DEPRESSIONS Flashcards
Define a depression
A closed off area of relatively low atmospheric pressure. Also referred to as a “Low” or “cyclone”.
Nil defined pressure value that constitutes a low
What are the four characteristics of a depression?
- Convergence at low level (due friction near earths SFC - causes wind to blow across isobars towards lower pressure)
- Divergence at upper levels
- Upward motion due to the above two factors
- thick cloud formation& precip due upward motion (concentrated around the front)
How is absolute vorticity affected by ridges vs. troughs & what does this result in?
Troughs: Tab = 7x absolute vorticities (since vorticity due to shear, curvature & the earth are all cyclonic).
Ridges: Rab = 3x absolute vorticities (since curvature & shear relative vorticities are anticyclonic & therefore cancel out 2x cyclonic vorticities)
Therefore, the faster spinning air moving from a trough must slow down before reaching a ridge. This occurs through expansion (divergence) (e.g. throwing arms out to slow) & vice versa.
What are the four factors that increase the intensity of upward motion?
- the sharpness/curvature around the upper trough, sharper = greater vorticity changes
- and increase in “shear” vorticity by a jetstream passing over
- Increasing diffluence (if isobars spread out downstream of the upper trough).
- Cloud development> releases latent heat > destabilises air = accelerated updrafts
How would an existing high intensify in terms of convergence vs. divergence
downstream from an upper ridge is a zone of convergence which results in downward motion & resulting high pressure at the surface. If upper level convergence exceeds sfc divergence, the high will intensify.
What charts would you look at for jet stream levels (in mid-latitudes)?
FL300-FL340 (250 or 300hPa)
What are the two most basic processes that lead to the formation of most depressions in NZ?
In cold air & in warm air.
Describe the “in cold air” formation process
Enhanced by?
Sometimes occurs in NZ in an area of cold air following the passage of a major cold front.
- Upper trough approaches large area of Cb/Cu (generated by SFC trough & upper level divergence already exisits)
- Extra ascent intensifies the convection & Cu/Cbs are enhanced.
- when upper DIV moves immediately above the SFC trough, the cloud system becomes comma shaped.
Further enhanced by the presence of a jet
Describe the “in warm air” formation process
Involves the advection (horizontal movement) of warm, moist air.
Resulting cloud sheet is wide & deep.
Depression may become intense with high amounts of latent heat released through condensation in rising air (unstable).
Often characterised by cirrus on leading edge, As & Ns in centre & embedded CBs trailing
When would you expect a lee depression to occur?
formed in strong wind conditions in the lee of a mountain range.
How does a lee depression form?
-Airmass is forced over/around a mountain barrier
- Build up of pressure on windward side = resulting ridge.
- On leeward side there will be a resulting drop in pressure & an orographic trough.
3 important aspects of lee troughs for aviation?
- Strong winds = mech turb & lee wave activity in lee of ranges.
- likely to be sig cloud & vis problems associated with precip on windward side of ranges. In the lee, wx should be dry with high cloud bases.
- windward ridge vs. leeward trough can result in large QNH changes over short distances.
How does a thermal low form?
Shallow system caused by sun heating of land mass in Summer. (requires light winds)
1. air near SFC is heated & becomes more bouyant which leads to upwards motion of air.
2. DIV therefore occurs aloft at ~2000-5000ft & SFC pressure lowers.
3 important aspects of thermal lows for aviation?
- Convective turb
- Poss arvo CBs with SH, TS & GS
- Warm temps & low pressures, increasinf the DA & reducing T/O performance
How are depressions affected when crossing a mountain barrier?
Weather effects?
Low level circulation is physically stopped on the windward side and will dissipate over a few days, however the upper trough continues to move & a new SFC cyclonic circulation will develop in the lee of the range.
Results in extensive cloud & precip on both sides
