A MULTI-REANALYSIS CLIMATOLOGY OF

A MULTI-REANALYSIS CLIMATOLOGY OF INTERMOUNTAIN CYCLONES
by
Matthew E. Jeglum
A thesis submitted to the faculty of
The University of Utah
in partial fulfillment of the requirements for the degree of
Master of Science
Department of Atmospheric Science
The University of Utah
May 2010
Copyright © Matthew E. Jeglum 2010
All Rights Reserved
ABSTRACT
The topography in and around the Intermountain West strongly affects the
migration, genesis, and lysis of extratropical cyclones. Existing climatologies describe
only the characteristics of Intermountain cyclones within the broader context of global or
North American cyclone activity, or concentrate on scales that do not fully resolve
regional orographic effects.
Here we examine the characteristics and hydroclimatic impacts of Intermountain
cyclones using the ERA-Interim global reanalysis, the North American Regional
Reanalysis (NARR), and NCEP/NCAR Global Reanalysis 1 (GR1) from 1989-2008.
Cyclones are defined and tracked objectively as geopotential height (sea level pressure)
depressions of ≥40 m (4 hPa) that persist for ≥12 h. Sea level pressure reduction results
in an artificially high cyclone count, so the analysis concentrates on 850 hPa, which is
near the mean altitude of the valleys and basins of the Intermountain West.
Intermountain cyclone frequency and genesis in all three reanalyses follows a
bimodal distribution with maxima in spring (absolute) and fall (secondary). Although the
results are sensitive to differences in topography and assimilation methodologies, both
the ERA-Interim and NARR produce two regions of high cyclone frequency and genesis
over the Intermountain West: (1) the Great Basin cyclone region, which extends
downstream from the highest portion of the Sierra Nevada to northern Utah and (2) the
Canyonlands cyclone region, which lies over the upper Colorado River Basin of Utah and
western Colorado. The GR1 produces a lower cyclone count and fails to resolve the two
distinct cyclone regions. The lysis distribution from all three reanalyses suggests that
most Intermountain cyclones decay within the region or move discretely into the lee of
the Rockies. Although Intermountain cyclones may be accompanied by heavy
precipitation they contribute less than 1/3 of the cool-season mountain precipitation in the
region (at most locations, much less than 1/3). They are, however, responsible for several
recent blowing dust events that have contributed to early snowpack ablation in western
mountain ranges.
v
1
TABLE OF CONTENTS
ABSTRACT....................................................................................................................... iv
ACKNOWLEDGMENTS ................................................................................................ vii
Chapter
1 INTRODUCTION ..........................................................................................................1
2 DATA AND METHODS ...............................................................................................7
Reanalyses................................................................................................................7
Cyclone Identification..............................................................................................8
Caveats and Limitations.........................................................................................12
SNOTEL Data........................................................................................................15
3 RESULTS .....................................................................................................................17
Cyclone Frequency, Genesis, and Lysis in the Western US and Surroundings ....17
Intermountain Cyclone Frequency, Genesis, and Lysis.........................................23
Evolution of High-Amplitude Intermountain Cyclone Events ..............................36
Linkages to Regional Hydroclimate ......................................................................42
4 CONCLUSIONS ..........................................................................................................49
REFERENCES ..................................................................................................................51
1
ACKNOWLEDGMENTS
First of all I would like to thank my advisor, Dr. Jim Steenburgh, for his guidance
and support throughout the duration of this project. I would also like to thank the other
two members of my committee, Drs. Larry Dunn and Courtenay Strong.
I am indebted to numerous fellow graduate students to their help with various
computer-based issues, but I especially want to thank Greg West, Trevor Alcott, and Dan
Tyndall for their help with GrADS, MATLAB, and other problematic programs.
Many thanks also go to my friends and family for their support in all the nonacademic parts of my life that allow me to complete my degree.
This research was supported under National Science Foundation Grant ATM0627937. Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the authors and do not necessarily reflect the views of the National
Science Foundation.
1
CHAPTER 1
INTRODUCTION
The Intermountain West, which lies between the Sierra Nevada and Cascade
Mountains to the west and the Rocky Mountains to the east (Figure 1a,b), has largely
been overlooked as an area of cyclone research. Primarily a footnote in studies
concentrating on global or regional cyclone activity (e.g., Petterssen, 1956, Whittaker and
Horn, 1981), it is in fact a region of frequent cyclogenesis (e.g., Lee, 1995). Hazardous
weather produced by Intermountain cyclones can include strong cold fronts and their
attendant high winds and blowing dust (e.g., Shafer and Steenburgh, 2008), which during
Utah’s 2002 Tax Day Storm toppled tractor trailers, forced the closure of Interstate 15,
and caused a wildfire run near Parowan, Utah that forced the evacuation of several homes
(NCDC 2009, West and Steenburgh 2010).
From a hydroclimate perspective, the
contribution of snows produced by Intermountain cyclones to mountain snowpack
remains unquantified, as are potential linkages between Intermountain cyclones and
episodic dust transport and deposition that contributes to early snowpack ablation over
western mountain watersheds (Painter et al. 2007).
There is little consensus in previous studies with regard to Intermountain cyclone
frequency and genesis. In part, this reflects differences in analysis methodology and
spatial resolution. Using data compiled in 5º lat-lon grid boxes from daily sea level
2
Figure 1. Topography (m, shaded following inset scale) of (a) the western U.S. and
surrounding region and (b) the Intermountain West. Geographic features and
locations discussed in text annotated.
3
pressure analyses published by the National Weather Service from 1899-1939, Petterssen
(1950, 1956) identified cyclone frequency and genesis maxima over the Intermountain
West during the winter and summer seasons (Petterssen did not specify the specific
months in each season). He attributed these maxima to the influence of the Sierra Nevada
during winter and the presence of thermal lows during summer. In both seasons, the
amplitude of the cyclone frequency and genesis maxima in the lee of the Sierra Nevada
exceeded that in the lee of the Colorado Rockies. Petterssen (1956) further observed that
there is little downstream extension of the Intermountain cyclone frequency and genesis
maxima, suggesting that many of these cyclones decay over the Intermountain West.
Klein (1957) developed a global cyclone climatology on a 5º lat-lon grid using the
20 “best” data coverage years of the 40-year Historical Map Series (1899-1938). He
found that the highest frequency of Intermountain cyclones occurred in Feb and Mar, and
in many months exceeded the cyclone frequency in the lee of the Colorado Rockies.
Cyclogenesis maxima over the Intermountain West were present in all months, including
Jul and Aug, even after his attempt to remove thermal lows.
In contrast to Klein, Reitan (1974) failed to identify Intermountain cyclone
frequency or genesis maxima in any season. His work used cyclone path data published
by the National Weather Service for the period (1951-1970), with cyclone statistics
aggregated into grid boxes with ~740 km spacing (roughly a 7º by 9º lat-lon grid box) at
40ºN. In April, he found a broad cyclone frequency maximum centered over eastern
Colorado and Western Kansas, with only a weak hint of a westward extension upstream
over the Intermountain West. In the other months presented (Jan, Jun, Jul, and Oct),
there was no evidence of Intermountain cyclone activity. This lack of Intermountain
4
cyclones could reflect the coarse resolution of the dataset, or Reitan’s requirement that
cyclones exist for 24 h or more.
Using the same cyclone path dataset as Reitan (1974), but compiled on a 2° latlon grid for the years 1950-1977, Zishka and Smith (1980) found a weak Intermountain
cyclone frequency maximum over Utah and a broad cyclogenesis maximum over the
Intermountain West in Jan. They found little evidence of Intermountain cyclone activity
in July when quasi-persistent thermal lows not included in the National Weather Service
track data dominate the regional climatology, and did not present separate statistics for
spring or fall.
Whittaker and Horn (1981) used the same cyclone track dataset as Reitan
(1974) and Zishka and Smith (1980), but presented monthly statistics on a 5° lat-lon grid
for specific regions, including the Colorado and Intermountain (their Great Basin)
cyclogenesis regions. Their study revealed a strong seasonality in Intermountain
cyclogenesis, with a pronounced maximum in April and weaker maximum in October.
These results suggest that the Jan and Jul statistics presented by Zishka and Smith (1980)
fail to capture the primary Intermountain cyclogenesis seasons, whereas the extremely
coarse grid used by Reitan (1974) inadequately resolves Intermountain cyclone events.
Lee (1995) produced the most comprehensive climatology of Intermountain
cyclones to date using 11 years (1976-1986) of 3-h surface analyses produced by the
National Meteorological Center [NMC, now the National Centers for Environmental
Prediction (NCEP)]. He defined Intermountain cyclones as closed low centers (based on
sea level pressure contours at 4 hPa intervals) that form or deepen over Nevada, are
migratory, and endure for at least five consecutive analysis times (12 h). Statistics were
aggregated on a 2.5° lat-lon grid.
5
The resulting bimodal cyclogenesis distribution
features an absolute maximum in March and secondary maximum in November. The
absolute minimum is observed in July, with a secondary minimum in Dec.
This
represents a shift of about one month relative to Whittaker and Horn (1981). The
amplitude of the bimodal distribution found by Lee (1995) is also more pronounced. Lee
(1995) attributes the spring and fall maxima to the semi-annual migration of the polar jet
stream across the Intermountain West as it moves southward with the approach of winter
and then northward with the approach of summer. The enhancement of the spring
maximum is attributable to the decreased static stability during the spring when surface
sensible heating occurs beneath a relatively cold mid-troposphere. While Lee (1995)
found static stability to reach a minimum in mid-summer, the lack of dynamic forcing
precludes frequent cyclone formation at that time.
Geographically, Lee (1995) found that the region of most frequent cyclogenesis
extends north-south across Nevada, with the highest frequencies in north-central Nevada.
Overall Intermountain cyclone frequencies are a fraction of that in the Colorado genesis
area, in contrast to Pettersen (1950), Klein (1957) and Whittaker and Horn (1981).
Composite and case studies by Lee confirm the role of topography in both the genesis
and lysis of Intermountain cyclones.
Here we present a revised climatology of Intermountain cyclones based on three
modern reanalyses, which enables some assessment of the influence of analysis
uncertainty arising from inadequate terrain representation and limited data assets over the
region. We also examine the potential linkage between Intermountain cyclones,
6
mountain precipitation, and dust transport that ultimately results in early snowpack
ablation over mountain watersheds.
7
CHAPTER 2
DATA AND METHODS
Reanalyses
The three reanalyses used are the European Center for Medium-Range Weather
Forecasts (ECMWF) ERA-Interim global reanalysis, the National Centers for
Environmental Prediction North American Regional Reanalysis (NARR), and the
National Centers for Environmental Prediction/National Center for Atmospheric
Research Global Reanalysis1 (GR1). The ERA-Interim analyses are produced at spectral
T255 (0.7°) horizontal resolution on 37 pressure levels (Uppala et al., 2008), with the
data used here available on a 1.5° lat-lon grid every 6 h from 1989-2008. The ERAInterim is the successor to the ECMWF ERA-40 reanalysis and includes numerous
improvements.
The GR1 is generated at spectral T62 horizontal resolution on 28 vertical levels,
with analyses performed on a 2.5° lat-lon grid every 6 h (Kalnay, 1996). While the
period of record extends to 1948, for comparison purposes, we concentrate on 19892008.
The NARR provides analyses on a 32-km grid with 45 vertical levels (Mesinger
et al. 2006). The data is bilinearly interpolated, however, to the same resolution grid (1.5°
degree) as the ERA-Interim. Raible et al. (2008) found that interpolating the ERA-40
8
reanalysis to a much larger 2.5° grid produced small differences in the results. While our
experience was similar, we found this process to remove the small mesoscale lows
present in the NARR and allows direct comparison with the ERA-Interim. This resulted
in a reduction of cyclone counts relative to the raw dataset. Based on the ETA-model
data assimilation system (EDAS) and the Noah land-surface model, the NARR attempts
to improve upon the NCEP/NCAR Global Reanalysis through increased resolution and
the assimilation of additional datasets that include more detailed precipitation
observations. For comparison purposes, we use NARR analyses at 6 h intervals from
1989-2008, even though the NARR analyses extend to 1979 and are available every 3 h.
See Table 1 and Table 2 for a summary of reanalysis specifications.
Cyclone Identification
Previous studies employ a range of approaches to identify and track cyclones.
Here we identify cyclones at 850 hPa, a level near the mean elevation of the valleys and
basins of the Intermountain West, using a three-step approach based in part on Wernli
and Schwierz (2006). First, we identify height minima (or sea level pressure minima) at
grid points where the geopotential height is lower than the eight surrounding grid points.
Next, we determine if each height minimum is fully enclosed by a geopotential height
contour 40 m greater than the geopotential height of the height minimum. The 40 m
threshold is roughly analogous to the 4 hPa contour interval commonly used in sea level
cyclone identification and also helps remove thermal lows, which often have an
amplitude less than 40 m.
If enclosed by this +40 m contour, the height minimum is
considered a low center. Otherwise it is discarded. If multiple low centers are enclosed
9
Table 1. Reanalysis Summaries. Supplemented by Wang et al., (2006). See Table 2
for a description of acronyms.
10
Table 2. List of acronyms for reanalysis summaries.
11
by the same +40 m contour, the height minimum with the lowest value is selected as the
low center. Certain low center configurations can confuse the filter, although these
situations occur infrequently and are negligible. The cyclone amplitude is the difference
between the geopotential height of the low center and the outermost contour (at 10 m
intervals) completely enclosing it.
In the case of sea level cyclones, low center
identification follows the same procedure, except a +4 hPa closed contour is required.
Low centers are classified as cyclones if they persist for 12 h or more (i.e., 3 or
more analysis times). Low centers at successive analysis times are considered to be the
same cyclone if they are located within 8.5° of arc length of each other. The 8.5° limit,
which represents an area comparable in size to the length of Nevada, was subjectively
developed after extensive review of many cases and is large enough that low centers that
move continuously or discretely (i.e., one low center dissipates while an another forms
elsewhere) within the Intermountain West are considered the same cyclone. On the other
hand, if the low centers are more than 8.5° apart, they are considered different systems.
Cyclone frequency will be defined as the number of low centers observed at a
given grid point, expressed as low centers per 104 km2. This means a cyclone consisting
of 6 low centers will be counted 6 times. Cyclogenesis frequency is a sum at each grid
point of the first low center observed in each cyclone, also expressed as low centers per
104 km2. Cyclolysis frequency is the same as cyclogenesis frequency, except limited to
only the last low center observed in each cyclone.
To provide a large-scale perspective, cyclones are identified over a region
encompassing the eastern Pacific Ocean, the western U.S., and parts of western Canada
and Mexico. Intermountain cyclones, however, represent a subset of these cyclones that
12
at any time during their life cycle has their low center located between 35° and 42.5° N
and 119° and 108° W (Figure 1b). Thus, an Intermountain cyclone can undergo genesis
in the Intermountain West or simply move into or through it.
Caveats and Limitations
A major concern for Intermountain cyclone identification is thermally-induced
areas of low pressure (a.k.a., thermal lows). Outside of the winter months of Dec-Feb,
the incoming solar radiation over the Intermountain West is sufficient to sensibly heat the
surface and atmospheric boundary layer and form thermal lows.
Over the desert
Southwest, Rowson and Colucci (1992) found that thermal low occurrences are rare from
Dec-Apr and most common in Jul and Aug. At 850 hPa, most thermal lows do not
maintain an amplitude of more than 40 m for 12 h, and are largely, but not completely,
eliminated by our cyclone amplitude and persistence requirements. Issues related to
thermal lows are discussed where appropriate in the manuscript.
Any cyclone identification algorithm necessarily involves the use of thresholds,
whereas a continuous spectrum is observed in nature. The threshold to which our method
is most sensitive is the +40 m amplitude requirement. The +40 m threshold was chosen,
as discussed earlier, for its common use in synoptic meteorology and because it
represented the best compromise between effective removal of thermal lows while
preserving those that are dynamically forced. Lowering this threshold leads to gains in
cyclone frequency in all months and at all locations, but especially during the summer
months as thermal lows are included. A higher threshold lowers frequencies in all
months, but also magnifies the bimodal annual distribution of cyclone events by leaving
13
only the strongest storms, which occur preferentially in spring and fall, as will be shown
later.
Our results are also influenced by the underlying reanalysis orography. The ERAInterim offers moderate resolution, with the high terrain of central Utah visible, along
with distinct depressions covering the Great Salt Lake basin and the Snake River Plain
(Figure 2a).
It also begins to resolve the Sierra Nevada, but still has high terrain
extending eastward across Nevada. Lowland areas along the Colorado River and Lake
Powell in southeastern Utah are only vaguely resolved. The NARR, with its smaller grid
spacing, provides the best representation of the regional topography (Figure 2b). The
Sierra Nevada are a distinct range. Major terrain features are distinguishable, if lacking
detail. The lower Colorado River, Lake Powell, and Great Salt Lake Basin are now
apparent.
The GR1, with the coarsest resolution, displays the least accurate
representation of the true terrain (Figure 2c). The western US appears as a broad plateau
that reaches its maximum elevation in western Colorado. A lower plateau lies across
central Nevada.
With its sparse population located in geographically concentrated metropolitan
areas, the number of surface observations available for assimilation is limited over the
Intermountain West. The ERA-Interim ingests surface temperature, pressure and
moisture
from
SYNOP
reports,
whereas
the
GR1
ingests
WMO
Global
Telecommunication System (GTS) data, with no indication of excluded variables, as seen
in the NARR. By contrast, the NARR assimilates only 10-m wind and 2-m moisture
values from surface synoptic stations.
14
Figure 2. Topography (m, following inset scale) from the (a) ERA-Interim, (b) NARR,
and (c) NCEP/NCAR GR.
15
SNOTEL Data
Contributions of Intermountain Cyclones to the hydroclimate of the Intermountain
West are examined using precipitation observations provided by the SNOpack
TELemetry (SNOTEL) network maintained by the United States Department of
Agriculture Natural Resources Conservation Service (NRCS). The SNOTEL network
consists of over 750 stations across the Western States that provide precipitation and
snowpack observations for water supply forecasts and water resource management. Here
we use a subset of 22 stations over the Intermountain West to enable manual quality
control while at the same time ensuring representation for every geographic region within
the Intermountain West. Located primarily in upper-elevation catchment basins, they are
equipped with an unheated storage-type precipitation gauge with an Alter wind shield
(Serreze 1999). Precipitation is measured with a manometer and pressure transducer.
Undercatch due to wind or snow bridging can be a problem, but is minimized with
shielding and a relatively large 81 cm aperture. Daily (24 h) precipitation amounts (i.e.,
water equivalent) during the cool-season (Nov-Apr) and snow pillow-derived snow
water-equivalent (SWE) were used as precipitation datasets. Twenty-four h SWE is
simply the midnight to midnight difference in recorded by the snow pillow. This dataset
will control for snow bridging issue common to the precipitation gauges during heavy
snowfall events. The 24 h precipitation dataset was derived from water-year accumulated
precipitation and quality controlled using a simple approach. Specifically, days with
negative precipitation values or amounts greater than 7.5 cm were subjectively examined
and replaced with a missing data flag if deemed questionable based on subjective timeseries and proximal station consistency checks. Negative values are nearly always
16
attributable to gain/loss events where a constant precipitation level is punctuated by an
erroneous one-day increase or decrease in the accumulated precipitation. This produces a
2-day gain/loss couplet in the data. In these cases the precipitation on the anomalous day
was set to missing. Twenty-four h snow pillow SWE was similarly quality controlled.
17
CHAPTER 3
RESULTS
Cyclone Frequency, Genesis, and Lysis in the
Western US and Surroundings
The highest 850 hPa cyclone frequencies over the western US and surrounding
region are found over the Gulf of Alaska, over the Intermountain West, and over
southeast Colorado (Figure 3). The ERA-Interim and the NARR produce two mesoscale
regions of frequent cyclone activity over the Intermountain West: (1) the Great Basin
cyclone region, which extends downstream from the highest portion of the Sierra Nevada
to northern Utah and (2) the Canyonlands cyclone region, which lies over the upper
Colorado River Basin, a low-elevation region between the mountains/plateaus of central
Utah and the Colorado Rockies (Figure 3a,b). The NARR produces a much stronger
maximum over the latter. The low resolution GR1 produces only a broad frequency
maximum over the Intermountain West and fails to capture these mesoscale cyclone
regions. The ERA-Interim sea level pressure cyclone frequency artificially inflates the
cyclone count over the mountain west, while remaining in agreement over the ocean.
For all three reanalyses, the mean monthly 850-hPa cyclone frequency for the
western U.S. and surrounding region exhibits a weak bimodal distribution with a broad
18
Figure 3. Cyclone frequency (events/104 km2, 1989-2008). (a) ERA-Interim 850 hPa
cyclones. (b) NARR 850 hPa cyclones, (c) GR 850 hPa cyclones. (d) ERA-Interim sea
level pressure cyclones.
19
maximum in spring (Mar-May) and a weaker secondary maximum in fall (OctNov)(Figure 4). The ERA-Interim and NARR are fairly similar in magnitude, but the
GR1 has fewer events and less pronounced by modality. In particular, the spring to
summer decrease in 850-hPa cyclone frequency is much less abrupt in the GR1. There
are more ERA-Interim sea level pressure cyclones than 850 hPa cyclones. The increase is
largest in the late summer (Jul-Sep) when reduction erroneously enhances the mean
pressure trough found over the region.
Cyclogenesis at 850 hPa is preferred over the Gulf of Alaska, the Intermountain
West, and downstream of the Colorado Rockies (Figure 5). Although the frequency of
cyclogenesis is similar in all three reanalyses over the Gulf of Alaska, it is much lower in
the GR1 over the Intermountain West and downstream of the Colorado Rockies (Figure
5c). This is likely due to the inability of the GR1 to resolve the development of weaker
orographically forced systems. On the mesoscale, the NARR and ERA-Interim show
frequent cyclogenesis in the aforementioned Great Basin and Canyonlands cyclone
regions, with the NARR producing a stronger maximum over the latter (Figure 5a,b). A
cyclogenesis maximum is also found over Wyoming in both the ERA-Interim and
NARR. The frequency of sea level pressure cyclogenesis is much greater than found at
850 hPa (Figure 5d).
The distribution of monthly cyclogenesis frequency for the western US and
surrounding region is similar to that of cyclone frequency (Figure 6). The ERA-Interim
and NARR produce broad maxima in the spring and fall, peaking in May and Nov.
Minima occur in winter (Dec-Feb), and Aug. The GR1 has fewer events overall and fails
to capture the full amplitude of the semiannual cyclogenesis cycle, primarily because it
20
Figure 4. Mean monthly cyclone frequency over the western U.S. and surrounding
region (Fig. 1a domain) for ERA-Interim 850-hPa cyclones (circle), ERA-Interim sea
level pressure cyclones (triangle), NARR 850-hPa cyclones (square), and
NCEP/NCAR GR 850-hPa cyclones (asterisk).
21
Figure 5. 850-hPa cyclogenesis frequency (events/104 km2, 1989-2008) from the (a)
ERA-Interim, (b) NARR, (c) NCEP/NCAR GR, and (d) ERA-Interim SLP.
22
Figure 6. Mean monthly 850-hPa cyclogenesis frequency over the western U.S. and
surrounding region (Fig. 1a domain) for the ERA-Interim (circle), NARR (square),
NCEP/NCAR GR (asterisk), and ERA-Interim SLP (triangle).
23
produces fewer cyclone events in spring and fall. The ERA-Interim produces far more sea
level cyclones than found at 850 hPa in all months. For brevity, sea level pressure results
are not presented hereafter.
With the spatial distribution of 850 hPa cyclolysis (Figure 7), the ERA Interim
and NARR both produce maxima of lysis in the cyclone regions of Intermountain West
as well as the Gulf of Alaska. The higher lysis frequency in the Intermountain West
reflects the greater number of analyzed cyclones, as well as the fact that many
Intermountain cyclones cannot be traced continuously downstream of the Rockies as
coherent height minima with +40 m amplitude. Instead, these systems either decay over
the Intermountain West and/or redevelop discretely more than 8.5 degrees from their
decay point. The GR1 shows lysis almost exclusively in the Gulf of Alaska, central
Canada, and the Mississippi Basin. Weaker lysis maxima are found over the
Intermountain West and east of the Colorado Rockies. Monthly frequency of lysis very
closely approximates that of genesis (cf. Figures 6,8).
Intermountain Cyclone Frequency, Genesis, and Lysis
The spatial distribution of Intermountain 850-hPa cyclone frequency is strongly
tied to the regional orography and sensitive to the terrain resolution of each reanalysis
(Figure 9). In the ERA-Interim, cyclone frequency maxima lie downstream of the Sierra
Nevada and Colorado Rockies (Figure 9a), with the latter somewhat stronger. A third
maximum is centered downstream of the mountains of western Wyoming, which are
represented as only a broad, smooth plateau by the ERA-Interim. Frequent cyclone
activity is found within the Great Basin cyclone region, which extends northeastward
24
Figure 7. 850-hPa cyclolysis frequency (events/104 km2, 1989-2008) from the (a)
ERA-Interim, (b) NARR, and (c) NCEP/NCAR GR.
25
Figure 8. Mean monthly 850-hPa cyclolysis frequency over the western U.S. and
surrounding region (Fig. 1a domain) for the ERA-Interim (circle), NARR (square),
and NCEP/NCAR GR (asterisk).
26
Figure 9. 850-hPa Intermountain cyclone frequency (events/104 km2, 1989-2008) from
the (a) ERA-Interim, (b) NARR, and (c) NCEP/NCAR GR. Topography shaded.
27
from a cyclone frequency maximum located east of the Sierra Nevada across northern
Utah, where a secondary frequency maximum is found over the Great Salt Lake Basin,
and into eastern Idaho. A secondary maximum in cyclone activity extends across the
Canyonlands cyclone region of southeast Utah. Even though these SW-NE oriented
bands of high frequency have the appearance of cyclone tracks, not all cyclones move
through these corridors. In some instances, they traverse the region west-to-east, with
individual low centers identified discretely in each band.
The NARR cyclone frequency climatology is quite similar to the ERA-Interim.
The maxima in the lee of the high Sierra and Rocky Mountains are still present, although
they are shifted to the west in both cases. The NARR also positions the Great Basin
cyclone region further to the west, lacks a pronounced extension into eastern Idaho, and
fails to produce a pronounced secondary cyclone frequency maximum over the Great Salt
Lake Basin. The NARR analyzes more frequent cyclone activity in the Canyonlands
cyclone region, which may be related to the improved representation of the surrounding
high mountains and depth of the Canyonlands basin (Figure 2). The relative strengths of
the maxima in the lee of the Colorado Rockies and Sierra Nevada are reversed compared
to the ERA-Interim, with the NARR generating more frequent cyclogenesis to the lee of
the latter.
The GR1 fails to capture the mesoscale details found in the ERA-Interim and
NARR. The whole of the Intermountain West is covered by a broad maximum in
frequency (Figure 9c), and the Colorado lee maximum is centered over NW Kansas. (not
explicitly shown) The positions of these maxima are probably related to the coarse,
smooth orography of the GR1.
28
The monthly frequency of Intermountain 850 hPa cyclones is bimodal in all three
reanalyses, with an absolute maximum in May or Jun and secondary minimum in Oct
(Figure 10). Minima occur in Jul or Aug and Dec. The bimodality in the ERA-Interim
and NARR, which agree well, is much stronger than produced by the GR1 and also more
amplified than for the western U.S. and surrounding region (cf. Figures 10 and 4). Lee
(1995) attributes this bimodality to the semi-annual migration of the polar jet stream
across the Intermountain West as it moves southward with the approach of winter and
then northward with the approach of summer. He attributes the spring enhancement
relative to November to the decreased static stability during the spring when surface
sensible heating occurs beneath a relatively cold mid-troposphere. The bimodality stands
in stark contrast, however, to the cyclone frequency upstream over the eastern Pacific,
which we use as a proxy for the ambient cyclone activity, and features a single maximum
in February, a minimum in Jun, and only a slight increase in cyclone activity in the spring
(Figure 11, see Figure 1 for comparison region location).
The ERA-Interim produces frequent Intermountain cyclogenesis downstream of
the Sierra-Cascade Ranges over western Nevada, within the Great Basin cyclone region,
and within the Canyonlands cyclone region (Figure 12a). Maxima are found directly
downstream of the high Sierra and at the western edge of the Great Salt Lake Basin.
Frequent cyclogenesis in this region may be related to vortex stretching as the flow
moves into the Great Salt Lake Basin, which is one of the lowest basins in the Great
Basin. The precise positioning of the maximum may be related to the coarse resolution of
the ERA-Interim terrain.
The NARR also produces an elongated maximum in
cyclogenesis, within the Great Basin cyclone region, but with the maximum oriented
29
Figure 10. Mean monthly 850-hPa Intermountain cyclone frequency for the ERAInterim (circle), NARR (square), and NCEP/NCAR GR (asterisk).
30
Figure 11. Mean monthly 850-hPa cyclone frequency over the oceanic region
identified in Fig. 1a for the ERA-Interim (circle), NARR (square), and NCEP/NCAR
GR (asterisk).
31
Figure 12. 850-hPa Intermountain cyclone genesis (events/104 km2, 1989-2008) from
the (a) ERA-Interim, (b) NARR, and (c) NCEP/NCAR GR. Topography shaded.
32
more SW-NE. Unlike the ERA-Interim, the NARR does not produce as frequent
cyclogenesis maximum over northwest Nevada or a secondary cyclogenesis maximum
over the western Great Salt Lake Basin (Figure 12b). It does, however, produce much
more frequent cyclogenesis within the Canyonlands cyclone region. The GR1 generates
only an elongated maximum along the Great Basin cyclone region (Figure 12c).
By month Intermountain cyclogenesis frequency exhibits a bimodal distribution
similar to that of cyclone frequency (Figure 13). In the NARR the absolute maximum is
shifted one month later from the cyclone frequency maximum to June, with a secondary
maximum in October. The ERA-Interim maxima occur in May and October. The GR1 is
also bimodal, but with less amplitude. Over the period 1989-2008 the ERA-Interim
produced 290 cases of Intermountain cyclogenesis (14.5 yr-1) while NARR produced 320
(16 yr-1). For comparison, Lee (1995) identified 250 cyclogenesis events in sea level
pressure analyses over an 11 yr period (22.7 yr-1). As with cyclone frequency, the sharp
bimodality in cyclogenesis stands in stark contrast to the cyclogenesis frequency
upstream over the eastern Pacific (Figure 14).
Petterssen (1956) found that most cyclones that develop in the Intermountain
West also undergo lysis there. The ERA-Interim (Figure 15a) shows that the lysis of
Intermountain cyclones occurs primarily over the Intermountain West. NARR lysis
(Figure 15b) is slightly more widespread, with lysis events occurring in Wyoming and
New Mexico. Very few Intermountain cyclones travel east of the Colorado/Kansas
border. These results are likely dependent on our search radius, which does not continue
tracking cyclones that reform or move discretely more than 8.5° between reanalysis
times. Since it is extremely unlikely that a pressure minimum could move further in 6 h,
33
Figure 13. Mean monthly 850-hPa Intermountain cyclogenesis frequency for the
ERA-Interim (circle), NARR (square), and NCEP/NCAR GR (asterisk).
34
Figure 14. Mean monthly 850-hPa cyclogenesis frequency over the oceanic region
identified in Fig. 1a for the ERA-Interim (circle), NARR (square), and NCEP/NCAR
GR (asterisk).
35
Figure 15. 850-hPa Intermountain cyclone lysis (events/104 km2, 1989-2008) from the
(a) ERA-Interim, (b) NARR, and (c) NCEP/NCAR GR. Topography shaded.
36
we believe these cyclones either decay in place over the region or decay as a new low
center forms downstream.
The frequency of higher-amplitude cyclones in the Intermountain West is greatest
in Apr-Jun, with a secondary maximum around October in both the ERA-Interim and
NARR (Figure 16; GR1 not presented). The most extreme events are produced by the
higher resolution NARR, and they occur preferentially from Mar-Jun. A marked
difference between the ERA-Interim and NARR occurs in the summer, where the higher
summer frequency of cyclones in the NARR is evident.
Evolution of High-Amplitude Intermountain Cyclone Events
A subset of Intermountain cyclones in the 90th percentile of amplitude (36
cyclones) was selected for further study. Each case was examined individually and,
following Lee (1995), classified based on the large-scale 500 hPa flow across the Sierra
Nevada. Thirty (83%) of the events occurred in westerly to southerly (SW) flow, 4 (11%)
in northwesterly (NW) flow, and 2 (6%) in northeasterly (NE) flow. For comparison,
75% and 24%, of the cases examined by Lee (1995) fell into the SW and NW categories,
respectively.
Composites of the 30 SW events show that 24 h prior to cyclogenesis a 500 hPa
short wave trough is found just off the Pacific coast, inducing weak upward motion over
much of the Western US (Figure 17a). West to southwesterly flow is present over the
Sierra Nevada at 700 hPa (near crest level), with relative humidity increasing to the north
and a weak baroclinic zone draped across the northern part of Intermountain West
(Figure 17b). A weak positively tilted trough exists downstream of the high Sierra at 850
37
Figure 16. Two-dimensional histogram of the maximum 850-hPa cyclone amplitude
(m) vs. month from the (a) ERA-Interim and (b) NARR.
38
Figure 17. ERA-Interim composite of the 90th percentile SW flow Intermountain
cyclones, hour -24 relative to genesis time. (a) 500-hPa geopotential height (every 60
m) and vertical velocity (every 1 cm s−1, negative values contoured, positive values
shaded and contoured); (b) 700-hPa temperature (contoured every 2°C), wind [full
(half) barbs denote 5 (2.5) m s−1], and relative humidity (shaded and contoured); (c)
Mean 850 hPa geopotential height (every 20 m), wind [full (half) barbs denote 5 (2.5)
m s−1], and surface potential temperature (shaded every 5°C according to scale at
right).
39
hPa over the Intermountain West and is collocated with confluence to the lee of the Sierra
Nevada (Figure 17c), a feature identified as the Great Basin Convergence Zone (GBCZ)
by West and Steenburgh (2010).
At h 0 (Figure 18) the 500 hPa trough has amplified and dug southward, rotating
the 500 hPa geostrophic wind close to normal to the Sierra Nevada barrier. Rising
motion over Nevada and Utah is now greatly enhanced relative to h -24. Concurrently,
cross-barrier westerly flow impinges on the Sierra Nevada at 700 hPa. The winds at 700
and 850 hPa are strongly confluent downstream of the high Sierra, with the 850 hPa low
center near the Utah-Nevada border (Figure 18b,c). The 700 hPa baroclinic zone has
rotated into a more meridional alignment, consistent with an amplifying thermal wave.
Little advection of temperature is present at 700 hPa at the location of the 850 hPa low.
However, it is flanked by cold advection to the west and warm advection to the east.
By 24 hours after cyclogenesis the 500 hPa trough has further intensified and is
now centered over central Nevada and southern California with a neutral tilt (Figure 19a).
Strong ascent and forcing for surface cyclogenesis has also shifted downstream and is
now found along the Utah-Colorado boarder. At 700 hPa, the flow has veered and
become nearly parallel to the high Sierra (Figure 19b). As the upper-level ascent shifts
downstream and the cross-barrier flow weakens, the 850 hPa low center shifts eastward
and is now located near the Canyonlands cyclone region (Figure 19c). The leading edge
of cold advection at 700 hPa over western Colorado and New Mexico is well downstream
of the low center and surface wind shift over central Utah, a configuration that contrasts
greatly from what one might expect from the Norwegian Cyclone Model (Bjerknes and
Solberg 1922).
40
Figure 18. ERA-Interim composite of the 90th percentile SW flow Intermountain
cyclones, hour 0 relative to genesis time. (a) 500-hPa geopotential height (every 60 m)
and vertical velocity (every 1 cm s−1, negative values contoured, positive values
shaded and contoured); (b) 700-hPa temperature (contoured every 2°C), wind [full
(half) barbs denote 5 (2.5) m s−1], and relative humidity (shaded and contoured); (c)
Mean 850 hPa geopotential height (every 20 m), wind [full (half) barbs denote 5 (2.5)
m s−1], and surface potential temperature (shaded every 5°C according to scale at
right).
41
Figure 19. ERA-Interim composite of the 90th percentile SW flow Intermountain
cyclones, hour +24 relative to genesis time. (a) 500-hPa geopotential height (every 60
m) and vertical velocity (every 1 cm s−1, negative values contoured, positive values
shaded and contoured); (b) 700-hPa temperature (contoured every 2°C), wind [full
(half) barbs denote 5 (2.5) m s−1], and relative humidity (shaded and contoured); (c)
Mean 850 hPa geopotential height (every 20 m), wind [full (half) barbs denote 5 (2.5)
m s−1], and surface potential temperature (shaded every 5°C according to scale at
right).
42
By 48 hours after cyclogenesis the 500 hPa trough has moved further west and
begun to fill (Figure 20a). The 500 hPa vertical velocities have weakened as the trough
fills. The 700 hPa baroclinic zone is now well downstream of the Rocky Mountains
(Figure 20b), while the 850 hPa closed low is over southeast Colorado. Some of this
weakening may be due to decreasing robustness of the composite two days from the
cyclogenesis time.
Cold air is now resident over the Intermountain West, having
advanced steadily southward over the last 72 h.
Upper-level dynamics and terrain-induced circulations both contribute to
development and evolution of Intermountain cyclones. The predominance of crossbarrier southwesterly flow at 700 hPa and 500 hPa indicates cyclogenesis occurs in
response to terrain-induced vortex stretching in conjunction with the dynamic forcing
accompanying the strong 500 hPa trough. Following cyclogenesis, the low center moves
downstream with the upper-level dynamical forcing as the crest-level flow veers to
increasingly terrain parallel.
Linkages to Regional Hydroclimate
Cool-Season Mountain Precipitation
Cyclones are generally viewed as producing much of the precipitation in the midand high-latitudes, but how much precipitation is produced by cyclones over the
Intermountain West? Recently, Myoung and Deng (2009) used cyclones identified in the
GR1 and precipitation from the NARR to show that 45-75% of the wintertime
precipitation over the Pacific States (CA, OR, WA) is generated by coastal cyclones, but
over the Intermountain West the cyclone attributable precipitation varies from a
43
Figure 20. ERA-Interim composite of the 90th percentile SW flow Intermountain
cyclones, hour +48 relative to genesis time. (a) 500-hPa geopotential height (every 60
m) and vertical velocity (every 1 cm s−1, negative values contoured, positive values
shaded and contoured); (b) 700-hPa temperature (contoured every 2°C), wind [full
(half) barbs denote 5 (2.5) m s−1], and relative humidity (shaded and contoured); (c)
Mean 850 hPa geopotential height (every 20 m), wind [full (half) barbs denote 5 (2.5)
m s−1], and surface potential temperature (shaded every 5°C according to scale at
right).
44
maximum of 60% near Reno, NV to a low near 15% in Western CO. We know of no
other effort to estimate the amount of precipitation produced exclusively by cyclones
over the Intermountain West, particularly during the cool-season when the high-elevation
snowpack that serves as the source for most of the region’s water resources is generated.
Here we use a simple approach to provide an estimate of the contribution of
Intermountain cyclones to the cool-season (Nov-Apr) mountain precipitation over the
Intermountain West.
Specifically, we define an Intermountain cyclone day as any day
during which an Intermountain cyclone is present during the 0000-0000 PST period and
examine the fraction of the precipitation produced on these days to the total cool-season
precipitation at the 22 SNOTEL sites. The fraction of cool-season precipitation produced
on Intermountain cyclone days is somewhat higher for ERA-Interim cyclones than
NARR cyclones, even though the latter produces more total cyclones (Figure 21). Using
the precipitation gauge, the ERA-Interim (NARR) derived fraction varies from as low as
8% (5%) to as high as 32% (19%). When snow pillow SWE is used (Figure 22), the
fractions are generally higher, but differ from the precipitation gauge figures by a
maximum of only 5%. While the results show no strong geographic pattern, stations
situated on west-east oriented terrain barriers appear to get a higher percentage of their
precipitation on Intermountain cyclone days. The overall low fractions suggest that the
bulk of the mountain cool-season precipitation over the Intermountain West is produced
by storm systems that do not produce surface cyclones, at least as defined here. This
approach does not account for precipitation induced by cyclones that do not cross into the
Intermountain domain, and although the frequency of cyclones outside of the
Intermountain region is relatively low (Figure 9) contributions from these systems could
45
Figure 21. Fraction of cool-season precipitation produced on NARR 850 height
Intermountain cyclone days. SNOTEL precipitation (top) and SNOTEL SWE
(bottom)
46
Figure 22. Fraction of cool-season precipitation produced on ERA-Interim 850 height
Intermountain cyclone days. SNOTEL Precipitation (top) and SNOTEL SWE
(bottom).
47
be important.
Dust Transport and Deposition
Recent research has shown that dust deposition on the mountain snowpack of
Western Colorado’s San Juan Mountains can result in premature snowmelt and up to a 35
day decrease in snow-cover duration (Painter et al. 2007). It is our experience that many
of the cool-season dust storms in this region occur in conjunction with Intermountain
cyclones (e.g., West and Steenburgh 2010). To examine this potential linkage, the dates
of major dust deposition episodes in the San Juan Mountains were compared to the ERAInterim and NARR 850 hPa cyclone climatologies (Table 3). The San Juan dust episodes
were identified by manual observation of dust on the snow surface in Senator Beck
Basin, CO by Christopher Landry of the Center for Snow and Avalanche Studies in
Silverton, CO. An ERA-Interim (NARR) Intermountain cyclone was identified within 24
h on either side of 0000 UTC on the day of the dust deposition on 17 (16) of the 33
events. While the wind speeds necessary for dust transport vary due to soil moisture,
seasonal vegetative growth and other factors, these results suggest that Intermountain
cyclones may provide the strong winds necessary to loft dust for a substantial fraction of
episodic dust deposition events in the San Juan Mountains of Colorado, and perhaps other
ranges of the Intermountain West.
48
Table 3. List of dust events in the San Juan Mountains, CO.
49
CHAPTER 4
CONCLUSIONS
In this study we examined the climatology of Intermountain cyclones using an
objective cyclone identification algorithm applied to the ERA-Interim, NARR, and GR1
reanalyses. The climatology concentrates on 850-hPa cyclones because that pressure
level is near the mean elevation of the valleys and basins of the Intermountain West and
sea-level reduction results in an artificially high cyclone count. At 850 hPa, the ERAInterim and NARR provided generally consistent results reflective of the regional terrain,
whereas the GR1 was handicapped primarily by a lack of terrain resolution.
The area of southern Nevada in the lee of the high Sierra is a preferred area of
cyclone and cyclogenesis frequency in the ERA-Interim and NARR. Secondary maxima
are found over the Great Basin cyclone region and Canyonlands cyclone region. The
ERA-Interim favored the former, while the NARR favored the latter.
Intermountain cyclone frequency, genesis frequency and amplitude all peak in the
spring and early summer (Apr-Jun) months, with minima in Jul-Aug and Dec-Jan, and a
secondary maximum in Oct-Nov. The ERA_Interim, NARR and GR1 differ primarily in
the amplitude of this bimodal pattern.
Cyclone statistics over the Intermountain West were compared to that over the
50
eastern Pacific directly upstream. Cyclone statistics in the Oceanic domain do not feature
the aforementioned bimodal distribution and cyclogenesis is far less frequent. Relatively
good agreement among all three reanalyses in the Oceanic domain contrasts with the high
inter-reanalysis variability in the Intermountain West. This is likely due to a combination
of terrain and land-surface differences over the Intermountain West, combined with
differing data assimilation strategies.
Hydrologically, no more than 32% of the cool-season precipitation at selected
mountain precipitation gauges falls on days with an Intermountain cyclone present.
Intermountain cyclones occur in conjunction with roughly 50% of recent dust deposition
events in the San Juan Mountains of Colorado, suggesting that strong winds
accompanying Intermountain cyclones is a common cause of dust transport from the
desert areas of Utah and Arizona to mountain snowpacks.
Further research should focus on further elucidating the role played by
Intermountain cyclones in the hydrologic cycle of the Intermountain West. Currently this
linkage is circumstantial and could benefit from explicit analysis.
In addition to
hydrologic factors, any number of sensible weather phenomena could be examined for
their relationship to Intermountain cyclones and cyclogenesis using this climatology.
51
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