Relationship between a Weakening Cold Front, Misocyclones, and Cloud Development on 10 June 2002 during IHOP

Abstract

The finescale structure and evolution of a cold front in the presence of small-scale circulations are examined using overdetermined dual-Doppler syntheses of mobile radar data along with thermodynamic data and cloud imagery collected on 10 June 2002 during the International H2O Project (IHOP). Linear clear-air reflectivity maxima and open cellular convection intersect the cold front, causing spatial variations in convergence along the front. Small-scale vertical vorticity maxima (misocyclones) often are coincident with these intersections and are associated with vertical velocity maxima. Throughout the deployment, trajectory analyses indicate that parcels entering the frontal circulation travel predominately in the along-front direction. During the first hour of the deployment, upward motion is nearly continuous along the front. Consequently, parcels remain in regions of upward motion as they move along the front, and many eventually ascend to cloud base. In response, a line of cumulus clouds develops along the front. Later in the deployment, however, enhanced warming behind the cold front causes the frontal circulation to weaken. Small-scale features such as misocyclones play a larger role in organizing upward motion along the front than when the front was stronger. Misocyclones contort the weakened cold front and are associated with kinks in radar reflectivity and fractures in upward motion. Parcels moving along the front now experience regions of both upward and downward forcing due to this fracturing. Hence, many of these parcels do not retain upward motion long enough to reach cloud base, and clouds along the cold front dissipate in the dry air above the boundary layer. Parcels that remain coincident with upward motion maxima, such as those maxima associated with misocyclones, however, often still reach the top of the radar domain and presumably cloud base.

The Formation of Small-Scale Atmospheric Vortices via Horizontal Shearing Instability

“…6.2) are consistent with observed misocyclones (e.g. Pietrycha and Rasmussen 2004;Arnott et al 2006;Marquis et al 2007). The misocyclonic structure, which is characterized by an absolute maximum in vertical vorticity near the surface and a relative maximum, that decreases and slopes downshear with height, is similar in both the simulation and the observations (Fig.…”

“…High-resolution observations of the planetary BL have revealed the presence of small-scale misovortices (i.e., ~1-4 km in width) along various surface-based boundaries (Crook et al 1991;Atkins et al 1995;Pietrycha and Rasmussen 2004;Kawashima and Fujiyoshi 2005;Arnott et al 2006;Murphey et al 2006;Marquis et al 2007;Buban et al 2007). Vortices having a core circulation diameter between 40 m and 4 km are termed "misocyclones" (Fujita 1981, Wakimoto andWilson 1989).…”

Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

“…Many studies have demonstrated that misocyclones along air mass boundaries distort the boundaries and produce local maxima of low-level convergence and vertical velocity in their vicinity, which often could be preferred regions to form clouds and/or radar reflectivity cores (e.g., Wilson et al 1992;Atkins et al 1995;Lee and Wilhelmson 1997b;Friedrich et al 2005;Arnott et al 2006;Murphey et al 2006;Marquis et al 2007;Wakimoto and Murphey 2009). Although well-defined vortices with quasi-circular vorticity contours do not develop in the present simulations, it is suggested that in the U15-S2 and U20-S2 cases the concentrated vertical vorticity distorted the horizontal wind field and enhanced the low-level convergence in the vicinity of the vorticity maxima, leading to formation of the precipitation maxima north of the bow apexes.…”

Tornado Generation in a Narrow Cold Frontal Rainband -Fujisawa Tornado on April 20, 2006-

“…Carbone (1982) pointed out the possibility of tornado generation in the frontal zone horizontal shear. Recently, Arnott et al (2006) showed that misocyclones are associated with vertical velocity maxima in convergence along the cold front. Friedrich et al (2005) discussed characteristics of misocyclones generated in the gust front and relationship between horizontal wind shear and convection initiation.…”