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CHAPTER 6
PRECIPITATION EXTREMES
CHAPTER 6
PRECIPITATION EXTREMES
Why doesn’t it rain every time it’s cloudy? (in other words, why don’t all cloud droplets fall out as rain?)
What are the different types of precipitation, and how do they form?
How is precipitation measured?
Cloud droplets are liquid water, but are much, much smaller than raindrops!
Most cloud droplets are very tiny (20 μm)
The force of gravity on the tiny drops is small, as is the air resistance: the terminal velocity is small
For very large drops, the force of gravity is larger, as is the air resistance – a large terminal velocity
Therefore: Larger drops fall much faster than small drops
Now, think of these drops in a cloud (formed by an updraft), with temperature above freezing
The terminal velocity of the tiny drops is only 0.01 m/s – any updraft will be stronger than this, and the cloud drops will remain aloft (or rise even farther)
The terminal velocity of a drop with diameter 2mm/2000 μm has terminal velocity of 6.5 m/s – as long as the updraft is not especially strong, this size of drop can fall as rain
But how do the drops get to be 2mm/2000 μm?
Because large drops fall faster than small ones, they will collide as they fall through the cloud
Many times, these colliding drops will stick to each other – coalescence
Which will form larger raindrops, a thick cumulus cloud or a thin stratus cloud?
Stepped Art
Fig. 7-5, p. 169
Most important – Liquid Water Content of Cloud
ALSO◦ Range of droplet sizes◦ Thickness of cloud◦ Updrafts◦ Electric charge of droplets and electric field in
cloud
Collision-coalescence only works where the cloud is warmer than freezing – what about in the north where much of cloud is cold?
To form cloud drops, need “cloud condensation nuclei” (dust, etc.) To form ice crystals, need “ice nuclei”
However, CCN are much more common in nature than IN
It must be extremely cold (-40°C) for “spontaneous freezing” to occur
In the region between 0 °C and -40 °C, there is a mix of supercooled water and ice crystals
How do these ice crystals grow big enough to fall as precipitation?
At a given subfreezing temperature, the saturation vapor pressure over water is greater than that over ice (more molecules, more vapor pressure)
Because of this, ice crystals grow at the expense of liquid cloud droplets
Swedish meteorologist, 1891-1977
One of the leaders of the “Bergen school” of meteorology in Norway, which made many of the pioneering advances in our field
Along with Wegener and Findeisen, discovered that ice crystals grow to form precipitation because of the different saturation vapor pressures above supercooled water and ice
From Liljequist (1981)
The ice crystal reaches its terminal velocity and starts to fall
It can collide with supercooled water drops, which freeze to it – accretion or riming – leads to graupel
It can collide with other ice crystals, and break apart, forming even more ice crystals
It can collide with other ice crystals, which can stick to it – aggregation – this produces snowflakes
As it gets near the ground, these might fall as snow, or melt and fall as rain
Precipitation and Cloud Types
• Precipitation in Cold Convective Clouds– Starts quickly (Either C-C or Bergeron)– Most precipitation formed through accretion– Many times rain starts as ice
• Cold Layered Clouds (NS and AS)– Less liquid water - C-C less effective
• Warm Stratus– Usually just drizzle
Seeding (natural and man-made)• Cloud Seeding
– Inject cloud with small particles that act as condensation/ice nuclei, triggering the precipitation process
– NEED CLOUDS: seeding does not generate clouds– Cold clouds with a low ratio of ice crystals to
droplets best – Man-made: Dry ice, silver iodide
• Tricky business – mixed results– Inadvertent seeding
Fig. 6.8, p. 151
Stepped Art
Fig. 6.9, p. 151
Fig. 6.10, p. 152
In layered clouds with less intense updrafts, rain will fall at a fairly steady rate and raindrop sizes will be uniform
In a cumulonimbus cloud, there may be no rain in the updraft portion (updraft is stronger than the terminal velocity of rain), but very heavy rain in the downdraft
Temperature and vapor content in the cloud determines the type of snowflake that forms – “habit”
DRY
WET
Fig. 6.11, p. 154
The temperature near the surface can also affect the type of snowflake we see at the ground:◦ If it falls through warm air (near or just above
freezing), it can start to melt, and the wet flakes can stick to each other to create “aggregates” – big fluffy flakes – good for snowballs
◦ If it falls through cold (well below freezing), dry air, we get powder
Snowfall is measured in two ways – depth of snow, and water equivalent◦ Wet snow will have 6 inches of snow per 1 inch of
water◦ Dry snow will have 20 inches of snow per 1 inch of
water New, dry snow is a very good insulator: can
actually protect plants from extreme cold
Fig. 6.13, p. 155
Sleet: snowflake melts as it falls, then freezes again before reaching the ground◦ Requires a deep subfreezing layer near the
ground Freezing rain: snowflake melts as it falls, remains melted,
but freezes on contact when it hits the cold ground, trees, cars, power lines, etc.
Rime: similar to freezing rain, but with drops that are even smaller (such as fog): small drops freeze on contact
Fig. 6.18, p. 158
Unlike sleet and snow, which form in relatively shallow clouds, hail forms in deep cumulonimbus clouds
Hail grows when particles accumulate huge numbers of supercooled water droplets – accretion
Large hail only occurs where there are very strong updrafts -- they keep the heavy stones aloft so they can continue to grow – must remain in the cloud for 5-10 minutes to become golf-ball size
Hail is “severe” if it is greater than 1.00” in diameter (quarter size)
June 22, June 22, 20032003Aurora, NEAurora, NE7” diameter7” diameter
Large hail often has a layered structure◦ In cold upper part of
cloud, drops freeze immediately – opaque areas with air bubbles: dry growth
◦ In wetter/warmer part, droplets create a layer of water that freezes without air bubbles – clear: wet growth
◦ The more rings, the more of these cycles the stone has been through
Frequency of severe hail in U.S.Frequency of severe hail in U.S.
Hailpads developed at CSU toHailpads developed at CSU tomeasure hail size and amountmeasure hail size and amountwww.cocorahs.orgwww.cocorahs.org
Fig. 6.25, p. 162
Stepped Art
Fig. 6.23, p. 161
Fig. 6.22, p. 159
Fig. 6.21, p. 159
Table 6.1, p. 156
Fig. 6.15, p. 156
EXTREME WEATHER
• Aircraft Icing– Aviation hazard is created by the increase in
weight as ice forms on the body of the airplane
– Most dangerous – clouds with all supercooled liquid, freezing drizzle
– Mitigation:• Spray aircraft with anti-freeze prior to flight (de-
icing• Avoid deep layers of icing
p. 160
PRECIPITATION—EXTREME EVENTS
• The Influence of Mountains– Promote convection – Force air to rise along their windward slopes
(orographic uplift)– Rain shadow
Fig. 6.27, p. 163
Fig. 6.28, p. 164
PRECIPITATION—EXTREME EVENTS
• Wet regions, dry regions, and precipitation records– “Rainiest” places located on the windward
side of mountains– Snowfalls heavier where cool, moist air rises
along the windward slopes of mountains– Driest regions of the world lie in the frigid
polar region, the leeward side of mountains, and in the belt of subtropical high pressure
Table 6.2, p. 165
Table 6.3, p. 167
PRECIPITATION—EXTREME EVENTS
• Floods– Flood waters that rise rapidly with little or no
warning are called flash floods; results when thunderstorms stall or move very slowly, causing heavy rainfall over a relatively small area
– Flooding occurs primarily in the spring when heavy rain and melting snow cause rivers to overflow their banks
Fig. 6.30, p. 167
Fig. 6.31, p. 170
EXTREME WEATHER
• “Almost” Record Snowfall– Montague NY– Heavy lake effect snow– 77 inches in 24 hours
INSTRUMENTS for Precip
• Rain Gauge– Manual (e.g. CocoRahs)– Tipping Bucket
• Doppler Radar– Radio detection and ranging– Doppler effect: change in frequency with direction of
movement• Circulation and hook echos (severe indicators)
• Satellite (microwave)
PRECIPITATION—EXTREME EVENTS
• Drought– A period of abnormally dry weather that produces
a number of negative consequences, such as crop damage or an adverse impact on a community’s water supply (agricultural drought, hydrologic drought)
• Drought is relative (to normal rainfall)
– Palmer Drought Severity Index (PDSI)– African Drought: Sahel– North American Drought: Dust Bowl Years
Table 6.4, p. 170
Fig. 6.32, p. 170
Fig. 6.33, p. 171
Palmer for Dust Bowl Drought summer
Fig. 6.34, p. 172
http://atmo.tamu.edu/osc/drought/
http://www.ncdc.noaa.gov/oa/climate/research/prelim/drought/palmer.html
Current Palmer Index Maps:
Texas Maps (OSC at TAMU)
The Most-Used Drought Indicator(The U.S. Drought Monitor)http://droughtmonitor.unl.edu/