FINAL EXAM Flashcards
What is Convection?
- Vertical displacement of air-masses under the effect of buoyancy.
- Air warmed @ the surface to the point that it becomes positively buoyant -> rises.
- Air is cooled @ mid-tropospheric levels to the point that it becomes negatively buoyant -> sinks
- Alternative term: vertical overturning.
Differ moist convection from dry convection.
Moist: most easily observed since it leads to cloud formation and typically to precipitation.
Dry: Characteristic to atmospheric boundary layer: thermals, thermal rolls, thermal plumes.
From buoyancy driven convection + background flow β> 3D circulation develops producing +/- complex convective phenomenon.
Write from less to more complex.
Fair weather cumulus - Shower producing cumulus - Cumulonimbus clouds - Squall lines - Supercell storms - ...
What is characteristic of he convective circulation?
The organisation of the elements (individual clouds/thunderstorms) in a way that the convective circulation maintains itself and is long-lived
Describe the scale of convective phenomena
Thermals/cumulus/thunderstorm : L<10km T<1hr
Squalls/Supercells: 101000km T: monthly-seasonal
What is the mean global and annual rate of decrease of temperature in the lower 10 km of the atmosphere? Its name?
It is the mean environmental lapse rate ~ -7.0 deg/km
What sets the value of the mean environmental lapse rate?
Atmospheric column is heated through: radiation/conduction/convection β> value is the result of the thermal equilibrium achieved.
By what is convection triggered?
If excessive heating through radiation/conduction create a lapse rate exceeding equilibrium lapse rate β> convecion is triggered to return the atmospheric column to equilibrium
What are the two processes in play when re-equilibrating atmospheric column
1- Exchange of air-masses in the vertical (warm from low to higher levels/cold from higher to lower)
2- Mixing: mixing with environmental air along the ascent/descent and once air-mass has attained final position
Describe the energy as convection occurs
As temperature profile is perturbed from equi value -> reservoir of available potential energy (APE) created in low tropospheric levels.
Convection releases this energy (APE) by converting it to kinetic energy (KE) : updrafts/downdrafts. Eventually the KE is dissipated into microscale motions
Where does convection happens?
1. Away from the tropics
- Following insolation cycle (thunderstorms, squall lines, tornadoes, warm season/daytime)
- Cold air moving over a heat source (over water near polar regions dur to continental air travelling over the warmer ocean)
- Air mass becoming buoyant as result of lifting (frontal: mid-latitude frontal rainband, orographic lifting)
Draw a sketch of convection happening due to the insolation cycle
destabilisation of equi lapse rate due to heating at the surface.
Insolation @ surface
Draw a sketch of the polar lows
Often a cold pool of air @ mid-levels contributing to destabilising equi lapse rate
Cooling of surface
Draw a sketch of convection as result of forced lifting
- forced lifting up to intersection point A
- advancing cold air behind cold front -> receding warm air ahead of cold front
How can we measure convection?
- Experimental Campaings
- Radar Imagery
- Satellite Imagery
About Radars:
- What do radars measure?
- What do the antennas send?
- What is their target?
The echoreflectivity is measured to estimate the type and distribution of precipitation in the cloud/its distance and height
They send out directional pulses of microwave radiation
They target rain/snow.graupel/hail particles (some can even detect cloud drops/ice) such that the pulse is reflected back to the antenna. Distance is a few kilometers
About Satellite imagery:
-Visible Imagery (VIS)?
Images are obtained by reflecting sunlight
- > high reflectance (white): dense cirrus from Cumulonimbus clusters, fresh snow, nimbostratus clouds
- > Low reflectance: much of the earthβs surface (dark grey or black)
About Satellite imagery:
- Infrared wavelengths?
- What do radiometers sense?
- Water Vapour Imagery?
Radiometers sense the intensity of the heat emission of the earth/ atmosphere components @ IR wavelengths
- > produce infrared images from which we can determine cloud top heights
- low intensity (white) : colder (higher cloud tops)
- high intensity (grey): warmer (lower cloud tops)
Detect water vapour in the infrared spectrum. @upper-mid levels of troposphere where winds are ruled by large-scale air masses
Describe what forms the ITCZ
What is the ITCZ a tracer of?
ITCZ: InterTropical Convergence Zone
-Trade winds from NH and SH come together @ equator, picking up moisture along their paths
- Convergence + Intense solar heating (sunβs zenith point)
- > buoyant, H2O loaded air masses
- > vertical upward lifting of air masses
- > vigorous convective activity manifesting as clusters of thunderstorms along the ITCZ belt
Tracer of Hadley circulation
Sketch the convecting development of ITCZ
South-North cross-section
- sensible heating @ surface along Southward trajectory
- radiative cooling in upper troposphere (anvil top)
- strong ascent in convective dev @ equatorial zone
- gentle sinking motion to the north of convecting zone (descending branch of Hadley cell)
What are Monsoons?
What is the most extensive Monsoon?
They are seasonally driven large-scale circulation.
Winter: from land-> ocean
Summer: from ocean-> land
The summer Asian monsoon: dominates the East Asian Sector, starts in June ends in September
Describe the circulation of monsoons
- air heated more rapidly over land
- air masses become buoyant and rises
- air mass of cool humid air is drawn from neighbooring ocean to the land
- humid air heated over land
- becomes buoyant
- rises
- its moisture condenses
- gives deep convection and heavy rains
What is the hyposometric equation
derive
Give the step by step circulation of monsoon
- By hyposometric eq delta Z land > delta Z ocean
- pressure surface pn is pushed higher up over land than ocean
- @ constant altitude P land>Pocean
- PGF towards ocean @ upper levels
- Air mass accumulation @ mid-upper levels
- Divergent flow @ mid-upper levels
- Low pressure center over Asia
- PGF towards land in response to the surface thermal low
- Convergence @ low level. This flow advects water vapour from the maritime to the continental boundary layer
- Humid air-masses become rapidly heater over continent -> lapse rate over land larger than equi -> favourable conditions for convective development
- Latent heat release on a massive scale over Asia