Liquid Water Flux Research Articles

Drying model and drying characteristic analysis of multiphase porous medium for flake materials

The main purpose of this paper is to optimize the drying process and gain a fundamental understanding of the movement of different phases during the drying process of cigarette flakes. Therefore, the drying model of a multiphase porous medium for cigarette flakes has been established in this paper. Binary diffusion of water vapor and gas pressure transport, as well as capillary diffusion of liquid water and gas pressure transport, are considered in this model. The non-equilibrium equations are used to calculate the evaporation rate, and the fluxes of water vapor and liquid water are also obtained. The results indicated that the stochastic stacking model is suitable for studying cigarette flake drying. Because water evaporation absorbs more heat than hot air, the temperature of the cigarette flakes decreases during the drying process. The water activity, vapor density, and vapor mole fraction of the medium layer of cigarette flakes were significantly higher than those of the upper and lower layers. The highest water vapor and liquid water fluxes occur in the medium of the drying process. Additionally, it was observed that the saturated vapor pressure was higher than the equilibrium vapor pressure and steam pressure, non-equilibrium evaporation occurs in the medium of drying. The correlation between the evaporation rate and the drying temperature and humidity of the cigarette flakes was found to be positive.

Design and Performance Evaluation of a Specialized Bubbler System for Accurate and Repeatable Solid Oxide Electrolysis Cell (SOEC) Testing

Accurate assessment of Solid Oxide Electrolysis Cell (SOEC) performance demands standardized testing conditions, particularly in stable steam delivery. Fluctuations in steam concentration and flow rate can significantly impact measured open-circuit voltage (OCV) and polarization behavior. Although direct steam injection is straightforward, it is often hindered by instability resulting from intermittent liquid water flux, pressure fluctuations, and fractional distillation effects. Bubbler humidification systems have the potential to offer consistent steam, but tailored designs for SOEC testing are currently absent.This study introduces a specialized bubbler system designed to deliver stable steam to SOEC test stands accommodating both oxygen-conducting (O-SOEC) and proton-conducting (H-SOEC) cells. The bubbler components include a refill water reservoir, a level control module, heating elements, pressure relief valves, check valves, tubing, and fittings integrated with the test stand. The system humidifies the sweep gas, transporting steam to the cell test fixture situated within a high-temperature furnace.The design process involved meticulous considerations of materials compatibility, hydrogen safety, and overtemperature protection. All wetted components employ stainless steel, glass, or PTFE materials in their construction construction. Redundant thermocouples ensure safe process temperatures with automated overtemperature shut off. The refill reservoir and bubbler headspace operate near ambient pressure, facilitated by an exhaust line vent sized for minimal backpressure.In various executed O-SOEC tests, the bubbler maintained OCV stability within ±5 mV over several hundred hours for each cell operating at 750°C, eliminating spikes caused by steam fluctuations observed with direct injection methods. This facilitates precise electrochemical analysis and determination of area-specific resistance losses. Polarization curves align with expected behavior based on the Nernst equation. The system has demonstrated safe and stable functionality for over thousands of hours across various operating conditions.The bubbler design presents researchers with notable advantages over existing steam supply options for SOEC test stands, addressing performance, safety, and flexibility concerns. Online instrumentation enables closed-loop control of steam conditions, enhancing stability. Standardization of test equipment is crucial for quality results, and this work contributes progress toward reproducible protocols in the international SOEC community. The modular bubbler approach, integrated with safety features, also signifies safer methodology choices for lab-scale experimental research.

Impact of Meteorological Factors on the Wire Icing Thickness and Growth Rate in Mountain Areas under Dry and Wet Growth Patterns

Wire icing events pose a significant threat to the southern power grid’s transmission lines in China. Fifteen such events were identified from 2018 to 2020 on the Guilin-Haiyang Mountain transmission line. Hourly measurements of ice thickness and concurrent meteorological data were analyzed using the Makkonen model’s freezing rate formula to categorize the events into distinct growth patterns: dry and wet. The relationship between wire icing and meteorological factors across different micro-topography (windward slope, leeward slope, and pass) was further explored. Several key conclusions can be drawn. First, the altitude is positively correlated to the icing thickness, but relatively independent of the icing rate; however, such independence between the icing rate and altitude cannot be interpreted by the negative correlation of altitude with temperature and the positive relationship between wind speed and liquid water content. Second, a pronounced connection of the icing rate with meteorological factors is not shown until the wet and dry patterns are separated. Notably, the correlations differ between these two patterns, with icing rate being negatively correlated with temperature for the wet growth process, but positively correlated with wind speed and liquid water content for the dry growth process. Third, both wet and dry growth processes exist across the icing events. A shift from wet to dry growth was evident with increasing altitude. At the mountain’s base, wet growth predominates, with the icing rate determined by the temperature close to the freezing point, whereas the higher temperature and lower liquid water flux account for the shorter wire icing duration, lower icing rate, and thus the thinner icing thickness at the leeward slope compared to the windward slope at a similar altitude. This study sheds light on the variations in icing rates under different micro-topographies and the underlying physical mechanisms governing icing growth patterns and provides a much-needed understanding of these distinct growth processes on the development of a more sophisticated predictive model for conductor icing.

Pioneering the preparation of porous PIM-1 membranes for enhanced water vapor flow.

In this study, porous polymers of intrinsic microporosity (PIM-1) membranes were prepared by non-solvent induced phase inversion (NIPS) and investigated for water vapor transport in view of their application in membrane distillation (MD). Due to the lack of high boiling point solvents for PIM-1 that are also water miscible, the mixture of tetrahydrofuran (THF) and N-methyl-2-pyrrolidone (NMP) was found to be optimal for the formation of a membrane with a developed porous system both on the membrane surface and in the bulk. PIM-1 was synthesized by using low and high temperature methods to observe how molecular weight effects the membrane structure. Low molecular weight PIM-1 was produced at low temperatures, while high molecular weight PIM-1 was obtained at high temperatures. Several membranes were prepared, including PM-6, PM-9, and PM-11 from low molecular weight PIM-1, and PM-13 from high molecular weight PIM-1. Scanning electron microscopy (SEM) was used to image the surface and cross-section of different porous PIM-1 membranes. Among all the PIM-1 membranes (PM) obtained, PM-6, PM-9, PM-11 and PM-13 showed the most developed porous structure, while PM-13 showed large voids in the bulk of the membrane. Contact angle measurements showed that all PIM-1 porous membranes are highly hydrophobic. Liquid water flux measurements showed that PM-6, PM-9 and PM-11 showed minimal water fluxes due to small surface pore size, while PM-13 showed a high water flux due to a large surface pore size. Water vapor transport measurements showed high permeance values for all membranes, demonstrating the applicability of the developed membranes for MD. In addition, a thin film composite (TFC) membrane with PIM-1 selective layer was prepared and investigated for water vapor transport to compare with porous PIM-1 membranes. The TFC membrane showed an approximately 4-fold lower vapor permeance than porous membranes. Based on these results, we postulated that the use of porous PIM-1 membranes could be promising for MD due to their hydrophobic nature and the fact that the porous membranes allow vapor permeability through the membrane but not liquid water. The TFC membrane can be used in cases where the transfer of water-soluble contaminants must be absolutely avoided.

Numerical modeling of drying from saturated to unsaturated conditions with two-phase partitioning boundaries at the medium surface and interface-dependent phase change

To accurately predict the drying of a porous medium from saturated to unsaturated conditions over time, this paper proposes two-phase partitioning boundaries for liquid water evaporation and water vapor transport at the medium surface. A non-isothermal two-phase (liquid water and gas mixtures), two-component (dry air and water vapor) transport model with nonequilibrium and interface-dependent phase change is then developed. The governing equations are then numerical solved using COMSOL Multiphysics software and applied to describe a series of laboratory soil drying tests as examples. The performance of the proposed boundaries at the surface is illustrated by comparison with other three common used boundaries. The results show that a specific flux of vapor at the surface with a zero flux of liquid water cannot be adopted to describe the drying process of initially saturated media due to the lack of interconnected gas-filled pores. The merged liquid-vapor boundary applied in the Darcy–Richards model significantly prolongs the constant rate stage when the liquid water evaporation is dominant. Then, the effects of air/gas–water interfacial area and water retention parameters on the drying process are investigated. A faster reduction in the air–water relative interfacial area at the medium surface will shorten the drying process with earlier dominance of the vapor transport. An increased entry capillary pressure and a decreased pore size distribution index for the water retention curve will result in slower drying, more loss by vapor transport and a more homogeneous distribution of pore water respect to depth. The current study can promote the understanding of the evolution of drying mechanisms of a porous medium from saturated to unsaturated conditions.

Synergistic mass transfer and performance stability of a proton exchange membrane fuel cell with traveling wave flow channels

As one of the critical components of the proton exchange membrane fuel cell (PEMFC), the flow channel design in the bipolar plate is crucial to improve the mass transfer performance and stability. In this study, the effect of the structural characteristics and location arrangement of traveling wave flow channels, with a concave surface as the mass transfer surface, on the fuel cell performance is investigated, and the evaluation indexes, such as oxygen mass flux, net power density, synergistic coefficient, and unevenness coefficient, are established to evaluate the flow law of fluid, distribution law of pressure, synergistic mass transfer, and performance stability of the fuel cell. Results show that the synergistic mass transfer performance is better in the traveling wave channels with identical structural characteristics and uniform arrangement. Furthermore, a 21.20 % increase in the average fluid flow velocity can enhance oxygen and liquid water flux at the bipolar plate (BP)/gas diffusion layer (GDL) interfaces by about 198.48 % and 207.94 %, respectively, resulting in an 87.13 % increase in net power density and 70.95 % improvement in the performance stability of the fuel cell.

Validating coupled flow theory for bare‐soil evaporation under different boundary conditions

AbstractEvaporation from bare soil is an important hydrological process and part of the water and energy balance of terrestrial systems. Modeling bare‐soil evaporation is challenging, mainly due to nonlinear couplings among liquid water, water vapor, and heat fluxes. Model concepts of varying complexity have been proposed for predicting evaporative water and energy fluxes. Our aim was to test a standard model of coupled water, vapor, and heat flow in the soil using data from laboratory evaporation experiments under different boundary conditions. We conducted evaporation experiments with a sand and a silt loam soil and with three different atmospheric boundary conditions: (i) wind, (ii) wind and short‐wave radiation, and (iii) wind and intermittent short‐wave radiation. The packed soil columns were closed at the bottom (no water flux) and instrumented with temperature sensors, tensiometers, and relative humidity probes. We simulated the evaporation experiments with a coupled water, vapor, and heat flow model, which solves the surface energy balance and predicts the evaporation rate. The evaporation dynamics were predicted very well, in particular the onset of stage‐two evaporation and the evaporation rates during the stage. A continuous slow decrease of the measured evaporation rate during stage‐one could not be described with a constant aerodynamic resistance. Adding established soil resistance parametrizations to the model significantly degraded model performance. The use of a boundary‐layer resistance, which takes into account the effect of point sources of moisture, improved the prediction of evaporation rates for the sandy soil, but not for the silt loam.

Effects of seasonal rainfall variations on the hydrothermal state and thermal stability of the permafrost active layer in the central Qinghai-Tibet Plateau of China

Rainfall significantly impacts the hydrothermal state and thermal stability of the permafrost active layer by modifying surface energy boundary conditions. The quantitative characterization of hydrothermal changes in the Qinghai-Tibet Plateau has long been a crucial issue, yet the mechanisms underlying the effects of seasonal rainfall on the hydrothermal state within the permafrost active layer remain poorly understood. Therefore, this study investigated the influence of seasonal rainfall variations on the hydrothermal state within the permafrost active layer based on a water-vapour-heat coupled model that optimises the surface energy balance boundary conditions. The results show that regardless of whether the spring rainfall changes or not, the Bowen ratio is >1, indicating that the main energy exchange mode between the atmosphere and surface is sensible heat flux. But following the increase in rainfall in summer and autumn, the Bowen ratio is <1, indicating that the main energy exchange mode is latent heat flux. Meanwhile, increased seasonal rainfall leads to an increase in liquid water flux and a decrease in water vapour flux, whereas decreased seasonal rainfall leads to the opposite trend. Heat conduction, water vapour thermal convection, and water vapour diffusion latent heat decrease with increasing rainfall in various seasons. However, liquid water thermal convection increases with increasing rainfall during various seasons. Seasonal rainfall variations ultimately affect permafrost thermal stability by influencing water-vapour migration and energy changes within the permafrost active layer. Among them, the active layer thickness remains basically unchanged after the spring rainfall increases by 2.0 times, whereas it decreases by 4 and 6 cm after the summer and autumn rainfall increase by 1.5 and 2.0 times, respectively. Additionally, when the rainfall during spring, summer, and autumn decreases by 2.0, 1.5, and 2.0 times, respectively, the active layer thickness increases by 7, 6, and 3 cm, respectively. Therefore, increased seasonal rainfall can alleviate permafrost degradation. Otherwise, it is unfavourable for the preservation of permafrost thermal stability.

The role of hydraulic and thermal properties of soil on evaporation: a numerical insight

Evaporation from geological formations results from the interaction between the geomaterial and the atmosphere. Geotechnical engineering issues, such as slope stability, pollution containment, and soil heave/shrinkage, require a deep understanding of the soil-atmosphere interaction ruled by evaporation. Evaporation is a multi-phase thermo-hydraulic phenomenon that includes liquid water, vapour, and heat fluxes. It is generally modelled considering unsaturated soils’ thermal energy and water mass balance equations. This paper presents a numerical model to reproduce evaporation processes under controlled environmental conditions. The model was implemented in Comsol Multiphysics Finite Element software and first validated against experimental data from the literature. Then it was used to investigate the role of the hydraulic and thermal properties in the evaporative response. The numerical results revealed differences in the evolution of the water content profiles over time due to the interplay between the hydraulic conductivity and the retention properties. Hydraulic conductivity mainly impacts the shape of the water content isochrones: fast drying of the superficial layers and slow desaturation of deeper layers occur for decreasing hydraulic conductivity values. On the other hand, the moisture capacity primarily impacts the thickness of the desaturating layer, which decreases for higher values of the moisture capacity.

Experimental and numerical studies of dynamic variation of liquid water and water vapor flow in the vadose zone

In arid and semi-arid regions, the depth of water table has a significant effect on evaporation. The soil in these regions is unsaturated, and the transport of liquid water and water vapor is accompanied by variation of the water table depth. In this study, we designed a laboratory experimental apparatus to conduct evaporation experiments of soil under shallow and deep water table conditions through continuous monitoring of water content and temperature variation in the vadose zone. A numerical model was used to simulate the coupled transport of liquid water, water vapor and heat in the vadose zone under two different water table elevations. The experimental and numerical results were in good agreement. Liquid water and water vapor fluxes driven by pressure head and temperature gradient were calculated and analyzed to illustrate the distribution of the water content. Both isothermal liquid flux and thermal vapor flux dominates the variation of water content. By determining the location of the evaporation front and analyzing its dynamic variation, we found that the evaporation front ranged from the surface to a depth of 2.5 cm under a shallow water table. For a deep water table, the evaporation front was nearly at a constant depth of 15 cm.

Estimation of soil thermal properties using conduction and conduction–convection heat transfer equations in the Brazilian Pampa biome

Soil temperature, composition, and structure directly affect the heat transfer mechanisms in soil. Understanding the thermal behavior of soil is fundamental for describing the processes of mass and energy exchange in the soil-plant-atmosphere system. In addition, estimating thermal property values is necessary to determine the amount of heat transferred, stored, or transmitted by conduction and convection between the soil surface and deeper layers. This study used two soil heat transfer models (the conduction and conduction-convection methods) to estimate soil thermal properties. Soil data were collected in a natural pasture area in the Pampa biome in southern Brazil over 1.5 years. The results showed that soil thermal diffusivity (k) has an inverse seasonal variation with soil temperature (Ts). The k and soil thermal conductivity (λ) values are higher as the soil depth and soil moisture (θ) increase, confirming the vertical variation of thermal properties. The liquid water flux density (W) represents the vertical movement of water in the soil and decreases in periods of higher Ts. Increasing values of W led to an increase in the variability of k with depth. The conduction–convection method provides the best estimates for Ts with R2 = 0.99 for deeper layers compared to the conduction method. However, we found that both models gave similar results for near-surface layers since, in this case, the conduction process was responsible for most of the heat transfer from the surface to the ground. The results obtained here allowed us to evaluate heat transfer at different depths and estimate thermal properties as a function of soil moisture in a natural pasture area in the Brazilian Pampa biome. Furthermore, the results can be integrated into global land surface models intended to represent the behavior of the energy exchange between soil, surface, and atmosphere.

Disentangling the Impact of Event‐ and Annual‐Scale Precipitation Extremes on Critical‐Zone Hydrology in Semiarid Loess Vegetated by Apple Trees

AbstractThe extent and mechanisms by which precipitation extremes affect hydrological processes in the critical zone (CZ) of loess possessing a thick unsaturated zone remains poorly understood. To this end, we here employed a coupled liquid‒vapor‒heat‒airflow STEMMUS (simultaneous transfer of energy, mass, and momentum in unsaturated soil) model to investigate the impact of extreme precipitation, at both event and annual scales, on CZ hydrological processes within a semiarid loess site vegetated by apple trees on China's Loess Plateau. At the event scale, the vapor flux was two orders of magnitude lower than the liquid water flux. However, the thermal vapor flux penetrated to depths of 200 cm, whereas the isothermal liquid water flux only infiltrated to 100 cm during the study period, implying that thermal‐gradient‐driven vapor transfer is an important mechanism for deep‐layer recharge (DLR). At the annual scale, the DLR below 200 cm during extremely wet years was 6.5 times larger than that during extremely dry years. In contrast, extreme changes in climate had only limited impacts on evapotranspiration; the difference between extremely wet and extremely dry years averaged only 35 mm, much less than the difference in precipitation, which averaged 310 mm. However, the extremely dry years showed a higher ratio (0.58) between transpiration and evapotranspiration than did the extremely wet years (0.51). The findings reported here improve our understanding of CZ hydrological processes related to precipitation extremes in semiarid loess regions.

Lysimeter and In Situ Field Experiments to Study Soil Evaporation Through a Dry Soil Layer Under Semi‐Arid Climate

AbstractDuring droughts, soil evaporation is often constrained by water vapor transport through an air‐dry soil layer (DSL). Fick's water vapor diffusion is widely regarded as the only process for such transport; however, field studies conducted in arid and semi‐arid conditions showed measured evaporation rates higher than those predicted by diffusion. Therefore, transport processes other than diffusion could be relevant. To study the evaporation through a DSL, the same lysimeter column with 70 cm thick DSL as earlier applied in laboratory in Balugani et al. (2021), was installed in the field in Spain applying an original weighing setup to measure evaporation. The correlation between the measured evaporation and possible drivers of the water vapor transport were evaluated. With the DSL thickness of 70 cm in 2012 and 12 cm in 2015, the lysimeter recorded similar groundwater evaporation rates: 1.25 and 1.05 mm days−1, respectively; these rates were much larger than the laboratory recorded rates (0.3 mm days−1) and those estimated in this study using Hydrus1D accounting for non‐isothermal liquid water fluxes and water vapor diffusion. The main forcing driver of the field lysimeter evaporation was the soil profile temperature fluctuation, which concealed other less important forcing factors, that is, atmospheric pressure fluctuations and diffusion. A multivariate regression model to estimate evaporation was proposed, based on the profile temperature fluctuation, that, when added to the atmospheric pressure fluctuations, yielded reliable estimates of the cumulative evaporation measured in both 2012 and 2015.

Thermal–moisture dynamics and thermal stability of active layer in response to wet/dry conditions in the central region of the Qinghai–Tibet Plateau, China

The amount of rainfall varies unevenly in different regions of the Qinghai-Tibet Plateau, with some regions becoming wetter and others drier. Precipitation has an important impact on the process of surface energy balance and the energy-water transfer within soils. To clarify the thermal-moisture dynamics and thermal stability of the active layer in permafrost regions under wet/dry conditions, the verified water-vapour-heat coupling model was used. Changes in the surface energy balance, energy-water transfer within the soil, and thickness of the active layer were quantitatively analyzed. The results demonstrate that rainfall changes significantly affect the Bowen ratio, which in turn affects surface energy exchange. Under wet/dry conditions, there is a positive correlation between rainfall and liquid water flux under the hydraulic gradient; water vapour migration is the main form under the temperature gradient, which indicates that the influence of water vapour migration on thermal-moisture dynamics of the active layer cannot be neglected. Concurrently, regardless of wet or dry conditions, disturbance of the heat transport by conduction caused by rainfall is stronger than that of convection by liquid water. In addition, when rainfall decreases by 1.5 times (212 mm) and increases by 1.5 times (477 mm), the thickness of the active layer increases by 0.12 m and decreases by 0.21 m, respectively. The results show that dry conditions are not conducive to the preservation of frozen soil; however, wet conditions are conducive to the preservation of frozen soil, although there is a threshold value. When this threshold value is exceeded, rainfall is unfavourable for the development of frozen soil.

Effect of vapour transport on soil evaporation under different soil textures and water table depths in an arid area of Northwest China

AbstractIn arid area, the liquid water and water vapour states in soil profiles and fluxes at the upper and bottom interfaces are extremely complex due to heterogeneity of soil textures and the driving forces of heat and matrix potential. In this study, we used Hydrus‐1D to simultaneously simulate liquid water, water vapour, and heat transport based on the observed datasets of atmosphere, soil and groundwater at three soil profiles in an arid area of northwest China. Based on the comparison of the observed and simulated results at the three soil profiles, we show that there are diurnal vapour entry and outlet fluxes at the dry surface layer of 30 cm in the summer season. The vapour entry and re‐evaporation account for about 14% of annual precipitation for the heterogeneity soil profile with a mean groundwater depth of 210 cm. Because of limited soil moisture in this arid area, vapour induced re‐evaporation occurs shortly in the early daytime. Moreover, the extent of vapour entry, condensation and re‐evaporation are strongly dependent on soil properties and water table depth. The deeper water table produces the drier soil surface, allowing more vapour entry, condensation and re‐evaporation. Whereas the finer grained soil layers benefit the vapour fixation to produce zero fluxes that substantially inhibit the upward liquid water and vapour fluxes, and thereby reduces soil actual evaporation (AE). The reduced AE correspondingly decreases the capillary effect on phreatic evaporation, given that AE decreases slowly with decline of water table and the large extinct depth of phreatic evaporation for the finer grained soil profiles. The estimated extinct depths are 180 and 200 cm for the soil profiles consisting of silt loam and loamy sand, respectively, much larger than 100 cm of the typical sandy soil profile. Additionally, as water table is comparably higher and lower than the extinct depth, the models neglecting the vapour – heat function could, respectively, overestimate and underestimate soil evaporation.

A multiscale finite element method for coupled heat and water transfer in heterogeneous soils

Large-scale modeling of coupled heat and water transfer (CHWT) is challenging due to the spatial variabilities of soil hydraulic and thermal properties. A multi-scale finite element method (MsFEM) has been designed for simulating liquid water fluxes in unsaturated soils. In this study, the MsFEM approach is expanded as a new scheme that can handle CHWT in soils. Two groups of MsFEM basis functions are constructed to incorporate the heterogeneities in soil hydraulic conductivity and thermal conductivity, and a Petrov-Galerkin formulation is adopted to implement the proposed MsFEM scheme. The MsFEM scheme is also exploited as a sequential solver when heat transfer and water transfer are expressed in a partially coupled formulation (Wang et al., 2022a). Numerical examples illustrate that, without essentially increasing the computing load, the MsFEM scheme can improve the accuracy by up-to 30% compared to the standard finite element method (FEM), especially for thermally driven water transfer. Some fine scale spatial variations in soil temperature can only be revealed with the MsFEM scheme. If the MsFEM sequential solver is applied, the coupled heat and water transfer model can be rewritten into a sequence of modules, where liquid water transfer, heat transfer and vapor transfer are solved step-by-step, but the thermally driven liquid water is omitted due to its relatively small value (Wang et al., 2022a). With the MsFEM sequential solver, a flexible modeling architecture can be achieved at the cost of a relatively small increase of error (<5%). Therefore, the MsFEM scheme presented in this study is an effective numerical approach to simulating CHWT in heterogeneous soils.

Surface melting of snow-firn-ice structures and estimation of extended transient energy and mass balances using a liquid solid phase-change model

Parameterized energy and mass balance models are often applied to estimate energy fluxes at the surface that are available for melting and sublimation of snow and ice surfaces. In this study we evaluate the suitability of quantifying transient energy and mass fluxes on frozen surface using the finite volume method in comparison with the traditional glaciological energy balance approach. To assess the model application, we apply the methods to two datasets: a glacier in the semiarid Andes, and an Antarctic ice shelf, estimating the effect of time varying surface irradiation, wind velocity, air temperature and humidity on the history of temperature, liquid water and heat fluxes during 30 months. Results of energy and mass balances are validated with previously published results of global balances and meteorological datasets.

Modeling vapor transfer in soil water and heat simulations: A modularized, partially-coupled approach

Coupled water and heat transfer models are widely used to analyze soil water content and temperature dynamics, evaluate agricultural management systems, and support crop growth modelling. In relatively dry soils, vapor transfer, rather than liquid water flux, becomes the main pathway for water redistribution. However, in some modularized soil simulators, e.g., 2DSOIL (Timlin et al., 1996), vapor transfer is not included, which may induce errors in soil water and heat modelling. Directly embedding vapor transfer into existing water and heat transfer modules may violate the modularized architecture of those simulators. Therefore, the objectives of this study are to design a vapor transfer model, evaluate its performance, and implement it as a separate module in a coupled soil water and heat simulator, e.g., 2DSOIL. The efficacy of the vapor transfer model is evaluated by comparing the simulated soil water content and temperature before and after including the new vapor transfer model, and the soil water content and temperature simulated with the standard Philip and de Vries (1957) model. By implementing vapor transfer as a separate module in 2DSOIL, modifications to existing water and heat transfer modules can be minimized and the modularized model architecture can be maintained. Numerical examples of 2DSOIL with the new vapor transfer model are presented to illustrate the effects of vapor flux on soil water and temperature redistributions. In conclusion, the new vapor transfer model provides an effective and easy-to-use method to account for the effects of vapor transfer on coupled soil water and heat simulations.

Bidirectional Turbulent Fluxes of Fog at a Subtropical Montane Cloud Forest Covering a Wide Size Range of Droplets

Size-resolved turbulent fluxes of fog droplets are investigated above a subtropical montane cloud forest in Taiwan. By integrating an aerosol spectrometer into an eddy-covariance set-up, we measure droplet number fluxes and liquid water fluxes in a size range of aerosol particles and droplets with diameters ranging from 0.25 {upmu }!mathrm{m} to 17.3 {upmu }!mathrm{m}. We find two flux-direction changes within this size range: a downward flux occurs for accumulation-mode aerosols of diameters between 0.25 {upmu }!mathrm{m} and 0.83 {upmu }!mathrm{m}, an upward flux occurs for hydrated aerosols with diameters between 1.1 {upmu }!mathrm{m} and 2.4 {upmu }!mathrm{m}, and a downward flux occurs again for activated fog droplets between diameters of 3 {upmu }!mathrm{m} and 17.3 {upmu }!mathrm{m}. The droplet size distributions can be modelled by a trimodal log-normal distribution, and the modes correlate with the different flux directions. The formation of the three modes and the establishment of the respective flux directions can be explained by combining the Köhler theory on the basis of measured ion concentrations in fog with the turbulent transport of droplets. Finally, from the combined analysis of droplet fluxes and size distributions, we infer relevant processes of droplet development and dissolving during various phases of the life cycles of the fog events.

Droplet size distribution, liquid water content and water input of the seasonally variable, nocturnal fog in the Central Namib Desert

In the arid Central Namib desert, rainfall is rare, but fog frequently occurs at night and exhibits a seasonal pattern. We deployed an eddycovariance setup at two different inland locations, namely Gobabeb and Vogelfederberg. The setup consisted of a cloud droplet probe and at times of up to two sonic anemometer-thermometers. It was deployed for 1.5 years, collecting data during more than 150 fog events at either one of the sites. We characterize the different fog regimes at the two stations with co-located measurements of visibility, fog collector mesh precipitation by fog collectors, and non-rainfall water input by microlysimeters for selected periods. At similar levels of visibility, fog collector mesh precipitation at Vogelfederberg is generally higher, likely due to its higher elevation and exposed location.Fog events appear front-like with a decrease of visibility and an increase of droplet numbers above 10 μm and the associated liquid water content. Fog events regularly lasted 6 h, with the first two to three hours exhibiting higher liquid water content. The droplet distribution, especially droplets above 20 μm, and thus liquid water content varied in numbers between the locations. Liquid water content correlates with fog collector mesh precipitation at Gobabeb, where the droplet distribution spans the whole resolved spectrum. While liquid water flux was bi-directional, the net sum was still a net gain for the surface. We found only a low correlation between liquid water flux and non-rainfall water input, which demonstrates that the liquid water flux can only partially connect the measured input in the fog collectors and the microlysimeters despite similar values. We therefore presume that drizzle, that is droplets out of the CDP sampling range, may play a role in the Central Namib fog, especially when fog is intermittent as fog deposition in microlysimeters continues when no droplets of the respective size range are present near the ground. Furthermore, the contrast of higher fog collector mesh precipitation at Vogelfederberg compared to Gobabeb despite lower liquid water content is potentially a result of the more numerous small droplets as a consequence of the interception with the upper parts of the stratus.