Mountain waves and sundowners

“All hell broke loose" City Fire Chief Andrew DiMizio, May 8 2009

Important concepts to review

• Adiabatic lifting (or adiabatic expansion) • Adiabatic sinking (or adiabatic compression)• Saturation mixing ratio, temperature, dew

point and relative humidity• Stability of the atmosphere

H=0 T=30oC

H=100m,T=29oC

H=200m,T=28oC

H=300mT=27oC

H=400mT=26oC

Air parcel lifts due to increase in buoyancy (warmer and less dense than the surrounding environment) Volume expands and work is done against the environment. This occurs too fast to transfer or receive heat from the environment Because no heat is transferred from or to the air parcel, volume increases and the air mass inside the air parcel cools downThis is known as adiabatic lifting

Adiabatic lifting (adiabatic expansion)

Dry adiabatic lapse rate:10oC/km or 1oC/100m

Adiabatic compression Air parcel sinksVolume decreases due to the work done by the environment This occurs too fast to transfer or receive heat from the environment Because no heat is transferred from or to the air parcel, volume decreases and the air mass inside the air parcel warms upThis is known as adiabatic compression or adiabatic sink

H=0 T=30oC

H=100m,T=29oC

H=200m,T=28oC

H=300mT=27oC

H=400mT=26oC

Dry adiabatic lapse rate:10oC/km or 1oC/100m

The air’s susceptibility to uplift is called its static stability.

Statically unstable air becomes buoyant when liftedand continues to rise if given an initial upward push.

Statically stable air resists upward displacement and sinksback to its original level when the lifting mechanism ceases.

Statically neutral air neither rises on its ownfollowing an initial lift nor sinks back to its original level;

it simply comes to rest at the height to which it was displaced.

When a parcel of unsaturated or saturated air is liftedand the Environmental Lapse Rate (ELR)

is greater than the dry adiabatic lapse rate (DALR), the result is absolutely unstable air.

T=9 T=9 ooCC

T=8 T=8 ooCC

T=7 T=7 ooCC

Note that in this case the air parcel is warmer than the environment and will continue to rise

When a parcel of unsaturated or saturated air is liftedand the Environmental Lapse Rate (ELR) is less thanthe saturated adiabatic lapse rate (SALR), the result

is absolutely stable air and the parcel will resist liftingwill resist lifting.

77oo C C

88ooCC

99ooCC

Assume the ELR is 0.7 °C/100 m and the air is unsaturated.

As a parcel of air is lifted, its temperature is less thanthat of the surrounding air, so it has negative buoyancynegative buoyancy.

77ooCC

88ooCC

99ooCC

9.39.3ooCC

8.98.9ooCC

8.88.8ooCC

10.010.0ooCC10 C

9.0 C

8.0 C

7.0 C

Force parcel upward

Temperature inversionStable atmosphere

V

V

V

Horizontal Atmospheric motion

Tridimensional view

Northern Hemisphere

• Pressure (height) variations in upper atmosphere mainly caused by temperature variations in lower atmosphere

• Ridges have warm air below• Troughs have cold air below

Typical temperature Humidity daily variation

Relative humidity, RH, relates the ACTUALACTUAL amount of water vaporin the air to the maximum possible at the current temperature.

RH = (specific humidity/saturation specific humidity) X 100%RH = (specific humidity/saturation specific humidity) X 100%

Winds accelerate along canyons

The Jesusita Fire

Although it is composed of gases, in many ways the atmosphere behaves like a fluid. Many atmospheric disturbances occur as waves. These atmospheric wave disturbances result from the interactions of several forces including pressure gradients, Coriolis force, gravity, and friction. Atmospheric waves occur on a full range of spatial scales. You are probably familiar with large-scale planetary and synoptic waves found on constant pressure charts. In these waves, the horizontal motions exceed the vertical motions by several orders of magnitude.

Sundowner and Santa Ana winds are considered mountain waves. When wind speeds are intense, they are often referred to as “wind storms”

• What is a mountain wave?

Mountain waves form above and downwind of topographic barriers when strong winds blow with a significant vector component perpendicular to the barrier in a stable environment

Wave activity

Cap clouds may indicate mountain waves

The vertically-propagating wave is often most severe just downwind of the mountain barrier. These waves frequently become more amplified and tilt upwind with height. Tilting, amplified waves can cause aircraft to experience turbulence at very high altitudes. Clear air turbulence often occurs near the tropopause due to vertically-propagating waves.

Vertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere. Wave-breaking can result in severe to extreme turbulence within the wave-breaking region and nearby, typically between 20,000-40,000 feet (6000-12000 m). If a vertically-propagating wave doesn't break, an aircraft would likely experience considerable wave action, but little turbulence.

Strong downslope wind cases are associated with strong cross-barrier flow, waves breaking aloft, and an inversion near the barrier top. This may be double or triple the wind speed at mountaintop level. These high winds frequently lead to turbulence and wind shear at the surface. Downslope windstorms often abruptly end at the "jump region”. The jump region is an extremely turbulent area that can extend up to 10,000 feet.

Rotors are also called horizontal roll vortices because they form a complete rotational pattern, with the axis of rotation parallel to the ground. They exist immediately downstream of the jump region and under a wave crest. Smaller-scale rotations embedded within the low-level turbulent zone can cause rolling that exceeds an aircrafts ability to stay level.

Lee waves whose energy does not propagate vertically because of strong wind shear or low stability above are said to be "trapped." Trapped lee waves are often found downstream of the rotor zone, although a weak rotor may exist under each lee wave. Strong turbulence can develop between the bases of associated lenticular clouds and the ground. Lenticular clouds form near the crests of mountain waves. As air ascends and cools, moisture condenses, forming the cloud. As that air descends in the lee of the wave crest, the cloud evaporates.

This photo, taken during the Sierra Wave Project in the 1950s, shows a dramatic example of a downslope windstorm in the Owens Valley, California. The wind accelerates down the lee slope of the Sierra Nevada on the right. Dust is seen being picked up off the floor of the valley and lifted in the "jump region" into the highly turbulent flow under rotor clouds.

The warming of downslope winds typically occurs when low-level air upwind of a barrier is blocked and does not proceed up and over. Instead, the downslope winds originate near or above mountain-top level. As this air descends the lee slope, it warms following a dry adiabatic lapse rate of 10°C per kilometer. Under stable atmospheric conditions, the lapse rate upstream of the barrier will be substantially less than 10°C per kilometer. The end result is warmer temperatures on the lee side of the mountain barrier, much warmer if an inversion is present on the windward side.

Cold downslope winds are called bora winds. They result from a deep and very cold upstream air mass that spills over a barrier and displaces a warmer air mass. Unlike a foehn wind, the upstream air mass is so cold that the air reaching the ground along the lee slope feels cool, despite warming adiabatically as it descends.

Santa Barbara Sundowner Winds

Generic Classification (Einar Hovind, Gary Ryan)

SMX – SBA SLP Gradient

Wind Ranges (m s-1)

2 mb No downslope winds

2-3 mb 10-15

3-4 mb 15-20

4-5 mb 20-25

5 mb 25

They typically occur in spring and summer seasons. North or northwest winds across Santa Inez Mountains with strong surface pressure gradient (e.g. Santa Maria and Santa Barbara). Increase in surface temperature and decrease in relative humidity

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History of Severe Sundowners

17 June 1859 T 138ºF (???)21-24 Sept. 1885 T 108ºF (4PM) T 102ºF (8PM)2-3 July 1907 T 88ºF (Midnight)15-17 June 1917 T 115ºF (5PM) all time SB T record12-14 July 1925 T> 100ºF Gusts 40-60 mph26 July 1977 Gusts 60-70 mph

06-07 May 2009 will likely be part of the history of strong eventsT> 100oF and Gusts 40mph

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Santa Barbara Painted Cave Fire

27 June 1990

•Point of origin: Old San Marcos Rd. and Hwy 154 6:02 pm PDT. T=96°F, RH=10%, 10-20 mph

•6:45 pm fire advanced 2 miles, winds 60 mph

•SBA T=109°F (42.7 °C) 1:30 pm. 30 mph (13 ms-1) 3:48pm.

•El Capitan 116 °F (46.6 °C).

•San Marcos summit 10-20 mph. South side 60 mph

•Tucker’s Grove 80 mph (35 ms-1)

1 death, consumed over 600 structures, burned 4,900 acres.

Arson ignited

Numerical simulation of Sundowner windshttp://www.icess.ucsb.edu/asr/painted_cave_plots.htm

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Conclusions:• Work with your group on the main conclusions

about this lecture:• Can you explain what is stability in the atmosphere

and its importance for cloud formation?• Can you explain the reasons for turbulence in the

atmosphere? • Can you explain the reasons why sundowners are

associated with dry and warm days in SB?• Do you remember what are the main dynamical

factors associated with sundowners?