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Analysis of Meteorological Precursors to Ordinary and Explosive Cyclogenesis in the Western North Paci®c

JOHN R. GYAKUM AND RICHARD E. DANIELSON Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

(Manuscript received 30 July 1998, in ®nal form 26 March 1999)

ABSTRACT Thirty-®ve cases of cyclogenesis that occurred during the cold seasons from 1975 to 1995 in the western North Paci®c Ocean are studied to determine common and disparate dynamic and thermodynamic structures in both the ordinary and rapid developments. An analysis of 1000-hPa height and 1000±500-hPa thickness anomalies with respect to the 20-yr climatology reveals the following results. Though each sample of cyclogenesis is characterized by a favorable-appearing thickness trough±ridge structure, important differences are found. Both the upstream surface anticyclone and the downstream precedent cyclone are preferentially stronger at the beginning of the most rapid cyclogenesis in the strong sample. Because of the consequently stronger equatorward ¯ow, the 1000±500-hPa thickness anomaly in the strong sample is colder by approximately 40 m (ϳ2ЊC) in the region of incipient cyclogenesis and eastward by 1500 km. A harmonic time series analysis of NCEP gridded ®elds partitions the geopotential height ®elds into high- (corresponding to synoptic-scale waves) and low-frequency wave components. This analysis shows the 500-hPa synoptic-scale disturbances that trigger both ordinary and rapid cyclogenesis are easily tracked as early as 72 h prior to the event. These triggering disturbances, 72 h prior to the most rapid cyclogenesis, are found most typically in central Siberia. Additionally, the synoptic-scale trough±ridge couplet is stronger at the onset of development for the explosive sample, suggesting a stronger large-scale forcing for cyclogenesis. To gain insight into possible physical mechanisms associated with these structural differences, the SST anomalies (with respect to a 30-yr climate) in the rapid developments are compared with those of the weaker systems. Though there is no statistically signi®cant difference in SST anomalies, the preferentially colder tropospheric air mass in the strong sample suggests this sample to be characterized by stronger surface ¯uxes. Indeed, the NCEP reanalyses reveal both the sensible and latent heat ¯uxes to be 50±75 W mϪ2 greater in the rapid development cases in the region along their subsequent cyclone tracks. These statistically signi®cant differences are also re¯ected in moisture budget analyses, which reveal surface evaporation to be larger in the explosive cases. This evaporation component contributes importantly to the computed precipitation in each class of cyclogenesis.

1. Introduction ger antecedent development (Gyakum et al. 1992). Kelly et al. (1994) emphasized the role of ¯anking cold pools The purpose of this research is to understand better as being particularly prominent in the rapid cyclogenesis the dynamic and thermodynamic distinctions between cases. Additional physical insight was limited in these cyclogenesis of an ordinary nature, and the more dan- studies, owing to the relatively limited amount of di- gerous explosive process. Much of the extensive extra- agnostic information available from the National Me- tropical cyclone research since 1980 has focused on the teorological Center [NMC, now National Centers for latter category of development in terms of case studies Environmental Prediction (NCEP)] octagonal grid that (Gyakum 1991) or climatological studies (e.g., Sanders is compiled on the compact disk (Mass et al. 1987). and Gyakum 1980; Roebber 1984; Rogers and Bosart Our study examines the possible interactive roles that 1986; Lackmann et al. 1996). the ocean and the atmosphere play in the development Bullock and Gyakum (1993), studying cyclogenesis of especially rapid cyclogenesis in a climatologically in the western North Paci®c Basin, have concluded that active region of the western North Paci®c Ocean. This preferentially strong cyclogenesis is preceded by stron- examination is accomplished with an initial study of the large-scale geopotential height and temperature structures of rapid and ordinary cyclogenesis that occurs in approximately the same location and in the same season. Corresponding author address: Dr. John R. Gyakum, Dept. of At- mospheric and Oceanic Sciences, McGill University, 805 Sherbrooke The goal is to identify systematically different atmo- Street West, Montreal, QC H3A 2K6, Canada. spheric circulations associated with each cyclogenesis E-mail: [email protected] type. Since especially rapid cyclogenesis occurs pref-

᭧ 2000 American Meteorological Society

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TABLE 1. List of cases considered for this study (an asterisk means not used for reasons discussed in the text). Strong (moderate bomb) Weak (strong nonbomb) Time Intensi®cation Time Intensi®cation Date (UTC) (Bergerons) Date (UTC) (Bergerons) 19 Oct 1975 1200 1.5 *1 Dec 1979 1200 0.8 *22 Oct 1975 0000 1.3 6 Dec 1979 1200 0.7 9 Nov 1978 0000 1.4 29 Oct 1980 1200 0.8 26 Jan 1979 0000 1.4 6 Dec 1980 1200 0.6 1 Dec 1981 1200 1.4 26 Dec 1980 1200 0.9 1 Feb 1982 0000 1.5 3 Mar 1983 0000 0.8 7 Feb 1982 0000 1.4 29 Jan 1985 1200 0.6 20 Mar 1984 1200 1.5 1 Mar 1985 1200 0.7 14 Dec 1985 1200 1.5 24 Oct 1985 1200 0.6 5 Mar 1986 1200 1.3 1 Jan 1986 1200 0.6 1 Mar 1987 1200 1.3 29 Mar 1987 1200 0.6 14 Jan 1992 0000 1.4 7 Mar 1988 0000 0.9 8 Feb 1992 1200 1.8 27 Dec 1989 1200 0.9 10 Jan 1993 0000 1.7 23 Feb 1991 0000 0.9 1 Feb 1993 0000 1.2 18 Feb 1992 1200 0.9 14 Feb 1993 0000 1.4 15 Oct 1992 1200 0.6 5 Feb 1995 1200 1.9 25 Oct 1992 0000 0.8 10 Feb 1995 0000 1.5 28 Dec 1992 1200 0.8 28 Mar 1995 0000 0.8 erentially over oceanic regions (Sanders and Gyakum in order to establish the importance of surface evapo- 1980), we also study the sea surface temperature (SST) ration in the rapid cyclogenesis process. structures associated with our samples of cyclogenesis. Finally, we examine diagnostic ®elds of surface sensible 2. Methodology and latent heat ¯uxes to evaluate the qualitative con- sistency between these structures and those of the at- The cases considered for this study, along with the mosphere and the SSTs. The surface latent heat ¯ux is maximum intensi®cation rates, are listed in Table 1. The also a component of the moisture budget, and we eval- choice of domain is such that each system must begin uate the moisture budget for each case of cyclogenesis its maximum deepening in the region bounded by 35Њ±

FIG. 1. Geographic locator map showing the features discussed in the text. The heavy-solid box encloses the region in which the maximum cyclogeneses begins.

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FIG. 2. Composite 1000-hPa height (solid, interval of 60 m) and 1000±500-hPa thickness (dashed, interval of 60 m) for 17 cases of strong cyclogenesis (MB) for (a) Ϫ24 h and (b) 0 h, and for 18 cases of strong, but nonexplosive (SN) cyclogenesis at (c) Ϫ24 h, and (d) 0 h. The heavy-solid box encloses the 35Њ±40ЊN, 145Њ±150ЊE region shown in Fig. 1, and in subsequent ®gures.

40ЊN and 145Њ±150ЊE. This region is shown as the The speci®c cases are derived from the North Paci®c boxed area of Fig. 1, which is also used as a locator Ocean cyclone dataset as originally compiled by Gyak- map for the geographic features discussed in the text. um et al. (1989), but these have been updated to include The choice of the region is based upon its location 20 cold seasons. These data are derived from the NMC downstream of good surface and upper-air data coverage ®nal manual analyses that are available at 6-hourly in- in a climatologically active area of the Kuroshio Current tervals, except for 1991±92, when 12-hourly maps only in the western North Paci®c Ocean (Sanders and Gyak- are archived. While the 6-hourly time interval helps in um 1980; Roebber 1984). The selection criterion used the determination of an individual low's continuity, the is based upon each case's maximum deepening rate: 1.2± cyclone data are archived at the 0000 and 1200 UTC 1.9 Bergerons for the strong cases, and from 0.6±0.9 times. Bergerons for the weak cases. We will also refer inter- The 37 surface cyclones found for the 120 months of changeably to these respective samples as ``strong'' or data are divided nearly equally among the strong and ``explosive'' (or ``MB'' for moderate bomb) and weak cases with respective numbers of 18 and 19. We ``weak'' or ``ordinary'' (or ``SN'' for strong nonbomb). remove one of the strong cases (22 October 1975) be- This terminology is consistent with that used by Bullock cause of its short time lag after the ®rst case (2.5 days). and Gyakum (1993), though the intensi®cation criteria We then remove one case from the weak set (1 Decem- used are slightly different in this study. Following Sand- ber 1979) that occurred only 5 days prior to another ers and Gyakum (1980), a Bergeron is de®ned as the system. These case deletions are designed to ensure a product of 24 hPa and the quotient of the sine of the reasonable semblance of independence between the cas- latitude and the sine of 60Њ. Therefore, a 24-h central es. The speci®c choice for removing these speci®c cases, pressure fall of 16.9 hPa at 37.5Њ of latitude corresponds as opposed to their ``nearest neighbor,'' is that we retain to one Bergeron. The period considered consists of 20 the system that maximizes the separation in intensi®- cold seasons (1 October±31 March) from 1975 to 1995. cation between the categories. These two systems are

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FIG. 3. Anomalies of 1000-hPa height with respect to climatology for the MB composite at (a) Ϫ24 h and (b) 0h. Difference (MB minus SN) ®elds of 1000-hPa height anomalies at (c) Ϫ24 h and (d) 0 h. Shaded areas in (c) and (d) illustrate signi®cance levels computed from the Student's t-test of the differences with progressively darker regions showing thresholds of 90%, 95%, and 99%. Contour interval is 20 m, with no zero contour, and positive/negative values are solid/dashed. listed with an asterisk in Table 1. Therefore, we use 17 dition to regions indicating statistical signi®cance at var- strong and 18 weak cases in our composites. ious levels of con®dence (90%, 95%, and 99%). The The use of a relatively small geographic region allows reader will note that various small regions are high- us to produce composites of relevant features with a lighted as being different at the 90%, or higher, con®- minimum of smearing at the onset of most rapid cyclo- dence level. Such regions, may simply be different due genesis. These features will be shown at varying inter- to chance. We will discuss only those regions of statis- vals from Ϫ72 h to the onset of maximum deepening, tically signi®cant differences in which there is a good thus emphasizing the dynamical conditioning processes apparent physical difference among the cases. prior to the cyclogeneses. Atmospheric data are provid- The next section discusses the atmospheric climato- ed by the NCEP reanalyses (Kalnay et al. 1996) com- logical anomaly ®elds associated with each class of cy- piled during the 1975±95 period. As the authors discuss, clogenesis. A technique for partitioning of the meteo- the NCEP reanalyses are being produced from the his- rological ®elds that documents the role of synoptic-scale torical data archives back to 1957 in order to provide processes in the cyclogenesis is discussed in section 4. consistent diagnostic information whose changes are not SSTs and surface ¯uxes are examined in section 5. Mois- a consequence of analysis changes. These data include ture budgets are studied in section 6. A concluding dis- heights at 1000 and 500 hPa, and water vapor and winds cussion follows in section 7. at mandatory levels from 1000 to 100 hPa. The data used are those compiled on the 2.5Њ lat ϫ 2.5Њ long grid, 3. Composites of circulation anomalies having been interpolated from the native T62 spectral data. Surface sensible and latent heat ¯uxes, averaged Figure 2 shows the composite strong and weak 1000- over 6-h time periods are also used. Differences between hPa height and 1000±500 hPa thickness ®elds at the the strong and weak composites are displayed in ad- time of onset of most rapid intensi®cation, hereafter

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FIG. 4. As for Fig. 3 except for ®elds of 1000±500-hPa thickness. referred to as ``0 h,'' and 24 h prior to this onset. Though line. The difference of the strong and weak cases, seen both composites show a persistent surface ridge ex- in Figs. 3c and 3d, reveal the most extensive regions of tending from Mongolia into China, the downstream dif- statistically signi®cant difference to be the surface low ferences between MB and SN are more apparent. The east of the Kamchatka Peninsula, and the upstream an- strong sample is characterized by a strong, persistent ticyclone. These two anomalous features, signi®cantly cyclone between the Kamchatka Peninsula and the date stronger in the MB sample (based upon a two-sided line with geostrophic cold advection extending equa- Student's t-test at the 90%±95% con®dence level), ac- torward to 30ЊN. The weak sample, in contrast, has a count for more equatorward geostrophic ¯ow into the more typical climatological position of the persistent region of incipient cyclogenesis. low near the Aleutian Islands. Additionally, the incipient The 1000±500-hPa composite thickness anomalies composite low is slightly weaker than in the explosive for the strong sample (Figs. 4a,b) illustrate an extensive sample at 0 h. These features have been noted previously region of anomalously cold air, averaging 4 dam (2ЊC) by Kelly et al. (1994). colder than climatology in a small region upstream in Since there is a large variability of dates among the eastern China at Ϫ24 h and in the Sea of Japan at 0 h. cases in each sample, we show the 1000-hPa height and A more extensive region of relatively cold air extends 1000±500-hPa thickness ®elds in terms of composite eastward from the Kuroshio to the date line. This par- anomalies from the appropriate half-monthly mean de- ticular area of cold air is absent in the weak sample rived from the NCEP data of 1975±95. Figure 3, show- (Figs. 4c,d), and is a consequence of the stronger north- ing the composite 1000-hPa height anomalies for the erly geostrophic ¯ow (and cold advection) associated strong cases, reveals the expected region of negative with the anomalously strong surface low east of Kam- anomaly covering the Kuroshio region. Additionally, chatka in the explosive cases (Figs. 3a,b). Sanders and Figs. 3a and 3b show a large upstream anticyclonic Davis (1988) have noted the presence of prominent anomaly, a relatively weak downstream ridge, and a large-scale cold 1000±500-hPa thickness anomalies that particularly large cyclonic anomaly that con®rms the were unique to the more extreme cases of explosive westward shift of the Aleutian low to west of the date cyclogenesis in the western North Atlantic basin at sim-

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FIG. 5. Composite MB 500-hPa height partitioned as the 14-day time mean (discussed in the text), plus the ®rst and second harmonics (light solid, interval of 60 m), and the remaining harmonics (heavy, interval of 20 m, with dashed showing negative values) for (a) Ϫ72 h, (b) Ϫ48 h, (c) Ϫ24 h, and (d) 0 h. The ``ϫ'' symbols mark the locations of the high-frequency height minima discussed in the text, with the exception of panel d in which the ϫ is omitted for clarity. ilar latitudes. Though the authors strati®ed the cases time series in the subsequent 156-h period. We then differently (their ``strong'' being greater than 1.8 Ber- produce a Fourier decomposition on this doubled time gerons and ``weak'' being less than 1.4 Bergerons), the series. This partitioning includes the 1) time mean, 2) similarity in results is clear in terms of especially rapid the ®rst harmonic (with a 14-day period) plus the second cyclogenesis being preferentially associated with anom- harmonic (with a 7-day period), and 3) the remaining alously cold airmasses slightly poleward of the devel- 12 higher frequency harmonics that consist of the 4.67- opment region. Thus, a stronger westerly thermal wind day to 24-h periods. The slowly varying trend for the (thermal gradient) is shown. The additional detail in our period from Ϫ156to0hisincluded in the ®rst harmonic results, showing the unique downstream cold pool of of the re¯ected time series as one-half of its 14-day air has potentially important implications for the surface period. ¯uxes, as will be discussed. We present these composite partitioned 500-hPa ®elds in Fig. 5 for the strong sample of cases at 24-h intervals from Ϫ72 h to the onset of the rapid cyclogenesis. This 4. Composites of partitioned 500-hPa heights composite is characterized by a robust high-frequency A means by which we may illustrate the structure of negative anomaly about 700 km. upstream of the cy- the 500-hPa heights, is to decompose the total ®eld time clogenesis centered over central Japan and the Sea of series at each grid point. We ®rst consider a time series Japan. Its signature may be traced upstream to eastern that consists of 14 consecutive 12-hourly data extending China at Ϫ24 h, east of Lake Baikal at Ϫ48 h, and from 0 h earlier to Ϫ156 h. Considering that we intend central Siberia at Ϫ72 h. Its movement is readily de- to discuss the synoptic-scale disturbances preceding the tected along the slowly varying northwesterly advecting cyclogenesis, and also want to avoid the presentation ¯ow. Sanders (1986) also documents southeastward- of spurious waves, we produce an arti®cial mirror-image traveling ®nite-amplitude 500-hPa vorticity maxima

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FIG. 6. As in Fig. 5 except for the 18-case sample of weak cyclogenesis (SN). several days in advance of U.S. east coast cyclogenesis. Comprehensive Ocean±Atmosphere Data Set (COADS; Though the weak composite structure (Fig. 6) appears Woodruff et al. 1987) that consists primarily of mobile similar to that of the strong composite throughout the ships and buoys. The 2Њ lat ϫ 2Њ long grid is chosen 72-h period, the synoptic-scale disturbances are gen- because the COADS 30-yr monthly climatologies are erally weaker. These differences appear in Fig. 7, in archived on this grid. The anomaly for each case is which at 0 h, the relatively ampli®ed trough-ridge cou- computed as the difference between the value analyzed plet of the strong composite has a half-wavelength of on the grid and the daily climatology that is derived 1500 km. Further, the relatively stronger negative height from the monthly means. This daily climatology is anomalies in the strong sample are different from that found for each grid point from a ®t to the ®rst, second, of the weak sample for each time with at least the 90% and third harmonics of the annual time series. con®dence level. The strong composite shows no SST anomalies of amplitude 1Њ or greater within the domain (Fig. 8a). Chen et al. (1992) have found enhanced rapid cyclo- 5. Sea surface temperatures and surface ¯uxes genesis in the Kuroshio Current region to be prefer- Composite strong and weak SST and anomaly ®elds entially associated with relatively warm SST anomalies. are shown in Fig. 8. The analyses are performed with Furthermore, the weak composite (Fig. 8b) shows slight- a Cressman (1959) objective analysis scheme using a ly colder SST anomalies that extend eastward of the variable in¯uence radius. The nearest 15 observations cyclogenesis region. The SST±anomaly differences are are used for each grid point (2Њ lat ϫ 2Њ long) and the shown in Fig. 9. A broad region of positive difference largest distance of the observation, plus 1 km, consti- extends from the cyclogenesis region and approximately tutes the in¯uence radius. If the data density does not 800 km eastward. However, there are only a few grid allow for at least 15 observations within 400 km of the points in this region in which the strong case composite grid point, then no analysis is performed. The bound- SST anomalies are signi®cantly (at the 95% con®dence aries of the analyses seen in Fig. 8 are de®ned using level) warmer than those of the weak case composites. this convention. The observations are taken from the Nevertheless, given the broad region of anomalously

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FIG. 7. Differences between the MB and SN high-frequency 500-hPa heights (interval of 10 m, with solid/dashed contours indicating positive/negative values). Shaded areas in each panel illustrate signi®cance levels computed from the Student's t-test of the differences with progressively darker regions showing thresholds of 90%, 95%, and 99%. cold air eastward of the cyclogenesis region that is es- sible and latent heat ¯uxes. The ®elds are 6-hourly pecially characteristic of the strong cases (Fig. 4), the means for the period beginning with the stated times. surface ¯uxes may be strongly affected by the combi- Thus, a reference to ``0 h'' means the 6-h period from nation of the SST and thickness anomaly differences. 0toϩ6 h. The sensible heat ¯uxes reveal a strength- The implications of these differences are discussed in ening during the period extending from Ϫ12to0hin the next section. the explosive cases, particularly in the upstream region Among the potentially important processes in cyclo- of the Sea of Japan, and in the region of the surface genesis are those of the surface ¯uxes of sensible heat cyclogenesis. The ®elds of difference between the MB and moisture. Although these processes are not univer- and SN ¯uxes (Figs. 10c,d) show positive differences sally accepted as crucial to cyclogenesis, Kuo et al. everywhere in the vicinity of the cyclogenesis. Differ- (1991) have identi®ed surface energy ¯uxes to be es- ences in excess of 50 W mϪ2 are found in the vicinity pecially crucial during the early development phases of of and eastward of the region of cyclogenesis. Similar a sample of seven explosive cyclones in the western structures and differences in the surface evaporation North Atlantic Basin. We address the issue here on the ®elds are shown in Fig. 11, except that the values of basis of the ¯uxes derived from NCEP's reanalysis. As the latent heat ¯uxes are approximately twice those discussed by Kalnay et al. (1996), these ¯uxes are de- shown for the sensible heat ¯uxes in the region of cy- rived from the global model ®elds that are forced by clogenesis. Bosart (1981) and Bosart et al. (1995) have the data assimilation. Kanamitsu (1989) discusses the identi®ed the downstream surface ¯uxes of sensible and surface ¯ux parameterization used in the global model, latent heating as being particularly important to the dy- which is a bulk aerodynamic formula proportional to namics of individual cases of U.S. east coastal cyclo- the differences between the values of either temperature genesis. He suggests that the surface lows would ingest or moisture at the surface and the adjacent atmosphere. warm, moist, and weakly strati®ed air as they travel Figures 10 and 11 show the respective ®elds of sen- toward such conditioned regions. More recently, Carrera

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FIG. 9. Difference of MB and SN SST anomalies (solid/dashed for positive/negative, interval of 1ЊC with no zero contours). Shaded areas exceed the 95% con®dence level, as computed from the Stu- dent's t-test of the differences. The region of analysis is bounded by the heavy dashed curve.

to be important to the processes distinguishing ordinary from explosive cyclogenesis. Other regions have been used in the moisture budget computations, and have revealed qualitatively similar results. We compute the terms in the expression (see Kuo and Anthes 1984) wץ hPa 1000 1 (qv) dp ϪϩE, (1)´ P ϭϪ١ tץ ͵ g 300 hPa is the horizontal gradient ١ ,where P is the precipitation operator, g is the gravity, v is the horizontal wind vector, FIG. 8. SST (solid, interval of 2ЊC) and its anomalies from the 30- q is the speci®c humidity, p is the pressure, t is the time, yr daily climate (discussed in the text) with light solid/dashed con- E is the evaporation, and w is the precipitable water tours corresponding to positive/negative values at an interval of 1ЊC de®ned by with no zero contours for the (a) MB and (b) SN samples. Progres- sively darker shading shown for increasing anomaly amplitudes. The 1 1000 hPa solid box encloses the 35Њ±40ЊN, 145Њ±150ЊE region shown in earlier w ϭ (q) dp. (2) ®gures. The region of analysis is bounded by the heavy dashed curve. g ͵ 300 hPa We de®ne the vertical integral of the water vapor transport vector as et al. (1999) have shown a case of rapid cyclogenesis along the U.S. east coast to be crucially dependent on 1 1000 hPa Q ϭ (qv) dp, (3) local evaporation as a source for its condensational heat- g ͵ ing. 300 hPa so that (1) may be rewritten as wץ (Q ϪϩE. (4 ´ Moisture budgets P ϭϪ١ .6 tץ To understand further the possible implications of the differences in the surface latent heat ¯uxes, we utilize The evaluation of Eq. (3) and the ®rst term on the a water vapor budget for the region bounded by 30ЊN right-hand side of (4) is shown in Fig. 12. The results, to the south, 40ЊN to the north, 145ЊE to the west, and similar for each of the ordinary and explosive samples 155ЊE to the east. The region encompasses 1.01 ϫ 106 at Ϫ12 and 0 h, include west-southwesterly vapor trans- km2, and covers the region of cyclogenesis and regions ports extending from the South China Sea, where there extending 5Њ lat equatorward and 5Њ long eastward. The is the largest amount of surface evaporation (Fig. 11). eastward and southward asymmetry in the budget region Newell and Zhu (1994) have termed these deep-tro- is based upon our results in which the downstream thick- pospheric water vapor transport vectors as atmospheric ness structures and latent heat ¯uxes (Figs. 4, 11) appear ``rivers'' that are also related to extratropical cyclogen-

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FIG. 10. Composite MB surface sensible heat ¯ux for (a) Ϫ12 through Ϫ6 h, and (b) 0 through ϩ6 h (contour interval is 50 W mϪ2). Difference of the composite MB and SN sensible heat ¯uxes for (c) Ϫ12 through Ϫ6 h, and (d) 0 through ϩ6 h (contour interval is 25 W mϪ2, with solid/dashed contours for positive/negative values). Shaded areas in panels c and d illustrate signi®cance levels computed from the Student's t-test of the differences with progressively darker regions showing thresholds of 90%, 95%, and 99%. esis in the Atlantic basin. Additionally, in each sample convergence of the moisture transport is computed at at 0 h, there is strong convergence of the vertical integral Ϫ12 h and at 0 h, and time averaged. The precipitable of the transport, located eastward of the cyclogenetic water changes are computed from 12-h ®nite differences area, which exceeds 10 mm (12 h)Ϫ1 in each sample at from Ϫ12 to 0 h, and the evaporation is computed from 0h. the surface latent heat ¯uxes, averaged for the Ϫ12 to Each of the terms on the right side of (4) is evaluated Ϫ6 h, and for the Ϫ6 to 0 h periods. for the 35 cases in our sample. The composite of each For each of the samples in the precipitation equation term in the budget is shown in Table 2. The precipitation (4), the convergence of moisture transport is over- term is computed as the sum of the convergence of whelmed by the storage increase. The evaporation, in moisture transport, the loss due to storage changes, and fact, contributes importantly to the implied condensation the evaporation. The water vapor budget is computed in both of the samples. What distinguishes the explosive for the 12-h period extending from Ϫ12hto0h.The sample from that of the ordinary cases is the enhanced evaporation in the former sample. The 0.9 mm difference in evaporation over the 1.01 ϫ 106 km2 domain TABLE 2. Moisture budget results for the 12-h period from Ϫ12 to 0 h (mm). Numbers marked with an asterisk indicate a statistically is statistically signi®cant at the 95% con®dence level. signi®cant (95% con®dence level) difference in the same column from Though the computed precipitation is also 0.9 mm larger that of the other sample. in the strong composite, this difference is not signi®cant. w The implied importance of surface evaporation to latentץ Ϫ Precipitation heating is evident during the earliest phases of surface (t Evaporation (computedץ Q ´ Sample Ϫ١ cyclogenesis in both samples. Moreover, the moisture MB 3.3 Ϫ3.9 3.2* 2.6 budget composites suggest the possible importance of SN 3.1 Ϫ3.7 2.3* 1.7 local surface ¯uxes of water vapor to latent heating.

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FIG. 11. As in Fig. 10 except for the surface latent heat ¯uxes.

7. Concluding discussion Second, we have utilized a harmonic analysis method We have documented several structural distinctions that is capable of tracking the composite precedent trig- between explosive and ordinary cyclogenesis in the gering disturbances in the lower and the upper tropo- western North Paci®c basin. It should be emphasized sphere. This method allows for the isolation of the fea- that the conclusions found herein are strictly valid only tures that correspond to the timescales of synoptic-scale for the systems that develop in this region. However, disturbances. This analysis reveals that high-frequency since we are studying a sample of cases that occurred (periods ranging from 4.67 to 1.0 days) negative 500- during a 20-yr period in a region that is climatologically hPa height anomalies can be traced northwestward to active, we believe the conclusions are applicable to cy- the vicinity of Lake Baikal in each sample (Figs. 5, 6). clogenesis in other topographically similar regions, such A notable difference is the strong sample's stronger up- as in the vicinity of the Gulf Stream Current of the North stream negative height anomaly, combined with its Atlantic Ocean. stronger downstream ridge (Fig. 7d). This enhanced up- First, we have identi®ed planetary and synoptic-scale per-level trough±ridge couplet, combined with the back- structures that occur preferentially in the strong sample. ground westerly thermal wind, provide for a more fa- Although the basic patterns of 1000-hPa height and vorable development environment (e.g., Bluestein 1992, 1000±500-hPa thickness appear similarly con®gured in p. 339). each sample (Fig. 1), the strong set is characterized by Third, the thermodynamic preconditioning in the ex- a stronger large-scale and persistent surface cyclone lo- plosive cases appears to be crucial to their preferentially cated between Kamchatka and the date line (Fig. 3). strong development. The signi®cantly stronger sensible This feature represents a westward shift of the clima- and latent heat ¯uxes (Figs. 10 and 11) are a manifes- tological Aleutian Low. At the onset of maximum cy- tation of at least two factors: the slightly warmer SST clogenesis (Fig. 3d), there is also a slightly stronger anomalies in the explosive cases (Fig. 9), and the pref- surface anticyclone in eastern China. These two features erentially colder air mass downstream of the explosive combine to advect anomalously colder air into and east- systems (Fig. 4). Additionally, the possibly faster wind ward of the Kuroshio (Fig. 4). speeds associated with the incipient strong cases (Figs.

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FIG. 12. Vertical integral of the water vapor transport (reference vector shows 700 kg mϪ1 sϪ1), as de®ned by Eq. (3), and the horizontal divergence of this quantity [contour interval of 2 mm (12 h)Ϫ1 with dashed/solid showing negative/positive] for (a) MB at Ϫ12 h, (b) MB at 0 h, (c) SN at Ϫ12 h, and (d) SN at 0 h.

2b, d; Fig. 3d) may be a factor in enhancing the surface appreciate the constructive comments made by the three ¯uxes, since the bulk aerodynamic representation is pro- anonymous reviewers. portional to the square of the wind speed. The importance of surface ¯uxes as crucial conditioning processes in North American east coastal surface cyclogenesis has REFERENCES been discussed by Bosart et al. (1995) and by Kuo et al. (1991). The dynamical consequences of these added Bluestein, H. B., 1992: Principles of Kinematics and Dynamics. Vol. I, Synoptic-Dynamic Meteorology in Midlatitudes, Oxford Uni- sensible heat ¯uxes may include static destabilization, versity Press, 431 pp. and ampli®ed lower-tropospheric baroclinicity (if these Bosart, L. F., 1981: The Presidents' Day snowstorm of 18±19 Feb- ¯uxes are occurring preferentially in the warm sector), ruary 1979: A subsynoptic-scale event. Mon. Wea. Rev., 109, both of which are favorable for development. The pos- 1542±1566. , C. C. Lai, and E. Rogers, 1995: Incipient explosive marine sible implications of the ampli®ed surface latent heat cyclogenesis: Coastal development. Tellus, 47A, 1±29. ¯uxes (Fig. 11) include their contributions to latent heat Bullock, T. A., and J. R. Gyakum, 1993: A diagnostic study of cy- of condensation in the vicinity of the cyclogenesis. The clogenesis in the western North Paci®c Ocean. Mon. Wea. Rev., moisture budget, as computed in this region, suggests 121, 65±75. that surface evaporation contributes importantly to the Carrera, M. L., J. R. Gyakum, and D.-L. Zhang, 1999: A numerical case study of secondary marine cyclogenesis sensitivity to initial latent heating in both ordinary and rapid cyclogenesis error and varying physical processes. Mon. Wea. Rev., 127, 641± (Table 2). 660. Chen, S.-J., Y.-H. Kuo, P.-Z. Zhang, and Q.-F. Bai, 1992: Climatology Acknowledgments. This work has been supported by of explosive cyclones off the east Asian coast. Mon. Wea. Rev., 120, 3029±3035. a Natural Sciences and Engineering Research Council Cressman, G. P., 1959: An operational objective analysis system. research grant, and by a subvention from the Atmo- Mon. Wea. Rev., 87, 367±374. spheric Environment Service of Canada. The authors Gyakum, J. R., 1991: Meteorological precursors to the explosive

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