Friday map discussion for
the
coast
of
Web links used in the discussion
included:
1.
NOAA/ESRL interactive web site (daily data maps):
2.
McTaggart-Cowan GFS animation builder (dynamic tropopause (DT) maps):
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
3.
Steenburgh GFS PV animation link (DT maps):
http://www.met.utah.edu/jimsteen/gfs/framesGFS.html
Attached images are as follows:
1.
Mean and anomaly North American 1000 hPa height, 850 hPa temperature and 250
hPa height for 14 Sep to 4 Oct 2007.
2.
Mean and anomaly NH 250 hPa height for 21 Sep to
3.
DT pressure/winds, SLP and 925 hPa potential temperature for selected times.
.........................................
The NA circulation pattern for the
latter half of September and the first few days of October was dominated by
anomalously strong anticyclones off the west
coast
of the
anomalously
strong SW/WSW jet from the
since
1895 (start of CONUS temperature records) once NCDC reports the September
climate statistics.
A less appreciated aspect of the
circulation pattern over the CONUS the last three weeks is the anomalously
strong and persistent E/NE flow that has
prevailed
from SC to
anticyclone
displaced westward and poleward to generate strong easterlies on the east coast
of
An additional perspective on the
large-scale flow regime for the last two weeks is provided by the attached NH
mean and anomaly 250 hPa height maps. An
anomalously
strong westerly jet is evident at higher midlatitudes from northeast
to
near
along
with tropical heating anomalies (negative OLR anomalies were concentrated
between 10-15 N and 110-160 E) played in the observed configuration of the
upper
tropospheric
flow across the Pacific and downstream across
...................................
The DT/SLP/925 maps for 12Z/1 show an
elongated trough (axis of higher pressure on the DT that defines a PV tail)
that extends from east of Newfoundland SW
across
southern FL, western
cyclonic
circulation situated north of central
of
higher pressure over the surface circulation center, corresponding to positive
PV advection over the circulation center, is likely the forcing mechanism for
surface
development in conjunction with very weak low-level warm-air advection.
Although in some situations, (e.g., Diana 1984; Bosart and Bartlo 1991;
Bosart
2001, 2002) TT ensued from a similar configuration of surface and upper-level
features, a TT event did not occur in this case despite my best attempt at
wishcasting.
At issue is why TT failed.
By
00Z/3, the DT pressure distribution features a "donut hole," manifest
by lower pressure on the DT and weaker vertical wind shear through the column,
over
the eastern
to
central FL is indicative that deep convection has been insufficient to build a
ridge (and associated reduced PV region) poleward and eastward of the surface
cyclone
center. Although very weak baroclinicity is still present to the NW of the
surface cyclone, there is little, if any, implied positive PV advection over
the
cyclone
center. By 00Z/5, a very weak surface cyclone is situated south of LA while the
triggering PV anomaly, now more compact and better organized, is well to
the
SW on the Mexican coast. Critically, near the surface cyclone center
baroclinicity is absent at low levels, positive PV advection is absent aloft,
and no
evidence
of a convective outflow channel aloft is evident. Sound the taps for the TT
pathway to a TC. Finally, by 12Z/6 all evidence for a surface cyclone in the
00Z/1
is alive and well over eastern
The early October example of a failed
TT discussed above was replicated several other times in September (and also
early in the season, e.g., Andrea).
These
failed TT datasets should provide a good opportunity to study null events to
help identify and distinguish between the combination of dynamical and
thermodynamical
processes that must align to favor and disfavor TT. The occurrence of null TT
events prompts consideration of the following science questions:
1.
Was the air aloft associated with the PV anomaly sufficiently cold to help to
contribute to destabilization and
2.
Was the configuration of the PV anoamly aloft sufficiently robust so as to
favor strong enough positive PV advection to generate sufficient midlevel
ascent to
moisten
midlevels and destabilize the atmosphere?
3.
Was the boundary layer air warm and moist enough to create sufficient
4.
As noted by James Belanger, was the air in the lower and middle troposphere too
dry over too large an area to permit concentrated deep moist convection to get
organized?
5.
Was the triggering PV anomaly moving southwestward too fast to "phase
lock" with the developing surface cyclone so as to permit a sufficient
number of air
parcels
to reach their LCL and LFC, thus hindering midlevel moistening and deep
convection formation?
6.
Related to 5), what processes govern the phase locking of upper level PV
anomalies and low-level disturbances in a weakly baroclinic and warm, moist
unstable
environment?
7.
Once the surface cyclone redeveloped westward into the
of
oceanic heat and moisture fluxes operating on the NE inflow? Similarly, did the
approaching trough over the upper
surface
pressure gradient (and the associated surface wind speeds) by weakening the SW
end of the western Atlantic surface anticyclone over southern GA and the FL
panhandle?
Lance
Response
from John Molinari:
All
great questions Lance! Formation via TT
has all the questions
associated
with TC formation in general, plus a few more.
I would add a few more variations on your list of 7 above:
8.
You and Ed Bracken showed that TCs in the western
9.
What is the role of the scale of the upper anomaly--is small better than large
because vertical shear is more localized, or is large better because it has a
larger penetration depth and can excite a broader region of upward motion?
10.
And how important are thermodynamic effects (like higher
As
usual, a good dissertation topic for someone!
You and Chris Davis have done some great TT case studies; I wonder if
the time has come for idealized modeling of trough interactions, which to my
knowledge has rarely been attempted...
Cheers,
John
Response
from Lance:
Hi
John,
With regard to 9), there is the
additional issue of whether
the
PV anomaly associated with the transient upper-level trough is
quasi-circular
or elongated. Quasi circular PV anomalies will tend to
be
associated with an increase in the magnitude of positive PV
advection
locally and a greater Rossby penetration depth everything
else
being equal (which, of course, it seldom is). As to whether a
"small"
or "big" PV anomaly is better (where better means more likely to
trigger surface development) the answer appears to be yes. I would turn the question around and ask under what
conditions you get more bang from your
buck with a smaller vs. larger PV anomaly. The answer is probably dependent upon the CAPE/shear
configuration (and the latitude and
underlying SST). When it comes to PV anomalies, supersizing them is no
guaranteee of a successful MacStorm development.
For example, the stronger deep layer shear associated with a larger-scale PV
may result in any growing surface disturbance being whisked poleward too fast
to benefit from being situated over higher SSTs long enough to take on tropical
characteristics in conjunction with oceanic heat and moisture fluxes.
With regard to 10), this is what I had
in mind with question
5
but you posed it more coherently.
Oops, I forgot 8). This is the question
we have been debating
since
before we wrote our Opal paper in 2000. A very good and as yet fully unanswered question.
I agree with you that we haven't fully
exploited idealized
models
yet to address some of these questions.
Thanks for your comments!
Lance