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
David NooneJoseph GalewskyZ. D. SharpJohn R. WordenJohn BarnesDoug BaerAdriana BaileyD. P. BrownL. E. ChristensenE. CrossonDong FengJohn V. HurleyLeah R. JohnsonM. StrongD. W. TooheyA. Van PeltJonathon S. Wright
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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.Keywords:
Mixing ratio
Diurnal cycle
Outflow
Water cycle
Mixing ratio
Precipitable water
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Ground‐based spectral line measurements of the 22.2‐GHz water vapor atmospheric emission line are used to deduce the mesospheric water vapor profile. The results generally indicate that the water vapor mixing ratio is independent of height or increases slowly in the 50‐ to 60‐km range with typical values of between 5 and 8 parts per million by volume (ppmv). (Thacker et al. (1981) give an analysis of the 1980 data sets obtained in this experiment, which indicated the existence of a pronounced layer of water vapor near 65 km. The reanalysis of these data contained in this paper, which includes the estimation and removal of spectral baselines, results in marked smoothing of the water vapor mixing ratio profile in the lower mesosphere, and the peak is much less pronounced.) Above 65 km the mixing ratio is seen to decrease rather rapidly to values of near 1 ppmv at 85 km. This decrease is far steeper than that expected from photochemical considerations. There has also been a large amount of variability observed in the water vapor profiles, especially on the day‐to‐day time scale. The water vapor retrievals have been used to estimate the vertical component of the upper mesospheric eddy diffusion coefficient ( K z ). This analysis has indicated either that vertical transport time scales in the mesosphere are perhaps an order of magnitude longer than previous studies have shown or that present understanding of the factors important in controlling the vertical distribution of water vapor in the mesosphere is inadequate.
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Water vapor burden and in situ mixing ratios above and at high altitude flight levels were inferred from observations in the rotational water vapor spectral band (19.0–35.0 µm). Flight levels ranged from 15.3 to 20.0 km during 12 meridional traverses of a W‐B57‐F jet. The method of radiance observation, while radiometric, involved emission observations rather than absorption spectra analyses employed by McKinnon and Morewood [1971]. The flights made during September, October, November 1973 and January 1974 provide profiles of lower stratospheric water vapor by overlapping tracks from 50°S latitude to 75°N latitude. The objective of the research is to describe the method of recovery of the water vapor burden and in situ mixing ratio by inference from infrared emission observations and to present the results as a latitudinal profile.
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The stratospheric and upper tropospheric water vapor levels have been measured with a new aluminum oxide sensor: Aquamax II®. This sensor is briefly described along with details of its calibration and performance. Data obtained from six balloon flights are shown. Almost all the results show a constant water vapor mixing ratio, in agreement with other data for the mid‐latitude regions. Moreover, considerable structure is seen in the present results.
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