Hi Folks,
Friday map discussion for
the unnamed eastern Pacific tropical storm (TS) that formed via a
tropical transition (TT) process beneath a cold trough on
This post will concentrate on the TT development period. A second
post will discuss the antecedent conditions prior to TS formation and
downstream impacts over the
cyclogenesis event over eastern
interest is that the northern stream trough that phased with the
southern stream to produce the eastern
event was originally part of the same trough system that ultimately
was responsible for the formation of the TS.
Attached please find the following
images:
1. Dynamic tropopause (DT, 2 PVU surface) pressure (hPa), shear (850
hPa to DT) delineated by conventional wind barbs (kt) and 925-850 hPa
layer-mean relative vorticity (black contours every 0.5 x 10-4 s-1
starting at 0.5 x 10-4 s-1) for 0000 UTC 26, 28 Oct, 1200 UTC 28 Oct,
0000 UTC 29-30 Oct, and 1200 UTC 2 Nov. The
maps is provided below.
2. Quikscat winds and enhanced IR imagery of the TS on 1-2 Nov'06
from Mark Lander. Relevant email posts from Mark Lander and a link to
eastern Pacific buoy observations is listed below.
I. The salient points from these analyses are as follows:
1. At 0000 UTC 26 Oct'06 an antecedent axis of high relative
vorticity extends from near 35 N, 170 W ENE to near 50 N and 130 W.
This vorticity axis lies ahead of a deepening trough over the
Aleutians. Two areas of relative high pressure (lowered DT) found on
the DT at this time can be equated with subsynoptic-scale
disturbances. The leading disturbance is situated near 52 N and
150-155 W. This disturbance later merged with the more compact DT
disturbance seen over the southwestern US at this time to produce the
major eastern
(details of this event were discussed in the special
map discussion of 27 Oct and posted to map earlier in the week). The
trailing disturbance preparing to round the basis of the trough near
175 W will be responsible for initiating the cyclogenesis that
ultimately resulted in TS formation via the TT process. Consider the
larger scale trough containing both disturbances a "twofer" in view
of its important impact on the weather.
2. At 0000 UTC 28 Oct'06 the trailing DT disturbance is located at
the southwestern end of an elongated SW-NE oriented potential
vorticity (PV) tail that has sharpened and strengthened. Note the
850-DT shear values > 150 kt on the forward side of this PV tail. How
do you spell strong baroclinic forcing? The 925-850 hPa vorticity
band ahead of the encroaching PV tail shows little evidence (yet) of
buckling.
3. By 1200 UTC 28 Oct'06 the PV tail has thinned to the north, has
compacted to the south, and has begun to induce low-level
cyclogenesis as indicated by the onset of buckling of the low-level
vorticity field.
4. Between 0000 UTC 29-30 Oct'06 a remarkable baroclinic cyclogenesis
event unfolds. By 1200 UTC 29 Oct'06 the 925-850 hPa relative
vorticity pattern resembles a classic occluded cyclone complete with
a ben-back front. The development of this relatively small-scale
cyclone occurs with a decrease in pressure on the DT and the collapse
of the deep-layer (850 hPa to DT) shear in the vicinity of the storm
by 1200 UTC 29 Oct'06 (shear values < 20 kt are denoted by dark blue
wind barbs).
5. At 1200 UTC 29 Oct'06 there is evidence of the existence a
warm-core seclusion. The thermal vorticity maximum has shifted to a
position 200-300 km to the south of the 925-850 hPa vorticity maximum
while a thermal vorticity minimum appears to have developed over and
to the north of the low-level vorticity maximum. By 1800 UTC 29
Oct'06 the deep-layer shear over the storm has decreased to < 20 kt
over a relatively wide area and the cyclonic vorticity has compacted
and concentrated near 38 N and 151 W. This low-level vorticity
signature is consistent with the formation of a vortex at the end of
the bent-back warm front.
6. By 0000 UTC 30 Oct'06 the vortex appears to have mostly separated
itself from the bent-back warm front that served as its umbilical
cord. This separation process is complete by 0600 UTC
by 1200 UTC 30 Oct'06 a deep-layer thermal vorticity minimum is
situated over the low-level vortex/vorticity maximum, consistent with
the formation of a weak warm-core disturbance (images not shown but
see web link below). The apparent TT of a baroclinic system into a
weak TS occurred over SSTs estimated to be between 18-19 C (not
shown).
7. Over the next 84 h the TS/vortex performed an anticyclonic loop
while strengthening somewhat (see attached imagery from Mark Lander).
By 1200
the basis of the low-level vorticity field. Anticyclonic shear is
still evident over the center of the TS and shear values are < 20 kt
poleward and eastward of the storm.
8. The development of the weak TS over the eastern Pacific occurred
within the broad envelope of a cold upper-level cyclonic circulation.
II. The following scientific issues and questions, and research
directions, are suggested by the preceding analyses:
1. To what extent does this eastern Pacific weak TS development
compare with midget TS/TC developments over the western Pacific as
discussed by Mark Lander in the past?
2. To what extent is this eastern Pacific weak TS development similar
to the occasional Mediterranean TS development as documented by Kerry
Emanuel (among others)?
3. The growth of the vortex and its separation from the bent-back
warm front in the low-level vorticity field as a warm seclusion
occurred on a time scale of ~12 h. How does this vorticity separation
and subsequent vortex vorticity growth occur? What role did deep
convection play in the vortex separation and growth process (I lack
detailed Pacific satellite imagery for the period in question to be
able to address this question)?
4. Can the role of cooling aloft ahead of the PV tail in steepening
the tropospheric lapse rates over the developing baroclinic system be
quantified?
5. How critical is the cooling aloft associated with the advance of
the PV tail to the formation of deep convection in the vicinity of
the storm?
6. Can a combination of cooling aloft in conjunction with heat and
moisture fluxes over a relatively cool ocean (SSTs ~18-19 C) produce
sufficient destabilization to support the development and
concentration of deep convection near the vortex center?
7. What determines when low-level vorticity concentrated near the tip
of a bent-back warm front will attempt to encircle the storm? Is the
presence of deep convection a necessary condition for the encircling?
What are the distinguishing features between a null event like the
intense southeastern
where bent-back warm front vorticity attempted to encircle the storm
but ultimately failed to do so from this eastern Pacific TS event
where vorticity was able to become concentrated in a vortex at the
tip of the bent-back warm front? In both events there appears to be a
warm-core seclusion but the warm-core seclusion by itself does not
appear to be a necessary and sufficient condition for warm-core
vortex growth
8. What was the pathway for the conversion of a cold-core baroclinic
cyclone to a warm-core disturbance in this case and how can this
pathway be compared with documented North Atlantic TT event pathways
for which there have been several numerical simulations (e.g., Davis
and Bosart 2003, 2006 in MWR and QJ, respectively)?
Further comments on related issues
will follow later in a
Part II message.
Lance
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
East Pacific buoy obs from Derrick Herndon via Mark Lander
http://amsu.ssec.wisc.edu/recon/npac_low_buoys.txt