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observed structure of the mean planetary-scale extratropical circulation

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observed structure of the mean planetary-scale extratropical circulation 1. surface energy balance disparities 2. mean sea-level structure 3. mean structure aloft – PowerPoint PPT presentation

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Title: observed structure of the mean planetary-scale extratropical circulation


1
observed structure of the mean planetary-scale
extratropical circulation
1. surface energy balance disparities 2. mean
sea-level structure 3. mean structure aloft 4.
baroclinic variability 5. the movement of
synoptic-scale systems
  • The images shown herein are based on NCEP/NCAR
    reanalysis dataset,
  • accessed thru the Climate Diagnostics Center
    website,
  • http//www.cdc.noaa.gov/cgi-bin/DataMenus.pl?datas
    etNCEP

2
We aim to answer these questions
  1. What explains the observed climatological SLP
    (sea level pressure) distribution and its
    seasonal variation?
  2. What explains the upper-level height and flow
    pattern?
  3. How does the upper-level structure relate to the
    surface lows and highs?
  4. How do upper-level trofs move in the baroclinic
    storm track?

3
suggested further reading
  • Holton Chapter 6.1
  • Bluestein 1993, esp.
  • section 1.1.8 Climatology of cyclogenesis and
    anticyclogenesis
  • section 1.2.5 Climatology of lows and highs
  • Palmen and Newton (1969)
  • various chapters are useful. The book is somewhat
    outdated, but it is very legible.
  • Peixoto and Oort (1992)
  • 4.1 transient and stationary eddies
  • 7.2 mean temperature structure
  • 7.3 mean height structure
  • 7.4 mean atmospheric circulation
  • An introductory, descriptive overview of the
    atmospheric general circulation (Word file), if
    you are not familiar with this.

4
1. background basics the Earths energy budget
(global, annual mean)
the units are in of the TOA incoming solar
radiation, i.e. S/4 (Ssolar constant 1380 W/m2)
R Sn Ln
R ? H LE
surface energy terms R net radiation Sn net
solar radiation Ln net terr. radiation H
sensible heat flux LE latent heat flux
and
R 51 21 30
R 7 23 30
30 net radiation
-30
5
the net solar radiation varies considerably with
latitude and season
6
zonal mean
net surface energy flux En
net incoming solar radiation Sn
En Sn Ln -H-LE
net outgoing terrestrial radiation (-Ln)
note the green-shaded area should equal the
orange one. This is true since the area of the
green latitudes is far larger than it appears.
note the linear scale
En
Source Trenberth and Caron 2001 Estimates of
Meridional Atmosphere and Ocean Heat Transports.
Journal of Climate, 14, 34333443
proportional to Earth surface area
7
The required total heat transport in order to
maintain an annual-mean steady state (RT), and
estimates of the total atmospheric transport AT
from NCEP and ECMWF re-analyses
What about the shortfall??
answer that heat is transported by oceans
Seasonal variation of the zonal mean of the
meridional heat transport by the atmosphere
Source Trenberth and Caron 2001 Estimates of
Meridional Atmosphere and Ocean Heat Transports.
Journal of Climate, 14, 34333443
PW petawatt or 1015 W
8
zonal annual mean ocean heat transports
solid mean dashed 1s (standard deviation)
0
30
-30
latitude
Source Trenberth and Caron 2001 Estimates of
Meridional Atmosphere and Ocean Heat Transports.
Journal of Climate, 14, 34333443
9
The poleward energy transfer that is needed to
offset the pole-to-equator net radiation
imbalance is accomplished partly by the
troposphere, partly by the oceans.
annual mean, northern hemisphere
(ºN)
this graph seems to overestimate the heat
transport by oceans (Source Ackerman and Knox
2003)
10
seasonal march of Sn, Ln and R
notice how - the latitudinal variation of Sn is
far larger than that of Ln and dominates that of
R - the zonal asymmetry of R (land-sea contrast)
is rather small - the desert areas over land are
radiatively deficient (anomalously low R for
their latitude, on account of the large Ln loss)
11
seasonal march of surface energy fluxes
Note how LE and H vary tremendously with season,
between land and ocean, and even over land and
over ocean. H and LE tend to compensate each
other. Their variation can be explained in terms
of forested regions vs deserts, warm vs cold
ocean currents, the sea ice edge, continental
airmasses advected over water, etc. Note that
oceans absorb and release far more heat than land
(storage change)
12
seasonal march of surface air temperature
note that the amplitude of the annual temperature
range is higher at - higher latitudes - over
land rather than over water this does NOT occur
in terms of net radiation Rn - over large land
masses, especially their eastern side
13
2. Structure of SLP, winds, temperatureseasonal
march of sea level pressure and sfc winds
  • northern oceans
  • polar lows Aleutian, Icelandic
  • subtropical highs Pacific, Bermuda
  • northern continents
  • - winter highs Siberian, Intramtn
  • - summer lows Pakistan, Sonoran
  • southern oceans
  • - circumpolar (southern) low
  • - subtropical highs (3 oceans)

observations - A see-saw SLP variation dominates
over the northern continents, with highs in
winter and lows in summer. The seasonal variation
of the polar lows and subtropical highs over the
northern oceans is also large, and is in
opposition to SLP variations over land at
corresponding latitudes. - The southern
hemisphere is far more zonally symmetric. - Note
the extremely low SLP around the Antarctic ice
dome.
14
polar perspective
  • the following plots are all polar stereographic
  • the images based on NCEP/NCAR re-analysis
    dataset, accessed thru the Climate Diagnostics
    Center website
  • either winter or summer is shown, either the
    northern hemisphere (NH or the southern (SH)
  • some maps display zonal anomalies, i.e. the
    departures from the zonal (constant latitude) mean

15
SL pressure NH winter
16
1000 mb height NH winter
What SLP is that? Z1000 280 m
answer 280 m 1035 mb
17
1000 mb temperature NH winter
18
1000 mb temperature, NH winter departure from
zonal mean
keep the magnitude of the zonal anomalies in mind
19
hydrostatic balance implies
  • negative surface temperature anomaly ? high SLP
  • positive surface temperature anomaly ? low SLP
  • Proof this assuming a flat pressure surface above
    the cold (warm) anomaly. The depth of this
    anomaly typically is 2km.

800 mb
height
warm? Z1000-850 large
cold? Z1000-850 small
ground (sea level)
1000 mb
low
high
20
1000 mb height
guess when
21
1000 mb temperature, NH summer departure from
zonal mean
keep the magnitude of the zonal anomalies in mind
22
1000 mb height, SH
guess when
note how the zonally rather symmetric subtropical
high is interrupted over land
23
1000 mb temperature, SH summer
departure from zonal mean
note the subtropical warm pools over land,
coincident with a low SLP anomaly
note the unusually low SSTs in the eastern
subtropical ocean basins
24
1000 mb height, SH
guess when
25
1000 mb height, SH winter!
400 m 1050 mb
26
1000 mb height, SH Antarctic high removed
ignored
27
1000 mb temperature SH winter
28
1000 mb temperature, SH winter
departure from zonal mean
ignored
note that these zonal anomalies are relatively
small
29
3. upper-level climatological structure
  • focus on winter in the northern hemisphere (NH)

30
seasonal march of the 500 mb heightwind
speed
31
500 mb height NH winter
note the seasonal-mean trofs, coincident with the
cold anomalies at low levels
1000 hPa temperature
32
500 mb height (zonal anomaly) NH winter
two quasi-stationary trofs ? wavenumber 2 pattern
33
Temperature 850 mb, NH winter
34
Wind _at_300 mb, NH winter
m s-1
m s-1
35
Verify qualitatively that climatological fields
are roughly in thermal wind balance. For
instance, look at the meridional variation of
temperature with height (in Jan)
36
Around 30-45 ºN, temperature drops northward,
therefore westerly winds increase in strength
with height
37
The meridional temperature gradient is large
between 30-50ºN and 1000-300 mb
thermal wind
Therefore the zonal wind increases rapidly from
1000 mb up to 300 mb
38
Question
  • Why, if it is colder at higher latitude, doesnt
    the wind continue to get stronger with altitude ?

39
There is definitively a jet ...
40
Answer above 300 mb, it is no longer colder at
higher latitudes...
tropopause
41
Tropopause pressure (hPa), NH winter
42
Wind 300 mb, NH winter
80 W
43
Zonal-mean wind, 80ºW, troposphere and lower
stratosphere
44
Wind _at_300 hPa, NH winter
B
A
45
section A, Japan temperature
Japan
tropopause
46
section A, Japan zonal wind speed
Japan
the polar-front jet (PFJ) has merged with the
subtropical jet (STJ)
47
section B, West Coast temperature
West Coast of N America
48
section B, West Coast zonal wind speed
West Coast of N America
STJ
PFJ
49
Wind 300 mb, NH winter
note that at most longitudes (esp. Asia and the
Pacific), a single jet is present
STJ
PFJ
PFJ
STJ
50
GP height _at_ 300 mb, NH winter
51
Potential vorticity _at_345 K
stratospheric air
tropo-spheric air
52
Schematic zonal-mean cross section (after Palmen
Newton)
? ITCZ
53
The Palmen-Newton model has three meridional
circulation cells in each hemisphere Note that
the three-cell pattern ignores seasonal variation
and land-sea contrast.
54
500 mb omega
note that blue is upward motion (wlt0) note the
rising motion near the ITCZ and subtropical
sinking, the latter mainly in the winter
hemisphere note the seasonal march of the ITCZ
(monsoon) note the rising motion in the
baroclinic storm track note the sinking (rising)
on the lee (upwind) side of mountain ranges
55
How strong are the meridional cells? (zonal
mean)
Jan
NH winter
Hadley
Ferrel
note the broad belt of subsidence (12-52ºN) in
winter and the broad belt of ITCZ ascent (0-30ºN)
in summer.
ITCZ
In the NH winter, over continents, the northern
Hadley cell rising branch crosses the Equator
into the SH ITCZ, and its sinking branch extends
between 12-50ºN
July
NH summer
Ferrel
NH Hadley
SH Hadley
Effectively ascent dominates in the summer
hemisphere, and sinking in the winter hemisphere,
and the Hadley cell that straddles the equator is
the strongest.
ITCZ
56
ageostrophic flow secondary circulation near
jet streak
Does this synoptic pattern apply to the mean
circulation?
57
Wind 300 hPa, NH winter
look for evidence of a secon-dary meridional
circulation around the Japan jet streak
A
B
58
Circulation, Section A
59
Specific humidity, Section A
Thermally direct!
x
Jet stream
60
Precipitation rate
Note the vertical velocity field in jet cores
will be revisited later for synoptic jets.
Jet core
ITCZ
61
4. SLP and 500 m height intraseasonal
variability, as a measure of the frequency of
storms due to baroclinic instability(the
midlatitude storm track)a 3-10 day bandpass
filter is used to highlight synoptic
disturbances.In theory this may
include tropical cyclones, but they are rare
62
SLP variability (3-10 day)
Northern hemisphere storm track, SLP, winter
Northern hemisphere storm track, SLP, summer
hPa
hPa
Northern hemisphere storm track, 500 mb, winter
Northern hemisphere storm track, 500 mb, summer
hPa
m
m
63
Southern hemisphere storm track, SLP, winter
hPa
64
Southern hemisphere storm track, SLP, summer
hPa
65
summary 3-10 day variability
  • there is a baroclinic storm track between
    40-60º latitude
  • storms appear vertically coupled
  • 1000 mb variability similar to 500 mb
    variability
  • storm track intensity (synoptic SLP variability)
    relates to meridional T gradient
  • stronger in winter than in summer
  • strongest east of the continents
  • storm track intensity also is stronger over ocean
    than land
  • the NH storm track has larger seasonal
    variability and is less zonally symmetric than
    the SH storm track
  • Lagrangian perspective where do lows and highs
    form and decay??

66
NH baroclinic storm trackcontour standard
deviation of GPHvector phase propagation vector
1000 mb
500 mb
67
Sea level cyclone formation and decay around
North America
Winter cyclolysis
Winter cyclogenesis
(Bluestein 1993, p 20)
68
anticyclone formation and decay around North
America
Winter anticyclolysis
Winter anticyclogenesis
(Bluestein p. 25)
69
5. On the movement of trofs and ridges
  • Rossby waves result from the conservation of
    vorticity. The restoring force is b or, more
    generally, the meridional gradient of the
    absolute vorticity. The resulting circulation
    causes the wave to propagate westward.
  • c is the Rossby wave propagation speed, u
    zonal wind, k and l are zonal and meridional
    wavenumbers
  • In short waves, the advection of relative
    vorticity dominates. The wave propagation speed
    is slow and they move with the prevailing
    westerly flow.
  • In long waves, the advection of planetary
    vorticity dominates. Their speed is large and
    they are generally stationary, or may retrograde.

70
On the movement of trofs and ridges
  • Trofs may be cut-off from the westerly flow and
    they become stationary at lower latitudes.
    Cut-off lows may dissipate or they may be kicked
    back into the main current
  • The kicker trof needs to approach the cut-off to
    within 2000 km (Henrys rule)

71
On the movement of trofs and ridges
  • If the max cyclonic shear is on the upstream side
    of the trof, the trof will tend to move
    equatorward, and may deepen.
  • If the max cyclonic shear is on the downstream
    side of the trof, the trof will tend to move
    poleward.

72
Hovmöller diagrams
24 hr average
departure of the 24 hr average from the zonal mean
Winter 02-03 500 mb height (m) 30-40ºN all
longitudes
question how does the stationary long-wave
pattern shown here matches with the 500 mb
anomalies from zonal mean, discussed earlier ?
Alpine lee cyclogenesis
Rockies lee cyclogenesis
red short-wave trofs blue stationary long-wave
ridge
Source http//www.cdc.noaa.gov/map/clim/glbcir.sh
tml
73
blocking flow aloft
  • occurs most frequently in spring around 50N
  • results from an anomalous long-wave ridge that
    blocks the progression of short waves
  • Occurs during low-index cycles
  • high index strong zonal flow
  • low index zonal flow weak, meridional flow
    strong
  • 3 types, qualitatively separated
  • High-over-low block
  • Omega block
  • upper-level ridge

74
high over low block
upper-level ridge
omega block
75
conclusions
  • A seasonally-variable net surface energy
    imbalance exists, with Engt0 at low latitudes and
    Enlt0 at high latitudes.
  • The atmospheric component of the meridional heat
    transfer is achieved in part by the meridional
    mean circulation (Hadley), but mainly by the
    mid-latitude eddy circulation associated with the
    high variability observed on the 3-10 day
    timescale (baroclinic eddies).
  • The Palmen-Newton model of the general
    circulation of the atmosphere
  • Highly simplified (seasonal variations/ land-sea
    contrast are very important) - applies better in
    the SH
  • The only strong meridional cell in the zonal mean
    circulation is the Hadley cell
  • The seasonal variation of SLP in the NH is
    dominated by the land-sea contrast
  • The annual temperature range over land, esp the
    eastern end of the land masses, is far larger
    than that over the oceans. This is not explained
    by the annual range of net radiation, which shows
    weak zonal anomalies.
  • Subtropical (30ºN) ocean highs and continental
    lows dominate in summer
  • Polar (60ºN) ocean lows and continental highs
    dominate in winter
  • A jet stream exists in the upper mid-latitude
    troposphere
  • Its climatological position and strength are in
    thermal wind balance, thus it is stronger in
    winter than in summer
  • The separation between STJ and PJ is not present
    at all longitudes, nor in the zonal mean, in both
    hemispheres seasons
  • Highly zonally asymmetric in the NH, due to
    continents and topography
  • Quasi-stationary trofs along east coasts of N
    America Asia
  • Very symmetric in the SH
  • Jet stream separates reservoirs of very different
    air (in terms of potential vorticity)
  • The jet stream carries quasistationary long waves
    and eastward-moving, baroclinic short waves
  • baroclinic storm track most apparent in the 3-10
    band-pass filtered data, at LL and UL
  • stronger in winter than summer,

76
meridional cross section (potential temperature,
zonal wind speed)
W
pressure (mb)
E
latitude
Holton (2004) p.145
77
Relationship between surface cyclone and UL wave
trof, during the lifecycle of a frontal
disturbance
500 mb height (thick lines) SLP isobars (thin
lines) layer-mean temperature (dashed) The
deflection of the upper-level wave contributes to
deepening of the surface low.
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