Contribution of recycled moisture to precipitation and its influencing factors in the subalpine zone of Qilian Mountains

Recycled moisture, the part of precipitating vapor that comes from surface evapotranspiration, plays an important role in the hydrological process and the feedback of the land surface-atmosphere system. In this study, we use the δ²H and δ¹⁸O data of precipitation, plant water, and surface water systematically collected in the subalpine area of the Qilian Mountains from May to October, 2019, and assess the contribution of recycled moisture to precipitation based on the three-component isotopic mixing model. The results indicated that the main source of precipitating vapor comes from the advected vapor, and its contribution reaches up to 87.8 ± 1.6%. The contribution of local recycled moisture is 12.2 ± 1.6%, in which the proportion of transpiration and evaporation are 8.9 ± 0.6% and 3.3 ± 1.0%, respectively. In terms of the correlation between recycled moisture and its influencing factors, the temperature has a positive effect on evapotranspiration and promotes moisture recycling, while precipitation and humidity have a certain inhibition effect on evapotranspiration and reduce the contribution of recycled moisture.

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

The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long-term datasets and comprehensive modeling capabilities now available. Theories for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large-scale mixing, deep convection, and kinetic fractionation. The technologies for in-situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite- and ground-based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope-enabled microphysics schemes using higher-order moments for water- and ice-size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.

Transport of ice into the stratosphere and the humidification of the stratosphere over the 21st century

Climate models predict that tropical lower-stratospheric humidity will increase as the climate warms. We examine this trend in two state-of-the-art chemistry-climate models. Under high greenhouse gas emissions scenarios, the stratospheric entry value of water vapor increases by ~1 part per million by volume (ppmv) over this century in both models. We show with trajectory runs driven by model meteorological fields that the warming tropical tropopause layer (TTL) explains 50-80% of this increase. The remainder is a consequence of trends in evaporation of ice convectively lofted into the TTL and lower stratosphere. Our results further show that, within the models we examined, ice lofting is primarily important on long time scales - on interannual time scales, TTL temperature variations explain most of the variations in lower stratospheric humidity. Assessing the ability of models to realistically represent ice-lofting processes should be a high priority in the modeling community.

A new photolysis laser-induced fluorescence instrument for the detection of H2O and HDO in the lower stratosphere

We present a new instrument, Hoxotope, for the in situ measurement of H(2)O and its heavy deuterium isotopologue (HDO) in the upper troposphere and lower stratosphere aboard the NASA WB-57. Sensitive measurements of deltaD are accomplished through the vacuum UV photolysis of water followed by laser-induced fluorescence detection of the resultant OH and OD photofragments. The photolysis laser-induced fluorescence technique can obtain S/N>20 for 1 ppbv HDO and S/N>30 for 5 ppmv H(2)O for 10 s data, providing the sensitivity required for deltaD measurements in the tropopause region. The technique responds rapidly to changing water concentrations due to its inherently small sampling volume, augmented by steps taken to minimize water uptake on instrument plumbing. Data from the summer 2005 Aura Validation Experiment Water Isotope Intercomparison Flights (AVE-WIIF) out of Houston, TX show agreement for H(2)O between Hoxotope and the Harvard water vapor instrument and for HDO between Hoxotope and the Harvard ICOS water isotope instrument, to within stated instrument uncertainties. The successful intercomparison validates Hoxotope as a credible source of deltaD data in the upper troposphere and lower stratosphere.

Water isotope ratios D/H, 18O/16O, 17O/16O in and out of clouds map dehydration pathways

Water isotope ratios have been measured by laser absorption spectroscopy in and out of cirrus clouds formed in situ and convectively generated in anvils over subtropical regions. Water vapor in the tropical and subtropical upper troposphere shows a wide range of isotopic depletion not observed previously. The range suggests that dehydration of upper tropospheric air occurs both by convective dehydration and by gradual dehydration mechanisms. Twenty-five percent of upper tropospheric water sampled is in ice particles whose isotopic signatures are used to identify those grown in situ from those lofted from below.