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Q. J. R. Meteorol. Soc. (2005), 131, pp. 603–623
doi: 10.1256/qj.03.201
Observational synthesis of mesoscale structures within an explosively
developing cyclone
By K. A. BROWNING∗
Department of Meteorology, University of Reading, UK
(Received 4 November 2003; revised 6 October 2004)
S UMMARY
An observational case-study is presented of a rapidly developing UK cyclone on 30 October 2000 to reveal
the rich diversity of coexisting mesoscale structures in the vicinity of the cloud head, dry slot and polar-front
cloud band. The mesoscale structures include: multiple rain bands, due to upright convection originating in the
boundary layer; stacked slantwise convective circulations at middle levels; cloud-top striations, due to upright
convection originating at high levels above the polar-front cloud band; multiple sub-structure within the cloud
head; and inertia–gravity waves in the region of the tropopause fold. The emphasis of the analysis (synthesis) is
on the inter-relationships between the various mesoscale structures. Issues raised by this study include: (i) the
interaction between upright and slantwise convection, including the effects of evaporation in the dry slot; (ii) the
possible roles of inertia–gravity waves and conditional symmetric instability in triggering/organizing the dry-slot
convective rain bands and the sub-structure within the cloud head; and (iii) the relationship between the dryslot rain bands and the multiple structures within the cloud head. This study is limited to documenting these
relationships; controlled experiments with high-resolution mesoscale models will be required to establish cause
and effect and to assess their importance.
K EYWORDS: Cloud head Convective rain bands Dry slot Inertia–gravity waves
1.
I NTRODUCTION
Recent studies of extratropical cyclones in the UK have identified several different
types of embedded mesoscale phenomena, all of which tend to occur in the form of
multiple circulations or wave trains. These include:
• multiple rain bands due to upright convection originating in the boundary layer
(Browning et al. 1997a, 2002; Browning and Roberts 1999);
• stacked slantwise convective circulations (Browning et al. 2001a,b (the latter is
referred to hereafter as BDGW));
• cloud top striations due to convection originating at high levels (Dixon et al. 2000;
Browning and Wang 2002, hereafter BW);
• cloud heads with multiple sub-structure (Browning et al. 1997b);
• inertia–gravity (IG) waves in the region of tropopause folds associated with the
developing cyclone (Browning et al. 2002).
Most of the cyclones in the above studies were intense, and a preliminary impression
has been gained that the richest diversity of mesoscale sub-structures does occur in
association with the most intense cyclones.
On 30 October 2000 a rare opportunity occurred to study all these phenomena
coexisting in a cyclone that was developing explosively just as it was passing over the
dense surface and weather radar networks in the UK, and also over a VHF and two
UHF wind-profiling radars. This has enabled not only the structure and evolution of
the individual mesoscale phenomena to be analysed, but also their interrelationships.
Because of the coexistence of so many phenomena the overall structure was very
complex; thus the writer made a start by publishing separately the evidence for some
of the individual phenomena (Browning et al. 2001b; Browning and Wang 2002).
∗
Corresponding address: Joint Centre for Mesoscale Meteorology, Department of Meteorology, University of
Reading, PO Box 243, Reading, Berkshire RG6 6BB, UK. e-mail: K.A.Browning@reading.ac.uk
c Royal Meteorological Society, 2005.
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K. A. BROWNING
Figure 1. Met Office surface pressure and frontal analysis at 0600 UTC 30 October 2000. Isobars are at 4 hPa
intervals.
The purpose of the present paper is to bring together a summary of the results from
those studies along with other data for this storm, in order to show the overall mesoscale
structure. Interesting relationships are found between the different types of mesoscale
structure. Some of these relationships are already well established, such as a line of
upright convection feeding a band of slantwise convection. Other relationships are found
that are less well known; these include relationships between convective lines, cloudhead sub-structures, and IG waves possibly descending within a major tropopause fold.
Some ideas regarding causal links are suggested, but controlled experiments with a highresolution mesoscale model will be required to establish their validity. The subtlety of
the mesoscale interactions will, however, severely test the capability of even the best
current models, so it is important to document the observed phenomena in order to
provide a dataset with which the models may be validated and developed; this paper is
a first step in this process.
2.
T HE CASE - STUDY
(a) Context
The cyclone of 30 October 2000 was a rapidly deepening cyclone that underwent
frontal fracture according to the life-cycle model of Shapiro and Keyser (1990). It
deepened by 60 hPa in 24 hours as it travelled east-north-eastwards towards and across
the British Isles, and attained an estimated depth of 941 hPa in the North Sea during the
early afternoon. The present study focuses on an earlier period as the cyclone crossed
Wales and western England. Figure 1 shows the surface analysis at 0600 UTC when
the cyclone was over England with a central pressure of 958 hPa, having deepened by
18 hPa in the 6 hours while travelling from the south coast of Ireland.
SYNTHESIS OF MESOSCALE STRUCTURES
605
Figure 2. Surface wet-bulb potential-temperature, θw , analysis for 0600 UTC 30 October 2000, with individual
station reports plotted in ◦ C (an adjacent dot means an increment of 0.5 degC). Note that the intense surface cold
front, passing through Cherbourg and the Isle of Wight, does not meet the surface warm front (as does the cold
front shown in Fig. 1); this is a case of frontal fracture, and air with high θw extends within a shallow moist zone
rearwards to the bent-back front over Wales close to the cyclone centre (marked L).
The analysis of surface reports for 0600 UTC, in Fig. 2, shows values of wet-bulb
potential temperature (θw ) of 13 ◦ C ahead of the main surface cold front, which was
sharply defined where it extended from Brittany to central southern England. North
of this, high-θw air with values of up to 12 ◦ C extended westwards into a region near
the England–Wales border; this was later to become cut off as a warm seclusion. The
western boundary of this high-θw region was marked by a bent-back front, across parts
of which θw dropped abruptly by as much as 7 ◦ C. The high-θw region just ahead of the
bent-back front was actually a shallow moist zone (SMZ) which was being overrun by
dry-intrusion air to give a pronounced ‘dry slot’, as shown in the satellite imagery.
Figure 3 is a Meteosat infrared image from a polar orbiting satellite at 0630 UTC.
It shows the dry slot over the England–Wales border, with a large area of polar-front
cloud to the south-east of it, warm-frontal cloud to the north and a cloud head bounding
it to the west. It is shown later that the extent of the dry intrusion was significantly
greater than that of the dry slot evident in Fig. 3, because part of it was undercutting
a deck of high cloud associated with rearward sloping ascent in the polar-front cloud
band. Three convective rain bands analysed in this paper were associated with this dry
intrusion; a shallow part of one of the convective bands occurred where the dry intrusion
was visible from satellite imagery as a dry slot, but the two other convective bands were
entirely masked by the deck of high cloud and could be seen only by means of radar.
Another feature shown in Fig. 3, but mentioned here only in passing, is the
series of transverse cloud-top striations orientated north-west to south-east above the
polar-front cloud bands in central England. According to BW these are attributable to
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Figure 3.
K. A. BROWNING
Meteosat infrared image for 0630 UTC 30 October 2000. See key for brightness temperatures (◦ C).
upper-tropospheric convection organized in rolls perpendicular to the underlying strong
thermal-wind shear.
Figure 4(a) shows a weather radar network plot of the rainfall distribution at 0630
UTC , close to the time of the satellite image in Fig. 3. It shows a rather larger dry slot
than in Fig. 3, because it is not affected by the upper-level cloud deck that partially
obscures the dry slot in the satellite image. Of course, the absence of precipitation
does not necessarily imply a true dry slot, i.e. the total absence of cloud, but evidence
given later shows that the precipitation-free air in this region lies beneath a precipitation
layer which does imply evaporation within dry air below it. This does not rule out the
possibility of non-precipitating cloud at some even lower level within this part of the
dry slot. To the south-east of the dry slot in Fig. 4(a) there is a large region of rain
which is accounted for below. To the north-west of the dry slot in Fig. 4(a) is the region
of rain associated with the storm’s large cloud head; the rain extends to the very edge
of the cloud head on its south-eastern side but stops far short of the cloud edge on its
north-western side owing to evaporation at low levels.
Figure 4(b), covering exactly the same area and time as Fig. 4(a), shows the
corresponding conveyor-belt analysis carried out as in Bader et al. (1995) and Browning
(2003). According to the conveyor-belt paradigm, the system-relative flow within a
cyclone can be visualized conveniently in terms of four principal flows. These consist
of primary and secondary warm conveyor belts, a cold conveyor belt and a dry-intrusion
flow. Figure 4(b) shows the following features that characterize the overall structure of
the cyclone system.
SYNTHESIS OF MESOSCALE STRUCTURES
607
Figure 4. For the cyclone at 0630 UTC 30 October 2000: (a) the distribution of rainfall from the weather radar
network (see key for rainfall rates); and (b) a schematic of conveyor-belt and rain band analyses. In (b): the broad
arrows show the system-relative flow of the primary and secondary warm conveyor belts (labelled W1 and W2)
and the cold conveyor belt (CCB); dotted arrows represent the dry intrusion (DI); surface warm and cold fronts
are shown by conventional symbols, and the upper cold front by open symbols. The sharp surface cold front was
associated with line convection labelled C1; two other convective rain bands are plotted as highly segmented
notched lines labelled C2 and C3.
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K. A. BROWNING
• The surface warm front, turning into a bent-back front near the cyclone centre, L
(consistent with the raw data in Fig. 2) with a cold conveyor belt (labelled CCB) skirting
it at low levels to form the lower part of the cloud head.
• The main surface cold front (SCF), labelled C1 in Figs. 4(a) and (b), coinciding
with the narrow cold-frontal rain band (NCFR) in Fig. 4(a) which, as discussed later,
turns into an upper cold front (UCF) at its northern end where the surface front has
undergone frontal fracture (Shapiro and Keyser 1990).
• The primary warm conveyor belt labelled W1, responsible for the polar-front
cloud band (PFCB; see Fig. 3) and for much of the rain in the broad band in Fig. 4(a)
extending from Brittany to the Wash on the east coast of England. The high-θw air in
W1, which extends down to the surface east of the SCF, rises abruptly as line convection
at the SCF, and (as shown later in Fig. 7) ascends above drier air to the west.
• The secondary warm conveyor belt labelled W2, which peels off from W1 and
flows westwards within the boundary layer in the SMZ (see Fig. 2) before rising at and
behind the bent-back front, and then flows northwards to form the upper part of the
cloud head.
• The dry-intrusion flow labelled DI, which, after an earlier period of descent as
it approaches from the west and north-west, undercuts the W1 flow, with some of it
penetrating to the surface behind the SCF and some of it overrunning the SMZ behind
the UCF. Comparison of Fig. 4(a) with Fig. 3 shows that the DI flow causes sufficient
evaporation for a large region beneath the W1 flow and its associated PFCB to be
devoid of rain reaching the surface. (Further evidence of this evaporation is given later
in connection with Figs. 7 and 8.)
• The two post-cold-frontal convective rain bands, labelled C2 and C3, which are
analysed in detail in later subsections. Orientated roughly parallel to the main NCFR,
C1, they travelled at approximately the same velocity as C1, maintaining their distance
from it. Each of the bands, C2 and C3, shows a break where the DI air was suppressing
the convection. (As discussed later, the DI was playing a dual role: it was generating
potential instability but was also tending to inhibit its release in places by creating
a subsidence inversion.) The justification for the analysis of convective rain bands in
Fig. 4(b) is not self-evident from inspection of the rainfall patterns in Fig. 4(a) alone;
however, it becomes more convincing in the light of the narrow and linear nature of the
rainfall bands at certain other times, and the vertical structure of the airflow observed
during their passage over the UHF radars (see Fig. 7 of BDGW). Evidence of the reality
of these convective bands is also provided by their temporal continuity. This is shown in
Figs. 5(a) and (b), which depict successive 15-minute plots of C2 and C3, respectively.
The stippled strips in Fig. 5 show the paths swept out by gaps in the rain bands. It is
shown shortly that these gaps occurred where the DI air had subsided sufficiently to
restrict the convection to a very shallow layer. The rain band C2 occurred in a region
where the overall flow was veering with time; thus the gap in C2 was travelling in
the direction of the DI flow from the west-south-west (Fig. 5(a)), whereas the gaps
in C3 were travelling within a diffluent flow from between west and west-north-west
(Fig. 5(b)). As the cyclone intensified further (beyond the period analysed in this paper),
the dry slot grew in extent and the convective bands, C2 and C3, were even more
suppressed.
(b)
Relationship of the convective rain bands to the neighbouring areas of slantwise
ascent and the dry intrusion
The Meteosat infrared image and the weather radar network display in Figs. 6(a)
and (b), respectively, depict the distributions of cloud and rainfall at 0430 UTC. This was
SYNTHESIS OF MESOSCALE STRUCTURES
609
Figure 5. Successive 15-minute positions of convective rain bands: (a) C2, and (b) C3, as determined from the
weather radar network. Numbers indicate hourly positions: 3 for 0300 UTC etc. The rain bands extended farther
to the south-west, as shown in Fig. 4, but their continuity was ambiguous and so these parts are not plotted here.
Stippled swaths indicate where the dry intrusion was keeping estimated rainfall rates below 0.5 mm h−1 . The
long-dashed lines after 0500 UTC in (b) are where Band C3 became indistinct owing to the widespread outbreak
of showers in its vicinity.
two hours before the plots in Figs. 3 and 4, and close to the time when the cold-frontal
and post-cold-frontal rain bands were passing over the two UHF wind-profiler radars.
Data from these profilers are used in this section to interpret the vertical structure of
these rain bands. Figure 6(c) shows an analysis of some of the rainfall features in
Fig. 6(b); it also shows the rear edges (scalloped) of bands of high cloud derived from
Fig. 6(a). Other annotations in Fig. 6(a) to aid interpretation are explained later.
Convective rain bands C1, C2 and C3 are identified in Fig. 6(b), and their positions
are plotted in Fig. 6(c) using the same convention as in Fig. 4(b). The well-defined
band of very heavy rain, C1, embedded within the broader zone of rather heavy rain, is
the NCFR associated with the main SCF. The next convective band, C2, extends as a
narrow rainfall maximum along the western boundary of the main cold-frontal rainfall
area, across Cornwall and then through the dry slot into the Gower peninsula in South
Wales. Band C3 extends from a position west of the Isles of Scilly to the south coast of
Dyfed near the southern tip of the cloud-head rain; it is situated in the dry air behind
the rear edge of the overall PFCB. Besides the convective rain bands, C1, C2 and C3,
Fig. 6(b) also shows two wider frontal rain bands characterized by bands of relatively
heavy rain (orange and red) about 80 km wide: one of these is situated just ahead of C1
and is identified as S0 in Fig. 6(c), and the other is just behind C1 and is identified as
S1. As will become clear, S1 is due to the slantwise ascent fed in part by the outflow
from the upright convection associated with C1.
The nature of bands S0 and S1 and their relationship to the convective rain bands
will now be explained by means of the time–height cross-sections from the UHF wind
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K. A. BROWNING
Figure 6(a) and (b).
For caption see (c).
SYNTHESIS OF MESOSCALE STRUCTURES
611
Figure 6. For 0430 UTC 30 October 2000: (a) Meteosat infrared image, (b) weather radar network rainfall display
and (c) mesoanalysis. The annotation in (a) is as follows. Scalloped lines represent the rear edges of upper-level
cloud features associated with mesoscale slantwise circulations S0 and S1, and cloud head CH. The cluster of
broken lines running from south of south-west England to the Thames estuary identify a higher-level cirrus
streak that was partially obscuring the cloud bands of interest. The solid lines orientated south-west to northeast represent the height (km) of the demarcation between rearward sloping ascent and forward sloping descent
within S1, as inferred from UHF wind profilers at Camborne, C, and Pendine, P. Curved dashed lines through C
and P show approximate locations of the time–height sections in Figs. 7 and 8, where time has been converted to
distance according to the local velocity of the mesoscale systems. Locations of Aberystwyth and Aberporth are
labelled A and a, respectively. The surface cold front is plotted conventionally. The mesoanalysis in (c) is derived
largely from (a) and (b). Annotations in (c) are as follows: fronts and rear edges of upper-level cloud layers are
plotted as in (a), and the surface warm front and upper cold front have been added; rain band C1 is associated
with the surface cold front and the other two convective rain bands, C2 and C3, identified in (b), are shown by
notched lines. The main areas of heavy stratiform rain are delineated schematically, hatched and labelled WF
(warm frontal rain), CH2 and CH3 (heavy rain associated with two sub-areas of the cloud head), and S0 and S1
(wide cold frontal rain bands associated with these mesoscale slantwise circulations).
profilers at Camborne and Pendine (locations C and P, respectively, in Fig. 6(a)). The
effective spatial locations of these two sections can be determined to a useful first
approximation by using the (time varying) system velocity of the salient mesoscale
features: the dashed lines labelled in hours (2 to 6 UTC) in Fig. 6(a) have been derived in
this manner. Figure 7 shows the cross-section for Camborne and Fig. 8 for Pendine. Each
cross-section is a synthesis of a set of diagrams showing various parameters derived
from the profilers in earlier studies: Fig. 7 is based on Figs. 4 and 8 in BDGW, whilst
Fig. 8 is based on Figs. 3(a) and (b) in BW. Several features, listed below, which are
evident in Figs. 7 and 8, account for the patterns of cloud and rain shown in Figs. 6(a)
and (b). As explained in BDGW, the layers of slantwise ascent labelled S0, S1 and S2
in Figs. 7 and 8, were inferred from UHF radar data from sloping layers of maximum
rearward flow relative to the system (as shown in Fig. 7 of BDGW). Similarly the dryintrusion flows labelled D1 were inferred from sloping layers of maximum forward flow
relative to the system (again, see Fig. 7 of BDGW). Upright convection features, labelled
C1, C2 and C3, were diagnosed from the UHF radar data from the combination of strong
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K. A. BROWNING
Figure 7. Convective circulations overplotted on a time–height cross-section of echo intensity from the UHF
wind-profiler at Camborne on 30 October 2000 (based on Figs. 4 and 8 in Browning et al. (2001b)). The position
of the section is shown by the dashed line through location C in Fig. 6(a). The rearward sloping arrows labelled
S0, S1, S2 and S3 represent the ascending parts of mesoscale slantwise convective circulations (the reality of S3
is uncertain). The dry intrusion (DI) feeds the forward sloping descent that undercuts flows S1, S2 and S3, which
are fed by upright convection represented by the vertical arrows C1, C2 and C3.
cyclonic shear in the front-parallel wind component (Fig. 6 of BDGW) and confluence
in the front-normal component of the boundary-layer winds (Fig. 7 of BDGW). The
heavier precipitation associated with the upright convection shows up as a maximum in
echo intensity on the C-Band weather radars, but is not always evident in the UHF echo
which is strongly affected by returns from humidity and temperature inhomogeneities.
The features analysed in Fig. 7 and 8 are as follows:
(i) An ana-cold-frontal system of the kind described by the conceptual model in Fig. 8.8
of Browning (1990), consisting of upright line convection C1 feeding a broad band of
slantwise convection S1. This combination of C1 + S1 shows up in the Camborne crosssection (Fig. 7) which was affected by the SCF, but the C1 component is missing from
the Pendine cross-section (Fig. 8) which is farther north and intersects the region where
the SCF becomes an UCF. The line convection C1 is marked in Fig. 6(b) by a NCFR,
which consists of line elements of very heavy rain (known from in situ measurements
to have briefly reached 100 mm h−1 ). There was an abrupt temperature fall of about 3
degC with the passage of C1 (see Fig. 3(a) in BDGW). Figure 7 shows that the slantwise
convection S1, along with S2 (see later), extended to the rear edge of the cloud deck,
150 km behind C1. Figures 6(b) and (c) show that the associated wide cold-frontal rain
band (WCFR) was about 80 km wide and situated behind the NCFR; it remained behind
the NCFR throughout the period of study. The WCFR did not extend to the rear edge of
the cloud deck associated with S1, because of evaporation within DI air undercutting S1.
Clear evidence of evaporation within the undercutting DI flow is shown in the vertical
structure of the radar echo power plotted in Figs. 7 and 8. The full set of time–height
cross-sections for Camborne and Pendine, reproduced in BDGW and BW, shows that
SYNTHESIS OF MESOSCALE STRUCTURES
613
Figure 8. Convective circulations overplotted on a time–height cross-section of echo intensity from the UHF
wind profiler at Pendine on 30 October 2000. (based on Fig. 3 in Browning and Wang (2002)). The position of
the section is shown by the dashed line through location P in Fig. 6(a). The rearward sloping arrows labelled S1
and CH represent the ascending parts of the mesoscale slantwise convective circulations. The dry intrusion (DI)
feeds the forward sloping descent that undercuts S1 and CH. The DI flow caps the upright convection, C2 and
C3, restricting it to below 3 and 4 km, respectively; C1 was completely suppressed at this location. There was a
shallow moist zone beneath the DI between 0300 and 0500 UTC . The upright convection above S1 was responsible
for the cloud-top striations discussed in Browning and Wang (2002).
the plane of demarcation between the slantwise ascent S1 and the underlying descent
was coincident with a maximum in wind strength (see, for example, Figs. 5 and 7 of
BDGW). Isopleths of the height of this plane of demarcation, as deduced from the
combined wind-strength plots at Camborne and Pendine, are plotted in Fig. 6(a) as solid
lines labelled at their northern ends in kilometres (2, 3, 4 and 5 km). These show that
this surface and, by inference, the base of the slantwise ascent S1, sloped upwards from
a height of 2 km just behind the line convection C1 to above 5 km near the rear edge of
the associated upper-cloud deck (with a mean slope of 1 in 25).
(ii) A further ana-cold frontal system characterized by another broad band of slantwise
convection S0.This produced the WCFR, labelled S0 in Fig. 6(c), extending up to the
rear edge of the corresponding upper-cloud deck. There is no evidence of significant line
convection feeding S0 at this time, but the possibility that this had been the case several
hours earlier cannot be ruled out.
(iii) A dry intrusion. The DI shows in Figs. 7 and 8 as a region of forward sloping
descent which is most easily seen where it undercuts S1 (and S2, see later). There is clear
evidence in Figs.7 and 8 of evaporation of precipitation falling into it from the overlying
slantwise ascent. In the radar-network picture in Fig. 6(b) the DI gives rise to the rainfree zone 60 km wide approaching the south coast of Wales. In the corresponding
satellite infrared image in Fig. 6(a) the dry slot is displaced slightly farther to the west,
over the west coast of Wales. As explained earlier, this displacement is the result of two
factors: first, the DI air undercutting the upper-cloud deck S1 to the east; and second,
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K. A. BROWNING
the western edge of the DI air overrunning the eastern parts of the cloud head (labelled
CH) where the cloudy ascent, although deep enough to produce rain close to the west
coast of Wales at 0430 UTC, was too shallow to show up clearly in the infrared imagery
with the particular colour enhancement scheme adopted here. The leading edge of the
DI where it overruns the SMZ corresponds to an UCF. The UCF is plotted in Fig. 6(c)
with open cold-frontal symbols. It is almost continuous with the sharp SCF associated
with Cl which also marks the leading edge of DI air, the difference being that whereas
the DI air behind the SCF descends down to the surface, as shown in the Camborne
cross-section in Fig. 7, the DI air in the region of the UCF remains above a SMZ in the
boundary layer as shown in the Pendine cross-section in Fig. 8. The UCF passed over
Pendine at about 0300 UTC where, according to evidence in BDGW summarized here in
Fig. 8, it did not have much effect on the air flow close to the surface, thereby implying
that any upright convection at the UCF would have been confined to middle levels.
(iv) The two post-cold-frontal convective rain bands C2 and C3. Convective band C3
was between 3 and 4 km deep where it passed over Camborne (Fig. 7) and Pendine
(Fig. 8). At both locations DI air separated C3 from the precipitation due to slantwise
ascent S1; the separation is evident both in the horizontal (Fig. 6(b)) and in the vertical
(Figs. 7 and 8). Convective band C2, on the other hand, is less clearly separated from
the precipitation associated with S1; Fig. 6(b) shows C2 as a narrow line of heavier rain
at the rear edge of WCFR S1 which emerges into the dry slot as a narrow south–north
orientated rain band over the Gower peninsula of South Wales (see analysis in Fig. 6(c)).
The wind-profiler data from Camborne (Fig. 7) also show that C2 precipitation blended
with that from S1. In fact, there is very little evidence in Fig. 7 of the distinct maximum
in precipitation intensity that might have been indicative of the convection associated
with C2; rather, the principal evidence for convection at C2 is to be found in the
characteristic patterns of cyclonic shear and convergence there at low levels, which
closely resemble those associated with C1 and the other examples of line convection
in the literature. Figures 6 and 7 in BDGW show abrupt transitions in both the frontparallel and front-normal components of the wind occurring across an upright boundary.
The abrupt transitions extended from about 1.9 km down to the ground, indicating that
the convection was being fed by air from the boundary layer. These same wind-profiler
plots led BDGW to deduce that the upright convection C2 extended through the DI air
and reached above 4 km, where the outflow from C2, labelled S2, appeared to merge
with the S1 slantwise ascent (Fig. 7). Farther north at Pendine, Fig. 8 suggests that the
DI flow, characterized by a forward flow relative to the system, retained its identity
whilst penetrating more deeply beneath the S1 flow; indeed, Fig. 8 shows that it led to
the evaporation of much of the S1 precipitation falling into it and restricted the depth
of the C2 convection to below 3 km, thereby accounting for the gap in Band C2 shown
earlier in Fig. 5(a). This gap was briefly filled in as it approached and crossed the South
Wales hills between 0430 and 0500 UTC, presumably because the local orographic uplift
compensated for the overall subsidence within the DI.
In summary: bands C2 and C3 were embedded within a descending DI flow. This
flow had two opposing effects: on the one hand, it had a low θw and created the potential
instability necessary for the convection to occur; on the other hand, because it was
dry it had a rather high dry-bulb temperature which tended to inhibit the release of
the potential instability. No radiosonde ascent was made in the appropriate location,
but the magnitude of the potential instability within the rain bands has been estimated
from the value of θw measured at the surface, minus a value of θw aloft calculated from
the height of the radar-derived melting level. At Pendine during the passage of band C2,
SYNTHESIS OF MESOSCALE STRUCTURES
615
the wind-profiler radar showed that the increase in precipitation terminal fall speed due
to melting occurred at 1.6 km, indicating a θw of 8 ◦ C in the DI at that level. This was
4 degC colder than the θw of 12 ◦ C at the surface within the underlying SMZ. Band C2
occurred where the DI was undercutting the rearward-ascending S1 flow. Ice crystals
falling from S1 evaporated within the DI; thus the release of the potential instability is
likely to have been aided by evaporative cooling of the DI flow.
(c) Relationship of the convective rain bands to the cloud-head substructure
Multiple structure within cloud heads is common; for example, see Fig. 7 of
Browning et al. (1997b), Fig. 3 of Lean and Clark (2003) and, on a smaller scale,
Fig. 11 of Browning (2004). There is some evidence in the satellite imagery of smallscale substructure within the cloud head in the present study, but the clearest evidence of
such substructure is to be found in the rainfall patterns from the weather radar network.
In Fig. 6(b), for example, there is a demarcation between the major part of the cloudhead rain over the Irish Sea and a smaller newly developing area of heavy cloud-head
rain extending from Cardigan Bay to Snowdonia in North Wales, the two parts being
separated by a narrow strip of lighter rain (green). Temporal continuity lends credibility
to the reality of this substructure. Soon after, yet another sub-area developed; this is
shown in the sequence of three tracings in Fig. 9 covering the period 0345 to 0615 UTC.
The three cloud-head rainfall sub-areas are identified in Fig. 9 by dashed envelopes
labelled CH1, CH2 and CH3. The numbering order for these features may seem perverse
since CH3 formed before CH2, and CH2 before CH1; however, this numbering order
has been adopted because of a perceived correspondence with convective bands C1, C2
and C3 which are also plotted in Fig. 9. At 0345 UTC (Fig. 9(a)) CH3 is seen to be
located just behind (to the west of) the northern end of C3, and the embryonic CH2 is
just behind the northern end of C2. The presumption is that each cloud-head rainfall
sub-area is due to rearward sloping ascent of air, at least some of which originates as
air lifted from the boundary layer by the line of upright convection ahead of it; this
corresponds to the usual ana-cold-frontal configuration associated with a cloud head
(e.g. Browning et al. 1997b). This situation, with the cloud-head sub-areas behind their
respective lines of upright convection, still prevails at 0515 UTC (Fig. 9(b)) by which
time a new cloud-head rain area, CH1, has formed just behind the northern end of the
UCF-extension to C1. The situation remains similar at 0615 UTC (Fig. 9(c)) except that
the spacing between the cloud-head features and the convective bands has increased,
especially for CH3. This is indicative of an increasingly shallow slope to the slantwise
convection between the respective line convection and cloud-head features. Why the
new cloud-head areas should be triggered downwind of the previous areas, rather than
upwind, is uncertain.
(d)
Relationship of the convective rain bands to mesoscale structure within the
tropopause fold
The principal sources of data used in this subsection are the time–height records
from a VHF wind-profiling radar, known as a Mesosphere–Stratosphere–Troposphere,
or MST, radar at Aberystwyth (location A in Fig. 6(a)) and a timely radiosonde ascent
released from nearby Aberporth (location a in Fig. 6(a), 60 km south-west of A). Interpretation of the MST radar data is assisted by using information on cloud-top heights
inferred from Meteosat infrared imagery, and information about rain band structure
inferred from the weather radar network and UHF wind-profilers. The cyclone centre
passed a few kilometres to the west of the MST radar at 0430 UTC; the radar obtained
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K. A. BROWNING
Figure 9. A sequence of analyses at (a) 0345, (b) 0515 and (c) 0615 UTC 30 October 2000, showing convective
rain bands C1, C2 and C3, and the sub-structures CH1, CH2 and CH3 within the cloud head. These features
have been derived from an analysis of the heavy-rain areas revealed by the weather radar network. The narrow
cold-frontal rain band C1 corresponds to the surface cold front; it has been drawn so as to follow the axes of all
the individual line elements/precipitation cores. The stippled area enclosed within scalloped lines corresponds to
the overall extent of light rain. Successive frames are displaced eastwards to keep the overall system centred.
vertical profiles successively through the PFCB, then through the dry slot containing
the extreme northern ends of convective rain bands C2 and C3, and finally through the
southern end of the cloud head. The Aberporth radiosonde provided confirmation for
some of the interpretations of the MST results, because it was released at 0500 UTC
just as the western boundary of the polar-front cloud band was passing overhead at high
levels and just after the leading edge of the cloud head had passed at low levels.
The time–height data from the MST radar are presented in Fig. 10. Figure 10(a)
shows the radar echo intensity for 0000 to 2400 UTC, and Figs. 10(b) and (c) show for
the same period the wind strength and the southerly component of the vertical wind
shear, respectively. The most striking feature in Fig. 10(a) is the series of multiple echo
layers above 7 km with vertical spacings of 1 km or less. According to Yamanaka et al.
(1996) and Pavelin et al. (2001) such echoes are characteristic features of the stably
stratified lower stratosphere in the vicinity of upper-level jet streams and are attributable
to IG waves probably triggered by the jet-stream behaviour. The differential excursions
of air parcels due to IG waves set up shallow layers of locally intensified static stability
and shear, often manifested as looped hodographs, which produce locally increased echo
intensity by a combination of Bragg scattering and specular reflection. Below these echo
layers there is for most of the time a region of weak echo in Fig. 10(a) corresponding to
the upper troposphere. After 1000 UTC the height of the lowest echo layer in Fig. 10(a)
indicates a tropopause at 7 km (Vaughan et al. 1995). Before 0400 UTC the echo layers
are seen to terminate at a rather higher level indicating a tropopause close to 9 km.
Between 0400 and 1000 UTC the stratospheric echo layers plunge downwards into a
major tropopause fold.
The long-dashed lines superimposed on Fig. 10(a) denote the height of the satellitederived cloud tops associated with the PFCB before about 0500 UTC and with the cloud
head between 0500 and 0730 UTC. The tops of the polar-front cloud coincide with
SYNTHESIS OF MESOSCALE STRUCTURES
617
(a)
c3 c2
(b)
(c)
()
c3c2
Figure 10. Time–height cross-sections derived from the MST (see text) radar at Aberystwyth (location A in
Fig. 6(a)) on 30 October 2000 of: (a) echo power, (b) wind strength, and (c) southerly component of vertical wind
shear. Long-dashed lines on (a) represent the tops of cloud derived from Meteosat imagery; from 0000 to about
0500 UTC this cloud is associated with the polar-front cloud bands which evidently extended up to tropopause
level, and from 0500 to 0730 UTC the observed cloud top is associated with the cloud head. The short-dashed
line on (a) from about 1000 to 2400 UTC is derived solely from the echo-power signature, and corresponds to
the tropopause. The locations of shallow convective rain bands C2 and C3, as derived from the weather radar
network, are marked on both (a) and (c). Regions of strongly fluctuating shear (alternating white and red/blue) in
(c) correspond to noise above 13 km due to poor signal/noise, but below 8 km they are an indication of real wind
variability due to some of the radar beams intersecting upright convection.
618
K. A. BROWNING
the tropopause as inferred from the MST radar. The dotted line between about 1000
and 2400 UTC shows the tropopause as inferred solely from the MST echo intensity.
The tropopause depression/fold region lies between 0500 and 1000 UTC, and it can
be seen to have been penetrated by the cloud head, the top of which extends upwards
from 3 km shortly after 0500 UTC to 6 km between 0700 and 0730 UTC. Between the
polar-front cloud and the cloud head, the intense echoes (yellow) are associated with
the northernmost ends of the convective rain bands C2 and C3. The high intensity of
these echoes is probably due to the strong refractivity gradients that are generated at the
turbulent interface between the moist convection and the dry-intrusion air descending
within the tropopause fold. The passage of convective bands C2 and C3 also coincides
with narrow, almost vertical spikes of noisy shear in Fig. 10(c), probably attributable to
disruption of the signal processing by the convection itself. (Similar regions of noise in
the wind-shear display occur from 0200 to 0400 UTC between 6.5 and 8 km during the
passage of the convective rolls at the top of the polar-front cloud band analysed by BW,
and from 1400 to 2300 UTC during the passage of deep convective showers.)
The MST radar plot of wind strength in Fig. 10(b) depicts a jet maximum of 65
m s−1 at tropopause level, at the top of the PFCB. This wind maximum descends from
9 km at 0300 UTC to 6 km at 0500 UTC and weakens rapidly thereafter. Between
0400 and 0600 UTC, within the tropopause fold, there is an inclined zone of very
strong wind shear. Of particular interest is the substructure within this shear zone which
exhibits layering broadly similar to that in the pattern of echo intensity. The wind shear
substructure shows up most clearly in the southerly component of the vertical wind
shear, depicted in Fig. 10(c). Note in particular the inclined shear layers in Fig. 10(c)
descending below 6 km within the fold region, and apparently emanating from the lower
stratosphere where the shear layers are inclined more gently and are attributable to IG
waves. A series of half-hour-mean perturbation wind hodographs in the fold region from
0600 to 1000 UTC (not shown) reveals loops with a cyclonically rotating vector between
5.5 and 8 km, having a peak-to-peak amplitude of up to 5 m s−1 . This is indicative of
downward energy propagation due to IG waves, possibly from a source at jet-stream
level. Below 5.5 km there are some regions with cyclonically looped hodographs but the
pattern of shear is more noisy. It is not clear whether or not the IG waves propagate all
the way down in this region, or whether they perhaps trigger the instabilities that lead
to the observed upright convection (C2 and C3) and slantwise convection (cloud head
substructures CH2 and CH3).
Dry-intrusion air was shown earlier to have penetrated downwards within the
tropopause-fold region, undercutting the slantwise ascent within the PFCB. The bands of
upright convection and the slantwise convective circulations within the cloud head were
all intruding upwards into this dry-intrusion air. Another indication that dry, possibly
stratospheric, air may have been undercutting the rearward ascending air, both in the
PFCB and in the cloud head, is provided by the vertical air velocity measurements
obtained from the vertically pointing beam of the MST radar (not shown). Normally
one uses such measurements very cautiously, because of possible contamination by
local topographic effects, but on this occasion there were systematic features that were
meteorologically plausible: small pockets of very strong descent (∼50 cm s−1 ) were
detected close to the rear edge of both the polar-front cloud and the cloud head. The
strong descent on the edge of the polar-front cloud occurred within the tropopause fold,
from 0500 to 0540 UTC between 5.5 and 7 km, i.e. just outside the cloud on the dry side
of the jet maximum. The strong descent associated with the cloud head occurred in an
almost identical position with respect to the cloud head, i.e. close to and slightly beneath
the edge of the cloud, from 0700 to 0740 UTC between 4.5 and 5 km.
SYNTHESIS OF MESOSCALE STRUCTURES
619
Figure 11. Tephigram showing the radiosonde ascent released at 0500 UTC 30 October 2000 at Aberporth
(location ‘a’ in Fig. 6(a)). Winds are given following the normal convention on the left-hand side.
The radiosonde released at nearby Aberporth at 0500 UTC (Fig. 11) went through
the shallow leading edge of the cloud head, where it encountered saturated coldconveyor-belt air, with θw = 8 ◦ C up to 2 km (800 hPa). The sonde also intercepted
a very shallow saturated layer with θw = 13 ◦ C at 6 km (470 hPa) corresponding to
the western boundary of the polar-front cloud. At altitudes between 2 and 6 km the
sonde encountered unsaturated air. From 2 to 4.6 km the air was moderately dry (relative
humidity RH ∼ 75%). This layer was capped at 4.6 km (570 hPa) by a very stable layer
above which there was a layer of very dry air (RH ∼ 10%) probably due to stratospheric
air in the fold. Figure 11 shows the strong increase in wind across the dry layer. This
corresponds to the multiple shear layers that were associated with (and perhaps were
triggering) the convective bands, but it is not possible to determine the extent to which
these shear layers were stratospheric in origin and hence attributable to IG waves. Some
of these shear layers may have been generated within the tropospheric air just beneath
the intruding stratospheric air and, hence, perhaps they were due to some process other
than IG waves, such as conditional symmetric instability. This is very similar to the
situation observed in a vigorous cold-air comma-cloud cyclone system analysed by
Browning et al. (2002).
The inter-relationships of the various mesoscale structures in the tropopause-fold
region are summarized in Fig. 12, which corresponds to a zoomed-in portion of Fig. 10.
620
K. A. BROWNING
Figure 12. Schematic synthesis of mesoscale structure in the vicinity of the tropopause fold, as inferred from
Fig. 10 and other data. Bold solid lines represent the tropopause. The main upper-level jet extends between the
two Js. The top of the polar-front cloud is at tropopause level between the two Js. The bold dashed line represents
the top of the cloud head inferred from Meteosat imagery. The regions of slantwise ascent associated with the
polar-front cloud and the cloud head are shown by the broad arrows. Thin vertical arrows show the locations of
upright convection: at low levels between 0430 and 0500 UTC , and at the top of the polar-front cloud between
0200 and 0400 UTC . The gently inclined thin solid lines represent the two most intense MST (see text) radar-echo
layers associated with inertia–gravity waves, with energy propagating downwards into the tropopause-fold region.
Related layers of maximum vertical wind shear are shown dotted; above 6 km these are gently inclined, whilst
below 6 km they blend into more steeply inclined shear layers roughly parallel to the top of the cloud head. There
was considerable fine structure to the shear pattern and the axes of only the stronger shear layers are shown.
For a detailed explanation of the diagram refer to the figure caption. The thin solid lines
in Fig. 12 represent two of the most intense echoes due to IG waves propagating energy
downwards into the tropopause-fold region. Below 5 or 6 km the echo layers become
steeper and less distinct, and are dominated by a layer that appears to be associated with
the upper boundary of the cloud head (long-dashed line) which intrudes upwards into the
otherwise dry air in the fold region. Multiple shear layers (dotted) are gently inclined,
like the echo layers, above 6 km; but below this they are more steeply inclined, roughly
parallel to the cloud head. The steeply inclined shear layers extend into the region of the
convective bands C2 and C3 and, as suggested above, may have been responsible for
triggering them. What is not clear, however, is the relationship between the shear layers
below 5 or 6 km and those above that level which are almost certainly associated with
IG waves. The shear layers below 5 km are not characterized by wind vectors with a
consistent sense of rotation in the way that the IG waves are, and so it is possible that
they were generated by a different mechanism.
3.
C ONCLUSIONS
An observational analysis has been carried out of a brief period during the development of an extratropical cyclone associated with a strong (65 m s−1 ) upper-level jet. The
cyclone was deepening rapidly (18 hPa in 6 h), displayed a prominent cloud head (Bader
et al. 1995) and had undergone frontal fracture according to the cyclone life-cycle model
SYNTHESIS OF MESOSCALE STRUCTURES
621
of Shapiro and Keyser (1990). A rich variety of convective and mesoscale structures has
been identified within the cyclone, and the relationships of these to each other have
been described. The path of the cyclone took it close to a good network of observations
in the UK, but the limitations of a purely observational study of this kind are such
that firm conclusions cannot be drawn regarding the nature of the mesoscale processes.
Nevertheless, the study is useful in directing attention towards possible new mechanisms
and interactions needing to be addressed in the future. This cyclone produced sustained
hurricane-force winds as it crossed the North Sea six hours after the period studied in
this paper. It remains to be seen to what extent a numerical weather prediction model
has to represent faithfully any or all of the many mesoscale processes described here in
order to provide a detailed forecast of such hazardous events.
A central feature of the cyclone of 30 October 2000 was a dry intrusion, which
produced a dry slot that grew rapidly with time. Parts of the dry intrusion were composed
of tropospheric air, but other parts were identified as being composed of stratospheric air
within a tropopause fold, descending at 50 cm s−1 in places. IG waves were detected in
the lower stratosphere, perhaps caused by the unbalanced flow in the vicinity of the jet
stream. The IG waves propagated energy downwards, and the associated layers of high
static stability and strong wind shear descended some way towards the ground within
the tropopause fold. Below 5 km the layers became more steeply inclined; they may
have triggered the convective rain bands, and the cloud-head substructure was associated
with some of them, but here the layers may have been due to mechanisms other than IG
waves,
Two distinct regions of potential instability and upright convection occurred in this
cyclone. One was at the top of the polar-front cloud on the warm (south-east) side of
the dry slot; the convection here occurred in the form of bands transverse to the main jet
axis within the polar-front cloud. According to Browning and Wang (2002), the strong
slantwise circulations generate the potential instability by distorting the θw surfaces in
the manner described by Bennetts and Hoskins (1979).
The other region of upright convection in this cyclone occurred as a result of the
low-θw air in the dry intrusion overrunning high-θw air in the boundary layer. The
resulting convection gave rise to rain bands that extended from the dry slot, close to
the cyclone centre, southwards behind the main surface cold front. Carr and Millard
(1985) have shown that in the USA these are locations which are prone to convectionrelated severe weather. However, much of the convection in the present case was not
severe, and along parts of the convective rain bands in the dry slot the convection was
inhibited and the surface precipitation suppressed, presumably because of the strength
of the dry descent and adiabatic warming in this region.
The (upright) convective rain bands associated with the dry intrusion were situated
close to the slantwise convective circulations responsible for the bulk of the polar-front
cloud. The first convective rain band occurred in the form of a narrow cold-frontal rain
band with rain rates of 100 mm h−1 . It was associated with very intense line convection
at the sharp cold front, and it fed a layer of slantwise ascent which produced a canopy
of high-level cloud extending rearwards (north-westwards) relative to the system. Dryintrusion air descended beneath this canopy, leading to the complete evaporation of
precipitation falling from it in parts of the dry slot close to the cyclone centre, and
to partial evaporation in regions father south behind the trailing cold front.
The second (upright) convective rain band formed beneath the cloud canopy due
to the slantwise ascent associated with the first convective rain band. In places, behind
the trailing cold front, the second line of convection penetrated through the descending
dry-intrusion air to reach, and augment, the overlying rearward ascending outflow
622
K. A. BROWNING
from the first convective band; in other places in the dry-slot, close to the cyclone
centre, the second convective rain band was strongly suppressed. Some convection
still occurred, and so the considerable evaporative cooling here appears to have been
sufficient to overcome the stabilizing effect of subsidence warming. As shown by Jewett
et al. (2003), evaporation of precipitation can sometimes play an important role in
maintaining mesoscale circulations in the dry slot of a cyclone. In the case they studied,
the evaporation led to a strong mesoscale gravity wave. Such gravity waves are seldom
seen in dry slots in the UK, and were not observed in this case, but they are common in
the USA (Uccellini and Koch 1987) where convective rain bands within dry intrusions
overrun and perturb the stable layer ahead of the surface warm front.
The rich diversity of convective and mesoscale processes within an extratropical
cyclone poses stiff challenges for mesoscale modelling and numerical weather prediction. Future work must concentrate on the application of higher-resolution mesoscale
models to events such as this, in order to determine whether the different phenomena
can be reproduced and, if so, what are the likely mechanisms and modes of interaction. Lean and Clark (2003) presented encouraging evidence of the ability of a nonhydrostatic mesoscale model to resolve multiple slantwise convective circulations and
multiple cloud-head structures, provided the model has approximately 100 levels in the
vertical and a 2 km horizontal grid length. However, additional levels may also be required in the stratosphere to resolve the IG waves. Moreover, the realistic representation
of the triggering of the upright convection and its interaction with slantwise convection
is likely to be particularly problematic.
ACKNOWLEDGEMENTS
I wish to thank Chang-Gui Wang (University of Reading) for her help in generating
the diagrams, Peter Clark (JCMM) for helpful discussions, John Nash (Met Office)
for providing data from the UHF wind-profilers at Camborne and Pendine and for
drawing attention to relationships between structures observed by the two profilers, Ed
Pavelin for generating the hodographs discussed in subsection 2(d), Geraint Vaughan
for suggesting the use of such hodographs, and David Hooper (Rutherford Appleton
Laboratory) for providing data from the Aberystwyth MST radar and carrying out
special quality-control measures to remove spurious parts of the Doppler spectra thought
to be caused by severe-wind-induced sea clutter. This study was partially supported
by the Natural Environment Research Council through its funding of the Universities
Weather Research Network (UWERN)
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