Hi Folks,
The focus
of the
The
following images are attached for reference:
1. WV mosaics for selected times between from 0000 UTC
2 Nov and 1200 UTC
Source: http://cimss.ssec.wisc.edu/tropic/real-time/tpw2/global/main.html
Source: http://www.rap.ucar.edu/weather/
2. High-resolution WV winds for 1200 UTC
Source: http://cimss.ssec.wisc.edu/tropic2/
3. GFS initialized SLP and 1000-500 hPa thickness
analyses for 1200 UTC
Source: http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/
4. Dynamic tropopause (DT) analyses and forecasts for
selected times between 1200 UTC 2 Nov and 0000 UTC
Source: http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
5. Bob Hart cyclone phase space diagrams updated as of
0600 UTC
Source: http://moe.met.fsu.edu/cyclonephase/
6. Buoy 44004 meteogram:
Source: http://seaboard.ndbc.noaa.gov/
A few
salient points related to the attached images and associated loops:
1. The many flavors of ET were on display with Noel.
The beginnings of dry slot development east of Noel and asymmetric flow
structure poleward of Noel could be seen in the WV imagery as early as 18Z/2,
even as the deep-layer (850 hPa to DT) shear remained anticyclonic with an
estimated thermal vorticity minimum immediately over Noel within a broader
anticyclonic shear environment.
2. The passage of multiple troughs of different scale
and intensity, some of which interacted with Noel directly and some of which
helped to define the larger-scale environment poleward of the storm, played a
part of the ET process. For example, a trough passage across the Northeast near
00Z/2 helped to pull tropical moisture poleward to Atlantic Canada along an
offshore baroclinic zone in its wake that served
as a conduit for the future track of Noel.
3. The evolution of Noel and its subsequent ET exposed
uncertainties and limitations with the ET definition. One "classical"
ET definition is that it represents the physical process that results in the
transformation of a warm-core TC into a cold-core extratropical cyclone (EC). Asymmetries in total precipitable water,
cloud distribution and deep convection that are consistent with increasing
vertical wind shear and the beginnings of ET were already present by 18Z/2. In
this context a fairly rapid ET occurred in the 12 h period ending 00Z/3.
4. However, the GFS sea level pressure (SLP) and
1000-500 hPa thickness initialized analysis for 12Z/3 clearly shows a closed
576 dam closed thickness contour is still colocated with the storm. On the
mesoscale the storm still appears to be warm core. However, on the synoptic scale
the thermal ridge (trough) lies just to the northeast (to the southwest) of the
storm as is characteristic of a baroclinic system and EC. The storm-scale
1000-500 hPa warm core shown in the 12Z/3 GFS initialized analysis is also
consistent the 850 hPa to DT mesoscale thermal ridge (anticyclonic shear) and
thermal vorticity minimum seen in the DT analysis for this time. It is not
until 00Z/4 that there is a clear signature of a mesoscale thermal trough
(cyclonic shear) and thermal vorticity maximum immediately to the southwest of
now EC Noel. On the synoptic scale, however, EC Noel now lies between the
downstream thermal ridge and the upstream thermal trough as is characteristic
of any TC that has gone through an ET worthy of the name.
5. Cyclone phase space (CPS) diagrams obtained from
Bob Hart's web link for the 06Z/4 GFS run can be used to shed some light on the
Noel ET. The CPS diagram comparing the 900-600 hPa thermal wind versus the
900-600 hPa storm-relative thickness symmetry shows that in the lower troposphere
that Noel passed from the symmetric warm core to asymmetric warm core phase
around 06Z/2 as an asymmetric structure was developing in the precipitable
water pattern around Noel. By 06Z/4 (denoted by the "C" in CPS), Noel
was crossing the boundary from asymmetric warm core to asymmetric cold core on
its way (in forecast mode now) to an ordinary baroclinic EC. The CPS diagram
for the 900-600 hPa thermal wind versus the 600-300 hPa thermal wind shows that
Noel was never a deep warm core system. It was moderate warm core at the outset,
consistent with a Cat 1 at its best, and was already shallow warm core by
12Z/2.
6. Explosively deepening oceanic ECs can also have
shallow warm cores in what otherwise are classic baroclinic environments. An
example is the first Presidents' Day storm (PSD1) of 19 Feb.79. Figure 11 from
Bosart (1981) shows the presence of a 1000-700 hPa thermal ridge over the cyclone
center at 12Z/19 at which time the cyclone was deepening rapidly. Shallow
warm-core storms such as Noel and PSD1 illustrate what should be obvious to any
veteran TC/EC watchers: Cyclones of all types inhabit a rich spectrum of
baroclinic and barotropic phase space and seldom, if ever, can be categorized
as 100% pure baroclinic or barotropic systems.
The Presidents' Day Snowstorm of 18-19 February 1979:
A Subsynoptic-Scale Event
Lance F. Bosart
Monthly Weather Review
Volume 109, Issue 7 (July 1981) pp. 1542-1566
7. On the synoptic scale, and in agreement with the
transition of symmetric cloud/WV structure to an asymmetric structure, Noel
experienced a fairly rapid ET in the 6-12 h period ending 18Z/2-00Z/3. However,
on the storm scale the ET began later (after 00Z/3) and was mostly complete by
06Z/4. Clearly, the process of ET, and the timing and duration of ET, is scale
(horizontal and vertical) and time dependent.
8. Noel passed just to the east of buoy 44004 (38.5 N
and 70.4 W) as it accelerated northeastward to
9. A fascinating and unresolved aspect of the Noel ET
is the role of the weak upper-level trough over the southeastern US that
appears to initiate the ET process. At 00Z/2 this trough appears as a weak PV
tail that extends from
tail/anomaly develops the classic "S" shape
of a system associated with the early stages of cyclogenesis. It is
hypothesized that the impact of this smaller-scale upper-level trough and PV
anomaly was to initiate the ET process by developing asymmetries in the
precipitable water structure and satellite cloud signatures, and to help reduce
the moderate deep warm-core symmetric structure of Noel to a weak shallow warm-core
structure
Lance