because two of the trough types (i.e., subsynoptic and mesoscale) are typically associated with an individual lake or a small number of lakes while cold fronts often intersected several lakes during their evolution through the region. Ac- counting for lake surface area to examine impact frequency, a marked reversal in distribution occurs, with Lakes Erie and Ontario experiencing greater coverage of fronts and troughs per unit area (Fig. 4b). This indicates that people along the shorelines of the eastern Great Lakes are impacted more by the changes in atmospheric conditions and any severe weather associated with fronts and troughs than are people along the shorelines of the western Great Lakes. b. Fronts, troughs, and baroclinic zones Overall, 3248 fronts and troughs in the Great Lakes region were examined to determine their relationship to surface baroclinic zones. Each front and trough at an individual time period (0000, 0600, 1200, and 1800 UTC) was used as an independent sample for analysis. This provided a database of 13 221 samples. Considering all of the front and trough samples, 54.1% were associated with a moderate or strong baroclinic zone. There were considerable differences in this relationship among the front types (Fig. 5a). In analyses with cold fronts, 49.9% were associated with a moderate or strong baroclinic zone. In comparison, warm and stationary fronts were each associated with moderate or strong baroclinic zones in about 64.0% of their occurrences. Occluded fronts differed greatly from the other front types, with only 38.7% of the analyzed time periods being associated with a moderate or strong baroclinic zone. The overall low percentage of occurrences with a surface baroclinic zone in proximity to a front can par- tially be explained by the criteria that NCEP analysts follow when identifying fronts on surface analyses. The FIG. 4. (a) Number of fronts and troughs that intersected an Uniﬁed Surface Analysis Manual (NWS 2006) states, individual Great Lake during the time period January 2000– December 2005. (b) Front and trough frequencies normalized by ‘‘over the continent a minimum of 68C over 500 km lake surface area. is usually needed for a frontal zone.’’ This value is approximately equivalent to 1.28C (100 km)21 which warm (35.6 6 7.1 yr21), stationary (30.1 6 7.9 yr21), would fall into the deﬁned category for this study of and occluded (20.3 6 4.9 yr21) fronts. All front types a weak baroclinic zone. As a reference, nearly every- occurred more frequently over the western lakes (Su- where on the map included in Fig. 1 has a potential perior, Michigan, and Huron) than the eastern lakes temperature gradient of at least 1.28C (100 km)21. (Erie and Ontario). Overall, Lake Michigan experi- Interestingly, only 9.7% of fronts were associated with enced the greatest number within each front type, ex- a strong baroclinic zone. The occurrences of a cold or cept for occluded fronts. The greatest frequency of occluded front having been analyzed in the vicinity of troughs occurred in association with Lake Superior and a strong baroclinic zone were 5.6 and 5.5%, respectively. progressively decreased for each lake in the eastward di- Stationary and warm fronts were most frequently asso- rection. Although the number of troughs was generally ciated with strong baroclinic zones with 15.5% and 560 WEATHER AND FORECASTING VOLUME 26

in cloud cover and water vapor. As an example, a stationary front that began at 1800 UTC 8 June 2000 lasted 84 h (3.5 days) and was associated with moderate and strong baroclinic zones throughout its lifetime, with a strong baroclinic zone more common later in the evolution (not shown). Moderate or strong baroclinic zones were associated with 34.1% of troughs present in the Great Lakes region. This small percentage likely reﬂects the criteria provided in the Uniﬁed Surface Analysis Manual (NWS 2006) for the analysis of a trough that does not require the presence of a thermal gradient. Synoptic, subsynoptic, and mesoscale troughs were associated with moderate or strong baroclinic zones in 23.5%, 46.1%, and 80.2% of the analyses, respectively (Fig. 5b). Since the thermal contrast between the water of the Great Lakes and surrounding land often aids in the development of subsynoptic and mesoscale troughs in the region, one might expect a larger percentage of association to baroclinic zones for these trough types. Sanders and Hoffman (2002) and Hoffman (2008) found a seasonal signal in the occurrence of baroclinic zones across North America, with an increased frequency of baroclinic zones during the cold season. In the current study, the association of analyzed fronts to baroclinic zones was also found to vary by season (Fig. 6). During the winter and spring, fronts were more often associated with moderate baroclinic zones (Figs. 6a and 6b); however, during the summer weak baroclinic zones dominated (Fig. 6c). As an example, during the winter cold fronts were associated with moderate baroclinic zones in 57.2% of their occurrences. During the summer, only 33.8% of analyzed cold fronts were associated with moderate baroclinic zones while 65.1% were associated with weak baroclinic zones. The association of fronts with strong baroclinic zones is greatly reduced during the summer to 4.1% of fronts and troughs analyzed having strong baroclinic zones as compared to 14.4% in the winter. FIG. 5. (a) Distribution of each frontal type and troughs within three categories of baroclinic zone strength. (b) Distribution of Finally, nonfront- and nontrough-related baroclinic each trough type (synoptic, subsynoptic, and mesoscale) within zones were also observed in the region; however, these three categories of baroclinic zone strength. The baroclinic zone were not included in our database. Most commonly, strength categories include weak [potential temperature gradient, these zones were present along the shorelines of Great 21 21 $u , 3.58C (100 km) ], moderate [$u $ 3.58C (100 km) ], and Lakes due to the thermal contrast between the lakes and strong [$u $ 7.08C (100 km)21]. surrounding land during both warm and cold seasons. For example, the potential temperature analysis for 16.5% of the analyses, respectively. Stationary fronts 0000 UTC 19 May 2000 shows the entire shoreline may be more commonly associated with regions of of Lake Superior with a moderate or strong gradient strong potential temperature gradient because these (Fig. 1). The presence of nonfront-related baroclinic fronts generally remain situated in the same location for zones in the Great Lakes region is consistent with the several hours or even several days. This allows for the relatively large frequency of baroclinic zones for the thermal contrast across the front to build with time and state of Michigan and adjacent water areas presented by perhaps have diurnal contributions due to variations Sanders and Hoffman (2002). AUGUST 2011 P A Y E R E T A L . 561

FIG. 6. Seasonal frequency of fronts and troughs and the association with baroclinic zone strength for each type of front.

4. Conclusions and 2.58C (100 km)21, respectively, during the warm season (1 April–14 September). The analyses in the The results of the current study suggest that use of current study were performed with consistent thresholds surface potential temperature analyses may be benefi- during both the warm and cool seasons; however, re- cial when analyzing fronts in order to more efficiently ducing the threshold of potential temperature gradients locate zones of intense temperature contrasts. Due to during the warm season would have likely increased the the observed seasonality in baroclinic zones, when in- association of fronts with surface baroclinic zones. corporating such analyses in the forecast process it may The current study significantly extends our under- be useful to implement different thresholds of strong standing of the relationship between fronts, troughs, and and moderate potential temperature gradients for the baroclinic zones originally developed from two sub- cool and warm seasons. As an example, online analyses stantially shorter durations by Sanders and Hoffman of surface potential temperature produced by the Uni- (2002) and Hoffman (2008). These previous studies versity at Albany, State University of New York, reduce demonstrated that there was considerable variability the threshold of strong and moderate potential tem- between fronts as analyzed by NCEP and surface baro- perature gradients from 78 and 3.58C (100 km)21 to 58 clinic zones. The results from the current study show 562 WEATHER AND FORECASTING VOLUME 26 that this variability was similar for the Great Lakes re- due to the higher density low-level air in their wake, gion over a 5-yr period. Cold fronts continued to be least which helps drive their forward motion. Over the con- often associated with moderate or strong baroclinic tinent, a minimum of 68C (108F) per 500 km (300 n mi) zones, while warm and stationary fronts were more of- is usually needed for a frontal zone, with smaller dif- ten associated with moderate or strong baroclinic zones. ferences needed over the oceans. In addition, the current study has shown that this type of variability extended to troughs as well. b. Warm front These results provide useful insights into the current A warm front is the equatorward edge of a density operational determination of analyzed surface fronts discontinuity behind a retreating–modified cool–dry air and troughs by quantifying their association with surface mass. This type of frontal zone is significantly broader baroclinic zones in the Great Lakes region. Addition- than a cold front, due to the slower erosion of the su- ally, this study is one of very few to present the clima- perior density air mass ahead of the boundary. Over the tological variability of surface fronts of all types across continent, a minimum of 68C(108F) per 500 km (300 n mi) a region. Awareness of the temporal and spatial fre- is usually needed for a frontal zone, with smaller differences quencies of fronts and troughs in the Great Lakes can needed over the oceans. also provide the operational forecasting community with a long-term understanding about the possibility of c. Occluded front weather-related issues associated with frontal passages, a topic to be investigated in future studies. Finally, con- An occluded front forms southeast–east of a cyclone siderable alongfront mesoscale temporal and spatial that moves deeper into colder air, in the late stages of variabilities of surface baroclinic zones and atmospheric wave-cyclone development. Cold occlusions result when conditions were observed in association with the fronts the coldest air surrounding the cyclone is behind its cold examined for this study. Further understanding of this front, and are normally seen on the west sides of ocean topic would be important for forecasters and analysts of basins and with clipper systems descending from the short-term forecasts and nowcasts. Arctic. Warm occlusions form when the coldest air surrounding the cyclone is ahead of its warm front, Acknowledgments. The first and third authors con- forcing the cold front aloft. Warm occlusions are nor- ducted this research during the 2008 summer un- mally seen on the east side of the ocean basins and just to dergraduate research program at Hobart and William the lee of the U.S. portion of the continental divide. Smith Colleges (HWS). Helpful comments were received at the 2008 Cyclone Workshop and from Dr. d. Stationary front Stephen Colucci of Cornell University. This research A stationary front is the equatorward edge of a slow- was supported by the National Science Foundation un- moving density discontinuity with a motion of less than der Grant ATM-0709526. We gratefully acknowledge 10 kt (12 mi h21). Winds tend to lie parallel to these support by the Office of the Provost at HWS and ev- boundaries. Over the continent, a minimum of 68C(108F) eryone associated with the summer research program at per 500 km (300 n mi) is usually needed for a frontal HWS. Any opinions, findings, conclusions, and recom- zone, with smaller differences needed over the oceans. mendations expressed in this publication are those of the authors and do not necessarily reflect the views of the e. Trough National Science Foundation. A trough is an elongated area of low pressure with no distinct low-level center. Winds usually flow cyclonically APPENDIX through it, outside of terrain influences.

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