Lecture 13: Precipitation W & H: Sections 6.4 and 6.5

Cloud Droplets vs. Raindrops

Exercise What is the ratio of the volume of a typical raindrop to the volume of a typical cloud droplet? r droplet =.01 mm r droplet =.01 mm r raindrop = 1 mm r raindrop = 1 mm Answer: 10 6

Diffusional Growth In condensation, water vapor molecules migrate toward the droplet by diffusion To grow a raindrop by diffusion would take weeks! There must be a faster way!

Growth by Collection Droplets collide and coalesce, forming a larger droplet Problem: How can droplets collide?

If all droplets were the same size, probability of collisions would be very small.

Collisions Requires large “starter droplets” Have larger terminal velocities than smaller droplets

Large droplets overtake smaller ones and collide with them. The droplets may coalesce, forming a larger droplet.

As large droplet falls, its speed and size increase  rate of collection of smaller droplets increases  rate of collection of smaller droplets increases Result: Growth rate of collector drop increases rapidly.

t = 0 t =  t t = 2  t t = 3  t

Diffusional Growth vs. Collection Diffusional Growth Growth by collection

Growth of Ice Particles Diffusional growth (deposition) Accretion (riming) Aggregation

Diffusional Growth Consider a mixed population of supercooled droplets and ice crystals Concentration of droplets >> number of crystals Concentration of droplets >> number of crystals Ambient vapor pressure  e s,w > e s.i  ice crystals will grow rapidly  ice crystals will grow rapidly

Diffusional Growth of Crystal For example, T = -10  C e s,w = 2.87 hPa; e s,i = 2.60 hPa e s,w = 2.87 hPa; e s,i = 2.60 hPa Ice crystal finds itself in a highly super- saturated environment  super-saturation with respect to ice = 10% Result: rapid growth

Evaporation of Droplets As ice crystals grow, they deplete water vapor  vapor pressure falls below e s,w  vapor pressure falls below e s,w  droplets begin to evaporate  droplets begin to evaporate

e s,i e s,w ambient vapor pressure Initially

e s,i e s,w ambient vapor pressure Later Air is super-saturated with respect to ice, but sub- saturated with respect to water

e s,i e s,w ambient vapor pressure Eventually No droplets left; ice-crystals stop growing

Ice crystal growing at expense of surrounding supercooled droplets. Fig in W & H

Crystal Shapes Crystal shapes determined by temperature and supersaturation

W & H: Fig Hexagonal Plates Column Dendrite Sector Plate Bullet Rosette

Accretion (Riming) Ice particles collide with super-cooled droplets Droplets freeze onto ice crystals Produces a rimed ice crystal Produces a rimed ice crystal

Rimed ice crystals W & H, Fig Graupel

Aggregation Clumping together of ice crystals (This is how snowflakes are formed) (This is how snowflakes are formed)

Precipitation Initiation 1.Drop growth by collection Growth of ice crystals by diffusion, accretion, and aggregation. #1 is dominant in the tropics (T > 0  C) #1 is dominant in the tropics (T > 0  C) 1 & 2 are important in the middle latitudes 1 & 2 are important in the middle latitudes

Precipitation Types Rain, snow, sleet & freezing rain, hail In cold clouds, precipitation starts as snow in cloud Precipitation at surface depends on temperatures below the cloud Can get rain, snow, sleet, or freezing rain Can get rain, snow, sleet, or freezing rain

Sleet Formation

Hail Forms in cumulonimbus clouds Starts as small ice crystal Ice crystal moves through region of supercooled water & grows by accretion AMS Glossary AMS Glossary

Hail Growth

Weather Radar: Purposes 1.Detection of precipitation 2.Detection of tornadoes

Detection of Precipitation Radar transmits microwaves Strength of return signal depends on precipitation intensity Radar unit does a 360  scan at various elevation angles Called a “volume scan” Called a “volume scan”

Example A 14-level volume scan mode

Base vs. Composite Reflectivity Base reflectivity just shows the lowest angle scan Composite shows the strongest echo from any level Comparison /comprefl.htm /comprefl.htm /comprefl.htm /comprefl.htm

Sample Reflectivity Display

Velocity Display Shows radial velocity of precipitation particles Uses Doppler effect Frequency of return signal is different from frequency of transmitted signal. Frequency of return signal is different from frequency of transmitted signal.

Straight-Line Motion On velocity display, red indicates motion away from the radar Green indicates motion toward the radar

Example Radar Storm movement

Detection of Rotation Radar Motion toward radar Motion away from radar

NWS Tutorial /doppler/doppler_intro.htm /doppler/doppler_intro.htm