precipitation processes*
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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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: simple flow chart
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Precipitation formation: complex flow chart
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A classification scheme based on vertically pointing radar data(Tokay et al 1999)
bright band
spectrum width Z > 10 dBZ
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Examples of automated precipitation classification scheme based on the preceding algorithm. From Tokay et al (1999, JAM).
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From Tokay et al (1999, JAM).
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From Tokay et al (1999, JAM).
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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 > 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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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
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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
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Conceptual picture of precipitation growth in (a) stratiform and (b) convective clouds
Fig. 6.1 from Houze
Quasi-steady state process, function of height
Time-dependent process, but also a function of height
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Rain
Ice crystals
Snow
Idealized stratiform cloud system
Aggregates
Melting layer:Water-coated or spongy ice
6 km
4 km
0.4 km
Mean diameter (mm)0 2 4 6 8 10
Hei
ght (
km)
0
10
Vertical variation of particle types within a nimbostratus stratiform cloud system
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Melting within stratiform precipitation produces the radar bright band
Growth of pristine ice by deposition
Some growth by deposition, rimingPrimary growth by aggregation
melting
Ice crystals
aggregates
rain
Change in Z due to various processes (Wexler 1955), p. 200 in R&Y
Melting VT Shape Condensation TotalSnow to bright band +6 -1 +1.5 0 +6.5 dBBright band to rain +1 -6 -1.5 +0.5 -6 dB
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Idealized radar profiles around the 0 C level
Growth by vapor deposition
Deposition, riming (?) and aggregationAggregation + melting
Conversion to raindrops,breakup of aggregates (?)
Some notes:Z for ice is lower than Z for snow of the same water contentbecause of difference in dielectric constant.When all ice converts to raindrops, the particle concentrationis reduced due to increase fall speeds.
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Tropical Storm Gabrielle
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0548
MIPS
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1247
MIPS
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Variability in the bright band (stratiform regions)
• 0548 UTC– thick
– enhanced SW layer above
– uniform VT
• 1247 UTC– thin
– greater SW below
– decreasing VT
SNR W v
0 C
0 C
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Top panels:Reflectivity shows the bright band, Doppler velocity shows the increase in fall speed as snow/aggregates melt to form rain drops.
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Hurricane Isaac
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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.
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Structure of a stratiform rainband, showing dynamical and microphysical processes. Fig. 6.8 from Houze (1993)
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Numerical simulation design of precipitation processes in frontal stratiform precipitation. Fig. 6.9 from Houze
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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)
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Conceptual model of the development of nimbostratus associated with deep convection. Fig. 6.11 from Houze
Fig. 6.10 Houze
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Schematic of the precipitation mechanisms in a MCS. Solid arrows are hydrometeor trajectories. From Fig. 6.13 of Houze
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Stratiform/convective clouds associated with midlatitude cyclones and fronts
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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
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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.
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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.
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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 (e decreases with height). Fig. 11.38 of Houze.
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UAH/NSSTC ARMOR 10/27/2006: 3-D View of light, stratiform
Horiz.-oriented iceLight. Rain
Wet Snow
Dry Snow
Irreg. Ice
Drizzle
Polarimetric Hydrometeor ID Radar Reflectivity
Melting Layer
He
igh
t (k
m)
10
0
5
125 km
Vertical Cross-Section 300o
Plan view
Rain Rate
Profile of liquid dependent on ice process/types
Proprietary information, Walter A. Petersen, University of Alabama Huntsville
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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
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ARMOR: 10 January 2011 – Tennessee Valley Thundersnow
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ARMOR: 10 January 2011 – Tennessee Valley Thundersnow
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bright bandlarge aggregates (5-10 mm)
large raindrops (2-3 mm) small raindrops (1-2 mm)
Snow (1-2 mm)
Stratiform precipitation within a midlatitude cyclone
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.
Small ice crystals
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Vertical air motion is required for precipitation production
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Schematic cross section of a wide cold frontal rainband. From Hobbs et al 1980.
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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)
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Reflectivity
Radial Velocity (vertical)
Spectrum Width
Generating CellsDry Slot
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Reflectivity
Radial Velocity (vertical)Generating Cells
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Precipitation1. 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 precipitationa) 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)
Precipitation Paths: Possible scenarios