Precipitation growth (Ch. 9, last section)

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Precipitation growth (Ch. 9, last section)

• Coalescence vs. ice crystal process
• Which produces a faster growth?
• Diffusional growth to precipitation size is
  possible for ice crystals, but not for water
  drops.
• Need collection processes to get water to
  precip sizes
• For mixed phase clouds, where all processes may
  be active, which process dominates warm or cold
  cloud growth processes?
• Mixed phase precipitation growth
• At what T is growth at a maximum?
• -15C (in a cold cloud)

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Fig. 9.8 from RY example calculations showing
relative growth times for ice crystal and water
droplet

• Assumptions
• Ice crystal
• - stellar dendrite, C 2r/p
• - water saturation
• - T -15 ?C (near optimum T)
• initial mass 10-8 g
• (eq. 9.4)
• Water droplet
• - r0 25 mm
• continuous collection growth
• (eq. 8.15)
• - collected drop radii, 10 mm
• - LWC 1 g m-3

Precipitation threshold
What grows faster ice by diffusion or water by
collection? Initially, ice crystal grows faster
and gets to precip size quicker. However,
raindrop can exceed the growth rate of ice crystal
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Ice Crystal Ice crystal grows rapidly by
diffusion Fractional rate of increase in mass,
m-1(dm/dt) is large initially Grows more rapidly
than the droplet up to 7 min. Growth to
precipitation size (4 mg) in about 10 min Water
droplet Initial droplet growth is slow due to
low values of collection efficiency (collection
kernel is small) m-1(dm/dt) is small, but
increases rapidly to a nearly steady value. After
30 min, the droplet mass matches that of the ice
crystal Growth to precipitation size (4 mg) in
about 20 min.
Fig. 9.8. Times required for an ice crystal and
a water droplet (solid curves) to grow to the
indicated mass. Top scale gives the
corresponding drop radius. Dashed curves are for
the rates of fractional mass increase, referred
to the scale on the right. (Rogers and Yau, 1989)
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Precipitation processes

• Types of precipitation
• Stratiform
• Convective deep (mixed phase) and shallow
  (warm)
• Mixed stratiform-convective
• Organization of precipitation
• Precipitation theories
• Mesoscale structure of rain

Read Chap. 12 of RY
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Types of precipitation
Stratiform Large variations in the vertical,
small in the horizontal Weak w, lt 1 m s-1 (w lt
VT) Precipitation growth during the fall of a
precipitation particle Convective Less
substantial variations in the vertical, large in
the horizontal Strong w, 5-50 m s-1 Time
dependence Evolution to stratiform
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Stratiform vs. convective rain

• Stratiform
• Temporal variability (?Z/?t) is small
• Large gradients (?Z/?z) in the vertical
• Large horizontal extent (small horizontal
  variations in Z
• Presence of a bright band
• Growth occurs as the precipitation particle falls
• Particles pristine ice, snow, aggregates, rain
  drops
• Convective
• Temporal variability is large
• Horizontal gradients (?Z/?s) are large
• Growth occurs at steady height (may rise or fall
  slowly)
• Particles pristine ice, snow, graupel, hail,
  aggregates, raindrops

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A physical definition of convective vs.
stratiform precipitation

• Convective precipitation
• hydrometeors move upwards at some point during
  the growth phase
• growth time scale 20-30 min
• Rain rate, R gt 10 mm hr-1
• Stratiform precipitation
• hydrometeors fall during growth
• R typically 1-5 mm hr-1
• growth time scale 1-2 h for a deep Ns system
• significant stratiform precipitation likely
  requires an ice phase
• the exception is drizzle from Sc, but this is not
  significant

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Two examples illustrating convective vs.
stratiformt-z section of 915 MHz profiler
surface rainfall rate (Tokay et al 1999)
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Schematic of a convective cell the elementary
building blocks of convective precipitation.
This is a vertical cross section through the
core of an updraft cell. From Stalker and Knupp
(2002). Wpbl contour shows the cell origin is
within the PBL. Hd threshold cloud layer height
where a minimum updraft strength of Wd must
develop Wd threshold diluted updraft, i.e.
updraft in an actual environment that is diluted
by entrainment of subsaturated air into the cloud
volume ? criterion of identifying precipitating
convective cells Dd threshold cloud layer
depth Ad threshold updraft area
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The four parameters used by Stalker and Knupp
(2002) to identify convective cells
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An updraft cell will produce precipitation if the
updraft is sustained. At this point, the updraft
cell and precipitation (Z) cell are spatially
correlated. Note the 2-3 km horizontal dimension
of the cell.
Max 17 m/s
Max 40 dBZ
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Observations of two primary cells in a multicell
thunderstorm in Florida.
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Updraft measurements in a supercell storm (left)
and ordinary cell storm (right)
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A simple flowchart of precipitation growth
Remember this diagram
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A complex flowchart
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Production of precipitation in convective storms

• Storms that generate precipitation have (at the
  surface) precip sizes lt 10 mm (mostly rain, some
  small hail)
• warm cloud, cold cloud, and a mix of both
• Hailstorms, with hail size gt10 mm at the surface
• microphysics is more complicated by the presence
  of very large hydrometeors

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Rain production in convective storms

• Three cases
• pure warm cloud
• collision-coalescence is dominant
• pure cold cloud
• two primary growth processes (two-stage process)
• growth of ice crystals (snow) by diffusion
• growth of graupel by collection (accretion)
• Hybrid (combo of warm/cold processes)
• most complex of the three classes
• all three primary growth processes can be active

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Case 1 Precipitation growth in warm clouds
Fig. 8.10. Simplified schematic of the
precipitation processes active in clouds. Taken
from Lamb (2001).
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Formation of precipitation by coalescence
What eqns describe this chart?
Fig. 9.1. The activation of a population of CCN
in an updraft of 2 m/s. Particle mass (salt
water) is shown as a function of height. The
peak supersaturation of 1.14 at a height of 1.27
m above the start. The level at which S 1.0
(base) is 97 m. At the starting height, CCN were
assumed to be in equilibrium at S 0.95. Above
the dashed line, particles are larger than their
critical size. Note the sharp distinction
between activated and non-activated CCN. From
Young (1993)
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Radar measurements of initial raindrop formation
from nucleation on giant CCN (NaCl) in FL.
What is Z?
Fig. 13. NCAR CP2 X-band radar reflectivity
evolution of two small cumulus clouds on 5 and 10
Aug 1995. Reflectivity calculated from SCMS
composite droplet distributions are shown for
their corresponding 0.5-km layers on (B), (C),
(F), and (G). Radar scan times (UTC) and azimuth
angle are shown for each panel. From Laird et al
2000.
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Time required to produce precipitation warm
cloud
No entrainment ? precip develops quickly
Quasi-stochastic model of coalescence that
involves activation of CCN (Young
1975) Precipitation threshold in terms of radar
reflectivity factor, gt 20 dBZ Z ?niDi6 (mm6
m-3) dBZ 10 log10 (Z/Z0) Dependent on the
CCN spectrum, cloud base T, and updraft speed.
Change in curvature is onset of
collision/coalescence
Define entrainment rate
Fig. 9.9. Radar reflectivity factor (dBZ) as a
function of time for different entrainment rates.
Cloud base T is 10 ?C, w 3 m/s. From Young
(1993).
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Warmer cloud base implies higher water vapor
mixing ratio, and hence higher adiabatic liquid
water content.
Maritime CCN promote higher precipitation
efficiency via the warm cloud process
Fig. 9.10. Z as a function of time for different
CCN spectra. Cloud base T is 15 C, w 3 m/s.
From Young 1993.
Fig. 9.11. Z vs time for different cloud base T.
w 1 m/s. From Young 1993
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Formation of precipitation by cold cloud processes
Glaciation conversion of supercooled droplets
into ice via introduction of ice (both nucleation
and multiplication)
p. 267 material here
Fig. 10.1 Temperature rise (contoured)
associated with glaciation at p 700 mb. The
broken line indicates that glaciation occurs with
water vapor phase balance. From Young 1993.
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Precipitation growth in cold convective clouds
All these processes can occur in cold convective
clouds
Fig. 8.10. Simplified schematic of the
precipitation processes active in clouds. Taken
from Lamb (2001).
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Growth of pristine ice and snow by deposition

• Growth of graupel/snow by
• Riming
• Deposition

Mixed phase
Limited raindrop growth
Melting of ice
Cloud nucleation
Rain core
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Simplified precipitation growth within deep
convection
Deposition
Because of descending air
A lot of cooling occurs due to melting/evaporation
? origin of gust front/cold pool in convective
cells
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Recall that growth at -15 ºC can be rapid

• Bergeron process For a mixed phase cloud, very
  high ice supersaturations are maintained as long
  as supercooled water exists (via nucleation
  within the updraft)
• Around -15 ?C, two precipitation growth processes
  are active
• Ice crystal growth by diffusion (deposition is
  optimum
• Accretion is active and quite efficient
• Ice crystals grow at expense of supercooled
  droplets

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Bergeron process
Fig. 6.36 Laboratory demonstration of the growth
of an ice crystal at the expense of surrounding
supercooled water drops. Photograph courtesy of
Richard L. Pitter. Taken from Wallace and Hobbs
(2005).
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Fig. 10.7 Comparative growth of a water drop and
a frozen drop. Particle growth trajectories are
shown for a uniform updraft of 5 m/s with cloud
base T 20 C. Both particles are introduced at
the -8 C level as 0.25 mm water drops with one
allowed to freeze at the start of the
calculations. From Johnson (1987), taken from
Young 1993.
Graupel is growing faster ? accretion very
efficient
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Defining three cloud zones for hail growth EFZ
embryo-formation zone -Weak updrafts so graupel
particles have time to grow HGZ hail growth
zone -Embryos that grow sufficiently enter this
zone. -Stronger updraft needed to hold the
hailstone in suspension -Updraft too weak,
hailstone falls out, updraft too strong, pushes
updraft into anvil where there is little liquid
water FOZ fallout zone Bottom figure shows
suggest pathway for hail growth
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• Evolution of a multicell thunderstorm in Florida.
  Developing stage, precipitation is significant
• Mixed phase cloud, with most of the initial
  precipitation development by collision-coalescence
  process
• How do we know?
• Dual-pol variable (ZDR)

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Dual-Polarization

• In the past, most radars only had the capability
  to transmit/receive horizontally polarized waves
• Targets sampled only in the horizontal dimension
• Dual-pol radars allow the transmit/receive of
  both horizontally vertically polarized waves.
• Targets sampled in both the horizontal and
  vertical dimension

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Dual-Polarization

• Using dual-pol radars, we can learn more about,
  the size, shape, and composition of precipitation
  particles
• Benefits include
• Improved radar based rainfall totals
• Improved ability to identify areas of heavy
  rainfall
• Improved detection and mitigation of non-weather
  echoes
• Easier identification of the melting layer during
  winter weather
• Ability to classify precipitation type
• New severe thunderstorm signatures

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Differential Reflectivity (ZDR)

• Ratio of the reflected horizontal and vertical
  power returns
• Highly dependent on the shape and size of
  hydrometeors
• Values typically range from -7.9 to 7.9 dB
• ZDR can aid in identifying
• Hail
• Melting layer
• Rain/snow transition
• Frozen precipitation types

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• ZDR Rain
• Strong relationship between raindrop diameter and
  shape
• Smaller drops tend to be spherical
• Horizontal and vertical pulses are similar
• Low ZDR
• As drops become larger, they become more oblate
• Higher ZDR

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• Evolution of a multicell thunderstorm in Florida.
  Developing stage, precipitation is significant
• Mixed phase cloud, with most of the initial
  precipitation development by collision-coalescence
  process
• How do we know?
• Dual-pol variable (ZDR)
• Large ZDR at lower levels suggests large drop
  development through C-C

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• ZDR Hail
• Unlike rain, hail does not have a definite
  relationship between size and shape
• Hail tends to tumble as it falls, appearing as an
  effective sphere to the radar
• ZDR is biased near 0 dB
• Classic hail signature is high reflectivity
  collocated with low ZDR

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REF
ZDR
High Reflectivity
Near 0 ZDR
Hail Spike
KHTX at 2053 UTC from 3/2/2012
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Hytop 88D
ARMOR
REF
REF
ZDR
ZDR
KHTX and ARMOR Examples from March 2, 2012
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Late mature stage Updraft now found at extreme
upper levels of storm Largest drops near the
surface ? drop fallout, precip loading, Storm
is dying
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Cell A evolution t-z sections of radar parameters
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Linear Depolarization Ration (LDR)

• Ratio of a vertical power return from a
  horizontal pulse or a horizontal power return
  from a vertical pulse
• Detects tumbling, wobbling, canting angles, phase
  and irregular shaped hydrometeors
• Large Rain Drops (gt -25 dB)
• Hail, hail and rain mixtures (-20 to -10 dB)
• Wet Snow (-13 to -18 dB)

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• Mature phase
• w, Z and ZDR warm and cold microphysical
  processes are active
• LDR indicates the presence of wet, tumbling ice.
• X-band attenuation is most substantial for
  water-coated ice.
• Good example of mixed phase growth.

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Vigorous growth of convective cell Strong updraft
lofts large drops (high ZDR) above the freezing
level The frozen drops experience rapid growth
by accreting cloud water. This leads to what is
known as an LDR cap, indicating mixed phase
precip.
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Examples from MIST

• Microburst and Severe Thunderstorm (MIST)
• A single-cell storm from 20 July 1986
• Well studied Wakimoto and Bringi (1988) Goodman
  et al. (1988) Tuttle et al. (1989) Kingsmill
  and Wakimoto (1991) Zeng et al. (2000)
• Produced hail within 10 min of radar detected Z gt
  10 dBZ extremely efficient accretional growth
  processes
• Microburst
• Focus is on the development and interaction of
  supercooled water, graupel and hail as related to
  the rapid development and demise of the dominant
  accretional growth period.

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• Early stage development
• Zmax (40-45 dBZ) located at 4-4.5 km
• ZDR gt 1.5 coincides with Z gt 45 dBZ
• Indicates Z core consists of raindrops with D gt
  1.8 mm
• Elevated ZDR exists slightly above 0C level
  indicate supercooled drops
• This suggests the formation of initial
  precipitation core was dominated by coalescence

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• 6 minutes later the storm has intensified and
  grown vertically
• Zmax gt 55 dBZ at 7.5 km
• Strong LDR values (-18 to -13 dB not shown) was
  associated with the Zmax, indicating substaintial
  depolarization caused by frozen drops or tumbling
  irregular shaped hail
• ZDR values were weak to moderate (0.5 to 2.5 dB)
  in the Z core combination of all three variables
  suggests coexistence of liquid water and hail
• Updraft enters the left side of the cell and
  slants slightly upward vertical velocity center
  is collocated with supercooled water and hail is
  present at top of the updraft
• Hail formed rapidly between the two periods
  initial hail embryos were likely large drops that
  formed by coalescence of liquid water below 6 km,
  rose, froze into large drops, and continued to
  collect small cloud droplets

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• 6 minutes later storm continues to strengthen
  and grow vertically
• Zmax gt 65-70 dBZ between 6-8 km
• ZDR in top half of the core (6-10 km) were
  negative, indicating hail or graupel
• Large amount of supercooled water seen previously
  has now glaciated in the elapsed 6 minutes
• Rapid growth of echo top suggests the latent heat
  release during glaciation may have played a role
  in the rapid growth of the upper portion of the
  cell
• Hydro ID shows bottom of hail region has sank
  below 0C, associated with negative vertical
  velocity on SW side
• Large ZDR (gt3.5 dB), located under Z core from 3
  km to sfc is likely caused by melting of hail
  into large raindrops

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• 6 minutes later Z core has descended
  dramatically
• Positive ZDR column on west side of cell
  indicates raindrops elevated positive ZDR
  indicates the updraft was still active in this
  part of the cell, despite storm collapse
• East side of cell, horizontal 0.5 dB ZDR contour
  is clear boundary b/w ice and water
• Despite hail fallout, graupel remains elevated in
  upper portion of the storm, suspended by positive
  vertical motion

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• Final image showing continued decay of updraft
  and collapse of storm
• Graupel aloft now extends toward surface
• Microburst