ATOC 2

convectiont
air rising rapidly in a way that causes storms
air parcel
distinct blob of air
Stable
if the parcel is displaced vertically, it will RETURN to its original position
Neutral
if the parcel is displaced vertically it will REMAIN in its new position
Unstable
if the parcel is displaced vertically, it will accelerate away from its new position
Adiabatic process
process in which a parcel of air does not mix with its environment or exchange heat energy with its environment
-air parcel will either compress to heat or condense to cool
-expand as they rise, compress as they fall (pressure)
-use lapse rate to predict temperature
Lapse rate
the rate at which temperature decreases (lapses) with increasing altitude
Dry adiabatic lapse rate
-the rate at which an air parcel that is not saturated will cool
10C/km
Adiabatic rate for SATURATED parcels
rate at which a saturated parcel will warm as it falls
-lower atmosphere: 4C/km
-In the middle atmosphere: 2C/km
Moist adiabatic lapse rate
-determined by the combined effects of expansional cooling and latent heat
-lower= 10C/km-4C/km= 6C/km
-Middle: 8C/km
-near tropopause: about 10C/km
Environment
the atmosphere outside the parcel
Environmental lapse rate
the rate at which temperature decreases with increasing altitude
-varies from 4C/km-9C/km
density and temperature
in the atmosphere, a parcel of air is less dense that its environment if the parcel is warmer than the environment
Determining stability of air parcels
we determine stability by comparing the parcel’s temperature after lifting to the temperature of the surrounding environment
-If its warmer, it will continue to rise
-if its cooler, it will sink
both unstable
stable environment
-if the temperature of the air parcel is different than that of the air, it will sink/rise back to its original position
Neutral stability
a parcel will have the same temp as its environment at all levels only if the environmental lapse rate is 10C/km
Conditionally unstable
the condition required for instability is that the displaced parcel of air be saturated
condensation level
the level where saturation first occurs
-seen at cloud base
Lifting condensation level
the air is lifted to cloud base (where saturation occurs)
level of free convection
level at which an air parcel first becomes buoyant (its temperature first exceeds the surrounding environment’s temperature)
Lifting mechanisms
what lifts air parcels or causes them to rise in the atmosphere
Cold Front
the boundary between a cold airmass and warm airmass when the cold air is advancing forward, lifting the warmer airmass
Sea Breeze
a circulation that develops along shore lines during daytime in which warm air rises over land and moves shoreward aloft, while cool air offshore descends and moves onshore to replace the warm air
upslope storm
a snowstorm along the east slope of the rocky mountains caused by air flowing westward across the plains and up the slope of the rockies
Convergence
net inflow of air molecules into a region of the atmosphere with the result of increasing surface pressure. convergence can be caused by changing wind speeds and or changing direction of air flow
Lifting Index
a measure of atmospheric stability, defined as the difference between the temperature of the environment at 500mb and an air parcel lifted from near the surface to 500mb
LI= T(environment)- T(parcel)
-the smaller the value, the warmer the parcel is relative to its environment
-negative LI indicates instability
pressure gradient
a horizontal or vertical change in pressure divided by the distance over which the change occurs
Pressure gradient force
the force applied to a small parcel of air due to the variation of pressure over a small distance around the parcel
-acts from high pressure (south) towards low pressure (north) perpendicular to isobars, accelerates air parcel northwards
Gravitational force
attractive force between any two objects. earth’s attractive force acting on air parcels has essentially a constant magnitude throughout the atmosphere and is directed toward the center of earth
Frictional Force
drag force acting in a direction opposite the motion of air; always acts to reduce the speed of the flow
Coriolis Force
an apparent force associated with the rotation of the Earth. causes air to deflect to the right of the motion in the northern hemisphere
Mechanical turbulence
rapid variations of air motion that develop when moving air encounters obstructions associated with ground roughness
Thermal Turbulence
turbulence created in the atmosphere by the ascent of buoyant air particles and subsequent mixing of the parcels with the environmental air
Wind Shear
the rate of change of wind speed and/or direction with distance or altitude
Shear induced turbulence
turbulence created in the atmosphere under conditions where winds increase rapidly with altitude
Boundary Layer
layer of the atmosphere adjacent to the earth’s surface where friction is important. depth can vary from a few hundred to a few thousand meters. also called friction layer
Angular momentum
the product of an object’s mass, rotational velocity, and radius from the axis about which it is rotating
Ang Momentum= M x V x R
Newton’s 2nd Law of Motion
force exterted on an object equals its mass times its acceleration F= ma
Hydrostatic balance
the balance between the upward pressure gradient force and the downward force of gravity
Geostrophic balance
the balance between the pressure gradient force and the coriolis force
Geostrophic wind
the wind that would exist if the pressure gradient and coriolis forces were in balance
-in northern hemisphere, higher pressures are to the right of the geostrophic wind
jetstream
band of strong winds that encircle the earth in the mid latitudes
Extratropical cyclone
a large swirling low pressure system that forms along the jetstream due to imbalances between the coriolis and pressure gradient forces
convergence
net inflow of air molecules into a region of the atmosphere with the result of increasing surface pressure. convergence can be caused by changing wind speeds and or changing direction of air flow
-means increasing surface pressure because the mass/unit area increases with time
Divergence
net outflow of air molecules from a region of the atmosphere with the result of decreasing surface pressure
-can be caused by changes in wind speeds and directions
Low pressure center (cyclone)
a location at which the pressure is less than the pressure at all surrounding points; generally associated with rising air, clouds, and precipitation
High pressure center (anticyclone)
a location at which the pressure exceeds the pressure at all surrounding points; generally associated with subsiding air and clear skies
centripetal acceleration
the acceleration toward the center of rotation experienced by a parcel of air in a rotational fluid
gradient wind balance
the state of curved flow in which the residual between the pressure gradient and coriolis forces exactly balances the centrifugal acceleration experienced by an air parcel
cyclonic curvature
a curved pattern either in the pressure or height field in which air flows counterclockwise in the N Hemisphere
Anticyclonic curvature
a curved pattern either in the pressure or height fields in which air flows clockwise in the n hemi
Curvature effect
the creation of divergence within the jetstream due to the change of flow curvature from cyclonic flow (around trough) to anticyclonic flow (around a ridge)
jetstreak
a region of exceptionally strong winds within the jetstream. convergence and divergence associated with jetstreaks directly influence the development of cyclones
Entrance region
the side of a jetstreak in which the height gradient is increasing along the flow direction and the air is accelerating
-air parcels displaced NORTHWARD in jetstream
Exit region
the side of a jetstreak in which the height gradient is decreasing along the flow direction and air is deccelerating
-air parcels displaced SOUTHWARD in jetstream
Curvature vs. Jetstreak (divergence and convergence)
-South of the jetstream axis, divergence and convergence of jetstream and curvature counteract each other
-north of the jetstream axis, the effects reinforce each other (same)
forces in friction layer
-forces are imbalanced unlike geostrophic balance
-friction disrupts balance by reducing wind speed- therefore coriolis force is reduced
-since PGF is unchanged, it is the dominant force
friction and wind
friction always acts to deflect wind direction to the left of the geostrophic wind direction, toward the low pressure side of isobars
-friction always acts to turn the wind such that the flow has a component from higher to lower pressure
Diabatic Process
a process in which an air parcel mixes with its environment and or exchanges heat with its environment
dynamic processes
a process that involves forces
Thermodynamic Process
a process involving heating or cooling of the atmosphere
Divergence (temp and pressure)
-surface pressure decreases over time
-low pressure system will develop
-changes in the curvature in the flow, jetstreaks, and heating of the atmospheric column
Convergence (temp and pressure)
high pressure systems are enhanced by convergence associated with the jetstream as these cool airmasses move out of their source regions
-air flows clockwise aroudn the high, outward and away due to friction
Airmass
a large body of air with relatively uniform thermal and moisture characteristics
-typically several hundred thousand km in area
-forms over relatively flat regions with homogenous surface characteristics
Arctic airmass
an airmass characterized by bitter cold temperatures
-considered arctic if it forms over the Arctic Ocean
Polar Airmass
a large mass of cold, dry air that decelops over the northern part of the North American continent
Tropical Airmass
an airmass that originates in tropical regions and is characterized by very warm to hot temperatures
Maritime airmass
an airmass that has formed over the oceans and hence is relatively moist
Continental airmass
airmass that forms over land and is characterized by low moisture content
Front
boundary between airmasses of differing density
-classified on the basis of the thermal and moisture characteristics of the airmasses, the direction of movement of the airmasses, and whether teh boundary between the airmasses in in contact with the ground or can only be found aloft
Cold Front
the boundary between a cold airmass and a warm airmass when the cold air is advancing forward, lifting the warmer airmass
-frontal boundary is typicall shaped like a dome
-if the wind on the cold side blows towards the front, its a cold front
-if warm air is unstable = thunderstorms
arctic front
a cold front that marks the boundary of an extremely cold airmass in wintertime
warm front
a boundary between airmasses where the cold airmass is retreating and the warm air is advancing at the surface
-if warm air is conditionally unstable, thunderstorms will form
-if the wind on the cold side blows away from the front its a warm front
Stationary Front
a boundary between airmasses where the colder airmass is neither advancing nor retreating at the surface
-unstable air can cause a line of thunderstorms
-if the wind on the cold side blows along the front, its stationary
Occluded Front
the airmass boundary that occurs when the air behind a cold front comes in direct contace with the cool air north or east of a warm front.
-develops in the latter stages of an extraropical cyclone’s life cycle when the cold front catches up to the warm front
cold occlusion
an occluded front in which air behind the cold front is more dense (colder) than the air behind the warm front
Warm occlusion
an occluded front in which air behind the cold front is less dense (warmer) than the air behind the warm front
Dry Line
a front characterized by a sharp moisture contrast, but little temperature change. often found in the southern plains
Upper-level front
a boundary between 2 airmasses that is present in the middle or upper-troposphere but does not extend to the earth’s surface