1 how weather and climate affect astronomical viewing and site selection dr. edward graham,...

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How weather and climate affect astronomical viewing and site selection

Dr. Edward Graham, University of the Highlands and Islands

Astroclimatology:

Where is University of the Highlands and Islands (“UHI”)?

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Scotland !!

E. Graham et al., 2010 3

The Highlands and Islands

University of the Highlands and Islands (“UHI”)

E. Graham et al., 2010 4

University of the Highlands and Islands (“UHI”)

Outline of my presentation today

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Two parts:

1) General Meteorology & Climatology

BREAK / PAUSE

2) Application of above to Astronomy


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Two parts:


BREAK / PAUSE


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Weather is the state of the atmosphere at any one particular place at a particular time.

Two separate places never have the exactly same weather, nor does the weather ever repeat itself

Every moment of weather is unique in space and time

Definition: Weather

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• Is the « average » of the weather, over « reasonably » long period of time (e.g. 30 years)

• Actual weather is usually chaotic, but is contained within certain boundaries, climate is the « average »

Definition: Climate

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The scale of weather and climate systems

• Weather and climate phenomena operate over huge temporal and spatial scales;

• Spatially: 10-3 m (millimetres) to 106m (thousands of kilometres)

• Temporally: 10-3 secs (milliseconds) to 108 secs (decades)

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Is climate steady?

«Traditional» (deterministic) climatologists (until ~1980s) viewed climate as being reasonably steady.Present view is contrary to this: Climate itself may not be stable & there can be sudden « shifts » or «step-changes»…

Temperature (red) of last 20,000 years on Greenland ice cap

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Rate of current climate change…

The rate of global climatic change is much faster than anything Earth has experienced in at least the last two millions years… (x 10 times faster)

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Intergovernmental Panel on Climate Change (IPCC) scenarios for 21st century

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Surface air temperature increase ~2090s

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It’s not just a temperature increase…. An increase in Extremes too!

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How does the Climate System work?

Polar regions receive less solar radiation because:• Ground surface area over which radiation is distributes gets larger

towards poles• Rays have a longer path length through atmosphere

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How does the Climate System work?Result:• Unequal heating of the Earth’s surface by the sun, which

varies according to day, season and latitude• The tilt of the Earth’s axis causes the seasons• The distribution of continents, mountains and oceans also

play a key role• Atmosphere is a fluid, but 1000 times less dense than water• P = ρRT (Ideal gas equationl P=pressure, ρ=density,

T=temperature, R=Universal gas constant)• Result is heat and moisture transfer towards the poles

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On average, there’s 342 Wm-2 incident and outgoing radiation at the top of atmosphere, but clouds & aerosols alter the balance depending on location

Hence there are energy transfers from equator to poles

The Earth’s Energy Balance

Main methods of Energy Transfer on Earth

The principal mechanisms driving this transfer of energy from the equator to the poles are the atmosphere and the oceans…

Both transport about the same, despite sluggishly-moving ocean currents…

Atmospheric Energy Transport: Wind

HL

Wind is just air moving from high pressure to low pressure (e.g. bicycle tyre) i.e. caused by a pressure gradient.

As air gets warmer, it expands, becomes less dense and therefore pressure decreases.

Helped by fact that air at the equator has greater relative angular velocity (40,000km per day) than air nearer the poles (0km/day).

But there is the CoriolisEffect

HL

HL

The Coriolis Effect:

The resulting balance between the Coriolis Effect (due to the Earth’s rotation) and the Pressure-Gradient Force is “Geostropic balance”

It means frictionless airflow is deflected by 90°….

Only for air that doesn’t “feel” the Earth’s rotation

For air that “feels” the Earth’s rotation (Geostrophic balance)

Wind and the Coriolis Effect

HL

Wind, the Coriolis Effect, and Friction

But differing amounts of surface friction (land, sea) result in a reduction in speed and a deflection reduced by 10-30°…

Air moving across latitudes in the Northern Hemisphere will swing to the right (clockwise).

Air moving across latitudes in the Southern Hemisphere will swing to the left (anti-clockwise).

Wind and the Coriolis Effect

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EquatorNorth Pole

-20C

-10C

0C

+10C+20

C

H L

“Slopes” in pressure pattern then cause winds / the jetstream:

The Jetstreams

Differences in air temperature / air pressure cause the weather patterns:

Latitudional (zonal) air circulation systems

Q: Why do low pressure turn anti-clockwise in the northern hemisphere? (and vice versa…)

1) 2) 3)

L L L

Rotation in weather systems - Lows

Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…)

1) 2) 3)

H H H

Rotation in weather systems - Highs

General global pattern of surface air pressure

The locations and intensities of these “highs” and “lows” vary with altitude.

Geostropic flow around these weather systems is permitted for cases of no friction e.g. >1km above surface.

But…. Non-geostropic flow can occur!

• On small scales i.e. local or regional flow may not be geostrophic!

• Especially true near mountains and coasts!

• 1 deg latitude is roughly equivalent to 18km/h (11mph) difference in relative velocity!

HL

HL

Non-Geostrophic Flow: The Sea Breeze

SEA LAND

Warm air over land rises

Pressure difference forces air out to sea

the sea-breeze moves onshore

The logarithmic wind profile

Where:

u = windspeed (ms-1)

u* = friction velocity (ms-1)

k = Von Karman’s constant (0.4)

z = height (m)

d = zero-displacement height (m)

z0 = roughness length

(after Oke, 1976)= stability term

The logarithmic wind profile

“Free Atmosphere” (Geostrophic)

“Boundary Layer” (Non-Geostrophic)

~300-1000m

But turbulence can still form in “free atmosphere”: Windshear!



~300-1000m

Windshear

Turbulence in the “free atmosphere”: Instability



Convection/bouyancy/instability

Turbulence in the “free atmosphere”: Gravity Waves



~300-1000m


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Two parts:


BREAK / PAUSE


Atmospheric Constraints on Astronomical Viewing

1. Clear skies / No Cloud

2. Stable Atmosphere / Little or no turbulence

3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)

4. Low night-time relative humidity (RH)

5. Gentle to moderate windspeeds, or less, throughout atmosphere

6. Moderate air temperatures, low variability

7. Low aerosol contamination

8. Infrequent or no severe weather (lightning, snow, hail)

9. Low light pollution

1. Cloud Cover: Clouds indicate ascending air

It's cooler in the atmosphere as you go up, and cold air cannot hold as much water vapour as warm air. So, when air is forced to rise, the excess water vapour (gas) in the air condenses into liquid droplets.Three main processes which lift andcool air to form clouds

1) Sea / Sun heating (thermals)2) Weather fronts (gentle)3) Mountains

Overall, the global upward movements of air are equally balanced by the downward movements, result is about 40-50% global cloudiness at any one time.

• Clouds occur on the local to synoptic

(national/international) scales i.e. ~102 to ~105m

spatial scale) and on temporal scales of 101 to 105

secs.

• Vertical extent depends on forcing and stability

• Local clouds occur especially daytime over mountain

tops, and night-time in valleys (so good for

astronomical observation)

• Satellite (e.g. EUMETSAT) and climate model data

(“reanalyses”) can be used to estimate cloud cover

1. Cloud Cover: Astronomical Observation

1: Cloud Cover : Contrails

1: Cloud Cover : EUMETSAT satellite (1km nadir)

1: Cloud Cover : UK Met Office African model (12km)

1: Cloud Cover : FriOWL / Re-analyses data

ERA40 reanalyses July total

cloud cover (above 2,000m

only)



2. Stable Atmosphere / Little or no turbulence








2. A stable atmosphere with little turbulence

• Covered by David / Aziz yesterday….

• Wobbling/scintillation of the stellar image is mostly

due to the vertical temperature gradient i.e. when

dT/dz is large - > unstable -> turbulence

• But also mechanical turbulence due to mountains or

obstacles

• Descending air usually descends gently (unlike most

ascending air, which ascends fast!)

• It so happens that there are preferential zones

zones of gently descending air around the globe…

Mean annual (1991-2000) ERA40 vertical velocities exceed 2.5 cm/sec (descent); these are indiciated by green / yellow/ red colours


ERA40 mid-to-upper tropospheric (775 to 200 hPa) vertical velocities in range 2.5 < > 5.0 cm /sec (i.e. gently subsiding air, turbulence less likely)

H H

H H

H H

H H



2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)










IWV is extremely height dependent, due to exponential relationship of WV with temperature


• Water vapour is the principal absorbing gas from visible to millimeter wavelengths in the atmosphere

• Also increases the refractive index of air, causing phase distortions• Decreases rapidly with vertical height; 2/3 less by a height of 2.5km• Sarazin (2003) quotes:• < 5mm IWV is suitable for visible astronomy• <3mm for infra-red• <2mm for microwave

Hence “High and Dry” sites are best…. Atacama, Rockies, Hawaii, Izana (Canarys), Morocco, Sutherland, African?

Only locations where mean annual IWV at 700 hPa is less than 4 mm (yellow) and greater than 4 mm (blue)


4. Low night-time Relative Humidity (RH %)

• RH is just the ratio of Vapour Pressure of Water Vapour ÷ Saturated Vapour Pressure at that same temperature

4. Low night-time Relative Humidity (RH %)

• RH usually reaches a maximum during the night and around dawn (minima during afternoon)

• If RH = 100% -> dew / condensation / mist / frost• Risk of dew/frost on mirror / lenses / optics

5. “Reasonably” low surface and jetstream windspeeds

Sarazin (2004) states 2-9m/sec are ideal surface windspeeds for the VLT

• <2m/sec -> no flushing of dome

• >9 m/sec -> shake!

Jetstream:

Sarazin & Tokovinin (2002) show that

the 200hPa jetstream is linearly related

to the speed of turbulent structures on

the stellar image

Isoplanatic Angle (θ): the angle subtended to the telescope becomes smaller as height increases

θ

6. Moderate Air Temperatures!

• Differences between dome

temperature and outside

temperature can lead to “dome

seeing”

-> SALT is ventilated to keep dT/dx

differences small!

• Extreme cold /heat can put a strain

on instrumentation, equipment and

personnel !

7. Low Aerosol Contamination

• Aerosols (dust, biomass

burning) contribute to

atmospheric extinction

• On-site wind-blown dust is

a hazard as it degrades

mirrors and optics rapidly

(Giordano & Sarazin, 1994)

• 22-years of TOMS aerosol

data available on FriOWL









9. Low light pollution & lots of others…. (infrastructure, culture,

geology, accessibility, political issues, etc..)


8. Infrequent Severe Weather !

• Much greater exposure to lightning at the top of a

mountain…

• But the choice of a dry desert with few storms mitigates

against chance of a lightning hit !

• Engineering needs to allow for specific loadings of snow !

Summary: There are links across a huge range of scales!

Milliseconds, millimetres (CN

2, CT2

CT2, seeing, r0, τ0)

Decadal cloudiness variability, Jetstream variations, Rossby waves (107m)

1010 differences in scale

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Thank You (1017 difference in scales!)

Spiral Galaxy, NGC 1232 21 Sep 1998, VLT Paranal (ESO)

Hurricane Epsilon, 3 Dec 2005, NASA

But 1017 times difference in scale!!

1 how weather and climate affect astronomical viewing and site selection dr. edward graham,...

Documents

weather slide

climate slide

atmosphere slide

astronomy slide

century slide

jetstreams slide

coriolis effect slide

islands uhi slide