38:250 Intro to Weather & Climate - Final Exam

Precipitation

• Occurs when water droplets or ice crystals grow large enough to overcome the updrafts of rising air that form clouds

• Must fall to the ground and not evaporate

Types of Precipitation

• Rain

• Drizzle

• Sleet

• Freezing rain

• Snow

• Hail

Temperature Profile of the Atmosphere

• Determines what type of precipitation reaches the ground

Typical Cloud Droplets

• Are 10 micrometres in radius

• Need to grow for precipitation to occur

• Terminal velocity is 0.01 m/s

Typical Raindrops

• Are 1000 micrometres (1 mm)

Terminal Velocity

• Is reached when the force of gravity is balanced by air resistance in still air

  • Smaller drops have larger surface area to mass ratio, so smaller drops have slower terminal velocities

Stratiform Updraft Velocity

• 0.1 m/s

Cumulonimbus Updraft Velocity

• 10 m/s

Large Cloud Droplet Characteristics

• Droplet radius → 50 micrometres

• Terminal velocity → 0.3 m/s

Drizzle Characteristics

• Droplet radius → 100 micrometres

• Terminal velocity → 0.7 m/s

Typical Raindrop Characteristics

• Droplet radius → 1.0 mm

• Terminal velocity → 6.5 m/s

Large Raindrop Characteristics

• Droplet radius → 2.0 mm

• Terminal velocity → 10 m/s

Mechanisms by Which Droplets Grow in Clouds

• Collision and coalescence

• The Bergeron-Findeisen process

Collision & Coalescence

• Happens in warm clouds, happens when temperature is above 0 degrees throughout

  • Large drops fall faster than small ones, collector drops grow by absorbing smaller drops

• Are promoted by a variety of drop sizes

Collector Drops

• Grow by absorbing smaller drops

Collision Efficiency

• Probability that two drops will collide

  • Increases for larger collector drops, must be >20 micrometres in size or when drops are of similar size (or if they are smaller), they will be deflected if these conditions are not met

Coalescence Efficiency

• Likelihood that two colliding drops will coalesce

  • Colliding drops do not always coalesce, it is more likely when drops are either very similar or different in size

Cloud Droplets Growth

• Grow larger the longer they remain within the cloud

Thins Clouds

• Produce small drops, such as drizzle

Deep Clouds

• With strong updrafts, can enhance droplet growth and produce large drops

Cumulonimbus Clouds

• Produce the largest drops

Cloud Temperature Between 0 deg C and -40 deg C

• Contain both ice crystals and water droplets

Saturation Vapour Pressure is Higher

• Over water than ice

  • Air can be saturated for ice but unsaturated for water

Water Droplets Shrink

• Through net evaporation

Ice Crystals Grow

• Through net deposition

Once Ice Crystals Reach a Certain Size Through Deposition

• They are likely to grow by:

1. Accretion

2. Aggregation

Accretion

• Ice crystals grow by colliding with supercooled water droplets

• Droplets immediately freeze onto the ice crystal

Aggregation

• Ice crystals grow by colliding with other ice crystals

All Cold Clouds

• Are mostly ice

• Temperature determines type of precipitation that reaches the ground

Virga

• Streaks or shafts of precipitation (rain or snow) fall from a cloud base but evaporate or sublimate before reaching the ground

  • Air below is very dry so precipitation can evaporate

Sleet & Freezing Rain

• When precipitation may melt in a warm layer, but then falls through a cold layer

  • If the cold layer is deep enough (>250 m), the precipitation will refreeze into ice pellets (sleet)

  • If the cold layer is shallow, the water may become supercooled (freezing rain) and rain freeze on contact

    • Layers of ice deposited on cold surfaces

Sleet is Likely to Form

• When the cold layer is >250 m deep

Freezing Rain is Likely to Form

• When the surface cold layer is <250 m deep

Hail

• Begins with small ice particles. Is generally larger than 5 mm and considered severe when larger than 2 cm

• Strong updrafts in cumulonimbus clouds cycle the particles through the cloud over and over

• Growth occurs through accretion

Action of Forces that Result in Wind

• Gravity

• Pressure gradient

• Coriolis force

• Centripetal force

• Friction

  • These forces explain the relationship between the winds we observe and the patterns of isobars or height contours on weather maps

Pressure Gradient (PG)

• Is the change in pressure (P) over a distance (x)

• Causes a pressure gradient force (PGF) directed from higher to lower pressure

• Is directed from equator to pole

Pressure Gradient Force

• Driving force of atmospheric motions

  • All other forces deflect or slow it down

• Is the force that starts the movement of air

The Coriolis Force

• Occurs due to Earth’s rotation

• On a large scale, it causes freely moving objects to deviate from a straight path

• Affects movements of air, water, aircraft, artillery shells that travel relatively quickly and/or over very long distances

• Is dependent on latitude and wind speed

• Produces upper-air westerlies

The Coriolis Force - Northern Hemisphere

• Apparent force deflects moving objects to the right

The Coriolis Force - Southern Hemisphere

• Apparent force deflects moving to the left

Earth’s Rotation at the Equator

• Causes purely translational movement of the surface

• Coriolis force is zero

The Coriolis Force - The Poles

• Rotation causes purely rotational movement of the surface

• Coriolis force is maximised

The Coriolis Force - In Between

• Force is between zero and maximum

The Coriolis Parameter

• Relates the strength of the force to the Earth’s rotation rate and latitude

Centripetal Force

• Force required to keep an object moving along a curved path

  • Must be larger for faster objects

  • Must be larger for smaller radii

Frictional Force

• Opposes movement

• Increases with wind speed and surface roughness

Planetary Boundary Layer (PBL)

• The lowest part of the troposphere, directly influenced by the Earth’s surface through turbulent exchange of heat, moisture, and momentum

• Is near the surface where winds are slowed by friction

• Is deeper for faster winds, rougher surfaces, unstable/rising air

Resulting Winds - As Air Accelerates

• The effects of other forces will increase because they depend on wind speed

  • Feedback processes operate among the forces until they become balanced

Geostrophic Winds

• Occur where friction is negligible, above the PBL

  • Resultant flow is parallel to the isobars, isobars are straight

When Isobars are Curved

• Balance of forces must include C_LF

  • Net force must be directed at the centre of rotation

Gradient Wind

• Occurs above the PBL

• Is the feedback between pressure gradient force (PGF), wind speed, and centripetal force (CF) keeps the flow parallel to isobars

Gradient Wind Around Areas of Low Pressure

• Is cyclonic in nature

  • PGF begins acting against the wind, so winds around cyclones are slower than geostrophic winds (subgeostrophic)

  • Rotation is cyclonic and spins counter-clockwise in the Northern hemisphere and clockwise in the Southern hemisphere

Gradient Wind Around Areas of High Pressure

• Is anticyclonic in nature

  • Centripetal forces (CF) begins to act against the wind, so winds around anticyclones are faster than geostrophic winds (subgeostrophic)

  • Rotation is anticyclonic and spins clockwise in the Northern hemisphere and counter-clockwise in the Southern hemisphere

Ridges

• Are areas of high pressure in the upper atmosphere

• Winds decelerate coming out (assuming a constant PGF)

Troughs

• Are areas of low pressure in the upper atmosphere

• Winds accelerate coming out (assuming a constant PGF)

As Air Slows Down

• It converges

Area Downwind of a Ridge

• There is speed convergence

• This area is often associated with high pressure at the surface

As Air Speeds up

• It diverges

Area Downwind of a Trough

• There is speed divergence

• This area is often associated with low pressure at the surface

Friction

• Acts opposite of pressure gradient force (PGF), which reduces wind speed

  • Coriolis force is now weaker than pressure gradient force (PGF) as a result

  • Resultant wind crosses isobars at an angle

Surface Wind

• Spirals into cyclones and out of anticyclone

Local Winds

• Are caused by pressure differences that are linked to spatial differences in surface temperature due to:

  • Variations in surface properties (land and sea breezes)

  • Variations in terrain (mountain and valley winds)

Local Winds - Differences in Thermal Properties of Land & Water

• Generate daytime sea breezes and night-time land breezes

Local Winds - Daytime Heating in High-Relief Terrain

• Generates upslope anabatic winds and valley winds

Local Winds - Nocturnal Cooling

• Generates downslope katabatic winds and mountain winds

Global Circulation - Theoretical Balance & Latitudinal Imbalance

• Drives atmospheric circulation, and in turn oceanic circulation

  • Temperature gradient determines the strength of circulation

Latitudinal Radiation Imbalance

• Dictates that there must be a poleward transfer of energy, there must also be poleward transfer of momentum (from tropical easterlies to mid-latitude westerlies)

Simplified Circulation on a Simple Earth

• Surface pressure systems: low pressure at the equator and high pressure at the poles

• Pressure patterns in the upper troposphere: high pressure at the equator and low pressure at the poles

Simplified Circulation on a Simple Earth in Rotation

• Surface pressure systems: low pressure a the equator and mid-latitudes, high pressure at subtropics and poles

• Pressure patterns in the upper troposphere: high pressure at the equator and low pressure at the poles

Earth’s Pressure Patterns Influences

• Seasonal shifts in declination of the Sun

• The location of large land masses Vs. bodies of water

• Differences in heating of land and ocean

• Topography

Tropical Circulation - The Hadley Cell

• Is a thermally driven cell with warm air rising near the equator and colder air forced to descend and warm adiabatically in the subtropics

Mid-Latitude Westerlies (Extra-Tropical - Mid-Latitude Circulation)

• Are produced by flow from subtropical highs northward towards subpolar lows

Polar Highs (Extra-Tropical - Polar Circulation)

• Produce southward pressure gradient towards subpolar lows, which results in polar easterlies

Polar Easterlies

• Are produced by polar highs producing southward pressure gradients towards subpolar lows

Regional Circulation - Monsoons

• ITCZ migrates northward over Southeast Asia in the summer and southward over the Indian Ocean in the winter

  • Results in seasonally reversing wind patterns

• Summertime onshore flows produce heavy rainfall

High Pressure Aloft

• Created by air expanding upwards at the equator

Low Pressure Aloft

• Created by air compressing downwards at the poles

The Subtropical Jet Streams

• Form over the subtropical highs

• Happen when poleward flow aloft becomes increasingly affected by Coriolis force and becomes westerly

  • Poleward movement brings air closer to the centre of rotation (Earth’s axis). Conservation of momentum causes velocity to greatly increase

Polar Jet Streams

• Form along the Polar Front

The Polar Front

• Is the boundary between cold/dry and warm/moist air masses

The Polar Front Jet Stream

• Is created when large latitudinal temperature gradients generate strong pressure gradients

• Are stronger in the winter due to greater temperature differences

  • Results in faster movement

Rossby Waves

• Are associated with the polar front jet stream

• Are 3 to 7 waves that are slowly circling the planet that increase and decrease in amplitude

• Strongly influence mid-latitude weather due to occurrence of cyclonic activity

Ocean Currents

• Account for about 1/3 of the poleward heat transfer from tropics to poles

  • Surface currents are driven by prevailing winds

  • Deep currents are driven by differences in density; called thermohaline circulation

• Operate over a long time:

  • Lateral circulation ca. 1 year

  • Deep ocean circulation ca. 1000 years

Thermohaline Circulation

• Deep currents driven by differences in density

Surface Currents

• Friction between water layers transfer momentum to a depth of about 100 m (Coriolis force acts on each layer)

  • Layers at successive depths are deflected further, producing an Ekman spiral

• Moves at ~45 degrees to the wind direction

Ekman Spiral

• When layers of water currents at successive depths are deflected further away than normal

• Describes how ocean water moves in layers, influenced by wind and the Coriolis effect, resulting in a spiral effect where water moves at different angles as you go deeper

Temperature & Salinity

• Affect ocean water density

Downwelling

• Occurs when water is more dense

Upwelling

• Occurs when water is less dense

Primary Zones of Downwelling

• North Atlantic Ocean

• Southern Atlantic Ocean

Areas of Upwelling

• Are less clear and poorly understood

Air Masses

• Are large bodies of air with uniform temperature and moisture conditions

• Develop over source regions where they adopt the characteristics of the land/water surfaces below

• Migrate from different source regions and meet at transition zones known as fronts

Continental Arctic

• Air-mass Symbol → cA

• Characteristics → very cold and dry

• Source region → arctic and antarctic (winter only)

Continental Antarctic

• Air-mass → cAA

• Characteristics → very stable

• Source region → (winter only)

Continental Polar

• Air-mass → cP

• Characteristics → cold and dry, stable in winter but slightly unstable in summer

• Source region → high-latitude continents and ice-covered oceans

Maritime Polar

• Air-mass → mP

• Characteristics → cool and moist, unstable

• Source region → high-latitude oceans

Maritime Tropical

• Air-mass → mT

• Characteristics → warm and moist, unstable on west side of oceans, stable on east side of oceans

• Source region → subtropical oceans

Continental Tropical

• Air-mass → cT

• Characteristics → hot and dry, very unstable

• Source region → subtropical deserts (in the summer only in North America)

Source Regions - Winter Pattern

• Continental arctic - very cold, dry, and stable

• Continental polar - cold, dry, stable, and high pressure

• Maritime polar - cool, humid, and unstable all year

• Maritime tropical - warm, humid, and unstable

Source Regions - Summer Pattern

• Continental tropical - hot, low relative humidity, stable aloft, and unstable at the surface

• Continental polar - cool, dry, and moderately stable

• Maritime polar - cool, humid, and unstable all year

• Maritime tropical - warm, very humid, and very unstable