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)
Fig. 9.8 from R&Y: 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:
Precipitation
threshold
- r0 = 25 mm
-continuous collection growth
(eq. 8.15)
- collected drop radii, 10 mm
- LWC = 1 g m-3
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
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)
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 R&Y
Types of precipitation
Stratiform
Large variations in the
vertical, small in the horizontal
Weak w, < 1 m s-1 (w < 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
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
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 > 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
Two examples illustrating convective vs. stratiform
t-z section of 915 MHz profiler + surface rainfall rate (Tokay et al 1999)
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
The four parameters used by Stalker and Knupp (2002) to identify convective cells
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
Observations of two primary cells in a multicell thunderstorm in Florida.
Updraft measurements in a supercell storm (left) and ordinary cell storm (right)
A simple flowchart of
precipitation growth
Remember this diagram
A complex
flowchart
Production of precipitation in convective
storms
• Storms that generate precipitation have (at the surface)
precip sizes < 10 mm (mostly rain, some small hail)
– warm cloud, cold cloud, and a mix of both
• Hailstorms, with hail size >10 mm at the surface
– microphysics is more complicated by the presence of
very large hydrometeors
Rain production in convective storms
Three cases:
1. pure warm cloud
•
collision-coalescence is dominant
2. pure cold cloud
•
two primary growth processes (two-stage process):
– growth of ice crystals (snow) by diffusion
– growth of graupel by collection (accretion)
3. Hybrid (combo of warm/cold processes)
•
most complex of the three classes
– all three primary growth processes can be active
Case 1
Precipitation growth in warm clouds
Fig. 8.10. Simplified schematic of the
precipitation processes active in clouds.
Taken from Lamb (2001).
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)
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.
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, > 20 dBZ
Z = ∑niDi6 (mm6 m-3)
Z

 N(D)D dD
6
0
dBZ = 10 log10 (Z/Z0)
Change in curvature is
onset of
collision/coalescence
Dependent on the CCN spectrum,
cloud base T, and 
updraft speed.
Define entrainment rate:

1 dM
M dz
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).
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.
Warmer cloud base implies higher water
vapor mixing ratio, and hence higher
adiabatic liquid water content.
Fig. 9.11. Z vs time for different cloud
base T. w = 1 m/s. From Young 1993
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.
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).
Growth of pristine ice
and snow by deposition
Mixed phase
Growth of graupel/snow by:
a) Riming
b) Deposition
Limited raindrop growth
Melting of ice
Rain core
Cloud nucleation
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
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
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).
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
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
Evolution of a multicell thunderstorm in
Florida. Developing stage, precipitation is
significant
Mixed phase cloud, with most of the initial
precipitation development by collisioncoalescence process
 How do we know?
 Dual-pol variable (ZDR)
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
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
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
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
Evolution of a multicell thunderstorm in
Florida. Developing stage, precipitation is
significant
Mixed phase cloud, with most of the initial
precipitation development by collisioncoalescence process
 How do we know?
 Dual-pol variable (ZDR)
 Large ZDR at lower levels suggests
large drop development through C-C
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
REF
ZDR
High Reflectivity
Near 0 ZDR
Hail Spike
KHTX at 2053 UTC from 3/2/2012
Hytop 88D
ARMOR
REF
REF
ZDR
ZDR
KHTX and ARMOR Examples from March 2, 2012
Late mature stage: Updraft now found at
extreme upper levels of storm
Largest drops near the surface  drop
fallout, precip loading,
Storm is dying
Cell A evolution: t-z sections of radar parameters
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 (> -25 dB)
– Hail, hail and rain mixtures (-20 to -10 dB)
– Wet Snow (-13 to -18 dB)
Mature phase:
a) w, Z and ZDR – warm and cold
microphysical processes are active
b) LDR indicates the presence of wet,
tumbling ice.
c) X-band attenuation is most substantial for
water-coated ice.
Good example of mixed phase growth.
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.
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 > 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.
• Early stage development
• Zmax (40-45 dBZ)
located at 4-4.5 km
• ZDR > 1.5 coincides with
Z > 45 dBZ
• Indicates Z core consists
of raindrops with D > 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
•
~6 minutes later… the storm has
intensified and grown vertically
•
Zmax > 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
•
~6 minutes later… storm continues to
strengthen and grow vertically
•
Zmax > 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 (>3.5 dB), located under Z
core from 3 km to sfc is likely caused
by melting of hail into large raindrops
• ~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
• Final image showing
continued decay of
updraft and collapse of
storm
• Graupel aloft now
extends toward surface
• Microburst