Precipitationand
Intro to Radar
ATS 351Lecture 7
October 19, 2009
Droplet Formation Recall the two types of nucleation
Homogeneous Nucleation Water molecules come together to form a cloud
droplet Heterogeneous Nucleation
Requires a cloud condensation Nuclei (CCN)
Heterogeneous Nucleation
Droplet Growth
Once a cloud droplet forms, there are 2 ways it can grow into precipitation
Growth by condensation Growth by Collision and Coalescence
Growth by condensation Very slow process
Growth by Collision and Coalescence More realistic mechanism
Collision and Coalescence
Coalescence occurs in clouds with tops warmer than 5°F (-15°C)
The greater the speed of the falling droplet, the more air molecules the drop encounters
Important factors for droplet growth High liquid water content within the cloud Strong and consistent updrafts Large range of cloud droplet sizes Vertically thick cloud Terminal velocity Droplet electric charge and cloud electric field
Collision and Coalescence
Homogeneous nucleation of ice
Freezing of pure water Enough molecules in the droplet must join together in
a rigid pattern to form an ice embryo The smaller the amount of pure water, the lower the
temperature at which water freezes Supercooled droplets
Water droplets existing at temperatures below freezing
1 ice crystal to 106 liquid droplets at -10°C Homogeneous nucleation (freezing) occurs at temperatures of –
40°C Vapor deposition
From vapor to solid Not likely to be sufficient in our atmosphere
Ice nuclei
Ice crystals (IN) form in subfreezing air on particles called ice nuclei
Ice nuclei are rare; only 1 out of 10 million aerosols is an effective ice nuclei
Fewer sources than CCN Desert and arid regions: silicate particle (dominant) Clay particles: for temperatures between –10 and –20°C Volcanic emissions Combustion products Bacteria
Oceans are NOT good sources of IN
IN requirements Insolubility
If soluble, cannot maintain molecular structure requirement for ice
Size Must be comparable, or larger than, that of a critical ice
embryo (typically 0.1 microns) Chemical bond
Must have similar hydrogen bonds to that of ice available at its surface
Crystallographic Similar lattice structure to that of ice (hexagonal)
Active Site Pits and steps in their surfaces
Growth mechanisms• Vapor deposition
Saturation vapor pressure over water greater than over ice Temperature affects saturation vapor pressure over ice the same
way that it affects saturation vapor pressure over liquid When ice and liquid coexist in cloud, water vapor evaporates
from drop and flows toward ice to maintain equilibrium Ice crystals continuously grow at the water droplet’s expense The process of precipitation formation in cold parts of clouds by
ice crystal diffusional growth at the expense of liquid water droplets is known as Bergeron process
Growth mechanisms
Diffusional growth alone not sufficient for precipitation formation
• Accretion/Riming Ice crystals collide with supercooled
droplets, which freeze upon impact Forms graupel (snow pellets) May fracture or split as falls, producing
more ice crystals
Growth mechanisms
Graupel from Accretion
Accretion of ice from ocean spray
Growth mechanisms• Aggregation
Collision of ice crystals with each other and sticking together
Clump of ice crystals referred to as a snowflake
Common in temperatures near freezing where there may be some liquid water on the surface of the crystal
Differing temperatures can cause aggregates to grow into different shapes
Precipitation Types
Rain - drop greater than 0.5 mm Rarely larger than mm because collisions break
them up What is the shape of a raindrop?
Drizzle - < 0.5 mm Usually from stratus
Snow - small ice of many forms Fallstreaks (like virga, but from cirrus) Flurries (no accumulation) Snow squalls Blizzard - winds > 30 kts
Precipitation Types
Sleet - tiny ice pellets formed from refreezing of rain drops
Translucent (unlike graupel), < 5 mm Freezing rain/drizzle - freezes upon
contact with the surface Can be extremely damaging Knocks out power Pulls down tree branches
Both are common along warm fronts
Damage from freezing rain
Precipitation Types Virga - any precipitation that evaporates before hitting the
surface
Graupel
Ice crystals falls through cloud, accumulating supercooled water droplets that freeze upon impact. Thus, graupel is an example of growth by accretion/riming.
Creates many tiny air spaces These air bubbles act to keep the density low and scatter
light, making the particle opaque
When ice particle accumulates heavy coating of rime, it’s called graupel
Hail An extreme example of growth by accretion Hailstones form when either graupel particles or large frozen
drops grow by collecting copious amounts of supercooled water Graupel and hail stones carried upward in cloud by strong
updrafts and fall back downward on outer edge of cloud where updraft is weaker
Hail continues to grow through updrafts until it’s so large that it eventually falls out bottom of cloud
Hail growth As hailstone collects supercooled drops which freeze on surface,
latent heat released, warming the surface of the hailstone Dry Growth
At low growth rates (caused by lower liquid water contents), this heat dissipates into surrounding air, keeping surface of stone well below freezing and all accreted water is frozen
Wet Growth If a hailstone collects supercooled drops beyond a
critical rate or if the cloud water content is greater than a certain value, latent heat release will warm surface to 0°C
Prevents all accreted water from freezing Surface of hailstone covered by layer of liquid water
Hail layers Alternating dark and light layers Wet growth
solubility of air increases with decreasing temperature so little air dissolved in ice during wet growth
Ice appears clear Dry growth
Hailstone temperature close to environmental temperature so at cold temperatures, large amount of air dissolved
Ice appears opaque
Hail Descriptors
Size (inches) Name0.25 Pea0.75 Quarter1.00 Golf Ball
1.75 Tennis Ball2.50 Baseball2.75 Grapefruit4.00 Giant> 4.00 Ruler measured
• RAdio Detection And Ranging• Transmits a microwave into the
atmosphere and measures the return power– 10, 5, 3 cm typical
• Size chosen depends on use
TransmitterTransmit/Receive
SwitchReceiver Display
Antenna
• Pulse of microwave energy sent out (emitted from antenna to parabolic dish reflector), dish focuses energy into beam
• Beam travels through atmosphere• If the beam hits an object, then some of the energy
is reflected back to the radar• Return power measured• Data processed to a visual display
• Radar measures the intensity of the returned signal, the frequency of the returned signal, and the elapsed time from the transmission of the pulse
• Energy beam travels at the speed of light– Knowledge of the elapsed time allows the
computation of the distance from the radar site
• Frequency uses doppler shift to determine movement
• Only a fraction of the emitted energy gets returned from reflection
• amplified and measured in decibels (dbz), Reflectivity– 1dbz = 10 log(p2/p1)
– p2 = power received at radar, varies
• Reflectivity is dependent upon the size of the object
• In meteorology, the objects are precipitation particles
• The return power (reflected beam) is dependent on the number of particles present, and the size of the particles
• Particle diameter^6 dependence• Number^1 dependence• Larger drops lead to larger reflectivities• Reflectivity mostly based on particle size
• Drizzle: 20 - 25 dBz• Light rain: 25 – 35 dBz• Heavy Rain: 35 – 50 dBz• Thunderstorm Heaviest Rainfall: >50
dBz• Light Snowfall: 15 – 25 dBz• Heavy Snowfall: 25 – 35 dBz
• How much rain falls to the surface in a given hour
• R=inches/hour
• a and b are constants• Higher reflectivity generally corresponds
to higher rainfall rates
Z e=aRb
Top Related