Met Office College - Course Notes
Easterly waves and Squall lines
Contents
1.
Easterly Waves
2.
Squall Lines
2.1
2.2
2.3
3.
Forecasting the occurrence of Squall lines
Comparing African Easterly Waves and Squall Lines
Satellite imagery sequence
Further Reading
Crown Copyright. Permission to quote from this document must be obtained from The
Principal, Met Office College, FitzRoy Road, Exeter, Devon, EX1 3PB
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1. Easterly Waves
Easterly waves are large synoptic scale waves, which appear during the
period of northern summer on the equatorial side of the ITCZ. The
influence of Easterly waves is a cause of variation in the structure of the
ITCZ, and in the associated surface weather. Easterly waves passing in
the vicinity of the ITCZ may be identified on satellite images as cloud
masses separated by cloudless zones. As they interact with the ITCZ
precipitation belt, they bring about fluctuations in intensity of the
monsoon.
They are a three dimensional phenomenon. Over East Africa they are
considered a continuation of waves of the same type which, having
crossed the south of the Arabian Sea, spread towards the west through
Central and West Africa. Their weak intensity over East Africa prevents
their detection on synoptic charts. On the other hand, over Central
Africa they can intensify to give the impression that they have their
origin there. They gain their barotropic energy through horizontal wind
shear and baroclinic energy through the vertical shear of the flow over
the region. Easterly waves reach their maximum intensity over West
Africa. They appear most clearly on 700hPa synoptic charts, where they
are very active as waves in the field of the flow lines. Easterly waves
have an average wave length (L) of 2500 km average period (T) of 3 – 4
days average speed of 7 – 9ms-1, (15 – 20 kt).
Easterly Waves owe their origin to the presence of the African Easterly
Jet (AEJ) during the period of northern summer in the low troposphere
over the regions of West and Central Africa. The vertical structure of the
average zonal component of the wind in these regions and at this time of
the year shows:
West winds associated with the monsoon in the low layers, the depth of
westerlies increases towards the south.
Above this relatively cold and humid air, Easterly winds are observed.
These are associated with hot dry air from the Sahara. In accordance
with the thermal wind equation, the easterly winds increasing with
altitude reach their maximum speed around 600 hPa before decreasing
again following the intrusion of cold dry air from the Sahara in the 600 –
300 hPa level. The mean position of the jet is around 15N.
The structure of Easterly waves is such that they are cold at the centre
between the surface and the level of the jet, and hot at the centre above
the level of the jet. They incline from the SW towards the NE to the
south of the centre of the jet. This indicates a transport of momentum
from the west towards the jet that tends to weaken the jet by reducing
the zonal kinetic energy. The kinetic energy lost in this way is
recuperated by easterly waves through a process of barotropic
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Easterly Waves And Squall Lines
conversion. The easterly waves incline towards the east at altitude in the
layer of vertical shear of zonal flow situated between the surface and the
level of the jet. This indicates a transport of sensible heat towards the
equator. As the average temperature increases towards the north
(dT/dZ0), this transport of heat against the pressure gradient leads to
the reduction of available zonal potential energy which leads to an
increase in the potential energy of the easterly waves. This takes place
through the process of baroclinic conversion.
The region of maximum vertical speed is found in the front of the axis of
the wave at 700 hPa, where the wave is most active.
Figure 1. Easterly wave model
A study by Andreas Fink and Andreas Reiner of African Easterly Waves
and Squall Lines between May and October of 1998 and 1999 confirmed
the existence of two AEW tracks at either side of the AEJ with a mean
latitudinal position of 17.1N for the northerly and 8.6N for the southerly
vortex (see fig 2). In only 15% of the 81 cases was a single vortex at
either the northern or southern flank of the AEJ discernible.
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Figure 2. AEW tracks for the 12 months from May –Oct 1998 and 1999. The
digits in the top (bottom) left-hand corners denote the numbers of northerly
(southerly) AEWs for the respective months. The northerly (southerly) tracks
are dashed (solid). A total of 81 AEWs were identified within the two sixmonth period.
2. Squall Lines
African squall lines play a crucial role in the agricultural activities of the
populations of the Sudan/Sahel zone. They are responsible alone for
close to 80% of the precipitation falls on that zone. The rest comes from
the monsoon rains.
Squall lines move from east to west across West Africa. They are the
leading portion of propagating disturbances which consist of a chain of
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convective storms aligned in a north – south direction, and the cloud
shield (anvil cloud) that trails behind. Each squall line is a few
kilometres wide but up to several hundreds of kilometres long. Its life
span is greater than that of an isolated storm (typically no more than a
few hours). The organised squall line can live for up to a few days.
Observational evidence suggests that they move about 16ms-1, on
average, which is faster than the winds at all levels, and can cover
several hundreds of kilometres. In contrast mid-latitude squall lines
travel at the approximate mean wind speed.
Air from the boundary layer enters at the front of the squall line rises in
the updraught and leaves at high levels at the rear (see fig 3).
Figure 3. Schematic cross sectional structure through squall line system (from
Houze, Mon Wea Rev, 1977). Dark shading shows strong radar echo in the
melting band and in the heavy precipitation zone of the mature squall-line
element. Light shading shows weaker radar echoes. Scalloped line indicates
visible cloud boundaries.
The noteworthy characteristics of these systems are:

Typical rain rates in convective region in excess of 30 mm per hour
for a period of about 30 minutes, with peak values ranging from 60
to 110mm per hour. Intense rains turn to continuous light rain in the
stratiform part with a characteristic intensity of 4mm per hour and
duration of 2 to 3 hours. Around half of the rainfall is associated
with the passage of the leading edge and the remainder due to
stratiform precipitation.
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
Evaporating air cools the dry air aloft generating both mesoscale and
convective scale downdraughts. Descending air produces a “gust
front” at the leading edge of the squall line which acts as a density
current, lifting warm moist air ahead and encouraging development
of new convective cells and accelerating the propagation of the
squall line.

Their association with the easterly waves. Fink and Reiner’s study
of AEWs and SLs between May and October of 1998 and 1999 found
that 42% of SLs were AEW-forced. Their analysis shows that 75% of
these SLs were initiated west of the 700 hPa trough and the rest
behind the trough in the region of maximum southerlies. They also
confirmed the result that squall lines move faster (15 ms-1) than the
easterly waves (9 ms-1).

Their initiation seems more important during the afternoon, thus
showing the importance of the diurnal cycle in convection. However
Fink and Reiner found that if SLs emerged at night time and during
the early hours then they mostly belonged to the sample of AEWforced SLs. They also demonstrated that the role of AEWs in forcing
SLs increases substantially west of the Greenwich Meridian and
peaks near the coast. This goes in line with the westward amplitude
growth of the wave. They also found that at 10W, the north-westerly
flow west of the trough is directed towards the Fjouta Djalon
Mountains and, thus, orography might be an additional factor in
triggering SLs west of the wave at this location. Their study also
confirmed that AEWs significantly influence SL generation at the
height of the Sahelian rainy season, especially in August and
September, but primarily west of the Greenwich Meridian, where
they are well developed.

Favoured locations for development: Rowell and Milford’s study of
SLs during August 1985 identified regions of higher total-generation
density, suggesting the influence of topographic and other features.
These included:

Just west of the Air Mountains (16 to 19N, 7 to 9E)

Just west of the Adrar des Iforas Mountains (20N, 2E)

Northernmost part of the river Niger. Generation possibly aided
by river and surrounding areas acting as a moisture source at a
latitude where surface evaporation is generally lacking.

Over the Jos Plateau area (9 to 12N, 7 to 10E)
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2.1 Forecasting the occurrence of Squall lines
Squall lines have similarities in structure and appearance with systems
in the same family observed in other parts of the world. Therefore the
following derived parameters guide the forecaster in predicting the
occurrence of squall lines.
The essential physical processes in the environment for these formations
are:

Instability in the lower layers of the atmosphere. This is always
present in the tropical African atmosphere. This process leads to a
vertical speed in the storm that one can express in terms of the excess
of temperature of a representative parcel of air over that of the
environment.

The presence of a mechanism to lift the parcel of air and to release
the convective instability.

The supply of humidity in the low layers.

The vertical wind shear must be sufficient.
2.2 Comparing African Easterly Waves and Squall Lines
African Easterly Waves
Squall line systems
Wavelength of 1700km, period of 3
to 4 days
Can be initiated by AEW
(convergence) as well as other
lifting mechanisms
Life span of 1-2 weeks
System can last hours to days
(individual cells have lifetime of
around an hour)
System speed of around 9 ms-1
System speed of around 16 ms-1 i.e.
faster than wind speed at any level
Significant weather can extend for
100s of kms behind trough axis
(some AEWs give rise to little
cloud and rain)
100s of kms long, up to 100 kms
wide
Precipitation lasting 1 to 2 days at
one location
Precipitation lasting 1 to 2 hours at
one location
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2.3 Satellite imagery sequence
Figure 1: Sequence of three-hourly Meteosat pictures showing the
movement and evolution of a squall cluster (S). The period shown is
from 1800 29th June to 0300 on 1st July 1979. The cross marks in the
pictures are at 10º N with a separation of 10º of longitude, the middle
cross indicating the Greenwich meridian. The top of the first picture is
about 16º N. Some cloud clusters (C) are also indicated in the top
picture.
Picture 1: The cluster originated sometime in the 15.00-18.00 period near
15º N, 5ºE over the plateau of Agadez, Niger, a favoured area for the
generation during the hottest hours. Also the squall line developed
where the moist layer was moderately deep (about 1500 m) with dry
easterlies above. This favoured the generation of strong precipitation
downdraughts. The front edge of the squall clusters was a (typical)
convex arc – a point of reference to measure its speed. Observed
association of maximum rainfall amounts is with the leading edge of the
cluster.
Pictures 2 & 3: The clusters grew and merged with the squall cluster in
the following six hours. The line structure of the squall line is not clearly
defined because of the cirrus shield formed by CB anvils.
Picture 12 (last): indicates that the line moved westwards with a mean
speed of about 20 ms-1, travelling a distance of about 2400 km (4º E to
18º W) during the 33-hour period. Somewhat faster than the 16 ms-1
average quoted.
Pictures 3–7: 0001 to 1500 30th June, a diurnal variation – more
disorganised and moving more slowly.
Pictures 8–11: 1500 to 2400 30th June it moved very fast, mean speed 28
ms-1.
Some of the pictures show new convective elements developing well
ahead of the downdraughts. These merge with the squall line as it
approaches. It is not known whether the squall cluster triggered the
cumulonimbi well in advance of its leading edge, or whether it simply
overtook deep convection formed by some other mechanism.
Figure 2: A sounding taken at Niamey, Niger (13.5º N, 02.2º E), a few
hours before the development of the squall cluster.
Figure 3: shows the surface changes observed at Niamey during the
passage of the squall cluster. This figure was constructed with hourly
data. The squall line arrived at about 00Z on 29th June. The third panel
down in Fig 1 shows the satellite picture at this time; Niamey is located
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near the S. With the arrival of the squall line and its associated rainfall, a
sharp decrease in temperature, an increase in relative humidity, a gust in
the wind speed, an abrupt change in wind direction and an increase in
pressure were observed. After the squall line had passed the station,
surface temperature and relative humidity stayed relatively constant for
many hours. Similar surface changes were recorded at Bamako (12.5N,
8W), Mali, and also have been observed during the passage of other
West African squall lines.
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Figure 1. Sequence of 3 hourly Meteosat images showing movement and
evolution of a squall cluster (S)
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3. Further Reading
Thorncroft, C D and Blackburn, M, 1999: Maintenance of the African
easterly jet. Q. J. R. Meteorol. Soc. 125, 763-786.
Fink, A H and Reiner, A, 2003: Spatiotemporal variability of the relation
between African Easterly Waves and West African Squall Lines in 1998
and 1999. J.Geophys. Res. 108, 5-1 – 5-17.
Houze, R A Jr., 1977: Structure and Dynamics of a Tropical Squall-Line
System. Mon. Wea. Rev. 105, 1540-1567.
Rowell, D P and Milford, J R, 199:. On the Generation of African Squall
Lines. J. Clim. 6, 1181-1193.
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