scholarly journals Climatology of Strong Intermountain Cold Fronts

2008 ◽  
Vol 136 (3) ◽  
pp. 784-807 ◽  
Author(s):  
Jason C. Shafer ◽  
W. James Steenburgh
Keyword(s):  
Sierra Nevada ◽  
Large Scale ◽  
Cold Front ◽  
Salt Lake ◽  
Salt Lake City ◽  
Pressure Rise ◽  
Intermountain West ◽  
Cold Fronts ◽  
Northern Utah ◽  
Upper Level

Abstract Motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern Utah, this paper presents a 25-yr climatology of strong cold frontal passages over the Intermountain West and adjoining western United States. Using conventional surface observations and the North American Regional Reanalysis, strong cold frontal passages are identified based on a temperature fall of 7°C or greater in a 2–3-h period, a concurrent pressure rise of 3 hPa or greater, and the presence of a large-scale 700-hPa temperature gradient of at least 6°C (500 km)−1. The number of strong cold frontal passages exhibits a strong continental signature with very few events (<10) along the Pacific coast and more than 200 events east of the Continental Divide. The number of events increases dramatically from the Cascade Mountains and Sierra Nevada to northern Utah, indicating that the Intermountain West is a frequent cold front breeding ground. A composite of the 25 strongest events at Salt Lake City (based on the magnitude of the temperature fall) reveals that confluent deformation acting on a broad baroclinic zone over central Nevada commonly initiates Intermountain frontogenesis. The confluent deformation develops in southwesterly large-scale flow and appears to be enhanced by flow deflection around the Sierra Nevada. Quasi-stationary development and intensification of the southwest–northeast-oriented cold front then occurs as a mobile upper-level trough approaches from the west. The front becomes mobile as cold advection and ascent associated with the upper-level trough overtake the low-level front. Cloud and precipitation observations suggest that differential diabatic heating contributes to the rapid frontal intensification in many events.

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

2009 ◽  
Vol 137 (6) ◽  
pp. 2000-2020 ◽  
Author(s):  
W. James Steenburgh ◽  
Colby R. Neuman ◽  
Gregory L. West ◽  
Lance F. Bosart
Keyword(s):  
Sierra Nevada ◽  
Cold Front ◽  
Salt Lake ◽  
Salt Lake City ◽  
Wrf Model ◽  
Moist Convection ◽  
Intermountain West ◽  
Cascade Mountains ◽  
Frontal System ◽  
Upper Level

Abstract On 25 March 2006, a complex frontal system moved across the Sierra–Cascade Mountains and intensified rapidly over the Intermountain West where it produced one of the strongest cold-frontal passages observed in Salt Lake City, Utah, during the past 25 yr. Observational analyses and numerical simulations by the Weather Research and Forecast (WRF) Model illustrate that the frontal system propagated discretely across the Sierra–Cascade Mountains and western Nevada. This discrete propagation occurs in a synoptic environment that features a mobile upper-level cyclonic potential vorticity (PV) anomaly that is coupled initially with a landfalling Pacific cyclone and attendant occluded front. The eastward migration of the upper-level cyclonic PV anomaly ultimately encourages the development of a new surface-based cold front ahead of the landfalling occlusion as troughing, confluence, and convergence downstream of the Sierra Nevada intensify preexisting baroclinity over Nevada. Trajectories show that the new cold front represents a boundary between potentially warm air originating over the desert Southwest, some of which has been deflected around the south end of the high sierra, and potentially cool air that has traversed the sierra near and north of Lake Tahoe, some of which has been deflected around the north end of the high sierra. Although diabatic processes contribute to the frontal sharpening, they are not needed for the discrete propagation or rapid cold-frontal development. Forecasters should be vigilant for discrete frontal propagation in similar synoptic situations and recognize that moist convection or differential surface heating can contribute to but are not necessary for rapid Intermountain West frontogenesis.

scholarly journals Life Cycle and Mesoscale Frontal Structure of an Intermountain Cyclone

2010 ◽  
Vol 138 (7) ◽  
pp. 2528-2545 ◽  
Author(s):  
Gregory L. West ◽  
W. James Steenburgh
Keyword(s):  
Life Cycle ◽  
Sierra Nevada ◽  
Great Basin ◽  
Salt Lake ◽  
Salt Lake City ◽  
Confluence Zone ◽  
Frontal Structure ◽  
Lake City ◽  
Weather Changes ◽  
Upper Level

Abstract High-resolution analyses and MesoWest surface observations are used to examine the life cycle and mesoscale frontal structure of the “Tax Day Storm,” an intermountain cyclone that produced the second lowest sea level pressure observed in Utah during the instrumented period and the strongest cold frontal passage at the Salt Lake City International Airport in the past 25 years. A key mesoscale surface feature contributing to the cyclone’s evolution is a confluence zone that extends downstream from the Sierra Nevada across the Great Basin. Strong contraction (i.e., deformation and convergence) within this Great Basin confluence zone (GBCZ) forms an airstream boundary that is initially nonfrontal but becomes the locus for surface frontogenesis as it collects and concentrates baroclinicity from the northern Great Basin, including that accompanying an approaching baroclinic trough. Evaporative and sublimational cooling from postfrontal precipitation, as well as cross-front contrasts in surface sensible heating, also play an important role, accounting for up to 40% of cross-front baroclinicity. As an upper-level cyclonic potential vorticity anomaly and quasigeostrophic forcing for ascent move over the Great Basin, cyclone development occurs along the GBCZ and developing cold front rather than within the Sierra Nevada lee trough, as might be inferred from classic models of lee cyclogenesis. Front–mountain interactions ultimately produce a very complex frontal evolution over the basin-and-range topography of northern Utah. The analysis further establishes the role of the GBCZ in intermountain frontogenesis and cyclone evolution. Recognition of this role is essential for improving the analysis and prediction of sensible weather changes produced by cold fronts and cyclones over the Intermountain West.

scholarly journals Multi-Reanalysis Climatology of Intermountain Cyclones

2010 ◽  
Vol 138 (11) ◽  
pp. 4035-4053 ◽  
Author(s):  
Matthew E. Jeglum ◽  
W. James Steenburgh ◽  
Tiros P. Lee ◽  
Lance F. Bosart
Keyword(s):  
Sierra Nevada ◽  
Great Basin ◽  
Large Scale ◽  
Intermountain West ◽  
The North ◽  
Evolution Analysis ◽  
Flow Splitting ◽  
Monthly Distribution ◽  
Upper Level ◽  
Regional Reanalysis

Abstract The topography in and around the Intermountain West strongly affects the genesis, migration, and lysis of extratropical cyclones. Here intermountain (i.e., Nevada or Great Basin) cyclone (IC) activity and evolution are examined using the ECMWF Re-Analysis Interim (ERA-Interim) the North American Regional Reanalysis (NARR), and the NCEP–NCAR reanalysis from 1989 to 2008, the period during which all three are available. The ICs are defined and tracked objectively as 850-hPa geopotential height depressions of ≥40 m that persist for ≥12 h. The monthly distribution of IC center and genesis frequency in all three reanalyses is bimodal with spring (absolute) and fall (secondary) maxima. Although the results are sensitive to differences in resolution, topographic representation, and reanalysis methodology, both the ERA-Interim and NARR produce frequent IC centers and genesis in the Great Basin cyclone region, which extends from the southern “high” Sierra to northwest Utah, and the Canyonlands cyclone region, which lies over the upper Colorado River basin of southeast Utah. The NCEP–NCAR reanalysis fails to resolve these two distinct cyclone regions and produces less frequent IC centers and genesis than the ERA-Interim and NARR. An ERA-Interim-based composite of strong ICs generated in cross-Sierra (210°–300°) 500-hPa flow shows that cyclogenesis is preceded by the development of the Great Basin confluence zone (GBCZ), a regional airstream boundary that extends downstream from the Sierra Nevada across the Intermountain West. Cyclogenesis occurs along the GBCZ as large-scale ascent develops over the Intermountain West in advance of an approaching upper-level trough. Flow splitting around the high Sierra and the presence of low-level baroclinicity along the GBCZ suggest that IC evolution may be better conceptualized from a potential vorticity perspective than from traditional quasigeostrophic models of lee cyclogenesis. Although these results provide new insights into IC activity and evolution, analysis uncertainty and the cyclone identification criteria are important sources of ambiguity that cannot be fully eliminated.

scholarly journals A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans

2015 ◽  
Vol 28 (17) ◽  
pp. 6743-6762 ◽  
Author(s):  
Catherine M. Naud ◽  
Derek J. Posselt ◽  
Susan C. van den Heever
Keyword(s):  
Large Scale ◽  
Cold Front ◽  
Conveyor Belt ◽  
Low Level ◽  
Scale Parameters ◽  
Convective Clouds ◽  
Cold Fronts ◽  
Surface Front ◽  
High Level ◽  
The Impact

Abstract The distribution of cloud and precipitation properties across oceanic extratropical cyclone cold fronts is examined using four years of combined CloudSat radar and CALIPSO lidar retrievals. The global annual mean cloud and precipitation distributions show that low-level clouds are ubiquitous in the postfrontal zone while higher-level cloud frequency and precipitation peak in the warm sector along the surface front. Increases in temperature and moisture within the cold front region are associated with larger high-level but lower mid-/low-level cloud frequencies and precipitation decreases in the cold sector. This behavior seems to be related to a shift from stratiform to convective clouds and precipitation. Stronger ascent in the warm conveyor belt tends to enhance cloudiness and precipitation across the cold front. A strong temperature contrast between the warm and cold sectors also encourages greater post-cold-frontal cloud occurrence. While the seasonal contrasts in environmental temperature, moisture, and ascent strength are enough to explain most of the variations in cloud and precipitation across cold fronts in both hemispheres, they do not fully explain the differences between Northern and Southern Hemisphere cold fronts. These differences are better explained when the impact of the contrast in temperature across the cold front is also considered. In addition, these large-scale parameters do not explain the relatively large frequency in springtime postfrontal precipitation.

scholarly journals CFS Prediction of Winter Persistent Inversions in the Intermountain Region

2010 ◽  
Vol 25 (4) ◽  
pp. 1211-1218 ◽  
Author(s):  
Robert R. Gillies ◽  
Shih-Yu Wang ◽  
Jin-Ho Yoon ◽  
Scott Weaver
Keyword(s):  
Salt Lake ◽  
Salt Lake City ◽  
Intraseasonal Oscillation ◽  
Weather Forecast ◽  
The United States ◽  
Intermountain West ◽  
Medium Range Weather Forecast ◽  
Forecast Models ◽  
Lake City ◽  
Intermountain Region

Abstract A recent study by Gillies and others of persistent inversion events in the Intermountain West of the United States found a substantive linkage between the intraseasonal oscillation (ISO) and the development of persistent inversion events. Given that NCEP’s Climate Forecast System (CFS) has demonstrated skill in the prediction of the ISO as far out as 1 month, it was decided to examine the CFS forecast’s capability in the prediction of such winter persistent inversions. After initial analysis, a simple regression scheme is proposed that is coupled to the CFS output of geopotential height as a way to predict the occurrence of persistent inversion events for Salt Lake City, Utah. Analysis of the CFS hindcasts through the period 1981–2008 indicates that the regression coupled with the CFS can predict persistent inversion events with lead times of up to 4 weeks. The adoption of this coupled regression–CFS prediction may improve the forecasting of persistent inversion events in the Intermountain West, which is currently restricted to the more limited time span (∼10 days) of medium-range weather forecast models.

scholarly journals Contrasting stable water isotope signals from convective and large-scale precipitation phases of a heavy precipitation event in southern Italy during HyMeX IOP 13: a modelling perspective

2019 ◽  
Vol 19 (11) ◽  
pp. 7487-7506
Author(s):  
Keun-Ok Lee ◽  
Franziska Aemisegger ◽  
Stephan Pfahl ◽  
Cyrille Flamant ◽  
Jean-Lionel Lacour ◽  
...  
Keyword(s):  
Large Scale ◽  
Cold Front ◽  
Heavy Precipitation ◽  
Convective Precipitation ◽  
Water Isotopes ◽  
Precipitation Event ◽  
Stable Water Isotopes ◽  
Heavy Precipitation Event ◽  
The North ◽  
Upper Level

Abstract. The dynamical context and moisture transport pathways embedded in large-scale flow and associated with a heavy precipitation event (HPE) in southern Italy (SI) are investigated with the help of stable water isotopes (SWIs) based on a purely numerical framework. The event occurred during the Intensive Observation Period (IOP) 13 of the field campaign of the Hydrological Cycle in the Mediterranean Experiment (HyMeX) on 15 and 16 October 2012, and SI experienced intense rainfall of 62.4 mm over 27 h with two precipitation phases during this event. The first one (P1) was induced by convective precipitation ahead of a cold front, while the second one (P2) was mainly associated with precipitation induced by large-scale uplift. The moisture transport and processes responsible for the HPE are analysed using a simulation with the isotope-enabled regional numerical model COSMOiso. The simulation at a horizontal grid spacing of about 7 km over a large domain (about 4300 km ×3500 km) allows the isotopes signal to be distinguished due to local processes or large-scale advection. Backward trajectory analyses based on this simulation show that the air parcels arriving in SI during P1 originate from the North Atlantic and descend within an upper-level trough over the north-western Mediterranean. The descending air parcels reach elevations below 1 km over the sea and bring dry and isotopically depleted air (median δ18O ≤-25 ‰, water vapour mixing ratio q≤2 g kg−1) close to the surface, which induces strong surface evaporation. These air parcels are rapidly enriched in SWIs (δ18O ≥-14 ‰) and moistened (q≥8 g kg−1) over the Tyrrhenian Sea by taking up moisture from surface evaporation and potentially from evaporation of frontal precipitation. Thereafter, the SWI-enriched low-level air masses arriving upstream of SI are convectively pumped to higher altitudes, and the SWI-depleted moisture from higher levels is transported towards the surface within the downdrafts ahead of the cold front over SI, producing a large amount of convective precipitation in SI. Most of the moisture processes (i.e. evaporation, convective mixing) related to the HPE take place during the 18 h before P1 over SI. A period of 4 h later, during the second precipitation phase P2, the air parcels arriving over SI mainly originate from north Africa. The strong cyclonic flow around the eastward-moving upper-level trough induces the advection of a SWI-enriched African moisture plume towards SI and leads to large-scale uplift of the warm air mass along the cold front. This lifts moist and SWI-enriched air (median δ18O ≥-16 ‰, median q≥6 g kg−1) and leads to gradual rain out of the air parcels over Italy. Large-scale ascent in the warm sector ahead of the cold front takes place during the 72 h preceding P2 in SI. This work demonstrates how stable water isotopes can yield additional insights into the variety of thermodynamic mechanisms occurring at the mesoscale and synoptic scale during the formation of a HPE.

scholarly journals Atmospheric Scale Interaction on Wintertime Intermountain West Low-Level Inversions

2010 ◽  
Vol 25 (4) ◽  
pp. 1196-1210 ◽  
Author(s):  
Robert R. Gillies ◽  
Shih-Yu Wang ◽  
Marty R. Booth
Keyword(s):  
Pacific Northwest ◽  
Prediction Models ◽  
Salt Lake ◽  
Weather Prediction ◽  
Salt Lake City ◽  
The United States ◽  
Intermountain West ◽  
The Pacific ◽  
Medium Range ◽  
Lake City

Abstract Persistent winter inversions result in poor air quality in the Intermountain West of the United States. Although the onset of an inversion is relatively easy to predict, the duration and the subsequent breakup of a persistent inversion event remains a forecasting challenge. For this reason and for this region, historic soundings were analyzed for Salt Lake City, Utah, with reanalysis and station data to investigate how persistent inversion events are modulated by synoptic and intraseasonal variabilities. The results point to a close linkage between persistent inversions and the dominant intraseasonal (30 day) mode that characterizes the winter circulation regime over the Pacific Northwest. Meteorological variables and pollution (e.g., particulate matter of ≤2.5-μm diameter, PM2.5) revealed coherent variations with this intraseasonal mode. The intraseasonal mode also modulates the characteristics of the synoptic (6 day) variability and further influences the duration of persistent inversions in the Intermountain West. The interaction between modes suggests that a complete forecast of persistent inversions is more involved and technically beyond numerical weather prediction models intended for the medium range (∼10 day). Therefore, to predict persistent inversions, the results point to the adoption of standard medium-range forecasts with a longer-term climate diagnostic approach.

Seismic Hazard in the Intermountain West

2015 ◽  
Vol 31 (1_suppl) ◽  
pp. S149-S176 ◽  
Author(s):  
Kathleen M. Haller ◽  
Morgan P. Moschetti ◽  
Charles S. Mueller ◽  
Sanaz Rezaeian ◽  
Mark D. Petersen ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Ground Motion ◽  
Salt Lake ◽  
Current Model ◽  
Salt Lake City ◽  
Intermountain West ◽  
Source Characterization ◽  
Motion Models ◽  
Lake City ◽  
Scientific Results

The 2014 national seismic-hazard model for the conterminous United States incorporates new scientific results and important model adjustments. The current model includes updates to the historical catalog, which is spatially smoothed using both fixed-length and adaptive-length smoothing kernels. Fault-source characterization improved by adding faults, revising rates of activity, and incorporating new results from combined inversions of geologic and geodetic data. The update also includes a new suite of published ground motion models. Changes in probabilistic ground motion are generally less than 10% in most of the Intermountain West compared to the prior assessment, and ground-motion hazard in four Intermountain West cities illustrates the range and magnitude of change in the region. Seismic hazard at reference sites in Boise and Reno increased as much as 10%, whereas hazard in Salt Lake City decreased 5–6%. The largest change was in Las Vegas, where hazard increased 32–35%.

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