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.
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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.
Z > 10 dBZ
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
Less substantial variations in the vertical, large in the horizontal
Strong w, 5-50 m s-1
Evolution to stratiform
Quasi-steady state process, function of height
Fig. 6.1 from Houze
Time-dependent process, but also a function of height
Water-coated or spongy ice
0 2 4 6 8 10
Vertical variation of particle types within a nimbostratus stratiform cloud system
Mean diameter (mm)
Growth of pristine ice by deposition
Some growth by deposition, riming
Primary growth by aggregation
Change in Z due to various processes (Wexler 1955), p. 200 in R&Y
Melting VT Shape Condensation Total
Snow to bright band +6 -1 +1.5 0 +6.5 dB
Bright band to rain +1 -6 -1.5 +0.5 -6 dB
Growth by vapor deposition
Deposition, riming (?) and aggregation
Aggregation + melting
Conversion to raindrops,
breakup of aggregates (?)
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.
Fig. 6.3 from Houze. Ice particle concentration obtained from aircraft flights through nimbostratus in tropical MCSs over the Bay of Bengal.
Fig. 6.10 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.
Profile of liquid dependent on ice process/types
Polarimetric Hydrometeor ID
Vertical Cross-Section 300o
Proprietary information, Walter A. Petersen, University of Alabama Huntsville
(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
Small ice crystals
Snow (1-2 mm)
large aggregates (5-10 mm)
large raindrops (2-3 mm)
small raindrops (1-2 mm)
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.
Fall speeds for snow vs. fall speeds for rain
Low in snow (not much variation in fall speeds), high in rain (greater variation in fall speeds)
Radial Velocity (vertical)
Radial Velocity (vertical)
Stratiform rain system with bright band and large aggregates near the bright band (relatively common)
Shallow convective cloud, small drops (0.5 mm diameter)
Shallow convective cloud, large drops (e.g., the Hawaiian shallow clouds that develop raindrops to diameters of 5-8 mm; Rauber et al 1991).
Deep convective cloud with graupel, snow, aggregates, and rain
Item (4), with the addition of hail
Clouds without large precipitation
Stratocumulus clouds, between 0.2 and 0.8 km above sea level (ASL), with 0.2 mm drizzle droplets (common)
Cirrus clouds, between 8 and 12 km ASL, with ice crystals up to 1 mm in diameter (common)