Synoptic Classifications

Question 1

Tropical disturbances in the easterlies

Answer 1

Easterly waves
Easterly lows

Question 2

Easterly waves

Answer 2

• warm humid air
• associated with ICTS
• disturbances in tropical easterly flow
• barotropic
• no slanting with height
• open waves and cold core closes lows
• warm cored high pressure / ridge in upper air
• lower level convergence
• upper leve divergence
• strong uplift
• wide spread rain to east of trough
• winds northerly component
• rain for few days

Question 3

Easterly lows

Answer 3

• surface convergence to east of low
• divergence higher than easterly wave
• very strong uplift
• heavy rains
• wind Northward component
• years with this type of system difference between dry/wet years

Question 4

Temperate disturbances in the westerlies

Answer 4

• westerly waves
• cut-off lows
• southerly meridional flow
• ridging anti cyclones
• west coast troughs
• cold fronts
• tropical cyclones

Question 5

Westerly waves

Answer 5

• baroclinic
• tilted westward with height
• closed circulation at surface may be present
• surface convergence to the rear of the trough
• conservation of absolute vorticity and effects
• upper air divergence occurs to the east and ahead of trough line
• ideal conditions for sustained gentle uplift
• cloud and precipitation occur in unstable air to the rear and surface trough
• produce heavy rains between October and April

Question 6

Cut-off lows

Answer 6

• more intense form of westerly wave
• cold-cored depression
• starts as trough and deepens into closed circulation
• unstable, baroclinic systems
• slope west with weight
• strong convergence and vertical motion
• comma clouds
• associated with floods
• peaks in march - may and September and November

Question 7

Southerly meridional flow

Answer 7

• strong zonal pressure gradient
• forms between high west and low east
• upper level westerly wave: divergence and vertical motion
• coastal rainfall
• sharp drop in temps

Question 8

Ridging anticyclones

Answer 8

• upper level westerly wave: widespread general rainfall over eastern SA
• strong pressure gradient: causes advection of moisture and allows for a lot of unstable air
• weakening inland pressure gradients, changing curvature, orographic forcing and upper level divergence combine
• general rains or thunderstorms
• ridging high produce extensive clouds over coastal areas (similar to westerly waves but a lot more extensive)
• sometimes high not deep enough to cross Drakensburg mountains (then you won’t have any upper level forcing and divergence)
• brings rainfall from October to may

Question 9

West coast troughs

Answer 9

• surface trough over the west coast combines with approaching westerly wave: widespread rainfall over western parts
• surface convergence combines with upper level divergence

Question 10

Cold fronts

Answer 10

• occurs together with westerly waves and cutoff lows
• should not be considered in isolation
• associated with cold invasion from the south
• distinct cloud bands
• distinct wind shift
• most frequent in winter with the northern movement of westerlies: because ICTZ moving northward due to summer being in the northern hem, we have a more frequent occurrence of the cold front system.
• ahead= divergence and subsidence
• convection to rear with max surface convergence

Question 11

Conditions required for the formation of tropical cyclones

Answer 11

1. Water temps of at least 26.5C down to depth of at least 50m
2. Rapid cooling with height
3. High humidity especially in the lower to mid troposphere
4. Low wind shear
5. Distance from the equator
6. A pre existing system of disturbed weather

Question 12

Composite of synoptic types

Answer 12

• interplay of different synoptic features that play together with one another and may cause a composite of synoptic features and inference.

Question 13

Fine weather and mildly disturbed conditions

Answer 13

• subtropical cyclones
• coastal lows
• berg winds

Question 14

Subtropical cyclones

Answer 14

• associated with anticyclone vorticity
• deep systems tilted towards northwest with height.
• divergence near the surface
• strong subsidence ( marked conditions in subtropical regions)
• max = June/July
• min = dec/Jan
• when prevailing = produce severe heat waves

Question 15

Coastal low

Answer 15

• cyclonic vorticity with eastward movement of plateau
• initiate on west coast and travel along the coast
• warm offshore air flow ahead
• cold onshore air flow behind
• shallow (850hPa / 1500 m)
• resemble cold fronts
• not a majority rain producing system, and can couple with wave in upper air to form depression and rainfall

Question 16

berg winds

Answer 16

• large scale prefrontal divergence
• dynamic warming
• caused by subsiding air
• highest winter temp along the coast
• several days or few hours