Transcript Tropical551EasterlyWavesNewFinal.ppt

551 Tropical
Advanced Topics
Trade Wind Inversion
One starts with subsidence in the
subtropics
But the subsidence is not zonally uniform:
greater in the eastern side of the oceans
The resulting greater subsidence on the eastern
sides depressing heights on the eastern sides of
the ocean relative to the central portions
But why more subsidence on the
eastern side?
• Asymmetry of meridional motions
– Northerly on western side
– Southerly on eastern side
• Thus, air parcels are experiencing
progressively smaller f on the eastern side.
• If relative vorticity is maintained, need
differential subsidence to balance.
Trade Wind or Subtropical
Inversion
• The height of the base of this inversion varies from about 500 m
at the eastern extremities of the subtropical highs to about 2000 m
at the western and equatorial extremities.
• In the equatorial trough zone and over the western portions of the
trade-wind belt, the inversion does not exist as a mean condition,
although it appears in certain weather patterns.
• The inversion is generally strongest when the height of its base is
lowest, and vice versa. The thickness of the inversion layer varies
from tens of m to more than 1000 m.
• On the average its thickness is about 400 m.
Tropical Analyses
• See both midlatitude style, isobaric analysis,
and streamline/isotach analyses.
• Why the latter is most popular?
Why streamline/isotachs better
than isobars/heights in the
tropics?
• Flow not geostrophic within roughly 5
degrees of the equator (so connection
between pressure and winds are not
necessarily straightforward)
• Lot of pibals (just winds) in the tropics
compared to radiosondes.
• Pressure variations normally weak in the
equator (large noise to signal)
Thus, why not analyze winds
instead of pressure?
Streamline/isotach analysis is
used but there are alternatives
(isogon, isotach)
Not intuitive
Tips on streamline/isotach
analysis (you will do this)
• Do streamlines first if possible.
• You will see a lot of familiar patterns
(circulation, deformation, convergence, etc.)
• Winds should go to zero at singular points:
such as centers of rotation,
convergence/divergence, and deformation.
Easterly Waves: The Major
Synoptic Disturbances in the
Tropics
•Westward moving synoptic waves characterize the whole tropics
• They are tropospheric waves that modulate clouds/rainfall and
move at about 5-10 m/s and have wavelengths of 2000-4000 km.
•Assignment: METED African Easterly Wave module
Composite African Wave Structures (Reed et
al., 1977)
African Easterly Waves Are
Closely Associated with and
Propagate in a Midtropospheric
Easterly Jet
• Centered around 650-700 hPa near
15N
• Associated with large temperature
difference between the hot Sahara
and cool Gulf of Guinea
The Mean State over West Africa: The African
Easterly Jet (AEJ)
Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa,
J. Atmos. Sci. 29, 77-90
Why is the AEJ there?
Hint: Surface temps
• Strong baroclinic zone 10o-20oN
Reed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as
observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333 (1977).
Zonal Variations in the Mean State
Mean 700hPa U wind, 16th July – 15th August 2000
Berry and Thorncroft 2005
Observations of African Easterly Waves
Carlson 1969ab
Carried out case studies of several AEWs
Peak amplitudes at 600-700mb and at
surface
Eastward tilt with height from the surface to
the level of the AEJ
Synoptic variations in cloud cover
Peak of cloudiness close to AEW trough
Observations of African Easterly Waves
Burpee (1970)
Eastward tilt beneath
the AEJ – Westward
tilt above the AEJ
Northerlies dry and
warm
Southerlies wet and
cold
Observations of African Easterly Waves
Reed et al, 1977
Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative
vorticity at the surface and 700hPa respectively with a contour interval of 10-5s-1. (c) and (d) show
percentage cover by convective cloud and average precipitation rate (mm day-1) respectively.
Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.
Origin of African Easterly Waves
• A joint baratropic/baroclinic instability on
the African easterly jet since there are large
vertical and horizontal shears.
• Release of latent heat in convection can
help initiate and support these waves.
Theories for the Genesis of AEWs
AEWs are generated via a linear mixed barotropic-baroclinic instability
mechanism
315K PV
AEJ satisfies the necessary
conditions for barotropic and
baroclinic instability:
Burpee (1972), Albignat and Reed,
1980; Charney and Stern 1962).
Therefore we expect AEWs to
arise
from small random perturbations
925hPa q
Barotropic Instability
• Associated
with reversal
of horizontal
gradient of
absolute or
potential
vorticity
Barotropic Instability Can Exist
in the Midlatitudes When
Horizontal Shear is Very Large
• Hakim examples:
• http://www.atmos.washington.edu/~hakim/
542/baro_wv.html
(1) Two Theories for the Genesis of AEWs
I: AEWs are generated via a linear mixed barotropic-baroclinic instability
mechanism (evidence against!)
• The AEJ is too short!
The jet is typically 40-50o long.
It can only support two waves at one time.
It is therefore not possible for AEWs to develop via a linear instability
mechanism.
• The AEJ is only marginally unstable!
Hall et al (2006) showed that in the presence of realistic boundary-layer
damping the AEW growth rates are very small or zero.
It is therefore not possible for AEWs to develop sufficiently fast to be
important.
Some have suggested that
barotropic/baroclinic instability is not a
complete explanation
• The AEJ is short
The jet is typically 40-50o long.
It can only support two waves at one time..
• The AEJ is only marginally unstable!
Hall et al (2006) showed that in the presence of realistic boundary-layer
damping the AEW growth rates are small.
It is may not be possible for AEWs to develop sufficiently fast to be
important.
So what can account for the existence of AEWs, their genesis and
intermittancy?
Perhaps AEWs are generated by finite amplitude
forcing upstream of the region of observed AEW
growth, with barotropic/baroclinic instability taking
over, assisted by latent heat release in the waves.
Carlson (1969) suggested the importance of convection and upstream topography for
the initiation of AEWs.
Thorncroft and Hodges 2001
The End