Influence of Terrain Resolution on Banded Convection in the Lee of the Rocky Mountains

Abstract

On 16-17 February 2007, snowbands formed in the lee of the Rocky Mountains in Wyoming, Colorado, and Nebraska on the anticyclonic-shear side of a midlevel jet streak. Two types of bands were prevalent: a longer, wider band associated with frontogenesis along an equatorward-moving cold front (major band) and multiple shorter, narrower bands farther poleward (minor bands). To understand how the upstream terrain affected the occurrence and intensity of the bands, multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed. The evolutions of the synoptic patterns were similar in all simulations that included topography, but the synoptic pattern differed and no bands developed in a simulation with a flat land surface. These results allowed a focus on the changes to the banded precipitation due to the terrain resolution. Remarkably, although the exact location of the bands differed from run to run, the bands in all simulations with topography were in roughly the same region where they occurred on 16-17 February 2007. The major band was associated with frontogenesis along an equatorward-moving cold front that became stalled against the terrain. The minor bands formed from the release of conditional, symmetric, and inertial instabilities by ascent up the large-scale topography, rather than by ascent up specific small-scale topographic features. Because the bands were not tied to specific terrain features, these results suggest that the precise location of the minor bands is unpredictable.

On the Role of Inertial Instability in Stratosphere–Troposphere Exchange near Midlatitude Cyclones*

“…2c,d). The negative PV regions over the Gulf of Mexico and northern Florida occur on the equatorward side of the subtropical jet and become more extensive in area upon ascending In agreement with previous studies of the relationship between symmetric instability and regions of enhanced rainfall (e.g., Hoggatt and Knox 1998;Schumacher et al 2010Schumacher et al , 2015, the region of negative PV over northern Illinois and southern Wisconsin (Figs. 2c,e) coincides fairly well with the lobe of maximum 24-h precipitation seen in Fig.…”

“…Convective bands initiated within the ascending branch of the secondary circulation were associated with elevated and near-surface frontogenesis. Schumacher et al (2015) reexamined this case and confirmed that the minor rainbands to the north of the low likely resulted from the combined effects of sloping topography and the inherent effects of symmetric instability. Their work is explicit about the presence of negative PV and inertial instability on the poleward side of the surface cyclone and anticyclonic side of the jet.…”

“…They found that the existence of distinctive layered perturbations in the UTLS corresponded with times of inertial instability. Schumacher and Schultz (2000) found that inertially unstable conditions are common in the subtropical troposphere. Knox and Harvey (2005) compiled a climatology of inertial instability and RWB and found that it is inertially unstable over 2%-5% of the midlatitude UTLS at any given time.…”

“…Simulations of the event indicated a quadrupole checkerboard pattern in the vertical motion field, signifying inertial instability, as in the equatorial middle atmosphere. Schumacher et al (2010) simulated quasi-stationary precipitation bands and found that they occurred in an environment with a well-mixed baroclinic boundary layer, positive convective available potential energy, and widespread negative PV. They argued that ascent caused by frontogenesis and banded moist convection produced narrow regions of negative absolute vorticity directly by the upward transport of low-momentum air.…”

“…EPV dynamics are a vital contributor in assessing conditions conducive to conditional symmetric instability (CSI). The concept of EPV has been widely used in studies of baroclinic systems since CSI was first introduced as a potential mechanism for the generation of frontal rainbands (Bennetts and Hoskins 1979;Emanuel 1979;Montgomery and Farrell 1991;Schumacher and Schultz 2000;Schultz and Knox 2007). EPV has proven useful in diagnosing squall lines (Zhang and Cho 1992), extratropical cyclones (Cao and Cho 1995;Cao and Zhang 2004;Brennan et al 2008), convective snowstorms (Halcomb and Market 2003), and dynamics of the UTLS (Hoskins et al 1985;Morgan 1999).…”