Advective Component Research Articles

Interaction of ocean temperature advection, surface heat fluxes and sea ice in the marginal ice zone during the North Atlantic Oscillation in the 1990s: A modeling study

A moderately fine‐resolution (0.4°, 40 vertical levels), global, coupled ice‐ocean model was configured and run for 24 years (1979–2002), forced with high‐frequency National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) atmospheric fluxes. The model consists of the Los Alamos National Laboratory Parallel Ocean Program (POP) and sea ice model (CICE). The fidelity of the simulated mean climatological state and variability of key variables such as ice concentration, total ice area, ice thickness and drift were compared to observational data sets from satellite and ice drift buoy measurements. Basin‐scale changes in the lower atmosphere/surface ocean/sea‐ice in the simulated Arctic and Nordic Seas before and after the North Atlantic Oscillation (NAO) phase switch in 1995 were examined using winter composite analyses over the period 1990–1999. Ice cover changes between the two NAO phases were consistent with observations in that reduced concentrations were found in the Nordic and Barents Seas and increased values occurred in the Labrador Sea. Next we regionally evaluated the relative importance of winter anomalies of upper‐ocean mixed layer net heat fluxes and of ocean temperature advection on marginal ice zone variability in the Irminger, Greenland, and Barents Seas for this ten‐year period. We found that the net heat flux winter anomaly was at least four times more important than the winter anomaly of ocean temperature advection in the Greenland and Barents Seas, while it was twice as important in the Irminger Sea. The Ekman ocean temperature advection component generally dominated the geostrophic component in all three regions.

On the Response of a Buoyant Plume to Downwelling-Favorable Wind Stress

Abstract Here, the response of a coastally trapped buoyant plume to downwelling-favorable wind forcing is explored using a simplified two-dimensional numerical model and a prognostic theory for the resulting width, depth, and density anomaly and along-shelf transport of the plume. Consistent with the numerical simulations, the analytical model shows that the wind causes mixing of the plume water and that the forced cross-shelf circulation can also generate significant deepening and surface narrowing, as well as increased along-shelf transport. The response is due to a combination of the purely advective process that leads to the steepening of the isopycnals and the entrainment of ambient water into the plume. The advective component depends on the initial plume geometry: plumes that have a large fraction of their total width in contact with the bottom (“bottom trapped”) suffer relatively small depth and width changes compared to plumes that have a large fraction of their total width detached from the bottom (“surface trapped”). Key theoretical parameters are Wγ/Wα, the ratio of the width of the plume detached from the bottom to the width of the plume in contact with it, and the ratio of the wind-generated mixed layer δe to the initial plume depth hp, which determines the amount of water initially entrained into the plume. The model results also show that the cross-shelf circulation can be strongly influenced by the wind-driven response in combination with the geostrophic shear of the plume. The continuous entrainment into the plume, as well as transient events, is also discussed.

Aspects of CO2 advection measurements

Observations of vegetation–atmosphere exchange of carbon dioxide (CO2) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO2 concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity (\(\emph{w}\)). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean \(\emph{w}\). Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE.

Mechanisms controlling mixed-layer temperature variability in the eastern tropical Pacific on the intraseasonal timescale

[1] The Madden Julian Oscillation (MJO) is a prominent mode of intraseasonal (∼35–95 day) variability in the tropical Indian and western Pacific Oceans. Its impacts on ocean mixed-layer temperatures (MLT) in the eastern tropical Pacific Ocean (ETPO) are not well-understood. A previous study of mooring data at (110°W, EQ) implies vertical advection forced by MJO-generated Kelvin waves is important to ETPO intraseasonal MLT; another, based on model output over (120–100°W, 3°S–3°N), indicates meridional advection dominates. We analyze an ocean state estimate at (110°W, EQ) and find combined subsurface processes, including a notable vertical advection component like that expected from MJO-forced Kelvin waves, dominate. However, when averaged over (120–100°W, 3°S–3°N), subsurface processes and horizontal advection are both important, and effects of thermocline forcing (e.g., Kelvin waves) are small compared to those by local wind driven mixing and horizontal advection. The results highlight the importance of understanding regional physics on different spatial scales.

Improved Riemann solvers for complex transport in two-dimensional unsteady shallow flow

The numerical solution of advection–reaction–diffusion transport problems in two-dimensional shallow water flow is split in three subproblems in order to analyze them separately. In the first part, the advection component is solved with the help of an extended Jacobian matrix for the coupled system of flow and advection conservation laws and focusing on the correct definitions of the approximate or weak solutions. Considering that one of the conserved quantities is the solute volume, nonphysical solutions for the solute concentration may appear in complex situations and a solute fix is proposed. This is formulated for first and second order schemes. In the second part of this work, the solution of problems with volumetric reaction terms is studied and the results of single-step as well as multi-step pointwise and upwind approaches are compared in order to establish their relative performance. The upwind treatment is done in 2D cases dividing cell volumes to transform reacting terms in singular source terms. The third part is concerned with the diffusion term. The focus of this part is put on the interference between numerical and physical diffusion. A simple form to estimate the magnitude of the numerical diffusion is proposed and it is shown to improve the accuracy of the results in first and second order approaches.

Processes contributing to the evolution and destruction of stratification in the Liverpool Bay ROFI

Liverpool Bay, a region of freshwater influence subject to semi-diurnal and enduring periods of stratification, is home to a long-term coastal observatory. The observatory provides a new array of data which include vertical profiles of velocity from an Acoustic Doppler Current Profiler and a high frequency radar system (which provides measurements of surface currents). Using this dataset in conjunction with an analytical potential energy model that uses advances in the formulation of a freshwater buoyancy term, the processes controlling stratification can be assessed. The results indicate that a depth-resolving freshwater buoyancy term should be used for the calculation stratification. Advection, in addition to depth-mean straining, is an important process affecting the stratification in Liverpool Bay. Specifically, when semi-diurnal stratification occurs, the two terms are in phase whilst when enduring stratification occurs, they are out of phase. The phase of the advective component, and thus its influence relative to depth-mean straining, was found to be a function of the vertical variation of the horizontal density gradient.

TRANSPIRATION AND NUTRIENT USE EFFICIENCY OF A SWEET PEPPER CROP UNDER TWO EXTERNAL SHADING SYSTEMS: MOBILE AND FIXED

High incident radiation levels during the summer cropping cycles in Mediterranean greenhouses generate microclimates of high temperature and evaporative demand so the incident radiation reduction by white-washing is usually applied. The aim of this work is to evaluate the transpiration and nutrient uptake of a sweet pepper crop (Capsicum annuum L.) in two greenhouses equipped with (i) external mobile shading (M) and (ii) external fixed shading (F). Transpiration was measured every five minutes in both greenhouses with two lysimeter balances. Leaf transpiration and intercepted radiation relationship was decreasing during the crop cycle. A multiple regression analysis, based on the simplified Penman-Monteith formula, was established between the measured transpiration and the radiative and advective components. A light extinction coefficient of 0.45 was determined and the leaf area index (LAI) was estimated in function of the days after transplanting. To get a good adjustment between observed and estimated transpiration it was necessary to change the P-M model coefficients through the crop development. Total nutrient uptake (N, K, Ca, Mg and P) was calculated weekly by the measure of supply and leached nutrient solution and its chemical analysis. It was initially similar in both treatments and finally higher in the F than the M treatment. By contrast, the nutrient uptake efficiency, in terms of fruit fresh weight (g g-1), was higher in the M treatment during the early yield period, although both treatments showed the same efficiency at the end of the crop.

Distinguishing Developing Versus Non-Developing African Easterly Waves Using Adaptive Observational Strategies

The impacts of dust loaded, Saharan Air Layer (SAL) during the life cycle of African Easterly Waves (AEWs) is a many-faceted scientific problem. It entails possible radiative effects of dust aerosols, impacts of dust on cloud physics, and the cloud nuclei of condensation, advective effects, that is, intrusions of dry dusty air versus humid air into the interior of storms. This paper addresses several such AEWs of the eastern Atlantic Ocean, where we have made use of special aircraft reconnaissance data sets that include wind and humidity profiles in the vertical. Using what are called adaptive observational strategies within a mesoscale model, we show the impacts of adding such observations in the analysis and in short-range forecasts of several AEWs. We do not have the direct and indirect effects of aerosols, but we do include the advective component. Our results show that the inclusion of humidity profiling distinguishes between developing versus nondeveloping AEWs from the use of these additional data sets via the adaptive observational strategies.

Rates of trace metal and nutrient diagenesis in an intertidal creek bank

Carbon mineralization in marine sediments is a key process involved in the cycling of carbon, nutrients and trace metals. However, as marine sediments are usually diffusion dominated, the pace of element and nutrient cycling is slow, because consumption of oxidants and/or nutrients in the pore waters via microbial activity often outpaces resupply. Adding an advective flow component to such a system should change the biogeochemical dynamics considerably. Numerical simulations show that shallow coastal aquifers affected by tidal forces can establish ground water velocities of up to 7 cm h −1, driving a circulation of sea water through the sediments with subsequent discharge. Although known to enhance solute exchange, the impact of advection on early diagenesis has not received much attention. To address this issue we mapped the interstitial water chemistry down to 2.5 m sediment depth along a transect on an intertidal creek bank that is subject to a periodic advective flow. Additionally a recently developed hydrogeological simulation of the creek bank was applied to calculate ages of the sampled pore waters. Sample ages obtained were used to quantify (flow path integrated) production or depletion rates for trace metals, nutrients, and sulphate. We find young sea water percolating relatively fast through sediments close to the creek showing strong signs of alteration, whereas pore waters from diffusion dominated regions are less altered. The increase in inorganic nutrients and some trace elements along the flow path requires high rates of turnover. Sulphate, molybdenum, and uranium are almost completely depleted after 200 days, while dissolved inorganic carbon (DIC), ammonia, and manganese increase. Averaged production rates for DIC appear to be three times higher when advection dominated the subsurface flow regime. Our results demonstrate that sites dominated by advection generally show signs of faster rates of diagenetic reactions.

Reduced model for characterization of solid wall effects for transient thermal dispersion in granular porous media

This paper is devoted to modelling of the thermal interface condition between a solid wall and a granular porous bed through which a fluid flows. Far from the wall, the porous medium, a bed of monodisperse glass beads, is homogeneous. An average enthalpic temperature, based on spatial averaging of the local field over a representative elementary volume, is defined, and a corresponding heat equation can be derived: it uses a Darcy velocity, dispersion coefficients that depend on this velocity, and two volumetric heat capacities (fluid and solid). The presence of a solid wall modifies the local arrangement of the beads, with a porosity that varies with the distance to the wall. In a plug flow situation in the core of the bed, the local velocity becomes larger in the near-wall region, creating a channelling effect that affects the wall-to-porous medium heat transfer. This transfer is studied, for transient wall heating, using heat transfer models of increased complexity and specific analytical methods. Finally a reduced model is proposed for this channelling effect, with its advection component that modifies heat transfer between wall and core region in a way that cannot be modelled by a single heat transfer coefficient.

Transport of nonsorbing solutes in a streambed with periodic bedforms

Previous studies of hyporheic zone focused largely on the net mass transfer of solutes between stream and streambed. Solute transport within the bed has attracted less attention. In this study, we combined flume experiments and numerical simulations to examine solute transport processes in a streambed with periodic bedforms. Solute originating from the stream was subjected to advective transport driven by pore water circulation due to current–bedform interactions as well as hydrodynamic dispersion in the porous bed. The experimental and numerical results showed that advection played a dominant role at the early stage of solute transport, which took place in the hyporheic zone. Downward solute transfer to the deep ambient flow zone was controlled by transverse dispersion at the later stage when the elapsed time exceeded the advective transport characteristic time t c (= L/ u c with L being the bedform length and u c the characteristic pore water velocity). The advection-based pumping exchange model was found to predict reasonably well solute transfer between the overlying water and streambed at the early stage but its performance deteriorated at the later stage. With dispersion neglected, the pumping exchange model underestimated the long-term rate and total mass of solute transfer from the overlying water to the bed. Therefore both advective and dispersive transport components are essential for quantification of hyporheic exchange processes.

Thermodynamic and Dynamic Mechanisms for Large-Scale Changes in the Hydrological Cycle in Response to Global Warming*

Abstract The mechanisms of changes in the large-scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project phase 3 and used for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report are analyzed by computing differences between 2046 and 2065 and 1961 and 2000. The contributions to changes in precipitation minus evaporation, P − E, caused thermodynamically by changes in specific humidity, dynamically by changes in circulation, and by changes in moisture transports by transient eddies are evaluated. The thermodynamic and dynamic contributions are further separated into advective and divergent components. The nonthermodynamic contributions are then related to changes in the mean and transient circulation. The projected change in P − E involves an intensification of the existing pattern of P − E with wet areas [the intertropical convergence zone (ITCZ) and mid- to high latitudes] getting wetter and arid and semiarid regions of the subtropics getting drier. In addition, the subtropical dry zones expand poleward. The accentuation of the twentieth-century pattern of P − E is in part explained by increases in specific humidity via both advection and divergence terms. Weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to decreased P − E in the ITCZ and to increased P − E in the descending branches of the Walker and Hadley cells. The changing mean circulation also causes decreased P − E on the poleward flanks of the subtropics because the descending branch of the Hadley Cell expands and the midlatitude meridional circulation cell shifts poleward. Subtropical drying and poleward moistening are also contributed to by an increase in poleward moisture transport by transient eddies. The thermodynamic contribution to changing P − E, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity.

Estimating 3D Movements from 2D Observations Using a Continuous Model of Helical Swimming

Helical swimming is among the most common movement behaviors in a wide range of microorganisms, and these movements have direct impacts on distributions, aggregations, encounter rates with prey, and many other fundamental ecological processes. Microscopy and video technology enable the automated acquisition of large amounts of tracking data; however, these data are typically two-dimensional. The difficulty of quantifying the third movement component complicates understanding of the biomechanical causes and ecological consequences of helical swimming. We present a versatile continuous stochastic model-the correlated velocity helical movement (CVHM) model-that characterizes helical swimming with intrinsic randomness and autocorrelation. The model separates an organism's instantaneous velocity into a slowly varying advective component and a perpendicularly oriented rotation, with velocities, magnitude of stochasticity, and autocorrelation scales defined for both components. All but one of the parameters of the 3D model can be estimated directly from a two-dimensional projection of helical movement with no numerical fitting, making it computationally very efficient. As a case study, we estimate swimming parameters from videotaped trajectories of a toxic unicellular alga, Heterosigma akashiwo (Raphidophyceae). The algae were reared from five strains originally collected from locations in the Atlantic and Pacific Oceans, where they have caused Harmful Algal Blooms (HABs). We use the CVHM model to quantify cell-level and strain-level differences in all movement parameters, demonstrating the utility of the model for identifying strains that are difficult to distinguish by other means.

Advective Heat Transport in an Unconfined Aquifer Induced by the Field Injection of an Open-Loop Groundwater Heat Pump

Problem statement: The increasing diffusion of low-enthalpy geothermal open-loop Groundwater Heat Pumps (GWHP) providing buildings air conditioning requires a careful assessment of the overall effects on groundwater system, especially in the urban areas. The impact on the groundwater temperature in the surrounding area of the re-injection well is directly linked to the aquifer properties. Physical processes affecting heat transport within an aquifer include advection (or convection) and hydrodynamic thermodispersion (diffusion and mechanical dispersion). If the groundwater flows, the advective components tend to dominate the heat transfer process within the aquifer and the diffusion can be considered negligible. This study illustrates the experimental results derived from the groundwater monitoring in the surrounding area of an injection well connected to an open-loop GWHP plant which has been installed in the "Politecnico di Torino" (NW Italy) for cooling some of the university buildings. Groundwater pumping and injection interfere only with the upper unconfined aquifer. Approach: After the description of the hydrogeological setting the authors examined the data deriving from multiparameter probes installed inside the pumping well (P2), the injection well (P4) and a downgradient piezometer (S2). Data refers to the summer 2009. To control the aquifer thermal stratification some multi-temporal temperature logs have been performed in the S2. Results: After the injection of warm water in P4 the plume arrived after 30 days in the S2. That delay is compatible with the calculated plume migration velocity (1.27 m d-1) and their respective distance (35 m). The natural temperature in the aquifer due to the switching-off of the GWHP plant has been reached after two month. The Electrical Conductivity (EC) values tend to vary out of phase with the temperature. The temperature logs in the S2 highlighted a thermal stratification in the aquifer due to a low vertical dispersion of the injected warm water. Conclusion: Experimental evidences seem to confirm the prevalence of heat advective transport component respect the dispersive phenomena. This hypothesis appears validated by the following evidences: (i) the calculated advective migration velocities are compatible with the calculated retardation factor and the temperature revealed in the S2, (ii) both the groundwater and the heat tend to flow horizontally due to the different values of horizontal and vertical hydraulic conductivity in the Unit 1 (thermal stratification) and (iii) the flowing water highlighted different geochemical characteristics during the time.

Edge behaviour in a minute parasitic wasp

1. Changes in the density of an organism near a boundary elements (edge effects) are often thought to be generated by changes in movement behaviour, but in most cases the mechanism underlying these effects is unknown. 2. We quantified the movement behaviour of a minute parasitic wasp, Anagrus columbi, in relation to edges in its habitat. This wasp attacks eggs of the planthopper Prokelisia crocea, which inhabits a wet prairie ecosystem composed of patches of its host plant prairie cordgrass (Spartina pectinata) interspersed within a matrix of mudflat, smooth brome (Bromus inermis) and native grasses. Two edge types are common in this system, cordgrass-mudflat and cordgrass-brome. 3. We conducted mark-recapture experiments in which wasps were released at the cordgrass-matrix edge and 50 cm within cordgrass or matrix, for both edge types. The marked wasps were recaptured using a grid of sticky traps. We fitted an advection-diffusion model to these data, yielding estimates of the diffusion rate and advection coefficient for cordgrass and matrix, for each release position and edge type. 4. The spatial distribution of wasps was well-described by the advection-diffusion model. The pattern suggests that marked wasps strongly biased their movements towards the edge when released in matrix, and to a lesser extent when released in cordgrass, while edge releases showed little bias. The advection coefficients were similar for the two edge types, as were the diffusion rates for the three substrates (cordgrass, brome, mudflat). The diffusive and advective components of movement were of comparable magnitude for matrix and cordgrass releases, suggesting equal amounts of directed and random movement. 5. Our results suggest the wasps are attracted to cordgrass patches across short distances, and that bias in their movements may concentrate them at the patch edge. Their edge behaviour is qualitatively different from that of the host insect. 6. The methodology described here could be readily adapted to other systems, where direct observations of movement are difficult but mark-recapture studies are feasible.

Interannual variability in North Pacific heat and freshwater budgets

Abstract Transports of volume, heat, and freshwater in the North Pacific Ocean from 1992 to 2004 are analyzed using a long-term high resolution expendable bathythermography (XBT) dataset and output from a data-assimilating model. Estimates of geostrophic transport from the data are compared with the model transport to close the volume budget north of the trans-Pacific XBT track. Advective transport from both model output and data are combined with surface fluxes to determine budgets of heat and freshwater in the closed region. The northward heat transport across the XBT track is estimated to be 0.74 ± 0.1 pW , and has variability of almost 0.5 pW on 3–4 year time scales, while freshwater transport is estimated to be − 0.1 ± 0.06 Sv . The balance between northward advective heat transport and surface heat flux gives a time-varying estimate of heat storage that compares well with observations. A similar balance is found between model estimates of advective freshwater transport and surface freshwater flux. Despite a scarcity of observations and uncertainties in all components, this analysis results in nearly closed budgets of volume, heat, and freshwater. Mean estimates of advective transport of both heat and freshwater agree with previous estimates. An analysis of each component of the heat budget with latitude indicates that a relative lack of time-variability of the surface component is consistent throughout the North Pacific. The dominant advective component is driven by changes in the wind stress curl field. For both heat and freshwater storage, strong signals occur concurrently at all latitudes. This behavior could indicate that these signals are controlled by large-scale dynamics, rather than small-scale disturbances from which signals would need to propagate to be widely felt. The analysis demonstrates the value of bringing models and data together, resulting in budgets that are consistent with observations, yet provide a comprehensive look at the variability of North Pacific heat and freshwater storage that would be unavailable from data alone.

Movement Directionality in Collective Migration of Germ Layer Progenitors

Collective cell migration, the simultaneous movement of multiple cells that are connected by cell-cell adhesion, is ubiquitous in development, tissue repair, and tumor metastasis [1, 2]. It has been hypothesized that the directionality of cell movement during collective migration emerges as a collective property [3, 4]. Here we determine how movement directionality is established in collective mesendoderm migration during zebrafish gastrulation. By interfering with two key features of collective migration, (1) having neighboring cells and (2) adhering to them, we show that individual mesendoderm cells are capable of normal directed migration when moving as single cells but require cell-cell adhesion to participate in coordinated and directed migration when moving as part of a group. We conclude that movement directionality is not a de novo collective property of mesendoderm cells but rather a property of single mesendoderm cells that requires cell-cell adhesion during collective migration.

Budget Analysis of Escherichia coli at a Southern Lake Michigan Beach

Escherichia coli (EC) concentrations at two beaches impacted by river plume dynamics in southern Lake Michigan were analyzed using three-dimensional hydrodynamic and transport models. The relative importance of various physical and biological processes influencing the fate and transport of EC were examined via budget analysis and a first-order sensitivity analysis of model parameters. The along-shore advective flux of EC (CFU/m(2).s) was found to be higher compared to its cross-shore counterpart; however, the sum of diffusive and advective components was of a comparable magnitude in both directions showing the importance of cross-shore exchange in EC transport. Examination of individual terms in the EC mass balance equation showed that vertical turbulent mixing in the water column dominated the overall EC transport for the summer conditions simulated. Dilution due to advection and diffusion accounted for a large portion of the total EC budget in the nearshore, and the net EC loss rate within the water column (CFU/m(3).s) was an order of magnitude smaller compared to the horizontal and vertical transport rates. This result has important implications for modeling EC at recreational beaches; however, the assessment of the magnitude of EC loss rate is complicated due to the strong coupling between vertical exchange and depth-dependent EC loss processes such as sunlight inactivation and settling. Sensitivity analysis indicated that solar inactivation has the greatest impact on EC loss rates. Although these results are site-specific, they clearly bring out the relative importance of various processes involved.

Energy balance and evaporation loss of an agricultural reservoir in a semi‐arid climate (south‐eastern Spain)

AbstractA typical agricultural water reservoir (AWR) of 2400 m2 area and 5 m depth, located in a semi‐arid area (southern Spain), was surveyed on a daily basis for 1 year. The annual evaporation flux was 102·7 W m−2, equivalent to an evaporated water depth of 1310 mm year−1. The heat storage rate G exhibited a clear annual cycle with a peak gain in April (G ∼ 45 W m−2) and a peak loss in November (G ∼ 40 W m−2), leading to a marked annual hysteretic trend when evaporation (λE) was related to net radiation (Rn). λE was strongly correlated with the available energy A, representing 91% of the annual AWR energy loss. The sensible heat flux H accounted for the remaining 9%, leading to an annual Bowen ratio in the order of 0·10. The equilibrium and advective evaporation terms of the Penman formula represented 76 and 24%, respectively, of the total evaporation, corresponding to a annual value of the Priestley–Taylor (P–T) coefficient (α) of 1·32. The P–T coefficient presented a clear seasonal pattern, with a minimum of 1·23 (July) and a maximum of 1·65 (December), indicating that, during periods of limited available energy, AWR evaporation increased above the potential evaporation as a result of the advection process. Overall, the results stressed that accurate prediction of monthly evaporation by means of the P–T formula requires accounting for both the annual cycle of storage and the advective component. Some alternative approaches to estimating Rn, G and α are proposed and discussed. Copyright © 2009 John Wiley & Sons, Ltd.

A new mass conservation approach to the study of CO2 advection in an alpine forest

A new method is proposed for the computation of CO2 Net Ecosystem Exchange (NEE) and its components in a forest ecosystem. Advective flux is estimated by taking into account the air mass conservation principle. For this purpose, wind and dry air density values on the surface of the control volume are first corrected and then the advective flux is estimated on the surface of the control volume. Turbulent flux is also computed along the surface of the control volume while storage flux is computed inside the volume. Additional characteristics of this method are that incompressibility of the mean flow is not assumed a priori, and that vertical and horizontal advective fluxes are not treated separately, but their sum is estimated directly. The methodology is applied to experimental data collected with a three‐dimensional scheme at the alpine site of Renon during the Advex project (July 2005). The advection flux was found to be prevailing positive at night and negative during the day, as was found in previous studies on advection for the same site, but showed a lower scatter in half‐hour calculated values. We tested the effect of its summation on turbulent and storage fluxes to produce half‐hourly values of NEE. Nighttime NEE values were used in functional relations with soil temperature, daytime values with PPFD. The effect of addition of the advection component was an increase in the values of parameters indicating ecosystem respiration, quantum yield, and photosynthetic capacity. The coefficient of correlation between NEE and environmental drivers increased.