Tropical Transition of an Unnamed, High-Latitude,
Tropical Cyclone in the Eastern North Pacific
1. Introduction

Tropical cyclones (TCs) are not exclusive to the tropics.

While the environmental conditions deemed favorable for tropical cyclogenesis by
Gray (1968) and DeMaria et al. (2001) are typical observed at tropical latitudes,
environmental conditions can become favorable for tropical cyclogenesis in locations
removed from the tropics.

The global climatology of tropical cyclogenesis events constructed by McTaggartCowan et al. (2013) reveals that many TCs forming poleward of 30°N (25°S) in the
Northern (Southern) Hemisphere during 1948–2010 developed in the presence of an
upper-tropospheric disturbance in a baroclinic environment (their Fig. 7).

These cases of baroclinically induced tropical cyclogenesis are typically associated
with the tropical transition (TT) process (Davis and Bosart 2003, 2004), during which
an extratropical cyclone (EC) transitions into a TC.

In the initial stages of the TT process, vertical wind shear in a baroclinic environment
produces a region of upward motion that focuses deep convection and diabatic
heating.
Vertical wind shear values are subsequently reduced by the diabatic
redistribution of potential vorticity (PV) in the vertical (Raymond 1992) and by
divergent outflow in the upper troposphere, allowing the surface cyclone to intensify
via wind-induced surface heat exchange (Emanuel 1986, 1995).

TCs forming via the TT process have been documented in many of the ocean basins
discussed by McTaggart-Cowan et al. (2013), including the western North Atlantic
(e.g., Moore and Davis 1951; Bosart and Bartlo 1991; Bracken and Bosart 2000;
Davis and Bosart 2003, 2004; McTaggart-Cowan et al. 2006; Guishard et al. 2007;
Evans and Guishard 2009; Guishard et al. 2009; Hulme and Martin 2009a,b), western
South Atlantic (e.g., Pezza and Simmonds 2005; McTaggart-Cowan et al. 2006;
Evans and Braun 2012), and western South Pacific (e.g., Garde et al. 2010).
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
TCs forming via the TT process have also been documented over large bodies of
water that are not explicitly discussed by McTaggart-Cowan et al. (2013), including
the Great Lakes (e.g., Sousounis et al. 2001) and Mediterranean Sea (e.g., Ernst and
Matson 1983; Pytharoulis et al. 1999; Reale and Atlas 2001; McTaggart-Cowan et al.
2010).

Tropical cyclogenesis events occurring over the western South Atlantic, Great Lakes,
and Mediterranean Sea are extremely rare. The infrequent development of TCs in
these regions is likely associated with the presence of relatively cold sea surface
temperatures (SSTs) that do not exceed the 26.5°C threshold for tropical cyclogenesis
identified by Gray (1968).

In order to facilitate the development of deep convection necessary for an EC to
undergo TT, tropospheric lapse rates must steepen in response to upper-tropospheric
cooling associated with an encroaching upper-tropospheric disturbance (Davis and
Bosart 2003, 2004).

In late October 2006, an unnamed TC (hereafter Invest 91C) developed at ~40°N over
the eastern North Pacific. A weak EC, forming downstream of a thinning uppertropospheric trough over the Gulf of Alaska, served as the precursor disturbance that
would ultimately undergo TT.

The TT of Invest 91C, which took place between 0000 UTC 29 October 2006 and
0000 UTC 2 November 2006, was extremely unusual—occurring over ~16°C SSTs in
a region historically devoid of TC activity (Fig. 1).

A synoptic overview of the formation of Invest 91C is presented in the following
section to document the features and processes associated with its development and
TT.

Model simulations of the TT of Invest 91C will also be presented to explore how the
use of different microphysical parameterization schemes could affect the structure
and intensity of an EC undergoing TT within a numerical model.

Information on model configuration, as well as the data and methodology used to
construct the model simulations, will be presented in section 3.

Section 4 will discuss specific findings from the model simulations, including the
observed differences between model runs.
2

This paper will conclude with a brief discussion and presentation of ideas for future
research.
2. Synoptic overview

http://www.atmos.albany.edu/student/abentley/research_images/ne_pac_tc/cfsr_pacific.html

The upper-tropospheric flow pattern over the central North Pacific becomes highly
amplified in late October 2006 (hereafter, all dates are in 2006) in response to two
ECs that develop over eastern Asia.

An ~1012-hPa EC (EC1) begins to deepen off the southeastern coast of Russia at
0000 UTC 25 October, downstream of a progressive upper-tropospheric PV anomaly.
At the same time, an ~1008-hPa EC (EC2) forms along the east coast of Japan on the
southwestern edge of the remnants of a midlatitude cold front—beneath the fracturing
equatorward end of an upper-tropospheric trough.

EC1 moves ~1500 km to the east-northeast over the following 48 h, deepening to
~1004 hPa over the Kamchatka Peninsula. EC2 begins to approach EC1 during this
period, moving ~3000 km to the northeast and deepening by ~8 hPa.

The poleward advection of high potential temperatures on the dynamic tropopause
(DT) downstream of EC1 and EC2 aids in the formation and amplification of an
upper-tropospheric ridge over the central North Pacific between 0000 UTC 25
October and 0000 UTC 27 October. Enhanced northwesterly flow downstream of the
amplifying ridge aids in the formation and amplification of an upper-tropospheric
trough to the south of the Aleutian Islands during this period.

http://www.atmos.albany.edu/student/abentley/research_images/ne_pac_tc/cfsr_alaska.html

EC1 and EC2 merge together by 0000 UTC 28 October, forming a sub-992-hPa EC
(EC3) over the Bering Sea.

Persistent northwesterly flow on the eastern periphery of the central North Pacific
ridge aids in the stretching and thinning of the upper-tropospheric trough in the
southern Gulf of Alaska. Negative PV advection by the 300–200-hPa layer-averaged
irrotational wind occurs to the west and east of the upper-tropospheric trough at 0000
3
UTC 28 October (Fig. __), tightening the horizontal PV gradient and transforming the
upper-tropospheric trough into an upper-tropospheric PV streamer.

The surface cyclone that will ultimately become Invest 91C begins to develop along
the southeastern edge of the upper-tropospheric PV streamer by 1200 UTC 28
October, in the equatorward entrance region of a 250-hPa jet streak.

Divergent outflow over the center of the surface cyclone, indicated by the starburst
pattern in the 300–200-hPa layer-averaged irrotational wind field emanating from a
region of 600–400-hPa ascent, opposes the eastward progression of the southern
portion of the upper-tropospheric PV streamer.

The southern portion of the upper-tropospheric PV streamer fractures from the
northern portion by 0000 UTC 29 October.

The surface cyclone, now positioned slightly to the northeast of the uppertropospheric PV anomaly, deepens to ~1000 hPa between 1200 UTC 28 October and
0000 UTC 29 October.

Warm lower-tropospheric air, manifested as high 1000–500-hPa thickness values,
wraps around the east side of the surface cyclone, reversing the meridional
temperature gradient and producing a bent-back warm frontal structure on the
northwestern periphery of the surface cyclone by 1200 UTC 29 October.

As previously documented by Hulme and Martin (2009b), the bent-back warm front
plays an important role in the TT of an EC. Convection along the bent-back warm
front is believed to generate lower-tropospheric vorticity on the western half of the
cyclone. This enhanced lower-tropospheric vorticity intensifies cold air advection on
the northern and western sides of the cyclone and helps to isolate the cyclone’s
developing warm core. The diabatic redistribution of PV in the vertical along the
bent-back warm front, upshear from the center of the cyclone, also helps to reduce
vertical wind shear values over the center of circulation.

The coupling index, defined in Bosart and Lackmann (1995) as the difference
between the potential temperature of the DT and equivalent potential temperature at
850 hPa, is shown in Fig. ___. Figure ___ reveals extremely low values of the
coupling index (< −5 K) near the center of the surface cyclone at 1200 UTC 29
October, indicating the presence of highly unstable air in the midtroposphere.
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
A vertical cross section, taken through the center of the surface cyclone at 1200 UTC
29 October, emphasizes the upper-tropospheric contribution to the midtropospheric
instability.

An upper-tropospheric PV anomaly, associated with the southern portion of the
upper-tropospheric PV streamer, extends below 500-hPa just to the south of the
center of the surface cyclone.

Potential temperature contours beneath the upper-tropospheric PV anomaly are
widely spaced and bow upward, indicating (1) the instability of the midtroposphere
near the center of the surface cyclone depicted in Fig. ___ and (2) that the cyclone is
primarily cold core.

The upper-tropospheric PV anomaly becomes collocated with the center of the
surface cyclone between 1200 UTC 29 October and 0000 UTC 30 October.

Weakly negative 925–500-hPa thermal vorticity values associated with the center of
the cyclone have separated from the bent-back warm front by 0000 UTC 30 October,
suggesting that the cyclone is losing its frontal structure and is beginning to acquire
more TC-like characteristics.

The region of 925–850-hPa cyclonic relative vorticity associated with the center of
the cyclone breaks away from the warm-frontal band between 0000 UTC 30 October
and 1200 UTC 30 October.

The expansion of negative 925–500-hPa thermal vorticity values near the center of
the cyclone and the reduction of positive 925–500-hPa thermal vorticity values in the
surrounding area indicate that the cyclone is becoming less cold core.

The region of 925–850-hPa cyclonic relative vorticity associated with the center of
the cyclone remains collocated with the southern portion of the upper-tropospheric
PV streamer and separate from remnants of the warm-frontal band between 1200
UTC 30 October and 1200 UTC 31 October.

An upper-tropospheric trough approaches the transitioning cyclone from the central
North Pacific during this period, wrapping around the southwestern edge of the storm.

The central pressure of the transitioning cyclone falls below 996 hPa by 1200 UTC 31
October, indicating that the cyclone is deepening.
5

The cyclonic circulation associated with the upper-tropospheric trough approaching
the transitioning cyclone from the central North Pacific is associated with the
transitioning cyclone’s turn to the northwest between 1200 UTC 31 October and 1200
UTC 1 November.

Thermal vorticity values surrounding the center of the transitioning cyclone are
predominately negative by 1200 UTC 1 November, indicating that the storm has
become warm core in the lower-to-midtroposphere.

The transitioning cyclone’s lack of frontal structure and warm-core characteristics
caused insert correct agency here to label the storm “Invest 91C” at 1200 UTC 1
November 2006.

Invest 91C has completely transitioned into a sub-992-hPa, axisymmetric, warm-core
TC by 0000 UTC 2 November (Figs. __a–d).

GOES-10 visible satellite imagery, taken at approximately 0000 UTC 2 November,
reveals the presence of an eye-like feature over the center of the cyclone (Fig. __).

A vertical cross section, taken through the center of Invest 91C, reveals that the
cyclone has completely undergone TT at this time.

Potential temperature contours bowing down over the center of Invest 91C (Fig. __)
confirm the warm-core structure of cyclone suggested by the 925–500-hPa thermal
vorticity field (Fig. __).

The upper-tropospheric PV anomaly that extended below 500 hPa at 1200 UTC 29
October (Fig. __) has been eroded by deep convection and no longer exists at 0000
UTC 2 November.

A PV tower, also indicative of the diabatic redistribution of PV in the vertical, is
present over the center of the cyclone between 925 hPa and 400 hPa.

Equivalent potential temperature contours are vertically oriented on either side of the
PV tower, suggesting that the eye-wall of the TC is well mixed and that deep
convection has been occurring.

Despite obtaining the characteristic structure of a TC, Invest 91C was not upgraded
from an invest area to a TC during its life cycle.
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
Invest 91C weakens from ~992-hPa at 0000 UTC 2 November to ~1000 hPa at 1800
UTC 3 November, moving to the northeast at ~40 km h−1 and making landfall along
the northwest coast of Washington.

Figure ___ depicts surface data obtained from the Destruction Island, WA, buoy
(DESW1), during the landfall of Invest 91C.

DESW1 reported a ~6 hPa pressure drop between 1200 UTC and 1800 UTC 3
November as the center of Invest 91C passed. This ~6 hPa pressure drop coincides
with a ~30 kt increase in sustained wind speeds measured by the buoy, with a
maximum sustained wind speed of > 50 kts recorded at ~1600 UTC 3 November.
3. Model description and evaluation

To better understand how the use of different microphysical schemes could affect the
structure and intensity of an EC undergoing TT within a numerical model, a
numerical simulation of the TT of Invest 91C is performed using version 3.4 of the
Advanced Research Weather Research and Forecasting (WRF) modeling system
(ARW; Skamarock et al. 2008).

WRF simulations are initialized with 1° Global Forecast System final (FNL)
analysis data beginning at 0000 UTC 28 October (correct start date? Or 24 h
later? Up to you. See ATM611 paper images.) and ending at 0000 UTC 2
November.

A two-way nested grid is used in this study with 30 km (10 km) horizontal resolution
within the outer (inner) nest (Fig. __).

Thirty-five vertical levels are analyzed.

The WRF physics package allows the user to employ various combinations of
cumulous,
land
surface,
planetary
boundary
layer,
and
microphysical
parameterization schemes.

All WRF simulations performed in this study use the Kain-Fritsch cumulous
parameterization scheme (Kain and Fritsch 1993), Noah land surface scheme (Ek et
al. 2003), and and Mellor-Yamada-Janjic TKE planetary boundary layer scheme
7
(Mellor and Yamada 1982).
This is done to isolate the impact of varying the
microphysical parameterization (MP) scheme.

The warming of the core of a TC is usually due to a combination of the diabatic
heating in the eyewall and dry adiabatic descent within the eye. Stern and Nolan
(2012) suggested that changes in the structure of the core of the TC may be sensitive
to the distribution of diabatic heating and, therefore, the MP scheme used in a
numerical model.

Three simulations are performed in this study, each with a different MP scheme.

Simulations are run with either the WRF Single-Moment 6-class (WSM6) (Hong and
Lim 2006), WRF Single-Moment 3-class (WSM3) (Hong et al. 2004), or Kessler
(Kessler 1969) MP scheme, consistent with the simulations performed by Stern and
Nolan (2012).

WSM6 has the highest complexity, predicting six categories of hydrometers: vapor,
rain, snow, cloud ice, cloud water, and graupel.

WSM3 is simplified, predicting three categories of hydrometers: vapor, cloud
water/cloud ice, and rain water/snow. Melting (freezing) occurs instantaneously at
temperatures above (below) freezing. WSM3 is often referred to as a “simply ice”
scheme in which liquid and solid water cannot coexist.

The Kessler MP scheme is a warm cloud scheme that includes vapor, cloud water,
and rain.
The only mechanism that produces new precipitation in the Kessler
parameterization is the autoconversion process. Kessler is the simplest and most
unrealistic of the three schemes considered in this study.
4. Analysis

It is often necessary for WRF model output to be compared to surface observations
and/or satellite data from the analyzed event to test the accuracy and validity of the
solution.

There is an unfortunate lack of radar coverage, upper air data, and surface observation
stations over the portion of the eastern North Pacific where Invest 91C underwent TT.
8

For this reason, the output fields analyzed in this study will be compared to surface
analyses from the National Centers for Environmental Prediction (NCEP) and
archived GOES-11 infrared (IR) satellite imagery from the Cooperative Institute of
Meteorological Satellite Studies’ (CIMSS) satellite blog, available online at
http://cimss.ssec.wisc.edu/goes/blog/archives/211.

Maximum reflectivity and mean sea level pressure values from the final output time
of each model run (1200 UTC 1 November) are shown in Fig. __. The corresponding
IR imagery is depicted in Fig. __.

All three MP schemes correctly identified the asymmetry in convection surrounding
the center of Invest 91C in the IR satellite imagery (Fig. __), with the region of the
highest maximum reflectivity located just to the north of the TC center (Figs. __).

All three simulations also highlight, with varying degrees of accuracy, the remnants
of Invest 91C’s bent-back warm front that extends from the northwestern to
northeastern periphery of the cyclone.

The final position of the center of circulation is remarkably similar in all MP schemes
(~42°N, 146°W) and is consistent with observations.

The Kessler MP scheme produces the deepest surface cyclone, with a central pressure
bellow 984 hPa (Fig. __). In contrast, the WSM6 and WSM3 MP schemes produce
~988 hPa cyclones (Figs. __).
Unfortunately, there is insufficient observational
evidence in this portion of the eastern North Pacific to prove that Kessler has
overestimated the central pressure of Invest 91C compared to WSM6 and WSM3.

All three MP schemes indicate that Invest 91C was a warm core cyclone at 700 hPa at
1200 UTC 1 November. This warm core extends above 500 hPa in each model
simulation (not shown).

The asymmetry illustrated in the 700-hPa wind field matches that of the convection,
with the fastest winds consistently to the north of the cyclone center.

The Kessler scheme continues to produce the deepest cyclone with the warmest core
(~2°C).

WSM3 produces the strongest 700-hPa radial temperature gradient and the fastest
700-hPa winds near the cyclone center.
9

Figure ____ depicts model derived outgoing longwave radiation (OLR) at 1200 UTC
1 November.

WSM6 and WSM3 correctly identify the asymmetry observed in the cold cloud tops
in the corresponding IR image (Fig. __).

While the overall spiral structure of the OLR field is relatively similar between the
two schemes, WSM3 has consistently warmer cloud tops. This is likely a result of
how hydrometers are separated within each MP scheme. Only identifying three
classes of hydrometers (vapor, cloud water/ice, and rain water/snow) could result in a
reduction in condensate in the upper troposphere in WSM3 and an overall warmer
solution.

WSM6 yields the solution that most closely resembles the few available observations
of the event.

The WRF simulation that utilizes the Kessler MP scheme produces a highly
unrealistic solution in the OLR field (Fig. ___).

A vast expanse of cold cloud tops (> 90 W m−2 in some regions) covers the majority
of the domain.
These cloud tops do not correspond to regions of convection
displayed in the simulated maximum reflectivity field (Fig. __).

The reason behind the highly unrealistic solution observed in the in the OLR field lies
in the assumptions embedded within the MP scheme itself.

Only the autoconversion process can produce new precipitation particles in the
Kessler parameterization. The autoconversion process will not occur unless a critical
concentration of cloud droplets is exceeded.

Fovell et al. (2009) suggest that the typical updraft speeds observed in TCs produce
less condensation than continental convection, a smaller droplet concentration, and,
therefore, fewer new precipitation particles when using the Kessler MP scheme.

The unrealistic feature observed in the OLR field is likely an extensive anvil cloud.

Figure ___, adapted from Fovell et al. (2009), displays vertical cross sections of the
condensate distribution in TCs utilizing the Kessler and WSM3 MP schemes. Kessler
exhibits considerably more condensate above 10 km than WSM3, likely manifesting
itself as the spurious OLR field observed in the present study.
10
5. Summary and discussion

The TT of Invest 91C occurred at ~40°N in the eastern North Pacific between 0000
UTC 29 October 2006 and 0000 UTC 2 November 2006.

Despite the unusual location of TT, the physical processes associated with the
cyclone’s transformation from an asymmetric, cold-core, EC into an axisymmetric,
warm-core, TC are consistent with those found in previous studies.

The present study supports the findings of Davis and Bosart (2003, 2004) and Hulme
and Martin (2009a,b) that the precursor disturbance to TT is an EC that develops as
an upper-tropospheric trough approaches a lower-tropospheric baroclinic zone (Figs.
__).

The EC progresses through the life cycle of a marine extratropical frontal cyclone
described by Shapiro and Keyser (1990), developing a bent-back warm front on its
west/northwestern side (Figs. __).

As previously documented by Hulme and Martin (2009b), the bent-back warm front
plays an important role in the TT of the EC. Convection along the bent-back warm
front is believed to generate lower-tropospheric vorticity on the western half of the
EC, intensifying cold air advection on the northern and western sides of the cyclone
and helping to isolate the cyclone’s developing warm core.

The diabatic redistribution of PV in the vertical along the bent-back warm front,
upshear from the center of the cyclone, also helps to reduce vertical wind shear values
over the center of circulation (Bosart and Davis 2004; Hulme and Martin 2009b).

The bent-back warm front eventually separates from the center of circulation as the
EC transitions into an axisymmetric, warm-core, TC (Fig. __). Invest 91C would
ultimately make landfall as an unnamed TC along the northwest coast of Washington
at ~1800 UTC 3 November 2006.

The results of this analysis indicate that the structure of a cyclone undergoing TT is
somewhat sensitive to the complexity of the MP scheme used in numerical
simulations.

Many similarities in the structure of Invest 91C are observed across the three
simulations.
All three MP schemes considered (WSM6, WSM3, and Kessler)
11
produce warm core cyclones by 1200 UTC 1 November with warm-core signatures
evident above 500 hPa.

All simulations also accurately capture the asymmetry in convection surrounding the
cyclone center, with the deepest convection and the strongest lower-tropospheric
winds predominantly on the northern side of the vortex.

Subtle differences in the magnitude of the analyzed fields highlight the effects of the
different MP schemes. The Kessler parameterization produces the deepest cyclone
(~4 hPa deeper than either WSM6 or WSM3) with the warmest core at all analyzed
levels.

Despite having a cooler warm core, the steep radial temperature gradient observed in
WSM3 produces the strongest low-tropospheric winds of any numerical simulation.

The greatest disparity that results from the use of different MP schemes can be seen
in the OLR field.

The WSM6 parameterization produces the most realistic solution that is also the most
consistent with the corresponding IR imagery.

WSM3 captures the overall structure of the WSM6 OLR field, but fails to produce
sufficiently cold cloud tops in regions removed from north of the cyclone center.

The Kessler parameterization scheme produces the most unlikely solution, with the
vast majority of the domain covered in an expansive cloud field.
The simple
representation of precipitation formation in Kessler causes condensates to remain in
the upper troposphere once lofted. The result is the formation of the extensive and
unrealistic anvil seen in Fig. __.

The methodologies used in this study could be expanded upon to offer further insight
into the structural differences that arise from changing the complexity of the MP
scheme.

Performing a similar analysis at higher resolution would provide a more detailed look
at the structural disparities discussed about.

Newer, more state of the art, MP schemes should also be incorporated into the
investigation, specifically the Thompson MP scheme (Thompson et al. 2004).

The same methodologies utilized here could also be applied to an entirely different
EC developing over the continental United States.
12
Examining an EC over the
continental United States would increase the likelihood of obtaining the surface
observations, ground-based radar products, and upper-air data necessary to compare
against WRF model output.
13