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!

 Remember that pressure is related to the force exerted by air on a body  If that air has water vapor in it, those molecules are exerting some portion of that force ◦ This is called the “partial pressure” – water vapor is responsible for a part of the pressure  The more water molecules, the higher the vapor pressure

 The saturation vapor pressure is the vapor pressure at which the air is saturated ◦ Also known as the equilibrium vapor pressure – where the same number of molecules are evaporating and condensing  The higher the temperature, the higher the saturation vapor pressure  In other words, the higher the temperature, the more water vapor can be in the air

 The saturation vapor pressure over a curved surface is greater than over a flat surface ◦ (This is because the water molecules are less strongly attached to the curved surface)  Therefore – if the air is saturated over the flat surface, it is unsaturated over the curved surface!

 Therefore, for cloud droplets to stay liquid (not evaporate), the air actually needs to be supersaturated – relative humidity above 100%  The smaller the droplet, the greater its curvature, so the higher the RH needs to be  The greater the supersaturation, the more the droplets will grow by condensation

 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,  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

Fig. 6.9, 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, 2003 Aurora, NE 7” 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. Hailpads developed at CSU to measure hail size and amount

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

rch/prelim/drought/palmer.html Current Palmer Index Maps: Texas Maps (OSC at TAMU) The Most-Used Drought Indicator (The U.S. Drought Monitor)