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.

Stable isotopes in atmospheric water vapor and applications to the hydrologic cycle

“…Subcloud remoistening associated with partial evaporation of condensate and the so‐called isotope amount effect (see below) can yield δ values that are lower than expected for Rayleigh distillation and thus lie below the Rayleigh curve on a q − δ diagram [e.g., Noone , ]. This relationship has been used to diagnose the role of convective processes in setting the humidity upstream in remote sensing [ Samuels‐Crow et al , ] and in situ [ Galewsky and Samuels‐Crow , ] data sets.…”

“…Other sources of water vapor, such as sources from midlatitudes westerlies, can also provide their own isotopic signatures [ Tian et al , ]. Due to the specific isotopic signatures of these different water sources (Figure b), several studies have tried to use water vapor isotopic measurements to provide information on the contributions of the different water sources to the water budget [e.g., Risi et al , ; X. Lee et al , ; Galewsky and Samuels‐Crow , ]. Such studies typically combine water vapor measurements and modeling experiments using the water‐tracking capability.…”

“…Therefore, the amount effect is not a purely local phenomenon. In monsoon regions, for example, deep convection is associated with more isotopically depleted water vapor and precipitation in downstream air mass trajectories [Vuille et al, 2003;Vuille and Werner, 2005;Vimeux et al, 2005;Risi et al, 2008b;Vimeux et al, 2011;Gao et al, 2011;Tremoy et al, 2012;Samuels-Crow et al, 2014b;Galewsky and Samuels-Crow, 2015;He et al, 2015] and also with the degree of organization of the precipitation [Lekshmy et al, 2014;Pausata et al, 2011].…”

Seasonal Patterns of Water Cycling in a Deep, Continental Mountain Valley Inferred From Stable Water Vapor Isotopes

“…The recent development of laser-based instruments capable of high-resolution (e.g.,~0.1-1 Hz) measurements of ambient water vapor isotope ratios, however, has opened the possibility of relatively facile, near-continuous monitoring of water vapor isotope ratios. Early applications of measurements of water vapor isotope ratios have included investigations of moisture source directions (Galewsky & Samuels-Crow, 2015;Steen-Larsen et al, 2015;Wei et al, 2016), upwind convective intensity (Brown et al, 2008;Samuels-Crow et al, 2014), passage of frontal systems (Aemisegger et al, 2015;X. Lee et al, 2006), cloud microphysical properties (Bailey et al, 2015;Galewsky et al, 2011), boundary layer entrainment and mixing with the free troposphere He & Smith, 1999;Lai & Ehleringer, 2010;Noone et al, 2013;Sodemann et al, 2017), ET (Aemisegger et al, 2014;Good et al, 2014;Welp et al, 2008), and dewfall or frost Noone et al, 2013;Wen et al, 2012).…”

Importance of the Saharan heat low in controlling the North Atlantic free tropospheric humidity budget deduced from IASI <i>δ</i>D observations

“…Worden et al, 2007;Frankenberg et al, 2009;Risi et al, 2012;Yoshimura et al, 2014;Tuinenburg et al, 2015;Galewsky et al, 2016). This is because the water isotopologues (HDO, H 2 16 O, H 2 18 O) preferentially condenseevaporate during the phase changes of water, and therefore their isotopic ratio is sensitive to key processes of the hydrological cycle such as air mass mixing (Galewsky et al, 2007), convection (Risi et al, 2008), and transport (Galewsky and Samuels-Crow, 2015). The observation of water isotopologues in the vapour thereby provide useful information on the processes that affected the air masses downwind.…”