Chapter 7 Flashcards
The term stationary waves refers to the
zonally asymmetric features of the time-averaged atmospheric circulation.
The term stationary waves refers to the zonally asymmetric features of the time-averaged atmospheric circulation. They are also referred to as
standing eddies
standing eddies
where standing refers to the time averaging over a month to season, and eddy is a generic term for zonally asymmetric patterns.
The zonal asymmetries of the seasonal circulation are particularly interesting because
they occur despite the longitudinally uniform incidence of solar radiation on our planet.
Stationary waves must arise, ultimately, due to
asymmetries at the Earth’s surface – mountains, continent–ocean contrasts, and sea surface temperature asymmetries.
Understanding precisely how the stationary waves are generated and maintained is a fundamental problem in
climate dynamics.
Stationary waves have a strong effect on the climate through
their persistent northerly and southerly surface winds, which blow cold and warm air.
Advection of moisture by the stationary wave flow contributes to
hydro-climate variations over the continents.
Beyond their direct …………… impact
advective
Beyond their direct advective impact
- 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.
Stationary waves are thus a fundamental feature of the
general circulation of the troposphere
Stationary waves are stronger in
the Northern Hemisphere because of greater orography and continentality.
Stationary waves are stronger in the Northern Hemisphere because of greater orography and continentality. Wave amplitudes in the Northern Hemisphere are largest during
winter
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
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
summer
The Southern Hemisphere stationary waves and their seasonal variation are substantially
smaller
The Stationary waves dynamics are
- 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
Waves will appear to be stationary if
their phase speed is equal and opposite to the mean flow
c=-u
Stationary waves will have a frequency of zero, since they
do not oscillate in time, only in space.
For dispersive waves, the wavelength of the stationary wave will correspond to
that wavelength which has a phase speed equal and opposite to the mean flow.
An example of stationary waves is the
stationary wave pattern that forms in a river when it flows over a submerged rock or obstacle.
The flow of the obstacle generates many different waves of various
wavelengths
The flow of the obstacle generates many different waves of various wavelengths, but only the ones whose
phase speed is equal and opposite to the mean flow will remain stationary.
if ………………………….. the wavelength of the stationary wave will change
the flow speeds up or slows down,
……………………… can also generate standing waves
In the atmosphere, flow of a stably stratified fluid over a mountain barrier
Vertically propagating waves occur when
- Wind speed above the mountain does not increase significantly
- Stability increases above the mountain top stable layer.
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.
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
downslope wind storms such as Chinooks
This tells us that
the lines of constant phase tilt upwind with height
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
downward component of phase speed. This means that the group velocity is upward, so that topographically forced waves propagate energy upward.
evanescent waves:
waves that decay with height
In the real world, mountains are not
pure sinusoids. However, through Fourier analysis, we can approximate the real topography by its Fourier component
A very sharp, or discontinuous function has a
wave number) components in its transform.
A broad function has
few high frequency components, and is mostly made up of low frequency (low wave number) components
Flow over a mountain will generate a
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
Based on analysis of Fourier transforms, a narrow mountain will generate
a lot of high wave number gravity waves
while a broad mountain will generate
more low wave-number gravity waves
Depending upon ……………………, the flow will ……………….
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).
Trapped waves occur when
wind speed above the mountain increases sharply with height and when stability decreases above the mountain top stable layer
Lee waves are
atmospheric stationary waves
Lee waves are atmospheric stationary waves. The most common form is mountain waves, which are
atmospheric internal gravity waves
Wave clouds do not move
downwind as clouds usually do, but remain fixed in position relative to the obstruction that forms them.
(lenticularis
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.
rotor
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
A foehn wall cloud
may exist at the lee side of the mountain, however this is not a reliable indication of the presence of lee waves.
A pileus or cap cloud
similar to a lenticular cloud, may form above the mountain or cumulus cloud generating the wave.
wave window
“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
On mountain waves, we’ve assumed that
the mean flow does not have vertical shear
static stability is constant with height
Since wind speed normally increases with height it is possible that m^2 = N^2/u^2-k^2 will yield
vertically propagating waves in the lower layer, but have vertically decaying waves in part of the atmosphere.
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
distinct vertical layers
Scorer parameter
N/u —> constant if we have distinct vertical layers
though N and u themselves may vary within the layer
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
waves that are ‘trapped’ in the lower layer downwind of the mountain.
