2310 MONTHLY WEATHER REVIEW VOLUME 129 A Diagnostic Study of an Explosively Developing Extratropical Cyclone and an Associated 500-hPa Trough Merger JENNIFER L. S. STRAHL* AND PHILLIP J. SMITH Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana (Manuscript received 7 August 2000, in ®nal form 16 March 2001) ABSTRACT This paper presents a diagnosis of an explosively developing cyclone (1.3 Ber) that occurred in conjunction with a 500-hPa synoptic-scale trough merger over the eastern United States in November 1999. The explosive development occurred in response to cyclonic vorticity and warm air advections that maximized in the 250± 200-hPa layer and latent heat release. Explosive development commenced 12 h before trough merger began and then accelerated through the 12-h merger period. During merger, development was enhanced by increased warm air advection above 300 hPa and reduced adiabatic cooling. Finally, comparison of this case with other explosive cyclogenesis±trough merger cases suggests that at the synoptic scale trough merger is an evolutionary process that requires as little as 12 h to complete and that, although trough merger is not required to initiate explosive cyclogenesis, the occurrence of merger can be expected to increase the development rate. 1. Introduction (1990) suggests that trough mergers are associated with surface cyclogenesis, that this cyclogenesis is often ex- It is well known that inclement weather in midlati- plosive, and that the merger usually occurs 12 to 48 h tudes is often associated with deep extratropical cyclone prior to the time of minimum cyclone central pressure. systems. Therefore, it is not surprising that such systems Of the 21 merger cases they examined, only one was have been extensively researched since the nineteenth associated with a weakening cyclone and only two ex- century (see reviews by Kutzbach 1979; Hoskins 1990; hibited minimum cyclone pressures concurrent with Newton 1990; Uccellini 1990; Bosart 1999; Volkert merger, one an explosive developer, one not. It is this 1999). From all of this it has become clear that extra- latter, rarer circumstance that is the subject of this paper, tropical cyclones evolve in various ways, very often in in which a case of explosive cyclone development that association with baroclinic zones and upstream upper- occurred over the United States in November 1999 in level jet streams and troughs, all of which are under- association with a 500-hPa trough merger is examined. going their own evolution. Note that, as for most of the background trough merger Much contemporary research has focused on the ``ex- studies cited, this paper examines only the synoptic- plosively developing'' variety of extratropical cyclones scale features of the merger. (Sanders and Gyakum 1980; Uccellini 1990), shorter- wave upper-level troughs known as mobile troughs (Sanders 1988; Lefevre and Nielsen-Gammon 1995; 2. Data and methodology Nielson-Gammon and Lefevre 1996), and trough merg- a. Data ers (Lai and Bosart 1988; Gaza and Bosart 1990; Hakim et al. 1995, 1996; Dean and Bosart 1996). Of particular North American surface station reports and rawin- interest relative to the present paper are the relationships sonde reports comprise the data for this case study. For between trough merger and surface cyclogenesis con- the period examined, pertinent variables were extracted tained in the latter papers. from the 0000 and 1200 UTC surface and upper-air The limited climatological study by Gaza and Bosart reports, including pressure, geopotential height, tem- perature, and wind speed and direction. In addition, sea level pressure at 0000, 0600, 1200, and 1800 UTC and * Current af®liation: SAIC, Monterey, California. 6-h accumulated precipitation amounts from U. S. sta- tions at 0000 and 1200 UTC were retrieved from surface reports. Precipitation data were not available from Ca- Corresponding author address: Prof. Phillip J. Smith, Department of Earth and Atmospheric Sciences, Purdue University, 1397 Civil nadian stations. This, however, did not affect the di- Engineering Building, West Lafayette, IN 47907-1397. agnosis during explosive cyclone development. E-mail: [email protected] Data from all reported levels were interpolated lin- q 2001 American Meteorological Society Unauthenticated | Downloaded 10/01/21 04:41 PM UTC SEPTEMBER 2001 STRAHL AND SMITH 2311 early in ln(p) to yield data in 50-hPa intervals from strophic vorticity on an isobaric surface as a function 1050 to 50 hPa. Winds below the ground were set to of explicit dynamic and thermodynamic forcing mech- zero. Geopotential heights were calculated from the anisms at all levels of the atmosphere. Introduced in hypsometric relationship and the interpolated tempera- quasigeostrophic form by Zwack and Okossi (1986), a tures. Below-ground temperatures were calculated as- generalized form of the equation was developed by suming a lapse rate of 6.58Ckm21. Lupo et al. (1992), as well as a simpli®ed form, known as the ``extended'' form, appropriate for synoptic-scale diagnoses. This simpli®ed form of the equation neglects b. Surface cyclone dynamics forcing that is largely subsynoptic in space and time. In addition, frictional effects have been neglected due to This study employs the Zwack±Okossi (Z±O) equa- the dif®culty of accurately capturing effects of friction tion as the primary diagnostic tool for assessing cyclone with routine station observations. The resulting extend- dynamics. This equation describes changes in geo- ed Z±O equation is ]z ppplllRQdpÇ gl 2 5 PD 2V ´ =za dp 2 PD ¹2V ´ =T 11Sv dp, (1) ]tfcpE EE p ppp12 tt[] (A) (B) (C) (D) where pl is the near-surface pressure level, pt the upper stability, S, is combined with vertical motion in this pressure level (50 hPa), zgl the near-surface geostrophic term, the opposition to development decreases, and thus vorticity, za the absolute vorticity (z 1 f ), f the Coriolis development is enhanced, for less stable air. parameter, R the dry air gas constant, V the horizontal All data analysis and Z±O equation calculations fol- Ç wind vector, Q the diabatic heating/cooling rate, cp the low the procedures described in Rolfson and Smith speci®c heat at constant pressure, S the static stability (1996). Refer to that paper for details. The analysis do- parameter (S 52(T/u)(]u/]p), where u is potential main is a 25 by 17 grid with 230-km grid spacing in temperature), v the vertical motion in isobaric coordi- the x and y directions, encompassing the continental United States (Fig. 1). Output is displayed on the 21 by nates (v 5 dp/dt), and PD 5 1/(pl 2 pt). The dynamic forcing term in (1) is term A, which 13 interior grid shown in Fig. 1. The gridding scheme represents the effect of horizontal vorticity advection on used in translating the sounding data to grid point values the geostrophic relative vorticity tendency. This term is a two-pass Barnes objective analysis (Barnes 1964, describes the adjustment of the mass ®eld to the chang- 1973). Vertical velocities at the grid points were cal- ing wind ®eld as represented by vorticity changes. culated using the omega equation in extended form, Terms B and C represent the effects of horizontal namely, temperature advection and diabatic heating on geo- f 22] strophic relative vorticity, respectively. The combined ¹12 (z 1 f ) v interior integral and inverse pressure weighting result 12s ]p2 in greater weight being given to temperature changing f ] RRQÇ processes that occur lower in the troposphere (Rausch 2 2 5 (V ´ =za) 1¹(V ´ =T) 2¹ . (2) and Smith 1996), while the Laplacian operator indicates s ]p sp spc12p the importance of the horizontal distribution (Smith This form of the omega equation is nearly identical in 2000). In contrast to the dynamic forcing, the thermo- form to the quasigeostrophic (QG) omega equation giv- dynamic terms describe the adjustment of the wind ®eld en in Holton (1992, 166±170). The primary differences to the changing mass ®eld. are the inclusion of diabatic heating and reliance on the Term D represents the effect of adiabatic temperature extended equation approach (Tsou et al. 1987). The ex- change on geostrophic vorticity. Upper-level divergence tended equations [(1) and (2)] are the same as the cor- (convergence) patterns initiated by cyclonic (anticy- responding QG equations except that the geostrophic clonic) vorticity advection and nonuniform heating winds and relative vorticity and isobarically constant (cooling) also result in upward (downward) motion. static stability in the latter are replaced by analyzed This upward (downward) motion results in adiabatic winds and vorticity and variable static stability. cooling (warming), which yields vorticity and height Diabatic heating was assumed to be solely latent heat changes that are usually of opposite sign to those of release, calculated from the 6-h precipitation rates. The terms A, B, and C. Therefore, term D often opposes the scheme for vertically distributing the latent heat release development prescribed by the other terms. Since static is described in full in Rolfson and Smith (1996); it yields Unauthenticated | Downloaded 10/01/21 04:41 PM UTC 2312 MONTHLY WEATHER REVIEW VOLUME 129 heating maxima at or near 700 hPa for grid points in the eastern portion of the United States and Canada. Horizontal derivatives were calculated using fourth-or- der ®nite differencing at grid points within and along the display domain box of Fig. 1. Second-order ®nite differencing was used to determine the horizontal de- rivatives at points one grid point out from the indicated box. Vertical derivatives were computed using second- order differencing, and vertical integrals were obtained using the trapezoidal rule. The Z±O computations produce instantaneous geo- strophic vorticity tendency values on isobaric surfaces. Subsynoptic-scale noise that occurs in the vorticity ten- dency calculations was reduced using a Shapiro (1970) ®lter that removes signals at wavelengths less than 1000 km and retains 90% of signals with wavelengths greater FIG.