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

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

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

Fig. 6.1 from Houze

Quasi-steady state process, function of height

Time-dependent process, but also a function of height

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

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

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.

Tropical Storm Gabrielle

0548

MIPS

1247

MIPS

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

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 (e decreases with height). Fig. 11.38 of Houze.

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

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

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

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

Radial Velocity (vertical)

Spectrum Width

Generating CellsDry Slot

Reflectivity

Radial Velocity (vertical)Generating Cells

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