Lecture 11 & 12 - orographic effects 2 & 3: thermal winds and dynamic winds

Question 1

Daytime wind is

Answer 1

Anabatic Winds (upslope)

Question 2

Nighttime wind is

Answer 2

Katabatic Winds (downslope)

Question 3

A. in the early morning the upward slope winds are already developing but the night time mountain wind is still apparent.
B. with continued heating, the mountain wind ceases.
C. The valley wind develops
D. In late arvo the valley wind is fully developed

Answer 3

Mountain valley wind surface evolution in the daytime

Question 4

E. shortly before sunset the radiation balance turns negative. Cold air drainage forms on the slopes
F. with continued cooling the valley wind ceases, and a lateral circulation develops.
G. continued accumulation of cold air at the valley floor initiates the mountain wind
H. shortly before sunrise, the mountain wind is most developed

Answer 4

Mountain valley wind surface evolution in the daytime

Question 5

Conditions required for the occurrence of mountain valley wind

Answer 5

Calm, cloud-free conditions

Question 6

Katabatic winds in cold climates

Answer 6

Cold ice/snow surfaces can create a semi permanent inversion

Question 7

Airflow seperation

Answer 7

Sudden discontinuity causing seperation

Question 8

What is the threshold for the boundary layer adjusting to slopes without seperating?

Answer 8

Up to 17°

Question 9

What factor is flow separation dependent on?

Answer 9

stability

Question 10

Example of blocking / flow splitting in NZ

Answer 10

In a very stable atmosphere air from Australia cannot flow up and over the Southern Alps. It must go around through the Foveaux and Cook Strait channels, which causes wind jetting

Question 11

Some air flows over the topographic obstacle and reaches the inversion layer. The stable layer forces the downward movement of the air parcel. The air parcel ‘overshoots’ the original level and stability/ convection results in subsequent upward forcing

Answer 11

Lee wave formation.

Question 12

2 key factors of Föhn effect

Answer 12

• cooling at a slower rate of downwind side
• losing water

Question 13

NZ Föhn - North westerly associations…

Answer 13

warm conditions, low humidity (in the 30’s), and high windspeeds ( >50ms-1)

Question 14

Why don’t north westerly winds lead to föhn conditions?

Answer 14

Blocking - air is prevented from flowing over.
Insufficent moisture in the atmosphere to cause precipitation as air rises and cools as it passes over the Southern Alps