Observed structure of mesoscale convective systems and implications for largescale heating

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Observed structure of mesoscale convective
systems and implications for large-scale heating
Houze, R. A., Jr., 1989 Observed structure of
mesoscale convective systems and implications
for large-scale heating . Quart. J. Roy. Meteor.
Soc., 115, 425-461.
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1.Introduction

• The convective regions contain numerous deep
  cells that are often but not always arranged in
  lines.
• The stratiform regions is an outgrowth of the
  convective towers. Sometimes it lies to the rear
  of a propagating convective line, while on other
  occasions it surrounds the convection.
• The heating of the large-scale environment by a
  mesoscale convective system is affected by both
  the convection and stratifoem regions. The net
  heating by a system is dominated by condensation
  and evaporation associated vertical motions.
• In stratiform regions, the mean vertical
  velocity that is upward in the upper troposphere
  and downward in the lower troposphere. The level
  separating upward from downward motion is located
  from 0 to 2 km above 00c level, depending on
  location within the stratifoem regions.
• Diagnostic calculations show that these vertical
  motion profile imply heating of the upper and
  cooling of lower troposphere by stratifoem-region
  processes.
• In convective regions, the vertical motions are
  less consist from case to case.

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1.Introduction

• Deep cumulonimbus clouds are the agents by which
  high energy air is conveyed from the planetary
  boundary layer to the upper troposphere.
• The deep convection inters with the large-scale
  environment during the vertical transport and
  exactly how the heating of the large-scale
  atmosphere remain poorly documented and
  understood.
• Some empirical information is needed regarding
  the actual vertical distribution of heating by
  deep convection.
• Houze(1982), summarized the cloud and
  precipitation structure of MCSs in GATE and MONEX
  field experiments.
• To reassess the tenets and calculations of
  Houze(1982).

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2. Review of conceptual model (a)
Idealized cloud system structure
Houze(1982) The idealized tropical MCS.

• Early stage Cluster consists of isolated
  precipitating convective tower.

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• Mature stage
• Ac - cloud shield
• Ah - convective cell
• As - stratiform precipitation
• Ao - upper-level cloud overhang

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• Weakening stage
• convective cell have disappeared
• stratiform precipitation remains
• upper cloud become thin and breaking up

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• Dissipating stage
• no precipitation remains
• upper cloud become more thin and breaking up

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• Houze(1989) has added detail to the conceptual
  model by describing aspects of the relationship
  between deep convective cells and the associated
  stratiform region.
• Houze(1989)s conceptual model emphasizes
  microphysical and kinematic aspects of
  precipitation process in the convective and
  stratiform regions.

00c
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• Precipitation ice is generated as growing drops
  are carried up past the 00c level by the
  convective updraughts.
• The ice particles grow by riming as they accrete
  supercooled cloud drops forming in the updraught
  at mid-toupper levels.

00c
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• Some of particles become quite heavy and fall
  out as part of convective cell.
• More slowly falling particles are spread
  laterally through the stratiform cloud region by
  the horizontal wind as they drift downward.

00c
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• The detrainment of snow from the convective
  cells is thus the mechanism by which
  precipitating ice particles are introduced into
  the stratiform potion of cloud system.

00c
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• The snow particle drift downward while growing
  by vapor dsposition.
• In the layer between 0 -120c, the particles
  aggregate to form large snowflakes and apparently
  sometime grow by riming as well.

00c
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• Rutledge and Houze(1987) Withou the lateral
  influx of snow, their model stratiform cloud was
  incapable of producing significant rain.
• Without the mesoscale ascent within the
  stratiform cloud, only ablou ¼ stratiform
  precipitation reached the ground.

00c
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• The deep convective cells seed the stratiform
  cloud with snow generated in the convection, but
  mean ascent in the stratiform region leads to a
  large increase in the mass of precipitating
  condensate through vapor deposition- a process in
  which a large amount of latent heat is released.

00c
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2. Review of conceptual model (b) Diabatic
heating associated with the idealizded cloud
system
(i) (ii)
(iii) (iv) (v)
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(i) (ii)
(iii) (iv) (v)

• scloud fraction of A covered by cloud
• Qrc net radiative heating in cloud
• sc fraction of A covered by convective
• precipitation region
• ss fraction of A covered by stratiform
• precipitation aera
• Lv and Lf latent heats of vaporization
• and fusion
• ccu condensation in convective updraughts
• ecd evaporation in convective downdraughts
• cmu condensation in meaoscale updraughts of
  stratiform region
• emd evaporation in meaoscale downdraughts of
  stratiform area
• m melting in stratiform region
• w dp/dt
• s dry static energy

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(i) (ii)
(iii) (iv) (v)
The dominant terms are (ii) and (iii), which are
in tern approximately proportional to the mean
profile of vertical velocity (w) in the
convective and stratiform region respectively.
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Houze (1982)
Convection
Stratiform
Radiative processes
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Houze (1982)
Convection
Stratiform
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The total heating in large-scale by idealized MCSs

• In upper level, the total heating are more
  significant then convection alone.
• In lower layer, the stratiform tend to cancel
  the heating from convection.
• (evaporation and
  melting)

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Johnson and Young(1983) rawinsonde data from
Winter MONEX in Stratiform region
Johnson and Young(1983)
Conceptual model
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Johnson (1984) reexamine the total diabatic
heating from Yanai(1973) for a region on western
Pacific.
Relative lower-level maximum may caused by small
non-precipitating cumulus at lower altitudes in
his estimation.
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3. Applicability of the conceptual model
(a) Tropical cloud clusters

• Williams and Houze(1987)
• 85 cumulative cloud cover
• was associated with clusters
• that had a maximum size
• 3104km2 in winter MONEX.

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EMEX
GATE squall line
stratiform
-470c
38 dBZ
stratiform
Winter MONEX
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3. Applicability of the conceptual model
(b) Bay of Bengal depressions Consistent with the
conceptual model , updraughts are strong enough
to support growth of ice by vapor deposition
above melting level.
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3. Applicability of the conceptual model
(c) Mid-latitude mesoscale convective systems
The secondary maximum reflectivity is related to
the fallout of ice particles from the convective
line.
Squall line in Oklahoma-Kansas PRE-STORM
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3. Applicability of the conceptual model
(d) Hurricanes
Marls and Houze (1987)
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3. Applicability of the conceptual model
(e) Summary of applicability of the conceptual
model It is evident that the conceptual model
designed initially to describe tropical cloud
clusters applies to a wide variety of MCSs,
including various types of tropical cloud
clusters, mid-latitude convective complexes,
subtropical monsoon depression and hurricanes.
In all of these precipitation systems, large
areas of stratiform rain accompany deep
convection. The different types of systems vary
in the horizontal arrangement of the convective
and stratiform precipitation. However, in
vertical cross-section they all resemble to the
conceptual model.
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (a) stratiform region profile
• (i) tropical oceanic and island cases
• Rawinsonde and aircraft-observed wind
• Single-Doppler weather radar
• Synthesis of dual-Doppler weather radar
• Vertical-incidence profiler

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All show upward motion in the upper troposphere.
J82South China Sea (rawinsonde
data) HRTZ,HRSF,GH85eastern tropical Atlantic
(rawidsonde and aircraft data) B87single island
station in the west Pacific (vertical-incidence
profiler)
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4. Observed vertical velocity profiles in
convective and stratiform region (a)
stratiform region profile (ii)
continental tropical cases
Squall line system over western equatorial Africa
(single Radar VAD)
80 km 120 km 150 km
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (a) stratiform region profile
• (iii) Mid-latitude continental cases
• OL rawinsonde composite analysis
• SH1 (behind the convective line)
• SH2 (behind the SH1 region)
• dual Doppler radar synthesis
• These cases exhibit maxima and minima
• that are a factor two greater then
• tropical oceanic cases.

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4. Observed vertical velocity profiles in
convective and stratiform region (a)
stratiform region profile (iii)
Mid-latitude continental cases Kansas and
Oklahoma squall line cases from
single Doppler radar VAD Curve12 northern edge
of the stratiform Curve3
center of the stratiform region Illinois
single Doppler radar VAD
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (b) convective region profile
• (i) individual updraughts and
  downdraughts
• The results of convection regions are not as
  consist as in straatiform.
• One difficulty with the convective regions is
  the small scale of the individual updraughts and
  downdraughts and the problems associated with
  sampling them, either individual or as a group.
• LeMone and Zipser(1980) GATE aircraft
  measurements of w
• w gt 1 m/s
• segment gt 500m

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• The convective downdraughts were primarily that
  in the lower troposphere and were the result of
  precipitation loading and precipitation into dry
  environmental air entrained at low to middle
  levels.
• Upper level downdraught adjacent to the upper
  portions of intense convective updraughts have
  now been found in mid-latitude squall lines.

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PRE-STORM, dual-Doppler radar analysis
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (b) convective region profile
• (ii) a mid-altitude continental case.
• OL (Oklahoma) rawinsonde
• composite analysis
• SH85 single Doppler radar
• VAD
• The height of maximum w are
• similar .
• The maximum value (0.4 0.5
• m/s) is considerably less then
• estimated from individual
• draughts.

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4. Observed vertical velocity profiles in
convective and stratiform region (b)
convective region profile (iii)
Tropical oceanic island cases Gamache and Houze
(1982,1985), Houze and Rappaport (1984)
composite ship rawinsondes and aircraft data in
and near GATE squall line
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GH82 AVE hand-analysed data which only have
eight levels of data GH82 PEAK analysed from a
hurricane case where have maximum w GH85 from
12 Sep. 1974 storm HR84 hand-analysed data
which only have five levels of data
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (b) convective region profile
• (iv) Tropical continental cases
• From western Africa squall systems during COPT81
  
• Chong et al. (1983) dual-Doppler analysis
• Chong (1983) single-Doppler VAD (R 40km)
• The continental convection have a higher
  altitude of maximum w

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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (b) convective region profile
• (v) Tropical island cases
• Balsley(1988) averaging the Phnpei profiler
  data for all heavy rain
• Bgt12.5 accumulative rainfall exceeding 12.5 mm
• Bgt25 accumulative rainfall exceeding 25 mm
• Resemble the continental tropical (COPT81) data

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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (c) Consistency of observed vertical velocity
  profiles with large-scale heating profiles of the
  conceptual model MCS
• (i) stratiform-region heating profile
• The conceptual model heating rate of Houze(1982)
  were calculated from a large-scale region over
  which an average of 3 cm/day rain was falling.
• Johnson (1984) reassessed the western Pacific
  heating profile, he estimate the rain rate 1.4
  cm/day in ITCZ region.
• Adopt 1.4 cm/day as a typical rain rate and
  normalize both Johnson (1984) and Houze(1982).

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4. Observed vertical velocity profiles in
convective and stratiform region (c)
Consistency of observed vertical velocity
profiles with large-scale heating profiles of the
conceptual model MCS (i)
stratiform-region heating profile H
Houze(1982) nonradiative heating rate J Johnson
(1984) HR Houze(1982) new startiform vertical
velocity profile (HRSF)
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4. Observed vertical velocity profiles in
convective and stratiform region (c)
Consistency of observed vertical velocity
profiles with large-scale heating profiles of the
conceptual model MCS (i)
stratiform-region heating profile

• The mesoscale condensation and evaporation in
  the stratiform region dominate the diabatic
  heating in the stratiform region, this term can
  alone be used to estimate stratiform-region
  heating profile.
• The vertical velocity profiles of mid-latitude
  and continental tropical squall line are a factor
  2 or 4 greater then in tropical oceanic and
  island stratiform region.

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4. Observed vertical velocity profiles in
convective and stratiform region (c)
Consistency of observed vertical velocity
profiles with large-scale heating profiles of the
conceptual model MCS (i)
stratiform-region heating profile SH
stratiform region of a mid-latitude
squall line MCS
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4. Observed vertical velocity profiles in
convective and stratiform region (c)
Consistency of observed vertical velocity
profiles with large-scale heating profiles of the
conceptual model MCS (ii)
convective-region heating profile H
Houze(1982) J Johnson (1984) both normalized to
a 1.4 cm/day rain rate The difference in
the vertical profile of heating may be associated
with the inclusion of the effects of smaller
cumulus in Johnsons estimate but might also
result from Houzes assumption weakly entraining
jet to represent the vertical velocity profile.
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• 4. Observed vertical velocity profiles in
  convective and stratiform region
• (c) Consistency of observed vertical velocity
  profiles with large-scale heating profiles of the
  conceptual model MCS
• (ii) convective-region heating profile
• The convective-region vertical velocity profile
• SH, HR84 and GH85 replaced the
• Houzr(1982)
• Suggest a lower tropospheric maximum of
• heating.

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4. Observed vertical velocity profiles in
convective and stratiform region (c)
Consistency of observed vertical velocity
profiles with large-scale heating profiles of the
conceptual model MCS (ii)
convective-region heating profile
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• 5. Conclusions
• The mesoscale convective systems responsible for
  most tropical rainfall exhibit a structure in
  which regions of deep convection are accompanied
  by mesoscale stratiform precipitation areas.
• The stratiform cloud is based in the
  mid-troposphere and contains a mesoscale
  updraught, while mesoscale subsidence occurs
  below the cloud base.
• Ice particles detrained or left aloft by the
  deep convective towers are advected horizontally
  into the stratiform cloud by storm-relative winds
  and grow through deposition of water vapour
  condensed in the mesoscale updraught.
• The ice particles melt into raindrops that
  partially evaporate in the region of subsidence
  below the stratiform cloud base.

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• 5. Conclusions
• Houze(1982) demonstrated that this heating is
  determined largely by the condensation and
  evaporation associated with the vertical air
  motion in the convective and stratiform regions.
• The level of zero vertical air motion, assumed
  by Houze to occur at the 00c level, actually
  occurs at or slightly above the 00c level. The
  height of w0 can vary within a single storm from
  0 to 2 km above the 00c level.
• It appears that this reduction of the net
  heating occurs through a slightly deeper layer
  than previously thought.
• Data on vertical motion profiles in the
  convective regions of MCSs show more variation
  from study to study than do the observations of
  stratiform-region vertical motions.

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• 5. Conclusions
• Heating profile in stratiform regions are
  consistent with the estimation from Houze
  heating at upper levels and cooling at lower
  levels
• However, a variety results have been emerged for
  the convective regions, as a result of the
  various vertical motion profile.
• Future work should be focused on the variation
  of the convective-region profiles from one
  large-scale situation to the next and on the
  environmental factors controlling these changes.