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1 How weather and climate affect astronomical viewing and site selection Dr. Edward Graham, University of the Highlands and Islands Astroclimatology:

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Presentation on theme: "1 How weather and climate affect astronomical viewing and site selection Dr. Edward Graham, University of the Highlands and Islands Astroclimatology:"— Presentation transcript:

1 1 How weather and climate affect astronomical viewing and site selection Dr. Edward Graham, University of the Highlands and Islands Astroclimatology:

2 Where is University of the Highlands and Islands (UHI)? 2 Scotland !!

3 E. Graham et al., 20103 The Highlands and Islands University of the Highlands and Islands (UHI)

4 E. Graham et al., 20104 University of the Highlands and Islands (UHI)

5 Outline of my presentation today 5 Two parts: 1)General Meteorology & Climatology BREAK / PAUSE 2) Application of above to Astronomy

6 Outline of my presentation today 6 Two parts: 1)General Meteorology & Climatology BREAK / PAUSE 2) Application of above to Astronomy

7 7 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

8 8 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

9 9 The scale of weather and climate systems Weather and climate phenomena operate over huge temporal and spatial scales; Spatially: 10 -3 m (millimetres) to 10 6 m (thousands of kilometres) Temporally: 10 -3 secs (milliseconds) to 10 8 secs (decades)

10 10 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

11 11 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)

12 12 Intergovernmental Panel on Climate Change (IPCC) scenarios for 21 st century

13 13 Surface air temperature increase ~2090s

14 14 Its not just a temperature increase…. An increase in Extremes too!

15 15 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

16 16 How does the Climate System work? Result: Unequal heating of the Earths surface by the sun, which varies according to day, season and latitude The tilt of the Earths 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

17 17 On average, theres 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 Earths Energy Balance

18 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…

19 Atmospheric Energy Transport: Wind H L 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.

20 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 Coriolis Effect

21 H L H L The Coriolis Effect: The resulting balance between the Coriolis Effect (due to the Earths rotation) and the Pressure-Gradient Force isGeostropic balance It means frictionless airflow is deflected by 90°…. Only for air that doesnt feel the Earths rotation For air that feels the Earths rotation (Geostrophic balance) Wind and the Coriolis Effect

22 H L 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°…

23 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

24 24 Equator North Pole -20C -10C 0C +10C +20C HL Slopes in pressure pattern then cause winds / the jetstream: The Jetstreams

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

26

27 Latitudional (zonal) air circulation systems

28 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

29 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

30 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

31 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

32 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.

33 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! H L H L

34 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

35 The logarithmic wind profile Where: u = windspeed (ms -1 ) u* = friction velocity (ms -1 ) k = Von Karmans constant (0.4) z = height (m) d = zero-displacement height (m) z 0 = roughness length (after Oke, 1976) = stability term

36 The logarithmic wind profile Free Atmosphere (Geostrophic) Boundary Layer (Non-Geostrophic) ~300-1000m

37 But turbulence can still form in free atmosphere: Windshear! Free Atmosphere (Geostrophic) Boundary Layer (Non-Geostrophic) ~300-1000m Windshear

38 Turbulence in the free atmosphere: Instability Free Atmosphere (Geostrophic) Boundary Layer (Non-Geostrophic) Convection/bouyancy /instability

39 Turbulence in the free atmosphere: Gravity Waves Free Atmosphere (Geostrophic) Boundary Layer (Non-Geostrophic) ~300-1000m

40 Outline of my presentation today 40 Two parts: 1)General Meteorology & Climatology BREAK / PAUSE 2) Application of above to Astronomy

41 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

42 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

43 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 and cool 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.

44 Clouds occur on the local to synoptic (national/international) scales i.e. ~10 2 to ~10 5 m spatial scale) and on temporal scales of 10 1 to 10 5 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

45 1: Cloud Cover : Contrails

46 1: Cloud Cover : EUMETSAT satellite (1km nadir)

47 1: Cloud Cover : UK Met Office African model (12km)

48 1: Cloud Cover : FriOWL / Re-analyses data ERA40 reanalyses July total cloud cover (above 2,000m only)

49 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

50 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…

51 Mean annual (1991-2000) ERA40 vertical velocities exceed 2.5 cm/sec (descent); these are indiciated by green / yellow/ red colours 2. A stable atmosphere with little turbulence

52 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. A stable atmosphere with little turbulence

53 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 Atmospheric Constraints on Astronomical Viewing

54 3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV) IWV is extremely height dependent, due to exponential relationship of WV with temperature

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

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

57

58 Only locations where mean annual IWV at 700 hPa is less than 4 mm (yellow) and greater than 4 mm (blue) 3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)

59 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 Atmospheric Constraints on Astronomical Viewing

60 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

61 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

62 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 Atmospheric Constraints on Astronomical Viewing

63 5. Reasonably low surface and jetstream windspeeds Sarazin (2004) states 2-9m/sec are ideal surface windspeeds for the VLT 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

64 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 Atmospheric Constraints on Astronomical Viewing

65 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 !

66 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 Atmospheric Constraints on Astronomical Viewing

67 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

68 1.Clear skies / No Cloud 2.Stable Atmosphere / Little or no turbulence (~10 -3 to ~10 5 m) 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 & lots of others…. (infrastructure, culture, geology, accessibility, political issues, etc..) Atmospheric Constraints on Astronomical Viewing

69 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 !

70 Summary: There are links across a huge range of scales! Milliseconds, millimetres (C N 2, C T 2 CT2, seeing, r0, τ0) Decadal cloudiness variability, Jetstream variations, Rossby waves (10 7 m) 10 10 differences in scale

71 71 Thank You (10 17 difference in scales!) Spiral Galaxy, NGC 1232 21 Sep 1998, VLT Paranal (ESO) Hurricane Epsilon, 3 Dec 2005, NASA But 10 17 times difference in scale!!


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