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/