LOCAL WINDS

Question 1

What is a katabatic?

Answer 1

A “down slope” wind

Question 2

How does a katabatic form?

Answer 2

1. Hills/mountains cool at night, the air in contact with the ground also becomes cooler (conduction)
2. Colder air is heavier than the free air above.
3. Ths heavier air slides downhill due gravity.

Question 3

When are katabatics most common?
Onset?

Answer 3

Winter months, however they can occur in summer on clear nights in large catchments.
Onset from just after sunset in winter, to just before dawn.

Question 4

When do Katabatics tend to die out?

Answer 4

By mid to late morning or when replaced by a geostrophic wind.

Question 5

Conditions associated with Katabatics?

Answer 5

• Wind speed varies from barely detectable to 15-20kts in some gorges. (In antarctica they can be gale-force in winter)
• Katabatics can either be too strong for radiation fog to develop (OH), or in some cases, just right to assist in the development (WP)

Question 6

NZ Named katabatic wind?

Answer 6

Greymouth Barber

Question 7

Standard katabatic wind in Ohakea? Whenuapai?

Answer 7

030 deg, 5-10kts.
NW’ly, 2kts

Question 8

The 3 requirements for Katabatic to develop?

Answer 8

1. Clear nights
2. Sloping ground
3. Pressure gradient not too strong

Question 9

What is an Anabatic flow?
How does it develop?

Answer 9

• Opposite to katabatic.
• Sunny day, warm air in contact with the sloping surface rises and has an upslope component.

Question 10

Characteristics of Anabatic flows?

Answer 10

1. Very light, 2-3kts
   2, Normally only found on side slopes of valleys
2. Flow is strongest in afternoon on slopes facing the sun

Question 11

Anabatics vs. Convection?

Answer 11

If conditions are unstable, ascent of air is realised through Convection & not anabatics.

Question 12

How does a sea breeze develop?

Answer 12

1. Coastal location (Sea or lake), slack pressure gradient & no pressure gradient through depth = little to no wind at low level.
2. After sunrise, sun heats land faster than water. Air above the land (through conduction), is heated faster & thermal expansion occurs = High pressure aloft over land (in comparison to out at sea).
3. Pressure gradient has been created at ~1000ft = corresponding OFFSHORE flow at this altitude.
4. Shift of air from H to L at 1000ft causes a corresponding L to form at the surface over land.
5. Thus, the remaining three legs of the circulation kick in = onshore breezee

Question 13

What seasons do we see sea breezes form?
Time of day?

Answer 13

Common in summer, occasionally in spring & autumn, rarely in winter.
Develops between mid AM & late afternoon.

Question 14

Conditions associated with sea breezes?

Answer 14

• Cool air flowing off the sea causes low level inversion.
• Inversion kills convection, often characterised by fair weather Cu inland, and clear skies out to sea (sinking branch of the circulation).
• Showers or TS can develop along the boundary if air is unstable enough. (sea breeze front).
• Turbulence differs depending on your location within the cycle. Expect light - mod convective turbulence over land. If CBs have developed, expect severe turb

Question 15

List the four conditions favourable for sea breeze development

Answer 15

1. Coastal situation (sea or lake)
2. Fine wx, especially summertime
3. Slack pressure gradient
4. period mid morning - late arvo for onset

Question 16

Formation process of terrain channelling?

Answer 16

1. Occurs when air is forced through a constriction by adjacent higher ground (e.g. valley/pass/saddle) (same effect as a venturi)
2. If air is stable, it resists being lifted, and a large portion (instead of rising up & over), will divert around the obstacle.

Question 17

3 requirements for terrain channelling to occur?

Answer 17

1. Wind speed as a function of pressure gradient
2. Steepness and proximity of adjacent high ground
3. Stability of the atmosphere (stable layer will enhance the effect).