Water Vapor Gradients Research Articles

The poorly-explored stomatal response to temperature at constant evaporative demand.

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

Moisture swing frequency response method for characterization of ion-transport kinetics of CO2 adsorption

Direct air capture of CO2 (DAC) sufficiently mitigates atmospheric CO2 to prevent the impacts of global warming of 1.5 °C above preindustrial levels. To overcome the challenges of the kinetics of sorbent under the ultra-low partial pressure (40 Pa) of DAC, the underlying mechanisms need to be constructed for better designs, simulations, and developments of the separation processes of carbon dioxide. Herein, a transient model based on the diffusion-reaction of ions at the molecular scale is developed for the moisture swing adsorption (MSA) process, disclosing the mechanism of mass transfer of multi-ions of sorbents. To compare the model with the experiment quantitatively, Fourier-transformed moisture-swing frequency response is applied to accurately measure the H2OCO2 concentration response, ensuring a systematic approach for unknown kinetic parameters for the model. The results reveal that the gradient of water vapor causes a counter gradient of CO2 concentration, generating the spontaneous transportation of CO2 of the MSA membrane from one side to another. Specifically, the diffusion coefficient of HCO3− drives the CO2 adsorption process predominantly, where the diffusion coefficient of HCO3− increases about ten times, leading to a nearly 12 times enhanced CO2 separation rate accordingly. Notably, CO2 adsorption kinetics can be stimulated by controlling specific ion conductivity in the moisture swing sorbent. With the enhancement of adsorption kinetics and low capital cost, the progress of CO2 mitigation using Moisture Swing Adsorption can be achieved for direct air capture of CO2.

Moisture-driven CO2 pump for direct air capture

Direct Air Capture, the chemical separation of carbon dioxide (CO2) from air, is considered a necessary approach to generate negative carbon emissions and in turn limit global warming, though energetic and monetary costs must continue to decline to meet the needed gigaton scale. The promise of energy-efficient and continuous carbon dioxide membrane separation has only recently been investigated with the development of facilitated chemical transport and novel electrochemically driven methods. This work demonstrates the potential for continuous pumping of CO2 from air using commercial anion exchange membranes driven solely by a water vapor gradient. A reactive transport model that couples ionic transport with moisture sensitive bicarbonate chemistry in charged polymers, also termed moisture swing sorption, was developed alongside experimentally measured equilibrium and transport properties of CO2 and water vapor in Fumasep FAA-3 to determine the mass transport limits of such a system. The results show that the commercial membrane is kinetically limited by the moisture sensitive chemistry, enabling a CO2 current of 1.1 μmol/m2∙ s. In diffusion limited materials, a CO2 pumping flux can reach 11 μmol/m2∙ s in and peak performance is expected with a dry air relative humidity around 50% pumping into a saturated water vapor permeate. In all configurations, water loss becomes the major cost in such a separation and must be managed through optimal polymer design or water recovery mechanisms. The work demonstrates the applicability of a new driving force, the evaporation of water, to meet the energetic demand of CO2 separation from air.

Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells.

We present a robust estimation of the CO2 concentration at the surface of photosynthetic mesophyll cells (cw ), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments. We introduce calculations to estimate cw using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances. The cw was lower than adaxial and abaxial estimated intercellular CO2 concentrations (ci ). Differences between cw and the ci of each surface were usually larger than 10 μmol mol-1 . Differences between adaxial and abaxial ci ranged from a few μmol mol-1 to almost 50 μmol mol-1 , where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial ci and the ci estimated by mixing both fluxes ranged from -30 to +20 μmol mol-1 , where the largest differences were found under high ASD or high ambient CO2 concentrations. Accounting for cw improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO2 and water vapor gradients within the leaf.

Transepidermal water loss and stratum corneum hydration in forearm versus hand palm.

Skin measurements of transepidermal water loss (TEWL) and stratum corneum hydration (SCH) reflect different aspects of skin physiology. Since epidermal water loss depends on epidermal-to-air water vapor gradients, a possible quantitative relationship between TEWL and SCH may exist. This investigation's purpose was to test the possible TEWL-SCH relationship. SCH and TEWL were measured noninvasively on forearm and palmer thenar eminence (hand) in 40 young adults (20 males) along with total body fat percentage (FAT) via bioimpedance. A significant positive nonlinear correlation (p<0.001) was detected between SCH and TEWL in hands of the male cohort that occurred when SCH exceeded a threshold level. This threshold level was not exceeded in male or female forearms and forearms did not display a SCH-TEWL correlation. There was a weak inverse dependence of TEWL on FAT on both forearm and hand (p<0.05), but no SCH-FAT relationship was observed. TEWL values on the forearm and hand were moderately correlated with each other (p=0.002) but SCH values were not. The findings clarify the relationship between forearm and palmer hydration and TEWL values, and their relationship to total body fat percentages in young healthy adults. The significant correlation between palmer stratum corneum hydration and palmer TEWL that was discovered in the male but not the female cohort suggests a threshold hydration level for which TEWL depends both on skin barrier function and stratum corneum hydration. This implies that conditions with increased SCH may in part account for elevated TEWL values.

Dry air valley in the upper troposphere over the eastern Mediterranean-western Tibetan Plateau

Based on AIRS satellite observations and ERA-Interim reanalysis data, we investigated the seasonal evolution of the dry air valley (DAV) in the upper troposphere over regions from the eastern Mediterranean to the western Tibetan Plateau (EM-WTP). We find that monsoon-related descending motion is the main driver for the DAV, and its contribution to the DAV is 61%. The intensity of DAV increases rapidly in June, reaches its maximum in late July and early August, then weakens gradually until it disappears in early October. From May to July, zonal temperature gradient over the Mediterranean Sea increases and the southerly wind in the western South Asia High enhances, resulting in an enhancement of descending motion over the Mediterranean. Meanwhile, the northerly wind and zonal vorticity gradient over south of the Aral Sea strengthen due to the thermal-dynamical effect of the Tibetan-Iranian Plateau, consequently, descending motion over south of the Aral Sea intensifies. From August to September, the descending motion weakens quickly over the Mediterranean but remains nearly steady over south of the Aral Sea. As a result, the east-west water vapor gradient increases over the DAV, and water vapor in the DAV converges horizontally due to the transport of westerlies. Accordingly, the DAV intensity decreases. Apart from the descending motion, the dry air intrusion from the lower stratosphere associated with tropopause fold events also has an impact on the DAV intensity. However, the contribution of the tropopause fold events to the DAV is rather small, no >10%.

Estimation of the Surface Fluxes for Heat and Momentum in Unstable Conditions with Machine Learning and Similarity Approaches for the LAFE Data Set

Measurements of three flux towers operated during the land atmosphere feedback experiment (LAFE) are used to investigate relationships between surface fluxes and variables of the land–atmosphere system. We study these relations by means of two machine learning (ML) techniques: multilayer perceptrons (MLP) and extreme gradient boosting (XGB). We compare their flux derivation performance with Monin–Obukhov similarity theory (MOST) and a similarity relationship using the bulk Richardson number (BRN). The ML approaches outperform MOST and BRN. Best agreement with the observations is achieved for the friction velocity. For the sensible heat flux and even more so for the latent heat flux, MOST and BRN deviate from the observations while MLP and XGB yield more accurate predictions. Using MOST and BRN for latent heat flux, the root mean square errors (RMSE) are 107 Wm^{-2} and 121 Wm^{-2}, respectively, as well as the intercepts of the regression lines are approx 110 Wm^{-2}. For the ML methods, the RMSEs reduce to 31 Wm^{-2} for MLP and 33 Wm^{-2} for XGB as well as the intercepts to just 4 Wm^{-2} for MLP and -1 Wm^{-2} for XGB with slopes of the regression lines close to 1, respectively. These results indicate significant deficiencies of MOST and BRN, particularly for the derivation of the latent heat flux. In fact, in contrast to the established theories, feature importance weighting demonstrates that the ML methods base their improved derivations on net radiation, the incoming and outgoing shortwave radiations, the air temperature gradient, and the available water contents, but not on the water vapor gradient. The results imply that further studies of surface fluxes and other turbulent variables with ML techniques provide great promise for deriving advanced flux parameterizations and their implementation in land–atmosphere system models.

Water Vapor Flux‐Profile Relationship in the Stable Boundary Layer Over the Sea Surface

AbstractComprehensive marine atmospheric turbulence observation data, meteorological sounding and sea surface conditions in the South China Sea were employed to analyze and parameterize the vapor profile in the stable marine atmospheric boundary layer. The observations involved a three‐dimensional ultrasonic anemometer, water vapor carbon dioxide analyzer, radiosonde, and buoy. This paper theoretically determined that the water vapor profile function φq differs from the temperature profile function φh and that φq should be independently parameterized. A linear relationship existed between the dimensionless water vapor gradient and stability parameters based on the observation results, and φq was then obtained as φq(z/L) = az/L + b, in which the stability covered the stability range (z/L &gt; 1). This result was applied in the Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment bulk flux algorithm, and the simulation of the latent heat flux was improved.

The effect of gas diffusion layer on electrochemical effective reaction area of catalyst layer and water discharge capability

A gas diffusion layer (GDL) is one of the main components of a proton exchange membrane fuel cell (PEMFC), which helps discharge the water generated by the electrochemical reaction and promotes the diffusion of reactant gases. The GDL typically consists of a microporous layer (MPL) and a substrate, and the MPL is mainly composed of carbon black and PTFE (polytetrafluoroethylene). In this study, to optimize the effective reaction area of the catalyst layer by increasing the specific surface area of the MPL and improve the water discharge capability of GDL by increasing the porosity of the MPL, the GDLs are manufactured by varying the manufacturing methods of the MPL. The MPL design parameters considered in this study are the particle size of carbon black, the addition of smaller carbon black particles to the MPL, and the number of cracks in the MPL. The performance tendencies are quantitatively identified through polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, electrochemical surface area, liquid water, water vapor, gas permeabilities, and capillary pressure gradient. The results showed that the MPL has a dominant effect on the electrochemical surface area (ECSA) of the catalyst layer and the mass transport resistance of the PEMFC.

Comprehensive Study on the Tropospheric Wet Delay and Horizontal Gradients during a Severe Weather Event

GNSS meteorology is today one of the most growing technologies to monitor severe weather events. In this paper, we present the usage of 160 GPS reference stations over the period of 14 days to monitor and track Hurricane Harvey, which struck Texas in August 2017. We estimate the Zenith Wet Delay (ZWD) and the tropospheric gradients with 30 s interval using TOMION v2 software and carry out the processing in Precise Point Positioning (PPP) mode. We study the relationship of these parameters with atmospheric variables extracted from Tropical Rainfall Measuring Mission (TRMM) satellite mission and climate reanalysis model ERA5. This research finds that the ZWD shows patterns related to the rainfall rate and to the location of the hurricane. We also find that the tropospheric gradients are correlated with water vapor gradients before and after the hurricane, and with the wind and the pressure gradients only after the hurricane. This study also shows a new finding regarding the spectral distribution of the gradients, with a clear diurnal period present, which is also found on the ZWD itself. This kind of study approaches the GNSS meteorology to the increasing requirements of meteorologist in terms of monitoring severe weather events.

Spatio-Temporal Variations of Precipitable Water Vapor and Horizontal Tropospheric Gradients from GPS during Typhoon Lekima

GPS data during Typhoon Lekima at 700 stations in China were processed by the Precise Point Positioning (PPP) method. A refined regional Tm model was used to derive the precipitable water vapor (PWV) at these GPS stations. Spatio-temporal variations of PWV with the typhoon process were analyzed. As the typhoon approached, PWV at stations near the typhoon center increased sharply from about 50 mm to nearly 80 mm and then dropped back to about 40–50 mm as the typhoon left. Comparisons of GPS, radiosonde, the Global Data Assimilation System (GDAS) Global Forecast System (GFS) analysis products and ERA5 reanalysis products at four matched GPS-RS stations show overall overestimations of PWV from radiosonde, GFS and ERA5 compared with GPS in a statistical perspective. An empirical orthogonal functions (EOF) analysis of the PWV during the typhoon event revealed some different patterns of variability, with both the first EOF (~36.1% of variance) and second EOF (~30.3% of variance) showing distinctively large anomalies over the typhoon landing locations. The typhoon caused a large horizontal tropospheric gradient (HTG) with the magnitude reaching 5 mm and the direction pointing to the typhoon center when it made a landfall on mainland China. The magnitude and the consistency of the HTG direction decreased overall as the typhoon weakened.

Modelling of effects of water vapor and temperature gradient on moisture and gas transfer in unsaturated landfill cover

Modelling of effects of water vapor and temperature gradient on moisture and gas transfer in unsaturated landfill cover

The competing effects of terrestrial evapotranspiration and raindrop re-evaporation on the deuterium excess of continental precipitation

The deuterium excess (d-excess) of precipitation, which tracks kinetic fractionations during water phase changes, has been used to trace the regions and conditions of oceanic moisture sources, in particular from polar ice-core records. Still, many observations suggest that precipitation d-excess varies significantly across terrestrial environments, both above and below the global average value 10. These variations are often interpreted to reflect either moisture recycling via terrestrial evapotranspiration or sub-cloud raindrop re-evaporation, respectively. Despite being frequently mentioned in literature, however, little work has been carried out to quantify these two competing effects on the widespread variations of d-excess. Here, we use a one-dimensional model of water vapor transport to interrogate the relative controls on d-excess of continental precipitation. We show that when the water vapor gradient is coupled with decreasing temperature, d-excess increases with net rainout and δ18O depletion along the model transect, while the magnitude of increase is controlled by the water balance, evaporation/transpiration ratio, and transport type. Raindrop re-evaporation functions as an additional flux of recycled moisture and further increases the d-excess downwind. Alternatively, when the water vapor gradient is coupled with decreasing relative humidity, d-excess may decrease along the model transect wherein upwind evapotranspiration is overwhelmed by local raindrop re-evaporation effects. This local effect becomes even stronger under a regime of turbulent eddy transport with high transpiration fractions, resulting in a pronounced decrease of d-excess without notable changes in δ18O. Finally, we demonstrate that model processes capture the isotopic variations in precipitation across the altitudinal gradient of the Andes as well as the South American low-level jet zone. Broadly, this study presents a novel framework for understanding the dynamical controls of precipitation d-excess and for linking spatial isotopic variations with ecohydrological fluxes and processes in both modern and paleo-environments.

Sunlight-absorbing aerosol amplifies the seasonal cycle in low-cloud fraction over the southeast Atlantic

Abstract. The mean altitude of the smoke loading over the southeast Atlantic moves from the boundary layer in July to the free troposphere by October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass burning aerosol loading at Ascension Island (8∘ S, 14.5∘ W) from July to October, based on island measurements, satellite retrievals, and reanalysis. In July and August, the smoke loading predominantly varies within the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol–cloud interactions explain the cloudiness changes. September marks a transition month during which midlatitude disturbances can intrude into the Atlantic subtropics, constraining the free tropospheric aerosol closer to the African coast. Stronger boundary layer winds on cleaner days help deepen, dry, and cool much of the marine boundary layer compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. The September free troposphere is better mixed on smoky days compared to October. Longwave cooling rates, generated by a sharp water vapor gradient at the aerosol layer top, encourage a small-scale vertical mixing that could help maintain the well-mixed smoky September free troposphere. The October meteorology primarily varies as a function of the strength of the free tropospheric winds advecting aerosol offshore. The free tropospheric aerosol loading is less than in September, and the moisture variability is greater. Low-level clouds increase and are more stratiform in October when the smoke loadings are higher. The increased free tropospheric moisture can help sustain the clouds through a reduction in evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region, increasing cloud droplet number concentrations, may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free tropospheric winds at Ascension Island supports slightly higher cloud tops during smokier conditions. Overall, the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology. This is climatically important, as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

A Model for Adsorption and Diffusion in Water Vapor Barrier Films

A model for describing the diffusion behavior of water vapor in barrier films is developed and applied to the cases of 1) pure polyethylene terephthalate (PET); 2) an inorganic thin film (ITF) with defects on a PET substrate (ITF/PET); and 3) PET/ITF/PET. The normalized concentration profile and the relative mass of water vapor in the ITF/PET and PET/ITF/PET barrier films are simulated as a function of diffusion time by considering the sorption and desorption of water vapor. The number of atoms penetrating the barrier films is quantitatively evaluated by the concentration gradient of water vapor, which is related to the water vapor transmission rate (WVTR). The simulated WVTR values in pure PET, PET/SiOx, and PET/SiOx/ORM layered structures are in good agreement with the experimental ones.

Tunable self-jumping of melting frost on macro-patterned anisotropic superhydrophobic surfaces

Controllable fast self-removal of frost pieces from surfaces can effectively reduce energy consumption and residual droplets during defrosting. In this work, a tuning strategy of melting frost directional self-jumping on regular macro-patterned anisotropic superhydrophobic surface (MASS) was proposed. Under high-rate cooling condition, patterned frost on MASS with significantly different thickness distribution was observed, due to the water vapor gradient in and out of the deep grooves. During defrosting process, L-shaped melting frost on MASS shrank independently with two arms retracting inward, which further caused the melting frost's centroid/mass center to shift out of the pillar top. Partial contact with pillar tops and circular fillet introduced directional lifting force and enabled the directional self-jumping of the melting frost. Influencing factors on directional self-jumping of melting frost on MASS were also discussed. Utilizing macro-patterned anisotropic structure, the frost splitting process and its jumping direction were both tuned, which could promote the self-jumping of melting frost and thus improve the efficiency of defrosting.

A new concept for generating mechanical work from gas permeation

Large amounts of energy are available from gas concentration gradients in thermal power plant exhaust. For example, the energy potential of carbon dioxide and water vapor gradients from flue gas and cooling towers of coal-fired and natural gas-fired power plants represents 1–2% of the plant's overall capacity. In this paper, we introduce a novel membrane-based gas permeation process for using concentration gradients of gas mixture to generate useful mechanical work in the form of compressed gas flow. The concept is experimentally validated using a simple binary gas mixture of nitrogen and water vapor with commercially available polydimethylsiloxane membranes. Power density from water vapor flux of 57 mW/m2 and power density from net gas flux of 11.5 mW/m2 are observed under relatively conservative laboratory test conditions. Fundamental transport dynamics are modelled and used to simulate the response of the system to membrane properties and feed concentrations, and up to 600 mW/m2 is reported for the cases considered. The possibility of recovering energy from exhaust gases may open up new opportunities for improving power plant efficiency and sustainability.

Modeling Arctic Boundary Layer Cloud Streets at Grey-zone Resolutions

To better understand how model resolution affects the formation of Arctic boundary layer clouds, we investigated the influence of grid spacing on simulating cloud streets that occurred near Utqiagvik (formerly Barrow), Alaska, on 2 May 2013 and were observed by MODIS (the Moderate Resolution Imaging Spectroradiometer). The Weather Research and Forecasting model was used to simulate the clouds using nested domains with increasingly fine resolution ranging from a horizontal grid spacing of 27 km in the boundary-layer-parameterized mesoscale domain to a grid spacing of 0.111 km in the large-eddy-permitting domain. We investigated the model-simulated mesoscale environment, horizontal and vertical cloud structures, boundary layer stability, and cloud properties, all of which were subsequently used to interpret the observed roll-cloud case. Increasing model resolution led to a transition from a more buoyant boundary layer to a more shear-driven turbulent boundary layer. The clouds were stratiform-like in the mesoscale domain, but as the model resolution increased, roll-like structures, aligned along the wind field, appeared with ever smaller wavelengths. A stronger vertical water vapor gradient occurred above the cloud layers with decreasing grid spacing. With fixed model grid spacing at 0.333 km, changing the model configuration from a boundary layer parameterization to a large-eddy-permitting scheme produced a more shear-driven and less unstable environment, a stronger vertical water vapor gradient below the cloud layers, and the wavelengths of the rolls decreased slightly. In this study, only the large-eddy-permitting simulation with gird spacing of 0.111 km was sufficient to model the observed roll clouds.

Analysis of GNSS sensed precipitable water vapour and tropospheric gradients during the derecho event in Poland of 11th August 2017

Nowadays, one of the techniques that meets the increasing requirements of meteorologist in terms of monitoring of severe weather events is global navigation satellite systems (GNSS), which can provide information about the tropospheric state independently of the weather conditions and even in real time. In this paper we present the usage of GNSS sensed tropospheric data to monitor sudden and intense weather events. Our analyses were performed on the example of a derecho event in Poland of 11th August 2017. We used GPS/GLONASS observations from 278 GNSS reference stations located in Poland and, using Bernese GNSS Software ver. 5.2, we estimated the zenith tropospheric delays (ZTD) and tropospheric gradients with 5 and 15-min intervals respectively. Next, using meteorological data from the synoptic stations, we estimated the precipitable water vapour (PWV) which gives information about the amount of water vapour in the atmosphere. By applying a dense network of GNSS receivers, we were able to create the PWV, rate of the PWV (ROP), and the tropospheric gradients maps, which in turn allowed us to monitor the derecho event. We also conducted case studies for the selected GNSS stations where a comparison between PWV, reflectivity, and microwave radiometer were performed. During the main phase of the event we obtained the maximum value of PWV equal to 52.1 mm at 20:30 UTC. We also acquired high agreement between the PWV/ROP maps and reflectivity derived from meteorological radars. This applies to both the direction and speed of the phenomena as well as the time and place of the occurrence of the main phase of the derecho. This was possible only owing to the use of data derived from the dense network of the GNSS receivers.

Numerical investigation of the effect of moisture on buoyancy-driven low turbulence flow in an enclosed cavity

This paper reports a numerical investigation of low turbulence buoyancy-driven flow of moist air and heat transfer inside a rectangular cavity with differentially heated vertical walls. The variations of the flow, temperature and moisture inside the cavity has been analysed together with heat transfer coefficients for a range of mass fraction of water vapour and temperature gradients between the vertical walls of the cavity. The accuracy of the numerical methodology was also scrutinised by conducting a rigorous validation study of benchmark experimental data for the average Nusselt number for similar buoyancy driven cavity flow.The results of this investigation showed that during the natural convection process, the change in moisture content in the moist air has a significant influence on the flow and temperature fields inside the enclosure and the variation of the vertical wall temperature gradients have also shown to affect the moisture concentration inside the cavity. The percentage change in the average heat transfer varied significantly depending on the mass fraction of moisture in the air and the temperature gradient between the vertical walls. The results also showed a 3.5% increase in the average heat transfer for every 0.02 kg/kg increment in the mass fraction of water vapour. The findings from the study are significance to scientists and practitioners who are responsible for moisture management in enclosed space at the design threshold and for optimisation of energy used in buildings.