Mountain Weather Flashcards
What influences the flow of wind (5 - 8)
- Wind strength
- Angle of wind flow near the ranges
- Shape of the mountain range
- Stability of the air
- Vertical profile of wind speed and direction
(6. Orientation of valley systems - Location of passes and higher peaks
- Effects of multiple ranges)
Requirements for lee wave development (5)
- Min 20kts at ridge top level
- Increasing wind speed with height (creates overturning motion)
- Wind perpendicular to ridge line (or within 30 degrees to perpendicular)
- Wind direction not varying with height
- Stable layer at about ridge top level.
Features of lee waves (7)
- Bands of lifting and sinking air parallel to range
- AC Lenticularis and rotors present (visible w sufficient moisture)
- Severe turb in rotor zones
- High level clear air turb if jet stream is present
- Smooth flying in wave system above friction layer (downdrafts can exceed climb performance of a/c)
- Downdrafts can touchdown to SFC = localised strong winds
- Severe icing in updrafting portion of wave clouds under special circumstances.
What can the diversion of airflow be affected by?
Atmospheric stability.
(if lower layers of atmosphere are stable, winds will try to flow around the range rather than over it. As a result of this, winds around the ends of the ranges are significantly stronger).
As air tends towards instability, more air will flow over the range, resulting in stronger winds at ridge top level.
Describe winds across the South Island in a NW flow.
- Winds below 3000ft: will hit the range, creating a high pressure windward (corresponding Low pressure on East/Lee side of Range). These winds will bend right/South to pass b/t stewart island and the bottom of the South island. A portion will also make its way up and through the cook strait.
- Winds above 3000ft: Will climb up and over the range, following river valleys etc.
- Areas where the wind ‘splits’ (e.g. Karamea) have calm winds/climate
- Areas where the wind ‘joins’ (e.g. Nugget Pt) have extremely strong winds.
Describe the Barrier Jet
Low level winds below 3000ft that travel along the west coast of the South island that combine to create a clockwise rotating tube of wind averaging 60kts.
This jet forces low level wind to rise over the top of the jet before climbing the ranges, producing more rainfall.
Describe Lee Wave formation
- Air displaced upwards over the Range hits a stable layer at ridge - height, this provides restoring force for the descent of the air on the leeward side.
- A pendulum effect is formed downstream.
Lee waves in NZ
Are common and well developed in NZ in NW flows.
What does the wavelength and amplitude of a lee wave depend on
Wavelength: Speed of the wind/stability of the air (generally 10 - 40km)
Amplitude: Shape of range.
What are the dangers of rotor clouds
They indicate severe turbulence and wind shear which can destroy aircraft.
What impact does multiple ranges have on lee waves?
Can:
- Cancel out the wave
- Double the amplitude and shorten the wavelength.
- Also result in counter rotating flows.
Define Hydrualic Jump
Occur when extremely strong down-slope air meets slower downstream winds. This results in a ‘jump’ in the wind. (think about rivers)
Define the formation of Rotor streaming
Occurs when strong winds blow across a ridge line (Easterlies), with a marked decrease in speed above ridge level (Westerlies). Lee waves will not form. However, a single rotor will format ridge top, lee of the range and can migrate further lee of the range.
Conditions required for rotor streaming
- Winds near perpendicular to ridge line
2. Strong winds at ridge top height, but decreasing above.
Features of rotor streaming
- Severe turb in rotor zone, lee and in line with or just below of the ridge line.
- Relatively smooth airflow above ridge top level.
Define the Fohn wind
Where does it occur often in NZ?
A warm, dry, very gusty wind blowing down the lee side of a major mountain range.
The Strong NW that blows across the canterbury plains to CHCH is a Fohn wind.
How is a Fohn wind formed
How is this related to the lapse rate.
- Air rises on windward side of a range
- Cloud forms, releasing latent heat (from condensation)
- Rainfall will occur on the windward side, reducing water vapour in the air.
- wind on lee side is therefore hot, dry and strong
DALR - SALR - Top of range - DALR.
Features of a Fohn wind
- Fohn gap (band of clear skies in lee of the ranges due descending air)
- High cloud bases on lee side
- Turbulent conditions
- Lee wave activity
- Warm temperatures to the east of the country
How are cloud bases impacted in NZ mountains?
- Bases are lower on the windward side of the range.
- Bases are higher on lee side, with exception of rotor clouds (which are to be avoided)
- Clouds change rapidly in mountainous areas (due changes in moisture content of air and lifting triggers)
How is turbulence impacted by NZ mountains?
- Windward: light to moderate turb. (severe turb is rare)
- Lee: Severe turb is relatively common, especially in rotor zones. And also possible in the friction layer. Hydraulic jumps can also be found in the lee.
How is visibility impacted by NZ mountains?
- Windward: Poor vis due to rain, drizzle or snow.
- Lee: Good vis, however curtains of rain can exist downstream
Dangers of flying in Lee Wave conditions
- Strong downdrafts
- Rising ground
- Low ground speed
- Risk of rotors with sever turb.
