African Easterly Wave dynamics in a mesoscale numerical model: Part (ii):
Synoptic modulation of convection.
Gareth J. Berry and Chris Thorncroft.
University at Albany, State University of New York.
Corresponding author: Chris?
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Abstract.
Physical mechanisms that describe the observed spatio-temporal relationship of African
easterly waves (AEWs) and convective systems are explored using a numerical weather
prediction model simulation. This simulation, described in a companion paper, is
conducted for a single case from September 2004 using the mesoscale weather research
and forecasting (WRF) model. Using hypotheses laid out in previous research, specific
pathways for the synoptic organization of convection by the AEW are investigated.
The results suggests that hypotheses from previous research, including adiabatically
forced ascent, synoptic advection of the environment and the AEW thermodynamic
structure all promote convection with the observed distribution. In this study an analytical
distinction is made between processes that promote the triggering of new convection and
those that act to modulate the intensity of that that already exists. The results are
presented in a probabilistic framework and indicate that new convective cells are more
likely to be triggered near the AEW trough axis due to the synoptic scale distribution of
low tropospheric moisture and temperature within the AEW. Conversely, it is suggested
that convection is likely to have maximum amplitude ahead (west) of the AEW trough
axis due to favorable synoptic scale modification of the environment in this region. A
conceptual model of the average convective system lifecycle within an AEW that fits
well with previous research is also presented.
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(1) Introduction.
In this paper, the 50km horizontal resolution numerical simulations of an African
Easterly Wave (AEW) case, conducted in part (i) of this research , is used to further
examine the relationship between the synoptic AEW and convection. Part (i) considered
the upscale impact of deep convection on the synoptic dynamics of the AEW, using a
PV- thinking approach. This work found that the aggregate effect of convection was an
essential part of the AEW structure. In this paper the aim is to look more closely at the
AEW-convection relationship and scale interaction in the opposite sense by examining
the downscale impact that the synoptic AEW has on the location of convection.
It has been long established that AEWs have a modulating effect on West African
precipitation (e.g. Simpson et al, 1968) and a coherent spatial relationship with the
occurrence of organized mesoscale convective systems (MCSs; e.g. Fink and Reiner,
2003). Composite AEW structures presented in the literature (e.g. Carlson (1969), Reed
et al, (1977)) tend to show that precipitation and deep convection tends to be more
prevalent south of the African Easterly Jet (AEJ) axis and close to, or slightly ahead of,
the mid-tropospheric AEW trough axis. By objectively tracking both AEWs and squall
lines during two summers, Fink and Reiner (2003) found that the squall line genesis
frequency was peaked close to or slightly ahead of the AEW trough and squall lines
tended to move faster than the parent AEW, before dissipating in the ridge proceeding the
synoptic AEW, consistent with what has been seen by Berry and Thorncroft (2009).
While the spatial relationship is well established, the mechanisms responsible for
providing this relationship are one of the least well understood and controversial aspects
of AEWs. To date, relatively few studies have presented results that relate the synoptic
AEW to the triggering and maintenance of deep convection. At present, there are three
main plausible hypotheses put forward by recent research:
(i) Synoptic scale ascent forced by the AEW adiabatic AEW structure.
Following concepts originating in mid-latitude meteorology it is often predicted that
ascent exists at low-levels ahead of the ahead of the synoptic AEW trough. Thorncroft
and Hoskins (1994a, b) demonstrated that low-level adiabatically forced ascent exists
ahead of idealized AEWs. The mechanism casing this vertical motion can be
conceptualized in a number of ways, such as using the quasi-geostrophic equations, PVThinking (Hoskins et al, 1985). Perhaps the simplest concept is that of isentropic motion
as the AEW are mid-level cyclonic circulation centers that propagate through the West
African monsoon that to first order consists of low-level isentropes that slope towards the
surface with increasing latitude (see e.g. Hall and Peyrille, 2006). The synoptic scale
circulation associated with the AEW would promote isentropic ascent to its west and
descent to its east, consistent with the observed location of maximum convection.
Previous studies that have examined adiabatic motion (e.g. Thorncroft and Hoskins, 1994
a, b) have found that its magnitude tends to be small (of the order 3-7 hPa h-1). These
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values are orders of magnitude less than a typical updraft in the planetary boundary layer,
which casts some doubt as to whether this adiabatic effect is significant.
(ii) Synoptic scale advection of the background environment.
On the basis of a case study of a single AEW using global analysis products, Berry and
Thorncroft (2005) suggested that the synoptic scale advection of the low level
temperature and moisture distribution by the AEW was able to alter the thermodynamic
stability of the atmosphere and favor convection near the AEW trough. Here, the low
level (~925hPa) circulation associated with a mature AEW acts in the correct sense to
advect the zonally orientated monsoon structure (see their Fig. 3) to produce equivalent
potential temperature anomalies in the locations where convection is most frequently
observed. However, this study only considered a single, exceptionally intense event and
only inferred the convective instability from a single level, rather than examining the
thermodynamic structure of the entire troposphere.
(iii) Synoptic scale ‘cold core’ of AEWs.
Following the results of Jenkins (1995), who determined that the AEW trough region
required the presence of a low level cold core to maintain the dynamical balance of the
AEW, Mapes (1997) hypothesized that a local reduction in convective inhibition due to
this would allow convection to develop near the trough more easily. This interpretation is
conceptually simple and fits well with the PV analysis shown in part (i), as a cold core
must exist below the mid level PV maxima that are characteristic of AEWs. The primary
shortcoming of this hypothesis is that it suggests that convection should be peaked near
the AEW trough, rather than ahead of the trough as in observations (e.g. Reed et al,1977).
The aim of this paper is to use the numerical model output from the experiments
performed in part (i) to investigate the mechanisms that lead to convection being initiated
and maintained in the regions close to and ahead of the AEW trough. As the verification
presented in part (i) demonstrated the simulation is realistic, it is hoped that the results
will provide answers that address the missing part of the scale interactions between AEW
and convection. This paper is organized as follows; section 2 describes the methodology,
section 3 presents the main results and these are discussed in detail with a synthesis and
conceptual model presented in section 4.
(2) Methodology.
The research conducted in this paper uses the same numerical simulations conducted in
part (i) of this research using simulations from the weather research and forecasting
(WRF) model for analysis. The simulations were conducted for a single AEW case from
September 2004 using a 50km horizontal resolution domain and a full suite of physical
parameterizations. A perturbation experiment was conducted in which the cumulus and
microphysical simulations were switched off half way through the model integration to
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provide a ‘dry run’ (i.e. without moist processes) to aid further analysis. A full
description of these simulations is found in part (i).
As was noted in part (i), the WRF simulation was able to capture the gross spatiotemporal
relationship between mesoscale convection and the synoptic AEW that was observed in
satellite imagery and analysis products. Here, the WRF model output will be examined in
the framework of the three hypotheses described in the introduction to indentify key
dynamic or thermodynamic quantities which explain the location and timing of
convection. Whilst specific convective ‘bursts’ will be shown in this analysis, the precise
cause of their triggering will not be examined in great detail. Instead, the approach used
here is to identify synoptic scale characteristics that promote the increased probability of
MCS genesis and maintenance in the mean location. This is because organized deep
convection can and does exist over tropical North Africa in the absence of synoptic
AEWs and will aid application in other instances.
Following the convention used in part (i), an AEW relative domain is again used in this
analysis. This domain measures 2000x2000km and is centered upon the point at which
the mean objective AEW and AEJ axes (see Berry et al, 2007) intersect one another
(dubbed the “AEW Centre”). The terminology “t+xhrs” is again used to describe the time
since the beginning of the original 50km simulation. For orientation, time averaged
quantities from the WRF simulation are reproduced in this system relative domain in
Fig.1. The mean OLR (Fig. 1(a)) and precipitation (Fig. 1(b)) show that the most active
convection is latitudinally confined to the region from the objective AEJ to
approximately 500km equatorward (-500km on the y axis in Fig.1). Based on this,
quantities will be examined averaged over this range of latitudes, which is henceforth
referred to as the “convective region”.
(3) Results.
The results from part (i) (c.f. Fig. 1) have demonstrated that the mean spatial distribution
of convection and precipitation in the WRF simulation closely matches that seen in
composite structures. The temporal distribution of deep convection is summarized in Fig.
2, which shows the lowest outgoing longwave radiation (OLR) values, maximum vertical
motion and mean diabatic heating rate from the cumulus parameterization at 8km, in the
2000km x 2000km system following domain used in Fig. 1. The 8km level is used due to
the vertical motion and heating in the organized MCSs being peaked near this level.
Figure 2 indicates a strong variation of the convective activity on the diurnal timescale,
with the onset of deep convection occurring near local noon (annotated by dashed line on
the figure).
Maps of OLR and surface topography at the approximate onset of the each diabatic
heating impulse from Fig. 2(c) are shown in Fig. 3 in order to give an overview of how
the triggering of simulated convective systems is related to the AEW and topography.
The frames each indicate the location of organized convective systems with letters to
enable the individual convective systems to be followed between frames. The first
convective system, labeled ‘A’ is triggered at t+48hrs and shown in Fig. 3(a). This
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system is triggered near the trough axis (in the centre of the domain) over relatively low
and flat terrain, close to the southern edge of Lake Chad, and dissipates approximately 12
hours later. At t+72hrs (the second impulse of diabatic heating in Fig. 3(b)) the second
and third convective systems (labeled ‘B’ and ‘C’) are triggered as the parent AEW
passes 8°E. The westernmost system (‘B’) is initiated approximately 200km ahead (west)
of the mean AEW trough longitude, over relatively flat terrain. The eastern of the two
systems (‘C’) is initiated approximately 100km behind (east) of the mean AEW trough
longitude, near the isolated Jos Plateau, which rises to heights over 1km. These two
convective systems are both long-lived and propagate westwards with the AEW and
reach the West African coast a few days later. The next diurnal pulse of diabatic heating
(at t+96hrs in Fig. 2(c)) does not initiate any new convective systems, only the
reinvigoration of those that already exist. At t+120h (Fig. 3(d)) the long lived systems
(‘B’ and ‘C’) have propagated further ahead of the AEW trough and undergo
enhancement over the elevated terrain of the Guinea highlands. At this stage another
convective system (labeled ‘D’) is initiated approximately 100km ahead (west) of the
AEW trough axis, over the Guinea highlands.
To examine the first hypothesis linking the synoptic AEW to the distribution, whereby
convection is forced by the adiabatic AEW structure, the isentropic ascent is computed in
the box following the AEW centre. A time mean cross-section, averaged over the
‘convective region’ of the total isentropic vertical motion is shown in Fig. 4(a). Overlaid
on this figure is the water vapor mixing ratio, shown as a zonal deviation across the same
range of latitudes to emphasize where the most active convection is located in the mean.
The justification for looking only at the isentropic vertical motion as opposed to e.g.
quasi-geostrophic vertical motion as these types of calculation tend to have a dependence
on the Coriolis parameter and the gradients of PV, which are very small compared to
typical midlatitude values. Figure 4(a) very clearly shows that the entire area ahead of the
trough between 1 and 4km is characterized by isentropic ascent, maximized near 2km,
and the region behind the trough is dominated by isentropic descent, maximized near
1km. The peak values are of the order 5 cms-1, similar to the values found in the idealized
modeling of Thorncroft and Hoskins (1994a). A horizontal map of isentropic vertical
motion at 2km (the level of maximum ascent) is shown in Fig 4(b). This reveals that the
convective region is on the southern fringe of a synoptic scale dipole of the isentropic
vertical motion. It is evident that the largest values are located in the dry environment
poleward of the AEJ, where the isentropic surfaces are highly tilted (see e.g. Hall and
Peyrille, 2006). The mean three dimensional isentropic vertical motion field is consistent
with the conceptual description of a synoptic scale circulation imposed on a sloping
isentropic surface.
In order to give an appreciation of how the isentropic motion evolves during the AEW
lifecycle, a Hovmöller space-time diagram of isentropic vertical motion, averaged in
latitude over the convective region is shown in Fig. 4(c). This diagram differs from a
traditional Hovmöller diagram as rather than showing a fixed geographical region, the
domain moves with the AEW, such that AEW centre is always in the middle of the
domain (at x=0km). The locations of the convective systems at the start of the diabatic
heating impulses (see Fig. 2(c) and 3) are signified by the corresponding letters on this
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Hovmöller diagram. During the first 70 hours of integration a clear diurnal modulation
across the entire range of relative longitudes is observed in, with ascent maximized
during the overnight hours. This large scale signal is in association with the strengthening
of the meridional monsoon circulation in the overnight hours as part of the diurnal cycle
of the monsoon (Parker et al, 2005). During the remainder of the integration, following
the triggering of convective systems ‘B’ and ‘C’ the development of a synoptic scale
dipole of isentropic vertical motion can be observed, such that by the end of the
integration there is a very clear difference either side of the trough axis with ascent ahead
of the AEW trough axis and descent behind. The period from approximately 70 hours
onwards was noted in Chapter 6 as where the synoptic scale AEW attained its mature
structure and began to intensify (see part (i)).
In terms of the convective systems that are triggered in the WRF simulation, there is
evidence that isentropic ascent occurs at this level in the few hours prior to the genesis of
three of the four convective systems (B, C and D), perhaps suggesting that isentropic
motion may be acting to provide a more suitable environment by synoptic scale cooling
of the mid troposphere. However, this ascent occurs across much of the domain, without
specific maxima near the genesis locations. After triggering, the mesoscale impact of
each convective system on the isentropic motion can also be observed in Fig. 4(c) by
mesoscale maxima of isentropic ascent moving westwards relative to the AEW trough.
This reflects the relatively large impact each convective has on the potential temperature
and sub synoptic wind fields. It is suggested that this enhancement is associated with
local intensification of the meridional flow along the sloped monsoon isentropes and also
the isentropic motion resulting from the PV anomalies associated with the convective
systems propagating in sheared flow (see Raymond and Jiang (1991)).
To fit with the probabilistic viewpoint laid out in the methodology section, Fig. 4(d) is a
cross section of the isentropic vertical motion in the convectively active region, but
displayed as a zonal deviation across the domain, rather than absolute values. This figure
shows that in terms of the relative amounts, the isentropic ascent is peaked at the lowest
levels near 400km and that the line of zero zonal anomaly has a baroclinic tilt up to the
AEJ level (approximately 3.5km). When the output from the ‘dry run’ perturbation
experiment is displayed in the same manner (Fig. 4(e)) and account is made for the slight
changes in the structure (particularly the deformation of the mesoscale PV anomalies
which is indicated by the overlaid mixing ratio contours), the same synoptic pattern
remains. This comparison shows that this is a mechanism driven by the synoptic scale
adiabatic AEW that has the capability of modifying the nature of convection on the
synoptic scale.
In many aspects of tropical meteorology, anomalous low level divergence on the synoptic
scale is often cited as a potential forcing for anomalous regions of precipitation and
convection. For example, the geographic distribution of convection with respect to
equatorial wave modes (see e.g. Wheeler and Kiladis, 1999) is directly linked to low
level convergence maxima in idealized solutions (e.g. Matsuno, 1966). For direct
comparison with other studies, the divergence in the WRF simulation is examined for the
presence of a coherent synoptic scale pattern. Note that this is a non-hydrostatic
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simulation of a real case so that even though there may be some overlap between
isentropic ascent and divergence, they do not have to vary in phase with one another.
Figure 5(a) shows a cross section across the convective region the mean divergence as a
function of height and relative longitudinal distance. This figure indicates that in the
mean at low levels (below 2km) there is a broad region of convergence ahead of and
close to the AEW trough. A horizontal map of mean divergence at 1km is shown in Fig.
5(b), which clearly indicates a region of enhanced convergence that almost exactly
corresponds to the region of mean precipitation (Fig. 1(b)). Whilst this may appear to be
a straightforward relationship between the two, sub-synoptic scale convective systems
have their own divergence field (see e.g. Houze, 2004). Therefore it is of paramount
important to consider if this mean divergence has a significant contribution from the
convective systems themselves. Figure 5(c) shows a Hovmöller diagram of the
divergence field averaged over the convective region as a function of forecast time and
relative longitude. It is evident from this that outside the regions influenced by the
convective systems there is little evidence that a coherent synoptic scale pattern of
divergence exists. The convergence maxima are all closely associated with the individual
convective systems, consisting of mesoscale maxima that move relative to the trough
axis. Aside from these regions of convergence that are associated with the convective
systems, there are also two other significant convergence maxima that move eastwards
relative to the AEW trough, that start near x=-800km near t+50h and t+110h,
respectively. Because a system relative domain has been used, these maxima are actually
fixed in space and correspond to the location of the Jos plateau and the Guinea highlands.
These localized extrema can result purely from the presence of elevated heating, but the
Hovmöller diagram shows that the amplitude this geographically fixed convergence is
maximized ahead of the trough from approximately x=-700km to the trough axis. With
reference to the mean low level flow shown in Fig. 1(b) this is approximately where the
maximum low-level northerly winds occur, suggesting that there may be a mechanical
interaction between this flow associated with the AEW and the topography that further
boosts the probability of convective triggering.
Because the convective systems dominate the divergence field in Fig. 5(c), the ‘dry run’
perturbation experiment provides a good alternative to look for underlying synoptic
patterns. Figures 5(d-f) provide the same fields as Fig. 5(a-c) for the dry run experiment.
The mean cross section of divergence (Fig. 5(d)) shows convergence at low levels ahead
of the trough, divergence within the trough PV maximum (which is outlined
approximately by the positive mixing ratio perturbation and convergence at mid levels
behind the trough. Accounting for the distortion of the mean PV distribution via
advection, this distribution is reminiscent of the idealized vertical motion through a PV
maximum in vertically sheared flow (Raymond and Jiang, 1991). The mean horizontal
map of divergence at 1km in the ‘dry run’ experiment (Fig. 5(e)) is dominated by a
couplet of convergence and divergence poleward of the AEJ that is associated with the
interface between the Saharan and monsoon air masses. Equatorward of the AEJ there is
some semblance of a synoptic pattern that indicates convergence ahead (west) of the
AEW and divergence behind (east). The time evolution, summarized by the Hovmöller
diagram (Fig. 5(f)) most clearly shows the development of a synoptic distribution with
the region ahead of the AEW trough exhibiting convergence.
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Following the second hypothesis described in the introduction for the synoptic scale
modulation of convection by AEWs, maps of the mean system relative convective
available potential energy (CAPE) and convective inhibition (CIN) are shown in Fig. 6(a)
and (c) respectively. Here, the CAPE is defined as the total integrated positive buoyancy
for a parcel rising pseudo-adiabatically from the lowest layer and the CIN the total
integrated negative buoyancy. Each of these quantities are integrated between 1000 and
200hPa and the virtual temperature correction (Doswell and Rasmussen (1994)) has been
applied to the parcel ascents. The mean CAPE (Fig. 6(a)) is peaked in the region
approximately 500km equatorward of the AEJ axis, at the same latitude as the time-mean
convection (see Fig. 1). Within the entire monsoon region (i.e. equatorward of the AEJ)
the mean CAPE exceeds 1000 JKg-1. Peak values approaching 2000 JKg-1 are found
slightly equatorward of the AEJ core, where equivalent potential temperatures are
maximized (c.f. Berry and Thorncroft (2005), Williams and Renno (1993)). Poleward of
the AEJ axis there is a rapid decrease in the mean CAPE values associated with the
sloping interface between the monsoon and dry desert environments. In the zonal
direction there is a distinct wave like perturbation to the northern edge of the high CAPE
values that is consistent with the pattern of advection that is inferred by the low-level
winds in Fig. 1(b), with a poleward extension of higher CAPE values behind (east) of the
mean AEW trough. A minimum relative to the zonal mean is apparent in the region of
active convection (values decreased by approximately 100-300 JKg-1), consistent with the
release of the available potential energy within the convection.
The mean CIN distribution (Fig. 6(c)) is essentially the mirror image of the CAPE, with
highest values occurring poleward of the AEJ, over the Sahara, and low values within the
monsoon region. Looking more closely near the location of the AEW trough and mean
convection, there is significantly more CIN immediately ahead of the AEW trough (of the
order 100 JKg-1) than there is behind it (of the order 50 JKg-1), which aligns with the lowlevel temperature distribution seen in Fig. 1(a). This difference can be attributed to the
increased presence of Saharan air, advected equatorward by the mid level circulation of
the AEW. Assuming that CIN is a relevant quantity to the triggering of convection, this
shows that the synoptic scale AEW has a potential thermodynamic control on its
distribution. System relative Hovmöller diagrams of the CAPE and CIN values are shown
in Fig. 6(b) and (d). Both indicate a distinct diurnal cycle in the quantities with CAPE
peaking during the local afternoon hours and CIN peaking in the local morning,
consistent with regular daytime surface heating (recall the simulation was started at
12UTC, so that t+24, 48, 72, 96 and 120hrs corresponds approximately to local noon).
All the convective systems that are annotated on the figures are initiated when the CAPE
is high and the CIN is low, as expected. Once the convective systems form, mesoscale
signals associated with them can be observed in the CAPE and CIN fields, with lower
CAPE values higher CIN values being closely associated with them. This pattern arises
as the convective systems ‘consume’ the CAPE and generate surface cold pools that
increase the CIN on the storm scale. Some semblance of a synoptic pattern may be
discerned after the formation of the convective systems labeled ‘B’ and ‘C’ and before
the western part of the domain crosses the West African coast (near t+125h). In this time
period, CIN values are significantly reduced on the synoptic scale behind the AEW
trough. This occurs at the same time as the isentropic motion develops a strong synoptic
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dipole (Fig. 4(c)) and the synoptic scale system starts to amplify (see part (i)), suggesting
that there may also be a synoptic signal in the thermodynamic fields.
Although CAPE and CIN are well established metrics that are often used to explain or
predict the spatial-temporal variability of convection, it is important to recall that they are
vertically integrated measures of atmospheric stability and thus the direct interpretation
and attribution required in this study is made difficult. It is entirely possible to have a
thermodynamic sounding with a large amount of CAPE that can never be realized in
reality as it may exist in a layer well above the Earth’s surface. Because there is a
potential synoptic scale signal in these fields the buoyancy of a lifted parcel will be
considered as this further explains the CAPE/CIN distribution and allows insight as to
whether there are significant thermodynamic controls on the triggering (as opposed to
maintenance) of convection.
A time mean zonal cross-section, averaged across the convective region, of parcel
buoyancy is shown in Fig. 7(a), where buoyancy is defined as the virtual temperature
difference between a parcel rising pseudo-adiabatically from 1000hPa and the
environment. As might be expected for any thermodynamic diagram in a region that is
characterized by high CAPE and deep convection, the cross section shows that there is a
large amount of buoyancy present over the entire region throughout most of the
troposphere. Either side of the AEW trough (at x=0km) the level of maximum buoyancy
occurs near 700hPa consistent with the minimum environmental equivalent potential
temperatures occurring near this level. At all points there is positive buoyancy from
lowest layers of the atmosphere up to approximately 400hPa, showing that the entire
convective region (across all phases of the AEW) is theoretically capable of supporting
deep convection.
With regards to the initiation of deep convection and where it is more probable, the
buoyancy in the lowest layers must be considered as deep convection is rooted in the
boundary layer (Houze, 2004). From Fig. 7(a) there is a discernable tilt of the buoyancy
contours at low levels that hints the buoyancy is higher toward the rear (eastern) half of
the AEW. This is displayed more succinctly in Fig. 7(b), where the buoyancy is shown as
a deviation from the zonal mean. Here it can be noted that below 800hPa there is a clear
synoptic dipole where buoyancy is lower ahead of the AEW trough than it is behind it.
The corresponding mean map of buoyancy at 900hPa is shown as a zonal deviation in
Fig. 7(c) and summarizes this synoptic scale difference. Again, to show the evolution of
this field, a Hovmöller diagram in system relative coordinates of zonal deviation of the
buoyancy at 900hPa is shown in Fig. 7(d). The figure shows that the genesis of all four
convective systems in the WRF simulation occurs when there is anomalous positive
buoyancy at this level. Consistent with the isentropic motion and divergence fields, this
figure also shows that a synoptic scale dipole develops during the intensification phase of
the AEW (from approximately t+72h onwards), suggesting that this pattern is linked to
the developments at the synoptic scale.
On Fig. 7(d) there are mesoscale regions of negative (relative) buoyancy that appear to
emanate from the convective systems that have been annotated on the figure. It is
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suggested that these are the signal of surface cold pools spreading from the active
convective systems as they move both eastwards and westwards relative to the AEW
trough. It is therefore possible that the synoptic signal may arise from numerous cold
pools from the active convection aliasing onto the synoptic scale rather than being a true
synoptic scale feature. To examine this, the mean buoyancy the ‘dry run’ experiment is
examined. Further analysis of the sub synoptic scale features in the ‘dry run’ simulation
(not shown) suggests that the convective cold pools are eliminated rapidly (<24 hours) by
surface heating, mixing and advection. The mean cross section of the zonal deviation of
buoyancy in the ‘dry run’ experiment is shown in Fig. 7(e) and the corresponding
Hovmöller diagram is shown in Fig. 7(f). When compared to their counterparts from the
original simulation (Fig. 7(b) and (d)) these figures show this synoptic scale dipole even
more clearly, showing that these synoptic scale signal in the buoyancy field is
consequence of the synoptic scale AEW structure rather than the net effect of the
convective systems.
(4) Synthesis and discussion.
In this paper, numerical simulations conducted using the WRF model have been
examined in order to determine if mechanisms that lead to the synoptic scale modulation
of the triggering of deep convection can be discerned. This was motivated by consistent
observations that mean convection is maximized in the region close to and ahead (west)
of the mid level AEW trough axis. Rather than examining the triggering of each
convective event in great detail, a probabilistic approach was taken so that it could be
argued that a particular synoptic region was more or less favorable for the triggering or
maintenance of deep convection. This approach was used as it is recognized that deep
convection does not exclusively occur in association with AEWs, but does exhibit a
relationship in both observations (see e.g. Reed et al (1977)) and this simulation (see part
(i)) and also to allow the results and conclusions to be used in other instances.
The results have shown that the temporal distribution of the genesis of convective
systems was strongly tied to the diurnal cycle, with all convective systems being initiated
after local noon. Comparison with climatology (e.g. Duvel, 1989) indicates that this does
not stand out as different to African convection in the absence of AEWs, suggesting that
the AEW does not have a significant impact on the timing of convection. This result also
fits well with the work by Fink and Reiner (2003) who suggested that convective systems
associated with AEWs had no notable differences from those that were not. Therefore it
is argued that the AEWs primarily impact the spatial distribution of convective triggering
and the eventual lifecycles of convective systems. They appear to have little contribution
on the temporal distribution of triggering, which is predominantly governed by solar
heating.
A synoptic scale signal was noted in the isentropic motion field, which amplified at the
same time as the growth phase of the synoptic AEW (c.f. Fig .5 in part (i)). This result
showed a dipole around the AEW trough with ascent ahead (west) of the AEW trough,
within the latitudes of the convective region. Analysis of the horizontal maps indicated
that this pattern is conceptually consistent with motion along the sloping isentropic
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surfaces within the West African monsoon that would be imposed by a synoptic scale
mid level cyclonic circulation. Although there is this distinct synoptic pattern, the mean
values are of the order 0.05ms-1 which is similar to that found in the idealized simulations
of Thorncroft and Hoskins (1994a). These vertical velocities are two orders of magnitude
smaller than might be expected from a convective updraft in the boundary layer (e.g.
Greenhut et al, 1982). The cross section of the isentropic vertical motion (Fig. 4(a))
shows that these maximum values occur near 2km, near the top of the planetary boundary
layer and also at the relative longitude where the CIN is relatively high. Put together
these factors imply that the isentropic vertical motion is unlikely to be directly linked to
the triggering of convection as it would take of the order of half a day to lift an air parcel
from the surface to its level of free convection.
There is a paradox that also precludes isentropic motion being the primary mechanism
responsible for the observed distribution of convection, as the isentropic ascent
intensified with as the amplitude of the synoptic AEW increased. It was determined in
part (i) using the ‘dry run’ that the AEW does not grow without convection, therefore
convection is required for the AEW to reach a sufficient amplitude for this isentropic
mechanism to be strong enough to trigger convection. This does not indicate that
isentropic motion associated with the AEW does not have a significant role in
determining the nature of the convection. Instead it is suggested that this synoptic scale
vertical motion acts on a longer timescale to partially offset the stabilization of the
atmosphere by the active convection and the impact of the Saharan air layer in the region
ahead of the trough. It is argued that this effect would lead to convection being more
vigorous and long lived but is unlikely to directly impact the triggering of new convective
systems.
In the original simulation, no synoptic pattern was detected in the low-level divergence
field, as it is dominated by the sub-synoptic divergence associated with the active
convection. However, using the ‘dry run’ perturbation experiment a synoptic scale signal
was found that broadly shows convergence ahead of the AEW trough and divergence
behind. This is consistent with the observed distribution of convection and it is speculated
that this is forced adiabatically by the synoptic AEW. The divergence Hovmöller
diagrams (Fig. 5(c and f)) also indicated the presence of persistent areas of convergence
moving through the AEW relative domain that are tied to topographic features. Although
convergence associated with an elevated heat source is expected, these convergence
maxima are amplified as the AEW passes. This suggest that there are additional physical
processes (e.g. flow blocking or channeling) that may further enhance the probability of
convection being triggered in the leading part of the AEW as it encounters high terrain.
The enhancement or triggering of deep convection ahead of an AEW trough in
association with high terrain is noted in this simulation as the AEW approaches the
Guinea highlands and also in real observations (e.g. Berry and Thorncroft, 2005).
The mean CAPE and CIN maps shown in Fig. 6 do show a distribution that is consistent
with advection of the basic state by the mean low-level flow that is shown in Fig. 1(b).
More precise analysis of the thermodynamic stability using the buoyancy of a lifted
parcel has shown a significant difference between the regions either side of the AEW
12
trough at the lowest levels (within the boundary layer). The interpretation of this synoptic
pattern of relatively buoyancy comes from nature of both the temperature and moisture
fields as the ascent of lifted parcel is governed by both these factors. From the mean
synoptic structure in both the original simulation and the dry run (Fig. 1 and Fig. 14 of
part (i)) it is clear that in the region ahead of the trough there is the equatorward
advection of Sahara air over the monsoon layer by the adiabatic AEW structure. Because
the Saharan air layer is both extremely warm and dry it presents a thermodynamic barrier
and thus strongly negative buoyancy anomalies at low levels ahead of the AEW trough.
The baroclinic configuration of the low level potential temperature and mid level
potential vorticity structure means that at the mid level trough axis there is little
meridional advection of the Saharan air layer, suggesting that the CIN values are not
substantially different from the environment in the absence of AEWs. Coupled to this is
the presence of a relatively cold lower troposphere imposed by the adiabatic AEW
structure (i.e. a cold core beneath a positive PV anomaly, c.f. Jenkins, 1995).
Consequently the buoyancy at low levels is increased, which implies that the probability
of convective triggering from air parcels originating in the planetary boundary layer is
larger.
It is evident from the results presented in this chapter that the mechanisms that have been
postulated by previous studies to promote deep convection (described in section 1) are all
present in this case and do act in the correct sense to promote convection close to and
ahead of the AEW trough. This confirms that the hypotheses from previous research that
were investigated all present viable ways of influencing the location of new convection or
the intensity of existing convection. In this research a distinction has been made between
convective triggering and maintenance as factors conducive to the maintenance of deep
convection does not necessary have to be favorable for its formation. From these results it
has been hypothesized that the synoptic scale adiabatic dynamics (in this case isentropic
motion and divergence) act on too long a timescale to impact the triggering of new
convective systems by the lifting of individual air parcels. It is argued in this case that the
probability of triggering is linked instead to the thermodynamics at low levels (i.e. the
buoyancy) that are substantially modified by the presence of the synoptic AEW It is
suggested that the dynamical effects are more important in the mature phase of
convective systems, where they work to offset the tendency for the atmosphere to
stabilize, via large scale ascent and cooling that enables the convection to be more intense
and long-lived that it otherwise would be.
On the basis of these results it is possible to present hypotheses that combine these
modeling results with observations from previous research (e.g. Fink and Reiner (2003),
Berry and Thorncroft (2009), etc) to explain the mean relationship between AEWs and
convection. Again, in these arguments the distinction is underlined between the triggering
of new convective systems and the maintenance of those that already exist. Figure 8
present a simple schematic of the main arguments, based on the results presented in this
paper and part (i), used to describe how the synoptic AEW influences the triggering and
maintenance of deep convection. An idealized cross section through the ‘convective
region’ is shown in Fig. 8(a) and changes in the probability of the triggering and
maintenance of deep convection are shown in Fig. 8 (b and c).
13
For the triggering of a new convective system, air parcels from the boundary layer must
be lifted to their level of free convection (approximately 850hPa, 1.5km in the convective
region). As shown in the results the low-level ascent attributed to the dynamic factors are
weaker than turbulent updrafts in the boundary layer, so are unlikely to play a major role.
In the absence of other forms of mechanical lifting (e.g. by cold pools, lake breeze
circulations etc.), it is postulated that the probability distribution is likely to reflect the
relative buoyancy in the lowest layers of the atmosphere, which is a product of both the
potential temperature and moisture anomalies. As shown schematically in Fig. 8(b) the
temperature structure tends to tilt westwards with height (as in a typical baroclinic wave)
whereas the moisture field would be tilted eastward with height due to the maximum
moisture being located poleward. Under the AEJ level ridge (denoted by the letter ‘R’ in
Fig. 8(a)) the strongest surface northerlies are present due to the baroclinic tilt of the
AEW. The lower troposphere above this point is both relatively warm (in part due to the
mid level negative PV anomaly) and dry (due to the advection of Saharan air), which
implies that a rising parcel has a reduced probability of reaching its level of free
convection. Under the maximum AEJ level northerlies (‘N’ in Fig. 8(a)) there may be
some impact of the cold core beneath the upstream positive PV anomaly, but the mid
levels are relatively dry due to equatorward advection of the Saharan air layer at the AEJ
level, meaning that the probability of triggering is still relatively low. Near the trough
axis the maximum surface moisture and mid level temperature anomalies occur, meaning
that the probability of triggering is maximized. Where the maximum AEJ level
southerlies occur (marked ‘S’ in Fig. 8(a)), the maximum mid level moisture anomalies
occur and the lower troposphere is still relatively cool thus the probability of convective
initiation is still relatively high.
The net change in probability of convective triggering (shown by the dashed line in Fig.
8(b)) is therefore essentially comprised of a sine wave that is slightly skewed towards the
east due to the baroclinic tilt of the AEW (whereby the temperature and moisture
anomalies tilt in opposite directions) and slightly modified by the weak dynamic factors.
Essentially, it is hypothesized that probability of triggering is maximized in the region
between the AEJ level trough and maximum southerlies as a consequence of the tilted
synoptic scale temperature and moisture anomalies.
It is suggested that the probability of maintaining an existing system as a function of
AEW phase (Fig. 8(c)) is substantially different. Once a convective system is triggered
and attains steady state, it is able to mechanically lift boundary parcels (e.g. by gust fronts
etc) so that the relative importance of the low-level buoyancy is reduced. The intensity of
a mature convective system in this region is more likely to be dependent on the mid-level
winds and thermodynamics (see Houze, 2004), as the convective downdrafts
predominantly originate at theses levels. Since the net effect of convection is to drive the
atmospheric temperature profile toward neutrality, it is suggested that the large scale
ascent forced by the adiabatic structure acts to partially offset this tendency. It is
therefore hypothesized that the probability of maintaining an existing MCS is more
peaked immediately ahead of the AEW trough where the synoptic scale ascent and
moderate amounts of mid-level dry air (for the generation of downdrafts) exist. This
probability distribution is shown schematically by the lines in Fig. 8(c).
14
Based on this schematic and the physical characteristics of West African convective
systems (see e.g. Chapter 7), the lifecycle of an average convective system associated
with an AEW can be postulated. In the absence of geographic features, a convective
system is triggered close to the AEW trough axis in response to maximized low level
buoyancy (consistent with the observations from Fink and Reiner (2003)). Because of the
nature of the West African monsoon environment this convective system would rapidly
grow to become a squall line type feature and propagate westwards relative to the AEW
due it its intrinsic phase speed. This results in the convective system moving through the
region where the probability of maintaining a convective system is maximized, causing
the convective system to peak in amplitude. As the convective system moves further
westwards relative to the trough it would encounter a less favorable environment
associated with meridional advection of Saharan air and begin to weaken.
Although this is a very simple conceptualization of an average MCS embedded within an
AEW, it fits very well with the observations of MCS behavior. It must be reiterated that
this is a probabilistic argument that assumes that the large scale environment is uniform.
In reality the actual probability distribution is going to depend on these synoptic factors
as well as many local factors such as topography, differences in the land surface, local
circulations (e.g. lake breezes) and pre existing boundaries. The key for future case
studies and especially forecasting is to blend this synoptic understanding with knowledge
of how important these local factors are to further understand or predict the nature of
AEWs and their associated convection.
15
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17
Figure captions.
Figure 1 – Mean synoptic maps from the WRF simulation. (a) Top of atmosphere
outgoing longwave radiation (Wm-2, filled grayscale) overlaid with 3.5km potential
vorticity (colored lines), lkm potential temperature (thin black lines), 3.5km objective
trough axis (thick black line) and 3.5km objective jet axis (thick red line). (b) Mean
precipitation rate (mm day-1, shaded according to legend), Streamlines at 1km (blue
streamlines) and wind vectors at 3.5km. The same objective trough and jet axes as in (a)
are overlaid. Data are shown in system relative coordinates (plot axes labeled in km) and
the first 24 hours of simulation are discarded for spin up.
Figure 2 – Time series averaged over a 2000x2000km system relative box, centered on
the objective AEW centre of (a) Mean outgoing longwave radiation (OLR, units of Wm2
) (b) Maximum Vertical velocity at 8km (units of ms-1) and (c) Diabatic heating rate
from the cumulus parameterization at 8km (units of K day-1). Local noon is signified by
the dashed vertical lines in each panel.
Figure 3 – Maps displaying topography (colored in m) overlaid with winds (barbs in ms1
) and objective trough and jet axes (thick black and red solid lines, respectively) at
3.5km with Outgoing longwave radiation (thin black contour shown every 25Wm-2 below
200Wm-2) at (a) 48 hours (b) 72 hours (c) 96 hours and (d) 120 hours into the WRF
simulation. The locations of convective systems discussed in the text are indicated by the
letters (A-D).
Figure 4 – (a) Time mean cross section of isentropic vertical motion (colored in units of
ms-1) and the zonal deviation of water vapor mixing ratio (black contours in units of g/kg,
dashed negative) in the convective region, defined as between y=-500 and y=0 km. (b)
Map of time mean isentropic vertical motion at 2km overlaid with the mean 3.5km
objective trough and jet axes (thick black and red lines). (c) A relative latitude versus
forecast time Hovmöller space time diagram of isentropic vertical and zonal deviation of
water vapor (same convention as panel (a)) with the locations of convective systems
annotated by the letters (A to D). (d) As panel (a), except showing isentropic vertical
motion as a zonal deviation. (e) As panel (d) except from the ‘dry run’ perturbation
experiment (see text for details).
Figure 5 – (a) Time mean cross section of divergence (colored in units of 10-4 s-1) and
the zonal deviation of water vapor mixing ratio (black contours in units of g/kg, dashed
negative) in the convective region, defined as between y=-500 and y=0 km. (b) Map of
time mean divergence at 1km (colored in units of 10-4 s-1) overlaid with the mean 3.5km
objective trough and jet axes (thick black and red lines). (c) A relative latitude versus
forecast time Hovmöller space time diagram of divergence at 1km and zonal deviation of
water vapor at 1km (same convention as panel (a)) with the locations of convective
systems shown by the annotations (A-D). (d-f) as panels (a-c), except for the dry run
perturbation experiment.
18
Figure 6 – (a) Time mean map of convective available potential energy (CAPE, colored
in units of Jkg-1) overlaid with the mean 3.5km objective trough and jet axes (thick black
and red lines). (b) A relative latitude versus forecast time Hovmöller space time diagram
of CAPE and zonal deviation of water vapor at 900hPa (black contours in units of g/kg,
dashed negative) with locations of convective systems annotated by letters (A-D). (c-d)
as panels (a-b) except showing Convective inhibition (CIN).
Figure 7 – (a) Time mean cross section of buoyancy (colored in units of K) and the zonal
deviation of water vapor mixing ratio (black contours in units of g/kg, dashed negative)
in the convective region, defined as between y=-500 and y=0 km. (b) as panel (a), except
showing the zonal deviation of buoyancy. (c) Map of time mean zonal deviation of the
buoyancy at 900hPa (colored in units of K) overlaid with the mean 3.5km objective
trough and jet axes (thick black and red lines). (d) A relative latitude versus forecast time
Hovmöller space time diagram of the zonal deviation of buoyancy and water vapor at
900hPa (same conventions as panel (b)) with the locations of convective systems
annotated by the letters (A to D). (e and f) As panels (b and d), except from the ‘dry run’
perturbation experiment (see text for details).
Figure 8 – Conceptual model describing the spatial relationship between the synoptic
AEW and convection. (a) Schematic west-east cross section across the convectively
active region of an AEW. The AEJ level ridge, maximum northerly wind, AEW trough
and maximum southerly winds are indicated by the letters ‘R’, ‘N’, ‘T’ and ‘S’
respectively. The AEJ level PV anomalies are indicated by the labels PV+, PV- and
isentropes are shown by the black horizontal lines. Maximum flow into and out of the
page at the AEJ level and the surface are indicated by the letters ‘x’ and ‘o’. Axes of
maximum and minimum moisture are indicated by the green annotations. (b) Graph
showing the change in the probability of convective triggering as a function of the AEW
phase shown in panel (a). The grey line is the modulation of probability due to synoptic
scale dynamic factors (i.e. isentropic vertical motion, divergence etc), the black line is the
modulation due to thermodynamics (i.e. buoyancy) and the black dashed line is the
combined probability. (c) As in (b) except showing the change in probability of
maintaining an existing convective system.
19