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Downloaded 09/24/21 08:39 PM UTC 852 MONTHLY WEATHER REVIEW VOLUME 128 MARCH 2000 GYAKUM AND DANIELSON 851 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 be- ginning 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 (;28C) 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 anom- alies (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 m22 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 cy- clogenesis. 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 struc- tures 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- q 2000 American Meteorological Society Unauthenticated | Downloaded 09/24/21 08:39 PM UTC 852 MONTHLY WEATHER REVIEW VOLUME 128 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 358± 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. Unauthenticated | Downloaded 09/24/21 08:39 PM UTC MARCH 2000 GYAKUM AND DANIELSON 853 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) 224 h and (b) 0 h, and for 18 cases of strong, but nonexplosive (SN) cyclogenesis at (c) 224 h, and (d) 0 h. The heavy-solid box encloses the 358±408N, 1458±1508E region shown in Fig. 1, and in subsequent ®gures. 408N and 1458±1508E. 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 608.
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