section 4: easterly waves

Section 4: Easterly Waves

Section 4: Easterly Waves

4.1 Introduction

4.2 The Mean State over West Africa

4.3 Observations of African Easterly Waves

4.4 Theory

4.5 Modeling

4.6 Hot Topics:

4.6.1 Genesis

4.6.2 Scale Interactions

4.6.3 Relationship to Tropical Cyclogenesis

4.7 Easterly Waves in other Tropical Regions

4.8 Final Comments

4.1 Introduction

• Westward moving synoptic waves characterize the whole tropics

• They are tropospheric waves, that modulate the rainfall and move at about 8m/s and have wavelengths of 2000-4000km.

4.1 Introduction

• The environments that they are embedded in varies around the tropics, and so details of the wave characteristics also vary.

4.1 Introduction

The emphasis here will be on African Easterly Waves (AEWs)

Have a strong influence on daily rainfall patterns over Africa and tropical Atlantic

Most Atlantic Tropical Cyclones are generated in association with AEWs

AEWs

MCSs

SAL

TC


Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90

Notable Features: 600mb African Easterly Jet (AEJ)

Upper-level Tropical Easterly Jet (TEJ)

Low-level Monsoonal Westerlies

Low-level Easterlies north of the AEJ

Upper-level Westerly Jet to the North

4.2 The Mean State over West AfricaReed, 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).

PV View of the African Easterly Jet

Discussion

Consider the meridional contrasts in convection (next slide) and the diabatic source/sink term in the PV-equation.

Schematic of African Easterly Jet

θ

50oCθ

θe

90oC

θe

AEJ

20oC60oC


Thorncroft and Blackburn 1999

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

Zonal Variations in the Mean State

Berry and Thorncroft 2005

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

Zonal Variations in the Mean State


Carlson, T.N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev., 97, 256-276.

Carlson, T.N., 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97, 716-726.

Burpee, R.W., 1970: The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90.

Reed, R.J., Norquist, D.C. and Recker, E.E., 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, 317-333

Thorncroft, C.D. and Hodges: 2001 K.I., African easterly wave variability and its relationship to tropical cyclone activity, J. Clim. 14, 1166-1179 (2001).

Kiladis, G., C. Thorncroft, and N. Hall, 2006: Three-Dimensional Structure and Dynamics of African easterly waves: part I: Observations, J. Atmos. Sci., 63, 2212-2230.

Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: On the significance of African easterly waves on convection, J. Climate, 19, 5405-5421.

Berry, G., Thorncroft, C.D. and Hewson, T. 2006 African easterly waves in 2004 – Analysis using objective techniques Mon. Wea. Rev., 133, 752-766


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

Two cyclonic centers at low-levels

Synoptic variations in cloud cover

Peak of cloudiness close to AEW trough


Burpee (1970)

Eastward tilt beneath the AEJ – Westward tilt above the AEJ

Northerlies dry and warm

Southerlies wet and cold


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.

Reed et al, 1977


Thorncroft and Hodges (2001)

Three Dimensional Structure of Easterly Wave Disturbances over

Africa and the Tropical North Atlantic

George N. Kiladis 1

Chris D. Thorncroft 2

Nick M. J. Hall 3

1 NOAA Aeronomy Laboratory, Boulder, CO2 Dept. of Atmospheric Sciences, SUNY, Albany,

NY3 LTHE, Grenoble, France

Space-Time Spectrum of JJA Antisymmetric OLR, 15S-15N

Wheeler and Kiladis, 1999

Regression Model

Simple Linear Model:

A separate linear relationship between a predictor at a grid point and a parameter at every other grid point is

obtained:

y = ax + b

where: x= predictor (TD-filtered OLR at 10N, 10W)

y= predictand (u or v wind at any grid point)

Maps or cross sections at lag can then be constructed to show the evolution of the dynamical fields versus the

predictor

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993

Day 0Streamfunction (contours 1 X 105 m2 s-1)

Wind (vectors, largest around 2 m s-1)OLR (shading starts at +/- 6 W s-2), negative blue


Day-4Streamfunction (contours 1 X 105 m2 s-1)



Day 0Streamfunction (contours 1 X 105 m2 s-1)



Day+1Streamfunction (contours 1 X 105 m2 s-1)



All the previous slides refer to composite AEW structures

They say little about the significance of AEWs on convection and

They say little about how these structures might be manifested on a weather map or how they may vary in space and time.

The next slides address the significance issue from Mekonnen et al (2006)

This will be followed by some maps of individual AEWs (Berry et al 2007).

Shaded region: power > red noise

Central Africa

E. Africa

Significant time scales:2-6 days & at 1 day.

Peak periods change from west to east

E. Atlantic

W. Africa

TB variance (in K2)

E. Atlantic (5-10N, 40W-20W)Land (10-15N, 15W-40E)


Central Africa

E. Africa



E. Atlantic

W. Africa

TB variance (in K2)

Central Africa

E. Africa

E. Atlantic

W. Africa

TB variance (in K2)




2-6d TB variance (shaded >140K2)

2-6d contribution:25-35% over land,35-40% over ocean

west-east variance is nearlythe same

Variance explained by 2-6d TB

(shaded > 20%)

Comparison with dynamic measures …..

2-6d 700-hPa variance

2-6d 850-hPa variance

Variance in the west are higher than in the east!

(shaded >5m2s-2)

Land: maximum along 10N, south of the AEJ, near peak convective region.Ocean: near 20N

Land: maximum to the north of AEJ, and over the coast, near peak convective regionOcean: within ITCZ

315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

Diagnostics for highlighting multi-scale aspects of AEWs

Berry et al 2006

315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

Summary of the observed AEJ and AEWs

Summary

AEJ: Consists of two prominent PV anomalies; a positive PV anomaly on the cyclonic side of the AEJ that is diabatically generated in the region of peak rainfall and a negative PV anomaly that is diabatically generated in the heat low region.

AEWs: AEWs have significant circulation anomalies at the level of the AEJ and at the surface. They tend to tilt against the horizontal and vertical shear of the AEJ – this tells us something about the growth mechanisms to be discussed in the next section.

section 4: easterly waves

Documents

structure of easterly

easterly waves section

african disturbances

synoptic waves

tropospheric waves

mean state berry

tropical regions4

tropical cyclogenesis4