A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic

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JUNE 2001 WANG AND ROGERS 1481

A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic. Part I: Cyclone Structure and Evolution

CHUNG-CHIEH WANG* AND JEFFREY C. ROGERS Atmospheric Sciences Program and Department of Geography, The Ohio State University, Columbus, Ohio

(Manuscript received 24 May 1999, in ®nal form 26 September 2000)

ABSTRACT General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.5Њ resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland±Iceland region is higher than previously thought, and some of the events can be extremely violent. Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle. For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a ``parent cyclone'' with appreciable strength exists to the north/northeast of the incipient system.

1. Introduction 1 Ber ϭ 24 sin(␾)/sin(60Њ), (1) Rapid intensi®cation of extratropical cyclones, or ex- where ␾ is the latitude of the cyclone center. Since the plosive cyclogenesis, has been an area of active mete- criterion is latitudinally adjusted to re¯ect the increase orological research during the past two decades due to in geostrophic wind speed, the required 24-h pressure onshore and offshore hazards and the failure of early drop for 1 Ber increases from 13.9 hPa at 30Њ to 27.7 operational models to predict it (Silberberg and Bosart hPa at the poles. The deepening rate in some extreme 1982; Sanders 1986b, 1987; Sanders and Auciello cases can well exceed 1 Ber, and two famous examples 1989). Sanders and Gyakum (1980) ®rst studied explo- are the Queen Elizabeth II storm in 1978 (Gyakum sive cyclones (or ``bombs'') and de®ned them as low 1983a; Kocin and Uccellini 1985) and the Presidents' systems whose central sea level pressure falls at an av- Day cyclone in 1979 (Bosart 1981). eraged rate of at least 1 Bergeron (or 1 Ber, in hPa Sanders and Gyakum (1980) produced the ®rst ex- dayϪ1), and plosive cyclogenesis climatology, updated by Roebber (1984), showing it to be primarily a cold season maritime phenomenon with highest frequencies over the northwestern Paci®c and northwestern Atlantic along * Current af®liation: Department of Environmental Management, major baroclinic zones. Although they suspect that both Jin-Wen Institute of Technology, Taipei, Taiwan. the frequency and intensity are likely underestimated over regions of sparse data coverage, studies by them and several others (e.g., Gyakum et al. 1989; Chen et Corresponding author address: Dr. Chung-Chieh Wang, Depart- ment of Environmental Management, Jin-Wen Institute of Technol- al. 1992) provide us with the best explosive cyclogenesis ogy, 99 An-Chung Road, Hsin-Tien City, Taipei, Taiwan. climatologies to date. E-mail: [email protected] Studies during the past two decades indicate that ex-

᭧ 2001 American Meteorological Society

Unauthenticated | Downloaded 09/29/21 01:23 AM UTC 1482 MONTHLY WEATHER REVIEW VOLUME 129 plosive cyclones are like ordinary ones, fundamentally sive study is yet to be done. The purpose of this paper driven by baroclinic instability (Sanders 1986a; Man- is, therefore, to compare and contrast explosive cyclones obianco 1989b; Wash et al. 1992), with other factors in different sectors of the North Atlantic through a com- also contributing signi®cantly to the deepening rate. posite study using a large number of cases, with an These include 1) strong upper-level forcing (Rogers and emphasis on northeastern Atlantic. Bosart 1986; Macdonald and Reiter 1988; Hirschberg The present paper (Part I) employs a more qualitative and Fritsch 1991; Lupo et al. 1992), 2) intrusion of approach and focuses on cyclone structure and evolu- stratospheric high potential vorticity (PV) air (Bosart tion. Part II (in preparation) presents results of a vor- and Lin 1984; Uccellini et al. 1985; Zehnder and Keyser ticity budget analysis from a quantitative perspective. 1991; Reader and Moore 1995), 3) latent heat release Section 2 describes the data and methodology used to (Gall 1976; Anthes et al. 1983; Gyakum 1983b; Eman- identify, select, categorize, and composite our cases. uel et al. 1987; Mullen and Baumhefner 1988; Kuo et Composite results of key variables at various levels and al. 1991b; Whitaker and Davis 1994), 4) surface energy on vertical cross sections are presented in sections 3 and ¯uxes from the ocean (Atlas 1987; Davis and Emanuel 4, respectively. Section 5 provides further discussion, 1988; Fantini 1990; Nuss and Kamikawa 1990; Kuo et and section 6 summarizes the major ®ndings of this al. 1991a; Hedley and Yau 1991), and 5) enhanced local paper. baroclincity from differential diabatic heating (Bosart 1981; Rogers and Bosart 1991). Rapid development is the result of nonlinear interaction among all these pro- 2. Data and methodology cesses, although all factors need not be present (Uccel- a. ECMWF dataset lini 1990; Gyakum et al. 1995) and their importance could vary considerably in individual cases (e.g., Reed In this study, we use the European Centre for Me- et al. 1993). Nonetheless, since strong upper-level forc- dium-Range Weather Forecasts (ECMWF) operational ing exists both over land and ocean, it is quite clear that global analysis (Level III-b), archived at the National the effect of abundant moisture, supplied through ma- Center for Atmospheric Research from January 1985 to rine boundary layer processes, in lowering the stability March 1996. Data are twice daily (0000 and 1200 UTC) is what makes explosive cyclogenesis an almost exclu- on a regular 2.5Њϫ2.5Њ latitude±longitude grid, and sive maritime phenomenon. In more recent years, high- variables include geopotential, temperature, and three- resolution datasets obtained through ®eld experiments dimensional u, ␷, and ␻ (omega) wind components at (e.g., Dirks et al. 1988; Hadlock and Kreitzberg 1988) 14 pressure levels (1000, 850, 700, 500, 400, 300, 250, allow analysis of detailed mesoscale structure and evo- 200, 150, 100, 70, 50, 30, and 10 hPa), as well as relative lution (e.g., Neiman and Shapiro 1993; Neiman et al. humidity from the 1000±300-hPa level. The 925-hPa 1993; Blier and Wakimoto 1995). More complete re- level has been added since January 1992. Other surface views can be found in Uccellini (1990), Hoskins (1990), variables include mean sea level pressure (MSLP), sur- and Shapiro and Keyser (1990). face pressure and temperature, temperature and dew- Prior studies have focused on northwestern Atlantic point at 2-m height, and u and ␷ winds at 10-m height events due to their socioeconomical impacts along the (at 2 m since February 1994). North American coast and better data coverage. The The ECMWF operational model is a primitive-equa- Greenland±Iceland region has received much less at- tion-based global spectral model with T213 truncation tention and is traditionally thought to have little cyclo- (roughly 0.55Њ resolution) and 31 levels (L31) from sur- genetic activity with the exception of polar lows (e.g., face to 10 hPa (Simmons et al. 1995). It was T106/L19 Sardie and Warner 1985; Businger and Reed 1989; Nor- before September 1991 and T63/L16 before May 1985, deng and Rasmussen 1992). More recently, Serreze et with various improvements over the period (Simmons al. (1993) and Serreze (1995) have shown that cyclo- et al. 1989, 1995; Bengtsson 1991). The model employs genesis is common near Iceland during cold seasons, a four-dimensional data assimilation (FDDA) scheme to and Rogers (1997) found that when the storm track is analyze data with multivariate three-dimensional opti- active in extreme northeastern Atlantic, over 20% of the mum interpolation (3D OI) and an injection period of days experience 24-h pressure changes in excess of 20 6 h (Lorenc 1981), and is initialized four times a day hPa. Examples of rapid cyclogenesis in this region in- (0000, 0600, 1200, and 1800 UTC) using nonlinear nor- clude the record-setting case on 14±15 December 1986 mal mode initialization (Wergen 1989). (Gadd et al. 1990), the Braer storm (McCallum and The ECMWF FDDA system uses the 6-h forecast Grahame 1993), the one on 30 September 1995 (L. D. from the previous model run as the ®rst guess (FG) ®eld, Keigwin 1996, personal communication), and those then incorporates all available observations to produce studied by Sinclair and Elsberry (1986), Reed et al. the analysis before model initialization. The 3D OI (1988), GrùnaÊs (1995), KristjaÁnsson and Thorsteinsson scheme processes many data points and variables si- (1995), and Wernli (1997). These cases (and the present multaneously by using large analysis volumes, and ap- study) demonstrate that rapid cyclogenesis also occurs plies statistical methods to perform data selection and near Iceland and can be very violent, but a comprehen- quality control at the same time. All data are subject to

Unauthenticated | Downloaded 09/29/21 01:23 AM UTC JUNE 2001 WANG AND ROGERS 1483 checks for internal consistency, and against climatology, the FG ®eld, and nearby reports (Lorenc 1981; LoÈnnberg and Shaw 1984; Shaw et al. 1987). The system has been shown to be useful in monitoring the observational net- work (Hollingsworth et al. 1986). The analyses are gen- erally of high quality in the Northern Hemisphere ex- tratropics (Trenberth and Olson 1988, Lambert 1988), and have been used for diagnostic studies of explosive cyclones by Sinclair and Elsberry (1986), Wash et al. (1988, 1992), and Manobianco (1989a). b. Potential case identi®cation FIG. 1. Geographic distribution of 24-h explosive cyclogenesis in Potential explosive cyclogenesis cases in the dataset the Northern Hemisphere derived from ECMWF MSLP data from were identi®ed through an objective approach. Auto- Jan 1985 to Mar 1996. Numbers are ®rst counted at 2.5Њ resolution matic detection and tracking algorithms have been de- and aggregated into 5Њϫ5Њ quadrilaterals by adding them together. veloped individually by Serreze (1995) for MSLP ®elds Then, a smoothing method of 0.5 times the frequency at the central point plus 0.125 times the sum of frequencies at four adjacent points of gridded analyses from National Centers for Environ- is applied. mental Prediction (formerly the National Meteorologi- cal Center), and by Sinclair (1994) for vorticity centers based on ECMWF analyses. Here two programs, one and have similar system characteristics (such as central for storm detection and the other for tracking, are de- MSLP, pressure de®cit, and size, etc.). After this, the veloped to obtain a life history of low pressure systems program determines whether remaining lows at t1 dis- in the Northern Hemisphere. Since all 162 cases ®nally sipate or merge with other lows, and whether those at selected are manually checked and potential errors of t2 are new systems or split from existing ones. the programs are inconsequential to our composite re- Once the life history of each low is obtained, Sanders sults, the two programs are only brie¯y described below. and Gyakum's criterion of an averaged deepening rate The detection program takes the ECMWF MSLP ar- of 1 Ber for 24 h is used to identify possible explosive ray as input and identi®es synoptic-scale low pressure cyclogenesis cases north of 30ЊN. A total of 1369 events, systems between 15ЊN and the North Pole. A low is 800 in the North Paci®c and 569 in the North Atlantic, identi®ed when the MSLP at a grid point is the lowest are identi®ed in the dataset. The smoothed geographical within a 750-km radius, is at least 1 hPa lower than the distribution of cyclone positions at the midpoint of 24-h average at surrounding eight points, and has a positive periods (Fig. 1) shows three distinct frequency maxima Laplacian. With the low's extent de®ned by the outer- in the North Atlantic, including a main center off the most closed isobar at 0.5-hPa intervals, the program coast near Nova Scotia and two other centers south of discards lows smaller than 40 000 km2 and those smaller Greenland (near 50ЊN, 45ЊW) and slightly south of the than 120 000 km2 if the MSLP de®cit between the low's Denmark Strait. When compared with Sanders and center and boundary is less than 2 hPa after scanning Gyakum (1980, their Fig. 3) and Roebber's (1984, their the array. Such a setting excludes small and weak lows Fig. 7c), our ®gure shows a higher degree of spatial and helps keep the tracking easier and more accurate, variability in the North Atlantic, as well as a higher although on average there are still as many as 16.9 lows explosive cyclogenesis frequency in the Greenland±Ice- identi®ed at each time. Output includes time, central land region. MSLP, Laplacian, and position of the low, system size, We evaluated the performance of detection and track- MSLP de®cit from boundary to center, and an identi- ing programs using manual tracking over two 3-month ®cation number for tracking purposes. periods from December to February of 1988/89 and The tracking program follows individual lows by 1991/92. A total of 100 errors are found among matches matching systems at each time (designated t1) with those for 6120 identi®ed lows, and thus 98.4% of them were 12 h later (designated t 2). Once a low at t 2 is ``matched'' correctly matched. However, 24 of the errors involve with a t1 low (i.e., considered the same system by the bombs in the North Atlantic, including 18 occasions of program), the latter becomes ineligible for any other t1 misses, 3 of false generation, and 3 of underestimation. system. The details of the procedure are rather compli- Manual detection identi®ed 79 cases while the programs cated, but in short the program attempts to ®nd a match identi®ed 64; so, overall the frequency in the North for the larger, deeper, and stronger lows earlier, and gives Atlantic is underestimated by nearly 20%. This de®- higher priority to isolated pairs. For less obvious pairs, ciency arises mainly because explosive cyclones in the the program employs a ``score approach'' and matches North Atlantic often travel fast and exhibit rapid chang- the pair that yields the highest score, if it is higher than es in system characteristics, producing a lower score, a threshold value. In general, a potential pair gives a sometimes below the threshold. In the North Paci®c, the higher score if the two lows are closer to each other programs perform signi®cantly better during the eval-

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FIG. 2. Geographical sectors in the North Atlantic used to classify explosive cyclogenesis: NWA, NCA, and NEA. Sector boundaries are drawn in thick solid lines. uation periods, as 82 cases are identi®ed with only six group them into three intensity classes since it corre- errors. Thus, the North Atlantic distribution in Fig. 1 sponds to the highest temporal resolution of the data. somewhat underestimates the climatology that would be The classes are strong (ST, Ն1.80 Ber), moderate (MO, obtained using manual tracking on ECMWF data, al- 1.40±1.79 Ber), and weak (WE, 1.00±1.39 Ber), so cho- though the programs are no doubt useful in providing sen to yield roughly comparable numbers of cases in a large sample of potential cases. each category. Thus, all potential North Atlantic cases Since the program's discrepancy is a tendency to miss are grouped into a total of nine categories, three sectors rather than falsely generate bomb events, the North At- times three intensity classes. lantic maxima in Fig. 1 are quite real in ECMWF data. The higher frequency near Iceland is in broad agreement d. Final case selection and storm track correction with ®nding of Serreze et al. (1997) that there has been an increase in cyclone frequency in this region since the Several rules are applied to further select only the early 1980s, which might be linked to the recent highly most ideal cases for the composites, with a goal to positive phase of the North Atlantic oscillation (e.g., choose between 15 and 20 cases in each category. An Hurrell 1995). Moreover, later we will show that ex- ideal case 1) has only one dominant deepening period, plosive cyclones in the Greenland±Iceland region ex- whose maximum 12-h deepening rate is at least 0.5 Ber hibit a shorter life cycle and were more likely to be greater than that of any other deepening period it may missed by the sparse and somewhat random ship ob- have; 2) deepens primarily in its corresponding sector servations (and therefore in manual analysis) compared for at least 50% of its rapid development period, and to events at lower latitudes. This provides another ex- 3) occurs between September and April. After all three planation for their frequencies to be underestimated in rules were applied, cases farther away from their re- the past. gional maximum in Fig. 1 were also dropped for categories that still had too many cases (Ͼ20). A ®nal total of 162 cases were selected, but in this paper only the c. Composite categories results of the ST NWA (18 cases) and ST NEA (19 Following Fig. 1 we divide the North Atlantic basin cases) categories will be presented due to limited space into three sectors such that each frequency maximum (see Table 1 for a list of cases). For each case, data falls into either the northwest Atlantic (NWA), north- during a 7-day period from tϪ84 to tϩ72 for the Northern central Atlantic, (NCA), or the extreme northeast At- Hemisphere are extracted with t0 de®ned as the end lantic (NEA) sector (Fig. 2). While all cyclones in Fig. point of the maximum 12-h deepening period, and used 1 satisfy the Bergeron criterion for at least one 24-h to produce composites. period, their maximum 12-h deepening rate is used to During the ®nal case selection process, MSLP maps

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Table 1. Cases used in the strong northwest Atlantic (ST NWA) and strong northeast Atlantic (ST NEA) composites. Variables (left to right)

are time, longitude (ЊE), latitude (ЊN), and central MSLP (hPa) at t0. Time and date of each case are listed. ST NWA cases ST NEA cases Time Long Lat MSLP Time Long Lat MSLP 1200 UTC 5 Jan 1985 292.5 40.0 983.52 0000 UTC 13 Jan 1986 330.0 57.5 962.98 0000 UTC 12 Jan 1985 295.0 37.5 994.70 0000 UTC 24 Mar 1986 347.5 52.5 985.38 0000 UTC 14 Feb 1987 305.0 40.0 968.30 0000 UTC 15 Dec 1986 325.0 60.0 921.83 1200 UTC 12 Nov 1987 292.5 42.5 980.47 0000 UTC 9 Feb 1988 342.5 57.5 951.74 1200 UTC 29 Dec 1987 290.0 37.5 992.29 1200 UTC 8 Feb 1989 335.0 52.5 970.72 1200 UTC 5 Feb 1988 300.0 47.5 980.59 1200 UTC 14 Feb 1989 342.5 62.5 977.29 1200 UTC 14 Dec 1988 297.5 37.5 981.54 1200 UTC 18 Jan 1990 337.5 60.0 967.22 1200 UTC 4 Jan 1989 295.0 37.5 966.04 1200 UTC 3 Feb 1991 332.5 65.0 950.17 1200 UTC 21 Jan 1989 297.5 45.0 977.42 1200 UTC 30 Oct 1991 337.5 52.5 960.04 0000 UTC 20 Jan 1989 302.5 40.0 988.30 1200 UTC 23 Feb 1992 330.0 55.0 973.32 1200 UTC 1 Feb 1992 302.5 40.0 976.21 0000 UTC 25 Feb 1992 330.0 60.0 960.86 1200 UTC 21 Mar 1992 295.0 35.0 980.67 1200 UTC 10 Jan 1993 342.5 60.0 914.43 0000 UTC 6 Dec 1992 300.0 45.0 980.88 0000 UTC 3 Oct 1993 332.5 57.5 966.62 1200 UTC 30 Dec 1993 295.0 42.5 984.21 0000 UTC 15 Nov 1993 327.5 57.5 951.64 1200 UTC 14 Jan 1994 290.0 35.0 992.58 1200 UTC 21 Mar 1994 330.0 60.0 957.29 0000 UTC 25 Jan 1995 305.0 42.5 975.29 0000 UTC 30 Mar 1994 332.5 52.5 952.22 0000 UTC 5 Feb 1995 290.0 42.5 977.30 1200 UTC 6 Dec 1994 337.5 52.5 957.46 0000 UTC 27 Nov 1995 300.0 42.5 970.03 0000 UTC 17 Dec 1994 337.5 60.0 964.11 0000 UTC 30 Sep 1995 327.5 60.0 965.76 were used to manually correct any errors made by the ner] is always placed at the sea level low center. Thus, detection and tracking programs and to extend the storm values for grid points at the same latitude±longitude histories backward to early stages. Since composites are position relative to the sea level low center, and at the made using a moving coordinate (quasi-Lagrangian ap- same stage relative to t 0, are averaged to make the com- proach) with the sea level low center being the common posites. This differs from Manobianco's (1989a) method reference point, a cautious treatment of storm tracks is in which the 500-hPa vorticity center, rather than sea needed. Therefore, PV maps for a chosen potential tem- level storm center, is the reference point. Data near perature (␪) surface (usually between 300 and 309 K) northern (southern) domain edges are occasionally ex- were also used in conjunction with MSLP maps to make cluded from the composite near the end (beginning) of ®nal surface storm track corrections for the cases se- the 7-day period because the storm center is north of lected. Here, (Ertel's) PV is de®ned as 65ЊN (south of 25ЊN). Variables not provided by the ECMWF dataset are ␪ץ PV ϭϪg (␨ ϩ f ) , (2) calculated subsequently. For most of them (e.g., layer p ␪ thickness), the result is not in¯uenced by whether theץ΂΃ where g is gravitational acceleration, and f is Coriolis variable is computed before or after compositing. How- parameter and must be computed on isentropic surfaces. ever, this is not the case for nonlinear quantities (e.g., This is done by ®rst ®nding the pressure level of chosen advection), and these variables are always computed ␪ at each grid point then interpolating u and ␷ winds ®rst at all grid points in the Northern Hemisphere before onto the ␪ surface to obtain isentropic absolute vorticity, ®nite-domain data extraction and compositing. This ap- while the vertical ␪ gradient is computed as described proach also ensures better approximation by ®nite dif- in section 2e. This procedure only made a few minor ferences along domain boundaries, and is used for all variables unless speci®ed otherwise in the text. When changes at early stages (mostly before tϪ36) when the PV value near the system center is approximately con- taking derivatives in the zonal direction, we skip an served. increasing number of grid points between 62.5Њ and 87.5ЊN to avoid problems due to convergence of me- ridians near the North Pole. Here, only enough points e. Composite techniques and variables are skipped to give a spacing at least that at 60ЊN(ϳ139 Composites for a category are made for each variable km), and this same method is used for all calculations at each level by averaging values from all cases in that for consistency. Vertical derivatives at a level are com- category. Variables from tϪ84 to tϩ72 are ®rst extracted puted by ®nite differences using interpolated values at at all levels inside a ®nite domain constantly moving 5 hPa below and 5 hPa above. The two levels used are with the sea level system for each case. The domain is each bounded by 1000 and 10 hPa, such that layers of de®ned in latitude±longitude space with a ®xed size of 5-hPa thickness are used for these two ends while those 75Њ longitude ϫ 50Њ latitude (31 ϫ 21 grid points), and of 10-hPa thickness are used for all intermediate levels. the grid point (19, 11) [with (1, 1) at the northwest cor- Using such thin layers of equal thickness helps estimate

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TABLE 2. MSLP (hPa) and deepening (or ®lling) rates (B) of mean

ST NWA and ST NEA cyclones from tϪ84 to tϩ72. MSLP (hPa) Deepening rate (B) Time (h) ST NWA ST NEA ST NWA ST NEA Ϫ84 1009.3 1011.1 0.00 Ϫ0.06 Ϫ72 1011.1 1011.2 Ϫ0.21 0.00 Ϫ60 1012.6 1010.9 Ϫ0.18 0.05 Ϫ48 1013.8 1009.6 Ϫ0.15 0.15 Ϫ36 1012.8 1006.3 0.16 0.35 Ϫ24 1008.6 1000.6 0.53 0.57 Ϫ12 999.5 985.9 1.12 1.39 0 980.8 958.5 2.17 2.42 12 971.6 949.6 0.95 0.73 24 967.7 955.3 0.39 Ϫ0.44 FIG. 3. Mean tracks of explosive cyclones for the ST NWA and 36 967.3 964.0 0.03 Ϫ0.72 NEA cases. Cyclone positions are drawn interchangeably with solid 48 971.0 971.3 Ϫ0.31 Ϫ0.59 (at tϪ84, tϪ60,...,andtϩ60) and open (at tϪ72, tϪ48,...,t0,...,and 60 974.5 976.4 Ϫ0.30 Ϫ0.39 tϩ72) circles, with those at t0 enlarged. Sector boundaries are also 72 978.2 981.0 Ϫ0.30 Ϫ0.37 shown.

b. Mean sea level pressure and clouds gradients more accurately, since vertical data spacing is Figures 4 and 5 show the MSLP (hPa, solid) and uneven and rather large in middle troposphere. For the approximated cloud regions (shaded) from tϪ36 to tϩ24 cross-section analysis in section 4, composite values are for the ST NWA and NEA composites. The cylindrical interpolated onto levels at 50-hPa intervals for plotting. projection with the latitude to longitude ratio at 45ЊN, and a plotting domain of 45Њ longitude ϫ 30Њ latitude, are used for these and all remaining ®gures in section 3. Composite results 3. The ST NWA low (Fig. 4) deepens from 1008.6 to 999.5, then to 980.8 hPa from t to t (by 9.1 and 18.7 a. Mean storm track and evolution Ϫ24 0 hPa, respectively), and afterward its central MSLP con- Figure 3 presents mean storm tracks of explosive cy- tinues to decrease to 967.7 hPa at tϩ24 (also Table 2). clones for the ST NWA and ST NEA categories. The Throughout the period the system is accompanied by ST NWA mean storm track (solid) starts near the Rock- increasing clouds, which display a distinct ``comma'' shape from t onward (Figs. 4d±f). In the NEA sector ies at t , and extends eastward just to the south of 0 Ϫ84 (Fig. 5), the projection-induced distortion is more severe Nova Scotia at t 0 then northeastward across Newfound- land toward the southern tip of Greenland. The ST NEA toward later stages as the mean storm moves farther north (cf. Fig. 3). The ST NEA cyclone's central MSLP cyclones have a mean track (dashed) farther east, with values are consistently lower than that in NWA, with the storm center at t to the southwest of Iceland near 0 larger 12-h pressure drops from t to t (by 14.7 and 58ЊN and extending toward Scandinavia near t . These Ϫ24 0 ϩ72 27.4 hPa). Its central MSLP minimum of 949.6 hPa is mean t cyclone positions correspond well with the fre- 0 reached at t , then the system starts to ®ll (Figs. 5e quency maxima in NWA and NEA sectors in Fig. 1. ϩ12 and 5f). Table 2 shows that the MSLP of the mean ST NWA The NEA incipient low develops from a pressure cyclone drops more rapidly from t to t and slower Ϫ36 0 trough, with a ``parent cyclone'' to the northeast. The afterward to gradually reach a minimum value of 967.3 two lows move closer and the parent cyclone (989.7 hPa at tϩ36. The mean NEA storm has more dramatic hPa) is well inside the plotting domain at tϪ12 (Fig. 5c). pressure drops between tϪ24 and t 0, and obtains the low- They rotate around each other cyclonically, and merge est MSLP of 949.6 hPa at tϩ12. This difference in pres- sometime between tϪ12 and t 0 as the most rapid 12-h sure falls is due mainly to the latitudinal adjustment of deepening occurs (Fig. 5d). It is worthwhile to note that the Bergeron criterion. In terms of deepening rates, more since the composite origin is the incipient low center, signi®cant deepening (positive values) for ST NEA cy- the averaged strength of parent cyclones in reality is clones occurs from tϪ48 to after tϩ12, but extends longer stronger than it appears in Fig. 5. On the other hand, to near tϩ36 for NWA ones (Table 2). The averaged max- little indication exists for either the parent cyclone or imum deepening rate is 2.42 Ber for ST NEA cyclones pressure trough in NWA (Fig. 4). and 2.17 Ber for ST NWA ones, suggesting that NEA Other differences are also noticeable when the ST cases are on average slightly more violent than NWA NWA and NEA cases are compared. The ST NEA cy- ones. After explosive development, the ST NEA mean clone appears to be stronger with a tighter MSLP gra- deepening rate drops rapidly to below zero values by dient, especially immediately to its south and southeast tϩ24, indicating faster ®lling than NWA storms. before and during explosive deepening (but the increase

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FIG. 4. Composite MSLP (hPa, solid lines) and areas with 1000±500-hPa mean relative humidity (RH) greater than 75% (gray shades) from (a) tϪ36 to (f) tϩ24 for the ST NWA. Isobar intervals are 4 hPa. Labels along the left and bottom edges are latitude and longitude relative to the low pressure center in Њ, and the latitude to longitude ratio used corresponds to that at 45ЊN. The same plotting domain and label meaning are used in Figs. 5±11. Lines AB are those used to construct vertical cross sections in Fig. 12.

in near-surface wind should be comparable based on the 1989). By tϪ12 the self-development process (Petterssen geostrophic relationship). The NEA mature storm tends 1956a, b) has started to further amplify both temperature to weaken more rapidly after t 0 as its central MSLP rises and height waves while promoting frontogenesis (Fig. by tϩ24, while the NWA low has not started to ®ll (also 6c). The thermal wave becomes severely distorted by Table 2). Cloud area appears larger in NEA, but a greater t0 (Fig. 6d), with a ridge extending toward the cyclone portion of the difference after t 0 may be arti®cially in- center from the southeast, and the thermal gradient starts troduced by the distortion. to weaken by tϩ24 (Fig. 6f). In NEA (Fig. 7), the basic evolution is similar but the 850-hPa response is again greater than in NWA. The c. The 850-hPa height and temperature central height falls by 226 m from tϪ12 to t0 and reaches The 850-hPa-level geopotential height (m, solid), an impressive 892 m at tϪ12 (lowest among all categories, temperature (K, dash), and positions of absolute vortic- Figs. 7c±e). The height gradient and ␩ maximum (Table ity (␩) maxima are shown in Figs. 6 and 7. For ST 3) associated with the short wave are stronger in NEA

NWA, the disturbance at tϪ36 and tϪ24 appears in the and weaker in NWA. The temperature gradient at tϪ36 height ®eld as a short-wave trough with a well-de®ned and tϪ24 in NEA is stronger and concentrated in a narrow vorticity maximum (Figs. 6a and 6b). Signi®cant de- zone across the surface low, and its thermal ®eld dis- velopment starts by tϪ12, and the 850-hPa low deepens tortion is greater during rapid development (Figs. 7a±d). by 160 to 1180 m at t 0 and further to 1034 m at tϩ24, After t0 a warm core seclusion forms and the temper- with the strongest height gradient immediately to the ature gradient deteriorates dramatically in NEA (Figs. southeast of the center (Figs. 6c±f). The temperature 7e and 7f). The warm core seclusion, most often ob- disturbance has a rather small initial amplitude but lags served in rapid marine cyclogenesis, forms when warm- the height wave by about ¼ wavelength with the stron- er air is wrapped around the low center by colder air gest gradient near the surface low, ideal for baroclinic (e.g., Neiman and Shapiro 1993; Reed et al. 1994). The development (Sanders 1971; Warrenfeltz and Elsberry evolution of ST NEA cyclones at 850 hPa is quite sim-

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FIG. 5. Same as Fig. 4 but for ST NEA. Lines AB are those used to construct vertical cross sections in Fig. 13. ilar to the model proposed by Shapiro and Keyser sphere, the trough turns from positive (NE±SW) to neg-

(1990). In contrast, the NWA temperature gradient re- ative (NW±SE) tilt. Further wave ampli®cation after t 0 mains relatively strong (Fig. 6), and a warm core se- eventually causes the low to cut off by tϩ24 (Fig. 8f). clusion is not apparent in NWA composites, which is Due to the self-limiting process, the vorticity advection perhaps related to data resolution. In NEA, the parent dipole weakens even though the height gradient south- cyclone is again visible at 850 hPa at tϪ12, and merges east of the low remains strong. The evolution is in very with the incipient disturbance (Figs. 7c and 7d). good agreement with sea level (Fig. 4), as MSLP con-

tinues to fall through about tϩ24 when CVA directly above the low becomes too small. d. The 500-hPa height and vorticity advection Several differences are evident when Figs. 8 and 9

Figures 8 and 9 depict composite 500-hPa geopoten- are compared. First, at tϪ36 and tϪ24 the height gradient tial height (m, thick), absolute vorticity advection (10Ϫ9 across the region of development is stronger in NEA, Ϫ2 s , thin), and positions of vorticity maxima. At tϪ36 and consistent with a stronger 850-hPa thermal gradient. tϪ24 the incipient trough in NWA lies about 7Њ±10Њ west This produces stronger CVA and reinforces the stronger of the surface low, with cyclonic vorticity advection low-level incipient disturbance discussed earlier. Sec- (CVA) and divergence near the latter (Figs. 8a and 8b). ond, the 500-hPa CVA forcing is also stronger during

The 500-hPa wave ampli®es signi®cantly at tϪ12 (Fig. rapid development in NEA, where the CVA maximum Ϫ9 Ϫ2 8c), with an enhanced height gradient at the trough base exceeds 4.0 ϫ 10 s at tϪ12 (Fig. 9c) and reaches 4.9 Ϫ9 Ϫ2 and near the surface low, and concurrent development ϫ 10 s at t 0, with higher values (shaded) over re- occurs at lower levels (Figs. 4 and 6). This enhances gions of strong dif¯uence and along the narrow area the vorticity maximum (Table 3) and CVA, and helps just downwind from the trough (Fig. 9d). The dipole to initiate the self-development process. The 500-hPa structure is also stronger and the growth in vorticity Ϫ9 Ϫ2 Ϫ9 CVA grows from 2.8 ϫ 10 s at tϪ12 to 4.0 ϫ 10 larger during this stage in NEA (Table 3). Third, after Ϫ2 s at t0 (so does the vorticity maximum), and a strong rapid development the CVA in NEA weakens dramat- positive and negative vorticity advection dipole devel- ically and moves farther away from the surface center ops at the vorticity center (Fig. 8d). In response to the (Figs. 9e and 9f), while that in NWA remains strong increasingly distorted thermal ®eld in the lower tropo- and close. This is tied to the formation of a cutoff low,

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FIG. 6. Composite 850-hPa geopotential height (m, solid) and temperature (K, dash) from (a) tϪ36 to (f) tϩ24 for the ST NWA. Contour intervals are 40 m for height and 3 K for temperature. Positions of absolute vorticity (␩) maxima are also plotted as Xs.

which occurs shortly after t 0 in NEA but much later in tively small. As the jet core moves to the trough base, NWA, and in NEA is consistent with the diminishing placing the surface low under its left-front quadrant, the thermal gradient. Finally, the parent cyclone is present approaching trough ampli®es signi®cantly. This com- in NEA even at 500 hPa, suggesting its deep vertical bined CVA±jet streak forcing dramatically raises the Ϫ5 Ϫ1 extent, and shows interaction with the developing low. maximum divergence from 1.4 ϫ 10 s at tϪ24 to 3.3 Ϫ5 Ϫ1 Ϫ5 Ϫ1 As the parent cyclone approaches the incipient low from ϫ 10 s at tϪ12, and further to 4.1 ϫ 10 s at t 0 the northwest, the strong forcing causes the latter to (Figs. 10b±d). The divergent center gradually moves intensify and become the more dominant feature of the toward the northeast after rapid deepening, as the sur- two, then they merge (Figs. 9c±e). The role of the parent face low approaches its peak intensity (Figs. 10e and cyclone in the NEA sector will be further discussed in 10f). section 5. During the incipient stage the jet in NEA also intensi®es from 49 to 53 m sϪ1 but its center is northeast of

the surface low up to tϪ12 (Figs. 11a±c). The divergence e. The 250-hPa jet stream and divergence Ϫ5 Ϫ1 reaches a peak of 3.8 ϫ 10 s early at tϪ12 but remains Figures 10 and 11 show the jet stream forcing and comparable through t0, when a new wind maximum of evolution at 250 hPa with winds (streamlines and iso- 50msϪ1 forms south of the storm center (Fig. 11d). As tachs) and divergence (10Ϫ5 sϪ1, thin) plotted. In NWA, the cyclone migrates farther north, the separation be- the jet shows an overall southward movement relative tween the jet and the surface low increases after t0, and to the surface storm, and intensi®es from 45 to 55 m streamlines indicate formation of closed circulation at Ϫ1 s and becomes narrower between tϪ36 and t 0 as si- 250 hPa by tϩ24 (Fig. 11f). multaneous development occurs at all levels (Figs. Many differences are also found by comparing Figs. 10a±d). The 250-hPa trough inferred from the stream- 10 and 11. The stronger jet in NWA indicates that the line pattern evolves similarly to the 500-hPa trough, baroclinicity here is of greater depth, while that in NEA, except that it is slightly farther west and no closed cir- although stronger in a narrow zone at lower levels, is culation forms up to tϩ24. A region of divergence exists shallower. The primary jet center is southwest of the ahead of the trough but initially its magnitude is rela- surface low in NWA during development, suggesting

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FIG. 7. Same as Fig. 6 but for the ST NEA. the role of jet core dynamics. In contrast, the strongest the formation of closed circulation at 250 hPa, which

NEA winds appear near the ridge before t0, to the north- does not occur in NWA. east of the surface low center (Figs. 11a±c). In NWA, the divergence is relatively weak before t , but after Ϫ24 4. Vertical cross-section analysis the rapid deepening it remains comparable for a lengthy period, as the maximum decreases by merely 0.2 ϫ 10Ϫ5 Composite vertical cross sections are also constructed Ϫ1 s between t0 and tϩ24 (Figs. 10). The NEA divergence, along the lines AB in Figs. 4 and 5, along a direction on the other hand, is considerably larger than in NWA roughly perpendicular to the upper-level wind above the Ϫ5 Ϫ1 up to tϪ12, but drops rapidly after t0 to 1.4 ϫ 10 s surface storm center. The orientation of the AB line at tϩ24, consistent with the evolution in the MSLP ®eld rotates gradually and changes from northwest±southeast (Figs. 7 and 11). The jet and divergent region in NEA initially to nearly west±east at tϩ24. The sections pass also move farther away from the surface low center after through the sea level cyclone center at midpoint (marked explosive deepening, consistent with lower levels and as ``0''), while the horizontal axis distance at each time is computed using the mean storm latitude in Fig. 3.

TABLE 3. Absolute vorticity (␩) maximum (10Ϫ5 sϪ1) associated with developing cyclones for ST NWA and ST NEA categories from a. Northwest Atlantic composites tϪ36 to tϩ24 at 850 and 500 hPa. Figure 12 shows AB cross sections from tϪ36 to tϩ24 Time (h) for ST NWA. While the composites are unable to resolve features such as tropospheric fronts and tropopause folds t t t t t t Ϫ36 Ϫ24 Ϫ12 0 ϩ12 ϩ24 at their true intensity, they are useful for qualitative 850 hPa comparison. The approach of an upper-level PV [de®ned ST NWA 12.0 13.2 16.2 26.0 29.6 28.1 in Eq. (2)] anomaly, associated with a lower tropopause ST NEA 17.3 19.8 25.4 33.4 31.3 28.9 (approximated by the 2 PVU line), before t0 is evident 500 hPa (Fig. 12). By tϩ12, a tropopause undulation has clearly ST NWA 14.3 14.3 16.9 20.4 22.4 23.4 formed above the surface low in the composite (Figs. ST NEA 15.8 17.0 18.9 26.7 26.3 25.0 12e and 12f). Depicted by winds normal to the plane

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Ϫ9 Ϫ2 FIG. 8. Composite 500-hPa geopotential height (m, thick lines) and absolute vorticity (␩) advection (10 s , thin lines) from (a) tϪ36 to Ϫ9 Ϫ2 Ϫ9 Ϫ2 (f) tϩ24 for the ST NWA. Contour intervals are 60 m for height, and 10 s for vorticity advection with areas greater than 2 ϫ 10 s shaded. Positions of ␩ maxima are also plotted as Xs.

(thin lines), a jet maximum exists to the right (southeast) wind shear is quite dramatic between tϪ12 and t 0 near of the PV anomaly near the region of steepest tropo- the PV center, along with a rapid upward extension of pause slope, and shows an overall relative rightward winds left of the storm center (Fig. 13c and 13d). At movement. The horizontal wind shear increases signif- sea level the cyclone circulation reaches a peak of ϳ23 Ϫ1 icantly at tϪ12 and t0, as does the PV gradient across the ms at tϩ12 (Fig. 13e), but the associated PV anomaly tropopause (Figs. 12c and 12d). At low levels the evo- has started to weaken considerably from 1.2 PVU. The lution is characterized by PV generation, as a near-sur- ␪ cross section (not shown) indicates downward bending face maximum separated from the upper-level anomaly of ␪ surfaces over regions near the storm center by t 0, appears by t 0. Applying the ``isentropic PV viewpoint'' suggesting warm core development and a zone of weak (Hoskins et al. 1985), the interaction between the two stability, in good agreement with the 850-hPa results anomalies afterward is evident as both positive and neg- (Fig. 7). Such a downward bending of ␪ surfaces is ative winds associated with the PV anomalies extend imposed by a warm anomaly below (Bretherton 1966; vertically, creating a deep-tropospheric cyclonic shear Hoskins et al. 1985) and is commonly found in oceanic zone. The low-level PV center reaches a peak of 1.4 explosive cyclones (e.g., Gyakum 1991). PVU at tϩ12 then weakens marginally to 1.3 PVU at tϩ24 (Figs. 12e and 12f). c. Comparison between NWA and NEA b. Northeast Atlantic composites When NWA and NEA cross sections (Figs. 12 and Cross sections for the ST NEA, corresponding to Fig. 13) are compared, differences consistent with those 12, are presented in Fig. 13. The PV evolution in the found in section 3 can be seen. The upper-level PV AB cross sections is similar to that in NWA; however, values during development are higher in NEA than in the near-surface maximum (0.7 PVU) appears by tϪ36 NWA at the same pressure level (with a lower tropo- at 920 hPa, then grows to 1.0 PVU and rises slightly at pause), especially at tϪ12 and t 0.Att0, more high-PV tϪ12 (Figs. 13a±c). The increase in the jet's horizontal air has been advected over the cyclone center in NEA,

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FIG. 9. Same as Fig. 8 but for the ST NEA. while afterward the anomaly continues to grow more in wind shear in NEA diminishes after development, in- NWA. dicative of a more equivalent barotropic structure, but Regarding the upper-level jet, strong winds extend remains relatively strong in NWA (Figs. 12 and 13). In downward from its center in NEA during the incipient the ␪ cross sections (not shown), the warm core structure Ϫ1 stage, best depicted by the 20 m s isotach (Figs. 13a (downward bending of ␪ surfaces) by t 0 is more visible and 13b). This extension in NWA is weaker and isotachs in NEA, and the ␪ surfaces near the storm center are have a broader shape, suggesting a larger area with ap- separated by greater distances. The latter indicates a preciable thermal wind and thus baroclinicity on the lower static stability, which will be further discussed in plane (Figs. 12a and 12b). By the same reasoning, the the following section. thermal gradient in early NEA stages must be strongest where vertical wind shear is large and minimized where 5. Discussion isotachs are nearly vertical. The change in jet strength during development differs greatly, as there is virtually Many features seen in sections 3 and 4 are consistent no reduction from tϪ12 to tϩ24 in NWA but substantial between levels and among variables in a dynamical reduction (from 44 to 28 m sϪ1) in NEA (Figs. 12 and sense and, therefore, did not arise randomly from com- 13). positing. These structural and evolutionary character- The low-level PV anomaly, when it forms, is clearly istics of ST NWA and ST NEA composite explosive separated from the upper-level anomaly in both sectors, cyclones are further discussed below in different as- but the appearance of a distinct center occurs much pects, with an emphasis on the northeast Atlantic sector. earlier in NEA (before tϪ36) than in NWA (after tϪ12). Its strength is also greater in NEA partly due to stronger a. Cyclone evolution and life cycle shearing vorticity linked to the downward extension of strong winds. The NEA anomaly reaches maximum at For the ST NEA cyclones, results in sections 3 and t0 then weakens rapidly, but that in NWA continues to 4 indicate a faster evolution and shorter life cycle, com- grow through tϩ12 and remains comparable at tϩ24. The pared to ST NWA ones. This is manifested in the rel- upward extension of cyclone circulation left of center atively early peak of upper-level forcing during the de- between tϪ12 and tϩ12 is quicker in NEA. The vertical velopment and, after the rapid deepening, in 1) fast

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FIG. 10. Composite 250-hPa streamlines (thick lines with arrows), isotachs (m sϪ1, thick white lines), and divergence (10Ϫ5 sϪ1, thin lines) Ϫ1 Ϫ1 from (a) tϪ36 to (f) tϩ24 for the ST NWA. Isotachs start from 25 m s at5ms intervals, with heavier shades in between for stronger winds. Contour intervals for divergence are 1 ϫ 10Ϫ5 sϪ1, and dash lines indicate negative values (convergence). weakening of sea level cyclone and forcing components, energy (APE) to kinetic energy as the baroclinic insta- 2) deterioration of thermal and height gradients, and 3) bility is released (see, e.g., section 8.3 of Holton 1992). the rapid occlusion and establishment of a more equiv- This enhanced destabilization in NEA is apparently alent barotropic structure. In his pioneer work, Charney caused by both the faster approach of an upper-level (1947) showed that for waves with the same wavelength, cold low (or trough) and low-level warm core devel- baroclinic instability increases with vertical wind shear, opment. For instance, the 500-hPa height at cyclone lapse rate (i.e., decreases with stability), and latitude. center falls by 460 m from tϪ36 to t 0 in the ST NEA, Hence, further works were carried out to examine the but only by 270 m in the ST NWA (Figs. 8 and 9). evolution in stability and thermal wind during the life Time series of the composite 1000±500-hPa thermal cycle of composite cyclones. wind speed averaged over an 11 ϫ 11 box (ϳ2750 ϫ Figure 14 shows the 1000±500-hPa (pressure weight- 2750 km2) is plotted to represent the evolution of overall ed) stability factor computed from composite ␪ and av- environmental baroclinicity (Fig. 15). The ST NWA eraged over a 5 ϫ 5 box (ϳ1250 ϫ 1250 km2) centered thermal wind (solid) increases during early stages and Ϫ1 at the cyclone center from tϪ84 to tϩ72 for the ST NWA reaches a maximum of ϳ24 m s at t 0. In contrast, the p) in Eq. peak thermal wind speed in the ST NEA (dashed) isץ/␪ץ)and ST NEA. The stability factor is Ϫg Ϫ1 (2), and for the ST NWA (solid) its value is about 5.5 only 20 m s at tϪ48, and the implied baroclinicity de- Ϫ3 2 Ϫ1 ϫ 10 m Kkg at early stages, decreases slightly at creases through the development from tϪ48 to tϩ36,toa Ϫ1 the onset of rapid development, rises from tϪ12 to tϩ12, value less than 10 m s , as the release of baroclinic then gradually decreases again. On the other hand, the instability is clearly visible. Moreover, our results also

ST NEA stability factor (dashed), though comparable suggest that the temperature gradient near tϪ24 in ST to ST NWA values initially, decreases signi®cantly NEA is strong only in a narrow zone near the incipient through rapid deepening stage to reach about 3.4 ϫ 10Ϫ3 system in the lower and middle troposphere, while the 2 Ϫ1 m Kkg near tϩ48. Such a condition facilitates the gradient in NWA is less concentrated but deeper. This development of vertical motion, which plays a key role reduced overall environmental baroclinicity in NEA, during the conversion process from available potential combined with the weaker stability (Fig. 14), is likely

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FIG. 11. Same as Fig. 10 but for the ST NEA.

to cause a rapid depletion of APE as the thermal wind b. Initial low-level system strength vanishes [see Eq. (3.38) of Holton 1992], in accordance Gyakum et al. (1992) show that for explosive mari- with the evolution in Fig. 15 and the short life cycle of time cyclones the frictionless vorticity equation at a sur- NEA cyclones seen in earlier sections. Thus, the ST face low center can be reduced to NEA cyclones weaken immediately after t0, while the ␩ץ /NWA ones continue to develop for a longer period and (V, (3 ´ ϭϪ␩١ tץ .or to weaken more slowly The rapid evolution of explosive cyclones in the Greenland±Iceland area offers yet one more reason for where V is the horizontal wind vector and the cyclone the underestimation of their frequency in the past be- vorticity grows roughly exponentially under constant sides inadequate data coverage. That is, since NEA cy- low-level convergence (presumably induced by diver- clones have faster life cycles, rapid development was gence aloft). Therefore, an antecedent surface vorticity more likely to be missed (compared to NWA ones) in development can be a useful indicator for further, more rapid, development. Compared to NWA, the ST NEA manual analysis because the time span for a key ship composite incipient cyclone is considerably stronger at observation to capture the phenomenon is shorter. Per- sea level and 850 hPa, but weakens more rapidly after haps partly related to this underestimation, past studies t (Figs. 4±7). The distinct near-surface PV maxima in mainly focused on rapid cyclogenesis closer to the North 0 the ST NEA appears much earlier, before tϪ36, and is American coast (e.g., Bosart 1981; Gyakum 1983a, b; on average about 0.3 PVU stronger than in the ST NWA Anthes 1990; Uccellini 1990; Kuo et al. 1991a, b). More before tϪ12 (section 4c). The stronger initial NEA dis- recently, with the aid of satellite data, operational mod- turbances are at least partially maintained by the forcing els with dense enough grids can better capture these aloft (section 3), but could also be linked to the fact events at higher latitudes through internal dynamics, and that they have moved over warmer water well before some cases have started to appear in the literature (e.g., t0, much longer than have NWA systems (Fig. 3). The Gadd et al. 1990; McCallum and Grahame 1993; GrùnaÊs stronger initial system strength at low levels in NEA 1995; KristjaÂnsson and Thorsteinsson 1995). suggests that it might play a greater role in determining

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FIG. 12. Composite vertical cross section along AB lines in Fig. 4 from (a) tϪ36 to (f) tϩ24 for the ST NWA. Plotted are PV (PVU, thick white lines) and horizontal wind speed normal to the section plane (m sϪ1, thin lines). For PV, contour intervals are 0.5 below 8 PVU with one extra line at 0.75 PVU, 1 from 8 to 12 PVU, and 2 above. The 2 PVU line is drawn in black, and heavier shades indicate higher PV values between 0 and 2 PVU, and from 2 PVU up. Isotach intervals are 5 m sϪ1 with solid and dashed lines for positive (into the plane) and negative (out from the plane) values. The bottom portion is masked with gray if below sea level. the subsequent maximum deepening rate, and applying some rather extreme cases listed in Table 1 (such as the antecedent development concept could have more cases 3 and 12), still exists near Iceland at latitudes as success here, at least for stronger cases. Indeed, since high as 60ЊN. In this respect, the proximity of the NEA explosive cyclones in NEA evolve faster and only a sector to the stratospheric high-PV reservoir certainly limited time is available to achieve a large deepening favors the creation of very strong IPV forcing, but up- rate before environmental APE is depleted, the initial per-level northerly winds obviously must also exist to system strength must become important for a more vi- advect high-PV air southward to interact with the de- olent development. veloping low, as discussed by Hoskins et al. (1985). On the 315-K isentropic surfaces of our ST NEA category (not shown), the composite PV anomaly is also c. Role of parent cyclone created through advection by northwesterly winds. Ta- From earlier sections, the upper-level forcing we ex- ble 4 shows that the association between the incipient amined at the commencement of the explosive devel- low and its parent cyclone is closer in stronger cases in opment is stronger in NEA, in terms of 500-hPa CVA, the NEA sector. Near the onset of most rapid deepening

250-hPa divergence, or cross-section PV anomaly. The (tϪ12), the composite parent cyclone in the ST NEA has produced net divergence during rapid deepening is also deep vertical extent and is just to the north of the in- greater in NEA, as con®rmed by examining the evo- cipient low (section 3). At upper levels, the northerly lution of vertically integrated column mass ¯uxes at the branch of its circulation enhances the southward ad- cyclone center (not shown). These results are in agree- vection of high-PV air toward the incipient low. At low ment with the Bergeron de®nition, since a larger sea levels, the con¯uence along the leading edge of cold air level pressure drop is needed to achieve the same deep- advection associated with the parent cyclone also en- ening rate at higher latitudes. The interesting aspect, hances local baroclinicity (and promotes moisture con- however, is that the forcing, strong enough to trigger vergence and surface energy transfer). Thus, a strong

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FIG. 13. Same as Fig. 12 but along AB lines in Fig. 5 for the ST NEA. parent cyclone for NEA cases tends to create favorable of explosive cyclones in the extreme northeast Atlantic conditions at both upper and lower levels in a timely obtained here are to some degree similar to those of fashion, and help to create further rapid development. polar lows, at least in a qualitative sense. These include Sequential cyclogenesis, similar to the process just dis- a short life cycle, strong low-level baroclinicity, and cussed, is perhaps the origin of the ``cyclone family'' weak stability in the vertical (Businger and Reed 1989), concept developed at the Bergen School in the 1920s. and their developments are both initiated by an upper- level CVA (or PV anomaly) under northerly winds (Ras- mussen and Lystad 1987; Rasmussen et al. 1992). d. Other aspects Emanuel and Rotunno (1989) used a theoretical hurri- Although their scale and associated pressure pertur- cane model to simulate polar low development, and sug- bation are vastly different, a number of characteristics gest that the mechanism of wind-induced surface heat exchange (WISHE; Emanuel 1986a; Rotunno and

FIG. 14. The 1000±500-hPa pressure-weighted stability factor computed from composite ␪ and averaged over a 5 ϫ 5 grid point box FIG. 15. The 1000±500-hPa thermal wind speed (m sϪ1) of com- centered at the cyclone center from tϪ84 to tϩ72 for the ST NWA and posites averaged over an 11 ϫ 11 gridpoint box centered at the

ST NEA cases. cyclone center from tϪ84 to tϩ72 for the ST NWA and ST NEA cases.

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TABLE 4. Association between incipient and parent cyclones in ST, developments still occur in the extreme NEA sector, MO, and WE categories of the NEA sector. From top to bottom: Total and some can be very violent. The composite upper- number of cases, number of cases with a parent cyclone, average level forcing, as indicated by 500-hPa CVA, 250- MSLP of parent cyclone at tϪ12, number of cases with direct merger, and signi®cant northerly wind at sea level west of incipient low at hPa divergence, and cross-section PV anomaly, is no tϪ12. less intense at the onset of the explosive deepening Strong Moderate Weak in NEA than in the NWA sector off the North Amer- ican coast. Total 19 18 16 2) Strong NEA explosive cyclones are associated with Parent cyclone 18 13 15 Avg MSLP (hPa) 979.3 984.1 987.9 lower static stability and locally concentrated but Direct merger 14 2 5 shallower thermal gradient with less overall envi- Northerly wind 19 14 13 ronmental baroclinicity. These factors lead to a rapid depletion of APE and, thus, a faster evolution and a shorter life cycle, with thermal gradient and forcing Emanuel 1987) for tropical cyclogenesis, although not components diminishing immediately after rapid de- spontaneous, is capable of sustaining a minimum sea velopment. In contrast, the NWA cases, being closer level pressure of about 922 hPa (their ``warm and to the major land±sea boundary, tend to have greater moist'' experiment using estimated sounding in a polar overall environmental baroclinicity, higher stability, environment), a value comparable to those in some ex- and evolve slower. treme NEA cases in Table 1. Emanuel (1986b) also 3) The initial low-level disturbances in NEA also tend proposed that the relative importance of WISHE to bar- to be stronger with earlier formation of a distinct PV oclinic instability for different types of lows during their anomaly near the surface. This initial system strength most rapid deepening stage is, in decreasing order, as appears to play a greater role in determining the follows: tropical cyclones, polar lows (Bear Island type subsequent maximum deepening rate in NEA cy- and comma cloud type), and ordinary cyclones. Based clones, consistent with their shorter life cycle. The on our ®ndings that the low-level PV anomaly during warm core structure produced during rapid deep- the incipient stage of ST NEA cyclones is important to ening is also more pronounced in the NEA events. further development, we suspect that a mechanism sim- 4) For NEA cyclones, the close proximity of the Green- ilar to WISHE could also contribute more in strong land±Iceland region to the stratospheric high-PV res- explosive cyclogenesis in the northeast Atlantic than in ervoir is one favorable condition to generate a strong lower latitudes. This question will be further investi- upper-level forcing. In addition, a parent cyclone gated in Part II of this paper, which presents results of with appreciable strength through a deep layer to the a vorticity budget analysis from a quantitative perspec- north or northeast of the incipient low appears es- tive using all 162 cases selected. sential in producing a rather extreme explosive cyclogenesis. The suitable synoptic conditions are created as the parent cyclone's circulation advects high- 6. Conclusions and summary PV air southward at upper levels and enhances local The present study employs twice-daily, gridded baroclinicity at low levels simultaneously. ECMWF operational analysis at 2.5Њ latitude±longitude Acknowledgments. The authors wish to thank Drs. Jay resolution from January 1985 to March 1996, and car- S. Hobgood, Ellen Mosley-Thompson, A. John Arn®eld, ries out a composite analysis on explosive cyclones in and John N. Rayner for their valuable suggestions and the northwest (18 cases) and extreme northeast Atlantic comments; Dr. David Bromwich and Mr. Richard Cul- (19 cases). To select ideal cases for the composites, the lather of the Byrd Polar Research Center for assistance geographical distribution of explosive cyclogenesis de- in data processing; and Mr. Jim DeGrand and Jens Ble- rived from ECMWF data is plotted, and the results sug- gvad for technical support. The valuable comments of gest a higher explosive cyclogenesis frequency in the two anonymous reviewers helped improve the manu- Greenland±Iceland region, and therefore the frequency script greatly. Thanks also go to the NCAR Data Support was likely underestimated in the past. Section for making ECMWF data available. This re- Characteristics of the dynamical and thermal structure search is funded by NSF Of®ce of Polar Programs under and evolution of strong composite explosive cyclones OPP-9726417. 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