Hi Folks,
After a hiatus for
the end-of-semester break and the AMS annual meeting in SAT, Friday map
discussion resumed on
focus was on the European wind storm of 17-18 Jan'07 and the
ensuing major rearrangement of the large-scale flow pattern over the North
Images of the NAO/AO
index time series and a normalized polar (65-90 N) geopotential height anomaly
(GPH) cross section
(1000-10 hPa) time series, all taken from the NOAA/CPC web site,
are attached. The NAO index, after being positive for most of the time
since 16 Nov, switched to a negative value beginning 19-20 Jan.
The behavior of the AO index was similar to the NAO index, except much
more extreme. A comparison of the ensemble forecast NAO/AO indices
is revealing in that it suggests that the abrupt shift in the
indices from positive to negative after 15 Jan was near or outside
the lower end of the ensemble forecast envelope. This ensemble
forecast behavior suggests that the NAO/AO switch from positive to
negative in the wake of the big European wind storm posed a
predictability problem for the GFS model and is worthy of further
investigation for that reason alone.
The normalized GPH
time series shows that above normal heights in the polar troposphere and
stratosphere during most of the
autumn were replaced by below normal heights beginning in mid-Nov.
The below normal GPH regime then expanded downward into the lower
stratosphere by 1 Dec. Subsequently, the negative values of the
normalized GPH quickly reached the surface, indicative of strong
circumpolar westerly flow and a strongly positive AO. Evidence
also exists of a possible limited stratospheric warming event centered
near 1 Jan. The switch to a strongly positive AO shortly after 1
Dec is consistent with the widespread warmth that has been felt in
many middle and higher latitude regions of the NH with anomalously
strong westerly flow until a week ago.
To independently check
the European storm chronology appended below go to the McTaggart-Cown link and
choose your favorite
quantities to loop (select
best views). As Gary Lackmann suggested in a previous post to map,
the European wind storm was born inauspiciously over southern
on 14 Jan. It formed beneath potentially very warm ahead on the
dynamic tropopause (DT) along an old frontal boundary at the surface
and in advance of a rather weak upper-level DT disturbance. If you
check the aforementioned loops you will see that the storm went
through two "life altering" redevelopments before it
reached western Europe. The first redevelopment occurred as a secondary cyclone
formed south of
central
bringing widespread wind damage, mostly to the south of the track
of the storm in a region of very strong westerly wind maximum (the
formation of the warm front wave over the east-central
research).
The large-scale flow
regime change over the
coast late on 18 Jan. By 00Z on 19 Jan a concentrated area of
925-850 hPa layer-averaged relative vorticity east of NJ marks the
developing cyclone. It is organizing ahead of a rather
innocuous-looking trough that is beginning to cross the western
image). Of interest is that a rather ordinary trough triggered an
extraordinary large-scale circulation response.
By 12Z on 20 Jan
this cyclone has deepened to under 960 hPa and ridging is developing
over the
strongly negatively tilted, zonally constrained, and meridionally
elongated trough-ridge pair is evident over the
(image). The change from zonally to meridionally oriented flow
over the
"perfect ridge" on the DT is in place over the
Satellite-derived integrated precipitable water (IPW) values for
2345Z 22 Jan, 2248Z 25 Jan, and 1051Z 27 Jan (images attached) reveal
that the aforementioned vorticity bands closely correspond to
concentrated moisture axis freeways that originate in the tropics (5-day
loops of IPW are available at the link given below). These
concentrated moisture axis freeways were given the name "atmospheric
rivers"
by Zhu and Newell (1998) and have become objects of considerable
interest in the last few years as satellite-derived IPW fields have
become more generally available. McGuirk et al. (1987, 1988,
1989), Rasmusson and Arkin (1993) and Iskenderian (1995) were among the
first to conduct systematic studies of atmospheric rivers (which
were then called tropical cloud plumes).
An interesting development
during the 21-24 Jan period was the formation of a long, narrow NNW-SSE
oriented bands of low-level
cyclonic vorticity. These vorticity bands closely paralleled the
strong SSE jet axis aloft and were situated on the anticyclonic shear
side of this jet (an east-west cross section through these
vorticity bands would likely show that the concentrated region of cyclonic
vorticity slopes upward and westward). These vorticity bands
marked the primary frontal zone along which multiple cyclones formed and
moved NNW toward the
helped to amplify the new
given below) suggests that the high IPW corridors also closely
coincide with the leading edges of the vorticity bands.
An interesting
research problem would be to take advantage of modern high-resolution global
gridded datasets to study the
relationship between atmospheric rivers and poleward fluxes of
heat, momentum and potential vorticity during extreme weather events
(often associated with blocking; see excellent papers by Mullen
1986, 1987 for more information on this subject) and large-scale regime
changes. To understand the role that atmospheric rivers play in
weather and climate the thermodynamics of atmospheric rivers need to be
linked with the dynamics that governs the structure and evolution
of lower- and upper-level jets, frontal zones, deformation regions,
and cyclogenesis, frontogenesis, and vorticity genesis. This is
likely to be a very interesting and important predictability problem.
......................................
In conclusion, a
brief remark about the +65 F (+36 C) maximum temperature anomaly noted from a
station over northern
astride the arctic circle by Roger Brugge a few days ago that I
posted to map earlier in the week. I have attached maps of the mean and
anomalous 850 hPa temperature for 22 Jan (images were derived from
the NOAA/CDC interactive online data link). They show the remnants
of plume of highly anomalous warm air that was driven well inland
across
powerful storm that impacted western Europe on 17-18 Jan. The
maximum temperature anomaly was easily more than twice the maximum 850
hPa temperature anomaly. Given the anomalously strong westerly
flow that prevailed well inland at high latitudes in conjunction with
the passage of the storm, it is quite likely that the extreme
maximum temperature anomaly of + 36 C reflected strong mixing that
obliterated the climatological surface-based radiation inversion
as much as the anomalous warmth of the air column.
Chronology of European storm of 17-18 Jan'07:
a) 00Z/14: Percolating in southern TX (an AMS annual meeting
"I Love You" to
b) 00Z/15:
c) 12Z/15: First redevelopment:
Primary storm
located over
Developing warm
front wave (secondary cyclone) south of
d) 00Z/16: Secondary cyclone becoming dominant storm over the
southern
e) 00Z/17: Sub 988 hPa cyclone near 47 N and 47 E beneath 175+ kt
250 hPa jet.
f) 12Z/17: Second redevelopment:
Primary storm (sub
972 hPa) near 52 N and 37 W in left exit of 175+ kt 250 hPa jet.
Strong warm front
wave developing near 50 N and 25 W on anticyclonic shear side of jet.
g) 00Z/18: Sub 968 hPa cyclone (primary) near 54 N and 29 W; sub
972 hPa cyclone (secondary) near 54 N and 20 W; secondary cyclone is
now in the poleward jet-exit region (175+ kt 250 hPa jet).
h) 12Z/18: Sub 968 hPa cyclone is near 56 N and 2 E; very fast
westerly flow to the south.
Chronology of
a) 00Z/19: Cyclogenesis (sub 1024 hPa cyclone) begins off the
southeastern coast of the
b) 00Z/20: Explosive cyclogenesis underway; sub 984 hPa cyclone
near 45 N and 64 W; strong Atlantic ridging underway to the east.
c) 12Z/20: Sub 960 hPa cyclone is situated over eastern
to southern
d) 00Z/21: First new ridge-reinforcing sub 992 hPa cyclone is
located near 40 N and 55 W; original cyclone (sub 980 hPa) is located 50
N and 63 W; strong SSE geostrophic flow is present through a deep
layer to the east of both cyclones from 30-75 N.
e) 00Z/22: First new ridge-reinforcing cyclone (sub 980 hPa) is
located near 50 N and 51 W; strong SSE flow and warm-air advection is
evident to the east.
f) 00Z/23: Second new ridge reinforcing cyclone (sub 1000 hPa) is
located near 38 N and 67 W; strong SSE flow and warm-air advection
continues to the east.
g) 00Z/24: Second ridge-reinforcing cyclone (sub 980 hPa) is
located near 41 N and 56 W; strong SSE flow and warm-air advection
continues to the east.
h) 00Z/25: Second ridge-reinforcing cyclone (sub 976 hPa) is
located near 47 N and 53 W; strong SSE flow and warm-air advection
continues to the east.
i) 00Z/26: Second ridge-reinforcing cyclone (sub 972 hPa) is
located near 55 N and 60 W; strong SSE flow continues to the east. Third
ridge-reinforcing cyclone (sub 1000 hPa) is developing near 38 N
and 63 W.
j) 00Z/27: Explosive
development of second ridge-reinforcing cyclone (sub 956 hPa) is located near
44 N and 56 W; strong southerly
flow continues to the east.
...................................................
Iskenderian, H., 1995: A 10-Year Climatology of Northern
Hemisphere Tropical Cloud Plumes and Their Composite Flow Patterns, Journal of
Climate, 8, 1630-1637.
McGuirk, J. P., A. H.
Thompson, and L. L. Anderson Jr., 1989: Synoptic Scale Moisture Variation over
the Tropical
Monthly Weather Review, 117, 1076-1092.
McGuirk, J. P., A. H.
Thompson, and J. R. Schaefer, 1988: An Eastern Pacific Tropical Plume, Monthly
Weather Review, 116, 2505-2521.
McGuirk, J. P., A. H.
Thompson, and N. R. Smith, 1987: Moisture Bursts over the Tropical Pacific Ocean,
Monthly Weather Review, 115,
787-798.
Mullen, S. L., 1987: Transient Eddy Forcing of Blocking Flows,
Journal of the Atmospheric Sciences, 44, 3-22.
Mullen, S. L., 1986: The Local Balances of Vorticity and Heat for
Blocking Anticyclones in a Spectral General Circulation Model,
Journal of the Atmospheric Sciences, 43, 1406-1441.
Rasmusson, E. M., and P. A. Arkin, 1993: A Global View of
Large-Scale Precipitation Variability, Journal of Climate, 6, 1495-1522.
Zhu, Y. and R. E. Newell, 1998: A Proposed Algorithm for Moisture
Fluxes from Atmospheric Rivers,
Monthly Weather Review , 126, 725-735.
.....................................................
Links used in map discussion:
Link for 30-day loop of NH 500 hPa Z/Z':
http://www.cpc.ncep.noaa.gov/products/intraseasonal/z500_nh_anim.shtml
Link for loop of global integrated precipitable water (IPW):
http://amsu.cira.colostate.edu/TPW/global.htm
Link for McTaggart-Cowan dynamical tropopause maps and animations:
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php
Link to NCEP 3-, 7- and 14-day loops of NH surface analyses:
http://www.opc.ncep.noaa.gov/UA.shtml
Link for NOAA/CDC interactive plotting and analysis page:
http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl
..........................................
Link to attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_01-26-07/