23/2 Met 2 heads Up Flashcards
Describe the relationship between high level Divergence/Convergence with what occurs of the SFC and the type of pressure expected
Divergence upper level
= Convergence at SFC, Air ascent, Low px at SFC
Convergence upper level
= Divergence at SFC, Air subsidence, High px at surface
Clouds associated with sub-tropical jet streams
Cloud-free area on the polar side of the jet, and cirrus (high level) on the equatorial side
Where does NZ sit in the general global circulation
Mid-latitude cell, below the sub-tropical high pressure zone (30° south), and above the polar low pressure zone (60° south).
What is a monsoon?
Cross equatorial flow of trade winds associated with seasonal changes in equatorial trough/ITCZ. Large amounts of cloud and rainfall are associated with these winds.
Areas affected by monsoons: Australia and Indonesia (South), India and Asia (North).
Seasonal cross equatorial flow that leads to a trough of increased cloud and precipitation.
What is the Inter-tropical Convergence Zone (ITCZ)
ITCZ is the region that is formed by a clash of the southeast trade winds from the Southern Hemisphere with the Northeast trade winds on the northern side of the equator. This collision results in upwards, or ascending motion over a vast area and is accompanied by prolonged heavy rain and frequent violent thunderstorm activity.
Describe the Maritime, polar and tropical airmasses
Tropical Maritime – Warm and moist from the north, modified by becoming cooler and more stable as they move south. Associated with layer cloud and light drizzle/rain.
Polar Maritime – Cold and moist air from the south, modified by becoming warmer and more unstable as they move north. Associated with convective cloud formation and showers of rain, hail, snow.
Where does hail come from?
Ice crystal is carried in a cycle of updrafts and downdrafts within the Cb cloud. Growth is mainly due to coalescence with SCWD. Once it has grown to a size that can no longer be supported by updrafts, it will fall from the base of the cloud or is ejected out of the tops and sides of the cell.
Conditions necessary for fohn winds
Precipitation on the windward side of the mountain barrier – to reduce the amount of water vapour in the air so the lee side warms at the DALR.
Conditions for lee waves in the southern alps
Wind flow within 30° of perpendicular to the ridgeline
At least 20kts at ridge top level
Wind speed increasing with height
Little variation of wind direction with height
A stable layer in the atmosphere at about ridge top level (provides restoring force for waves)
Large SCWD – at what rate do they freeze and why
Slowly – because they release more latent heat, slowing down the freezing process
What is a jet stream?
A region of strong winds defined by the 60kt isotachs.
How do orographic troughs form?
Formed in strong wind conditions in the lee side of a mountain range. As an air mass is forced over or around a mountain barrier there is buildup of air on the windward side causing an area of high pressure. On the leeward side, there is a corresponding drop of pressure with a resulting orographic trough.
What do you do if you fly into volcanic ash?
Exit the ash cloud as quickly as possible; a 180 degree descending turn will be the quickest exit strategy.
Surface pressure changes before and after a cold front
Pressure decreases before the front due to the warm air ascending. Sharp pressure rise in colder heavier air mass that follows the front.
What conditions produce blowing dust after a dry period?
An extended dry period.
Marked instability
Strong surface winds
Enhancement by ‘dry’ cold fronts (which increase 2 and 3 above)
What is the total lifetime of a thunderstorm?
Each stage is 30 minutes on average, so the total lifecycle of an upright stationary Cb is around 1.5-2 hours.
Airframe Icing – at what temperatures is it an issue, and why is it not an issue when it is very cold?
0 to -40°C in cloud.
By the time the temperature has cooled to minus 40 celsius, all SCWDs have frozen out, so that at temperatures colder than -40, only ice crystal cloud exists in the atmosphere.
Lifecycle of a thunderstorm
Cumulus Stage -
Updrafts only
No downdrafts mean turbulence is light-moderate, and most other hazards are yet to materialize
Airframe icing can be severe due to large and numerous SCWD carried aloft by the updrafts.
Light rain possible
Mature stage-
Updrafts and downdrafts
Any of the 8 hazards can exist
Characterised by the onset of heavy precipitation at the surface
Anvil will start to form at the top of the cloud as the droplets spread out horizontally beneath the tropopause
Anvil or Dissipating Stage-
Downdrafts only
Hazards weaken and disappear
Anvil is glaciated with clear signs of cirrus cloud development
The bottom 2 thirds may evaporate leaving cirrus and anvil – this cloud is benign
The freezing level rises again
Fohn Wind temperature calculations using DALR/SALR
DALR - 3°/1000ft SALR – 1.5°/1000ft
Draw a lee wave diagram – rotors, stable layer, cloud etc.
I don’t have premium so can’t add images…
but it’s on page 188
Cold Anticyclones and how they form
When the ground cools considerably, the air above it cools and becomes denser. Surface pressure rises and an anticyclone is formed.
Very shallow systems that can occur over inland parts of NZ in winter with overnight cooling. Temperature inversion usually below 3000ft.
Vorticity – what is it, what are the 2 types?
A measure of how much rotation we have at a point. Two types:
Relative Vorticity – Sum of curvature and shear vorticity at any given point above the earth’s surface.
Earths Vorticity – Fixed and arises from the shear induced by the earth’s rotation, and like Coriolis force, it increases with latitude.
Define air mass
A vast area of air that has taken on the physical characteristics, namely temperature and humidity, of the surface underlying it.
