Water Vapor Depletion Research Articles

South Asian black carbon is threatening the water sustainability of the Asian Water Tower

Long-range transport of black carbon from South Asia to the Tibetan plateau and its deposition on glaciers directly enhances glacier melt. Here we find South Asian black carbon also has an indirect effect on the plateau’s glaciers shrinkage by acting to reduce the water supply over the southern Tibetan plateau. Black carbon enhances vertical convection and cloud condensation, which results in water vapor depletion over the Indian subcontinent that is the main moisture flux source for the southern Tibetan plateau. Increasing concentrations of black carbon causes a decrease in summer precipitation over the southern Tibetan plateau, resulting in 11.0% glacier deficit mass balance on average from 2007 to 2016; this loss rises to 22.1% in the Himalayas. The direct (accelerated melt) and indirect (mass supply decrease) effects of black carbon are driving the glacial mass decline of the so-called “Asian Water Tower”.

Rain Evaporation, Snow Melt, and Entrainment at the Heart of Water Vapor Isotopic Variations in the Tropical Troposphere, According to Large-Eddy Simulations and a Two-Column Model.

We aim at developing a simple model as an interpretative framework for the water vapor isotopic variations in the tropical troposphere over the ocean. We use large‐eddy simulations of disorganized convection in radiative‐convective equilibrium to justify the underlying assumptions of this simple model, to constrain its input parameters and to evaluate its results. We also aim at interpreting the depletion of the water vapor isotopic composition in the lower and midtroposphere as precipitation increases, which is a salient feature in tropical oceanic observations. This feature constitutes a stringent test on the relevance of our interpretative framework. Previous studies, based on observations or on models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts, rain evaporation, rain‐vapor diffusive exchanges, and mixing processes. The interpretative framework that we develop, valid in case of disorganized convection, is a two‐column model representing the net ascent in clouds and the net descent in the environment. We show that the mechanisms for depleting the troposphere as the precipitation rate increases all stem from the higher tropospheric relative humidity. First, when the relative humidity is larger, less snow sublimates before melting and a smaller fraction of rain evaporates. Both effects lead to more depleted rain evaporation and eventually more depleted water vapor. This mechanism dominates in regimes of large‐scale ascent. Second, the entrainment of dry air into clouds reduces the vertical isotopic gradient and limits the depletion of tropospheric water vapor. This mechanism dominates in regimes of large‐scale descent.

Water vapor mixing ratios and air temperatures for three martian years from Curiosity

The Mars Science Laboratory (MSL) Rover Environmental Monitoring Station humidity instrument (REMS-H) onboard the Curiosity rover is measuring daily minimum water vapor mixing ratios (min vmr), the respective pre-dawn air temperatures (T), and vmr at 2200LT. These are displayed for nearly three martian years (sols 10–2003) and compared with adsorptive column model simulations. The model was initialized with MSL-observed local column water contents, optical depths and surface pressures from sols 230–1291, assuming the same annual cycle outside this period.The first two and a half MSL years present rather similar annual cycles in the REMS-H data, whereas from about sol 1800 onward the min vmr and T suddenly increase and the 2200LT vmr values get closer to the min vmr, indicating less depletion of water vapor during the nights. Model experiments with typical regolith (ground thermal inertia of 300 SI units and porosity of 30% for adsorption) match the observed min vmr and T relatively well for the first 2.5 years. However, from about sol 1800 onward, when Curiosity started to climb onto Mt. Sharp, simulations with higher thermal inertia of about 400 SI units and very low porosity of ~0.3%, suggesting exposed bedrock, provide a far better fit. Some other periods of bedrock- and dune-dominated ground can be detected from the REMS-H vmr and air-T data along the Curiosity traverse.

Annual and diurnal water vapor cycles at Curiosity from observations and column modeling

Local column precipitable water contents (PWC) for more than a martian year from 113 Curiosity ChemCam passive-mode sky scans were used to force a column model with subsurface adsorption. ChemCam volume mixing ratios (vmr) and T, RH and vmr from REMS-H were compared with model results. The REMS-H observations point to decrease of vmr (i.e. depletion of near-surface water vapor) during every evening and night throughout the year. The model's pre-dawn results are quite similar to the REMS-H observations, if adsorption is allowed. The indicated porosity is about 30% and the night depletion ratio about 0.25. If adsorption is not allowed, RH and vmr become excessive during every night at all seasons, leading to ground frost between Ls 82°–146°; frost has not been observed. As brine formation is unlikely along the Curiosity track, adsorption thus appears to be the depleting process.During daytime the ChemCam vmr is in general close to surface values from the Mars Climate Database (MCD) vmr profiles for the Curiosity site when those profiles are scaled to match the ChemCam PWC. Our simulated daytime surface-vmr is in turn close to the ChemCam vmr when moisture is assumed well-mixed to high altitudes, whereas a low moist layer (15 km) leads to overestimates, which are worse during the warm season. Increased TES-like regional PWC also leads to large overestimates of daytime surface-vmr. Hence the crater appears to be drier than the region surrounding Gale and the results support a seasonally varying vertical distribution of moisture with a dry lower atmosphere (by Hadley circulation), as suggested by MCD and other GCM experiments.

A Model Based Analysis of Alkaline Membrane Fuel Cells

The implementation of reduced-cost catalyst materials in Anion Exchange Membrane Fuel Cells (AEMFCs) renders them an attractive alternative to Proton Exchange Membrane Fuel Cells (PEMFCs). An important challenge existing in the development of AEMFCs involves water management, which is especially problematic since in these systems (as compared to PEMFCs) water is produced in the anode and consumed in the cathode. We present a model-based analysis of AEMFC performance with an emphasis on water management. Using this approach it is possible to elucidate the relation between reduced performance and issues of hydration throughout the Membrane Electrode Assembly (MEA). We provide a brief overview of the model, validate our results against published experimental data and present results related to the impact of relative humidity, as well as the depletion of water vapor in the air flow stream, on system performance.

The Inland Penetration of Atmospheric Rivers over Western North America: A Lagrangian Analysis

Abstract Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern “high” Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (qυ). The larger initial qυ of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed υ for those initiated at 950 and 700 hPa, respectively. Inland- and interior-penetrating AR trajectories maintain large qυ over the interior partially due to increases in υ that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial qυ at the coast, limited water vapor depletion by orographic precipitation, and increases in υ over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs.

Clustering mesoscale convective systems with laser‐based water vapor δ18O monitoring in Niamey (Niger)

AbstractThe isotopic composition of surface water vapor (δv) has been measured continuously in Niamey along with the isotopic composition of event‐based precipitation (δp) since 2010. We investigate the evolution of water vapor and precipitation isotope ratios during rain events of the 2010, 2011, and 2012 monsoon periods. We establish a classification of rain systems into three types based on the δv temporal evolution. We find that 51% of rain events (class A) exhibit a sharp decrease in δ18Ov in phase with the surface air temperature drop, leading to a depletion of water vapor by −1.9‰ on average during rainfall. Twenty‐nine percent of rain events (class B) show a similar decrease in δ18Ov in phase with the temperature drop but are characterized by a progressive enrichment of the vapor in the stratiform region, resulting in a depletion of water vapor by −1.2‰ on average during rainfall. The last 20% of the rain events (class C) are associated with a progressive increase in δ18Ov during rainfall (+0.8‰). We also examine the temporal evolution of water vapor deuterium excess (dv) which shows a sharp increase as δ18Ov decreases, followed by a progressive decrease in the stratiform part for classes A and B. Using a basic box model, we examine for each class the respective roles that mesoscale subsidence and rain evaporation play on the evolution of δ18Ov. We show that those two processes are dominant for class A, whereas other processes may exert a major role on δ18Ov for classes B and C.

Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

Abstract Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November–April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon–Washington coast, a strong transition zone upwind (west) of and over the Cascade–Sierra ranges, and a broad minimum that extends from the “high” Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade–Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade–Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.

Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008

Abstract. The NASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic during the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and anthropogenic industrial pollution were sampled. Observations were spatially extensive (50–85° N and 40–130° W) and exhibit significant variability in aerosol and CCN concentrations. The chemical composition was dominated by highly oxidized organics (66–94% by volume), with a water-soluble mass fraction of more than 50%. The aerosol hygroscopicity parameter, κ, ranged between κ = 0.08–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08 ± 0.01, while the Arctic background had the highest and most variable κ of 0.32 ± 0.21, resulting from a lower and more variable organic fraction. Both fresh and aged (long-range transported) biomass burning air masses exhibited remarkably similar κ (0.18 ± 0.13), consistent with observed rapid chemical and physical aging of smoke emissions in the atmosphere, even in the vicinity of fresh fires. The organic hygroscopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (εWSOM), with a κ = 0.12, such that κorg = 0.12εWSOM. Assuming bulk (size-independent) composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all environments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an external mixture and size-dependent composition was required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predictions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing and therefore may not always be representative of the environments sampled. No correlation was observed between κorg and O : C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concentrations exceeded 1.5×104 cm−3 and introduced supersaturation depletions of ≥25%. Not accounting for supersaturation depletion in these high concentration environments would therefore bias CCN closure up to 25% and inferred κ by up to 50%.

Polydisperse aerosol condensation with heat and mass conservation II: Fluid dynamic couplings for an embedded aerosol zone

Aerosol condensational growth in a fluid field, including water vapor depletion and latent heat of condensation, is studied by a newly developed model coupling aerosol condensation with fluid dynamics. This model couples the fluid dynamics (via ANSYS FLUENT) and a time-dependent model [1] developed for nonsteady state, nonisothermal conditions. Mass and heat are effectively conserved throughout the computational domain with changes driven by aerosol condensation and evaporation. In turn, these changes promote fluid movement and its evolution. Communications between the air parcel, inside which the aerosol particles are introduced, and the outside environment are allowed by means of mass and heat transfer. Comparison between results of models with and without computational fluid dynamics (CFD) shows that the fluid movement due to aerosol condensation can have significant impact on the aerosol growth. By comparing the growth of both monodisperse and polydisperse (NH4)2SO4 particles of different number densities at various initial supersaturation conditions, particle number concentrations are shown to have great impact on the aerosol condensational growth with mass and heat exchange between the inside and outside of the air parcel.

A coupled observation – modeling approach for studying activation kinetics from measurements of CCN activity

Abstract. This paper presents an approach to study droplet activation kinetics from measurements of CCN activity by the Continuous Flow Streamwise Thermal Gradient CCN Chamber (CFSTGC) and a comprehensive model of the instrument and droplet growth. The model, which can be downloaded from http://nenes.eas.gatech.edu/Experiments/CFSTGC.html , is evaluated against a series of experiments with ammonium sulfate calibration aerosol. Observed and modeled droplet sizes are in excellent agreement for a water vapor uptake coefficient ~0.2, which is consistent with theoretical expectations. The model calculations can be considerably accelerated without significant loss of accuracy by assuming simplified instrument geometry and constant parabolic flow velocity profiles. With these assumptions, the model can be applied to large experimental data sets to infer kinetic growth parameters while fully accounting for water vapor depletion effects and changes in instrument operation parameters such as the column temperature, flow rates, sheath and sample flow relative humidities, and pressure. When the effects of instrument operation parameters, water vapor depletion and equilibrium dry particle properties on droplet size are accounted for, the remaining variations in droplet size are most likely due to non-equilibrium processes such as those caused by organic surface films, slow solute dissociation and glassy or highly viscous particle states. As an example of model application, data collected during a research flight in the ARCTAS 2008 campaign are analyzed. The model shows that water vapor depletion effects can explain changes in the observed average droplet size.

Modelling of HNO3-mediated laser-induced condensation: A parametric study

Based on both static (extended Köhler) and dynamic modelling, we investigate the influence of temperature, humidity, HNO(3) initial concentration, as well as of the particle concentration, on the efficiency of HNO(3)-mediated laser-induced condensation. This mechanism is most efficient for low temperatures, high HNO(3) concentration, and relative humidities. It is, however, still active up to 30 °C, down to 70% relative humidity, and below the ppm level of HNO(3). Furthermore, lower particle concentration minimizing the depletion of both HNO(3) and water vapor is more favourable to particle growth.

A strong control of the South American SeeSaw on the intra-seasonal variability of the isotopic composition of precipitation in the Bolivian Andes

Water stable isotopes (δ) in tropical regions are a valuable tool to study both convective processes and climate variability provided that local and remote controls on δ are well known. Here, we examine the intra-seasonal variability of the event-based isotopic composition of precipitation (δD Zongo) in the Bolivian Andes (Zongo valley, 16°20′S–67°47′W) from September 1st, 1999 to August 31st, 2000. We show that the local amount effect is a very poor parameter to explain δD Zongo. We thus explore the property of water isotopes to integrate both temporal and spatial convective activities. We first show that the local convective activity averaged over the 7–8 days preceding the rainy event is an important control on δD Zongo during the rainy season (~ 40% of the δD Zongo variability is captured). This could be explained by the progressive depletion of local water vapor by unsaturated downdrafts of convective systems. The exploration of remote convective controls on δD Zongo shows a strong influence of the South American SeeSaw (SASS) which is the first climate mode controlling the precipitation variability in tropical South America during austral summer. Our study clearly evidences that temporal and spatial controls are not fully independent as the 7-day averaged convection in the Zongo valley responds to the SASS. Our results are finally used to evaluate a water isotope enabled atmospheric general circulation model (LMDZ-iso), using the stretched grid functionality to run zoomed simulations over the entire South American continent (15°N–55°S; 30°–85°W). We find that zoomed simulations capture the intra-seasonal isotopic variation and its controls, though with an overestimated local sensitivity, and confirm the role of a remote control on δ according to a SASS-like dipolar structure.

Water Vapor Depletion in the DMT Continuous-Flow CCN Chamber: Effects on Supersaturation and Droplet Growth

The continuous-flow streamwise thermal-gradient cloud condensation nuclei counter (CFSTGC) is a commercially available instrument that is widely used for laboratory and field measurements of cloud condensation nuclei (CCN). All studies to date assume that the supersaturation profile generated in its growth chamber is not influenced by the condensation of water vapor upon the growing CCN. The validity of this assumption, however, has never been systematically explored. This work examines when water vapor depletion from CCN can have an important impact on supersaturation, measured CCN concentration, and droplet growth. A fully coupled numerical flow model of the instrument is used to simulate the water vapor supersaturation, temperature, velocity profiles, and CCN growth in the CFSTGC for a wide range of operation and CCN concentrations. Laboratory CCN activation experiments of polydisperse calibration aerosol (with a DMT CFSTGC operated in constant flow mode) are used to evaluate the simulations. The simul...

Comprehensively accounting for the effect of giant CCN in cloud activation parameterizations

Abstract. Large cloud condensation nuclei (CCN) (e.g., aged dust particles and seasalt) cannot attain their equilibrium size during the typical timescale of cloud droplet activation. Cloud activation parameterizations applied to aerosol with a large fraction of large CCN often do not account for this limitation adequately and can give biased predictions of cloud droplet number concentration (CDNC). Here we present a simple approach to address this problem that can easily be incorporated into cloud activation parameterizations. This method is demonstrated with activation parameterizations based on the "population splitting" concept of Nenes and Seinfeld (2003); it is shown that accounting for large CCN effects eliminates a positive bias in CDNC where the aerosol dry geometric diameter is greater than 0.5 μm. The method proposed here can also be extended to include the water vapor depletion from pre-existing droplets and ice crystals in global and regional atmospheric models.

Timescale analysis of aerosol sensitivity during homogeneous freezing and implications for upper tropospheric water vapor budgets

Using timescales for the generation and depletion of water vapor, we predict aerosol sensitivity in clouds formed by homogeneous freezing. Our timescale analysis explains why aerosol sensitivity increases dramatically with ice deposition coefficients (αi) ≪ 0.1, and also why aerosol sensitivity increases as vertical velocity increases, temperature decreases, aerosol number decreases, and aerosol size decreases. We combine existing in‐situ observations with adiabatic parcel modeling to constrain αi ≥ 0.1 for small ice crystals forming at high ice supersaturations. Two important implications for understanding and modeling upper tropospheric water vapor budgets emerge from our results: 1) aerosol sensitivity can be appreciable at low temperatures and moderate updrafts (∼5 cm/s) in the upper tropical troposphere, 2) reconciling our results with recent laboratory measurements supports theory that αi increases with ice supersaturation and/or decreases with ice crystal size.

Technical Note: Characterization of a static thermal-gradient CCN counter

Abstract. The static (parallel-plate thermal-gradient) diffusion chamber (SDC) was one of the first instruments designed to measure cloud condensation nuclei (CCN) concentrations as a function of supersaturation. It has probably also been the most widely used type of CCN counter. This paper describes the detailed experimental characterization of a SDC CCN counter, including calibration with respect to supersaturation and particle number concentration. In addition, we investigated the proposed effect of lowered supersaturation because of water vapor depletion with increasing particle concentration. The results obtained give a better understanding why and in which way it is necessary to calibrate the SDC CCN counter. The calibration method is described in detail and can, in parts, be used for calibrations also for other types of CCN counters. We conclude the following: 1) it is important to experimentally calibrate SDC CCN counters with respect to supersaturation, and not only base the supersaturation on the theoretical description of the instrument; 2) the number concentration calibration needs to be performed as a function of supersaturation, also for SDC CCN counter using the photographic technique; and 3) we observed no evidence that water vapor depletion lowered the supersaturation.

Response of Martian ground ice to orbit‐induced climate change

Variations in the orbit and spin axis of Mars drive climate changes that affect both surface temperatures and atmospheric water content, both of which affect the distribution of ground ice. A simple technique is presented to determine the atmospheric water content for different epochs, on the basis of the water carrying capacity of the atmosphere over surface ice. Also presented is a technique to correct the water vapor density just above the surface for depletion due to nighttime frost, reducing the effective water vapor density in contact with ground ice. Distributions of stable ground ice are generated for the present epoch with varying amounts of water vapor in the atmosphere; water vapor depletion restricts the extent of stable ground ice and ice never becomes stable at low latitudes. As the position of perihelion varies, the extent of ground ice changes several degrees in latitudinal extent. The extent of ground ice is more sensitive to obliquity; however, high obliquities are still not able to make ground ice stable at low latitudes. The finding that ice is never stable at low latitudes is consistent with the absence of ice‐related landforms, like terrain softening, at low latitudes. Correlations exist between the extents of stable ground ice and the distribution of various styles of mantle deposits.

Tropospheric Emission Spectrometer observations of the tropospheric HDO/H2O ratio: Estimation approach and characterization

We present global, vertical profile estimates of the HDO/H2O ratio from the Tropospheric Emission Spectrometer (TES) on the Earth Observing System (EOS) Aura satellite. We emphasize in this paper the estimation approach and error characterization, which are critical to determining the very small absolute concentration of HDO relative to H2O and its uncertainty. These estimates were made from TES nadir‐viewing (downlooking) thermal infrared spectral radiances observed on 20 September 2004. Profiles of HDO and H2O are simultaneously estimated from the observed radiances and a profile of the ratio is then calculated. This simultaneous, or “joint,” estimate is regularized with an a priori covariance matrix that includes expected correlations between HDO and H2O. This approach minimizes errors in the profile of the HDO/H2O ratio that are due to overlapping HDO and H2O spectroscopic lines. Under clear‐sky conditions in the tropics, TES estimates of the HDO/H2O ratio are sensitive to the distribution of the actual ratio between the surface and about 300 hPa with peak sensitivity at 700 hPa. The sensitivity decreases with latitude through its dependence on temperature and water amount. We estimate a precision of approximately 1% to 2% for the ratio of the HDO/H2O tropospheric densities; however, there is possibly a bias of approximately 5% in the ratio due to the HDO spectroscopic line strengths. These global observations clearly show increased isotopic depletion of water vapor at higher latitudes as well as increased depletion in the upper troposphere versus the lower troposphere.

Development and Test of a Cirrus Parameterization Scheme Using NCAR CCM3

Abstract Recent research has shown that depending on the cloud properties, cirrus clouds can either increase or decrease the overall heating of the earth–atmosphere system. Hence, the representation of cirrus clouds in GCMs is recognized as an important contemporary problem. In this study a new diagnostic cirrus parameterization scheme is developed with the intention of improving the simulation of cirrus macro- and microphysical properties in large-scale models. The scheme allows both large-scale motions and convective detrainment to be a source of moisture for cirrus. Water vapor depletion is calculated as diffusional growth of ice crystals with known size distributions, and the effective fallout from a model layer is estimated using mass-weighted fall velocities of the bulk precipitation. The scheme was implemented and tested with the NCAR Community Climate Model (CCM3). The seasonal means of cirrus cloud cover and ice water contents over the warm pool region, as simulated with the new cirrus parameteri...