Non-Convective High Wind Events (CONUS/UK + Weather)
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Saint Louis University
Summer 2012
Non-Convective High Wind Events
(CONUS/UK + Weather)
Investigating the Influence of Stratospheric Intrusions on
Non-Convective High Wind Events Using RGB Air Mass
Product
Technical Report – August 8, 2012
Nicholas Elmer, Saint Louis University (project lead)
Michelle Hogenmiller, Saint Louis University (project lead)
Mallory Cato, Saint Louis University
Sarah Trojniak, Saint Louis University
Dr. Emily Berndt, Saint Louis University (Science Advisor)
Dr. Michael Folmer, GOES-R Proving Ground (Science Advisor)
Dr. Timothy Eichler, Saint Louis University (Science Advisor)
Executive Summary
The public usually expects high winds to accompany severe weather, but
tends to ignore high winds associated with non-convective events. These events
are absent of familiar features such as thunderstorms, tornadoes, or tropical
cyclones. According to Ashley and Black (2008), fatalities associated with non-
convective events are commonly associated with passing extratropical
cyclones along the Northeast and Northwest coasts of the United States.
Additionally, Lacke et al. (2007) demonstrated a significant frequency of non-
convective high wind events in the Great Lakes region. There is no commonly
accepted explanation for non-convective winds associated with extratropical
cyclones; however, physical explanations include: topography, the isallobaric
wind, tropopause folds, or the sting jet (Knox et al. 2011).
The goal of the project was to examine the intensity and location of the
high winds in cyclones in conjunction with stratospheric intrusions. A secondary
goal was to determine the possible presence of the sting jet. The project utilized
NASA satellite data and NASA Modern-Era Retrospective Reanalysis for research
and applications (MERRA) reanalysis data to investigate three intense
extratropical cyclones: 1) a Great Lakes cyclone on October 26-27, 2010, 2) a
Nor’easter on October 29-30, 2011, and 3) a cyclone on January 2-3, 2012,
which impacted the United Kingdom.
The satellite imagery analyzed included the Red, Green, Blue (RGB) Air
Mass product and the Atmospheric Infrared Sounder (AIRS) ozone product. The
RGB Air Mass product was used to trace stratospheric intrusions. The AIRS ozone
product was used to confirm the presence of ozone-rich air associated with the
stratospheric intrusions. The storm structure was further investigated through the
Hybrid Single Particle-Lagrangian Trajectory Model (HYSPLIT) (Draxler and Hess
1997, 1998, Draxler 1999) and system relative stream lines generated with the
General Meteorology Package (GEMPAK) (desJardins 2004)
Since the RGB Air Mass product is new, MERRA reanalysis data is useful to
assess and confirm features we see in the RGB product. MERRA data was used
for analysis because it is a unique product that differs from other reanalysis
datasets, since it has the advantage of higher spatial resolution (Bosilovich et al.
2011). This advantage was used to diagnose six-hour winds, ozone, and
potential vorticity, to locate sub-synoptic regions of non-convective high winds.
Future research includes evaluating these regions to see if they may be
associated with the sting jet (Browning 2004, Cao 2009).
The long-range goal of our research is to develop a tool for our partner in
the GOES-R Proving Ground that will lead to increased skill in forecasting non-
convective high wind events. Future research will focus on how to best apply
the RGB product, AIRS, and MODIS imagery to provide operational forecasters
with additional understanding and tools to enhance forecasting capability.
Future research also includes evaluating these cyclones to see if they may be
associated with the sting jet (Browning 2004, Cao 2009).
Abstract
Intense extratropical cyclones are often associated with non-convective
high winds, which have devastating economic and societal impacts. This study
investigated the influence of stratospheric intrusions on the production of high
surface winds for three recent events: the October 26-27, 2010 Great Lakes
event, the October 29-30, 2011 early season Nor’easter, and the January 2-3
2012 United Kingdom storm. Rapidly intensifying cyclones are commonly
associated with tropopause folds, which can be identified by intrusions of
subsiding warm, dry, ozone-rich air. The link between subsiding upper-level air
and high surface winds has not been fully established.
This study uses unique NASA satellite products and MERRA reanalysis data
to diagnose the dynamical structure of non-convective high wind events. This
project also incorporates the use of the Spinning Enhanced Visible and Infrared
Imager (SEVIRI), MODIS, AIRS, and experimental RGB Air Mass imagery derived
from Aqua and Terra MODIS satellite products to diagnose the storm structure.
We compare six-hourly MERRA reanalysis geopotential height, temperature,
winds, and sea level pressure, as well as PV and ozone, for our three storms with
the satellite imagery to verify the various structures seen in the satellite imagery
pertaining to high-wind hazards. To provide even greater detail of the storm
structure and conveyor belts, we also utilize the Hybrid Single Particle
Lagrangian Trajectory (HYSPLIT) model to calculate trajectories.
By comparing the RGB Air Mass satellite analysis with the MERRA data analysis,
we aim to increase our understanding of the structure of intense extratropical
cyclones that produce high-impact non-convective winds. With the additional
information provided by the HYSPLIT model, a more detailed three-dimensional
picture of the cyclone structure is presented, which will ultimately lead to
improved forecasts of high-impact, non-convective wind events.
Keywords
Non-Convective High Wind Events, RGB Air Mass product, Stratospheric
Intrusions,
Conveyor Belts, Extratropical Cyclones, Ozone
Project Objectives
The project objectives are to confirm if the RGB Air Mass product can be
used to identify stratospheric intrusions, to gain a better understanding of what
causes non-convective wind events, and to discover if “sting jets” occur in the
United States or only with oceanic storms. Additionally, the project intends to
demonstrate the benefit of NASA MERRA reanalysis data used for a synoptic
analysis of each storm. The overall project objective is to gain a greater
understanding of the role of stratospheric intrusions in producing high-impact
non-convective wind events.
Introduction
A. Background Information
Currently, non-convective high wind events are not well forecasted.
Therefore, a better understanding of these events will help the public
become aware of the threat and hopefully lead to the protection of life and
property. Forecasters often know which storms have the potential to
produce near-hurricane force winds. However, the timing and location of
these high winds is often difficult to forecast. Recently there has been
research done suggesting that the cause for some of these events is a micro-
scale feature called a sting jet (Browning 2004). Browning and Field (2004)
used Meteosat imagery to identify cloud structures associated with
damaging winds due to the presence of the sting jet. It is unknown whether
the sting jet occurs in extratropical cyclones that impact the United States.
Although our current satellite imagery has helped forecasters track and
forecast these events, many improvements are still needed. One available
improvement is the RGB Air Mass product, which is currently on the Meteosat-
9 satellite and will be on the new GOES-R satellite. Since there is no
operational satellite over the United States that currently produces RGB Air
Mass imagery, the GOES-R Proving Ground has the capability to derive a
similar RGB Air Mass product using either the GOES-11/13 sounder or Aqua
and Terra MODIS.
B. Study Area
The study area included North America and the United Kingdom. Analysis
on North America included the Great Lakes region, New England, and the
southeastern coast of Canada.
C. Study Period
The study period begins October 2010 and ends January 2012.
D. National Application(s) Addressed
This study mainly addresses weather, more specifically non-convective
high wind events. Furthermore, it addresses the need for a better
understanding of how to use NASA reanalysis and satellite products to
forecast these high wind events.
F. Project Partners
GOES-R Proving Ground and NASA SPoRT are our partners. GOES-R
Proving Ground is interested in this project because they want to study the
applications of the RGB Air Mass product before it is available on GOES-R in 3
years. This includes an understanding of how to use the RGB Air Mass
product to trace stratospheric intrusions. Knowledge gained will be
incorporated into forecaster training and improve the forecasting of high
wind events.
Methodology
A. Data Acquisition
The following data sets were utilized:
• MERRA: NASA
• GOES-11/13 Satellite: Space Science and Engineering Center (SSEC)
in Wisconsin
• NARR: National Center for Environmental Protection (NCEP)
• ERA-Interim: Computational Information Systems Laboratory (CISL)
Research Data Archive—National Center for Atmospheric Research
(NCAR)
• RUC13: Earth System Research Laboratory
• Surface and Upper air observations: Unidata via Local Data
Manager at Saint Louis University
• Meteosat-9 RGB Air Mass imagery: GOES-R Proving Ground
• GOES-11/13 Sounder RGB Air Mass Imagery: NASA SPoRT and GOES-
R Proving Ground
• Terra and Aqua MODIS RGB Air Mass Imagery: NASA SPoRT and
GOES-R Proving Ground
• AIRS ozone product
We collected data for the following dates:
• Oct 25-27, 2010
• Oct 29-31, 2011
• Jan 1-4, 2012
Using this data, we created our own images via GEMPAK scripting, NMAP2,
and GRADS.
B. Data Processing
• The MERRA data was converted from Hierarchical Data Format to
binary and netCDF to binary.
• Observational data, NARR data, and RUC-13 data were converted to
GEMPAK format.
• The ERA-Interim data was converted to GEMPAK, which was then used
to create products and images.
• Code was written to analyze the data.
• Conventional satellite imagery was converted to GEMPAK format.
• NASA SPoRT processed the AIRS Ozone and MODIS RGB Air Mass
products to GEMPAK area files for our use.
• GOES-R Proving Ground provided the GOES-11/13 sounder RGB Air
Mass images.
• NARR data was converted to ARL format for use in HYSPLIT and NCEP
reanalysis.
C. Data Analysis
• NASA MERRA reanalysis data was used to verify synoptic features in the
RGB Air Mass imagery.
• NARR data, ERA-Interim data, and RUC-13 data were used to analyze
meteorological parameters.
• GEMPAK code was used to look at parameters such as potential
vorticity (PV), mean sea level pressure, surface winds, surface
temperature, and surface wind gusts. These parameters were overlaid
onto AIRS and MODIS products.
• NMAP2 was employed to look at the GEMPAK files, which aided in the
synoptic analysis of the cases.
• GOES-11/13 Satellite was used to identify cloud features.
• GOES-11/13 Sounder RGB Air Mass, MODIS RGB Air Mass, and
Meteosat-9 RGB Air Mass products were used to identify stratospheric
intrusions.
• AIRS ozone product was used to confirm that the dry intrusions in the
RGB Air Mass imagery were stratospheric in origin.
• GEMPAK coding was used to track the area of low pressure and
identify the u-wind and v-wind, which allowed for the creation of
streamlines.
• HYPSPLIT was utilized to create parcel trajectories to model the
conveyor belts within the cyclones and verify the location of the dry
intrusions.
Results
A. October 26-27, 2010
1. Societal and Economic Impact of the Event
The historic extratropical Great Lakes cyclone on October 26-27, 2010,
impacted much of the entire United States, including nearly every state east of
the Rocky Mountains. At the peak of the storm, the cyclone stretched from the
Atlantic Ocean to the Rocky Mountains and from Canada to the Gulf of
Mexico. The storm brought a wide variety of severe weather, including
tornadoes, damaging winds, hail, heavy snow, and heavy rain. Strong, non-
convective winds occurred across much of the northern Plains and Great Lakes,
with consistently intense wind gusts over 28 m/s, with a measured gust of 35 m/s
in Sherwood, Wisconsin. Non-thunderstorm wind gusts over 26 m/s were
recorded in 10 Midwestern and Great Lakes states. Over Lake Superior,
sustained winds of 30 m/s and gusts of 35 m/s contributed to the development
of very large waves 5.8 meters high. Widespread wind damage occurred
across much of the eastern U.S., including loss of power in several states.
Additionally, shipping was halted on the Great Lakes due to the strong winds
and high waves, greatly impacting shipping and commerce in the region.
The cold front spawned dozens of tornadoes throughout the southern U.S.
A total of 57 tornado reports were made in a total of 10 states: Wisconsin, Illinois,
Indiana, Ohio, Kentucky, Tennessee, Mississippi, North Carolina, Georgia, and
South Carolina. Banded snowfall throughout the comma head brought blizzard
conditions to the Dakotas and as much as 23 cm of snow. Additionally, large
amounts of rain south of the heavy snow led to moderate flooding in some
regions. The cyclone reached its lowest pressure of 955.2 hPa at 2130 UTC on
October 26 in Bigfork, Minnesota. This central pressure was just 0.2 hPa shy of the
lowest pressure ever recorded in a continental U.S., land-based, non-tropical
cyclone. The surface analysis of the cyclone at its peak, 2030 UTC on October
26, shows the intensity of the cyclone and the widespread area along the cold
front under severe weather warnings (Fig. 1).
Figure 1: Surface analysis at 2030 UTC 26 October 2010 provided by Unisys.
2. Synoptic Analysis from MERRA Reanalysis
For the most part, the storm was highly predictable and well forecasted
by several models, which predicted a deep cyclone over Minnesota well in
advance of its development. The models also predicted strong winds along the
cold front associated with thunderstorm activity and in the southern tip of the
comma head associated with the cold conveyor belt (CCB) downdraft and
strong low level jet (LLJ). While the anomalously high surface winds for these
locations were well forecasted by the models, the strong winds within the dry slot
were not. These dry slot winds occurred over Lake Michigan and Lake Superior
during the most intense period of the storm, and had previously occurred over
the tri-state region of Iowa, Missouri, and Illinois 12 hours earlier.
Key characteristics of this cyclone, which played large roles in its
development and production of strong surface winds, included a very strong
pressure gradient, a strong upper level jet with winds in excess of 77 m/s (Fig. 2),
an strong area of divergence aloft (Fig. 3), and a strong LLJ (30-35 m/s). While
these characteristics explain the presence of strong winds, they fail to account
for the strong surface wind gusts which occurred in the dry slot.
Figure 2: MERRA 300 hPa heights and winds at 1800 UTC 26 October 2010.
Figure 3: MERRA 200 hPa heights and divergence at 1800 UTC 26 October 2010.
At 300 hPa, there was strong divergence across Minnesota and Iowa as
the surface low deepened. The jet was located south across Oklahoma,
Kansas, and Missouri, placing the surface low in the left exit region of the jet,
providing essential upper level support and divergence so that the cyclone
could continue to deepen. Strong diffluence was present at 500 hPa and 300
hPa over a rapidly intensifying surface low, which stretched well into the upper
levels, with closed height contours reaching as high as 250 hPa.
The 400-200 hPa layer PV maximum was located at the point where the
dry intrusion began to strongly curve counterclockwise towards the center of
the low, not at the leading edge of the dry intrusion. This characteristic holds
true for the cyclone’s entire lifespan. At 1500 UTC on October 26, for example,
the leading edge of the intrusion was over central Wisconsin, while the PV
maximum was located over Missouri, Iowa, and Illinois (Fig. 4). The location of
the PV was nearly identical with the placement of the dry conveyor belt (DCB).
This region is located over a 20 m/s wind gust region at the surface with the
same basic shape, suggesting that the two are linked (See Fig. 11). The LLJ was
situated over the wind gust region to the west. The origin of the 400-200 hPa
layer PV comes from the Pacific Ocean, crossing the coast over Oregon. This
was the same location where PV was transported from Typhoon Megi into the
Continental U.S.
Figure 4: NARR 400-200 hPa layer PV at 1500 UTC 26 October 2010.
The PV maximum occurred at the exit region of the LLJ at both 700 and
850 hPa. This also corresponds with the tip of the comma head and the location
of winds within the dry intrusion. The dry intrusion can be separated into a
northern and southern branch. The northern branch is most clearly indicated by
the 400-300 hPa layer PV, suggesting this northern branch was between 300 and
400 hPa.
At 1800 UTC on October 26, a region of strong surface wind gusts was
located over Lake Michigan below the dry intrusion. By 0000 UTC on October
27, the point at which the cyclone began to dissipate, the strong surface windscease to appear below the dry slot. The strong surface wind gusts mainly
occurred in the same locations throughout the cyclone’s lifespan, because the
system stalled between the border of Minnesota and Ontario.
3. Satellite Analysis
i. Cloud Structures
Using the method outlined in Browning and Field (2004), cloud features
were tracked in satellite imagery to see if certain cloud features associated with
strong surface winds were present. During this event, only one chevron-shaped
cloud feature was identified, which appeared over northern Iowa around 1400
UTC on October 26 and tracked eastward into southwestern Wisconsin over the
next 3 hours. The cloud feature appeared on radar and was accompanied by
strong winds. This event was verified using the National Climatic Data Center
(NCDC) storm database, which shows that peak high winds were recorded in
northern Iowa while the satellite infrared images showed the chevron-shaped
cloud passing over the area (NDCD 2010). At four points in this time period, the
location of the chevron-shaped cloud coincided with reports of high winds (See
Fig. 5,6, and 7). This cloud feature occurred near the tip of the comma head at
the boundary of the dry intrusion. The chevron feature trailed just west of the
400-200 hPa layer PV maximum. This may be a possible indication of a sting jet
created as the DCB and CCB interact.
Location Date Time Event Speed
Figure 5: NCDC Storm Events database entries from 1045 to 1145 local time (1545 to 1645 UTC) for
counties in northern Iowa and southwestern Wisconsin.Figure 6: GOES 13 IR Imagery 1545 UTC 26 October 2010 (top left), 1615 UTC 26 October 2010
(top right), 1645 UTC 26 October 2010 (bottom left), 1715 UTC 26 October 2010 (bottom right).
The chevron-shaped cloud (circled in each image) can be seen propogating eastward across
northern Iowa from 1545 UTC to 1715 UTC.
Additionally, two cloud arcs propagated eastward over Wisconsin and
Michigan’s Upper Peninsula between 2100 UTC on October 26 and 0200 UTC on
October 27 (Fig. 7). These could not be verified with the NCDC database, either
because they were not associated with the peak wind for that area during the
storm, there were no surface wind gusts that accompanied them, or the gusts
occurred but were not reported to the local National Weather Service. These
two cloud features, similar to the chevron-shaped cloud feature, also trailed the
400-200 hPa layer PV maximum as it moved northward and cyclonically towards
the center of the low.
Another larger crescent-shaped cloud feature stemmed from the comma
head around 2000 UTC on October 26. This feature separated from the comma
head and propagated across Minnesota, Wisconsin, and Lake Superior during
the following 5 hours. The cloud arcs, which emanated from the comma headtip between 2100 UTC and 2359 UTC on October 26, occurred over the strong
surface winds in Wisconsin and northern Minnesota.
Figure 7: The surface wind gusts from the Great Lakes cyclone are shown in the Duluth National
Weather Service map, along with the paths of three cloud features, indicated by the color lines.
ii. RGB Air Mass Imagery
The RGB Air Mass imagery derived from the GOES-11/13 sounder showed
a strong pocket of dry air pushing towards the center of the low beginning on
October 26. The dry air matched up with the 400-200 hPa layer PV field, which
may indicate the hypothesized stratospheric intrusion. The RGB Air Mass imagery
(Fig. 8) clearly shows two different regions of red-orange coloring which indicate
sources of the dry air intrusion, one from the Pacific Northwest and another from
Canada.Figure 8: GOES-11/13 Soumder RGB Air Mass image 1700 UTC 26 October 2010.
The dry air coming off the Pacific actually originated in Southeast Asia
when it was stripped off a decaying typhoon, known as Typhoon Megi.
Typhoon Megi was one of the strongest typhoons in the Western Pacific on
record. It made landfall near Luzon, Philippines, just after 0000 UTC on October
18, 2010, bringing over 50 cm of rain to some parts of the country. The typhoon
killed 28 people and destroyed countless homes. Its lowest recorded central
pressure was measured at 885 hPa with accompanying 1-minute sustained
winds of 82 m/s. As a rare Category 5 super typhoon, Megi shares the record for
the second strongest typhoon ever observed in the Northwest Pacific.MTSAT 1R water vaper imagery
(a) (b)
GOES-11 water vapor imagery
(c) (d)
Figure 9: Water vapor imagery from MTSAT 1R and GOES-11 was used to trace the dry air with
high PV from Typhoon Megi to the central U.S., where it played a role in the development of the
October 26, 2010 cyclone. Typhoon Megi is shown in (a) by the dark purple dot. (a) 2332 UTC 17
October 2010 (b) 1732 UTC 21 October 2010 (c) 0000 UTC 25 October 2010 (d) 1200 UTC 26
October 2010
Typhoon Megi influenced the Great Lakes cyclone because it provided
PV, which allowed the rapid intensification of the Midwest Cyclone. As Typhoon
Megi moved across the South China Sea and made landfall with the mainland,
it ran into the mid-latitude westerly flow at mid-levels. These winds stripped the
PV from the typhoon and transported it eastward along the 45 degree line oflatitude, where it arrived in the continental U.S. less than eight days later (Fig. 9).
By October 25, the PV crossed over the Rocky Mountains and interacted with a
broad area of moisture in a leeward trough. By October 26, the PV introduced
enough energy to spin up a strong mid-latitude cyclone over the upper
Midwest. Both of these storms are believed to have been influenced by
stratospheric intrusions, and it appeared as if they were affected by the same
pocket of mid-level PV. The introduction of large amounts of PV allowed for
their rapid intensification. Looking at the water vapor imagery from the MTSAT
1R stationed over the Pacific (Fig. 9), the dry region can be seen developing
over China, just north of Typhoon Megi as a result of a stratospheric intrusion.
In addition to the PV that was transported across the Pacific, there was
also another source of dry, stratospheric air in Canada. These two sources
combined over Nebraska and Kansas at 1200 UTC on October 26 and became
a major player in the development of the cyclone. The RGB Air Mass product
allows for a clearer analysis of the presence of dry air in this case than the water
vapor imagery, because the water vapor fails to pick up the northern source of
the stratospheric air. For example, at 0000 UTC on October 27 (Fig. 10a), the
water vapor image clearly shows dry air over the southern U.S., but nothing is
shown of the dry air coming from Canada. However, in the RGB Air Mass image
(Fig. 10b), the northern branch of dry air can clearly be seen arcing across
Montana and the Dakotas, meeting up with the other branch in Iowa and
Illinois. The RGB Air Mass also shows the origin of the southern branch, as the tail
trails across the western U.S. and into the Pacific Ocean, verifying that this is the
same ribbon of vorticity that was tracked across the Pacific from Typhoon Megi.
(a) (b)
Figure 10: Comparison between the (a) GOES-13 water vapor imagery and (b) GOES-11/13
Sounder RGB Air Mass imagery at 0000 UTC 27 October 2010.
Starting at 0900 UTC on October 26, the dry air pushed past the southern
tip of the comma head. Strong convection occurred mostly along the cold
front, while much of the comma head at this time was composed of weak
convection. The dry slot and 400-200 hPa layer PV had no role in creating thesurface wind gusts over the Dakotas. At 1200 UTC and 2100 UTC on October 26,
wind gust regions formed within the dry slot, shown in Fig. 11 and Fig. 12 by the
red colors on the imagery. The surface wind gusts at 1200 UTC on October 26
show strong surface winds underneath the dry intrusion over Illinois, Iowa, and
Missouri. At 2100 UTC, the dry intrusion is coincident with strong surface wind
gusts over Lake Michigan and Lake Superior. In addition to the CCB, there were
wind gusts over the Dakotas and warm conveyor belt (WCB) winds associated
with the squall line (Fig. 11b and Fig. 12b, respectively).
(a) (b)
Figure 11: (a) The GOES-11/13 Sounder RGB Air Mass imagery and (b) surface wind gusts
(shaded) at 1200 UTC 26 October 2010.
(a) (b)
Figure 12: (a) The GOES-11/13 Sounder RGB Air Mass imagery and (b) surface wind gusts
(shaded) from 2100 UTC 26 October 2010.
By 0100 UTC on October 27, the maximum 400-200 hPa layer PV maximum
and dry slot reached Ontario. In the next seventeen hours, strong winds were
recorded across the southern portion of the province, first in Pukaskwa Park and
Welcome Island on the north side of Lake Superior, and then including otherlocations northeast of Lake Superior and Lake Huron, such as WaWa, Chapleau,
Sault Sainte Marie, Earlton, Sudbury, Gore Bay, Killarney, Wiarton, and
Goderichin. Wind gusts at these locations were consistently above 21 m/s, even
reaching 29 m/s in Pukaskwa Park. Comparing these locations to the RGB Air
Mass imagery, they were coincident with the dry intrusion. The wind vectors
were also pointing in the same direction as the movement of the dry intrusion
and PV. Some of the winds were positioned beneath the remaining cyclone
cloud structures, but these were also overlaid by red in the RGB Air Mass
imagery, indicating PV was present as well.
After 0300 UTC on October 27, the cyclone began to dissipate and the PV
maximum weakened. At this time, the surface wind gusts no longer matched
up with the dry intrusion (Fig. 13). Notice that the region of strong wind over the
eastern half of Lake Superior correlates with the dry intrusion (red) in the RGB Air
Mass image. Instead, the decaying comma head and CCB brought dry, cold
air to the surface, leading to strong surface winds via mixing with the LLJ.
(a) (b)
Figure 13: (a) GOES 11/13 Sounder RGB Air Mass and (b) surface wind gusts (shaded) from 0300
UTC 27 October 2010 as the cyclone decayed.
In comparing the RGB Air Mass imagery with the upper level winds, it was
quickly evident that the jets did not influence the dry intrusion in a major way.
The 500 hPa jet was not associated with winds or the PV, as it lied mostly south of
the PV ribbon. The 300 hPa jet was situated across the same states as the 500
hPa jet, but slightly more to the north. Since the 500-400 hPa layer PV matched
with the northern branch of the red region, this is an indication that the
stratospheric intrusion may have reached at least to 500 hPa, possibly even as
low as 600 hPa.
The PV analysis showed that the 400-300 and 400-200 hPa layer PV
correlated the best with the red regions of the RGB Air Mass imagery. This
suggests that the top of the stratospheric intrusion lied mostly around the 300-500
hPa level, at least near the maximum PV region. There was very little upper levelvorticity over the region of strong wind gusts in the Dakotas, so these winds were
unrelated to the presence of the PV.
iii. AIRS Ozone Retrievals
AIRS ozone was compared to the 400-200 hPa layer PV in order to verify if
the red regions were stratospheric in origin and to see if the surface wind gusts
were in any way related to the presence of stratospheric air. The AIRS ozone
clearly showed high concentrations of ozone, with measurements above 350
Dobson units (DU) in the most concentrated areas. Figure 14 indicates a
stratospheric intrusion and links ozone and PV. The ozone concentrations
matched up exactly (when accounting for time discrepancies) with either the
400-300 hPa layer PV or the 300-200 hPa layer PV, depending on how far west in
the U.S. the ozone was analyzed. This suggested that the ozone-rich,
stratospheric air moved closer to the surface as it moved eastward as time
progressed. For a comprehensive comparison, the 400-200 hPa layer PV was
compared to the AIRS. This combined both levels and matched the best with
the RGB Air Mass imagery (Fig. 15). Since the ozone concentrations matched up
with the RBG imagery, this is a good indication that the red region in the RGB
imagery is showing a stratospheric intrusion of dry, ozone-rich stratospheric air.
Figure 14: AIRS ozone at 0842 UTC 26 October 2010 and NARR 400-200 hPa layer PV at 0900 UTC
26 October 2010.Figure 15: GOES-11/13 Sounder RGB Air Mass imagery from 0900 UTC 26 October 2010.
Notice also that there were two separate red regions in the RGB imagery,
indicating that two tropopause folds occurred and that two stratospheric
intrusions were present, which combined over Missouri and Iowa (Fig. 16). The
northern branch of the red region also shows high PV and represents ozone rich
air that originated in Canada (Fig. 17). Additionally, the northern branch can be
seen in the streamline analysis, indicated by a kink in the streamlines in Fig. 18,
and can be seen down to the 308 K theta level.Figure 16: GOES-11/13 Sounder RGB Air Mass imagery from 2100 UTC 26 October 2010.
Figure 17: The AIRS ozone at 1948 UTC and NARR 400-300 hPa layer PV from 2100 UTC 26
October 2010.Figure 18: The 320 K system-relative streamlines, created from RUC-13 data at 2100 UTC 26
October 2010.
iv. Hysplit and Streamline Analysis
National Oceanic and Atmospheric Administration Air Resources
Laboratory (NOAA ARL) HYSPLIT was used to model the locations of the
conveyor belts on October 27 at 0000 UTC. Composite images were created to
show all three conveyor belts, shown in Fig. 19. The DCB and CCB are in nearly
the same locations for both images. The WCBs are farther east in the NARR
trajectory image than in the NCEP/NCAR global reanalysis trajectory image.
Although the composite images failed to show that the DCB descended, the
individual DCB forward trajectory analysis did show that the dry air descended
below 700 hPa, shown in Fig. 20. The DCB originated over the Pacific, entering
the U.S. over Oregon and passing through Utah, Colorado, Kansas, Missouri, and
into Illinois and Michigan. Notice at 1200 UTC on October 26 (Fig. 20),
corresponding to the lowest point reached by the DCB, the air is located over
the same region as the strong surface winds, which includes the states of Iowa,
Illinois, and Missouri. The CCB originated in northern Ontario and Manitoba and
crossed through Montana, North Dakota, South Dakota, Minnesota, and
Wisconsin. The WCB originated in Louisiana and Mississippi, tracking northward
through Tennessee, Kentucky, Indiana, Ohio, and Michigan.(a) (b)
Figure 19: The 36-hour forward trajectories starting at 1200 UTC 25 October 2010 calculated from
(a) NARR and (b) NCEP/NCAR global reanalysis.
Figure 20: The DCB modeled in HYSPLIT with 24-hour forward trajectories starting at 0000 UTC 26
October 2010 calculated from NCEP/NCAR global reanalysis.The HYSPLIT results were also compared to a storm-relative streamline
analysis. The CCB, WCB, and DCB, for the Great Lakes cyclone were best
represented by the 290, 304, and 330 K theta levels, respectively (Fig. 21). The
locations of the conveyor belts were very similar to the Hysplit results, but a few
differences could be observed. The WCB was located farther to the east,
stretching from Mississippi, Georgia, and Tennessee, through Ohio and Ontario.
The DCB was slightly further to the south. The location of the CCB was
unchanged. The streamlines were overlaid onto the surface wind gusts, and the
DCB could be observed passing over the same location as a region of strong
wind gusts located over the Great Lakes and Michigan’s Upper Peninsula (Fig.
22).
Figure 21: 2100 UTC 26 October 2010 storm relative streamlines. The blue line represent the CCB,
the red lines represent the WCB, and the brown lines represent the DCB.Figure 22: 2100 UTC 26 October 2010 storm-relative streamlines and the surface wind gusts. The
red lines represent the DCB, the blue lines represent the WCB, the green lines represent the CCB,
and the shaded region shows the surface wind gusts in m/s.
4. Diagnostic Analysis from MERRA Reanalysis
The AIRS ozone image from 0842 UTC on October 26 (Fig. 14) shows the
connection between the upper level ozone and the 400-200 hPa layer PV.
MERRA 300 hPa ozone from 1200 UTC on October 26 (Fig. 23) verifies this as well,
showing that the PV and ozone match up exactly. The PV maximum and high
ozone concentrations also matched up with the red region in the RGB Air Mass
image from the same time (Fig. 15). Figure 24 shows strong near surface wind
gusts within the dry slot, specifically over southeastern Iowa, western Illinois, and
northeastern Missouri. These winds matched up with the red region in the RGB
Air Mass image and were located slightly east of the MERRA ozone and 300 hPa
PV maximum (Fig. 24).Figure 23: MERRA 300 mb heights, PV, and ozone from 1200 UTC 26 October 2010. Figure 24: MERRA mean sea level pressure and 925 hPa winds from 1200 UTC 26 October 2010.
B. October 29-30, 2011
1. Societal and Economic Impact of the Event
Nor’easters are common occurrences along the Northeast coast of the
United States and Canada. However in 2011 the East coast experienced a rare
early season nor’easter in late October. It coincided with Halloween and was
dubbed by the media as “Snow-tober”. This storm pounded the east coast from
Massachusetts to Newfoundland with heavy snow, rain, and high winds from
October 29-31st.
Record amounts of snow fell along the east coast. For example, Central
Park, New York City, received more than two inches of snow. This was the first
time over an inch of snow had fallen in the park on any given day in October
since records have begun (Pydynowski 2011). Additionally, some places in New
Hampshire and Massachusetts saw over 31 inches of snow. Figure 25 shows a
snowfall map for southern New England. The record amounts of snowfall alone
would have caused devastating effects; the storm caused more damage
because the trees had not dropped their leaves yet. The leaves, covered with
snow, caused additional stress on the trees and led to an abundance of
downed trees (Frank 2012). Significant amounts of snow also fell in Canada,
where parts of Nova Scotia received around 20 inches of snow (Environment
Canada 2011).
Although snowfall was the main threat in this event, wind gusts also
caused damage. Wind gusts greater than 22 m/s were recorded from Cape
Cod to Newfoundland. Along the east coast of the United States, winds did not
exceed 31 m/s (National Weather Service Taunton, MA 2011). However, as the
storm strengthened and moved into Canada, the wind gusts associated with
the storm increased greatly. In Nova Scotia, wind gusts greater than 31 m/s
were reported at the height of the storm (CBCnews Canada 2011). This
combination of strong winds and heavy snow weighing down tree foliage led to
wide spread power outages in both countries. In southern New England 1.5 to 2
million people were without power, some of which were without power for a
week. In Nova Scotia alone, there were 35,000 people left without power as the
storm traversed the area. In addition to power outages, the storm grounded
aircraft. In some cases, passengers were stuck on airplanes for several hours.
Schools closed and numerous automobile accidents also occurred. In Canada,
the Confederation Bridge, which connects Prince Edward Island and New
Brunswick, was closed to all high-sided vehicles and the ferry trips between
Prince Edward Island and Nova Scotia were canceled. Unfortunately, loss of life
accompanied the economic loss in both countries. The majority of loss was due
to automobile accidents. Although no loss of life was reported at sea, winds
pushed a manned boat into the rocks along the Canadian coast. Overall, the
storm caused an estimated 3 billion dollars in damage in the United States alone
(North American Electric Reliability Corporation 2012).Figure 25: Snowfall totals for southern New England provided
by the National Weather Service in Taunton, Massachusetts
2. Synoptic Analysis from MERRA Reanalysis
The storm was accurately forecasted well in advance, with the National
Weather Service (National Weather Service Taunton, MA 2011) issuing winter
storm watches and warnings ahead of the storm. Although the forecasted
snowfall totals were too low, the storm track and location of the heaviest snow
fall was well forecasted. Despite an accurate forecast of the storm track and
location, whole gale to hurricane force wind gusts that occurred in and near
the dry slot were not expected.
The storm began as a disturbance in southern Texas, then moved across
the Gulf States and strengthened off the North Carolina and South Carolina
coast. It then progressed up the East coast and went out to sea after crossing
Newfoundland, Canada. At its peak at 1800 UTC on October 30th, the central
pressure of the storm was 975 hPa and the occlusion was wrapped back to the
north and west of the low pressure center of (Fig 26). The surface map at this
time also shows an extremely strong pressure gradient around the low pressure
center, indicating that strong winds were present (Fig. 26).
Synoptically, several factors contributed to the development of this early
season storm. A strong 200 hPa jet, a well-defined 850 hPa jet, and divergence
aloft were all present to create this intense storm. For the duration of the storm,
the 200 hPa jet remained strong with winds in excess of 70 m/s east of the
northeastern U.S. east coast (Fig. 27). Divergence on the east side of the 200
hPa trough also aided in helping the cyclone intensify (Fig. 28). Around 1500 UTC
on the 29th, a strong 25 m/s low level jet developed at 850 hPa along the warm
and cold fronts. These factors, along with strong pressure and temperature
gradients explain most of the strong winds associated with the storm; however,
they fail to explain the near hurricane force winds observed in the dry slot.Figure 26: Surface analysis at 1800 UTC 30 October 2011 from Hydrometeorological
Prediction Center.
Figure 27: MERRA 300 hPa winds and heights at 1800 UYC 29 October 2011Figure 28: MERRA 200 hPa divergence and heights at 1800 UTC 29 October 2011
(a) (b)
Figure 29: 0600 UTC 30 October 2011 (a) GOES-13 Water Vapor Imagery (b) NARR 200-400 hPa
Layer PV
Investigation of the PV provides evidence supporting the development of
near hurricane force winds associated with the dry slot. For example, at 0600
UTC on the 30th, a 200-400 hPa layer PV maximum began to follow along the
southern tip of the comma head cloud, near the boundary of the dry air filtering
into the storm, and continued through the remainder of the storm’s evolution
(compare Fig. 29a, b). The PV stream ran along the DCB, which can be seen as
dry air on water vapor imagery (Fig. 29a). An examination of MERRA data (Fig.
30) indicates strong surface winds were collocated with the region of maximum
PV. The collocation of the PV, the DCB, and strong winds suggests there was a
coupling between PV and the DCB to produce strong surface winds in and nearthe dry slot as higher momentum air mixed to the surface (compare Fig. 29a, b
and 30).
Figure 30: MERRA mean sea level pressure and 925 hPa winds in m/s (shaded) at 0600 UTC 30
October 2011
5. Satellite Analysis
i. Cloud Structures
As mentioned in the October 26, 2010 case, Browning and Field (2004)
determined that high surface winds were correlated with chevron and arch
shaped cloud features near the comma head of the storm. For this particular
storm, only banded clouds were identified which rotated around the comma
head. The banded clouds were evident at 1145 UTC on the 30th (indicated by
red vertical lines in Fig. 31a) and by 1215 UTC had almost dissipated (Fig. 31b).
Unfortunately, there were few high wind reports associated with the banded
clouds, since they occurred in relatively data void areas offshore. Examination
of the 925 hPa MERRA winds showed a concentration of high winds associated
with the comma head (Fig. 32), although anomalously high winds associated
with the banded clouds were not present. Spatial resolution, and
model/assimilation techniques may be the cause, although a lack of surface
observations does not allow us to make this conclusion. .(a) (b)
Figure 31: (a) GOES-13 IR imagery with banded clouds at 1145 UTC 30 October 2011. (b)
Recorded wind gusts and GOES-13 IR imagery at 1215 UTC 30 October 2011.
Figure 32: MERRA mean sea level pressure and 925 hPa winds at 1200 UTC 30 October 2011.
ii. RGB Air Mass Imagery
RGB Air Mass imagery derived from both the GOES-11/13 Sounder and
MODIS showed a dry air intrusion originating from Wisconsin which filtered into
the Ohio Valley as the storm formed. The MODIS imagery provided better
coloring and a sharper image. However, the temporal and spatial sub-grid scale
processes of the storm prevent an in-depth analysis. Even with these
disadvantages, MODIS was beneficial near the beginning of the storm since it
showed the origin of dry air in western Ontario. The relationship between PV,high surface wind gusts, and the dry air intrusion was examined. The 400-300
hPa layer and 400-200 hPa layer PV correlated with the dry air intruding into the
storm. This further supports the hypothesis that the dry air was stratospheric in
origin.
When examining the GOES-11/13 Sounder RGB Air Mass imagery only one
stratospheric intrusion, identified by the red coloring in the imagery could be
found. It stretched down from Wisconsin to the Ohio River Valley (Fig. 33a).
During most of the 29th, the dry air intrusion trailed the comma head.
Throughout this time, wind gusts were not as strong as shown in Fig. 33b. By 0000
UTC on the 30th, the dry air had begun to intrude into the storm and the storm
became occluded. Since it is hypothesized that the dry air is stratospheric in
origin, it is possible that the stratospheric intrusion enhanced the development of
the occlusion. RGB Air Mass imagery shows a well-defined region of dry air over
New Jersey, New York City, and Long Island (Fig. 34a). Located just to the east
of this region, at the tip of the comma head, high wind gusts were observed
(Fig. 34b). This supports the hypothesis that the interaction of the dry air with the
comma head can help forecast the location of high surface winds gusts.
(a) (b)
Figure 33: 1800 UTC 29 October 2011 (a) GOES 11/13 Sounder RGB Air Mass Imagery (b) NARR
wind gusts (shaded) and wind speed and wind direction (arrows)(a) (b)
Figure 34: 0700 UTC 30 October 2011 (a) GOES 11/13 Sounder RGB Air Mass Imagery (b) NARR
wind gusts (shaded) and wind speed and wind direction (arrows)
Even though the GOES-11/13 Sounder RGB Air Mass imagery covered the
entire U.S., it did not extend far enough eastward to encompass the complete
track of the storm. Therefore, there was a limit to analyzing this imagery. This
obstacle was solved by analyzing MODIS RGB Air Mass imagery. Despite the
fact the MODIS RGB Air Mass imagery does not provide an image over the
entire U.S., it provides higher resolution granules of the storm at select time
periods when the polar orbit passed over the region. Therefore it was used to
investigate locations beyond the limb of the GOES-11/13 Sounder imagery. The
MODIS RGB Air Mass imagery showed additional dry air intrusions which
originated in southern Missouri and the Georgia (Fig. 35). At 1500 UTC on the
29th, a PV maximum was collocated with the dry air located over Missouri,
supporting the hypothesis that this dry air was of stratospheric origin (Fig. 36)
Figure 35: MODIS RGB Air Mass imagery at 1625 UTC 29 October 2011.Figure 36: NARR 400-200 hPa layer PV at 1500 UTC 29 October 2011.
One additional feature identified in MODIS RGB Air Mass imagery was a
North Atlantic storm. This storm appeared to have little impact on the October
storm’s development. In fact, it seemed as if the northern storm would be out of
the region by the time the storm made its way up the Canadian coast.
However, examination of the MODIS RGB Air Mass imagery showed a dry air
intrusion which extended southwest from the northern storm and entrained into
the center of the developing storm (Fig. 37a, b).
(a)(b)
Figure 37: MODIS RGB Air Mass imagery: (a) 2320 UTC 30 October 2011 showing the northern
storm. (b) 0100 UTC 31 October 2011 showing northern part of the storm’s comma head cloud.
The upper level jet was examined to define whether it influenced the dry
air intrusion or the 400-200 hPa layer PV. The 300 hPa jet was located to the east
of the dry air intrusion and the PV maximum, indicating it had little influence on
these features. This suggests that the PV was not associated with the jet but
rather some other disturbance. In this case, the dry air was believed to be
associated with a stratospheric intrusion. The 500 hPa jet was situated just to the
south of the dry air, closer to the dry air intrusion. However examination of the
400-500 hPa layer PV shows that very little PV was present at this level. Therefore
the 500 hPa jet did not have much influence on the dry air ribbon and its
associated PV.
iii. AIRS Ozone Retrievals
AIRS ozone was overlaid with 200-400 hPa layer PV to verify the hypothesis
that the red regions on the RGB Air Mass imagery were stratospheric in origin.
Additionally, it was used to determine if the near surface wind gusts were
related to the stratospheric air. Unfortunately, AIRS was only over the region four
times and in three of those times, it was not positioned over the storm. However,
at 0742 UTC on the 30th, AIRS was almost directly over the storm. The ozone and
PV correlated when considering slight time differences for the images. There
were concentrations of ozone as high as 424 DU just north of Lake Ontario down
to the New York, New Jersey border (Fig. 38a). At this time, dry air on the RGB Air
Mass imagery was closely correlated with the location of the AIRS ozone (Fig.
38b). The orientation of the ozone and the location of the PV suggest that the
higher ozone was present over that region. Additionally the ozone north of Lake
Ontario was closely correlated with MODIS RGB Air Mass imagery at this time asshown in Fig. 39. The collocation of the ozone and the red regions in the RGB Air
Mass imagery suggests that the dry air was ozone-rich stratospheric air. Since the
high surface winds were correlated with the upper level PV, analysis provides a
link between the PV, stratospheric intrusion and surface wind gusts. However,
further mesoscale analysis via cross sections will be necessary to determine how
the stratospheric air is related to the near surface wind gusts.
(a) (b)
Figure 38: (a) AIRS ozone at 0742 UTC and PV at 0900 UTC 30 October 2011. (b) GOES11/13
Sounder RGB Air Mass imagery at 0701 UTC 30 October 2011.
Figure 39: MODIS RGB Air Mass imagery at 0740 UTC 30 October 2011.
iv. Hysplit and Streamline Analysis
HYSPLIT was used to help identify the conveyor belts associated with
the October 2011 storm. To identify the location of the conveyor belts,
backward trajectories were calculated at 0600 UTC on the 30th. This time
was chosen because 0600 UTC on the 30th was before the storm began to
occlude and therefore the conveyor belts were not influenced by the
occlusion. Figure 40a shows the DCB originated near the Minnesota,North Dakota border. However, upon inspection of the forward
trajectories at 0600 UTC on the 29th (Fig. 41a), the DCB did not return to the
center of the storm due to difficulty rounding the base of a deep 500 hPa
trough located to the east of the DCB. The path of the DCB closely
followed the location of the 500-400 hPa layer PV, which implies the DCB
was associated with the dry air intrusion on the RGB Air Mass imagery
(compare Fig. 40b and 41a). Therefore, analysis shows that the DCB
analysis is correct, despite the air parcels not returning to the center of the
storm.
Both the composite and the individual trajectories show the DCB
descended toward the surface (Fig. 40a and 41a). Figure 40a shows the
DCB descended as low as 600 hPa. The location where the DCB
descended this low was collocated with high near surface winds, which
are just east of Martha’s Vineyard and Nantucket Sound (Fig. 41b). The
location of the high near surface winds suggests they were the result of
the descending DCB, since they were not located near fronts.
(a) (b)
Figure 40: (a) The 24-hour backward trajectories starting at 0600 UTC 30 Oct 2011 calculated
from NCEP/NCAP global reanalysis. (b) 0600 UTC 29 October 2011, NARR 500 hPa PV.(a) (b)
Figure 41: (a) 24-hour Forward trajectories starting at 0600 UTC 29 October 2011 calculated from
NCEP/NCAR Reanalysis. Green: DCB Red: WCB Blue: CCB. (b) 0600 UTC 29 October 2011 NARR
wind gusts (shaded) and wind speed and wind direction (arrows)
In conjunction with the HYSPLIT analysis, system-relative streamlines were
created with GEMPAK. For uniformity streamlines at 0600 UTC on the 30th were
analyzed, but the trajectories at 0900 UTC provided a better representation of
the conveyor belts (Fig. 42). The 288 K, 308 K, and 324 K theta surfaces provided
the best representation of the CCB, WCB, and DCB respectively. The streamlines
provide further proof that the DCB starts somewhere near the Minnesota, South
Dakota, and Iowa border. Although the DCB path is a bit further south, the
streamline path is nearly identical to the individual trajectory path constructed
with HYSPLIT.
Figure 42: NARR system relative streamlines at 0900 30 October 2011,Blue: CCB Red: WCB
Maroon: DCB.The streamlines corresponding to the CCB and the WBC on the other
hand differ from HYSPLIT. The general path of both conveyer belts match the
HYSPLIT trajectories, however there are noticeable differences. For example,
HYSPLIT has the CCB originating just south of Nova Scotia while the streamlines
went through the area but did not start there (Fig. 42). The streamlines have a
similar south to north trajectory as HYSPLIT for the WCB but the streamlines show
that it curves cyclonically into the storm as opposed to anti-cyclonically away
from the storm in HYSPLIT (Fig. 42). One reason for the discrepancy is the
method used to calculate the air streams on each platform. System-relative
streamlines only show the trajectories on one theta surface and do not illustrate
whether the air is ascending or descending. On the other hand, HYSPLIT plots
the trajectory and shows how the particles ascend or descend. It is important to
examine both types of trajectories since they are determined differently and
show different characteristics of the air flow. The analysis of both HYSPLIT
trajectories and system-relative streamlines confirmed the starting location of
the DCB and showed how the WCB split into separate branches.
4. Diagnostic Analysis from MERRA Reanalysis
AIRS ozone established a connection between ozone, 200-400 hPa layer
PV, and the dry air on RGB Air Mass imagery. These connections helped to verify
the hypothesis that the dry air was stratospheric in origin. This conclusion was
solidified upon examination of MERRA ozone and PV. As with AIRS, the MERRA
ozone and PV coincided. At 0600 UTC on the 30th, the dry air intrusion on the
RBG Air Mass imagery was over the Mid-Atlantic States and correlated with the
MERRA ozone and PV (Compare Fig. 43 and 44a). Ozone as high as 9X10-2 g/kg
was located right over the Pennsylvania, New Jersey border, confirming the
presence of stratospheric air. Figure 44b shows strong winds just off the coast of
New Jersey at the base of the comma head (Fig. 44b). The location of the
strong winds correlated with the dry intrusion, ozone, and PV maximum,
supporting the hypothesis that the descending DCB helped to produce the
strong surface wind.
Overall the analysis of both the NARR and MERRA reanalysis data of
ozone, PV, and wind gusts support the conclusion that the dry air seen in the
RGB Air Mass imagery is stratospheric in origin. The collocation of the near
hurricane force winds associated with this storm and the descending dry
conveyor belt suggest the mixing of this high momentum air led to the strong
winds seen. Additionally, examination of the correlation between the
stratospheric air on RGB Air Mass imagery and the surface gusts suggest that
forecasters can use the RGB Air Mass imagery to help with the forecasting of
strong non-convective surface winds.Figure 43: GOES 11/13 Sounder RGB Air Mass image at 0600 UTC 30 October 2011
(a) (b)
Figure 44: (a) MERRA ozone (contoured) and PV (red dashed lines) at 0600 UTC 30 October 2011.
(b MERRA mean sea level pressure and 925 hPa winds 0600 UTC 30 October 2011.
C. January 3-4, 2012
1. Societal and Economic Impact of the Event
On January 3, 2012, Cyclone Ulli, an extratropical cyclone named by the
Free University of Berlin, brought extreme winds to the United Kingdom and
Ireland. Cyclone Ulli not only impacted the United Kingdom and Ireland, but
also parts of the Netherlands, Germany, Norway, Denmark, and Sweden. The
United Kingdom was hit hardest by this extratropical cyclone, with southern
Scotland over the Central Belt region being the worst affected area. Thehighest reported gust of 47 m/s was recorded on a mountain summit in the North
Pennines of Scotland. The strongest gust not recorded by a mountain station
was 45 m/s at Edinburgh, Scotland. Across the Central Belt region winds gusted
from 36 m/s to 45 m/s and across the rest of the United Kingdom wind gusts were
reported between 26 and 31 m/s. The highest mean hourly wind speeds
exceeded 20 m/s. Along with the over 90 severe wind gust reports, two
tornadoes and heavy rainfall occurred along the squall line associated with the
cold front that moved over southern England. Heavy snowfall and blizzards
associated with the extratropical cyclone’s comma head also accompanied
the extreme winds that took place across parts of Scotland. It is clear to see
from Fig. 45 how intense both the winds and the precipitation were.
This severe weather, especially the intense winds, caused fallen trees which
blocked roads, railways, and damaged power lines. Travel throughout Scotland
was disrupted due to closed bridges, delayed ferry services, and cancelled
flights at the Glasgow and Edinburgh airports. Additionally, some buildings were
damaged due to the severe wind. Overall, this storm caused 306 million dollars
in damages, left over 100,000 people in Scotland and 10,000 people in Northern
Ireland without power, and resulted in three injuries and two fatalities.
(a) (b)
Figure 45: (a) Maximum wind gust speed in knots on January 3, 2012, and (b) radar image at
1030 UTC on January 3, 2012, showing the squall line associated with the cold front, and rain and
snow from the comma head (images from the UK Met Office website).
2. Synoptic Analysis from MERRA Reanalysis
Cyclone Ulli was well forecasted by many of the major weather models,
which accurately forecasted the location of strong winds on thesouth/southwest flank of the low pressure center in the comma head. However,
like the other storms, the intense winds in the dry slot were not well forecasted.
Cyclone Ulli began as a surface trough over the Midwest United States. It
moved offshore near New England on December 31 and progressed northeast
across the northern Atlantic. The models correctly predicted the track of Ulli
across Central Scotland and the timing of it crossing Scotland from 0600 to 1200
UTC. In addition, its minimum pressure of 952 hPa which was reached at 0600
UTC off the western coast of Scotland (Fig. 46) was accurately forecasted. The
location and timing of the heavy rain and high winds that occurred with the
squall line associated with the cold front of Ulli were also well represented by the
models. Numerous severe weather warnings were issued over a 24 hour period
in advance of the storm for strong, damaging winds, heavy rain, snow, and
flooding.
Figure 46: Surface analysis at 0600 UTC on January 3, 2012 (image from the UK Met Office
website)
Like the other storms studied, Cyclone Ulli had strong 300 hPa and 850 hPa
jets, 200 hPa divergence, and a strong surface pressure gradient, all of which
helped to make Ulli a rapidly intensifying mid-latitude cyclone with intense
surface winds. However, 200-400 hPa layer PV must also be taken into account
to explain the extreme winds that took place within the dry slot. An area of 200
hPa divergence remained over the low pressure center of Ulli as it moved over
the Atlantic and the United Kingdom. The upper level jet at 300 hPa associated
with Ulli sustained speeds of over 60 m/s for most of Ulli’s lifetime. The surface low
pressure center was positioned underneath the right entrance region of the 300
hPa jet until 0000 UTC on the 3rd when it moved beneath the left exit region of
the jet (Fig. 47b). Both of these locations were areas characterized by 200 hPa
divergence which intensified the surface low pressure center (Fig. 47a). The
lower level, 850 hPa, jet was situated over the cold and warm fronts of Ulli, with a
maximum speed of over 30 m/s. These factors all contributed to the strong
winds observed near the cold front and comma head.You can also read
