4.6 Hot Topics

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4.6 Hot Topics
4.6.1 Genesis 4.6.2 Scale Interactions 4.6.3
Relationship to Tropical Cyclogenesis
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4.6.1 On the Genesis of African easterly waves
Chris Thorncroft, Nick Hall and George Kiladis
(1) Two theories for the Genesis of AEWs (2)
Idealised Modeling Results (3) Conclusions and
Perspectives
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(1) Two Theories for the Genesis of AEWs
I AEWs are generated via a linear mixed
barotropic-baroclinic instability mechanism
AEJ satisfies the necessary conditions for
barotropic and baroclinic instability Burpee
(1972), Albignat and Reed, 1980). Therefore we
expect AEWs to arise from small random
perturbations consistent with a survival of
the fittest view. Continues to be the consensus
view.
315K PV
925hPa q
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(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.

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(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.
• It is therefore not possible for AEWs to develop
  sufficiently fast to be important.
• So what can account for the existence of AEWs,
  their genesis and intermittancy?

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(1) Two Theories for the Genesis of AEWs
II AEWs are generated by finite amplitude
forcing upstream of the region of observed AEW
growth. Carlson (1969) suggested the importance
of convection and upstream topography for the
initiation of AEWs. Others pushed the linear
instability hypothesis. More recent
observational evidence has been provided
by Berry and Thorncroft (2005) case study of
an intense AEW Kiladis et al (2006) composite
analysis Mekonnen et al (2006) climatological
view
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(2) Idealised Modeling Results

• More observational and modeling studies are
  required to explore the validity of the
  hypothesis that AEWs triggered by upstream
  forcing.
• Here we use an idealised modeling study
  (following Hall et al, 2006)
• Global spectral primitive equation model
• Resolution T31 and 10 levels in the vertical
• Low-level damping is included (AEWs are stable!)
• Basic state is fixed.

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(2) Idealised Modeling Results

Basic state is the observed JJAS mean flow from
NCEP (1968-1998)
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(2) Idealised Modeling Results
Most unstable normal mode for the observed
zonally varying basic state (Hall et al ,
2006). Structure compares well with previous
composites based on observations including
Kiladis et al (2006). Due to damping this normal
mode structure is stable! So why do we observe
AEWs?

Perturbation streamfunction at sigma0.850 (top)
and in cross section through 15N. Dark shading is
ascent, light shading is descent.
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(2) Idealised Modeling Results
We hypothesize that observed AEWs are triggered
by upstream heating due to convection. To explore
this hypothesis we apply heating in the jet
entrance for one day and consider the adiabatic
response to this. This heating is meant to
represent the integrated effect of several MCSs.
The half width of the circular heating is about
140km.

Stratiform
Deep
Shallow
Heating rate profiles (K/day) as a function of
sigma.
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(2) Idealised Modeling Results
Initial heating located at (15N, 20E)

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Basic state is the observed JJAS mean flow from
NCEP (1968-1998)
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(2) Idealised Modeling Results
Deep Heating Run

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(2) Idealised Modeling Results
Shallow Heating Run

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(2) Idealised Modeling Results

Stratiform Heating Run
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(2) Idealised Modeling Results

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(2) Idealised Modeling Results

• Summary of heating runs
• In all runs the atmospheric response to the
  heating takes the form of enhanced and coherent
  AEW-activity in the downstream AEJ.
• While the subsequent forced normal mode
  structure appears to be insensitive to the
  initial heating profile, the amplitude clearly
  is.
• A heating profile that creates more intense
  lower tropospheric circulations (closer to the
  AEJ) results in larger amplitudes at day 1and
  after this.
• The timing of the trough passage at 10W is also
  sensitive to the heating profile.

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(2) Idealised Modeling Results

• Summary of heating runs
• In all runs the atmospheric response to the
  heating takes the form of enhanced and coherent
  AEW-activity in the downstream AEJ.
• While the subsequent forced normal mode
  structure appears to be insensitive to the
  initial heating profile, the amplitude clearly
  is.
• A heating profile that creates more intense
  lower tropospheric circulations (closer to the
  AEJ) results in larger amplitudes at day 1and
  after this.
• The timing of the trough passage at 10W is also
  sensitive to the heating profile.
• So where is the best place to trigger AEWs?

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(2) Idealised Modeling Results
Influence function for each profile defined by
the root mean square streamfunction at sigma0.85
and day 10. Confirms greater efficency of
shallow and stratiform heating profiles compared
to the deep heating profile. Best location to
trigger an AEW is around 20N, 15E close to AEJ
entrance and slightly north of basic runs.

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(3) Conclusions and Perspectives

• Significance for weather prediction
• A significant convective outbreak in the Darfur
  region will favor the formation of a train of
  AEWs to the west over sub-Saharan Africa within
  a few days.
• For daily-to-medium range forecasts of AEWs, it
  is important to monitor, and ultimately predict,
  the nature of the upstream convection.

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(3) Conclusions and Perspectives

• Significance for longer timescales
• In addition to considering the nature of mean
  AEJ, we should consider the nature and
  variability of finite amplitude convective
  heating precursors.

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(3) Conclusions and Perspectives

• Future work
• To address issues that relate to variability and
  predictability of AEWs including their
  intermittency we should consider
• the nature of upstream finite amplitude heating
  triggers
• how the heating interacts with the wave itself.
  
• how the nature of the observed AEJ impacts the
  response to these triggers and to the convection
  within the waves.

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4.6.2 Scale Interactions
Studies like Reed et al (1977) and Kiladis et al
(2006) highlight the typical observed
relationship between the AEW dynamical fields and
the convection (and associated rainfall). They do
not directlly address how AEWs interact with
convection.
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4.6.2 Scale Interactions
The PV-Theta thinking framework is ideal to
explore these scale interactions. To introduce
this the following slides show some results
from Berry and Thorncroft (2005)
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Selection of case.
Case Study of an intense African easterly wave
700hPa Meridional (v) wind, averaged 5oN-15oN.
(ve values contoured, gt2ms-1 shaded)

• Chose the most intense AEW of summer 2000 from
  700hPa meridional wind hovmoller.
• Case chosen was later associated with Hurricane
  Alberto.

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Mean State.
Mean 700hPa U wind, 16th July 15th August 2000
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Mean State 16th July 15th August 2000.
925hPa q
315K PV

• PV strip present on the cyclonic shear side of
  AEJ.

• Strong baroclinic zone 10o-20oN

925hPa qe

• High qe strip exists near 15oN

Mean State supports Baroclinic waves and MCSs!
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Satellite imagery

• METEOSAT-7 Water Vapour channel.
• Shown every 6 hours from 30th July 2000 00z to
  4th August 2000 18z.

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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
1st August 00z
((((()))))
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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
1st August 12z
((((()))))
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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
2nd August 00z
((((()))))
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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
2nd August 12z
((((()))))
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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
3rd August 00z
((((()))))
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700hPa Meridional wind (shaded, ms-1), 850hPa
Relative Vorticity (contoured x10-5 s-1)
3rd August 12z
((((()))))
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PV-theta analysis of AEWs

• PV-theta highlight synoptic scales and
  structures associated with baroclinic growth
  (adiabatic)
• PV is generated in regions of moist convection
  in particular in the vicinity of MCSs (diabatic)

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
1/8/00 00UTC
(((((((())))))))

• PV structure very different to mean meandering
  strip with embedded PV maxima.

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
1/8/00 12UTC
(((((())))))

• System retains baroclinic growth configuration,
  PV maxima intensified by convection, 925hPa
  cyclonic flow strengthens.

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
2/8/00 00UTC
(((((()))))))

• 7K q anomaly, with strong (nearly 20ms-1) 925hPa
  circulation.
• PV generated over Guinea highlands.

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
2/8/00 12UTC
((((((((()))))))))

• Disintegration of baroclinic structure.
• Interaction between system PV and Guinea
  Highlands PV.

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
3/8/00 00UTC
(((((((())))))))

• Merger of PV maxima establishes a 925hPa
  circulation.
• q anomaly moves to North and West.

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315K (650hPa) PV (Shaded), 925hPa q anomaly
(contour), 925hPa Wind vectors.
3/8/00 12UTC
((((()))))

• Further development of PV maxima gives a strong
  vortex with significant circulation at 925hPa
  (22ms-1 on East side).

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A conceptual model for AEW life-cycles

• Phase I Initiation
• Phase II Baroclinic growth
• Phase III West coast developments

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Conceptual framework (i) Initiation.
q Max
In the Alberto case a large MCS or several MCSs
provides an initial disturbance on a basic state
that supports AEWs. Initial value problem?
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Conceptual framework (ii) Baroclinic growth.
q Max
700hPa Trough
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Conceptual framework (ii) Baroclinic growth.
q Max
700hPa Trough
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Conceptual framework (ii) Baroclinic growth.
q Max
700hPa Trough
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Conceptual framework (iii) Merger of PV maxima.
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PV-theta analysis of AEWs

• PV-theta highlight synoptic scales and
  structures associated with baroclinic growth
  (adiabatic)
• PV is generated in regions of moist convection
  in particular in the vicinity of MCSs (diabatic)

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PV-theta analysis of AEWs

• PV-theta highlight synoptic scales and
  structures associated with baroclinic growth
  (adiabatic)
• PV is generated in regions of moist convection
  in particular in the vicinity of MCSs (diabatic)
• To complete the analysis we need also to
  understand what aspects of the AEW encourage or
  discourage convection.

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PV-theta analysis of AEWs How do AEWs favour
convection?
Adiabatic forcing of ascent? Destabilzation and
reduced CIN? Recent modeling results (Berry,
2008) favour the latter. CIN decreases steadily
between the northerlies and the trough
convection gets triggered before the trough
though. But there may be strong case-to-case
variability.

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PV-theta analysis of AEWs What is needed for
growth?

Need ve PV anomalies to be located where the AEW
trough is. If they occur ahead they will only
affect propagation (cf Diabatic Rossby Waves
Parker and Thorpe (1995) If they occur in the
ridge then they will result in decay of the AEW.
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PV-theta analysis of AEWs Scale Interactions

Synoptic-Mesoscale Interactions
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PV-theta analysis of AEWs Scale Interactions

Synoptic-Mesoscale Interactions From a PV-theta
perspective, the heating rate profiles are
crucial to know and understand.
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PV-theta analysis of AEWs Scale Interactions

Synoptic-Mesoscale Interactions From a PV-theta
perspective, the heating rate profiles are
crucial to know and understand.
Mesoscale-Microscale Interactions Ultimately
these profiles are influenced by the nature of
the microphysics!
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4.6.3 Relationship to Tropical Cyclogenesis

West Africa
USA
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Atlantic Tropical Cyclone Variability (ATCV)
There exists marked interannual to decadal
variability in ATCV.
What are the causes? Are they predictable?
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Atlantic Tropical Cyclone Variability (ATCV)
Known factors Tropical Atlantic
SSTs ENSO West African rainfall Phase of
QBO!!!
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Atlantic Tropical Cyclone Variability (ATCV)
Known factors Tropical Atlantic
SSTs ENSO West African rainfall Phase of
QBO!!!
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Atlantic Tropical Cyclone Variability (ATCV)
Goldenberg and Shapiro (1996) Linear correlation
coefficients ENSO ATCV -0.41 Sahel
Rainfall ATCV 0.70 Why is West Africa so
important? large-scale environmental impacts
(e.g. shear) weather systems (possibly)
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Atlantic Tropical Cyclone Variability
vertical wind shear between 200mb and 925mb
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Atlantic Tropical Cyclone Variability
Understanding the processes that influence the
MDR shear and its variability is very
important West Africa and East Pacific both
provide important anomalous heat sources that can
impact the MDR shear through tropical
teleconnections
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Atlantic Tropical Cyclone Variability
What about variability in the weather systems?
Thorncroft and Hodges (2001)
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Atlantic Tropical Cyclone Variability
There is a hint that the number of strong
vortices leaving the West African coast impacts
ATCV but this is far from being a sure
case. Recent analysis in the ERA40 datset (Hopsch
et al, 2006) suggests this relationship to be
weak on interannual timescales - but not on
interdecadal timescales!
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West Coast Developments
Case Studies have indicated that there can be
significant enhancement of the circulations,
including at low-levels, just before the AEWs
leave the African coast. BIG question Do AEWS
matter? Composite analysis from Hopsch (2008)
shows some recent results relevant to this
question.

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Composite for developing AEWsPV at
600hPastreamfunction of 2-6 day filtered wind
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Composite for non-developing AEWsPV at
600hPastreamfunction of 2-6 day filtered wind
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Cross sectionfor coast storm AEWs(i.e. storms
forming east of 30W)all cross-sections are
along 40W-10E at 11.25N black arrow points
to composite trough location
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Day -2
Shaded VOR (Red pos, Blue neg) Horizontal
Wind at all levels Theta-e
Shaded RH (Red lt 50 Blue gt 60) Vertical
wind
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Day -1
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Day 0
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Day 1
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Day 2
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Cross sectionfor non-developing AEWs
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Day -2
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Day -1
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Day 0
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Day 1
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Day 2
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4.7 Easterly waves in other tropical regions
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4.7 Easterly waves in other tropical regions
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4.7 Easterly waves in other tropical regions
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4.7 Easterly waves in other tropical regions
Builds on Schubert et al 1991 Potential
Vorticity modeling of the ITCZ and the Hadley
circulation JAS, 48, 1493-1509 Consider PV
sign-reversals associated with ITCZ-convection
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4.7 Easterly waves in other tropical regions
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4.7 Easterly waves in other tropical regions
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4.7 Easterly waves in other tropical regions
ITCZ breaks down in association with barotropic
instability
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4.7 Easterly waves in other tropical regions
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4.8 Final Comments
Easterly waves are present everywhere in the
tropics They are particularly important over West
Africa where they grow through baroclinic and
barotropic energy conversions. The baroclinic
energy conversions are particularly strong
consistent with the strong baroclinic zone not
present in other tropical regions. AEWs are
likely forced by upstream heating associated with
convection. AEWs grow through interaction of
Rossby waves (adiabatic) and are enhanced by
convection (Extra PV-sources) although we can
expect marked case-to-case variability. Hot
Research Topics Genesis Scale
Interactions Variability (especially
intraseasonal-to-interannual) Relationship to
Tropical Cyclones
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4.8 Final Comments
Easterly waves are also studied in the Caribbean
and Pacific where they are often linked to
tropical cyclogenesis Some questions To what
extent are the EWs generated in situ or come from
upstream (e.g. West Africa)? What can explain
the observed phase relationships why do they
vary between Africa and the ocean? How do they
interact with equatorial waves?