Upload
dangphuc
View
227
Download
0
Embed Size (px)
Citation preview
1
ATMO 1300
Chapter 7 – Precipitation Processes
Understanding Weather and ClimateAguado and Burt
ATMO 1300
Precipitation
2
ATMO 1300
Why Cloud Droplets Don’t Fall
• Gravity vs. Updraft– Terminal Velocity – The final speed obtained
by an object falling through the atmosphere. A balance between gravity and the frictional forces (drag) on the object.
ATMO 1300
How Large is Large Enough?
3
ATMO 1300
Growth of Cloud Droplets
• Condensation• Warm Clouds
– Collision and Coalescence• Cool and Cold Clouds
– Bergeron Process– Riming and Aggregation
ATMO 1300
Condensation
• Can lead to quick growth for small water droplets.
• Once radii of 20 microns is passed, the efficiency of condensation is reduced.
• By itself condensation would produce few raindrops.
4
ATMO 1300
Collision
• Collisions between drops– Dependent on absolute
size of the collector drop and relative size of the droplets below.
– Most efficient when droplets are slightly smaller than the collector drop.
ATMO 1300
Coalescence
• When droplets stick together after colliding.• Coalescence efficiencies are usually
assumed to be near 100%
5
ATMO 1300
Bergeron Process
• Depends on the co-existence of supercooled water and ice in the same cloud.
• Saturation vapor pressure over ice is less than that over water at the same temperature.
• Leads to a continuous transfer, in which supercooled droplets surrender water vapor which is subsequently deposited onto the ice crystals.
ATMO 1300
Bergeron Process
6
ATMO 1300
Riming
• Analogous to droplet collision• Riming is the collision of ice crystals and
supercooled water droplets.• Causes rapid grow of ice crystals and
acceleration of their fall speeds.
ATMO 1300
Aggregation
• The joining together of two ice crystals to form a single larger crystal.
7
ATMO 1300
Types of Precipitation
• Snow• Rain• Graupel• Hail• Sleet• Freezing Rain
ATMO 1300
Snow
• Results from the growth of ice crystals through deposition, riming and aggregation.
• A snowflake’s structure depends on the temperature and moisture conditions that exist when the crystal is formed.– Warmer conditions provide for riming and a
wet snow pack– Colder conditions provide for less adhesion
(riming) and a “powder”
8
ATMO 1300
US Snowfall
ATMO 1300
Raindrop Shape
• Not tear drop shaped!
9
ATMO 1300
US Precipitation
ATMO 1300
Graupel
• When riming coats an ice crystal and its 6-sided sharp edges are lost into a milky-white spongy texture.
• Graupel can fall to the ground or remain in the cloud providing a nucleus for hail.
10
ATMO 1300
Hail
• Ice pellets forms in roughly concentric layers.
• Formed by a repetitive sequence, where ice falls into a region with liquid droplets and provides a liquid coating, upon lifting the ice crystal higher the coating freezes and the process restarts. (Figure 6-13 in text book)
ATMO 1300
Hail
11
ATMO 1300
Sleet
• Raindrops freeze in the air while falling to the surface.
• Must have a layer in the lowest portion of the atmosphere with a temperature below 0°C.
• Requires an inversion aloft.
ATMO 1300
Sleet
12
ATMO 1300
Freezing Rain
• Light rain or drizzle of supercooled water droplets fall through air at or slightly below 0°C. It freezes when it hits the ground.
ATMO 1300
Measuring Rainfall
• Raingauge – A instrument which collects the rain and enables its measurement– Tipping Rain Gauges – Weighing-Bucket Gauges
13
ATMO 1300
Measurement Errors
• Rain gauges only represent a point measurement.
• Errors:– Wind generated turbulence can deflect
precipitation from entering the gauge– Splashing– Snow drifts
ATMO 1300
Snow Measurement
• Measure depth• Water equivalency – depth of water which
would result if all of the snow was melted.• Conversion factors – 10:1• Snow pillows – big mattresses filled with
antifreeze liquid and connected to a pressure regulator.
14
ATMO 1300
Measuring Precipitation by Radar
• Intensity of backscattered radiation to the radar indicates the size of the particles (droplets, snow flakes, gruapel or hailstones)
ATMO 1300
Radar Estimates