, Air Masses, and Fronts

AT 351 Lab 8 March 13, 2008

Droplet Formation

• Recall… • Condensation on CCN at high RH

• This gets them started…

1 Droplet Growth by Collision and Coalescence

• Growth by condensation alone takes too long • CC occurs in clouds with tops warmer than 5°F (-15°C) • The greater the speed of the falling droplet, the more air molecules the drop encounters • Important factors for droplet growth – High liquid water content within the cloud – Strong and consistent updrafts – Large range of cloud droplet sizes – Vertically thick cloud – Terminal velocity – Droplet electric charge and cloud electric field

Collision and Coalescence

2 Water must freeze at -40°C

Homogeneous nucleation of ice • Freezing of pure water – Enough molecules in the droplet must join together in a rigid pattern to form an ice embryo – The smaller the amount of pure water, the lower the temperature at which water freezes • Supercooled droplets – Water droplets existing at temperatures below freezing – 1 ice crystal to 106 liquid droplets at -10°C • Homogeneous nucleation (freezing) occurs at temperatures of –40°C • Vapor deposition – From vapor to solid – Not likely to be sufficient in our atmosphere

3 Ice nuclei

• Ice crystals form in subfreezing air on particles called ice nuclei • Ice nuclei are rare; only one out of 10 million aerosols is an effective ice nuclei • Fewer sources than CCN – Desert and arid regions: silicate particle (dominant) – Clay particles: for temperatures between –10 and –20°C – Volcanic emissions – Combustion products – bacteria – IN may be de-activated when exposed to atmospheres with high concentrations of Aitken nuclei produced by industrial processes – Oceans are NOT good sources of IN

IN requirements

• Insolubility – If soluble, cannot maintain molecular structure requirement for ice • Size – Must be comparable, or larger than, that of a critical ice embryo (typically 0.1 microns) • Chemical bond – Must have similar hydrogen bonds to that of ice available at its surface • Crystallographic – Similar lattice structure to that of ice (hexagonal) • Active Site – Pits and steps in their surfaces • Freezing Nuclei – Immersion freezing, contact freezing

4 Growth mechanisms • Vapor deposition – Saturation vapor pressure over water greater than over ice – Tend to move from higher pressure to lower pressure • From water saturation vapor pressures to ice saturation vapor pressures. – When ice and liquid coexist in cloud, water vapor evaporates from drop and flows toward ice to maintain equilibrium – Ice crystals continuously grow at the water droplet’s expense – The process of precipitation formation in cold parts of clouds by ice crystal diffusional growth at the expense of liquid water droplets is known as Bergeron process

5 Growth mechanisms

• Diffusional growth alone not sufficient for precipitation formation • Accretion – Ice crystals collide with supercooled droplets, which freeze upon impact – Forms ( pellets) – May fracture or split as falls, producing more ice crystals

6 Growth mechanisms

• Aggregation – Collision of ice crystals with each other and sticking together – Clumping of ice crystals referred to as a snowflake – Common near freezing where there may be some liquid water on the surface of the crystal

7 Precipitation Types- Ice Habits

Environmental Crystal Habit Temperature (°C)

0 to -4 thin plates

-4 to -6 needles

-6 to -10 columns

-10 to -12 plates

-12 to -16 dendrites, plates

-16 to -22 plates

-22 to -40 hollow needles

8 Precipitation from Clouds

• Low-water clouds – IN growth by diffusion • Turns to large crystals • Then aggregates • Then snowflakes • May melt and fall as • High-water clouds – IN growth by accretion • Rimed crystals • Graupel • May melt or grow into

Precipitation Types • Virga - any precipitation that evaporates before hitting the surface

9 Precipitation Types

• Rain - drop greater than 0.5 mm – Rarely larger than mm • Collisions break them up – What is the shape of a raindrop? – Showers, cloud bursts • - < 0.5 mm – From stratus (short, often no ice layer) • Snow - small ice of many forms – Fallsteaks (like virga, but from cirrus)’ – Flurries, snow quall (nimbostratus), blowing snow – Blizzard - winds > 30 kts

Precipitation Types

• Sleet - tiny formed from refreezing of rain drops – Translucent (unlike graupel), < 5 mm • Freezing rain/drizzle - freezes upon contact with the surface – Can be extremely damaging – Knocks out power – Pulls down tree branches • Both are common along warm fronts

10 11 Graupel

• Ice crystals falls through cloud, accumulating supercooled water droplets that freeze upon impact – Creates many tiny air spaces – These air bubbles act to keep the density low and scatter light, making the particle opaque • When ice particle accumulates heavy coating of rime, it’s called graupel

Hail

• Hailstones form when either graupel particles or large frozen drops grow by collecting copious amounts of supercooled water • Graupel and hail stones carried upward in cloud by strong updrafts and fall back downward on outer edge of cloud where updraft is weaker • Hail continues to grow and carried into updraft until so large that it eventually falls out bottom of cloud

12 Hail growth

• As hailstone collects supercooled drops which freeze on surface, latent heat released, warming surface of stone • At low growth rates, this heat dissipates into surrounding air, keeping surface of stone well below freezing and all accreted water is frozen • Referred to as dry growth of hailstone

Hail growth

• If hailstone collects supercooled drops beyond a critical rate or if the cloud water content is greater than a certain value, latent heat release will warm surface to 0°C • Prevents all accreted water from freezing • Surface of hailstone covered by layer of liquid water • Referred to as wet growth of hailstone

13 Hail layers

• Alternating dark and light layers • Wet growth – solubility of air increases with decreasing temperature so little air dissolved in ice during wet growth – Ice appears clear • Dry growth – Hailstone temperature close to environmental temperature so at cold temperatures, large amount of air dissolved – Ice appears opaque

14 Hail Descriptors

Size (inches) Name 0.25 Pea 0.75 Quarter 1.00 Golf Ball 1.75 Tennis Ball 2.50 Baseball 2.75 Grapefruit 4.00 Giant > 4.00 Ruler measured

15 16 Lake effect snow

Lake effect snow

• Heating – Water warmer than land in fall and early winter – Unstable environment

17 Lake effect snow

• Air rises, quickly reaching saturation due to addition of moisture from lake (evaporation)

Lake effect snow

18 Lake effect snow

Lake effect snow

• Wind fetch – Length of trajectory of wind across lake – Greater the distance the wind blows over warm water, the greater the • Frictional difference – When wind moves from over water to land, friction slows it down, resulting in surface convergence and lifting • Large-scale forcing – Enhancement of lake-effect snow

19 Case study (Dec 1998)

Case study (Dec 1998)

20 Case study (Dec 1998)

Air Masses

• Named for their region of origin – Continental Polar/Continent al Arctic – Maritime Polar – Maritime Tropical – Continental Tropical

21 cP/cA

• Originate over ice- and snow- covered regions of northern Canada and Alaska – Long, clear nights allow for strong radiational cooling – brings extremely cold and dry of the surface conditions during the winter – Moves S as – Brings relief to hot, humid enormous shallow summer days and development high pressure area of fair cumulus in summer

cP/cA

22 cP/cA

• Very stable upper levels, with upper level subsidence causing a subsidence inversion • Usually confined to E of Rockies • Responsible for Texas northers • Produce lake effect over Great Lakes • Can reach as far south as Mexico and Florida, freezing crops

Texas norther event

Wichita Falls, TX: Temperature dropped 11C in one hour

23 Blue norther

mP • In winter, cP/cA air masses originating over Asia carried across Pacific around Aleutian low – Ocean adds warmth and moisture to – Typically conditionally unstable • Coastal force it to rise, producing heavy • As air moves over High Plains, referred to as modified Pacific air – Drier than originally, yet warmer than cP air – When it replaces retreating cold air from the N, Chinook winds form – Brings fair weather East of the Rockies

24 mP (west coast)

mP (east coast) • Not as common as Pacific counterpart • Air mass originates from the N Atlantic, steered by NE winds • Most often in late winter/early spring

25 mT

• Subtropical east Pacific – Very warm and moist by time arrive on east coast – Usually produces heavy precipitation – Called Pineapple Express – Directed by strong jet stream

mT

•Gulf of Mexico –Warm, humid, subtropical air –Confined to Gulf Coast in winter –Formation of dew, fog and low clouds along the coast –Could lead to record heat waves

26 mT

cT

• Source is N Mexico and the arid SW US • Exist only in summertime • Hot, dry, and conditionally unstable at low levels • Clear skies and hot weather – Severe drought if stalls over plains • Frequent dust devils

27 Air masses and fronts

•Frontogenesis –Formation and strengthening of a frontal system •Frontolysis –Weakening and death of a frontal system

Fronts

• Transition zone between two air masses of different densities • Extend both horizontally and vertically • Location on surface maps: – Sharp temperature changes over a relatively short distance – Changes in the air’s moisture content – Shifts in wind direction – Pressure and pressure changes – Clouds and precipitation patterns

28 Stationary Fronts • Essentially no movement • Surface winds blow parallel to front, but in opposite directions on either side of it • Separates two air masses • Seen often along ranges when cold air cannot make it over the ridge

Cold fronts

• Cold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT) • Steep vertical boundary due to surface friction slowing down the surface front • Has strong vertical ascent along the surface front • Can cause strong convection, severe weather, and squall lines. • Strong upper level westerlies push ice crystals far ahead of the front, creating Ci and Cs in advance of the front. • Air cools quickly behind the front

29 Cold fronts

• Cold, dense air wedges under warm air, forcing the warm air upward, producing cumuliform clouds • Rising motion causes decreased surface pressure ahead of the front – On a surface pressure map, frontal location can be seen by “kinks” in the isobars. – Pressure is lowest at the surface front.

Animation

30 Cold fronts Before While After Winds S-SW Gusty, shifting W-NW Temperature Warm Sudden drop Steady drop Pressure Steady fall Min, then Steady rise sharp rise Clouds Increasing, Ci, Cb Cu Cs, Cb Precipitation Brief showers Heavy rains, Showers, then severe clearing Fair to poor Poorweather Good

Dew Point High, remains Sharp drop lowering steady

Types of cold fronts • Arctic Front – Frigid air replaced cold air – Temperature change is only evidence of frontal passage • Back Door – Cold front moves to the SW instead of SE – High pressure in Canada brings NE winds as far south as Boston – Appalachians act to dam the cold air, possibly creating a stationary front On weather maps, cold fronts are indicated by blue lines with triangles pointing in the direction of frontal motion (towards warmer air)

31 Warm fronts

• At the leading edge of advancing warm, moist, subtropical air (mT) from the Gulf replaces the retreating cold, maritime, polar air from the N. Atlantic (mP) • Slowly advances as cold air recedes; moves at about half the speed of an average cold front – Speed may increase due to daytime mixing – Speed may decrease due to nighttime radiational cooling • Smaller vertical slope than cold front

Warm fronts • Warmer, less- dense air rides up and over the colder, more- dense surface air – “Overrunning” – Produces clouds and precipitation well in advance of the front

32 Animation

Warm fronts

Before While After Winds S-SE Variable S-SW Temperature Cool-cold Steady rise Warmer, then Slowly warming steady Pressure Falling Leveling off Slight rise, followed by fall Clouds (in order) Ci, Cs, Stratus-type Clearing with As, Ns, St, fog scattered Sc (Cb in summer) Precipitation Light-to- Drizzle or none Usually none moderate Visibility Poor Improving Fair Dew Point Steady rise Steady Rise, then steady

33 Occluded Fronts

• Cold fronts generally move faster than warm fronts • Occlusion occurs when cold front catches up to and overtakes a • Occlusions can be warm or cold

Cold occlusion

• Cold front lifts the warm front up and over the cold air • As approaches, similar to warm front precipitation sequence – High clouds lower and thicken with precipitation forming well in advance of the surface front • After front passes, weather then similar to a cold front – Heavy and showery precipitation • Violent weather can occur where the cold front meets the warm front and temperature contrast is greatest (triple point)

34 Cold Occlusion

Warm occlusion

• Less common than cold occlusion • Cold air behind the cold front not dense enough to lift cold air ahead of warm front • Cold front rides up and over the warm front • Similar weather to a warm front • Upper level cold front precedes low level occlusion

35 Dry lines • Separates warm, humid mT air in the southern Great Plains from warm, dry cT air • Drier air behind dry lines lifts the moist air ahead of it, triggering storms along and ahead of it

Dry Lines

• Unique to southern great plains of US because of mountains and Gulf of Mexico • Creates strong front with dew point differences of 50 degrees across the front • Also induces lifting along front • Often the initiation point of severe thunderstorms inin OklahomaOklahoma andand TexasTexas

36 A guide to the symbols for weather fronts that may be found on a weather map: #1 cold front #2 warm front #3 stationary front #4 occluded front #5 surface trough #6 squall/shear line #7 dry line #8 tropical wave

Upper-air fronts

• Present aloft; may or may not extend to surface • Forms when tropopause dips downward and folds under the polar jet stream • Marked by tightly packed isotherms • N side: slowly sinking air (ozone rich air from stratosphere descends into troposphere) • S side: slowly rising air • Can aid in development of mid-latitude

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