Terrain Influences on Synoptic Storm Structure and Mesoscale Precipitation Distribution during IPEX IOP3

Abstract

The influence of topography on the evolution of a winter storm over the western United States and distribution of precipitation over northern Utah are examined using data collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The analysis is based on high-density surface observations collected by the MesoWest cooperative networks, special radiosonde observations, wind profiler observations, Next-Generation Weather Radar (NEXRAD) data, and conventional data. A complex storm evolution was observed, beginning with frontal distortion and low-level frontolysis as a surface occluded front approached the Sierra Nevada. As the low-level occluded front weakened, the associated upper-level trough moved over the Sierra Nevada and overtook a lee trough. The upper-level trough, which was forward sloping and featured more dramatic moisture than temperature gradients, then moved across Nevada with a weak surface reflection as a pressure trough.Over northern Utah, detailed observations revealed the existence of a midlevel trough beneath the forward-sloping upper-level trough. This midlevel trough appeared to form along a high-potential-vorticity banner that developed over the southern Sierra Nevada and moved downstream over northern Utah. A surface trough moved over northern Utah 3 h after the midlevel trough and delineated two storm periods. Ahead of the surface trough, orographic precipitation processes dominated and produced enhanced mountain precipitation. This period also featured lowland precipitation enhancement upstream of the northern Wasatch Mountains where a windward convergence zone was present. Precipitation behind the surface trough was initially dominated by orographic processes, but soon thereafter featured convective precipitation that was not fixed to the terrain. Processes responsible for the complex vertical trough structure and precipitation distribution over northern Utah are discussed.

Discrete Frontal Propagation over the Sierra–Cascade Mountains and Intermountain West

“…A particularly remarkable event occurred on 25 March 2006 when temperatures plunged 88C in 15 min and 148C in 2 h with a cold-frontal passage at the Salt Lake City International Airport (KSLC). The coldfrontal temperature change and accompanying 6-hPa pressure rise would rate as the fifth and third largest at KSLC in the 25-yr cold-front and pressure change climatologies presented by Shafer and Steenburgh (2008) and Koppel et al (2000). The frontal system also moved from the northern California coast to KSLC in just 16 h [an apparent mean speed of ;60 km h 21 (16.7 m s 21 )] and intensified rapidly over Nevada where the temperature gradient increased from 3.58 to 138C (100 km) 21 in less than 10 h. Over much of northern Utah, postfrontal wind gusts exceeded 35 m s 21 .…”

“…Rapidly developing cold fronts produce strong winds and dramatic temperature falls several times each year over the Intermountain West (Shafer and Steenburgh 2008). A particularly remarkable event occurred on 25 March 2006 when temperatures plunged 88C in 15 min and 148C in 2 h with a cold-frontal passage at the Salt Lake City International Airport (KSLC).…”

“…Bryan and Fritsch (2000a) define discrete frontal propagation as ''a process whereby a midlevel front moves through an area aloft without evidence of a front moving through the same area at the surface, while the surface frontal properties (e.g., pressure trough, wind shift, and thermal gradient) dissipate at one location and simultaneously develop at another location.'' Recognition and understanding of discrete frontal propagation is important for operational weather forecasting, particularly over the Intermountain West where cold fronts are frequently accompanied by severe winds and surface frontal analysis and prediction is complicated by the region's complex topography (Shafer and Steenburgh 2008).…”

Quantitative precipitation forecasting in the Alps: The advances achieved by the Mesoscale Alpine Programme

“…In CAL-JET, orographically modified precipitation was studied in connection with the 'landfall' of oceanic cyclones and associated fronts (Kingsmill et al, 2006). The dynamical and microphysical data obtained in IPEX allowed detailed observational and numerical studies of precipitation over and upstream of the Wasatch Mountains in Utah (Colle et al, 2005;Shafer et al, 2006). IPEX and, to a larger extent, IMPROVE were more specifically targeted than MAP-P1 at measuring cloud microphysical properties related to orographic precipitation.…”

Snow Growth and Transport Patterns in Orographic Storms as Estimated from Airborne Vertical-Plane Dual-Doppler Radar Data

“…Orographic convection is not uncommon in winter storms (e.g., Marwitz 1980;Lee 1984;Steenburgh 2003;Shafer et al 2006;Kumjian et al 2014). Shallow, smallscale convection often occurs during cold-air advection in postfrontal flow (e.g., Abbs and Jensen 1993;Kusunoki et al 2004;Colle et al 2008;Geerts et al 2011;Cunningham and Yuter 2014), notwithstanding the limited surface heat flux in winter.…”