Weather Midterm #2

Water cycle

* Water evaporates from the ocean
* This water vapor enters the atmosphere and moves over the land, and comes down as rain or snow
* This merges into rivers
* This returns to the seas
* Finishes water cycle

Along the paths of the water cycle, the water carries

heat and energy, driving weather and climate disasters

Components of Global Water cycle

* Ocean water
* Covers 70% of the Earth’s Surface
* Heat reservoir
* Holds heat energy for global climate system
* Sea ice
* Part of the water system
* When sea ice melts, the sea level won’t rise because the sea ice is already inside the water. And sea ice has a lower density than water, that’s why it floats at the top
* So when the sea ice melts, the volume added to the ocean is less than the volume of the ice, so the sea won’t rise, it’ll sink a tiny bit
*  Land soil moisture, rivers, snow cover, ice 
* >1000 major rivers flow on the 7 continents
* Amazon River=#1 river
* River flow of this river is greater than the sum of the next 9 biggest rivers
* Mississippi river=#6 river
* Yangtze river: #4
* River basins or river catchments: catch rain and snowfall and merge it into rivers 
* sheet and glaciers
* Sheets are different than sea ice. When this melts, it will cause the ocean to rise
* Antarctic Ice sheet is the largest ice sheet on Earth
* 10 times larger than greenland ice sheet
* If totally melted, will ocean will rise up by 60 meters
* We have found it’s not melting, which is interesting because we have global warming
* Greenland ice sheet is melting though
* Only melting at the very top layer, upper 10 meters, but the greenland ice sheet is a kilometer thick
* Mountain glaciers: 
* They’ve disappeared by half or more of the total mass
* Most significant signal of global warming
* However, after we got satellite measurements, we got accurate weight of mountain glaciers around the whole world sums to .3 meters of global sea level rise. So if all of the mountain glaciers melted around the world, the ocean would only rise by .3 meters. Not a big of deal as ice sheets melting
* \
*  Atmosphere water vapor, clouds, precipitation
* Water vapor flows in the atmosphere
* Each person has a cube of 48x48x48 meters of water if it was divided equally
* Water vapor moves with the wind
* West to east and south to north
* Anything released in the global atmosphere comes back to us, nothing is isolated, that’s why we should protect our water
* Precipitation drives many atmospheric circulations which in turn transport water around the world
* Tropical circulations connect land and ocean
* Monsoons provide water supply to world’s population
* Hadley circulation brings water to the poles from the equator?
* Water in biosphere (including human beings)
* Every 16 days almost 100% of the water in our bodies is exchanged 
* 90.7% blood is water, 70% of our body is water
* The remaining: fat, protein, carbohydrate, other solids
* Therefore we need to protect the environment because any pollution we put into the environment may someday come back into our bodies

Condensation is the cold temperature cooling down the water in the air

* Controlled only by temperature
* When it’s cold, water condenses, when it’s hot, the water evaporates

Water (H2O ) is unique on earth because it can exist in all 3 states (phases)

* An H2O molecule
* Exists in all 3 states (gas, liquid, solid) depending on how the molecules are connected together (TEMPERATURE)
* Gas phase has the highest energy level, molecules can move without confinement
* Liquid phase has medium energy, molecules are loosely connected, they have some freedom to move around but not much
* Ice phase has lowest energy, they are more condensed into the formation of a crystal, which is a fixed structure, they don’t have freedom to move

Latent heat: heat released when water changes structure

* Drives weather and climate


* • Can change from any state to any other state. Latent heat is consumed or released in a phase change 
*      e.g. Evaporation -> liberation of water molecules, requires energy

Phase changes

* From gas to liquid=condensation
* Reverse process is evaporation
* From liquid to solid=freezing
* Reverse process is melting
* Water vapor to ice=sublimation
* Reverse process has the same name

Evaporation and Condensation

* Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates.
* • Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation.
* • There is an equilibrium between evaporation and condensation during saturation. Upon saturation, evaporation rate equals condensation rate.

\n

* Water vapor evaporates into the sky. This is very fast. A slower process is when this water condenses into water like rain. Condensation rate increases the more water is evaporated, until condensation rate=evaporation rate
* Reaches saturation, the maximum amount of water that can be held in the atmosphere

Sublimation and Deposition

* Water vapor (gas) can change directly into ice or snow (solid) during deposition.
* • Ice or snow (solid) can turn directly into water vapor (gas) during  sublimation.
* • There is an equilibrium between deposition and sublimation during saturation. Upon saturation, deposition rate equals sublimation rate.

\n

* Water vapor saturates in the sky as ice in the winter

Indices of Water Vapor Content

Humidity: amount of water vapor in air
• Humidity expressed in a number of ways Indices


1. Vapor Pressure: the partial pressure exerted by water vapor. the partial pressure exerted by water vapor. Saturation vapor pressure (SVP) – maximum amount of vapor that can exist at a given temperature, increase w/ Tair
2. Absolute Humidity: density of water vapor expressed in g/m3
3. \
Specific Humidity: mass of water vapor (g) per mass of air (kg) (in g/kg).Saturation specific humidity (qs): highest specific humidity for a given temperature and pressure
4. \
Mixing Ratio: amount of water vapor (g) relative only to mass of dry air (kg).Saturation mixing ratio: maximum mixing ratio
5. Relative Humidity: the amount of water vapor in the air relative to the possible maximum.
What we use in our daily lives
Ranges from 0 to 100%
100% means atmosphere is saturated with water vapor
40% saturation means there’s 60% more room for water vapor
Comfortable water vapor is 60% or more
6. Dew point temperature: temperature at which saturation occurs in air (generally colder than Tair, equals to Tair when saturated)
This is what the weather stations read, and then we convert this into the other 5 measurements

Saturation vapor pressure

* Saturation vapor pressure is temperature dependent.
* • Saturation vapor pressure increases with temperature. 
* Warmer air can hold more water vapor.
* • It’s a non-linear increase. At low temperatures the saturation vapor pressure increases slowly but it increases rapidly at higher temperatures.
* TEMPERATURE CONTROLS CONDENSATION OF WATER
* When your car windshield is foggy, turn on the heat, heat can hold more water vapor, so it won’t condense on your windshield anymore
* When there is condensation on your cold drink, it’s because there is alot of water in the atmosphere, and the cold air can’t hold all of that water so it condenses
* So to make it rain, cool down the temperature

Methods to achieve saturation and condensation

* Diabatic processes – add/remove heat
* – Conduction (e.g. movement of air mass over a cold surface)
* – Radiation (e.g. cooling of boundary layer air by longwave radiation)
* fog
* • Adiabatic processes - no addition/removal of heat
* – Add water vapor to air
* – Mix warm air with cold air
* – Cooling of air parcel when it rises (because air parcel expands when it rises, like a balloon)
* CREATES CLOUDS AND THUNDERSTORMS
* 1st Law of Thermodynamics  expanding air cools, compressed warms (like a manual hand air pump).

Forms of Condensation:

* saturation  droplets or ice crystals
* •  condensation/deposition  dew, frost, fog, clouds

Forms of Condensation:

* Dew 
* • liquid condensation on surface
* • occurs early morning on windless cloudless days
* • air immediately above ground cools, reaches dew point 
* • diabatic process
* COOLING DOWN OF TEMPERATURE OF WARM MOIST AIR FROM EARTH’S SYSTEM. 

Frost

*  \~ similar to dew BUT saturation occurs below 0 degrees Celcius
* •  deposits white ice crystals  known as hoar frost
* •  e.g. car windshield  
* •  phase change from vapor directly to solid (deposition)
* Same thing as dew but solid
* •  diabatic process

Frozen Dew

* results when saturation occurs slightly above 0 degrees celcius  liquid dew   formed, when Temp drops liquid dew freezes
* •  forms thin sheet of ice, tightly bound to surface
* •  dangerous – black ice
* 2 STEPS
* Formation of morning dew
* Then the temperature drops below freezing and the liquid turns into ice

Fog

* Just need cooling of cool, moist air
* can be considered a cloud with base at ground level
* •  air has either been:
* •  cooled to dew point
* •  had moisture added 
* •  mixed with warm moist air
* • 5 different types
* •  radiation
* •  advection
* Movement of warm moist air onto cold ocean surface, creating fog. Reason for fog in San francisco
* •  upslope
* •  precipitation
* •  steam

Radiation Fog

* What happens in in land places
* Earth’s surface cooling down air next to surface
* occurs when near surface air chills diabatically through loss of  longwave radiation  reaches Dew Pt
* •  requires cloudless nights and light wind to create mixed layer
* •‘burns’ off with sunrise – evaporates from below due to surface  Heating
* Happens in the morning because that’s when the temperature is the coldest

Advection Fog

* occurs when warm moist air moves across a cooler surface
* • air is chilled diabatically to saturation
* • common on the U.S. west coast  warm, moist air 
* from Pacific advects over the cold California current
* • Frequently develop near boundaries of opposing ocean temperatures
* • e.g: northeast coast of the U.S., Gulf Stream   and Labrador current

Upslope Fog

* Caused by wind blowing towards mountains, which is then pushed up. And temperature decreases with height, so water condenses
* develops due to adiabatic cooling
* •  occurs when air is lifted over topographic barriers, mountains
* •  air expands and cools as it rises
* •  common in region between Great Plains and Rocky Mountain foothills

Precipitation Fog

* Occurs when there are thunderstorms, because of addition of moisture in atmosphere
* Rain occurs and some evaporates as it falls toward Earth
* •  Sometimes this will lead to saturation near surface and cause fog
* •  Adiabatic process

Steam fog

* Direct contact of warm moist air from streams contacting with cold air
*  Mixing of warm, moist air with cold air
* •  Adiabatic process (no net change of energy)
* • e.g.,  common when cold air move over warm lakes/streams in autumn

Fog along the coasts are avection fog

* Inland is radiation fog
* Along the gulf is a mixture of avection and radiation fog
* Along the rockies is the upslope fog

Fog concentrations

* Huge concentrations of fog in the appalachian mountains
* Warm moist air from the ocean is lifted up along the mountains
* Least number of days of fog in the deserts, because it’s too dry for any moisture to condense

The most common atmospheric circulation structure

* Imbalance of heating 
* Through convection, radiation, conduction
* Imbalance of temperature
* Hot air moves to cold air
* Imbalance of pressure 
* High pressure to low pressure
*  Wind

Clouds are instrumental to the Earth’s energy and moisture balances, and constitute a wild card for climate change

* Clouds are also known as convection
* Vertical movement of atmosphere. Air rises up 
* Clouds are just Hot Air Balloons
* Clouds rise up because the air is hotter and lighter than surrounding air
* pressure=air density times air temperature.

Clouds are important because

* they create the heat in the upper atmosphere, and they create extreme weather by the latent heat they release
* Play key role in global warming because they reflect sunlight back to space, keeping our Earth cool
* A 4% increase on clouds could cancel out global warming completely

Satellite observation of clouds

* Knowledge of clouds came after the creation of satellites. 
* NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites. 1983-Now, cloud top height and optical depth.
* 1st satellite
* Geostationary satellites fly above the equator at the same speed as the Earth’s rotation
* Always be above the same spot
* Polar Orbiting satellites fly freely between the 2 poles
* Only pass the same location twice each day
* NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

Lack of cloud coverage over

* poles and deserts
* Lots of cloud coverage over oceans and around the south pole, the tropics, and around the north pole

Vertical Structure of Clouds

3 belts of clouds, over the equator, and the southern and northern hemisphere storm tracks

Cloud Properties (quantitatively)

1. Cloud top height/pressure
2. Cloud thickness (optical depth)
3. Cloud coverage (cloud amount)
When clouds comprise more than 9/10th of the sky = overcast
When coverage is between 6/10th and 9/10th = broken
When coverage is between 1/10th and 6/10th = scattered
Cloud coverage less than 1/10th = clear

NASA’’s International Satellite Cloud Climatology Project (ISCCP) s International Satellite Cloud Climatology Project (ISCCP)

* Cloud Classification - commonly used in climate researchCloud 
* High clouds above 8 km high, medium clouds between 3 and 8 km, short clouds are below 3km
* Stratus means layered clouds
* Thick clouds block incoming sunlight, so the darker the cloud, the thicker the sunlight
* Types of clouds
* Cirrus
* High top pressure
* Small cloud optical thickness
* Cirrostratus
* High top pressure
* medium cloud optical thickness
* Deep convection
* High top pressure
* High cloud optical thickness
* thunderstorm
* Altocumulus
* Medium top pressure
* Small cloud optical thickness
* Alto means tall
* Cumulus means convected clouds, clouds that rise
* Altostratus
* Medium top pressure
* Medium cloud optical thickness
* High layered clouds
* Nimbostratus
* Medium top pressure
* large cloud optical thickness
* Nimbo means windy?
* Cumulus 
* Low top pressure
* Small cloud optical thickness
* Stratocumulus
* Low top pressure
* Medium cloud optical thickness
* Stratus
* Low top pressure
* large cloud optical thickness

 Why do clouds constitute a wildcard for climate change?

* Clouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect).  
*  The net effect (cooling or warming) depends on the type of cloud
* In a changing climate, increases in high thin clouds would promote warming while increases in low thick clouds would cause cooling
* Climate models have difficulties in simulating clouds, especially low thick clouds (stratocumulus)
* Conclusion: Clouds cause the largest uncertainty in model simulations of future climate. 
* Clouds also trap in longwave radiation (greenhouse effect)
* Cooling effect of reflecting shortwave radiation back to space
* Net effect depends on thickness of clouds
* Thick clouds are climate refrigerators and keep out heat
* Think clouds let heat in and don’t let it out

Thick cloud produce rain, thin clouds

don’t

Formation of clouds

* Most clouds form as air parcels in boundary layer are lifted and cooled to saturation. 
*  The air parcels could be lifted by mountains, meeting of different air masses, surface convergence, and local convection
* Convection of hot air from the surface rising to the atmosphere

Lifting by local convection

* Static stability – refers to atmosphere’s susceptibility to being displaced
*  Stability related to buoyancy force  determined by density difference btw parcel and environment (FFBB= = ρρenvenvg – g – ρρparcelparcelgg)  determined by temperature difference btw parcel and environment (ρ = P/TR)
* When an air parcel rises, the cooling rate of the parcel (adiabatic lapse rate or ALR) relative to the cooling rate of  surrounding atmosphere (environmental lapse rate or ELR) determines the “stability” of a parcel.

\n

* If inside air is colder on the inside than outside air, it won’t rise up

The three types of stability

* Absolutely stable
* Inside temperature is always colder than the outside, so we don’t have enough heat to push cloud up, no clouds or thunderstorms, fair weather
* Absolutely unstable
* Inside temp is very hot, so clouds rise up and form thunderstorms
* Conditionally unstable
* In between stable and unstable. We do have some heating, but it’s not large enough. So cloud air is initially colder than surrounding environment and doesn’t rise, but if there’s a trigger that forces the cloud up, and water vapor will condense into liquid water and release latent heat, which warms up the clouds, and the cloud will rise up and form thunderstorms
* NATURE IS USUALLY CONDITIONALLY UNSTABLE, IT NEEDS A TRIGGER

When convection happens:

1. Rising up of air parcel (called updraft)
2. Formation of clouds and sometimes precipitation
3. Heating up the environment because parcel temperature is warmer than the environment

What stops ‘unstable’ air masses from rising indefinitely ?

* 1)  Entrainment
*  Turbulent mixing of ambient air into parcel 
*  Leads to evaporation along cloud boundaries
* Evaporation uses latent heat, cooling the cloud  reduces buoyancy
* ATTRACTING COOLER AIR INTO THE CLOUD
* Cools down inside air, making inside air equal outside air temp
* No longer warmer than surrounding environment and will stop where it is
*   2) Encountering a layer of stable air (inversion)
*  a rising parcel may reach a stable upper air environment 
*  the parcel cooling rate will exceed that of The ambient air
*  the parcel will slowly cease ascension and come to rest at some equal temperature level
*  three types: radiation, frontal, subsidence
* THERE'S A WARM LAYER OF AIR IN THE ATMOSPHERE, THE CLOUD WON’T BE AS WARM, AND WILL STOP RISING
* There will be a wide layer of clouds at this deck, because they can’t move any higher

In convection, an updraft is often associated with a downdraft – Overturning of the troposphere

* Air can be cooled down by radiation, evaporation of raindrops, melting of snowflakes, etc.
* Air that is cooler than its environment tends to sink, leading to the formation of downdrafts
* Sometimes precipitation drag enhances the downdrafts
* Downdrafts cool down the  environment (generally the lower troposphere)
* Downdrafts (also called downbursts) can cause significant damage at the ground
* UPDRAFTS ARE AIR PARCELS THAT MOVE UP, AND DOWNDRAFTS AT AIR PARCELS THAT ARE MOVING DOWN. 2 COMPETING PROCESSES THAT HAPPEN AT THE SAME TIME

Low stratocumulus clouds

* Generated by convection inside boundary layer
* Convection is driven by cloud-top longwave cooling and evaporative cooling
* WARM LAYER OF AIR THAT WON’T LET CLOUDS MOVE UP ANYMORE.

Satellite observation of precipitation

* ALL OF OUR KNOWLEDGE ABOUT RAIN AND SNOW COMES FROM SATELLITES
* Monitors the precipitation on Earth, monitors it using
* Infrared-derived or visible-derived (GPI)
* Only tells us about cloud top height and thickness, and with equations, we can indirectly get amount of rain fall
* Microwave-derived (MSU, SSM/I, TMI)
* Can actually see the rainfall and snowflakes
* Radar: Tropical Rainfall Measurement Mission (TRMM) 
* Best method. First satellite to carry radar in space
* Merged with surface gauge measurements and model Forecast
* Merges all 3 sensors into a unified instrument

Global distribution of precipitation

* We get the heaviest rainfall near the equator
* Also on East coasts of land masses

Precipitation formation - cloud drop growth

* Cloud droplets are formed in the sky. Their speed to fall to the ground is very slow because there’s the downward force of gravity, and the upward force of friction
* These forces balance. Creating a constant speed of terminal velocity, which is determined by the size of the cloud droplet. 
* Not all clouds precipitate due to their small sizes and slow fall rates
* Balance between gravity and frictional drag  eventually become equal to achieve terminal velocity VT, which is proportional to the square root of cloud drop radius  VT=c r0.5 ,where r is drop radius and c is a constant.
* For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up.
* Cloud drop growth is required for precipitation to form

Mechanisms for cloud drops to grow larger

1. Collision Coalescence (happens inside warm clouds, when temperature is greater than 0 C, form rain)
2. Bergeron Process (cool/cold clouds, temperature is colder than 0 C, form snow)
Entire cloud mass is colder than 0 degrees C
But in cool clouds, only the bottom part is colder than 0 degrees C

1\. Collision Coalescence: Growth in Warm Clouds, creates rain

* Not all droplets are the same size. Large droplets fall faster than smaller droplets. Large droplets will catch up to small droplets, and merge with small droplets. They’re known as collectors. Grow bigger and form raindrops. But, the collector drop can’t be too big, or else it’ll create a small wind under it, and push small droplets away from it, preventing small droplets from being able to merge. Most efficient when collector drop is slightly bigger than all other droplets
* Process begins with larger collector drops which have higher terminal velocities
* Collector drops collide with smaller drops and merge with them (coalesce). Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining.
*  If collector drop is too big: compressed air beneath falling drop forces small drops aside
*  If collector drop is too small (same size as other drops) it will fall at same speed and no collision will occur
* So, collection efficiency is greatest when the size of collector drop is slightly larger than the size of the other drops
* After the collector drops become large, the larger one among them can serve as a “super-collector” to collide with other collector drops

Raindrop shape and maximum size

* There’s an upper limit for rain drop caused by friction of surrounding air. When rain falls down, frictional force pushes rain drop and flattens it out. Shape of rain isn’t tear shaped, because of friction, it flattens it out. If the raindrop is too large, friction will break the raindrop in half, limiting its size to under .25 inches. 
* Determined by competition between surface tension and frictional drag. Frictional drag is larger at the bottom than at the top
* Small drop (<0.08in):  frictional drag << surface tension  Sphere shape 
* Medium-size drop (0.08in<size<0.25in): frictional drag approaches surface tension  Parachute shape 
* Large drop (>0.25in): frictional drag at bottom > surface tension  Split (The surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.)
* Maximum drop size of about 0.25in or 5 mm

Formation of snow and hails

* No 2 snowflakes the same
* Bergeron Process: occurs inside cool/cold clouds. At least some of the cloud mass is colder than 0 degrees Celsius. Formation of snowflakes needs a condensation nucleus to form a snowflake, from pollution, dust, aerosol, some sort of small particle. If the atmosphere is super clean, then it’ll be super cold water than won’t freeze down to the temperature of -40C. If you do have the nuclei, then the Bergeron Process starts, converting cool water droplets into snowflakes. Saturation vapor pressure of ice that’s smaller than the saturation vapor pressure of super cool water. (When we have the coexistence of water and ice, water vapor in the sky will jump to the lower energy process of ice). Water vapor freezes on condensation nuclei. Super cool water will evaporate, and enter the atmosphere as water vapor, and refreeze onto nuclei, making them larger and larger creating snowflakes. 
* 2. Bergeron Process: Growth in Cool/Cold Clouds
* Clouds are usually composed of: liquid water, super-cooled water, and/or ice (supercooled water exists down to T= -40C !!)
* Supercooled water can exist at T<0C because ice formation requires ice nuclei, which, unlike condensation nuclei, are rare unless the temp. is very cold
* Coexistence of ice and super-cooled water is critical to the creation of cool/cold cloud precipitation - the Bergeron Process
* Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature. 
* When air is in saturation wrt super-cooled water, it’s over-saturated wrt ice - deposition of water vapor over ice. 
* When air is in saturation wrt ice, it’s sub-saturated wrt super-cooled water - evaporation of super-cooled water into water vapor. 
* In this way, ice crystals grow rapidly at the expense of super-cooled drops http://www.uwsp.edu

Further growth: Riming and Aggregation

* Snowflakes created by bergeron process are still very small and will take forever to land on ground. To get faster, we need rimming and aggregation. Rimming is the freezing of liquid water onto the surface of ice particles. Because snowflakes are larger than water droplets and collide into them, causing water droplets to freeze layer by layer on them. Gives us hail. Aggregation is the direct joining of small snowflakes into a larger snowflake. 
* Bergeron Process usually not enough to produce large enough crystals for preciptation
* Further growth is due to collisions between falling crystals and drops  riming and aggregation
* Riming (or Accretion) = liquid water freezing onto ice crystals 
* Aggregation = the joining of ice crystals through the bonding of surface water builds ice crystals, producing snowflakes 
* Collision combined with riming and aggregation allow formation of crystals large enough to precipitate within 1/2 hour of initial formation

Shape of snowflakes depend on formation conditions (humidity and temperature)

Wilson Bentley was the first person to take pictures of snowflakes.

* Different shapes are controlled by environmental temperature and humidity
* If the atmosphere is dry and cold, shapes will be very compact. If it’s warm and moist, fluffy snowflakes. Less dense and fall to ground slower.

Change of falling ice crystal: dependent on atmospheric temperature and winds

* Raindrops and snowflakes form in upper atmosphere and change by the environment and wind as they come to the surface
* A snowflake could melt on the way down to the ground because the surface may be warmer than the atmosphere
* So snow may turn into rain or hail before hitting the ground because of the temperature of the path it takes

Snow

* When the temperature is colder than the freezing point, so snow will come all the way to the surface in their original shape
* Snow is precipitation that forms by the Bergeron process, riming, and aggregation, and reaches the surface without melting 
* Crystal form (habit) varies with T and RH
* Large, soggy snowflakes associated with moist air near freezing

Global Map of Snow cover

* Lots of snow cover in high latitudes, north of 40 degrees north
* And antarctica

Global Snowfall for North America

 around Canada and Great Lakes

Lake effect

* When a storm passes over lake, the water of the lake evaporates and rises into the air, creating more rain or snow over downstream part of lake (right side)
* Heat and moisture fluxes from warm lake enhance snowfall in downstream regions

Melted snowflake turning into rain

* Happens in the summer. Rain at the ground was originally snow in the sky, because thunderstorms are all formed about freezing level (which is at about 5km) The snowflakes are melted at ground level because it’s warmer at the surface. Takes half an hour for snowflakes to fall to ground and turn into rain. In a picture of a thunderstorm, the bright part of the cloud is the ice and snow, that’s why it’s so bright. Darker part is rain, where the ice has melted. 
* Much of the rain in mid-lats (even in summer) begins as snow!
* The falling snow begins to melt around the freezing level

Sleet

* Occurs in winter when there is a sandwich temperature structure. When the temperature is cold in the upper atmosphere, starting as snow. It than melts in the warm layer to rain. But than at the surface there is another cold layer. So the rain is refrozen into ice particles. 
* Sleet begins as ice crystals which melt into rain as they fall through the atmosphere. Before reaching the surface they solidify into a frozen state.

Freezing rain

* Most dangerous one. Just like sleet. But the warm layer is extremely large. And the bottom cold layer is very tiny, so there’s no chance to be frozen again until they reach the ground. So it falls as rain, meets the surface, and then freezes over. 
* Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface

Graupel

* In the summer, generated by strong wind in sky. In super cell thunderstorms, there’s strong wind which holds ice particles in the sky, performing extensive rimming. So liquid particles freeze on the ice particles, growing the haild larger and larger. Only occurs when there is supercell thunderstorms. Wind will hold the ice for more than half an hour. 
* Graupel – ice crystals that undergo extensive riming
* Lose six sided shape and smooth out
* Either falls to the ground or provides a nucleus for hail

Hail

* Hail isn’t as frequent because it only forms in the summer. More intense form of graupel?
* Hail – concentric layers of ice build around graupel
* Requires very strong updrafts
* graupel carried aloft in updrafts  high altitudes freezing temperatures
* water accreting to graupel freezes, forming a layer
*  Hail begins to fall, carried aloft again by updrafts, process repeats
* Hailstones are very heavy – high density 
* Capable of tremendous amounts of damage
* Great Plains = highest frequency of hail events
* Warm moist air from the gulf of Mexico gives enough moisture for hail. Supercell thunderstorm lifts the moisture into the sky and generates ice balls.

Summary: Different types of precipitation

* Surface precipitation type depends on atmospheric temperature and winds
* If cold temperature all the way down to surface, snow
* If temperature is warmer at surface, rain
* If there is a sandwich of cold, warm, cold, sleet, unless the bottom cold layer is very tiny, freezing rain
* If super cell thunderstorm is formed, ice balls are lifted in the sky, stay there for at least half an hour, and then hail is formed.

Processes of Lightning Formation

4 steps


1. Charge separation: need to accumulate positive and negative charges. Generation by collisions of ice particles in high clouds. Particles vary in size and temperature. Large particles are warmer, they fall quicker to the ground. Fall speed is proportional to radius of particle, it’s the square root of it. Larger particles are warmer than smaller particles because they have greater friction with surrounding air as they fall faster. Large particles collide with smaller particles. They transfer positive ions from warm particles to cold particles. So there is only negative particles left in the large ions. Now, at the cloud’s base, it lots of large negative ions. And at the top, small positive ions. Charges have seperated
2. Stepped leader: negative charges at cloud base attract positive charges from the ground, and accumulate under the cloud base. Creates an electrical field between ground and cloud base. This pushes the first part of lightning, to extend away from cloud base to the ground. Very weak lightning spark, you can barely see it.
3. Return Stroke: The step leader will eventually touch down. Opens circut, connecting cloud base and ground. All the ground’s positive charge comes up to the cloud base, colliding with negative ions, creating large lightning strike. (return stroke) peak of lightning event
4. Dart Leader: there are still leftover electrical charge. Generates secondary lightning spark that is weaker.

\

1. Charge separation. Charge layers in the cloud are formed by the transfer of positive from warmer graupel to colder ice crystal ions from warmer graupel to colder ice crystal when they collide with each other.
2. Stepped leader. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms.
3. Return stroke. A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud
4\.Dart leader. Not all of the first stroke neutralizes the negatively charged ions and results in another leader in 1/10 of a second

Lightning: Runaway discharges

* Lightning is a big explosion of the atmosphere. Generates heat. Temperature of atmosphere will be hotter than sun’s surface. That’s why an explosion is formed. Discharge is so strong, it can cause catastrophic event. Charges are colliding with each other in small volume of air. 
* High-speed moving electrons radiate light as lightning.
* Electrons approach the speed of light, gaining energy.  Fast moving electrons create others by colliding with atoms. This can result in an avalanche called runaway electrons.
* When a large number of runaway electrons accumulate in a small volume, the energy is released in a so-called runaway breakdown.

Thunder

* Lightning caues thunder. We don’t hear all thunder. Because the travel path of thunder isn’t always a straight line. It can be distorted by atmosphere. It travels in a curve. Lightning always travels in a straight line though, so we always see it. Every lightning generates thunder. Thunder travels at a  slow speed, speed of sound, which is slower than speed of light. That’s why you see the lightning first, and hear thunder after. It takes 3 seconds for thunder to travel 1 kilometer. 5 seconds a mile. 
* Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air creates an explosive shock wave called thunder. 
* •It takes about 3 seconds for thunder to travel 1 kilometer (5 sec per mile). A lag in lightning strike and thunder occurs due to sound traveling slower than light.  
* When thunder is farther away, the echoing of sound waves off of objects (like buildings and hills) causes thunder to sound rumbling.

Refraction of thunder

* Sometimes you see lightning and not hear thunder because of refraction of thunder by atmosphere. Path of thunder is a curve, not a straight line, it’s distorted. Dependent on temperature distribution in atmosphere. 
* Sound waves move faster in warm air than they do in cool air.
* Typically, the air temperature decreases with height. When this occurs, thunder will normally ave an audible range up to 10 miles (16 km).
* However, when the air temperature increases height, called an inversion, sound waves are refracted (bent back toward the earth) as they 
* move due to their faster motion in the warmer air effectively amplifying the thunder.

* Global distribution of lightning strikes

* Significant difference between continent and ocean
* Most lightning over africa 
* Warm tropical areas have more lightning that cold polar regions
* Lightning happens over land, very little over ocean
* Because there’s very little ice particles in clouds over ocean

Annual mean number of days with thunder

* Least amount near alaska because it’s too cold
* Also not alot near California. It’s too dry
* Most near florida

U.S. distribution of lightning time

* For the east coast, most lightning happens at night
* For the mountains, most lightning happens during the day

Lightning

* Super fast discharge of electricity. 2 types of lightning
* Cloud to cloud lightning: 80% of lightning. Not dangerous. Only in clouds
* Cloud to ground lightning: 20% of lightning. Heat the ground, dangerous 
* Lightning is a discharge of electricity, a giant spark
* 80% of lightning occurs when clouds discharge electricity. This is referred to as cloud-to-cloud lightning; occurring when voltage gradient overcomes the electrical resistance of the air.
* 20% of lightning occurs when electrical discharge travels between the base of the cloud and the surface. This is referred to as cloud-to-ground lightning.  

Duration of lightning flash

.00003 seconds

\n

* Climate impacts of lightning (1) Nitrogen cycle

* Important for formation of life, it fixed nitrogen to soil back in the development of earth. Nitrogen helps form life. 
* The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. 
* These dissolve in rain, forming nitrates, that are carried to the earth.Contributes some 5– 8% of the total nitrogen fixed.
* Was probably the only source of fixed nitrogen for the earliest forms of life.

Climate impacts of lightning (2) Ozone production

* Lightning also helps with Ozone production. Ozone protects us from UV light in the high atmosphere. When ozone is close to the ground though, it’s pollution, you don’t want to inhale it. Lightning generates ozone, that’s why you can smell it after a lightning strike. 
* Ozone was named after the Greek verb ozein (ὄζειν, "to smell"), from the peculiar odor in lightning storms.the peculiar odor in lightning storms.
* Nitrogen oxides produced by lightning can react with others in the presence of sunlight to produce ozone. 
* Since most lightning occurs inside a storm, the added ozone tends to show up several miles high rather than near the earth's surface, so it doesn't add significantly to ozone pollution at ground level.

Climate impacts of lightning (3) Damages

* Causes forest fires. Especially in California. Can kill people. 
* Annual mean fatalities 69 for U.S., 600 world-wide
* Hundreds of people are permanently injured
* Annual mean property loss $38 million for U.S.
* Lightning is the leading cause of wildfires (25,000 each year)
* Especially caused by lightning in dry thunderstorms (virga)

Lightning Safety

* DON’T BE THE HIGHEST OBJECT IN AN AREA. AVOID WATER BECAUSE IT’S A CONDUCTOR OF ELECTRICITY
* Always take cover in a building. 
* Do not stand under a tree or other tall object that might serve as a lightning rod.
* •Avoid standing on mountain summits, ridges, rooftops, or other high areas. 
* •Avoid caves
* •Avoid open water (pool, lake, or hot tub).

Convection Systems

* Tornadoes: about : about 100-600 m, last , last 1 minute to 1 hour 
* Wind stronger than hurricanes.
* Thunderstorms: about 10 Km, last , last 10 minutes a couple of hours. 
* 3 types: ordinary, multicell, supercell
*  Mesoscale convective systems (MCSs): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 Km or more in at least one direction, and often last for several hours to a couple of days
* Largest convection system. A cloud system that is larger than 100 km in at least one direction. Last for a longer time period. 
* The larger the system, the longer it lasts and the more damage it costs

Thunderstorms

* THE WARMER IT IS, THE MORE DAMAGING IT IS. We need a trigger to start a thunderstorm, that’s why we don’t have them everyday. We need a trigger to push the air parcel up and let it release latent heat. 
* A storm containing lightning & thunder
*  Convective; form when warm, humid air rises in conditionally unstable environment
*  The warmer the rising air parcel is relative to environment, the more buoyant force is driving it upwards (stronger convection)
*  Trigger to start uplift: warming sfc, terrain (orography), converging sfc winds, frontal zones, divergence aloft (or combination)

Global distribution of Thunderstorms

Most around tropical land areas. Mostly happens on land

Thunderstorms: 1. Ordinary Storms

Weakest thunderstorm. What we normally get. Don’t cause too much damage. 3 stages:


1. Developing stage: warming up of surface by incoming sunlight. Warms up the air parcel and it becomes warmer than the surrounding environment. And a trigger pushes the air parcel up.
2. Mature stage: cloud parcel has reached the upper atmosphere and has generated lots of precipitation. Rain falls and generates a down draft. The down draft hits the ground first with the wind, before the rain. Known as gust front. This is a signal of a thunderstorm. After 10 minutes, snow or rain comes.
Dissipating stage: Downdraft kills the updraft of heat and kills the thunderstorm by cooling down the surface.
Three stages have been identified in ordinary thunderstorms:
a) DEVELOPING: unstable atmosphere, vertical updrafts keep precipitation : unstable atmosphere, vertical updrafts keep precipitation suspended
b) Mature: entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts
c) DISSIPATING: weakening updrafts and loss of the fuel source after 15-30 minutes.

Thunderstorms II. Multicell Storm

* Stronger than ordinary thunderstorm. Ordinary thunderstorms are killed by downdrafts. But sometimes, cold air can act as a trigger and push warm air up, creating a new thunderstorm in the upper atmosphere. It’s a domino effect that creates a whole line of thunderstorms being made. Oldest one is the largest one. The one that is currently raining.
* Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward.
* Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

Thunderstorm III. Supercell Storm

* Strongest storm. Can generate tornadoes and hail
* Storms producing a minimum of 
* a) 3/4 inch hail and/or 
* b) wind gusts of 50 knots and/or  
* c) tornado winds, classify as severe.

Formation of supercell thunderstorms

We need 2 necessary conditions
A supercell thunderstorm is basically a vortex in the sky. We need a vortex to form by wind flowing in opposite directions on the surface and above the surface. We also need convection. Warm moist air to push the air parcel up. Helps generate strong storms. We always have warm moist air in the spring and summer. The vertical vortex is known as mesocyclone. This means the size of the vortex is large. Tornadoes than form in the mesocyclones when rotating fast and touching down on the ground.


1. Before thunderstorms develop, a change in wind direction and an increase iin wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
2. Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a mesocyclone.
3. Most strong and violent tornadoes form within this area of strong rotation.

Vertical structure of a supercell thunderstorm

* The thunderstorm is always tilted. Never a perfect vertical storm. This tilt is important for the strength of the storm. It keeps the updraft from being killed, and it only gets stronger. Lets it last for a long time
* In ordinary storms, the downdraft and falling precipitation cut off the updraft. But in . But in supercell storms, winds aloft push the rain away and the updraft is not weakened and the storm can continue maturing and maintain its structure for hours.

Cloud structure in a supercell thunderstorm

* Before a touchdown of a tornado, a predictor of it is a wall cloud, extending from the cloud base to the ground. It’s a rotating cloud. In the wall cloud, a smaller region touches down on the ground, the tornado.
* Rain and hail fall in the rear part of the storm.

Horizontal structure of a supercell thunderstorm

Tornado occurs in the front of the storm. Rear is where the rain and hail is. 2 regions of the strongest damage.

Radar echo of a supercell

Red part, looks like it’s rotating

Satellite image of a supercell

Cloud deck over storm. Looks similar to a hurricane. A round shape that’s rotating

Types of supercell thunderstorms

* Low precipitation
* When surrounding environment is dry
* Not as dangerous, but strong wind still causes problems
* Higher precipitation
* Most dangerous one

Effects of supercell Thunderstorms

* Large hails
*  Damaging winds
*  Flooding
*  Dangerous cloud-to-ground lightning
*  Deadly tornadoes

Tornadoes

* Small intense vortex rotating in the atmosphere. The center of the tornado is low pressure system. (Low pressure means intense weather. High pressure is high temperature and fair weather). This low pressure in the center with the high pressure on the outside converges in the low pressure center, generating the strong wind of the tornado. Size of a tornado is about the size of our stadium. The larger the tornado, the longer it lasts. Form along the boundary that separates the cold and warm air masses. Move from SW to NE in America. Not that way in all countries
* A rapidly rotating column of air blowing around intense low pressure with circulation reaching ground
*  Wind speeds between 105 km/hr(65mph) and 450 km/hr(280mph)
*  Rotation is almost exclusively cyclonic; a few spin in the opposite direction
*  Various sizes: most are 100-600 m in diameter; some just a few meters; some >1 mile
*  Various shapes: twisting rope-like funnels to cylindrical funnels, to massive black funnels
*  Usually last only a few minutes, but some lasted several hours
*  Most move ahead of cold fronts, from SW to NE; some move in other directions 
*  Moving speed is about 30 mph (some >70 mph)

Tornado Formation

* We still don’t know how to predict them, we can only warn people after the tornado has formed. Can only use remote sensing instruments to detect tornados from far away because they’re so dangerous. 
* Tornadoes can develop in any situation that produces severe weather – cold fronts, mesoscale convective systems, supercells, and tropical cyclones. 
* The processes leading to their formation are not well understood
* The most intense and destructive tornadoes come from supercells.

3 Stages of Supercell Tornado Formation

* Most tornadoes are generated by supercell thunderstorms. Formed the same way as supercell thunderstorms. There is a horizontal vortex that is the seed for tornado formation. This vortex then gets tilted up by convection of warm air. This vortex begins to rotate, mesocyclone. In the 3rd stage, the mesocyclone is rotating very fast and touches down at the ground. We don't know where and when, just in the mesocyclone. 
* Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect  in the lower atmosphere. 
* • Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into mesocyclone..
* •• Most strong and violent tornadoes form within this area of strong rotation.

Cloud Structure of tornado breeding and supercell storm

* Before every tornado there is a wall cloud extending from the base, and the tornado than forms from the wall cloud and touches down to the ground. The wall cloud is a short term predictor.
* Wall cloud forms underneath the mesocyclone when cold/moist downdraft air feeds into the updraft and condenses at a lower level than warm air

Wind structure of supercell and tornado

Supercell thunderstorm is a giant vortex with a mesocyclone rotating inside it in the atmosphere. In the center of the mesocyclone is a wall cloud and tornado. Wind structure of tornado is warm air rising up on the outside, and cold air going down in the center. Wind rotates very fast all the way to the upper atmosphere.

Most violent tornadoes

* Many strong storms typically have multiple vortexes and tornadoes. Very violent. 
* Most tornadoes rotate around a single core, some of the most violent ones have several small zones of intense rotations called suction vortices.

\n

* Tornado Damage

* Measured by Fujita Scale (they evaluate based on damage, and then decide on the wind speed, because they can’t measure the wind speed). Divide tornadoes into 6 categories of intensity. 
* EF-0 (65 mph) broken branches and signs, etc
* EF-1 (86-110 mph) damage to roofs and mobile homes and cars
* EF-2 (111-135 mph)
* EF-3 (136-165 mph)
* EF-4 (166-200 mph)
* EF-5 (over 200 mph). Concrete and steel made buildings bad damage
* Tornadoes are classified by the magnitude of damage they cause using the (enhanced) Fujita scale.

Tornado Outbreak

* When more than 6 tornadoes happen at the same time. A super outbreak is when more than 100 happen at the same time. 
* The forms causing the largest damages are families of 
* tornadoes; when many occur (>6), it is a tornado outbreak
*  The largest tornado outbreak on record, depending on the definition applied and time elapsed between breaks in tornadic activity, was the April 25–28, 2011 tornado outbreak, with as many as 358 tornadoes. 
*  It surpasses the 1974 Super Outbreak, in which 148 tornadoes were counted. 
*  Another big event is the November 2013 tornado outbreak, with 136 tornadoes reported.

Tornado Occurrence Globally

* All tornadoes form on land along the mid latitudes (the boundaries that separate warm tropical air masses and cold polar air masses) ALSO HAS THE CORIOLIS FORCE NEEDED TO HELP IT ROTATE
* America Has the largest occurrence of tornadoes
* We have perfect conditions for tornado occurrences, 
* We have warm moist air from the gulf of mexico
* We have a horizontal vortex every night in the great plains

\n

* Tornado Occurrence in America

* Most happen along the southern great plains (aka tornado alley). There’s an eastward shift of tornado occurrence currently. 
* Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes 
* annually, but the highest frequency is observed  in tornado alley of the Central Plains. Great setting for potent mixing of air masses.

The timing of Tornados

Nearly 75% of tornadoes form from March to July, when warm humid air is overlain by cooler drier air to cause strong vertical lift.

Trends in US tornado Occurrence

* It looks like we have an increasing trend of tornadoes, but really, our radar is getting better and is able to predict them more. We didn’t have any observations about tornadoes before 1950. The national weather service couldn’t predict any tornadoes and were worried about their reputation. So they forbid the word tornado. Then, in the 1950s, an airforce guy made a tornado report. When a tornado hit his airport, he recognized the signs when a tornado was coming again a week later. __**TORNADO OCCURENCE IS ACTUALLY DECREASING FOR THE STRONGEST KIND**__
* As population centers have expanded into formerly rural areas, there is a greater probability that a tornado would hit a structure or be observed