4.6 hot topics

4.6 Hot Topics

4.6.1 Genesis4.6.2 Scale Interactions4.6.3 Relationship to Tropical Cyclogenesis

4.6.1 On the Genesis of African easterly waves

(1) Two theories for the Genesis of AEWs

(2) Idealised Modeling Results

(3) Conclusions and Perspectives

Chris Thorncroft, Nick Hall and George Kiladis

I: AEWs are generated via a linear mixed barotropic-baroclinic instability mechanism

(1) Two Theories for the Genesis of AEWs

925hPa

315K PV

AEJ satisfies the necessary conditions for barotropic andbaroclinic instability:Burpee (1972), Albignat and Reed, 1980).

Therefore we expect AEWs to arise from small random perturbations consistent with a “survival of thefittest” view.

Continues to be the consensus view.

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.

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?

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

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.

Basic state is the observed JJAS mean flow from NCEP (1968-1998)

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 sigma=0.850 (top) and in cross section through 15N. Dark shading is ascent, light shading is descent.

-2 -1 0 1 2 3 4

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.

Heating rate profiles (K/day) as a function of sigma.

Stratiform

Shallow

Basic state is the observed JJAS mean flow from NCEP (1968-1998)

Initial heating located at (15N, 20E)

Deep Heating Run

Shallow Heating Run

Stratiform Heating Run

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.

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?

Influence function for each profile defined by the root mean square streamfunction at sigma=0.85 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.

• 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.

• 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.

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.

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.

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):

Selection of case.

• Chose the most intense AEW of summer 2000 from 700hPa meridional wind hovmoller.

• Case chosen was later associated with Hurricane Alberto.

700hPa Meridional (v) wind, averaged 5oN-15oN. (+ve values contoured, >+2ms-1 shaded)

Case Study of an intense African easterly wave

Mean State.

Mean 700hPa U wind, 16th July – 15th August 2000

Mean State: 16th July – 15th August 2000.

925hPa 315K PV

• Strong baroclinic zone 10o-20oN

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

925hPa e

• High e strip exists near 15oN

Mean State supports Baroclinic waves and MCSs!

Satellite imagery

• METEOSAT-7 Water Vapour channel.

• Shown every 6 hours from 30th July 2000 00z to 4th August 2000 18z.

1st August 00z

((((()))))

700hPa Meridional wind (shaded, ms-1), 850hPa Relative Vorticity (contoured x10-5 s-1)

1st August 12z

((((()))))

2nd August 00z

((((()))))

2nd August 12z

((((()))))

3rd August 00z

3rd August 12z

((((()))))

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)

315K (~650hPa) PV (Shaded), 925hPa anomaly (contour), 925hPa Wind vectors.

1/8/00 00UTC

(((((((())))))))

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

1/8/00 12UTC

(((((())))))

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

2/8/00 00UTC

(((((()))))))

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

2/8/00 12UTC

((((((((()))))))))

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

3/8/00 00UTC

(((((((())))))))

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

3/8/00 12UTC

((((()))))

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

A conceptual model for AEW life-cycles

• Phase I: Initiation

• Phase II: Baroclinic growth

• Phase III: West coast developments

’ Max

Conceptual framework (i) Initiation.

In the Alberto case a large MCS or several MCSs provides an initial disturbance on a basic state that supports AEWs. Initial value problem?

’ Max

700hPa Trough

Conceptual framework (ii) Baroclinic growth.

’ Max

700hPa Trough

Conceptual framework (ii) Baroclinic growth.

’ Max

700hPa Trough

Conceptual framework (iii) Merger of PV maxima.

• To complete the analysis we need also to understand what aspects of the AEW encourage or discourage convection.

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.

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.

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!

4.6.3 Relationship to Tropical Cyclogenesis

West Africa

Atlantic Tropical Cyclone Variability (ATCV)

There exists marked interannual to decadal variability in ATCV.

What are the causes? Are they predictable?

Known factors:

Tropical Atlantic SSTs

West African rainfall

Phase of QBO!!!

Known factors:

Tropical Atlantic SSTs

West African rainfall

Phase of QBO!!!

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)

Atlantic Tropical Cyclone Variability

vertical wind shear between 200mb and 925mb

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

Thorncroft and Hodges (2001)

What about variability in the weather systems?

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!

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.

Composite for developing AEWs:

PV at 600hPastreamfunction of 2-6 day filtered wind

Composite for non-developing AEWs:

PV at 600hPastreamfunction of 2-6 day filtered wind

Cross sectionfor coast storm AEWs

(i.e. storms forming east of 30°W)

all cross-sections are along 40°W-10°E at 11.25°N

black arrow points to composite trough location

0-10-20-30

Shaded:VOR (Red = pos, Blue = neg)

Horizontal Wind at all levels

Theta-e

Shaded:RH (Red < 50% Blue > 60%)

Vertical wind

Day -2

0-10-20-30

Day -1

0-10-20-30

Day +1

0-10-20-30

Day +2

Cross sectionfor non-developing AEWs

0-10-20-30

Day -2

0-10-20-30

Day -1

0-10-20-30

Day +1

0-10-20-30

Day +2

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”

ITCZ breaks down in association with barotropic instability

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

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?

4.6 hot topics

initiation of aews

existence of aews

linear instability mechanism

linear instability hypothesis

idealised modeling study

genesis of aewsii

varying basic state

modeling studies

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