Hi Folks,
The focus of Friday map discussion for
cycles of the blocking anticyclones
over the
Pacific that first formed in the third
week of January. Relevant web
links given below include Ron
McTaggart-Cowan's GFS analysis animation
builder and the CSU 30-day animation of
global total precipitable water
(TPW).
Following the big European storm of 17-18 Jan'07 (see Friday map
discussion synopsis from
large-scale circulation pattern
occurred over the
zonal westerlies (NAO/AO >0) were
replaced by an amplified flow pattern
that resulted in the formation of a
high latitude blocking anticyclone
near
the latter part of Jan before relaxing
closer to neutral values the last
couple of days.
I. Eastern
The first ridge development near southern
Strait occurred in conjunction with
cyclogenesis over Atlantic Canada in
advance of a moderately intense but
otherwise ordinary-looking trough on
the DT on 19-20 Jan. This trough swept
across the
northeastern US on 18-19 Jan.
Subsequent cyclogenesis events just east of
the North American continent and
associated downstream ridge
reinforcement and amplification occurred
on 21-22 Jan, 24-25 Jan, 26-27
Jan, 28-29 Jan, 30-31 Jan and 1-2 Feb
(see attached example for 0000 UTC
22). Each of these subsequent
cyclogenesis events, fish storms all
(alas), resulted in renewed downstream
ridge building and ridge
retrogression at high latitudes that
had the effect of helping to anchor
a long-wave trough over eastern
TPW (link below) shows repeated
poleward deep moisture surges from the
subtropics and tropics toward
conjunction with the parade of
poleward-moving "wide right" (hey, it's
Superbowl Sunday) oceanic cyclones.
II. Eastern Pacific and
The first North Pacific transient disturbance that helped to
launch and eventually sustain ridge
building over western
and
(downstream trough development over the
southwestern US provided the
impetus for the miserable weather and
icing in the SAT during the annual
AMS meeting). This ridge was like a
convective tower that looks promising
and then disappoints as it collapses
because it failed to amplify
sufficiently so as to anchor itself in
position, instead weakening and
moving eastward. Further ridge building
occurred in conjunction with
North Pacific cyclogenetic events on 18
Jan, 20-21 Jan (a major event),
22-23 Jan, 25-26 Jan, 27-29 Jan (two
events), and 30 Jan - 2 Feb (two big
events). An example for 0000 2 Feb is
attached. Each new cyclogenesis
event contributed to ridge
re-amplification and retrogression. An analogy
that comes to mind to describe this
process is watching new convective
bubbles continually form along a
flanking line and then become absorbed
by the main convective updraft with the
result that the main convective
tower appears to backbuild. As was the
case for the
global TPW loop showed repeated
poleward surges of deep moisture from the
subtropics and tropics toward the
III. Science Issues:
One science issue deals with the structure and evolution of
blocking anticyclones. Blocking
anticyclones come in two flavors
according to the classic literature: 1)
Rex blocks that are characterized
by an upper-level closed anticyclone
situated poleward of a closed
cyclone, and 2) omega blocks that are
characterized by an upper-level
closed anticyclone that lies to the
northeast and northwest (NH sense) of
lower latitude upper-level closed
cyclones. Rex blocks, sometimes called
dipoles or high-on-top-of-low blocks
(aka modons; see, e.g., Neven 2001),
are commonly seen on time-mean
(typically 7-14 days) upper-level flow
maps, occurring preferentially in the
NH over western Europe and the
eastern
the
an extratropical cyclone that became
Hurricane Catarina (2004) via the
tropical transition process
(McTaggart-Cowan et al. 2006). There is a
slight tendency for omega blocks tend
to occur more preferentially over
the
At issue is whether the distinction between Rex blocks and omega
blocks is somewhat artificial. Go to
the McTaggart-Cowan GFS animation
builder link below and construct an
extended time series of the 200 hPa
streamfunction and non-divergent wind
going back to mid-January (choose
northern polar for the geographical
domain). Note the strong and classic
omega block over the
classic Rex block by 29 Jan. This
observed change in the configuration of
the block raises the question as to
whether Rex and omega blocks
represent different phases of blocking
life cycles and/or whether there
is something physically distinctive
about each of the two commonly
observed blocking configurations.The
modon-like structure of Rex blocks
(e.g., Neven 2001) may reflect, in
part, repeated ridge building and
ridge retrogression by eddy heat,
vorticity and potential vorticity (PV)
fluxes in conjunction with
poleward-moving transient disturbances.
Likewise, similar eddy fluxes but of
opposite sign would help the lower
latitude trough to continue to be
progressive with resultant modon
formation as the ridge and trough
rotate cyclonically around one another.
Luo and Chen (2006) suggest that the dipole and omega blocking
structures may be critically dependent
upon the configuration of the oceanic
upper-level jets and the associated
eddy fluxes of heat, momentum and vorticity.
Not known yet (to my knowledge) is
whether the dipole (Rex) and omega-type
blocks represent distinct stable states
of blocking regimes. A few questions for
the theoreticians on the map list: 1)
is there any theoretical evidence to
suggest that an omega block should be
more or less stable than a Rex block and,
if so, what are the relevant dynamics?,
2)do omega and Rex blocks contribute
differently to the general circulation
of the atmosphere and, if so, how and
why?, and 3)how might blocking behavior
change in a future warmer climate?
The time may be ripe to take advantage of the vastly improved resolution
and quality of global gridded analyses
over oceanic regions to re-examine the
role that blocks play in the
atmospheric general circulation, thanks to huge
improvements in the quality and number
of satellite-derived remotely sensed
observations and rapid progress in
assimilating these observations by
operational centers like NCEP and
ECMWF. As the loops referenced above make
clear, it is now possible to see
important structural details in synoptic- and
subsynoptic-scale (and even mesoscale)
weather systems that are associated with
blocking formation, evolution and
dissipation. Also, given that the life cycles
of blocks can vary from one to three
weeks, it is important to appreciate that
blocks are one of the key links between
weather and climate. A comprehensive
analysis of how atmospheric heat,
moisture, vorticity, potential vorticity and
momentum fluxes contribute to episodic
ridging, blocking formation and blocking
evolution as discussed above is likely
to prove insightful and could also lead
to fresh perspective on the two
well-known blocking types.
A second science issue relates to the formation and maintenance of the
highly elongated, quasi-linear 925-850
hPa layer-averaged vorticity bands that
define frontal regions. With the
now-routine availability of 0.5 degree GFS
analyses it has become apparent that
these vorticity bands are ubiquitous
features of the lower atmosphere and
that more often than not they are
meridionally oriented, especially when
they are highly elongated. As the
aforementioned loops indicate, the
highly elongated meridional bands of
vorticity tend to occur along cold
fronts associated with cyclones moving
poleward to the west of building
high-latitude ridges. The upper tropopsheric
flow in the these situations tends to
be diffluent, longitudinally constrained,
and latitudinally expanded, all
consistent with an expected meridionally
oriented axis of dilatation downstream
of an oceanic jet-exit region. In these
flow configurations, cold (warm) fronts
are long and meridionally oriented
(short and zonally oriented),
consistent with results found by Schultz et al.
(1998).
Lance
Luo, D. and Z. Chen, 2006: The Role of
Land-Sea Topography in Blocking Formation
in a Block-Eddy Interaction Model,
Journal of the Atmospheric Sciences, 63,
3056-3065.
McTaggart-Cowan, Ron, L. F.
Bosart, C. A. Davis, E. H. Atallah, J. R. Gyakum,
and K. A. Emanuel, 2006: Analysis of
Hurricane Catarina (2004), Monthly Weather
Review, 134 3029-3053.
Neven, E. C., 2001: Linear Stability of
Modons on a Sphere, Journal of the
Atmospheric Sciences, 58 2280-2305.
the Atmospheric Sciences, 60, 743-755.
Schultz, D. M., D. Keyser, and L. F. Bosart, 1998: The Effect
of Large-Scale
Flow on Low-Level Frontal Structure and
Evolution in Midlatitude Cyclones,
Monthly Weather Review, 126 1767-1791
Links used during map discussion:
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
http://amsu.cira.colostate.edu/TPW/Global30/default.htm
Link to attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_02-02-07/