Chapter 7

Question 1

The term stationary waves refers to the

Answer 1

zonally asymmetric features of the time-averaged atmospheric circulation.

Question 2

The term stationary waves refers to the zonally asymmetric features of the time-averaged atmospheric circulation. They are also referred to as

Answer 2

standing eddies

Question 3

standing eddies

Answer 3

where standing refers to the time averaging over a month to season, and eddy is a generic term for zonally asymmetric patterns.

Question 4

The zonal asymmetries of the seasonal circulation are particularly interesting because

Answer 4

they occur despite the longitudinally uniform incidence of solar radiation on our planet.

Question 5

Stationary waves must arise, ultimately, due to

Answer 5

asymmetries at the Earth’s surface – mountains, continent–ocean contrasts, and sea surface temperature asymmetries.

Question 6

Understanding precisely how the stationary waves are generated and maintained is a fundamental problem in

Answer 6

climate dynamics.

Question 7

Stationary waves have a strong effect on the climate through

Answer 7

their persistent northerly and southerly surface winds, which blow cold and warm air.

Question 8

Advection of moisture by the stationary wave flow contributes to

Answer 8

hydro-climate variations over the continents.

Question 9

Beyond their direct …………… impact

Answer 9

advective

Question 10

Beyond their direct advective impact

Answer 10

• stationary waves control the location of storm-tracks – the preferred paths of synoptic weather systems in the midlatitudes, and the zone of tropical–extratropical interaction in the subtropics.
• important also on longer time scales, since interannual climate variability projects substantially on the zonally asymmetric component of the flow.
• Finally, stationary waves contribute significantly to the maintenance of the complementary zonally symmetric circulation, in both climatological and anomalous states; the contribution is through quadratic fluxes of meridional momentum and heat.

Question 11

Stationary waves are thus a fundamental feature of the

Answer 11

general circulation of the troposphere

Question 12

Stationary waves are stronger in

Answer 12

the Northern Hemisphere because of greater orography and continentality.

Question 13

Stationary waves are stronger in the Northern Hemisphere because of greater orography and continentality. Wave amplitudes in the Northern Hemisphere are largest during

Answer 13

winter

Question 14

Stationary waves are stronger in the Northern Hemisphere because of greater orography and continentality. Wave amplitudes in the Northern Hemisphere are largest during winter, modest during

Answer 14

the transition seasons of spring and autumn

Question 15

Stationary waves are stronger in the Northern Hemisphere because of greater orography and continentality. Wave amplitudes in the Northern Hemisphere are largest during winter, modest during the transition seasons of spring and autumn, and weakest during

Answer 15

summer

Question 16

The Southern Hemisphere stationary waves and their seasonal variation are substantially

Answer 16

smaller

Question 17

The Stationary waves dynamics are

Answer 17

• forced by mountains and land-sea thermal contrasts width of mountain range is important
• strongest in high latitude NH, winter
• poleward heat flux, upward EP flux centered ~60°N
• dispersion to lower latitudes at jet level
• there also exist equatorially-trapped planetary waves

Question 18

Waves will appear to be stationary if

Answer 18

their phase speed is equal and opposite to the mean flow

c=-u

Question 19

Stationary waves will have a frequency of zero, since they

Answer 19

do not oscillate in time, only in space.

Question 20

For dispersive waves, the wavelength of the stationary wave will correspond to

Answer 20

that wavelength which has a phase speed equal and opposite to the mean flow.

Question 21

An example of stationary waves is the

Answer 21

stationary wave pattern that forms in a river when it flows over a submerged rock or obstacle.

Question 22

The flow of the obstacle generates many different waves of various

Answer 22

wavelengths

Question 23

The flow of the obstacle generates many different waves of various wavelengths, but only the ones whose

Answer 23

phase speed is equal and opposite to the mean flow will remain stationary.

Question 24

if ………………………….. the wavelength of the stationary wave will change

Answer 24

the flow speeds up or slows down,

Question 25

……………………… can also generate standing waves

Answer 25

In the atmosphere, flow of a stably stratified fluid over a mountain barrier

Question 26

Vertically propagating waves occur when

Answer 26

• Wind speed above the mountain does not increase significantly
• Stability increases above the mountain top stable layer.

Question 27

Vertically propagating waves can become very turbulent in the

Answer 27

upper troposphere and lower stratosphere in the so-called “breaking wave” region, and also in the “hydraulic jump” region of the wave.

Question 28

Vertically propagating waves can become very turbulent in the upper troposphere and lower stratosphere in the so-called “breaking wave” region, and also in the “hydraulic jump” region of the wave. They are closely related to

Answer 28

downslope wind storms such as Chinooks

Question 29

This tells us that

Answer 29

the lines of constant phase tilt upwind with height

Question 30

This tells us that the lines of constant phase tilt upwind with height (see figure below). Since the wave is propagating upstream, the individual wave crests have a

Answer 30

downward component of phase speed. This means that the group velocity is upward, so that topographically forced waves propagate energy upward.

Question 31

evanescent waves:

Answer 31

waves that decay with height

Question 32

In the real world, mountains are not

Answer 32

pure sinusoids. However, through Fourier analysis, we can approximate the real topography by its Fourier component

Question 33

A very sharp, or discontinuous function has a

Answer 33

wave number) components in its transform.

Question 34

A broad function has

Answer 34

few high frequency components, and is mostly made up of low frequency (low wave number) components

Question 35

Flow over a mountain will generate a

Answer 35

whole spectrum of gravity wave. Each wave component generated will either propagate vertically, or decay vertically, depending on whether its vertical wave number is real or imaginary

Question 36

Based on analysis of Fourier transforms, a narrow mountain will generate

Answer 36

a lot of high wave number gravity waves

Question 37

while a broad mountain will generate

Answer 37

more low wave-number gravity waves

Question 38

Depending upon ……………………, the flow will ……………….

Answer 38

wind speed and the mountain width

generate or decay waves propagating vertically (waves propagating vertically will generate if the wind speed is slow and mountain is wide).

Question 39

Trapped waves occur when

Answer 39

wind speed above the mountain increases sharply with height and when stability decreases above the mountain top stable layer

Question 40

Lee waves are

Answer 40

atmospheric stationary waves

Question 41

Lee waves are atmospheric stationary waves. The most common form is mountain waves, which are

Answer 41

atmospheric internal gravity waves

Question 42

Wave clouds do not move

Answer 42

downwind as clouds usually do, but remain fixed in position relative to the obstruction that forms them.

Question 43

(lenticularis

Answer 43

Around the crest of the wave, adiabatic expansion cooling can form a cloud in shape of a lens (lenticularis). Multiple lenticular clouds can be stacked on top of each other if there are alternating layers of relatively dry and moist air aloft.

Question 44

rotor

Answer 44

The rotor may generate cumulus or cumulus fractus in its upwelling portion, also known as a “roll cloud”. The rotor cloud looks like a line of cumulus. It forms on the lee side and parallel to the ridge line. Its base is near the height of the mountain peak, though the top can extend well above the peak and can merge with the lenticular clouds above. Rotor clouds have ragged leeward edges and are dangerously turbulent

Question 45

A foehn wall cloud

Answer 45

may exist at the lee side of the mountain, however this is not a reliable indication of the presence of lee waves.

Question 46

A pileus or cap cloud

Answer 46

similar to a lenticular cloud, may form above the mountain or cumulus cloud generating the wave.

Question 47

wave window

Answer 47

“wave window” or “Foehn gap”.

Adiabatic compression heating in the trough of each wave oscillation may also evaporate cumulus or stratus clouds in the airmass

Question 48

On mountain waves, we’ve assumed that

Answer 48

the mean flow does not have vertical shear

static stability is constant with height

Question 49

Since wind speed normally increases with height it is possible that m^2 = N^2/u^2-k^2 will yield

Answer 49

vertically propagating waves in the lower layer, but have vertically decaying waves in part of the atmosphere.

Question 50

Since wind speed normally increases with height it is possible that (9) will yield vertically propagating waves in the lower layer, but have vertically decaying waves in part of the atmosphere. This is illustrated in the

Answer 50

distinct vertical layers

Question 51

Scorer parameter

Answer 51

N/u —> constant if we have distinct vertical layers

though N and u themselves may vary within the layer

Question 52

In each layer the quantity N/u (called the Scorer parameter) is constant (though N and u themselves may vary within the layer). This results in

Answer 52

waves that are ‘trapped’ in the lower layer downwind of the mountain.