ATOC 3050 Final

El Nino

Warm phase, ocean temps in central/eastern tropical Pacific warmer than average

La Nina

Cold phase, ocean temps in central/eastern tropical Pacific colder than average

Standard atmospheric pressure at sea level

1013.25 mb or spa

Depth of a column

Pressure decreases more rapidly with height in a cold dense column, rather than warm less dense

Hydrostatic Equation

Relationship between the change of pressure with height and density of air

Warm Column

At given height above surface, pressure in a warm column will be greater than cold column

• assuming both gave same surface pressure

PGF

Force exerted on the air by presence of a pressure gradient

• Pressure moves from high pressure to lower pressure

• Closest spaced isobars: strong PGF and winds

• Perpendicular to isobars

Horizontal pressure difference aloft

PGF is established at surface which causes air to move from the cold column to the warm

• warm air advection aloft

• cold air advection at surface

Constant height map

Map shows how pressure varies with horizontal location at fixed height

Constant pressure map

Map shows how the height of a constant pressure surface varies with horizontal location

• if temp does not vary horizontally and SLP is same at every location, then no change in height on constant pressure and no change in pressure on constant height

• related to surface pressure and temp of column

Height of constant pressure

• as air warms, constant pressure surface moves up

• as air cools, constant pressure surface moves down

Pressure contours on height map vs pressure map

On constant pressure map, areas of low height correspond to locations of low pressure (and vice versa)

Ridge

Region of high heights or pressure on constant pressure or height map

Trough

Region of low heights or pressure on a constant pressure or height map

Surface map

• wind is CCW and inward around L

• wind is CW and outward around H

Upper air map (500 mb)

Wind is parallel to height contours, with lower height to left of wind

Common pressure levels

1000 mb: provides similar info to SLP maps

500: middle of troposphere, steering level - far from surface

Common elevations

4800-9800 ft (1460-3000 m): shows lower troposphere state and temp, useful to identify fronts

30100-38700 ft (9180-11800 m): near tropopause, useful to identify jet streams

Centrifugal force

If an air parcel is turning, it experiences centripetal acceleration in direction it is turning

Coriolis deflection

• Earth rotates faster at Equator than at poles

• Shorter distance (x) to travel in same amount of time means slower speeds (v) closer to the poles

Coriolis force

• Acts to right in NH

• Acts to left in SH

• Magnitude is proportional to wind speed and greater at higher latitudes and zero at equator

• Only can change wind direction, not speed

Magnitude of CF

• Between two lows: weak gradient, low PGF and CoF, weak wind

• Between high and low: strong gradient, high PGF and CoF, strong wind

Friction Force (Turbulent Drag)

Result of turbulence transporting atmospheric momentum to the ground and acts in atmospheric boundary layer

• turbulent drag slows wind and acts in direction opposite to wind

Surface vs Upper Layer winds

• Upper layer: traveling in response to PGF and CoF, not influenced by surface friction

Wind Advection

Wind can transport air that has different velocity past a fixed location causing wind to change over time at that location

• not really a force due to describing movement/transport process itself

Advection

Depends on:

• gradient of what is being advected, larger gradients result in larger advective changes

• direction of wind relative to gradient, wind blowing in direction of gradient increases advective changes

• strength of wind along gradient, stronger wind increases advective changes

Geostrophic Wind

When CoF and PGF forces are equal and opposite (balanced)

• travel parallel to geopotential heights

Horizontal winds: Geostrophic wind

• When at rest, PGF is only force

• Straight flow: no centrifugal force, no turbulent drag due to being above boundary layer

• CoF will increase as wind speed increases and will turn to make it perpendicular

• Geostrophic: balance between PGF and CoF (parallel isobars)

How geostrophic wind speed changes as distance between isobars changes

• As PGF increases, Geostrophic speed increases

• As isobars are closer: PGF increases

• CoF must also increase for geostrophic balance, but can only increase as wind speed increases

Surface vs Geostrophic Winds

Upper Level winds: Aloft with parallel isobars, winds “stay in their lane”

Surface winds (near geostrophic): “cross their lanes” due to surface friction

Gradient Wind

A wind that blows at constant speed, parallel to curved isobars or height contours above level of friction

• PGF and CF not in balance

• net force that results from these forces cause CCW around L, CW around H

Horizontal winds: Gradient winds

• Accounts for air flow along a curved geopotential height contours

• Extension of concept of geostrophic wind (move along straight and parallel isobars

Same magnitude PGF, different pressure centers

Gradient WS around L (CCW flow) less than around H (CW flow)

• Low: PGF = Coriolis + Centrifugal

• High: PGF + Centrifugal = Coriolis

Gradient WS vs Geostrophic WS around a Low

PGF = Coriolis + Centrifugal

Geostrophic is PGF and CoF, while Gradient is PGF, CoF, and Centrifugal

High vs Low pressure systems and Gradient Wind

Low: PGF > CoF since centrifugal is balancing

• weaker CoF than PGF means wind turns in direction of PGF

High: PGF < CoF since centrifugal is balancing the difference

• stronger CoF than PGF means wind turns in direction of CoF

Winds on Upper Level Charts

• In gradient wind balance where height contours are curved

• Winds geostrophic where height contours are straight

Horizontal winds: Surface winds

• In atmospheric boundary layers, include turbulent drag force

• Turbulent drag opposes motion and slows wind speed leading to weaker CoF

• PGF > CoF, winds turn in direction of PGF

Winds influenced by Friction

1. slower than geostrophic or gradient winds

2. will blow at an angle across isobars from high pressure towards low pressure

3. therefore winds spiral inwards L, outwards H

Vertical Forces and Motion

Forces acting on air in vertical:

• PGF (from surface to upper atmosphere)

• Gravitational force

• Frictional Force

• Advection

Conservation of Mass

• Relationship between mass inflow and outflow and changes in density is expressed by continuity equation

Horizontal convergence or divergence is balanced by vertical motion

• Horizontal convergence leads to vertical divergence

• Horizontal divergence leads to vertical convergence

Vertical Motion

Result of friction: air coverages into L at surface, diverges from H at surface

• results in rising motion above L centers, sinking motion above H centers

Effect of Mass Conservation

Upper level DIV increases: more air from below has to replace diverging air aloft and surface CON decreases

Upper level DIV weakens: surface CON will increase

Three Cell Atmospheric Model

Hadley, Ferrel, Polar

Jet Stream

Long, narrow, and shallow band of fast moving air

• typically located just below the tropopause

• westerly

• polar jet stream: located near polar front at elevation of 9 km, very steady

• subtropical jet stream: located near 30 latitude at elevation of 12 km, extremely variable

  • decrease of altitude of tropopause from tropics to polar regions

Jet Stream Seasonal Variation

Larger temp gradient between pole and equator in winter than summer, cause stronger jet stream in winter

• strength of jet stream related to horizontal temp gradient

Baroclinicity

Baroclinic: density depends on both temp and pressure

• strong temp gradient: baroclinic

Barotropic: density depends on only pressure

• no temp gradient (ex. tropics): barotropic

Temp and Jet Streams

• decrease in temp from tropics to poles

• higher heights in tropics and lower heights in polar regions

• in stratosphere, temp increases from tropics to poles (opposite)

• Strongest geostrophic: occur where pressure surfaces have steepest slope, so where the jet stream is located

Polar Front

Largest temp gradient found near polar front (boundary separating cold polar air and warm tropical air)

• moves equatorward in winter, poleward in summer

• at any given moment, north/south temp gradient is concentrated along fronts

  • largest slope of constant pressure surfaces is located along front

Thermal Wind Effect

Relationship between horizontal temp gradient and changes in geostrophic wind speed with height

• Height of an upper-level constant pressure surface will increase as air below that pressure surface warms (from hypsometric equation)

• in area with large horizontal temp contrast, there will be a large horizontal height difference at given pressure level

Highs

Identified as relative maxima in pressure on constant height maps or height on constant pressure maps

• ridge: extended area of relatively high pressure (height)

Highs at Surface

Causes:

• general circulation, highs are located in subtropics or polar regions

• subsidence (upper level convergence)

  • monsoonal season temp contrasts: shifts in subsiding air due to changes in land/sea contrasts

• cooling air (highs form over cold regions)

Centrifugal Force and Upper Layer Winds

To compensate the imbalance and additional forces (CE), Coriolis must increase/decrease and in turn increase/decrease wind speed

• Gradient winds travel faster than geostrophic “over” ridges, travel slower “through” troughs

• Upstream side of trough tends to have slower wind speed but downstream side have stronger

Upper Level CON and DIV

Surface low/high is aligned with DIV/CON zone in upper atmosphere

• high and low centers are not aligned and generally shift to the west

Air Mass

Widespread body of air whose temp and humidity are fairly similar in any horizontal direction at a given altitude

• movement results in changing weather

Area over which air mass originates provides its characteristics

• best source is large flat areas with uniform temp or moisture

• longer it stays over area, more likely to adopt certain properties

Air Mass Characteristics

Classified based on temp and moisture characteristics:

Moisture

• Continental

• Maritime

Temp

• Polar

• Tropical

4 Most Common in NA

• Continental polar (cP)

• Maritime polar (mP)

• Continental tropical (cT)

• Maritime tropical (mT)

Formations/Modifications to Air Masses

Formation:

• transient weather patterns will cause air masses to move out of source region

Once moves out of region:

• modifications due to surface fluxes, radiative heating/cooling, flow over mountains

Continental Polar (cP)

• Winter: cold and dry, cold weather/clear skies

• Summer: cool and dry, relief from humid weather in central US

• Forms in northern interior of NA

Arctic (A)

• Winter only air mass: extreme old and dry

• Forms in snow/ice covered NA and Arctic

• Related to the extreme cold air outbreaks with extreme negative temps

Maritime Polar (mP)

• Cool/cold and moist (dependent on source regions and time of year)

• Forms over sub-polar cold oceans of north Pacific and north Atlantic

  • cP, flows over cold ocean, gain moisture, leads to mP

• Variable weather conditions dependent on season and locations

  • mP, move inland, lifted over mountains, rain (lower), snow (higher)

Modification of mP

• As air rises over mountains clouds form, latent heat released, precipitation falls on windward side of mountain

• On lee side, air warms adiabatically and is warmer/drier

Maritime Tropical (mT)

• Hot/warm and moist

• Forms over subtropical oceans (Pacific, Atlantic, Gulf of Mexico)

• Can lead to record heat and humidity

• Responsible for severe weather in central and gulf coast

  • West coast: warm with heavy rain

  • Central: warm/humid with flow/low clouds

Continental Tropical (cT)

• Hot/dry, no precipitation, can lead to drought in summer

• Forms over subtropical land areas (southwestern US, Mexican plateau)

Fronts

Boundary between two air masses of different density

Characterized by:

• Large horizontal temp gradients

• Large horizontal moisture gradients

• Strong horizontal wind gradients

• Relative minimum in pressure

• Clouds and precipitation

Identifying Fronts

• Change in temp (large gradient)

• Change in absolute humidity (Large dewpoint gradient)

• Pressure and pressure changes (min. pressure along front)

• Clouds/precipitation patterns

Cold Fronts

Zone of cold (polar or arctic), dry air replacing warm, moist air (tropical)

• Slopes upward towards pool of cold air

• Steep slope

• Speed: 20-30 mph

How to locate Cold Fronts

1. Strong temp gradient

2. Change in moisture content of air (large changes in dewpoint depression over short distance)

3. Shift in wind direction (large change in speed/direction over short distance)

4. Strong thunderstorms

5. Fast moving

Warm Fronts

Front where warm (tropical) air is advancing and replacing cold (polar or active) air

• Gentile slope

• Uplift confined to small region

• Speed: 10 mph

• Clouds become lower and precipitation begins as front approaches

  • High clouds far ahead of front

Weather as result of Warm Front

• T: cool/cold with increase in T after front passes (warm air replacing)

• Td: rises as front approaches and passes (warm and moist air), steady at higher value

• Pressure: falls as front approaches and rises after front passes

  • less dense air, convection, low pressure, warm air settle, higher pressure

• Overrunning warmer and less-dense air rides up and over colder more dense air

How to locate Warm Fronts

1. Strong temp gradients

2. Shift in wind direction

3. Change in pressure

Warm vs. Cold Fronts

Warm fronts slower than cold

• Advancing warm air is less dense than cold, warm moves slower

• Harder to replace dense air mass

• Cold front advancing cold air is more dense than warm air it is displacing

• Due to slower movement of warm fronts their slopes are usually shallower than cold fronts (1:300)

Frontal Inversion

• Cold air replaces warm , so T increases with height as you move up through front

• Warm air rides up and over cold air so T increase with height as you move up front

Stationary Fronts

Front not moving but separates cold and warm air masses

• Clouds and light precipitation along/near front

Dryline Fronts

Boundary that separates moist (maritime, mT) and dry (continental, cT) air

• thunderstorms common along front

• temp is warm on both sides

• high Td ahead of dryline since it is moist (mT) and low Td behind dryline (cT)

Frontogenesis

Strengthening of a front as the temp contrast across the front increases

Frontolysis

Weakening or dissipation of a front as the temp contrast across the front lessens

Processes for Frontogenesis/Frontolysis

Kinematics: wind might increase temp gradient

Thermodynamics: spatial differences in heating/cooling can strengthen/weaken front

Dynamics: flow associated with front can enhance kinematic/thermodynamic effects

Occluded Fronts

Front that forms when a cold front (dense) catches up to and overtakes a warm front (less dense)

• Clouds and precipitation: similar to warm front

• T: little change with slight decrease after front passes

• Td: little change

• Pressure: minimum at front

Stages of Occluded Front

1. Cold front moves faster than warm

2. Cold front catches up due to being denser than warm

3. Cold air completely surrounds low-pressure center, occluded front is the boundary between warm and cold

Extratropical Cyclones

Extratropical: refers to something outside of tropics (middle or high latitudes)

Cyclone: area of low pressure

Cold Core Low

Low pressure system with coldest temperature located near its center

• Strengthening cold core low is tilted with height

ETC Characteristics

• Pressure: low (950-1000 mb)

• Wind: CCW, inward spiral, rising air over low, moderate to strong speed

• Size: 500-1000 km

3 Dimension Airflow

• Behind cold front: descending cool, dry air

• Ahead of cold front (in warm sector): warm air moves towards/over warm front

  • This is the warm air conveyor belt

• Ascending warm, humid air: responsible for area of cloud cover ahead of warm front

Cyclogenesis

Development or intensification of a cyclone

• net DIV in a column of atmosphere will cause surface pressure to decrease and cyclogenesis to occur

Cycloysis

Weakening or dissipation of a cyclone

Polar Front Theory

Model of how mid-latitude storms develop: birth, growth, decay

• connects storms with dynamics of the polar front

• transition zone between cold in polar cell and warm in ferrel cell

Cyclone Evolution

1. Stationary Front

2. Frontal Wave

3. Open Wave

4. Mature Stage

5. Advanced Occlusion

6. Cut-Off Cyclone

1. Stationary Front

Cold and warm air (winds) are both flowing parallel to stationary front and have opposite direction

• initially a stationary front is located in mid latitudes

• cyclogenesis favors baroclinic zone (temp gradient creates this zone)

2. Frontal Wave

Perturbation occurs at the intersection of two air masses where there is large temp gradient causing a wave pattern to develop

• CCW flow begins to develop around low'

• hard to predict due to being small feature

• more DIV than CON

3. Open Wave

Frontal wave to open wave where pressure decreases and wind strengthens

• Warm sector: region of warm air between advancing cold and warm fronts

• westerly

• clouds and precipitation ahead of warm front and along/behind cold front

4. Mature Stage

As cyclone continues to move northeastward the pressure continues to fall, winds increase and waves develop into mature cyclone

Known as “Initial Occlusion”

• warm sector narrows

• strong wind, heavy precipitation

5. Advanced Occlusion

Cyclone is now occluded, cold air surround cyclone leading to lower temp gradient (decay stage)

• Dry tongue (intrusion): cool, dry air wraps around cyclone

• warm air is forced aloft

• surface low begins to be removed from upper level DIV and pressure begins to increase (cyclolysis)

• Triple point: where the occluded, warm and cold fronts meet (new low may form, AKA secondary low)

6. Cut-off Cyclone

Cyclone dissipates

• lasts several days to more than a week

• few storms directly follow this

Stages/Relationship between Upper/Surface

• Upper level CON leads to sinking air and suppresses clouds

• Upper level DIV helps to deepen cyclone

• Upward motion through DIV help to form clouds and precipitation

Lee Cyclogenesis

Development of a cyclone on the lee side of a mountain range

• Lee side low: low pressure center that develops on the downwind side of a mountain range

• Behind mountains: no rotation

• On mountains: DIV, CW, H, anticyclone

• Leeside: CON, CCW, L, cyclone

Rossby Wave

Height of mountain, driven by changing Coriolis parameter with latitude

Cause of DIV or CON

• Upper level processes (troughs or ridges)

• Diabetic heating (modifying temp independent from pressure changes)

• Boundary layer effects: friction, surface heat and moisture fluxes

• Jet streaks

Boundary Layer Effects and Cyclogenesis

• Low level CON into surface low results in rising

• Rising, air cools adiabatically, RH increases, clouds and precipitation from

• Friction in boundary layer causes wind to spiral causing CON

Diabatic heating

Modifying temp independent from pressure change

• Caused by latent heat release as clouds form

• Heating causes upper level pressure/height to rise which results in upper level DIV and causes surface pressure to fall