Chajnantor Plateau Research Articles

Constraining Supersaturation and Transport Processes in a South American Cold-Air Outbreak Using Stable Isotopologues of Water Vapor

Abstract In situ measurements of water vapor isotopic composition from the subtropical Chilean Andes, supported by mesoscale model simulations and diagnostic analyses, document the processes governing the transport of dry air and isotopically depleted water vapor from the midlatitudes into the subtropics during a South American cold-air surge in July 2014. On 23 July 2014, temperatures on the Chajnantor Plateau reached −18°C, among the lowest temperatures on record for the site. On 26 July 2014, water vapor δD and δ18O reached a low of −538 ± 1.6‰ and −71.7 ± 0.2‰, among the lowest values on record. Numerical simulations show that the dynamics of the event were consistent with previous studies of South American cold-air outbreaks. Back-trajectory analyses show that the isotopically depleted water vapor that reached Chajnantor on July 26 was last saturated over the South Pacific on July 23 at 300 hPa at a temperature of about −50°C under ice supersaturation with RHice of about 110%. The water vapor traveled to Chajnantor along a nearly isentropic path following saturation. Modeling of the isotopic data require condensation at temperatures between −50°C and −53°C under supersaturation with RHice between 112% and 118%, followed by less than 25% moistening during transport. These results show that measurements of water vapor isotopic composition can provide observational constraints on in-cloud processes that influence the humidity of the subtropics.

Comment on “Record solar UV irradiance in the tropical Andes, by Cabrol et al.”

Recently Cabrol et al. (2014), hereafter C14, reported measurements of the UV Index (UVI) reaching 43.3 at a high altitude location in the Andes (Licancabur; 22.5◦ S, 67.5◦ W, 5917m a.s.l.). This value is much larger than those previously reported from other high altitude tropical locations. For example, UVI maxima reported for Mauna Loa (19.5◦ N, 155.6◦ W, 3397m a.s.l.) (Bodhaine et al., 1997; McKenzie et al., 2001), La Paz, Bolivia (16.5◦ S, 68.1◦ W, 3420m a.s.l.) (Zaratti et al., 2014), and Chile’s northern Atacama Desert (Chajnantor Plateau; 23.0◦ S, 67.8◦ W, 5100m a.s.l) (Cordero et al., 2014) are less than 26. The large UVI values measured by C14 are surprising because they would simultaneously require atmospheric conditions—specifically low total ozone amounts and cloud enhancements of UV-B radiation—that are extremely unlikely if not impossible (Seckmeyer et al., 2010). An alternative explanation of the results of C14 is that the large reported UV-B changes stem from instrument problems or data analysis issues, as will be discussed later. There are two important reasons why these measurements should be questioned and re-examined more closely:

Summertime Moisture Transport to the Southern South American Altiplano: Constraints from In Situ Measurements of Water Vapor Isotopic Composition

Abstract Austral summer transport of water vapor to the southern South American Altiplano is investigated using in situ measurements of water vapor isotopic composition collected from 1 November 2012 to 10 February 2013 on the Chajnantor Plateau in the Chilean Andes. Onset of the wet season in December was associated with an increase in mixing ratios from an average of 1500 ppmv during the winter dry season to 5400 ppmv in early December. Water vapor isotopes δD and δ18O increased from dry season averages of −235‰ and −31‰, respectively, to wet season averages of −142‰ and −17‰, reaching as high as −70‰ and −17‰, respectively. The highest water vapor δ values were close to those measured in coastal settings, suggesting little condensation during transport to the site. About 5% of the wet season data have δ values that are lower than expected for Rayleigh distillation and are associated with high relative humidity (>75%), easterly winds, and periods of low outgoing longwave radiation over the Altiplano, consistent with moistening by deep convection. The remainder of the data have δ values that are greater than expected for Rayleigh distillation, up to 250‰ above the Rayleigh curve. These data are consistent with mixing between very dry air and moist air from the boundary layer. The data show intraseasonal variability coherently linked to the position of the Bolivian high, with moist air associated with a southward displacement in the Bolivian high. The humidity over the southern Altiplano during the wet season reflects a balance among advective drying, advective moistening with little condensation, and convective moistening.

Estimating and forecasting the precipitable water vapor from GOES satellite data at high altitude sites

In this work, we describe a method to estimate the precipitable water vapor (PWV) from Geostationary Observational Environmental Satellite (GOES) data at high altitude sites. The method was applied at Atacama Pathfinder Experiment (APEX) and Cerro Toco sites, located above 5000 m altitude in the Chajnantor plateau, in the north of Chile. It was validated using GOES-12 satellite data over the range 0–1.2 mm since submillimeter/millimeter astronomical observations are only useful within this PWV range. The PWV estimated from GOES and the Final Analyses (FNL) at APEX for 2007 and 2009 show root mean square error values of 0.23 mm and 0.36 mm over the ranges 0–0.4 mm and 0.4–1.2 mm, respectively. However, absolute relative errors of 51% and 33% were shown over these PWV ranges, respectively. We recommend using high-resolution thermodynamic profiles from the Global Forecast System (GFS) model to estimate the PWV from GOES data since they are available every three hours and at an earlier time than the FNL data. The estimated PWV from GOES/GFS agrees better with the observed PWV at both sites during night time. The largest errors are shown during daytime. Short-term PWV forecasts were implemented at both sites, applying a simple persistence method to the PWV estimated from GOES/GFS. The 12 h and 24 h PWV forecasts evaluated from August to October 2009 indicates that 25% of them show a very good agreement with observations whereas 50% of them show reasonably good agreement with observations. Transmission uncertainties calculated for PWV estimations and forecasts over the studied sites are larger over the range 0–0.4 mm than over the range 0.4–1.2 mm. Thus, the method can be used over the latter interval with more confidence.

Deuterium excess in subtropical free troposphere water vapor: Continuous measurements from the Chajnantor Plateau, northern Chile

AbstractWater vapor measured continuously by cavity ring‐down spectroscopy from July 2012 to March 2013 on the hyperarid Chajnantor Plateau, northern Chile (elevation = 5080 m, pressure ≈ 550 hPa), has a mean deuterium excess (d‐excess = δD − 8*δ18O) of 46‰ ± 5‰ and frequently exceeds 100‰ at low water vapor mixing ratios (q ≤ 500 ppmv). These measurements provide empirical support for theoretical predictions of free troposphere d‐excess. The d‐excess measured at this site can be understood in terms of supersaturation with respect to ice at relative humidities between 100% and 130%, followed by mixing with moist midtropospheric or lower tropospheric air en route to the plateau. The d‐excess measured at Chajnantor is consistent with predictions for d‐excess in the upper troposphere from isotope‐enabled general circulation models and with high vapor saturation over ice in cloud‐resolving and microphysical models.

Parque Astronómico de Atacama: An Ideal Site for Millimeter, Submillimeter, and Mid-Infrared Astronomy

The area of Chajnantor, at more than 5000 m altitude in northern Chile, offers unique atmospheric and operational conditions which arguably make it the best site in the world for millimeter, submillimeter, and mid-infrared observatories. Long-term monitoring of the precipitable water vapor (PWV) column on the Chajnantor plateau has shown its extreme dryness with annual median values of 1.1 mm. Simultaneous measurements of PWV on the Chajnantor plateau (5050 m) and on Cerro Chajnantor (5612 m) show that the latter is around 36% lower under normal atmospheric conditions and up to 80% lower than the plateau in the presence of temperature inversion layers. Recently, the Government of Chile has consolidated the creation of the Parque Astronómico de Atacama (Atacama Astronomical Park), an initiative of the Chilean Commission for Science and Technology (CONICYT). This new park offers an opportunity for national and international projects to settle within its boundaries, gain access to an extremely dry site that is suitable for observations over a broad spectral range, especially in the millimeter to mid-infrared wavelengths, and benefit from operational and logistical support within a secure legal framework.

Water vapor isotopic composition of a stratospheric air intrusion: Measurements from the Chajnantor Plateau, Chile

AbstractMeasurements of water vapor isotopic composition in stratospheric air intrusions can be used to constrain the dilution of the intrusion as it mixes into the middle troposphere. The intrusion studied here occurred on 17 and 18 August 2012 with measurements obtained at an altitude of 5 km in the Chilean Andes at the Atacama Large Millimeter Array astronomical observatory on the Chajnantor Plateau. Surface ozone concentrations rose 16 ppb in 6 h and were associated with a potential vorticity intrusion on the 330 K isentropic surface. A simulated stratospheric ozone tracer reaching Chajnantor also supports the interpretation of a stratospheric intrusion. Beginning around 18:00 UTC on 17 August, the mixing ratio dropped from 3000 ppmv to 430 ppmv as the water vapor δD values dropped from −153‰ to −438‰ over 13 h while the δ18O values dropped from −20‰ to −63‰. The average mixing ratio, δD, and δ18O values during August 2012 were measured to be 1500 ppmv, −250‰, and −34‰, respectively. The minimum water vapor concentration during the intrusion was in the driest 5% of measurements made during that month, while the minimum δD and δ18O values were within the lowest 0.5% of measurements made during that month. Simple two‐component models of mixing between stratospheric or upper tropospheric air with boundary layer air fail to reproduce observations, but a three‐component mixing model, in which the stratospheric intrusion mixes with an upper tropospheric background air mass, as it mixes with boundary layer air on Chajnantor, matches the observations.

An episode of extremely low precipitable water vapour over Paranal observatory

We report on an episode of extremely low precipitable water vapour (PWV) of approximately 0.1 mm with a duration of more than 12 h at European Southern Observatory’s Paranal observatory [2635 m above sea level (asl)]. Such conditions are more commonly expected at sites at much higher altitude such as ALMA on the Chajnantor plateau (5000 m asl) or otherwise particularly dry sites such as locations in Antarctica. We provide a full account of the measurements of PWV and other relevant atmospheric parameters. An explanation of the observed conditions is given in terms of the prevailing meteorological pattern. Based on statistical evidence from measurements by VLT spectrographs (UVES and CRIRES) covering more than a decade, we find that PWV <0.2 mm can be expected on less than 1 per cent of the nights, while <0.5 mm is encountered on 6–7 nights per year (≈ 2 per cent). The scientific potential of using this small but significant fraction of observing time is illustrated in the context of service mode observing.

The world’s highest levels of surface UV

Chile's northern Atacama Desert has been pointed out as one of the places on earth where the world's highest surface ultraviolet (UV) may occur. This area is characterized by its high altitude, prevalent cloudless conditions and relatively low total ozone column. Aimed at detecting those peak UV levels, we carried out in January 2013 ground-based spectral measurements on the Chajnantor Plateau (5100 m altitude, 23°00'S, 67°45'W) and at the Paranal Observatory (2635 m altitude, 24°37'S, 70°24'W). The UV index computed from our spectral measurements peaked at 20 on the Chajnantor Plateau (under broken cloud conditions) and at 16 at the Paranal Observatory (under cloudless conditions). Spectral measurements carried out in June 2005 at the Izaña Observatory (2367 m altitude, 28°18'N, 16°30'W) were used for further comparisons. Due to the differences in sun-earth separation, total ozone column, altitude, albedo, aerosols and clouds, peak UV levels are expected to be significantly higher at southern hemisphere sites than at their northern hemisphere counterparts.

FIRST SEASON QUIET OBSERVATIONS: MEASUREMENTS OF COSMIC MICROWAVE BACKGROUND POLARIZATION POWER SPECTRA AT 43 GHz IN THE MULTIPOLE RANGE 25 ⩽ $\ell$ ⩽ 475

The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and 95GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to measure the anisotropy in the polarization of the CMB. QUIET primarily targets the B modes from primordial gravitational waves. The combination of these frequencies gives sensitivity to foreground contributions from diffuse Galactic synchrotron radiation. Between 2008 October and 2010 December, >10,000hours of data were collected, first with the 19-element 43GHz array (3458hours) and then with the 90-element 95GHz array. Each array observes the same four fields, selected for low foregrounds, together covering ~1000deg^2. This paper reports initial results from the 43GHz receiver which has an array sensitivity to CMB fluctuations of 69uK sqrt(s). The data were extensively studied with a large suite of null tests before the power spectra, determined with two independent pipelines, were examined. Analysis choices, including data selection, were modified until the null tests passed. Cross correlating maps with different telescope pointings is used to eliminate a bias. This paper reports the EE, BB and EB power spectra in the multipole range ell=25-475. With the exception of the lowest multipole bin for one of the fields, where a polarized foreground, consistent with Galactic synchrotron radiation, is detected with 3sigma significance, the E-mode spectrum is consistent with the LCDM model, confirming the only previous detection of the first acoustic peak. The B-mode spectrum is consistent with zero, leading to a measurement of the tensor-to-scalar ratio of r=0.35+1.06-0.87. The combination of a new time-stream double-demodulation technique, Mizuguchi-Dragone optics, natural sky rotation, and frequent boresight rotation leads to the lowest level of systematic contamination in the B-mode power so far reported, below the level of r=0.1

Surface measurements of upper tropospheric water vapor isotopic composition on the Chajnantor Plateau, Chile

[1] Simultaneous, real-time measurements of atmospheric water vapor mixing ratio and isotopic composition (δD and δ18O) were obtained using cavity ringdown spectroscopy on the arid Chajnantor Plateau in the subtropical Chilean Andes (elevation 5080 m or 550 hPa; latitude 23°S) during July and August 2010. The measurements show surface water vapor mixing ratio as low as 215 ppmv, δD values as low as −540‰, and δ18O values as low as −68‰, which are the lowest atmospheric water vapor δ values reported from Earth's surface. The results are consistent with previous measurements from the base of the tropical tropopause layer (TTL) and suggest large-scale subsidence of air masses from the upper troposphere to the Earth's surface. The range of measurements is consistent with condensation under conditions of ice supersaturation and mixing with moister air from the lower troposphere that has been processed through shallow convection. Diagnostics using reanalysis data show that the extreme aridity of the Chajnantor Plateau is controlled by condensation in the upper tropical troposphere.

Effects of Oxygen Supplementation on Acute Mountain Sickness Symptoms and Functional Capacity During a 2-Kilometer Walk Test on Chajnantor Plateau (5050 Meters, Northern Chile)

The aim of this study was to test the hypothesis that administration of low-flow oxygen will improve physical performance in subjects unacclimatized to altitude. We evaluated the effects of oxygen supplementation on functional capacity and acute mountain sickness (AMS) symptoms in young, healthy male and female subjects who performed a 2-km fast walk test following rapid ascent to the Chajnantor plateau (5050 m above sea level) in Northern Chile. The participants were randomly distributed into 2 groups according to oxygen supplementation levels: 1 or 3 L O(2) · min(-1). Within each group, males and females were evaluated separately. A preliminary walk test was carried out at sea level on a 100-m long, flat track with 10 U-turns. For the first walk at altitude, subjects carried the supplementary oxygen system but did not breathe the oxygen. Subjects received oxygen through a facemask the following day during the second test. The nights prior to altitude tests were spent at 2400 m in San Pedro de Atacama. Supplementary oxygen administration during a 2-km walk test significantly improved walking times at 5050 m. We also observed a significant improvement in AMS symptoms. As expected, however, performance was poorer at altitude compared to test values at sea level, despite supplementary oxygen administration. Our findings demonstrate the beneficial effects of supplementary oxygen administration on physical capacity, reducing the incidence of AMS and, thus, improving health and safety conditions for high altitude workers following rapid ascent, when adequate acclimatization is not possible.

ATLASGAL: the APEX Telescope Large Area Survey of the Galaxy

Submillimeter dust continuum emission traces high molecular column densities and, thus, dense cloud regions in which new stars are forming. Surveys of the Galactic plane in such emission have the potential of delivering an unbiased view of high-mass star formation throughout the Milky Way. The location of the APEX telescope on the Chajnantor plateau in Chile is ideally suited for mapping the inner Galaxy. ATLASGAL, The APEX Telescope Large Area Survey of the Galaxy at 870 μm, is a survey of the Galactic plane using the Large APEX Bolometer Camera (LABOCA), in the Galactic longitude and latitude ranges of ±60 and ±1.5°, respectively. This survey is providing an unbiased sample of cold dusty clumps in the Galaxy at submillimeter wavelength and a variety of molecular line follow-up observations have been started to characterize the physical and chemical conditions in the newly found clumps. Here, first results from this survey and its follow-up programs are described.

The capabilities of the calculated approach for the astroclimatic assessment in radioastronomy

The work is dedicated to calculation of daily variability of monthly averaged full vertical at-mospheric absorption for six well-known moun- tain locations of sub-millimeter wave band ra-diotelescopes obtained with usage of chosen by authors models combination. Test locations we- re defined as follows: Chajnantor plateau in the Atacama mountain desert (Chile), Hanle (India), South Pole (Antarctic), Mauna Kea (Hawaii, USA), Sierra Negra (Puebla, Mexico) and El Leoncito (Argentine). The data of these calculations were compared with the data of long term radiometric observations of other authors. Se- arching for new alternative places to complement existing sub-millimeter telescopes locations was attempted too.

Submillimeter astrophysics from Antarctica in the ALMA era

The Atacama Large Millimeter/submillimeter Array (ALMA) is currently being constructed at the 5000 m Chajnantor plateau in the Chilean Andes. ALMA has been designed and is being built to deliver transformational science in the millimeter and submillimeter regime for many years to come. We briefly describe the project status and timeline. We discuss which niches in millimeter/ submillimeter astronomy will remain open for a possible facility in Antarctica.

ATLASGAL, the APEX Telescope Large Area Survey of the Galaxy

AbstractSubmillimeter continuum emission traces high molecular column densities and, thus, dense cloud regions in which new stars are forming. Surveys of the Galactic plane in such emission have the potential of delivering an unbiased view of high-mass star formation throughout the Milky Way. Here we present the scope, current status and first results of ATLASGAL, an ongoing survey of the Galactic plane using the Large APEX Bolometer Camera (LABOCA) on the Atacama Pathfinder Experiment (APEX) telescope at the Chajnantor plateau in Chile. Aimed at mapping 360 square degrees at 870 μm, with a uniform sensitivity of 50 mJy/beam, this survey will provide the first unbiased sample of cold dusty clumps in the Galaxy at submillimeter wavelengths. These will be targets for molecular line follow-up observations and high resolution studies with ALMA and the EVLA.

Star formation throughout the Universe with the ArTeMiS bolometer imager

Astronomical observations at sub-millimetre wavelengths are limited either by the angular resolution of the telescope or by the sensitivity and field of view of the detector array. New generation of radio telescopes, such as the APEX 12-m telescope on Chajnantor plateau in Chile, can overcome these limitations if they are equipped with large detector arrays made of thousands of sensitive bolometer pixels. Instrumentation developments undertaken at CEA and based on the all silicon technology of CEA/Leti are able to provide such large detector arrays. The ArTeMiS project at CEA Saclay, in partnership with IAS, IAP and CRTBT, consists in developing a 4000-pixel bolometer camera for ground-based telescopes that operates in atmospheric windows at 200–450 μm (Talvard et al. 2006, SPIE, 6275).

ALMA, APEX and beyond

The Atacama Large Millimeter/submillimeter Array (ALMA) is currently being constructed at the 5000 m Chajnantor plateau in the Chilean Andes. ALMA has been designed and is being built to deliver transformational science in the millimeter and submillimeter regime for many years to come. We briefly describe the project status and timeline. The Atacama Pathfinder Experiment (APEX), built at the same site, is already operational and proves to be an effective survey instrument. We discuss which niches in millimeter/submillimeter astronomy will remain open for a possible facility in Antarctica.

Infrared and Submillimeter Atmospheric Characteristics of High Antarctic Plateau Sites

ABSTRACTThe best ground‐based astronomical sites in terms of telescope sensitivity at infrared and submillimeter wavelengths are located on the Antarctic Plateau, where high atmospheric transparency and low sky emission are obtained because of the extremely cold and dry air; these benefits are well characterized at the South Pole station. The relative advantages offered by three potentially superior sites, Dome C, Dome F, and Dome A, located higher on the Antarctic Plateau, are quantified here through the development of atmospheric models using the line‐by‐line radiative transfer model code. In the near‐ to mid‐infrared, sensitivity gains relative to the South Pole of up to a factor of 10 are predicted at Dome A, and a factor of 2 for Dome C. In the mid‐ to far‐infrared, sensitivity gains relative to the South Pole up to a factor of 100 are predicted for Dome A and 10 for Dome C. These values correspond to even larger gains (up to 3 orders of magnitude) compared to the best mid‐latitude sites, such as Mauna Kea and the Chajnantor Plateau.