956 MONTHLY WEATHER REVIEW VOLUME 132 An Investigation of Extratropical Cyclone Development Using a Scale-Separation Technique KENNETH E. PARSONS Department of Meteorology, Embry±Riddle Aeronautical University, Prescott, Arizona PHILLIP J. SMITH Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana (Manuscript received 24 February 2003, in ®nal form 23 October 2003) ABSTRACT The explosive development phase of an extratropical cyclone (ETC) is examined using output generated by the ®fth-generation PSU±NCAR Mesoscale Model (MM5). A full-physics run of MM5 with 60-km grid spacing was used to simulate the intensive observation period (IOP)-4 storm of 4±5 January 1989 from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). A diagnosis of the simulated ETC is performed using the Zwack±Okossi (Z±O) equation to examine the forcing mechanisms in¯uencing development. A second- order Shapiro ®lter is used to partition the terms in the Z±O equation into synoptic-scale and subsynoptic-scale contributions to the near-surface synoptic-scale geostrophic vorticity tendency. Results con®rm that previous work using the Z±O equation at coarser resolutions correctly identi®ed synoptic- scale processes as the most important cyclone development mechanisms. However, the results also show that both adiabatic and diabatic subsynoptic thermal processes can make important contributions to synoptic-scale ETC development. 1. Introduction Hoskins 1990, 1999; Reed 1990; Uccellini 1990; Bosart 1999; Volkert 1999). In most respects, the development (i.e., the formation, Today, cyclone development is seen to be related to intensi®cation, and subsequent decay) of extratropical a variety of mechanisms. These mechanisms can be cyclones (ETC) is a well-understood process. That un- grouped into those related to dynamic effects, that is, derstanding has progressed from the thermal theory of cyclonic vorticity advection aloft (Sanders 1986; Mac- cyclogenesis that was prominent in the nineteenth cen- Donald and Reiter 1988), vorticity preconditioning tury (Kutzbach 1979), through the polar front theory of (Gyakum et al. 1992), and favorable jet streak posi- cyclones that was developed during the early twentieth tioning (Uccellini et al. 1984; Uccellini and Kocin century by Bjerknes and others (e.g., Bjerknes 1919; 1987); and thermal effects, that is, upper-tropospheric/ Bjerknes and Solberg 1922), continuing with the work lower-stratospheric warm-air advection (Hirschberg and of Sutcliffe (1939, 1947) and Petterssen (1955), and into Fritsch 1991a,b; Lupo et al. 1992), latent heat release contemporary interest in explosive cyclogenesis (e.g., (Pauley and Smith 1988; Reed et al. 1988), surface en- Sanders and Gyakum 1980; Roebber 1984; Sanders ergy ¯uxes (Kuo et al. 1991), and reduced static stability 1986; Lupo et al. 1992). Summaries of the progress that (Smith and Tsou 1988). Alternately, cyclogenesis can has been made in understanding the development of be viewed as a response to changing potential vorticity ETCs can be found in the volumes deriving from the (e.g., Davis and Emanuel 1991; Hakim et al. 1996). PalmeÂn Memorial Symposium on Extratropical Cy- These studies are examples of the remarkable pro- clones and the International Symposium on the Life gress that has been made over the past century and a Cycles of Extratropical Cyclones (e.g., Newton 1990; half in scienti®c understanding of the synoptic-scale forcing processes that are important in ETC develop- ment. Yet, an important question regarding ETC de- Corresponding author address: Dr. Kenneth E. Parsons, Embry± Riddle Aeronautical University, 3700 Willow Creek Road, Prescott, velopment remains to be answered, namely, ``What, if AZ 86301. any, is the role of subsynoptic processes in ETC de- E-mail: [email protected] velopment?'' While the synoptic-scale impact of syn- q 2004 American Meteorological Society Unauthenticated | Downloaded 09/24/21 12:22 PM UTC APRIL 2004 PARSONS AND SMITH 957 optic-scale ensembles of subsynoptic heating are well More recently, Newton and Holopainen (1990) ob- known, little is known about the impact of individual served, ``A question not explicitly considered at length subsynoptic heating elements nor about the role of sub- in this monograph concerns the feedback of subsynop- synoptic transport processes. This is not to say that the tic-scale features on the larger ¯ow.'' They then pre- potential importance of such processes has gone unrec- dicted, ``It is probably a safe forecast that scale inter- ognized by the scienti®c community. Holopainen and action questions of this type will come up at many meet- Nurmi (1979) warned that ``. forcing due to unre- ings in the future.'' Also in that volume, Shapiro and solved horizontal scales may need more attention than Keyser (1990) noted the debate over downscale versus is often believed.'' Using the dense network of rawin- upscale impacts between mesoscale and synoptic-scale sonde stations in Europe to investigate the impact of processes. They further observed that research into scale subgrid-scale processes on upper-air winds, they ob- interaction processes would and should continue in or- served that the total ¯ow ®eld was stronger than the der to advance the understanding of atmospheric de- smoothed ¯ow ®eld and described this effect as the velopment processes. smoothed ¯ow being ``. accelerated by the horizontal To address this question, Rausch and Smith (1996) sub-grid scale processes.'' The speci®c forcing mech- analyzed the Experiment on Rapidly Intensifying Cy- anism was ¯ux convergence of momentum. They further clones over the Atlantic (ERICA) intensive observation advocated the use of output from high-resolution nu- period (IOP)-4 cyclone. They presented a diagnosis that merical models to extend the study of scale interactions included, in addition to synoptic-scale forcing, the role past the use of real data from a synoptic-scale upper- that subsynoptic-scale processes played in extratropical air network. cyclone development. Their analysis looked at forcing Maddox (1980) presented a method of separating me- terms that included the exchange of ``information'' be- teorological data into macroscale and mesoscale com- tween the synoptic scale and subsynoptic scale as rep- ponents. In that study, he observed that a mesoscale resented by vorticity and temperature exchange pro- perturbation, that is, a mesoscale convective system, cesses. The objective of the work described herein is to could interact with the macroscale pattern to suf®ciently further explore the importance of such synoptic/sub- modify the upper-air pattern so as to be detectable in synoptic exchanges using a more successful simulation the synoptic-scale upper-air observations. In studying of the ERICA IOP4 storm. scale interactions with respect to the kinetic energy bud- get, Carney and Vincent (1986) used an enhanced net- work of upper-air observations from one synoptic case 2. Diagnostic technique during the Second European Stratospheric Arctic and a. Diagnostic equation Mid-latitude Experiment (SESAME) ®eld program to examine the in¯uence of organized deep convection on To conduct the study, the Zwack±Okossi (Z±O) equa- the synoptic-scale ¯ow. They found both signi®cant tion, ®rst introduced in quasigeostrophic form by Zwack generation of kinetic energy at the synoptic scale and and Okossi (1986), is used as the tool to diagnose the dissipation of kinetic energy to the subsynoptic scale. development of an explosively deepening extratropical In addition, they found evidence of momentum transport cyclone. Derived from the equation of state, the vorticity processes that were the result of scale interactions be- equation, and the First Law of Thermodynamics, the tween the synoptic-scale motion ®eld and the subsy- generalized Z±O equation is (see Lupo et al. 1992 for noptic-scale mass ®eld. development) ]z 1 ppssRQdpÇ ]v ]v ]y ]v ]u gds 2 5 (2V ´ =paz ) 2¹2ppV ´ = T 11Sv 1 z a22 ]tpst2 pfE E cpp ]p ]x ]p ]y ]p pp1212 r [ (a) (b) (c) (d) (e) (f) (g) ]za g ]]z ]zag 2 v 1 k ´ =p 3 F 1 b 2 dp, ]pf]y12]t ]t ] (1) (h) (i) (j) (k) where ]zgs/]t is the near-surface geostrophic relative vor- eter; za the absolute vorticity (z 1 f ); zag the ageostroph- 2 ticity tendency; V the horizontal wind vector; T the ic vorticity; =p and¹p the horizontal del and Laplacian temperature; v the vertical motion (dp/dt); p the pres- operators on an isobaric surface; Rd the dry-air gas con- 21 21 sure; ps 5 950 mb; pt 5 100 mb; f the Coriolis param- stant (287 J kg K ); cp the speci®c heat for air at Unauthenticated | Downloaded 09/24/21 12:22 PM UTC 958 MONTHLY WEATHER REVIEW VOLUME 132 constant pressure (1004 J kg21 K21); F the frictional verse pressure weighting and double integration of the force; QÇ the diabatic heating rate; S the static stability thermodynamic terms, which when combined reveal parameter (RdT/cpp 2]T/]p); g the acceleration of grav- that these mechanisms are more heavily weighted when ity (9.8 m s21); and b 5]f/]y. The terms in (1) are placed lower in the atmosphere (Rausch and Smith identi®ed as: (a) geostrophic vorticity tendency at p 5 1996), con®rming the results of Tracton (1973), Anthes ps; (b) horizontal absolute vorticity advection term; (c) and Keyser (1979), and Gyakum (1983) regarding the horizontal temperature advection term; (d) diabatic heat- release of latent heat. These studies demonstrate that the ing term; (e) adiabatic term; (f) divergence term; (g) three-dimensional distribution of heating can be as im- tilting term; (h) vertical absolute vorticity advection portant as the total amount of heating in determining term; (i) frictional term; (j) beta term; and (k) ageo- ETC development. strophic vorticity tendency term. The adiabatic term represents the cooling (warming) that results from ascent (descent) of air in a column. Because the static stability is nearly always positive b.