The Effect of Large-Scale Flow on Low-Level Frontal Structure and Evolution in Midlatitude Cyclones

Abstract

Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach. Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving into confluent, low-amplitude zonal flow becomes zonally elongated and possesses strong zonally oriented warm and bent-back fronts and a weak cold front. The frontal structure in this case is reminiscent of the Shapiro-Keyser cyclone model, exhibiting a fracture between perpendicularly oriented cold and warm fronts (i.e., the so-called frontal T-bone structure). The idealized simulations employ a nondivergent barotropic model in which potential temperature is treated as a passive tracer. When a circular vortex acts on an initially zonally oriented baroclinic zone, cold and warm fronts, a frontal fracture, a bent-back front, and eventually a Norwegian-like occlusion develop. When a circular vortex is placed in a diffluent background flow, the vortex and frontal zones become meridionally elongated, and the evolution resembles the Norwegian occlusion with a narrowing thermal ridge. When a circular vortex is placed in a confluent background flow, the vortex and frontal zones become zonally elongated, and the evolution resembles the Shapiro-Keyser model with a frontal fracture, frontal T-bone, and bent-back front. Although the idealized model qualitatively reproduces many of the frontal features found in the observed cyclones analyzed in the present study, one significant difference is that the maximum potential temperature gradient and frontogenesis along the cold and warm fronts may differ by a factor of 2 or more in the observed cases, but remain equal along the cold and warm fronts throughout the idealized model simulations. Possible reasons for this asymmetry in the strength of the observed cold and warm fronts are discussed.

Vertical moisture profile characteristics of severe surface drought and surface wetness in the western United States: 1973–2002

“…Synoptic systems do not emerge as important since they tend to rely upon both lower-and upper-level moisture dynamics (Schultz et al, 1998). Based on this, our results indicate that diminished convective precipitation in the West such as associated with a weakened monsoon circulation (e.g.…”

“…899 Ciesielski and Johnson, 2008), low-level moisture is key, while dry air aloft aids in 'overturning' (McCaul and Cohen, 2002;Parker and Johnson, 2004). In synoptic systems, contrasting airmasses must interact and generate both upper-and lower-level dynamics (Schultz et al, 1998); these airmasses extend through the troposphere, and their moisture profiles remain distinct. Previous literature on western US precipitation indicates that orographic influences reduce the impact of synoptic scale precipitation by forcing the storm track northward over mountainous terrain (Broccoli and Manabe, 1992;Leung et al, 2003).…”

Baroclinic development within zonally‐varying flows

“…Hines and Mechoso, 1993;Kuo and LowNam, 1994;Thompson, 1995;Rotunno et al, 1998). While this may be true, Schultz et al (1998Schultz et al ( , p. 1771 argue that numerical-modelling experiments that only vary the magnitude of surface friction are unable to capture the range of variability observed in extratropical cyclones.…”

“…These authors have employed observational (e.g. McCallum and Norris, 1990;Evans et al, 1994;Young, 1995;Hartmann, 1995;Schultz et al, 1998;Simmons, 1999;Shapiro et al, 1999;Sinclair and Revell, 2000;Shapiro et al, 2001;Martius et al, 2007) and idealized numerical-modelling (e.g. Davies et al, 1991;Thorncroft et al, 1993;Wernli et al, 1998;Hartmann and Zuercher, 1998;Schultz et al, 1998) (Shapiro and Keyser, 1990, figure 10.27), enhancing the zonal elongation of the cyclone and fronts and reflecting the continued existence of the frontal T-bone in stage IV.…”

“…Thus, this body of work suggests that variations in the acrossjet shear result in large-scale deformation that affects the regions of preferred frontogenesis, thus affecting the structure and evolution of baroclinic waves, see Wernli et al (1998, figure 3) . A complementary approach is described by Schultz et al (1998), who argue that zonal variations in the background horizontal wind speed control the structure and evolution of baroclinic waves. They obtain the background flow in these cases quantitatively by filtering the fields into a portion due to the cyclone and a remaining portion attributed to the background flow.…”