prepared by m. sarazin, eso · capped above by a cold top. ... to 18 months. 8 ... site is better...

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Climatology of ParanalPrepared byM. Sarazin, ESO

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Climatology of Paranal

• The Main Climate Actors• The Main Parameters• And their Climatology

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Main Climate Actors

•Bolivian High•El Nino Southern Oscillation•South American Monsoon•Pacific Decadal Oscillation (?)•Global Warming (no)

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Main Climate Actors1-The Bolivian High

The Bolivian high is characterized by a warm core below about 150 mb,capped above by a cold top.

The warm temperatures extend from the surface and maximize at around 300 mb.

M. Beniston, P. Casals & M. Sarazin; Perturbations to astronomical observations at the European Southern Observatory's very large telescope site in Paranal, Chile: analyses of climatological causes; Theoretical and Applied Meteorology, Volume 73 Issue 3-4 (2002) pp 133-150

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Main Climate Actors 2-South American Monsoon System (SAMS)

also called “Bolivian Winter”Over 60% of the annual precipitation of the Andes is concentrated in the austral summer (DJF) in the form of intensive convective rainstorms along the plateau, when easterly flow prevails over the central Andes allowing moisture transport from the interior of the continent up to the Altiplano.

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Main Climate Actors3- El Niño/Southern Oscillation (ENSO):

• El Niño/Southern Oscillation (ENSO): The term for the coupled ocean-atmosphere interactions in the tropical Pacific characterized by episodes of anomalous high sea surface temperatures in the equatorial and tropical eastern Pacific; associated with large scale swings in surface air pressure between the western and eastern tropical Pacific. These episodes recur at irregularly spaced intervals (2-7 years) and may persist for as long as 2 years (Niño=warmer water, Niña=colder water).

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Main Climate Actors4- Pacific Decadal Oscillation (PDO):

• The "Pacific Decadal Oscillation" (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months

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Main Parameters

•Seeing•Coherence Time•Precipitable Water Vapor

•Cloudiness: Observatory night reports

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Main ParametersThe Seeing

The seeingis measuredby dedicatedInstruments:

DIMM since 1988MASS since 2003

2004-09-25

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The seeingis measuredby dedicatedInstruments:

DIMM since 1988MASS since 2003

2004-10-15

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The seeing measured by DIMM compares well with UT4 active optics spot size

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Comparison with FORS Image QualityBecause its diameter is comparable to the finite outer scale of the turbulence, the

site is better for a VLT than predicted by DIMM by about 10% in the visible

FORS data, Roberto Mignani, ESO

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Comparison with ISAAC image quality

for a VLT, the site is better than predicted by DIMM by up to 30% in the IR

ISAAC data, Wolfgang Hummel, ESO

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Main ParameterThe turbulence Coherence Time, Tau0

(adaptive optics, VLTI fringe tracker)

The wavefront velocity is estimated from 200mb and ground wind velocityV0fit= Max(Vground, 0.4 V200mb )

(systematic trend observed during Paranal and Gemini balloon campaigns)

First light of SinfoniBonnet et al, The Messenger 117

Tau0=0.31 r0/ V0

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Main Parameters CLOUDS and RECIPITABLE WATER VAPOR

(Andre Erasmus, SAAO, South-Africa)Infra-red Window Channel and Water Vapour Channel at 11:45UT

on October 25, 2000Infra-Red Window (10.7µm) Water Vapor (6.7µm)

GOES Meteorological Satellites

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Main ParametersCLOUDS and PRECIPITABLE WATER VAPOR

Schematic showing a 9-pixel area centred on a potential site in plan view (left) and cross-section (right). At left, each square represents a pixel in the satellite image. At right, assuming a site altitude of 4km and a pixel resolution of ~10km, at Tropopause level (~12km), the “sky” encompassed by the 9-pixels corresponds approximately to an area of observation within 62o of zenith.

62o

4km

12km

678

5*Site1

9

432

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Main Parameters PRECIPITABLE WATER VAPOR

PWV (mm) statistics for Paranal (January - August 1998) and Chajnantor (January - September 1999) for satellite and ground-

based site monitor measurements of PWV

2.161.265.217.273rd Quartile0.770.723.664.05Median0.400.472.412.571st Quartile0.260.301.701.8310th percentile

24 hrsDayNightDayPeriod

Site MonitorSatelliteSite MonitorSatelliteSensorChajnantorParanalSite

Conclusion: The satellite-derived PWV for daytime hours gives a reliable and accurate absolute humidity measurement that is representative of daytime and nighttime moisture conditions

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Main ParametersPRECIPITABLE WATER VAPOR

Precipitable water vapor content of the atmosphere (ppwH2O) multiplied by the airmass of the standard star observation versus sensitivity. Data are shown whenever available during TIMMI2 operations in year: 2001 – 2004.R. Siebenmorgen, ESO

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Climatology of

•Cloud Cover ( )•Seeing ( )•Turbulence Coherence Time•Precipitable Water Vapor•Boundary Layer Flow

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Climatology ofThe Cloud Cover

Paranalphotometric night monthly fraction

Long term degradation?

Coupling with El Nino?

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Paranalphotometric night monthly fraction

Long term degradation(Bolivian High moving South)

Coupling with El Nino

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La Sillaphotometric night monthly fraction

Long term improvement

(Polar low moving South)


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Paranal photometric night monthly fraction

Long term degradation(Bolivian High moving South)


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Paranal-La Silladifferential

photometric night monthly fraction

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Climatology ofThe Seeing

Paranal & La Sillaseeing

Long term degradation?

Coupling with El Nino?

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Paranal seeingduring

the VLT life

Monthly Average0.91”

Monthly Median0.81” (0.66)5 percentile0.47” (0.38)

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Paranal seeing

Long term degradation

+Coupling with El

Nino

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Paranal & La Sillaseeing

Long term degradation

+Coupling with El

Nino

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Climatology ofThe turbulence Coherence Time, Tau0

Tau0=0.31 r0/ V0

Paranal Wind at 200mb

No long term climatic trend

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Tau0=0.31 r0/ V0

Paranal Wind at 200mb

No long term climatic trend

No coupling with el-nino

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Tau0=0.31 r0/ V0

Paranal yearly wind vertical

profile

The strong seasonal trends of Tau0 are due to Jet Stream variability

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Tau0=0.31 r0/ V0

Paranal

Median tau0 reaches 5ms in

summer, below 3ms the rest of the

year

80% of the dark time larger than

3ms in J-FM, only 40% in J-J-A-S

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Climatology ofPrecipitable Water Vapor

Paranal & La Silla3-hourly

PWV

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Climatology ofPrecipitable Water Vapor

Paranal & La Silla3-hourly

PWV

Winter is two times dryer than

Summer

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Looking for long term trends• The mechanism of the degradation of the seeing at Paranal is not

global but selective: the superb seeing conditions from the sea breeze (N-Westerly wind) are progressively replaced by turbulent boundary layer flow from the land (N-Easterly wind)

• This effect is amplified by the presence of other summits upwind

Study Boundary Layer Climatology

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Climatology ofBoundary Layer Flow

A MesoscaleModel can reproduce

regional patterns

MM5 analysis of the wind trajectories during bad seeing conditions at Paranal(source: Dept. of Meteorology, Univ. of Munich)

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A MesoscaleModel can reproduce

regional patterns

MM5 vertical cut along the NE wind flow during bad seeing conditions at Paranal(source: Dept. of Meteorology, Univ. of Munich)

Climatology ofBoundary Layer Flow (NCAR-NCEP re-analysis)

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ParanalWind Velocity at

700mb

Long term trend

El-Nino anti-coupling?

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Climatology of ParanalTwo lines of sight which make all the difference

prepared by m. sarazin, eso · capped above by a cold top. ... to 18 months. 8 ... site is better...

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