Hi
Folks,
Here is a delayed synopsis of Friday
map discussion for 12
Oct'07.
Conditions were pretty quiet on the first anniversary of the
start of
the remarkable lake-effect snowstorm of 12-13 Oct'06 in
The focus of map discussion was on
selective PV tails,
beginning
in the
and
in a
series of weak PV tails whose equatorward extensions reached to
between
15-25 over the last month. These PV tails, as typified by the
attached
DT maps for 0000 UTC 9, 11 and 13 Oct'07 drawn from the
McTaggart-Cowan
GFS animation builder, tended to be associated with
weak,
non-developing low-level (925-850 hPa) vorticity maxima.
At 00Z/9, a PV tail that ended with a
weak PV maximum on the
DT
near 27 N and 70 W is associated with a low-level vorticity
maximum
near 26 N and 66 W. The low-level system proved to be
non-developing,
perhaps because of the relatively strong westerly
shear
overhead (not shown). By 00Z/11, general ridging had occurred
over the
west-central
eastern
W,
had become elongated and separated from the remnant PV tail to the
east now
barely detectable on the DT. Likewise, the low-level
vorticity
maximum associated with the aforementioned PV anomaly had
also
become elongated NE-SW in response to the increasing SW flow
aloft.
Finally, by 00Z/13 the PV anomaly in question had been carried
equatorward
to near 24 N and 52 W where it showed signs of
reorganizing
beneath strengthening northerly flow east of the still
intensifying
mid-Atlantic ridge. Of interest is the obvious
decoupling
of the lower-level and upper-level disturbances as the
former
system is now near 30 N and 50 W, well to the ENE/E of its
position
48 h earlier.
The observed behavior of the upper- and
lower-level
disturbances
just described is typical of other cases in the North
Atlantic
over the last four to six weeks and raises a few scientific
issues.
1.
The 0.5 deg GFS gridded datasets suggest that weak PV anomalies
that form
at the ends of elongated PV tails (aka PV streamers)
are
relative
common, and perhaps more so than has previously been
suggested.
2.
What is the role of "PV deposits" (aka PV
anomalies that form at
the ends
of PV tails) in the general weather of the subtropics and
outer
tropics and are they relevant to day-to-day weather forecasting
in these
regions?
3.
What is the role of "PV deposits" in tropical transition (TT)? Ten
years ago
I might have argued that TT was relatively uncommon because
triggering
DT PV anomalies were relatively infrequent based on
earlier
coarse resolution data sets. It would now appear that my
earlier
thinking no longer holds vorticity because of the relative
ubiquity
of PV deposits on the DT in higher resolution gridded
datasets.
4.
What dynamical and thermodynamical processes control
whether a DT
PV
anomaly will capture and retain a lower-level disturbance long
enough
for deep convection to organize with subsequent weakening of
the DT PV
anomaly as the transition to a warm-core disturbance begins?
5.
What dynamical processes control the mutual interaction of
multiple
PV tails and how do such interactions contribute to the
consolidation
of one or two individual PV anomalies (this situation
occurs in
the example discussed above before 00Z/9 as can be
ascertained
from a DT loop).
........................................................
A loop of potential temperature on the
DT for the SH reveals
the
contraction of the Antarctic PV reservoir from mid-September to
mid-October
as spring arrives (not shown). This contraction can be
seen by
comparing the attached DT images for 0000 UTC
1200
UTC
strong PV
tail approaching southern
the
apparent concentration of low potential temperature values (< 290
K) near 42 S and 20 E in this PV tail (The 1000-500 hPa
thickness
beneath
this PV tail was < 516 dam; not shown). This PV tail had its
origins
in the Antarctic region (not shown, best seen in a loop). The
beginning
of this PV tail can be seen near 66 S and 25 W where the
potential
temperature is < 290 K in the DT image for 00Z/4. DT
potential
temperature images subsequent to 00Z/7 show rapid PV tail
thinning
and the eventual formation of an isolated PV anomaly over
southern
spring
cold-air outbreak. Science issues prompted by the examination
of this
case include:
1.
What does it mean to say that spring arrives from a PV perspective?
2.
What are the dynamical processes that govern the shedding of
Antarctic
cold pools with the arrival of spring and how is the
shedding
process modified by the extensive sea ice around
in
September and October?
3.
Do dynamical processes that help to determine the evolution of the
annual
spring breakdown of the Antarctic polar vortex also play a
role in
the shedding of PV tails and coherent tropopause disturbances
into
lower latitudes?
4.
Can the longitudinal timing and intensity of spring onset as
measured,
say, by temperature departures relative to the longer term
climatology,
be linked to the characteristic Antarctic PV tail
freeways
for an individual season and, if so, what controls the
establishment
of preferential longitudes for these PV tails?
5.
Does the arrival of spring in the SH differ from its NH
counterpart
and, if so, in what ways does it differ and how can the
differences
be understood within the context of PV thinking?
Lance
Response
from John Molinari:
Lance,
those are all great questions. In the
West Pacific forecasters talk
about
"TUTT cells" (TUTT being tropical upper tropospheric trough for the
uninitiated),
which I believe are small-scale lows that separated from
breaking
Rossby waves/PV tails/PV streamers. Some
people claim that TCs
can form
in association with those cells, and presumably the process
relates
to the tropical transition cases you and Chris Davis have described.
One
interesting question with regard to PV tails is how they vary by
season,
with respect to ENSO, etc. In the June
07 JAS there is a paper by
Hitchman
and Huesmann that gives a
climatology of breaking Rossby
waves. At the 350K level (roughly 200 mb), the central Pacific and the
western
tails. I was a little surprised that NH winter
events were generally less
common,
with only a North African maximum.
I
wonder if there is a way to collect all of your sets of questions over
the years
and offer them to ATM/MET grad students everywhere as Potential
PhD
Projects! There have certainly been a
good set, covering all scales of
motion
from tornadoes to the NAO (and maybe even PDO)...
Cheers,
John
Response
from Dave Schultz:
Olivia
Martius and Connie Schwierz
have also written several papers
about the
climatology of PV streamers. See
Olivia's web site at:
http://www.iac.ethz.ch/staff/olivia/
Dave