All
attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_12-08-06/
Hi Folks,
The last regular Friday map
discussion of the semester
featured: 1) brief comments on the very short-lived
arctic surge
across the Northeast on
explosive cyclogenesis over the western
of the NH SON06 climate based upon 300 hPa
geopotential height and
850 hPa temperature mean and anomaly fields, and 3) an extended
discussion of subtropical jet (STJ) development over
the Pacific east
of the Dateline prompted by an ongoing eastward surge
of potentially
warm air on the dynamic tropopause (DT) across the
subtropical
Pacific basin.
I. Arctic surge and cyclogenesis:
The official minimum temperature
of 10 F at ALB on Friday
morning
forecasts from the preceding few days. Forecast errors
as large as 10
F on the overnight minimum temperature are
comparatively rare today.
Conditions often associated with large errors on minimum temperature
in winter such as radiative
cooling under clear skies with light
winds and/or snow cover were notably absent. The
attached ALB
sounding for 12Z/8, courtesy of U-Wyoming, was taken
close to the
core of the arctic air mass. It shows a shallow mixed
layer, a 850
hPa temperature of -21.7 C, a 1000-500 hPa thickness
of 4999 m (could
be the only day the thickness will go below 500 dam in
what passes
for winter these days), and a DT below 500 hPa.
At issue is why the MOS forecast
minimum temperatures derived
from various operational models were systematically
too warm.
Questions come to mind as to whether the core of the low-level arctic
air was represented properly in the initialized fields
due to the
relatively coarse Canadian upper-air network and/or
whether the core
of the arctic air was modified in any way by
interaction with the
open waters of the
suggests there was enough "north" in the
low-level winds to miss the
being relatively infrequent these days, are worth
investigating to
help asses how much of the error is due to physics
versus
initialization problems.
The arctic air mass that crossed
the Northeast on Friday was
associated with a PV anomaly of arctic origin (easily
tracked on the
DT). The GFS consistently forecast explosive
cyclogenesis over the
northwest
18Z/8 an intensifying cyclone in advance of this PV anomaly with an
estimated central pressure of 990 hPa was situated
near the southern
coast of
(map also attached) intensified this cyclone to near
932 hPa just
east of southern
surface analyses indicated cyclone central pressures
of 935 hPa and
928 hPa, respectively at 18Z/9 and 12Z/10. The 24 h
GFS forecast was
almost dead-on, testimony to how much the models have
improved in
their capability to predict explosive oceanic
cyclogenesis since
Sanders and Gyakum (1980) published their pioneering paper
documenting the phenomenon for the first time. The
forecast SLP fall
of 58 hPa in 24 h equates to a remarkable 2.5 Bergeron
storm using
the intensification criteria of Sanders and Gyakum
(1980).
The spectacular cyclone
intensification appears to have
occurred in response to the interaction of the storm
with two arctic
PV anomalies, the first PV anomaly being the one that crossed the
Northeast on the 9th (see attached DT theta/wind maps for 12Z8,9) and
the second, and larger, PV anomaly being the one over
northern
first PV anomaly and the rapidly intensifying cyclone
after 00Z/9 and
by 12Z/9 was absorbing the first PV anomaly at which
time the
forecast cyclone central pressure was ~946 hPa
(NCEP/OPC had it at
950 hPa). This case appears to be another example of a
"multiple hit"
cyclogenesis event and prompts the question as to
whether the
extended tail of rapid deepeners in Roebber's (1984) climatological
frequency versus deepening plot for oceanic cyclones
could be
disproportionately impacted by cyclones that
experienced interactions
with multiple PV anomalies. A reworking of the Roebber
oceanic
cyclone climatology with a modern higher resolution
dataset to
address this question in "depth" might be
revealing.
Statistical Analysis and Updated Climatology of Explosive Cyclones
Paul J. Roebber
Monthly Weather Review
Volume 112, Issue 8 (August 1984) pp. 1577-1589
II. NH SON06 mean and anomaly fields:
The mean and anomalous 850 hPa
temperature maps for the NH
for SON06 (attached), constructed from the CDC
interactive web site,
show a pattern that has become common in recent
seasons when there is
an upward trend in the data: areas of above normal
temperatures
dominate areas of below normal temperatures. Notable
positive
anomalies are seen over western Europe (where it was
the warmest
autumn in 300+ years in the
northeastern
(no word yet on just how remarkable).
The corresponding mean monthly
mean and anomalous 300 hPa
geopotential height fields for Sep-Nov'06 (attached)
show, not
surprisingly, that middle and higher latitude regions
were dominated
by above normal heights while below normal heights
were confined to
more isolated regions. During Sep and Oct one of the
few below normal
300 hPa geopotential height regions was situated over parts of the
interior northern US and southern
flow enabled Canadian cool air masses to repeatedly
enter the
of the
As is evident from the 300 hPa mean and anomaly height patterns for
Nov however, the Sep and Oct pattern reversed significantly over
were replaced by above normal heights. The resulting
anomalous WSW
flow over the
anomalously strong onshore flow and the
record-breaking rains
observed in that region.
In the time-mean sense, a significant
large-scale regime
change occurred between Oct and Nov over
interesting scientific question is what contributed to
this
significant regime change. A good research project
might be to
examine the daily weather maps during Oct and Nov in
order to
document the structure and evolution of the
large-scale flow to help
understand how much of the regime change was driven by
tropical
convective heating anomalies (e.g., in conjunction
with the budding
El Nino) and how much was derived from middle latitude dynamical
processes.
III. STJ onset:
Beginning in very late Nov, a
plume of potentially warm air
on the DT began to work its way across the subtropical
toward
illustrated in a series of DT maps (attached)
beginning at 00Z/28. At
this time Supertyphoon
Durian was approaching the
plume of potentially warm air was first observed to
erupt poleward
and eastward of Durian at about this time. Whether the
eruption of
the plume of potentially warm air was the result of a
diabatic
heating "burp" associated with Durian and/or
occurred in conjunction
with an eastward-moving midlatitude disturbance in the
westerlies
further poleward is unknown at this time.
Subsequently, this plume of
potentially warm air propagated eastward in what
appeared to be a
series of discrete pulses (go to the below web link
and build the
appropriate loops to see the details). As this plume
of potentially
warm air expanded eastward across the Pacific the
meridional
temperature was increased on its poleward side as
evidenced by the
attached DT maps. The resulting strengthened
meridional temperature
gradient in the upper troposphere likely supported the
observed
eastward expansion and strengthening of the
subtropical jet (STJ).
In a typical El Nino year the STJ
is displaced eastward
across the Pacific. In strong El Nino years such as
1982-83 and
1997-98 a strengthened STJ is often observed to expand eastward
across northern
Ocean on time-mean upper-level height and wind maps.
Much less well
known are the physical and dynamical processes that
govern the
structure and evolution of the STJ on a daily basis.
For example,
questions remain as to what are the roles of tropical
and midlatitude
disturbances, anomalous tropical convection, and
anomalous oceanic
heat content regions in the observed evolution of the
STJ. The
current observed eastward expansion of a large plume
of potentially
warm air across the subtropical Pacific may be
symptomatic of the
onset of El Nino conditions across
watched to see if it is a transient or sustained
effect. The recent
arrival of the warm plume over western
least in the short run winter will be on hiatus across
most of the
US. Whither winter before the official start of
winter? Stay tuned.
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Regular Friday map discussion will
resume next semester after
the AMS annual meetings in SAT. Impromptu discussions
will occur in
the interim depending upon the level of the maproom excitement factor.
Lance
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
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