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Potential Vorticity Diagnosis of a Tropopause Polar Cyclone

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Potential Vorticity Diagnosis of a Tropopause Polar Cyclone 1358 MONTHLY WEATHER REVIEW VOLUME 137 Potential Vorticity Diagnosis of a Tropopause Polar Cyclone STEVEN M. CAVALLO AND GREGORY J. HAKIM University of Washington, Seattle, Washington (Manuscript received 9 June 2008, in final form 15 October 2008) ABSTRACT Long-lived coherent vortices located near the tropopause are often found over polar regions. Although these vortices are a commonly observed feature of the Arctic, and can have lifetimes longer than one month, little is known about the mechanisms that control their evolution. This paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex (TPV) using an Ertel potential vorticity (EPV) diagnostic framework. Results from a climatology of intensifying cyclonic TPVs suggest that the essential dynamics are local to the vortex, rather than a consequence of larger-scale processes. This fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex. A component-wise breakdown of EPV reveals that cloud-top radiational cooling is the primary diabatic mechanism that intensifies the TPV during the growth phase. Increasing amounts of moisture become entrained into the vortex core at later times near Hudson Bay, allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity. 1. Introduction In the context of PV, vortex amplitude on the dy- namic tropopause can be defined by the range of closed Tropopause vortices are extratropical, cold-core cy- contours of potential temperature.1 Therefore, changes clones or warm-core anticyclones, defined by closed in vortex strength require nonconservative diabatic or material contours. On the dynamic tropopause, taken frictional processes (e.g., Pedlosky 1998, his section 2.5). to be a potential vorticity (PV) surface, cyclones are This PV perspective has been applied extensively to characterized by relatively high pressure (lowered tro- studies of warm-core cyclones, where significant latent popause) and low potential temperature while anticy- heating has been shown to affect vortex intensity (e.g., clones are characterized by relatively low pressure Shapiro and Willoughby 1982; Schubert and Hack 1983; (raised tropopause) and high potential temperature. The Nolan and Grasso 2003). In these cases, latent heating Arctic is particularly favorable for these features, with produces low-level PV, which intensifies the vortex in relatively weak vertical and horizontal wind shear pro- regions near where the vortices are strongest. viding an environment conducive for vortex longevity. Several studies have addressed the effect of diabatic Horizontal length scales of these tropopause polar vor- processes on the PV distribution near extratropical cy- tices (TPVs) are most often less than 1000 km in radius, clones. For example, Davis and Emanuel (1991) used a and their life span can extend beyond one month (Hakim piecewise PV inversion methodology in a case study and Canavan 2005). Despite their ubiquity and longevity, quantifying the development of a low-level PV anomaly little is known about the mechanisms that control their produced by latent heating. They found that this low- evolution. This study focuses on mechanisms affecting level PV anomaly subsequently contributed to the cyclonic vortex intensity, and specifically examines the reduction of downstream tropopause-level PV, and sug- life cycle of an observed event using a numerical model gested that in addition to advection, destruction due to and a PV diagnostic framework. 1 Assuming adiabatic and inviscid flow, PV and potential tem- Corresponding author address: Steven Cavallo, Department of perature are both conserved following the fluid motion. Thus, the Atmospheric Sciences, University of Washington, Box 351640, existence of closed PV contours on potential temperature surfaces Seattle, WA 98195-1640. implies the existence of closed contours of potential temperature E-mail: [email protected] on PV surfaces. DOI: 10.1175/2008MWR2670.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 10/02/21 04:25 PM UTC APRIL 2009 C A V A L L O A N D H A K I M 1359 latent heating could also have been an important mech- are provided in section 5. A concluding summary is given anism in amplifying downstream PV. Using a similar in section 6. technique in a numerical modeling study, Stoelinga (1996) showed that latent heating destroyed upper-level 2. Tropopause polar vortex climatology of PV downstream and above the latent heating source in intensification the direction of the absolute vorticity vector, consistent with theoretical expectations. Vortices are identified here by the presence of closed Few studies have examined the effect of nonconser- contours of potential temperature on the dynamic tro- vative PV processes on the dynamics of isolated cold- popause (e.g., Hakim 2000), defined by a PV surface. core extratropical vortices. Cold-core cyclonic vortices Adiabatic conditions imply that fluid parcels located strengthen upward in the troposphere, and since low- within closed contours act as tracers of the vortex. A level latent heating typically destroys upper-level PV, change in the range of closed potential temperature latent heating leads to a weaker vortex (e.g., Hoskins contours on the PV surface representing the tropopause et al. 1985, their section 7). For example, Wirth (1995) provides one objective measure of vortex amplitude performed an idealized study of an axisymmetric tro- change, and we employ this method in our analysis. popause cyclone and found that a latent heat source Using the vortex dataset of Hakim and Canavan (2005), located in the cyclone core acted to weaken the vortex we extend the results of their climatology in this section considerably. Adding an idealized thin layer of modest to examine a TPV climatology of intensification, with cooling to simulate the effect of cloud-top radiational the goal of determining where vortex intensification cooling had little effect on the cyclone evolution. How- occurs and whether there exists a relationship between ever, increasing the magnitude of the cloud-top radia- intensification and large-scale flow patterns. tional cooling strengthened the vortex, suggesting that Regions of cyclonic TPV intensity change are deter- radiation may play a role in cyclonic vortex intensity mined from 2.58 National Centers for Environmental change, depending on the exact magnitude and location Prediction–National Center for Atmospheric Research of the cloud-top radiational cooling relative to the tro- (NCEP–NCAR) reanalysis data every 6 h during 1948– popause (Wirth 1995). Idealized radiational cooling in 99 (Kalnay and collaborators 1996). Tropopause vorti- isolation was shown in a subsequent idealized study of ces are identified and tracked using an objective algo- an axisymmetric anticyclonic vortex to create upper- rithm developed and described in Hakim and Canavan level PV and lower the tropopause (Zierl and Wirth (2005), which tracks TPVs on the 1.5 PVU (1 PVU [ 1997). The combined interactive effects of latent heating 1 3 1026 Kkg21 s21 m2) PV surface. Cyclonic vortex and cloud-top radiational cooling on vortex intensity do amplitude, defined as the difference between the local not appear to have been previously considered. minimum in potential temperature (which we call the Hakim and Canavan (2005) performed an objective core value) and the last closed contour, is used to de- census of tropopause vortices and found an average of termine the strength of the vortices relative to their sur- 15 unique cyclonic vortices per month, with a frequency roundings. TPVs are defined as in Hakim and Canavan maximum over the Canadian Arctic. Here we extend (2005) to include only those vortices that spend at least these results by examining the vortex census results of 60% of their lifetime north of 658N and last at least 2 Hakim and Canavan (2005) to identify regions of TPV days. To filter out spurious intensity changes, we discard genesis, growth, and decay. A numerical simulation of a any event for which there is a tropopause potential particular case is then analyzed from a PV framework temperature amplitude change of at least 10 K followed with the goal of investigating how diabatic and radia- by an amplitude change of at least 75% of the opposite tional processes affect vortex strength. sign within the following 12 h; this eliminates about 2% This paper is organized as follows. Section 2 provides of the vortices. Although an arbitrary threshold, high- a brief climatology of cyclonic TPV genesis and inten- frequency amplitude changes such as these are consid- sity change, the results of which suggest that the es- ered to be numerical artifacts, beyond the range of sential dynamics are local to the vortex rather than observed EPV tendency attributable to diabatic ten- dependent on large-scale patterns. This finding moti- dencies (e.g., Davis and Emanuel 1991; Davis 1992; vates the use of PV diagnostics on a numerical simula- Stoelinga 1996). Vortex genesis is defined to occur by the tion of a particular case. The theory and methodology identification of a local minimum in tropopause potential for the PV diagnostics are described in section 3, and an temperature that is colder than all other locations within overview of the case study is provided in section 4. A a 650-km radius, which is roughly the vortex length description of the numerical model and model simula- scale; results are not sensitive to this radius. Vortex lysis tion results from the case study, including a PV budget, is defined to occur when a local minimum in tropopause Unauthenticated | Downloaded 10/02/21 04:25 PM UTC 1360 MONTHLY WEATHER REVIEW VOLUME 137 potential temperature can no longer be identified with DP D 1 v Du $u F 5 v Á $u 5 a Á $ 1 Á $ 3 . an existing vortex track. Growth (decay) refers to the Dt Dt r a r Dt r r greatest increase (decrease) in amplitude during a 24-h (1) period over the vortex lifetime.
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