Hi Folks,
The focus of the
Oceans, and 2) cross-equatorial
upper-level flow linkages between the NH and SH.
I. Eastern Pacific Cyclones:
Go the the McTaggart-Cowan GFS analysis
animation builder (link is given below) and marvel at the rich complexity of
the
multiple cyclone events in the
eastern Pacific over the last week or so. Think of the flow in the upper
troposphere over the eastern
Pacific during this time as one
big turbulent cyclonic soup. Within this soup are a potpourri of cyclones. A
challenge is to try to
find *any* cyclone with a life
cycle that agrees with existing conceptual models of cyclone behavior. Attached
maps of potential
temperature and winds on the DT,
and 925-850 hPa layer-averaged relative vorticity, for 1200 UTC 7 and
challenge. At 1200 UTC on the 7th
the vorticity analysis suggest the existence of a triple point near 32 N,148 W.
New cyclonic
development often occurs along the
triple point, but not this time. Instead, the buckling of the NW-SE oriented
vorticity strip near 38
N, 153 W marks the location of a
new cyclogenesis event to the NW of the triple point. The precursor dynamic
tropopause (DT)
disturbance for this event is
comparatively modest. By 1200 UIT on the 8th a region of concentrated vorticity
near 38 N, 157 W marks
the position of the new cyclone
center. Of interest is the quasi-linear vorticity strip that has separated from
the new cyclone and
stretches from near 42 N, 156 W
ESE to near 36 N , 132 W. and then from there SW to near 25 N, 144 W. The apex
of this "wishbone"
vorticity structure (part of the
atmosphere's "wishbone offense?") marks the former triple point
(check the loops to see the details).
This vorticity apex is situated
beneath a DT diffluent mesoscale trough that is trying to break away from a
larger scale trough to the
west. Lots of mesoscale structure
can be seen within a broader, turbulent cyclonic flow envelope.
A particularly interesting feature was
a mesoscale dry vortex that formed along the eastern boundary of a narrow
NNW-SSE
oriented streamer of relatively
dry air in the mid- and upper-troposphere to the south of the
source of archived water vapor
imagery for the eastern Pacific for the details). As the vortex formed the dry
air became secluded
within it while moist to the north
and south wrapped around the vortex. In the WV imagery the vortex appeared as a
"donut" with drier
air defining the hole and moister
air surrounding the hole. The vortex, almost perfectly circular at times, had a
diameter of 200-300
km (a patch of slightly more moist
air resided in the interior of the vortex). A shear line running NW toward the
larger scale vortex centered near
46 N, 136 W in the attached DT image from 1200 UTC on the 7th coincided fairly
well with the initial
dry streamer. The mesoscale dry
vortex formed near the NW end of this shear line and can be found near 51 N,
150 W where it was moving slowly NW at this time. By 0000 UTC on the 8th the
mesoscale dry vortex had reached its most poleward position (53 N, 152 W; not
shown). It subsequently turned equatorward and is marked by a small patch of
potentially cooler air and a cyclonic circulation near 50
N, 153 W at 1200 UTC on the 8th.
The vortex remained located near the tip of the aforementioned shear line as it
continued equatorward (the shear line began curving cyclonically within the
outer envelope of the overall cyclonic circulation. By 0000 UTC 10 Feb the
mesoscale vortex was situated near 38 N, 146 W where it could be identified
with a quasi-circular patch of lower-level cyclonic
vorticity for the first time. As
of 1200 UTC 10th the mesoscale vortex could still be identified near 35 N, 138
W as it was being
reabsorbed into the trough that
had previously made it a free agent. What goes around comes around.....
Science Issues:
1. Close inspection of these (and other)
loops shows ample evidence for upscale and downscale behavior within the larger
scale envelope
of turbulent cyclonic flow. At
issue is how to quantify this observed behavior, represent it properly in
numerical models and
understand how it impacts weather
forecasting. Also of interest is defining a role for ensemble forecasts in the
untangling process.
2. Are new conceptual models
needed to account for cyclone evolution and changes in cyclone structure during
periods of upscale and
downscale behavior? If so, what
should be represented and how should it be represented?
3. What role do multiscale
deformation processes play in the evolution of cyclone structures during these
events?
4. How important is scale matching
between upper-level subsynoptic scale disturbances and low-level weak cyclones in determining
whether, and to what extent, a
cyclone will grow?
II. Eastern
A weak "Alberta Clipper"
cyclone produced a band of moderate snowfall (upwards of 15 cm) across parts of
IL, IN, OH, KY and WV
on 6-7 Feb'07. This cyclone began
to deepen moderately after it crossed the East Coast in response to the
approach of a fairly potent
subsynoptic-scale DT disturbance.
Explosive intensification occurred between 1200 UTC 8 Feb and 0600 UTC 9 Feb
(see attached images for
these times) along a strong
meridional thickness gradient over the west-central
this 18 h explosive
intensification period the central pressure in the cyclone had reached 948 hPa.
Interestingly, and perhaps
tellingly, the triggering DT
disturbance appeared to weaken during the 18 h period of most rapid
intensification.
[Carl Schreck forwarded me a
spectacular quickscat image (original source: Mike Brennan) that showed a wide
swath of 80-90 kt
winds around the south side of
this storm. Alas, I forgot to post this image to map. I will do so on Monday
morning when I can regain
access to my computer at school
where the image is saved.]
It is also evident from examining the loops that a parade of smaller
scale DT disturbances of arctic origin repeatedly cross
the
Science Issues:
1. In the original Manchurian
Candidate movie (the original is better than the remake) the hypnotized
political assassin is programmed
to "activate" when the
queen of hearts is turned over after he is told to play a game of
solitaire. What is the equivalent of the
queen of hearts in the atmosphere
in conjunction with explosive cyclogenesis? Is there one queen or are there
multiple queens?
2. To what extent is scale
matching of upper- and lower-level disturbances important for explosive oceanic
cyclogenesis? During the
early stage of oceanic
cyclogenesis on the 7th the surface cyclone was responding to a moderately
intense DT disturbance of comparable scale to the developing low-level
vorticity field associated with the cyclone. During the most explosive intensification
phase of the cyclone the upper-level disturbance appeared to weaken (it was
negatively tilted, however). Likewise, the 250 hPa jet rearranged itself and
split as the surface cyclone became sandwiched between the now-coupled jets
(see loops). This observed cyclone/jet/disturbance behavior prompts a few
questions:
a) Can diabatic Rossby
vortices/waves explain the observed evolution of the cyclone/jet/disturbance
(e.g.,
initially because the low-level
vorticity can grow quasi independently of the upper-level disturbance. Here
(7th), a precursor
disturbance appears to be of
critical importance to the initial cyclogenesis and is possibly of lesser
importance during the explosive
intensification phase (as always,
I am prepared to be wrong and Mike can help straighten me out). Scale matching
of the upper-level and
lower-level disturbances looks to
be important factor in permitting the explosive intensification in this case.
As an aside, a similar
scale matching of
jet/trough/hurricane interactions appeared to be one factor responsible for the
rapid intensification of Hurricane
Opal (1995) over the
on cyclone development from an
observational perspective, further numerical simulations of cases such as the
Atlantic event of 8-9
Feb'07 look to be one avenue for
continuing research progress.
b) What controls the creation of a
split/coupled jet in the absence of split flow? This question has been
investigated in conjunction
with FASTEX (e.g., Riviere and
Joly 2006; Pomroy and Thorpe 2000). The diabatic rearrangement of PV throughout
the troposphere (and the corresponding vertical wind shear) would appear to be
critical elements of this process. From a physical/forecasting perspective the
original self development ideas of Sutcliffe and Petterssen, suitably modified
by consideration of flow rearrangements associated with diabatic heating, would
appear to be one promising way to understand cyclogenesis/jet developments of
this type. See, for example, the Bosart and Lackmann (1995) for an application
of Sutcliffian self development principles to the reintensification of tropical
storm David (1979) over land.
c) Deformation, deformation,
deformation. The great and often neglected stepchild of meteorology. It keeps
saying "Look at me, look at
me!" (see also map discussion
posts of 26 Jan and
3. Investigate to what extent the
passage of mesoscale arctic disturbances as seen on the DT theta/wind/pressure loops
across the Great Lakes are critical to controlling the intensity, areal extent
and timing of heavy lake-effect snowfalls. As noted in a Friday map
discussion post from last year,
the spectacular
passage of a prominent arctic DT
disturbance across the
Lakes over the last 7-10 days have
been less prominent than the
been more numerous and more
frequent.
III. Cross-equatorial Flow
("Hemi-talk"):
New maps recently made operational online at
help monitor tropical
synoptic-scale weather systems, tropical-extratropical flow interactions, and
cross-equatorial air exchanges
between the NH and SH (aka
"hemi-talk"). Go to the first link below to find these new maps.
Scroll down to where it says GFS
SLP/Thickness/Moisture and
Isentropic Maps. The maps were generated from 1.0 deg GFS grids and consist of:
1) sea level pressure and 1000-500 hPa thickness contours and 700 hPa
relatively humidity (shaded), 2) 315 K pressure, winds and vertical motion
(shaded), and 3) 345 K pressure, winds and potential vorticity (PV, shaded).
The 315 K and 345 K surfaces were chosen to be representative of the middle
troposphere in midlatitudes and the upper troposphere in the tropics. Maps are
provided for the western and eastern hemispheres (WH and EH) between
approximately 60 N and 60 S. The last 15 days of maps (twice daily) are
available for animation. Individual maps (four-times daily) can also be
downloaded.
To illustrate the usefulness of these
new online maps, WH 345 K surface plots are attached for 0000 UTC 27 Jan and
and 1200 UTC
cyclonic flow reaches to the
equator. Both SH PV tails are matched by similar features in the NH. Between
them zonally constrained
near-equatorial jets are apparent.
At the later time the PV tail in the eastern South Pacific is broader and
weaker but the two PV
tails in the eastern North Pacific
are stronger. Cyclonic flow around the more eastern PV tail reaches into the SH
where it becomes
anticyclonic. In the
eastern
subtropical jet (STJ) that extends
into northwest
will be small this is clearly not
the case for selected longitude bands. I have not checked the extent of
convective anomalies/outflow
over northeastern
EH images on the 345 K surface are
attached for 0000 UTC 8-10 Feb'07. Of interest here is the eastward
continuation of the STJ
across
reaches across the equator into
the SH. The progressive downstream trough crossing northern
of severe weather (assuming there
is sufficient moisture and southerly flow at low levels from the
about these three images is how
amplified the flow is in the SH down to the deep tropics. A deep positively
tilted trough and PV tail
near 0 W, 30 S at 0000 UTC on the
8th narrows zonally and elongates meridionally over the next 48 h (the 540 dam
1000-500 hPa thickness contour reached almost to 30 S in this
trough....remarkable for mid-to-late summer). Downstream develop also clearly
occurs by 0000 UTC on the 10th (note PV tail reaching extreme northern
Australia) while in the NH the anticyclonic circulation over the Middle East
only moves very slowly eastward (the cross equatorial flow with this
circulation continues).
Science Issues:
1. A question I have asked before.
Just what are these cross-equatorial trough-ridges (or ridge-troughs) in the
upper troposphere and
what is their significance in the
grand scheme of things?
2. What is the significance of the
zonally confined near-equatorial jets at the longitudes of the aforementioned
upper-level
trough-ridges? Webster and Holton
(1982), Webster and Chang (1988), Zhang
and Webster (1989), and Chang and Webster (1990), among others, taught us that
deformation regions where du/dx was significant in the tropics were often
associated with "interesting" weather. Equatorial wave theory, derived from the
shallow water equations, has proved to be useful in understanding the behavior
and movement of convection-dominated tropical weather systems on daily, weekly
and intraseasonal time scales. At issue in my mind is whether knowledge of
tropical wave theory alone is sufficient to improve tropical weather
forecasting. Repeated instances of tropical-extratropical interactions of the
type described above prompt continuing questions about what role these
interactions and cross-equatorial air exchanges play in the structure and
evolution of tropical weather systems. It seems to me that we have to improve
our knowledge of these complex and interacting physical processes before we can
hope to do a better job of tropical weather forecasting.
3. My impression, perhaps
mistaken, is that cold air surges into the lower latitudes of the SH are more
zonally confined than similar
cold surges into the lower latitudes
of the NH. I base this impression on the apparent greater frequency of zonally
constrained
midlatitude troughs (PV tails) in
the SH as compared to the NH. If the structure of subtropical and tropical PV
tails from higher
latitudes differs between the NH
and the SH then what accounts for the difference and how is it important?
....................................................
And to conclude with a little
amusement, check out a story about "storm naming rights" uncovered by
Kevin Tyle (Kyrill was the
name given to the big European
storm of 18-19 Jan'07):
Date:
From: "Kevin R. Tyle"
<ktyle@atmos.albany.edu>
To: Lance Bosart
<bosart@atmos.albany.edu>
Subject: Storm naming rights
Hi Lance,
From the NYT on
-----------------------------------------------------------------------------------------------------------
The name Kyrill stems from a
German practice of naming weather
systems. Anyone may name one, for
a fee. Naming a high-pressure
system costs $385, while
low-pressure systems, which are more common,
go for $256. Three siblings paid
to name this system as a 65th
birthday gift for their father,
not knowing that it would grow into a
fierce storm.
------------------------------------------------------------------------------------------------------------
I of course love the fact that
anticyclones are worth more!
--Kevin
Full link:
http://www.nytimes.com/2007/01/19/world/europe/19europe.html?ex=1326862800&en=5f9a99190e2ab291&ei=5090&partner=rssuserland&emc=rss
Reflecting on Kevin's post, I'll offer
a few "modest" examples of where this storm naming thread could take
us......
High Bonnie and Low Clyde combine
to rake western Europe with machine-gun precision Al Capone style....
High Bonnie and Low Clyde combine
to produce high-octane entertainment and low-blow destruction over western
Europe.
...........................................
Tim Dunkerton asked,
tongue-in-cheek, how global warming is going in NY State........
A couple of places on the Tug Hill
Plateau have surpassed 100" (250 cm sounds better) of snow in the current
series of storms
(which is 100+" more snow
than we have here in ALB where the seasonal total sits at a paltry 6"
(seasonal normal is 64"). As remarkable as these storm totals are, they
are not unprecedented for the Tug Hill Plateau. Bigger snow dumps have been
recorded in past seasons. That said, one might expect that with the ongoing
global warming that the lake-effect snow season will gradually shift toward
from late autumn and early winter to mid winter and late winter as the
Lance
....................................................................
Web links used during map
discussion:
http://www.atmos.albany.edu/deas/nwp.html
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
....................................................................
References:
Bosart, L. F., and G> M.
Lackmann, 1995: Postlandfall Tropical Cyclone Reintensification in a Weakly
Baroclinic Environment: A Case
Study of Hurricane David
(September 1979), Monthly Weather Review, 123, 3268-3291.
Bosart, L. F., C. S. Velden,
W. E. Bracken, J. Molinari, and
P. G. Black, 2000: Environmental Influences on the Rapid
Intensification of Hurricane Opal
(1995) over the
Chang, H.-R., and P. J. Webster, 1990: Energy Accumulation
and Emanation at Low Latitudes. Part II: Nonlinear Response to Strong
Episodic Equatorial Forcing,
Journal of the Atmospheric Sciences, 47, 2624-2644.
Moore, R. W. and M. T. Montgomery,
2005: Analysis of an Idealized, Three-Dimensional Diabatic Rossby Vortex: A
Coherent Structure of the Moist Baroclinic Atmosphere, Journal of the
Atmospheric Sciences, 62, 2703-2725.
Moore, R. W. and Michael T.
Montgomery, 2004: Reexamining the Dynamics of Short-Scale, Diabatic Rossby Waves
and Their Role in
Midlatitude Moist Cyclogenesis,
Journal of the Atmospheric Sciences, 61, 754-768.
Rivière, G. and A. Joly, 2006:
Role of the Low-Frequency Deformation Field on the Explosive Growth of
Extratropical Cyclones at the Jet
Exit. Part I: Barotropic Critical
Region, Journal of the Atmospheric Sciences, 63, 1965-1981.
Pomroy, H. R. and A. J. Thorpe,
2000: The Evolution and Dynamical Role of Reduced Upper-Tropospheric Potential
Vorticity in Intensive Observing Period One of FASTEX, Monthly Weather Review,
128, 1817-1834.
Webster P. J., and J. R. Holton,
1982: Cross-Equatorial Response to Middle-Latitude Forcing in a Zonally Varying
of the Atmospheric Sciences, 39,
722-733.
Webster, P. J., and H.-R. Chang, 1988: Equatorial Energy
Accumulation and Emanation Regions: Impacts of a Zonally Varying
Zhang, C., and P J. Webster, 1989:
Effects of Zonal Flows on Equatorially Trapped Waves, Journal of the
Atmospheric Sciences,46,
3632-3652.
........................................................
Attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_02-09-07/