Download Extratropical Dry-Air Intrusions into the West African Monsoon

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JOURNAL OF THE ATMOSPHERIC SCIENCES
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Extratropical Dry-Air Intrusions into the West African Monsoon Midtroposphere:
An Important Factor for the Convective Activity over the Sahel
RÉMY ROCA
Laboratoire de Météorologie Dynamique, École Normale Supérieure, Paris, France
JEAN-PHILIPPE LAFORE, CATHERINE PIRIOU,
AND
JEAN-LUC REDELSPERGER
Centre National de la Recherche Météorologique, Toulouse, France
(Manuscript received 3 October 2003, in final form 28 June 2004)
ABSTRACT
This paper investigates the relationship between large-scale dynamics, water vapor, and organized convection over West Africa. Making use of a simplified condensation hypothesis, a back-trajectory model fed
by NCEP-analyzed winds is used to reconstruct the midtropospheric humidity field over Africa during July
to August 1992. The approach documents both the moisture content and the origin of the air masses.
Meteosat satellite infrared imagery is used to characterize the convective systems.
A case study analysis reveals that very dry air patches (RH ⬍ 5%) are located in the immediate
midtropospheric environment of a typical squall line. Such dry-air structures are shown to originate in the
upper levels (200–250 hPa) on the anticyclonic side of the polar jet stream at 50°N. Focusing on the Sahel
region, dry events are isolated using the time series of the 500-hPa relative humidity distribution during the
monsoon period. These dry events are shown to be composed of extratropical air. Composite analysis of the
convective activity indicator exhibits a strong positive association between dry intrusions and convection on
the eastern side of the Sahelian region. Organized convective systems that are fast moving and long lasting
are more likely over this region when a dry intrusion is present. This coincides with the well-established
theory that midtropospheric dry air, when combined with sufficient wind shear, can maintain and intensify
previously triggered deep convection through rain evaporation that feeds the cold pools, especially within
squall lines. This paper suggests that the extratropical dry-air intrusions modulate the occurrence and
duration of convective systems and, therefore, the mode of variability of rainfall over West Africa during
the monsoon.
1. Introduction
Strong variability of free tropospheric humidity has
been observed over the Pacific Ocean during the Tropical Ocean Global Atmosphere Coupled Ocean–
Atmosphere Response Experiment (TOGA COARE)
campaign with extremely dry layers found within regions that are usually convectively active (e.g., Mapes
and Zuidema 1996). These dry-air tongues were shown
to originate from the extratropical upper levels yielding
to the notion of extratropical dry-air intrusions in the
deep convective regions (Yoneyama and Parsons 1999;
Parsons et al. 2000). Locally, dry air inhibits deep convection through two processes: first, the radiatively
built thermal inversion due to the dry layer that pre-
Corresponding author address: Dr. Rémy Roca, Laboratoire de
Météorologie Dynamique, École Normale Supérieure, 24 rue
Lhomond, 75005 Paris, France.
E-mail: [email protected]
© 2005 American Meteorological Society
JAS3366
vents the development of convective clouds and second, the entrainment of dry air as the parcel rises that
decreases the parcel’s buoyancy (Mapes and Zuidema
1996; Sherwood 1999; Parsons et al. 2000; Redelsperger
et al. 2002b). Both processes control the cloud-top altitude. It has been suggested that these dry-air intrusions play an important role in the synoptic and intraseasonal variability of deep convection in the warm
pool region (Brown and Zhang 1997; Johnson et al.
2001). While such phenomena received a great deal of
attention in the TOGA COARE community, their distribution over the Tropics is still unclear. For instance,
the Indian Ocean Experiment (INDOEX) sounding
analysis revealed the existence of dry-air layers that
originate from extratropical upper-level flows over the
Indian Ocean during the boreal winter (Zachariasse et
al. 2001); these dry structures have also been seen over
the Atlantic Ocean in the summer (Pierrehumbert and
Roca 1998). In the present paper, we investigate the
humidity distribution of the environment of deep con-
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vection in the midtroposphere over the West African
monsoon region and show the existence of extratropical
dry-air intrusions.
Over West Africa a number of processes govern continental convection, ranging from the role of topography in the triggering phase to the role of wind shear for
moving convective lines, including the diurnal cycle,
etc. Much of the Sahelian seasonal rainfall is produced
by organized mesoscale convective systems (Lebel and
Lebarbé 1997; Mathon and Laurent 2001). The environmental midtropospheric water vapor content of the
West African squall lines has been investigated during
the Global Atmospheric Research Program (GARP)
Atlantic Tropical Experiment (GATE). Composites of
the environment of squall lines were formed using
soundings and by separating slow- and fast-propagating
lines. Barnes and Sieckman (1984) showed that fast
lines propagated in an environment where the midtropospheric equivalent potential temperature (␪e) was
smaller than that for the slow lines, and where the wind
shear was significant and normal to the fast lines while
it was parallel to the slow-moving cloud lines. This
midtropospheric dry environmental air, together with
the wind shear, was further shown to be an important
element of the mesoscale circulation associated with
convection. The evaporation of hydrometeors in the
dry-air layer acts as a fuel for the density current that
drives the new convective cells’ development (Redelsperger and Lafore 1988; Lafore and Moncrieff 1989)
and favors intense and long-lived systems, if the lowlevel shear is strong enough, according to the Rotunno
et al. (1988) theory. More recently, Diongue et al.
(2002) conducted numerical simulations of the 21 August 1992 squall line observed during HydrologyAtmosphere Pilot Experiment in the Sahel (HAPEXSahel). Their analysis indicates a strong sensitivity to
moisture patterns in the free troposphere of the initial
state of the simulations, further emphasizing the importance of the structure of water vapor in the environment of the squall line for its development. This suggests a specific relationship between dry-air intrusions
and convection over West Africa that is also investigated in this paper. The monsoon season of 1992 is
selected for the study because it is a well-documented
period corresponding to the HAPEX-Sahel campaign
(Goutorbe et al. 1994) and was shown to be an typical
monsoon season with respect to rainfall variability (Redelsperger et al. 2002a, hereafter R02).
The paper is structured as follows: section 2 presents
the data and methodology used to reconstruct the water
vapor content and the origin of midtropospheric air
parcels. Section 3 presents a case-study analysis of a
strong convective event that occurred on 21 August
1992 and how dry-air intrusions from the extratropics
occur in the midtroposphere of its environment. Section 4 is dedicated to the entire 1992 African monsoon
period and an analysis of the link between dry intru-
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sions and convection. Finally, a summary and discussion are given.
2. Data and method
a. Satellite observations
1) INDICATORS
OF CONVECTION ACTIVITY
AND ORGANIZATION
Deep convection over West Africa spans a wide
range of degrees of organization: local convection,
slow- and fast-moving squall lines, stationary systems,
and families of mesoscale convective systems (MCSs)
that sometimes become as large as mesoscale convective complexes (MCCs; Laing and Fritsch 1997; Mathon
and Laurent 2001). This hierarchy of systems is presently analyzed using a cloud-tracking technique developed at the Centre National de la Recherche
Météorologique (CNRM) applied to Meteosat infrared
imagery. The Instrument de Suivi de l’Imagerie Satellitale (ISIS) algorithm is an automated method for
MCS identification and tracking developed by Morel
and Senesi (2002a) to derive a climatology of MCSs
over Europe using IR imagery (Morel and Senesi
2002b). It allows organized convective systems (e.g.,
fast-moving, long-lasting systems) to be separated both
in space and time from other convective events (e.g.,
short-lived and stationary systems). Here the tracking
takes place from July to August 1992, using half-hourly
full-resolution Meteosat IR data. This description of
convection is further completed by a simple convective
activity index built from the full-resolution imagery (5
km at nadir) consisting of the number of pixels colder
than a given threshold in a 0.625° ⫻ 0.625° grid. This
cold-pixel surface is computed for an IR threshold of
253 K, which represents the upper-level cloudiness in
the Tropics (e.g., Duvel 1988; Roca and Ramanathan
2000). The National Oceanic and Atmospheric Administration (NOAA) outgoing longwave radiation (OLR)
2.5° ⫻ 2.5° daily interpolated fields (Liebmann and
Smith 1996) are further used to document the seasonal
variability of convection over West Africa.
2) WATER
VAPOR IN THE MID- TO
UPPER TROPOSPHERE
Mid- to upper-tropospheric water vapor is documented using the Meteosat observations in the water
vapor channel (5.3–7.1 ␮m). This channel is centered
over a strong water vapor absorption band and, in clear
skies, is sensitive mainly to the mid- to uppertropospheric moisture. Following the approach of
Soden and Bretherton (1993) and Schmetz et al. (1995),
the brightness temperatures are inverted in terms of the
mean relative humidity of the clear column weighted by
the captor weighting function. This free tropospheric
relative humidity (FTH) corresponds to the whole free
troposphere, although it is the most sensitive to the
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mid- to upper troposphere (600–200 hPa). Roca et al.
(2003) presents the technique in detail. Figure 1 shows
the July–August mean-FTH distribution. The very dry
areas (FTH ⬍ 10%) span a large portion of the region
and correspond with the large-scale subsidence regions
(Picon and Desbois 1990). A strong gradient separates
the moist ITCZ (FTH ⱖ 40%) from the dry areas.
These observations offer an estimate of the average
tropospheric humidity but not its vertical structure.
Processing the satellite data are hence completed by an
estimation of the 500-hPa water vapor field as discussed
below.
b. Reconstruction of the midtroposphere water
vapor field
Investigating the water vapor distribution over Africa using atmospheric analysis and reanalysis remains a
challenge (e.g., Schmetz and van de Berg 1994; Salathé
and Chesters 1995; Vesperini 2002). For instance, the
European Centre for Medium-Range Weather Forecasts (ECMWF) 15-year Re-Analysis (ERA-15) fields
were compared to Meteosat water vapor data for the
summer 1992 and shown to suffer from a longitudinal
shift in the tropospheric dry structures that impact initial conditions for mesoscale simulations (Diongue et
al. 2002). In this data-void region, only a few soundings
are available and assimilated by the ECMWF and the
National Centers for Environmental Prediction
(NCEP) making it difficult for a model to reproduce
details of the moisture field. Note that recent advances
in the assimilation of satellite moisture data have
yielded much better results in operational analysis systems (Hólm et al. 2002). In order to investigate both the
water vapor distribution and the origins of air masses
over West Africa, we use an idealized advection–
FIG. 1. Mean FTH (%) derived from 3-hourly Meteosat water
vapor data for Jul–Aug 1992.
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subsidence model (Pierrehumbert 1998; Pierrehumbert
and Roca 1998, hereafter PR98) that allows for the
reconstruction of water vapor fields in the clear-air region of subsidence using back trajectories.
The model is fully described in PR98 and references
therein; so, here, we only give a brief overview of its
functions and underlying assumptions, as well as the
slight changes made to the original approach. This
model belongs to the so-called last temperature of saturation models to estimate the water vapor distribution
(Held and Soden 2000). This fully Lagrangian transport
scheme relies on analyzed winds and simplified moisture source/sink parameterizations. No diffusion is considered, neither are the complex processes associated
with the cloud microphysics of the moistening/drying of
the air mass. It is assumed that the entire moisture
source is located in a saturated boundary layer and that
deep convection transports saturated air upward where
it is detrained and made available for the large-scale
flow to transport away from this source region. The
transport scheme is run as a reverse domain-filling technique (e.g., Sutton et al. 1994) using back-trajectory
computations and allows the reconstruction of the humidity field at any given time and level in the atmosphere. The NCEP-analyzed winds (Kalnay et al. 1996)
are used to compute the 3D back trajectories. Instead
of diagnosing the vertical velocities from the horizontal
winds using the continuity equation, as in PR98, the
vertical velocities from NCEP are retained. While the
reconstructed field is slightly noisier in the latter case,
the major features of the recomputed moisture field, as
well as the ensemble trajectories, are not greatly
changed. Comparisons with another trajectory model
run with ECMWF winds (P. Peyrillé and J. P. Cammas
2002, personal communication) also did not modify the
large-scale patterns we consider in this study.
The trajectories are run backward in time until they
encounter deep convection. Instead of using ancillary
information, like a global satellite archive, to determine
convection it is simply diagnosed using the NCEP precipitation field. Not only is this an easy procedure for
isolating active convection, it also insures consistency
between the wind field and the convection indicator,
although it does suffer from the limitations of the analyzed precipitation field. A threshold of 1 cm day⫺1 is
used to determine when convection occurred. At this
point, the air mass is considered saturated and its mixing ratio is set to the saturation mixing ratio. Furthermore, the mixing ratio along the trajectory is reset to
the minimum between the saturation mixing ratio and
the current mixing ratio, the excess moisture implicitly
being rained out. As a result, the mixing ratio at a given
point corresponds to the saturation mixing ratio at the
coldest point encountered along the back trajectories.
This accounts for the cold-air processing of the air mass
in the extratropical upper levels. Once the mixing ratio
of the individual air mass is obtained, relative humidity
with respect to water is computed using the NCEP-
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analyzed temperature profiles. The model is integrated
at the 500-hPa pressure level using a resolution of 0.5°
⫻ 0.5° over the studied region.
As discussed in PR98, the individual level reconstructed field exhibits small-scale filamentary structures
that are not discerned in the Meteosat imagery. These
small-scale structures result from the advection of a
passive scalar by the large-scale flow. While the existence of these very finescale filaments (100 km wide) is
difficult to establish, the large dry patterns that we focus on appear as robust features of the water vapor
field. Indeed a sensitivity analysis of the time of back
advection indicated that the features to be discussed
next appear and remain in the recomputed field from 5
to 25 days. The smaller-scale features are more sensitive to the integration time and becomes finer as the
advection time increases. A trade-off between the filaments’ resolution and the central processing unit
(CPU) time to perform the computations has been
made. As a result, the back trajectories are integrated
over 12.5 days.
c. Evaluation of the reconstructed water vapor field
On average, during July–August 1992, the reconstructed 500-hPa relative humidity field exhibits largescale structures similar to both the ECMWF ERA-40
reanalysis and the NCEP reanalysis (Fig. 2). The three
estimates qualitatively resemble the Meteosat-derived
FTH field (Fig. 1) characterized by strong latitudinal
gradients and a dry region located over the eastern
Mediterranean Sea as well as a moist ITCZ. Overall,
ERA-40 exhibits a moister distribution than NCEP and
the reconstructed field over the Sahara. The meridional
gradients between the moist ITCZ and the dry central
Sahara are weaker in ERA-40 than in the two other
analyses. The Lagrangian-reconstructed moisture field
shows very moist conditions (RH ⬎ 80%) in the ITCZ
that are not seen in any other analysis. This is due to the
simple saturation assumption, which strongly moistens
the convective regions.
Quantitative evaluation of the product is conducted
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using soundings performed at 1200 UTC during July–
August 1992 over a dry region at Tamanrasset, Algeria
(22.7°N, 5.5°E) as well as over a typical Sahelian region
at Niamey, Niger (13.48°N, 2.17°E). Figure 3 shows the
scatter diagram of the difference between the NCEP
analysis and the radiosonde RH and between the reconstructed relative humidity field and the radiosonde
RH at 500 hPa. Overall the NCEP RH is biased moist
by 5% over Tamanrasset and dry by 10% over Niamey,
while the reconstructed RH is biased dry by around
10% in both cases. In the dry regimes (RH ⬍ 20%),
NCEP bias is positive at around 10% in both regions
while the reconstructed field bias changes sign and
reaches 3.1% and 6% for Tamanrasset and Niamey,
respectively.
In summary, the reconstructed water vapor field
agrees reasonably well with the radiosonde observations in the dry region over the Sahel. Unlike the analysis, such reconstruction gives access to smaller scales,
which are of interest to our study, with consistent quality over the entire African subsidence regions. It is independent of the assimilation of moisture information
from the soundings. These comparisons complete and
confirm the results of Pierrehumbert (1998) and PR98
concerning the ability of this approach to yield realistic
and consistent water vapor content in the free troposphere subsidence areas of the Tropics.
3. A case study of the 21 August 1992 squall line
The strongest convective event of the 1992 season
over the Sahel started on 21 August 1992 (R2002). The
squall-line life cycle is described as follows: individual
cells form over the Aïr Mountains late in the morning
of the 21 August and they slide down the slope of the
topography in a southwesterly direction. The individual
cells organize into a well-shaped squall line that propagates to the west at a speed of around 17 m s⫺1. The
mature stage of the system is reached at around 1600
UTC and the squall lasts through the next morning
(Diongue et al. 2002).
FIG. 2. Maps of Jul–Aug 1992 mean RH (%) at 500 hPa. (a) ECMWF ERA-40, (b) NCEP,
and (c) the reconstructed humidity field.
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FIG. 3. Scatter diagram of the difference between analyzed and radiosonde 500-hPa RH as
a function of the radiosonde RH (%) over (top) Tamanrasset and (bottom) Niamey. (a), (c)
NCEP and (b), (d) the reconstructed field. The vertical dashed line delimits the dry regime
(RH ⬍ 20%).
Large-scale and regional context of the case study
The West African monsoon (WAM) region exhibits
a number of specific local and large-scale circulations
features. These features are important to the understanding of the water vapor transport during the days
preceding the squall line and are summarized in Figs. 4
and 5. The tropical easterly jet (TEJ) is located at latitudes around 5°N with a zonal extension across the
whole region of study. Other features of interest include the subtropical jet (STJ) located over the northern part of the Himalayas at latitudes around 40°N. The
polar jet stream is located over the North Atlantic
Ocean at a latitude of 50°N. The jet core is centered at
50°W and the jet extends eastward up to 10°E. These
upper-level westerlies spread across a wide region in
latitudes down to 20°N over the Atlantic Ocean and
West Africa. Another interesting structure is the local
FIG. 4. Averaged 200-hPa circulation over 11–21 Aug 1992 from NCEP. Contours show the
wind module (m s⫺1). Topography above 750 m is shaded.
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FIG. 5. Zonal circulation for 11–21 Aug from NCEP averaged over the 10°W–5°E longitude band. Thin
contours show potential temperature (K) and thick contours show the zonal wind module (m s⫺1).
anticyclonic circulation centered over 25°N, 30°E. The
zonal mean vertical slab over the 10°W–5°E band of the
zonal wind, together with the potential temperature,
shown in Fig. 5 denotes these structures in the vertical.
The polar jet stream core is located at 250 hPa around
55°N with a maximum velocity of 26 m s⫺1. The entry of
the STJ is centered at 200 hPa around 30°N with a
smaller velocity (13 m s⫺1). The TEJ is limited to the
150–250-hPa altitude with a velocity around ⫺18 m s⫺1.
The African easterly jet (AEJ) is located at 15°N with
its core centered on 600 hPa with a velocity of ⫺9 m
s⫺1. Another important feature of the West African
climate is the deep dry convective layer over the Sahara, which extends equatorward to Sahelian latitudes.
This deep mixed layer reaches up to a 6-km height over
some regions (Gamo 1996) and is believed to complement the role of the ITCZ in maintaining the AEJ
(Thorncroft and Blackburn 1999). This mixed layer
prevents us from investigating the water vapor transport below 500 hPa with the present methods because
of the difficulties in establishing meaningful trajectories
in the strong mixing regimes. Before the squall-line occurrence, the deep mixed layer extends up to 600 hPa
over the 20°–30°N region (Fig. 5).
This large-scale context determines the water vapor
distribution in the regional environment of the squall
line. Over the 12.5°–17.5°N band where convection occurs, the whole 10°W–5°E region is very dry during the
day preceding the squall line, while in the eastern part
of the region, moist conditions associated with active
convection prevail to the south of the region (Fig. 6a).
Extratropical air (at a latitude greater than 40°N) intrudes into the region on the night of 20 August, then it
spreads into the region during the 21st, reaching a maximum during the squall-line mature phase at 1800–2400
UTC (Fig. 6b). To the north of the region (17.5°–
22.5°N), extratropical dry-air conditions prevail up to
10°E from the 20th (0000 UTC) to the 22d (0000 UTC).
It is then interrupted by a moist plume (Fig. 6c) that
originates from the squall line as evidenced by the low
latitudes of origin (Fig. 6d). Accordingly, we investigate
the water vapor distribution and origins at 1800 UTC
on 21 August when the squall is fully developed.
Figure 7 shows the Meteosat-derived FTH map. The
squall line is clearly seen at 15°N, 5°E as well as the
large-scale dry zones surrounding it. The large-scale dry
features (FTH ⬍ 20%) are separated by a moist plume
extending from the convection to the northeast of the
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FIG. 6. Hovmöller diagram around 21 Aug 1992: (a) reconstructed RH, and (b) maximum latitude of air mass at
500 hPa averaged over the 12.5°–17.5°N latitude band. (c), (d) Same as (a), (b) at 500 hPa but averaged over the
17.5°–22.5°N latitude band. Isolines correspond to the fraction of Meteosat pixels colder than 253 K.
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FIG. 7. Meteosat-derived FTH (%) for 1800 UTC 21 Aug 1992. Note that cloudy regions are white and that the
estimate could be contaminated by thin cirrus on the edges of the convective systems.
region previously mentioned. The satellite product indicates relatively dry regions, both ahead and behind
the squall line, that are better characterized using the
reconstructed moisture field. The 500-hPa relative humidity map indeed exhibits a number of dry features
(Fig. 8a). The first feature (referred to here and later as
A) is located over the 12°–20°N, 8°W–0° region on the
western flank of the squall line and corresponds to a
large dry region almost 800 km wide. Relative humidity
is as low as 5% and does not exceed 15% to 20%. The
second dry spot (referred to as B) is located on the
eastern side of the squall line around 15°N. The relative
humidity is greater than in the previous dry tongue with
values ranging from 5% to 30%. The horizontal distribution of relative humidity in this second tongue is less
homogenous than in the first one. A moist plume separates the dry region, in qualitative agreement with the
satellite information (Fig. 6). The large-scale mixing between the Tropics and the midlatitudes is demonstrated
in Figs. 8b and 8c, showing the spatial distribution of
the maximum latitude encountered by the particles during their flights. The moist plume is associated with
low-latitude air exported from the Tropics toward the
northeast. The dry region ahead of the squall line originates north of 60°N. The longitude coordinates of the
air particles when they reach their maximum latitude
during their flight is shown in Fig. 8c. Overall, the western part of the region appears to be fed by western air
masses while the opposite holds true for the eastern
region. However these two-dimensional views do not
reveal the whole path of air particles which is now assessed for the two key dry features (A and B) west and
east of the squall line, respectively.
Figure 9 shows an ensemble of back trajectories for
these two regions. The air masses feeding the region
(A; see Fig. 9a) all originate from the polar jet stream
between 300 and 200 hPa, which explains their relative
dryness. A close examination of the trajectories’ position relative to the polar jet (Fig. 4) indicates that they
always stay on the southern anticyclonic flank of the jet
centered at 50°N. This is confirmed by the fact that
these air masses exhibit low potential vorticity (PV),
characteristic of the upper troposphere, all along their
trajectories (not shown). The air masses are transported eastward within the polar jet up to around 0°
(longitude) where they make a strong southward turn
and start to subside. They take around 5 days to subside
to 500 hPa at a rate of 40–60 hPa day⫺1, which is in
agreement with magnitudes usually quoted for tropical
subsidence. Note that the ensemble of 25 trajectories
spanning a 2.5° ⫻ 2.5° region remains very compact
during the 5 days of subsidence indicating the robust-
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ness of this feature. On the eastern side, the situation is
less clear. The plume from the 25 trajectories reveals
three regions of origin for the dry feature (B; Fig. 9b).
One corresponds to the local anticyclonic circulation
seen in Fig. 5, where particles take up to 12 days to
recirculate from the upper level of the region north of
the ITCZ to east of the squall line. The air masses
originate from the 300-hPa level and subside slowly.
The second region of origin is from the eastern part of
the analyzed area and is characterized by a compact
flow possibly associated with the upper part of the AEJ.
Owing to their lower altitudes of origin and their history, these particles have a greater water vapor content
than the previous ones. The third region only concerns
a few back trajectories originating for the extratropical
upper level (200 hPa) with subsiding characteristics
similar to the one from the region (A) ahead the squall
line.
In summary, this case study reveals a large signal of
intrusion of extratropical dry air originating from the
anticyclonic side of the polar jet stream and extending
down to the midtroposphere in the environment just
ahead of a typical WAM fully developed squall line.
The occurrence of these intrusions and their links with
convection for the whole monsoon season is investigated next.
4. Dry-air intrusions and convective activity during
the 1992 monsoon
The 1992 season corresponds to the HAPEX-Sahel
campaign (Goutorbe et al. 1994) and was shown to be
a typical monsoon with respect to the rainfall variability
over Sahel including synoptic (3–5 days) and longer
(10–25 days) variability (R02). First we present the
mean structure of convective activity and airmass origins. Then, we analyze the relative humidity variability
and link it to the large-scale dynamics as well as to the
convective activity.
a. Mean structure of convective activity and of
airmass origins
FIG. 8. Map of (a) relative humidity, (b) maximum latitude, and
(c) longitude at maximum lat for 1800 UTC 21 Aug 1992, at 500 hPa.
Areas where Meteosat IR temperature is colder than 273 K are
indicated with black horizontal lines. Relative humidity for initially ascending trajectories are not reported and are shown in white.
During the monsoon, the convectively active regions
form a zonal structure corresponding to the ITCZ
(shaded area in Fig. 10). Two centers of high-convective
activity are located over the eastern part of the domain
(15°E) at a latitude of around 7.5°N and over the Atlantic Ocean west of Guinea, respectively. The OLR
variability (isolines in Fig. 10) is at its highest on the
northern flank of the ITCZ centered on the large gradient of convective activity. The maximum variability
(standard deviation of 35 W m⫺2) appears over the
Sahelian region (12.5°–17.5°N, 10°W–5°E). Figure 11
shows the characteristics of the convective systems during the season. All the systems contribute up to 50 h
month⫺1 of occurrence over the centers of convection
previously highlighted (Fig. 11a). The distribution of
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FIG. 9. Three-dimensional view of an ensemble of back trajectories initially located at 500
hPa centered at (a) 15°N, 6°W and (b) 16°N, 18°E. The color scale indicates the duration of
the air mass along the trajectory in days since 1800 UTC 21 Aug 1992.
the systems with a life cycle longer than 12 h (Fig. 11b)
corresponds to the region where the convective variability is at its greatest (Fig. 10). These long-lasting systems make up more than 60% of the convective activity
over the Sahel. Figure 11c shows that the mean westward speed of these systems reaches up to 16 m s⫺1 on
average. Long-lasting systems are less common on the
southern flank of the ITCZ and move more slowly.
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acterized by two bimodal behaviors: dry air can originate from tropical/extratropical regions (Pierrehumbert
1998) and dry and moist air coexists at small and large
scales (Zhang et al. 2003). In the present case over the
Sahel, for example, dry-air structures that interact with
convection can originate from the upper levels either
from the Tropics or from the PJ. Similarly, saturated
moist air from local convection can be found simultaneously all over the region with dry air (e.g., Fig. 8a).
This strong bimodality in water vapor and origins precludes the use of a simple regional average to characterize the relative humidity distribution and its variability. Defining quantitatively what could be a dry intrusion remains a difficult task that is attempted in the
next section.
FIG. 10. Map of the mean Jul–Aug 1992 OLR (W m⫺2). Darkest
shading is 180 W m⫺2, and shading bands correspond to a 10 W
m⫺2 increment. Isolines show standard deviation of the daily
OLR over the period. The black rectangle highlights the key Sahelian region discussed in the text.
These findings are in agreement with previous climatological studies (Lebel and Lebarbé 1997; Mathon and
Laurent 2001) and confirm the typical characteristics of
the 1992 season (R02) for the MCS distribution. Thus,
over the Sahelian region convective activity is dominated by organized fast-moving convective systems,
lasting more than 12 h, presumably squall lines. Note
that this region corresponds to the mean position of the
AEJ (Fig. 5) resulting in strong low-level wind shear
underneath, which, with dry air is a key element for the
fast-moving squall-line occurrence.
Figure 12 shows the spatial distribution at 500 hPa of
the airmass fraction with an extratropical origin (the
maximum latitude encountered north of 40°N) for July–August. The distribution of the extratropical contribution is mainly meridional with a steady increase poleward from 10% at 12.5°N up to 80% at 35°N. Nevertheless, a strong nonmeridional structure is centered at
25°N, 10°E. This commalike feature indicates that more
than 70% of the air mass over Tamanrasset at 500 hPa
originates from the midlatitudes. The 20% isoline extends south down to 14°N over Sahel, where the variability in convection is at a maximum, as indicated by
the OLR variance overlaid in Fig. 11. Over this region,
these extratropical air masses originate from the southern anticyclonic flank of the PJ at 50°N, 250 hPa (Fig.
13). On average, these trajectories lie almost on the
isentropic surfaces starting at the 330–340-K level at the
jet altitude and reach the Sahel region over the 325–
330-K level where strong low-level wind shear prevails
below the AEJ, about 5 days later. The other regions
where the Sahelian midtropospheric air masses originate include the tropical latitudes with both midtropospheric and upper-tropospheric sources.
This large-scale mixing influences the distribution
and variability of the Sahel midtroposphere’s relative
humidity. Indeed, in the Tropics, water vapor is char-
b. Variability of the midtropospheric relative
humidity and airmass origins
Considering the 12.5°–17.5°N, 10°W–5°E Sahelian
region, a dryness index is built as the percent of the grid
points with RH below 5%. The mean value of the index
is 15% and its standard deviation is 11%. The index
varies from 0% to 55% during the season with no clear
seasonality (Fig. 14a). Figure 14b also shows the time
series of the fraction of extratropical air over the same
Sahelian region. The fraction ranges from 0% to 100%
over the season and exhibits a smooth seasonal cycle
with its greatest point toward the end of July. By defining the dry (moist) event as conditions where the
dryness index is greater (lower) than the seasonal mean
plus (minus) one standard deviation, one can further
inspect the link between dryness and origin. All of the
dry events are associated with a positive seasonal
anomaly of the extratropical fraction and all the moist
events are associated with a negative anomaly. On the
other hand, not all the positive (negative) anomalies in
the extratropical fraction are associated with a dry
(moist) event. In particular, the 18 July event (Fig. 14b)
does not produce a strong positive anomaly in the dryness index. Although this extratropical intrusion is associated with dry air, it is not dry enough to show up in
the dryness index. The case study of 21 August is associated with a 2-day dry intrusion where the dryness
index reaches 40% even though the extratropicalfraction anomaly is modest and involves only a third of
the region. The 23 July case, on the other hand, yields
a similar value of the index, with twice as much of an
extratropical fraction as that for the case study. These
examples remind us of the complex variability associated with the water vapor and the airmass origin and
confirm that the dry events are associated with intrusions of extratropical air in the region.
c. Dry-intrusion events and convective activity
In order to link relative humidity and convective activity, we need to account for the strong diurnal cycle of
convection observed over West Africa. Our case-study
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401
FIG. 11. Convective system distribution and characteristics for the Jul–Aug 1992: (a) occurrence of all systems
(h month⫺1), (b) occurrence of systems lasting more than 12 h (h month⫺1), and (c) mean westward speed (m s⫺1)
of systems in (b).
analysis reveals that dry air from the extratropics exists
over the Sahel for a whole day before the squall line
occurs (Fig. 6). So we propose to define a dry-intrusion
(moist exhaust) period as the 24 h period starting from
0800 UTC and continuing to the next morning (0800
UTC) when a dry (moist) event occurs; 0800 UTC corresponds to the time when convective activity is minimum in this region. Using this definition, the shadings
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FIG. 12. Spatial distribution of the 500-hPa airmass fraction (%) with an extratropical origin (max lat ⬎ 40°N)
over Jul–Aug 1992. The OLR variance from Fig. 10 is overlaid.
in Fig. 14 highlight the eight dry-intrusion periods
(dark) and seven moist-exhaust periods (light) that are
found during the 1992 season. The dry intrusions last
from 1 to 4 days and the moist exhausts from 1 to 3
days. The moist exhausts exhibit more stable structures
since they are always associated with almost no extratropical fraction and less than 7% of the region being
dryer than 5%. These moist exhausts could be related
to tropical surges over western North Africa as discussed by Knippertz et al. (2003), but a detailed analysis
of their fate is beyond the scope of this paper. Using
these two types of events, the composite of Meteosat
cold-cloud pixels is built for days with dry intrusions
and days with moist exhausts. In the dry-intrusionevents composite (Fig. 15a), the cold-pixel fraction indicates strong convective activity northeast of the region while the moist composite (Fig. 15b) shows an
opposite pattern with maximum convection on the west
side of the region. The difference between the dry and
the moist events (Fig. 15c) shows that the eastern reinforcement of convection is about 10%–15% in coldpixel fraction. The western decrease reaches also 10%–
15%. This is a significant modulation of the convective
activity, which in terms of cold-pixel fraction, reaches
about 30%. The composite analysis is further down-
scaled to the individual convective systems scale. When
dry intrusions are present over the Sahel, the systems
occurrence is enhanced on the eastern part of the region (Fig. 15d), and decreased in the south and west of
the region (Fig. 15e) in a similar way to the cold cloudpixel composite. South of 13°N all types of convective
systems occur less frequently during dry-intrusion
events. The next dry phase composite indicates a strong
occurrence of the long-lasting systems (with life cycles
greater than 12 h) on the eastern Sahel from 5°W to 5°E
(Fig. 15g) over the region where, on average, these systems are more frequent (Fig. 10). The moist phase is
associated with an increase of the long-lasting systems
occurrence on the western Sahel and south of the Sahel
(Fig. 15h). As a result, the composite of the different
dry-intrusion minus moist-exhaust periods (Fig. 15i)
shows a strong reinforcement of the long-lasting, fastmoving systems on the eastern Sahel during dryintrusion events, which explains a large part of the
mean distribution seen in Fig. 10. There between 5°W
and 5°E, the deep convection overall positive anomaly
during dry-intrusion events appears to be mainly due to
the long-lasting systems’ positive anomaly. Changing
the threshold of the dry-intrusion selection from 5% to
10% and keeping the same definition does not alter
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403
FIG. 13. Zonal circulation and thermal structure of the atmosphere from NCEP averaged over the 10°W–5°E
longitude band and the Jul–Aug period. Thin lines correspond to potential temperature (K) and thick lines to the
zonal wind module (m s⫺1). Shading represents the trajectories’ density in arbitrary units for the air mass with
trajectories ending at 500 hPa in the Sahelian target region (12.5°–17.5°N, 10°W–5°E). The density is integrated
over the 45°W–45°E longitude domain. The lighter shading corresponds to the higher density.
the patterns discussed above or the magnitude of the
signal.
It appears that the relationship between dry intrusions and convective activity over West Africa is more
complex than previously found over the west tropical
Pacific where the inhibiting effect of dry air is dominant
(Sherwood 1999; Parsons et al. 2000), and where dryintrusion events are associated with inactive phases of
convective activity (Brown and Zhang 1997). Over the
eastern Sahel, dry-intrusion events are associated with
more long-lasting and fast-moving MCS activity than
during moist exhausts; on the other hand, over the
western Sahel convective activity is diminished overall
by dry-intrusion events. Thus, the intense variability of
the convective activity over the Sahel region seems to
be related to the dry-intrusion occurrence that is associated with the long-lasting and fast-moving convective
systems.
5. Summary and discussion
Using a Lagrangian water vapor transport model, the
moisture distribution of the midtroposphere over West
Africa has been investigated together with the occurrence of the convective systems. The new findings of
this study are the following:
• Dry air structures (RH ⬍ 5%) exist in the Sahelian
midtroposphere and originate from the anticyclonic
flank of the polar jet stream.
• These dry-intrusion events are associated with longlasting convective systems, likely squall lines, over
the eastern Sahel.
This highlighted relationship between convection
and dry intrusions is not easy to explain and relies on
the following physical reasons. Midtropospheric humidity and low-level wind shear have been shown to be key
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FIG. 14. Time evolution over the 12.5°–17.5°N, 10°W–5°E region: (a) the surface fraction of
the grid points with RH (%) below 5%. The horizontal thick line is the mean and the thin lines
the mean plus or minus one standard deviation. (b) The extratropical (max lat greater than
40°N) fraction over the region (%). The thick line is the seasonal cycle corresponding to the
first two harmonics of the filtered signal. The dark shading indicates dry-intrusion periods and
the light shading the moist-exhaust periods. See text for details.
ingredients for intense and long-lived convective systems. This is well supported both by observations
(Zipser 1977; Barnes and Sieckman 1984) and by modeling studies (Redelsperger and Lafore 1988; Lafore
and Moncrieff 1989; Rotunno et al. 1988) resulting in
conceptual models and a theory. One important characteristic of the Sahel region during the monsoon period is that both the convective available potential energy (CAPE) and the convective inhibition (CIN) are
large (Diongue et al. 2002), so that specific processes
are needed to trigger and maintain the convection. As
extensively developed here, this region is affected by
midlevel dry-air intrusion events and by strong lowlevel shear induced by the AEJ fulfilling the necessary
conditions for strong and long-lived convective systems.
Based on these considerations we propose the following two-step scenario. First, a combination of forcing
mechanisms such as topography, strong diurnal cycles
over land, easterly waves, etc., allows the onset of local
convection within an environment that is dry in the
midtroposphere. The forcing is supposed to be strong
enough to break the CIN and it is further assumed that
enough CAPE is available for these cells to develop
(see Diongue et al. 2002 for a discussion of the earlier
stage of the 21 August 1992 case). In the second step,
these cells organize themselves into a convective line
that turns into a mature squall line because of favorable
conditions (CAPE, shear and dry air) encountered over
the region. At this time hydrometeor evaporation in the
dry midtroposphere fuels a density current and favors
the new cells’ development. Due to a good balance
between the low-level shear and the cold pool, the latter can become large, widespread, and well-organized
so that it can supply long-lived convection even in regions of strong CIN as well as during the night when
convective instability (or CAPE) is reduced. This scenario differs from the interaction of dry intrusions and
convection over the warm pool where the inhibiting
effect of dry air dominates (Sherwood 1999; Parsons et
al. 2000). We suggest that it is due simply to the fact
that the shear is generally weak over the warm pool so
it is not possible to build up efficient cold pools, except
on particular days when shear is present as observed
during TOGA COARE (LeMone et al. 1998).
The present analysis investigates the water vapor distribution and origins at a large scale but lacks the very
finescale details in the vicinity and within mesoscale
convective systems. The use of cloud resolving models
should make it possible to investigate the fate of dry air
in the environment very close to convection and establish how active this dry air is in these convective systems
so as to confirm the above scenario.
The analysis indicates that the long-lived, fastmoving systems are related to the dry intrusions. Most
of the Sahelian seasonal rainfall is produced by organized mesoscale convective systems, mainly squall lines.
FEBRUARY 2005
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405
FIG. 15. Composite maps of convection indices for the dry-intrusion and moist-exhaust phases. Cold-cloud
fraction at 253 K (%): (a) dry phase, (b) moist phase, and (c) dry minus moist. Occurrence of all the convective
systems (h month⫺1): (d) dry phase, (e) moist phase, and (f) dry minus moist. Occurrence of the convective systems
lasting more than 12 h (h month): (g) dry phase, (h) moist phase, and (i) dry minus moist.
As a result, the dry intrusions are expected to influence
rainfall and its variability over the Sahel through their
influence on the squall lines. Over the Sahel, rainfall is
characterized by two important modes of variability at
synoptic and suprasynoptic time scales (R02). The
large-scale circulation associated with the WAM is indeed perturbed at the synoptic scale (3–5 days) by African easterly waves (AEW) associated with the barotropic and baroclinic instabilities of the AEJ (Burpee
1972). These synoptic scales are also seen in the rainfall
variability (Reed et al. 1977; Diedhiou et al. 1999), although the link between these AEWs, the squall lines,
and rainfall is complex (Duvel 1990; Fink and Reiner
2003). Sultan et al. (2003) recently documented the second mode of variability between 10 and 20 days of rainfall over West Africa. The link between this suprasynoptic mode and squall lines is yet to be investigated.
The period of analysis is currently too short to fully
extract the spectral characteristics of the dry intrusions.
Nevertheless, from the present 2-month analysis, it appears that dry events occur in series that are wellseparated from each other. Figure 14 indicates three
dry-event series (early July, late July through the be-
ginning of August, and mid-August). This intraseasonal
variability of the dry intrusions could explain the 10–
20-day mode of variability of the Sahelian rainfall. Furthermore it could also affect the unclear relationship of
the AEW and squall lines and hence modulate the
AEW-rainfall efficiency (Diedhiou et al. 2003; Fink and
Reiner 2003). A long-term climatology of the Sahelian
dry intrusions should be established to provide the
needed database to investigate the relationship between their temporal characteristics and rainfall variability.
The other finding of this study concerns the extratropical origin and the trajectory of the dry midtropospheric air over the Sahel that raises many questions
about the dynamical processes at play in the midlatitudes. The analysis of such processes is clearly beyond
the scope of the present paper, and a detailed analysis
of the dynamics of this midlatitudes air outbreak will be
provided in another paper (D. Raymond 2004, personal
communication). Nevertheless, it is useful to stress that
Sahelian dry intrusions exhibit different characteristics
than intrusions from previously documented regions.
Previous analysis of the warm pool dry-intrusion origins
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suggests the instabilities of the boreal winter, Northern
Hemisphere subtropical jet, and the corresponding
Rossby wave breaking as one dynamical mechanism
associated with the dry-air intrusions (Yoneyama and
Parsons 1999; Yoneyama 2003). Here dry intrusions
originate from the upper troposphere and from the anticyclonic side of the polar jet. They are associated with
weak PV anomalies and interact in a specific way with
the subtropical jet, which differs from the warm pool
intrusion mechanism. Systematic comparison of the PV
anomaly and midtropospheric dry layer (e.g., Yoneyama 2003 for the warm pool) over Africa and Europe
could further our understanding of the dynamical processes at play in these extratropical dry-air outbreaks.
Such effort could complete previous analyses that
linked the upper-tropospheric high-PV intrusions with
convection over the tropical oceans (e.g., Kiladis and
Weickmann 1992; Kiladis 1998; Waugh and Polvani
2000; Waugh and Funatsu 2003) and contribute to form
a more complete picture for dry intrusions on a global
scale.
Acknowledgments. Discussions with D. Ramond, J.
P. Cammas, and P. Peyrillé on the midlatitude circulation are appreciated. The authors thank H. Brogniez
for her help with the Meteosat free tropospheric humidity data. The authors are grateful to C. Morel, S.
Senesi, and L. Labatut whose support and expertise
have been helpful in applying the ISIS algorithm to
Africa. The authors are deeply indebted to R. T. Pierrehumbert for providing us his model. This study was
partially funded by the Programme National Atmosphere et Océan à Méso-échelle of the French Institut
des Sciences de l’Univers et de l’Environnement. The
anonymous reviewers are acknowledged for their comments that helped us to clarify the paper.
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