Transcript Nimbostratus and stratiform precipitation

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
Nimbostratus and stratiform precipitation
• Classification of precipitation
– stratiform: |w| < Vice, where Vice is in the range 1-3 m s-1
• Vice refers to snow and aggregates
– convective: |w|  Vice
• A more detailed classification (see following figure)
–
–
–
–
shallow convection
deep convection
mixed convective/stratiform
(pure) stratiform
Precipitation formation: simple flow chart
Fig. 8.10. Simplified schematic of the
precipitation processes active in clouds.
Taken from Lamb (2001).
Compare with the complex diagram in
thefollowing frame.
Precipitation formation: complex flow chart
A classification scheme based on vertically pointing radar data
(Tokay et al 1999)
bright band
spectrum width
Z > 10 dBZ
Examples of automated precipitation
classification scheme based on the
preceding algorithm. From Tokay et
al (1999, JAM).
From Tokay et al (1999, JAM).
From Tokay et al (1999, JAM).
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
The importance of stratiform precipitation
• For the Huntsville region, stratiform precipiation
occurs ~99% of the time (large area)
• A much greater fraction of rain originates from
convective precipitation (40-60%)
– Some estimates:
•
•
•
•
DJF - 90% stratiform and 10% convective
MAM - 35% stratiform and 65% convective
JJA - 20% stratiform and 80% convective
SON - 35% stratiform and 65% convective
Types of precipitation: focus on stratiform
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
Conceptual picture of precipitation growth in (a) stratiform
and (b) convective clouds
Quasi-steady state process, function of height
Fig. 6.1 from Houze
Time-dependent process, but also a function of height
Idealized stratiform cloud system
10
Ice crystals
6 km
Height (km)
Snow
Aggregates
0.4 km
4 km
Rain
Melting layer:
Water-coated or
spongy ice
0
0
2
4
6
8 10
Mean diameter (mm)
Vertical variation of particle types within a
nimbostratus stratiform cloud system
Melting within stratiform precipitation produces the radar bright band
Ice crystals
Growth of pristine ice by deposition
Some growth by deposition, riming
Primary growth by aggregation
aggregates
melting
rain
Change in Z due to various processes (Wexler 1955), p. 200 in R&Y
Snow to bright band
Bright band to rain
Melting VT
+6
-1
+1
-6
Shape
+1.5
-1.5
Condensation
0
+0.5
Total
+6.5 dB
-6 dB
Idealized radar profiles around the 0 C level
Growth by vapor deposition
Deposition, riming (?) and aggregation
Aggregation + melting
Conversion to raindrops,
breakup of aggregates (?)
Some notes:
Z for ice is lower than Z for snow of the same water content
because of difference in dielectric constant.
When all ice converts to raindrops, the particle concentration
is reduced due to increase fall speeds.
Tropical Storm Gabrielle
0548
MIPS
1247
MIPS
Variability in the bright band (stratiform regions)
SNR
• 0548 UTC
– thick
– enhanced SW
layer above
– uniform VT
0 C
• 1247 UTC
– thin
– greater SW
below
– decreasing VT
0 C
W
sv
Top panels:
Reflectivity shows the
bright band, Doppler
velocity shows the
increase in fall speed as
snow/aggregates melt to
form rain drops.
Hurricane Isaac
Measured profiles of ice hydrometeors
Fig. 6.3 from Houze. Ice particle concentration obtained from aircraft flights
through nimbostratus in tropical MCSs over the Bay of Bengal.
Structure of a stratiform rainband, showing dynamical and microphysical
processes. Fig. 6.8 from Houze (1993)
Numerical simulation design of precipitation processes in frontal
stratiform precipitation. Fig. 6.9 from Houze
Results of a numerical simulation of precipitation processes in a
frontal stratiform rainband. Each panel shows the rates of
conversion for the process considered (10-4 g kg-1 s-1)
Conceptual model of the development of nimbostratus associated
with deep convection. Fig. 6.11 from Houze
Fig. 6.10 Houze
Schematic of the precipitation mechanisms in a MCS. Solid
arrows are hydrometeor trajectories. From Fig. 6.13 of Houze
Stratiform/convective clouds associated with midlatitude
cyclones and fronts
Examples of a four different narrow cold frontal rainbands. The
location of the cold front is shown. Note the different orientation
of the smaller elements within the rainband. Fig. 11.28 of Houze
Hypothesized airflow along a
cold frontal rainband, and the
development of wave
features due to horizontal
shearing instability. (Fig.
11.30 of Houze).
Schematic of the relative airflow across two
precipitation cores, and the gap between
them, in a narrow cold frontal rainband.
The airflow, represented as wind vectors,
was inferred from Doppler radar. Fig. 11.29
from Houze.
Cloud structure, air motions, and precipitation mechanisms within cold frontal
bands. This structure is derived from aircraft, Doppler radar, and other sources.
Fig. 11.31 of Houze.
Schematic of clouds, precipitation, and thermal field of a warm frontal rainband
as deduced from rawinsonde, aircraft and radar data. The region above the
elevated warm front is convectively unstable (qe decreases with height). Fig.
11.38 of Houze.
UAH/NSSTC ARMOR 10/27/2006: 3-D View of light, stratiform
Rain Rate
Plan view
Profile of liquid
dependent on ice
process/types
Vertical Cross-Section 300o
Polarimetric Hydrometeor ID
Radar Reflectivity
Height (km)
10
5
0
Dry Snow
Light. Rain
Irreg. Ice
Wet Snow
Drizzle
Horiz.-oriented ice
Melting Layer
Proprietary information, Walter A. Petersen, University of Alabama Huntsville
ARMOR: 27 October 2006 Bright band variability and precipitation
(RHI’s over MIPS wind profiler every 2-3 minutes)
+/- 500m oscillations in melting level height, and finally a rise with warm front!
DSD properties from combined profiler/radar retrieval
ARMOR: 10 January 2011 – Tennessee Valley Thundersnow
ARMOR: 10 January 2011 – Tennessee Valley Thundersnow
Stratiform precipitation within a midlatitude cyclone
Small ice crystals
Snow (1-2 mm)
large aggregates (5-10 mm)
large raindrops (2-3 mm)
bright band
small raindrops (1-2 mm)
time
Reflectivity factor measured by a vertically pointing X-band radar
Stratiform precipitation with both ice and water phase is common over large
regions in both the tropics (mesoscale convective systems and tropical storms)
and midlatitudes (within low pressure regions)
The bright band region could be especially problematic.
Vertical air motion is required for precipitation production
Schematic cross section of a wide cold frontal rainband. From Hobbs et al 1980.
Vertically pointing Doppler radar measurements within a stratiform rain band
Reflectivity factor:
Bright band
Rain streaks
Doppler velocity
Fall speeds for
snow vs. fall
speeds for rain
Spectrum width
Low in snow (not
much variation in
fall speeds), high
in rain (greater
variation in fall
speeds)
Reflectivity
Generating Cells
Radial Velocity (vertical)
Spectrum Width
Dry Slot
Reflectivity
Generating Cells
Radial Velocity (vertical)
Precipitation Paths: Possible scenarios
Precipitation
1. Stratiform rain system with bright band and large
aggregates near the bright band (relatively common)
2. Shallow convective cloud, small drops (0.5 mm diameter)
3. Shallow convective cloud, large drops (e.g., the Hawaiian
shallow clouds that develop raindrops to diameters of 5-8
mm; Rauber et al 1991).
4. Deep convective cloud with graupel, snow, aggregates,
and rain
5. Item (4), with the addition of hail
Clouds without large precipitation
a) Stratocumulus clouds, between 0.2 and 0.8 km above
sea level (ASL), with 0.2 mm drizzle droplets (common)
b) Cirrus clouds, between 8 and 12 km ASL, with ice crystals
up to 1 mm in diameter (common)