Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory

Abstract

[1] Water vapor in the subtropical troposphere plays an important role in the radiative balance, the distribution of precipitation, and the chemistry of the Earth's atmosphere. Measurements of the water vapor mixing ratio paired with stable isotope ratios provide unique information on transport processes and moisture sources that is not available with mixing ratio data alone. Measurements of the D/H isotope ratio of water vapor from Mauna Loa Observatory over 4 weeks in October-November 2008 were used to identify components of the regional hydrological cycle. A mixing model exploits the isotope information to identify water fluxes from time series data. Mixing is associated with exchange between marine boundary layer air and tropospheric air on diurnal time scales and between different tropospheric air masses with characteristics that evolve on the synoptic time scale. Diurnal variations are associated with upslope flow and the transition from nighttime air above the marine trade inversion to marine boundary layer air during daytime. During easterly trade wind conditions, growth and decay of the boundary layer are largely conservative in a regional context but contribute ∼12% of the nighttime water vapor at Mauna Loa. Tropospheric moisture is associated with convective outflow and exchange with drier air originating from higher latitude or higher altitude. During the passage of a moist filament, boundary layer exchange is enhanced. Isotopic data reflect the combination of processes that control the water balance, which highlights the utility for baseline measurements of water vapor isotopologues in monitoring the response of the hydrological cycle to climate change. Citation: Noone, D., et al. (2011), Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory,

Airborne in situ vertical profiling of HDO / H<sub>2</sub><sup>16</sup>O in the subtropical troposphere during the MUSICA remote sensing validation campaign

“…This suggests that a considerable fraction of the water content is originating from processes other than slow ascent and dehydration of boundary layer air. A model based on the method described by Noone et al (2011) that describes mixing of moist air typical of the MBL (H 2 O = 22 900 ppmv, δD = −75.6 ‰) and dry air from the upper troposphere (UT; H 2 O = 1100 ppmv, δD = −385 ‰) excellently fits our observations (solid line (4) in Fig. 11).…”

Characterizing moisture exchange between the Hawaiian convective boundary layer and free troposphere using stable isotopes in water

“…Nor was it consistent with modified distillation expected during moist convection (refer to Figure 5). Instead, large variations along the x axis in the isotopic mixing diagram (Figure 10) point to a probable role for large-scale transport, which previous studies have argued controls the isotopic composition of the free troposphere near Hawaii to first order [Galewsky et al, 2007;Noone et al, 2011;Hurley et al, 2012].…”

“…The trade wind temperature inversion typically limits convective activity on Hawaii's east side [Price and Pales, 1963;Garrett, 1980]; however, Noone et al [2011] observed strong convective activity when regional winds shifted to southwesterly. Upper air radiosondes Figure 13.…”

“…[18] Due to differences in the saturation vapor pressures of heavy and light isotopologues of water [Bigeleisen, 1961], the relationship between the isotope ratio and observed vapor mixing ratio provides a valuable diagnostic for moisture fluxes [Gat, 1996;He and Smith, 1999;Noone et al, 2011] and a means to distinguish moistening and dehydrating mechanisms [Worden et al, 2007;Noone, 2012]. When conditions are unsaturated, both q and the isotope ratio are conserved variables, and airmass mixing will cause observations to fall along a line in δ-1/q space [e.g., He and Smith, 1999;Noone et al, 2011].…”

“…When conditions are unsaturated, both q and the isotope ratio are conserved variables, and airmass mixing will cause observations to fall along a line in δ-1/q space [e.g., He and Smith, 1999;Noone et al, 2011]. In contrast, if an air mass experiences a pseudoadiabatic process, in which condensate is immediately removed as precipitation, its δ value follows a theoretical Rayleigh distillation [e.g., Dansgaard, 1964].…”