Advection Of Sensible Heat Research Articles

On atmospheric pressure and temperature correlation across various terrain types

Temperature (T), pressure (p), and density (ρ) are fundamental variables describing the atmospheric behavior. This study investigates the interdependence of these variables near Earth's surface in real-world conditions, by evaluating data from different European weather stations. It has been found that the correlation between pressure and temperature is strongly related to the dynamics of the PBL that facilitates a two-group classification. The first group includes all the stations in the plain or in the valley-floor and exhibits a weak correlation, R(p,T). 2D density plots representing hourly pressure against temperature have a distinctive triangular shape at these stations. Regardless of location, the upper boundary of this triangle consistently fits a linear equation with a constant slope and an intercept that scales with the average pressure of the station. This finding holds promising implications for enhancing the quality check of pressure and temperature data, enabling the identification of implausible measurements using a unified equation. In contrast, the second group includes stations with a strongly positive correlation R(p,T) and a more linear density plot; it includes all stations near a mountain-top. Their correlations exhibit identical features when compared to radiosounding data extracted at corresponding heights. The study concludes that: i) the first group of stations is significantly influenced by non-hydrostatic processes such as turbulence, friction and surface radiative heating/cooling in the PBL, resulting in weakly negative R(p,T) for shorter timescales that become null over longer durations; ii) the second group of stations has R(p,T) characteristics similar to the free atmosphere, predominantly regulated by hydrostatic balance and the advection of sensible heat.

The 3‐Week‐Long Transport History and Deep Tropical Origin of the 2021 Extreme Heat Wave in the Pacific Northwest

AbstractThe heat wave in late June of 2021 (PNW21) set new temperature records in the Pacific Northwest (PNW). In Lytton the highest temperature ever recorded in Canada was measured. Several studies have already explored this extreme event in detail, however, here we compare the atmospheric air mass transport and heating processes associated with this heat wave with the 34 other most extreme heat events in the same region during the period 1960–2021, using a long backtracking time of 25 days. We found significant differences in the heat waves. During PNW21 most of the air was coming from the Philippine Sea, with more than 40% of the air located south of 15°N, and anomalous advection of sensible and latent heat from the Tropics was the dominant cause of PNW21. The latent heat was efficiently converted into sensible heat by precipitation, which was unique, as most other extremes experienced net diabatic cooling.

Understanding wind-driven melt of patchy snow cover

Abstract. The representation of snow processes in most large-scale hydrological and climate models is known to introduce considerable uncertainty into the predictions and projections of water availability. During the critical snowmelt period, the main challenge in snow modeling is that net radiation is spatially highly variable for a patchy snow cover, resulting in large horizontal differences in temperatures and heat fluxes. When a wind blows over such a system, these differences can drive advection of sensible and latent heat from the snow-free areas to the snow patches, potentially enhancing the melt rates at the leading edge and increasing the variability of subgrid melt rates. To get more insight into these processes, we examine the melt along the upwind and downwind edges of a 50 m long snow patch in the Finseelvi catchment, Norway, and try to explain the observed behavior with idealized simulations of heat fluxes and air movement over patchy snow. The melt of the snow patch was monitored from 11 June until 15 June 2019 by making use of height maps obtained through the photogrammetric structure-from-motion principle. A vertical melt of 23 ± 2.0 cm was observed at the upwind edge over the course of the field campaign, whereas the downwind edge melted only 3 ± 0.4 cm. When comparing this with meteorological measurements, we estimate the turbulent heat fluxes to be responsible for 60 % to 80 % of the upwind melt, of which a significant part is caused by the latent heat flux. The melt at the downwind edge approximately matches the melt occurring due to net radiation. To better understand the dominant processes, we represented this behavior in idealized direct numerical simulations, which are based on the measurements on a single snow patch by Harder et al. (2017) and resemble a flat, patchy snow cover with typical snow patch sizes of 15, 30, and 60 m. Using these simulations, we found that the reduction of the vertical temperature gradient over the snow patch was the main cause of the reductions in vertical sensible heat flux over distance from the leading edge, independent of the typical snow patch size. Moreover, we observed that the sensible heat fluxes at the leading edge and the decay over distance were independent of snow patch size as well, which resulted in a 15 % and 25 % reduction in average snowmelt for, respectively, a doubling and quadrupling of the typical snow patch size. These findings lay out pathways to include the effect of highly variable turbulent heat fluxes based on the typical snow patch size in large-scale hydrological and climate models to improve snowmelt modeling.

Controls of contemporary (2001–2018) springtime waterflow dynamics in a Large, snowmelt-dominated basin in northeastern North America

The objective of this study was to characterise the primary forcing variables and system feedback responsible for daily waterflow dynamics within a large, international river system (Canada and USA) during 17 melt seasons from 2001 to 2018. An analysis based on extreme gradient boosting showed that daily waterflow in four subcatchments of the upper Saint John River (SJR, Wolastoq) basin during the 17 melt seasons was to a large measure controlled by the area’s seasonal warming associated with the springtime increase in regional incident global radiation and northeasterly advection of sensible and latent heat from southerly locations. Historically, seasonal surges in air temperature and cumulative snow degree-days were shown to contribute to roughly 60% of the control on subcatchment discharge by influencing the production and timing of snowmelt. Peak accumulation of snow on the ground provided the second most important control of discharge, accounting for about 15.6% of the overall control at a daily timescale. Cumulative short- and long-term forest cover losses in the four subcatchments provided some control, but at varying levels (i.e., 4.8–14.2%) dependent on the extent of total forest cover loss and other subcatchment traits. Convergent cross mapping confirmed the unidirectional, causal relationship between annual forest cover loss and daily discharge rates at the outlet of three of the four subcatchments. The strength of the annual-forest-cover-removal-to-daily-discharge signal within the four subcatchments varied, with the subcatchment with the least annual forest cover loss (<1%, over the 17 years), predictably displaying the weakest signal (p = 0.282). Forest cover removal was shown to increase springtime discharge for all subcatchments, albeit at different rates. This work provides a more comprehensive, mechanistic interpretation of daily snowmelt control of stream/river flow dynamics in northeastern North America.

Importance of regional indices of atmospheric circulation for periods of warming and cooling in Svalbard during 1920–2018

AbstractThe Arctic has experienced prominent climate warming, at the beginning of the 20th century and currently. Comparing the driving mechanism during these periods helps to explain the causes of contemporary climate change. Our study explores the impact of regional circulation on Svalbard's surface air temperature (SAT, 2 m above ground). We used air temperature data from Svalbard Airport, Bjørnøya stations, and three regional circulation indices that describe the frequency of cyclonic conditions, zonal circulation, and meridional circulation. The indices were calculated for four circulation areas with differing circulation conditions and, therefore, may have various impacts on long‐term changes in SAT. This was checked for the entire study period (1920–2018), and 30‐year sub‐periods representing the most prominent climatic events: the early 20th‐century Arctic warming (ETCAW), contemporary Arctic warming (CAW), and a cold period between them (CAP). In autumn and winter, the deviations in SAT from the long‐term average during warm and cold periods were almost twice as large at Svalbard Airport as at Bjørnøya. In these seasons, the ETCAW was significantly warmer than the subsequent cold period, which was not the case for summer and spring. However, long‐term trends in the regional circulation indices were more evident in summer and spring than in autumn and winter. Air temperature was the most strongly influenced by meridional circulation over the eastern circulation areas, with the exception of spring, when air temperature variability was more affected by zonal circulation. The recent warming weakened the relationship between SAT and the indices in summer. We attributed the ETCAW in autumn to a southerly advection of sensible heat. During the same historical period, the impact of the indices was much weaker in winter. In winter during the CAP, there was a higher frequency of northern air advection, particularly over the northern part of the Greenland Sea.

Thermal impact of magmatism in subduction zones

Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by ∼300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.

Local‐Scale Advection of Sensible and Latent Heat During Snowmelt

AbstractThe breakup of snow cover into patches during snowmelt leads to a dynamic, heterogeneous land surface composed of melting snow, and wet and dry soil and plant surfaces. Energy exchange with the atmosphere is therefore complicated by horizontal gradients in surface temperature and humidity as snow surface temperature and humidity are regulated by the phase change of melting snow unlike snow‐free areas. Airflow across these surface transitions results in local‐scale advection of energy that has been documented as sensible heat during snowmelt, while latent heat advection has received scant attention. Herein, results are presented from an experiment measuring near‐surface profiles of air temperature and humidity across snow‐free to snow‐covered transitions that demonstrates that latent heat advection can be the same order of magnitude as sensible heat advection and is therefore an important source of snowmelt energy. Latent heat advection is conditional on an upwind source of water vapor from a wetted snow‐free surface.

Axisymmetric front tracking model for the investigation of grain structure evolution during directional solidification

A recent numerical investigation by the present authors Battaglioli et al. (2017) had shown that the heat and mass transfer associated with steady-state Bridgman furnace solidification is dependent on advection of sensible and latent heat, and also on the axisymmetric geometry of the crucible. The present work extends the previous one by considering more complex non-steady solidification scenarios, where transient (limited-duration) Bridgman solidification is subsequently combined with a controlled power-down cooling process. A significant advancement with respect to the previous model is the inclusion of a front tracking method for the simulation of columnar growth. In the front tracking method the columnar mush region is demarcated by a series of markers that advance at a growth rate governed by the solutal undercooling at the dendrite tips. A classic micro-segregation law is used to govern the evolution of solid fraction in the columnar mush and the release of latent heat. The inclusion of the front tracking method has provided greater insights into how heat fluxes and thermal conditions can influence the final grain structure. Firstly, we show that by analysing the trajectories of the markers, it is possible to predict the transition from axial columnar growth (directional solidification) to unwanted radial columnar growth due to significant radial heat fluxes in the sample. Secondly, we show that model can simulate an undercooled liquid region ahead of the columnar front where equiaxed grains could possibly nucleate and grow, leading to a columnar to equiaxed transition (CET). Two indirect CET prediction methods from literature have been included to the model to assess the likelihood of a CET occurring in the as-cast grain structure of the alloy. The model was employed to simulate experimental scenarios from literature involving a γ-TiAl alloy where the samples were subjected to different transient cooling conditions in a Bridgman furnace. Examples of axial columnar growth, radial columnar growth, and CET are discussed in detail with elucidation from the model.

Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

The formation of new ocean crust at mid-ocean ridges is a fundamental component of the plate tectonic cycle and involves substantial transfer of heat and mass from the mantle. Hydrothermal circulation at mid-ocean ridges is critical for the advection of latent and sensible heat from the lower crust to enable the solidification of ocean crust near to the ridge axis. The sheeted dike complex (SDC) is the critical region between the eruptive lavas and the gabbros through which seawater-derived recharge fluids must transit to exchange heat with the magma chambers that form the lower ocean crust.ODP Hole 1256D in the eastern equatorial Pacific Ocean provides the only continuous sampling of in-situ intact upper ocean crust formed at a fast spreading rate, through the SDC into the dike–gabbro transition zone. Here we exploit a high sample density profile of the Sr-isotopic composition of Hole 1256D to quantify the time-integrated hydrothermal recharge fluid flux through the SDC. Assuming kinetically limited fluid–rock Sr exchange, a fluid flux of 1.5–3.2×106 kgm−2 is required to produce the observed Sr-isotopic shifts. Despite significant differences in the distribution and intensity of hydrothermal alteration and fluid/rock Sr-isotopic exchange between Hole 1256D and SDC sampled in other oceanic environments (ODP Hole 504B, Hess Deep and Pito Deep), the estimated recharge fluid fluxes at all sites are similar, suggesting that the heat flux extracted by the upper crustal axial hydrothermal system is relatively uniform at intermediate to fast spreading rates.The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ∼20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid–rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper hydrothermal system. The first requires the downward expansion of the upper crustal hydrothermal system ∼800 m into the lower crust in response to a downward migrating conductive boundary layer. The second scenario invokes a separate hydrothermal system in the lower crust for which fluid recharge bypasses reaction with the sheeted dikes, perhaps via flow down faults.

Numerical simulation of Bridgman solidification of binary alloys

A transient 2D axisymmetric numerical model for the Bridgman solidification process for a stationary furnace and moving sample is presented. The model is able to predict the evolution of temperature and solid fraction of binary alloys in cases where buoyancy induced convection is negligible, such as in microgravity conditions. A dimensionless form of the governing equations was derived in order to identify the dimensionless parameters that characterise the process, those being the Stefan, Péclet, and Biot numbers. The problem was solved using a finite volume method and an explicit time stepping scheme. To test the efficacy of the model, simulated results were compared with experimental data from the literature and acceptable agreement was obtained. Finally, a parametric analysis was performed for understanding the influence of the process parameters on solidification. One key feature of this study was the inclusion of a term describing the advection of latent heat due to the translation of the mushy zone with varying solid fraction. This thermal transport mechanism was shown to be significant, since its magnitude was comparable to the advection of sensible heat. It was also found that when small Biot numbers were due to low values of the heat transfer coefficients at the surface of the sample, rather than to small sample radii, advective mechanisms were enhanced resulting in more convex shapes of the liquidus isotherm. This highlighted the importance of considering both axial and radial heat fluxes when describing the process.

Comparison of lysimeter based and calculated ASCE reference evapotranspiration in a subhumid climate

The standardized form of the well-known FAO Penman-Monteith equation, published by the Environmental and Water Resources Institute of the American Society of Civil Engineers (ASCE-EWRI), is recommended as a standard procedure for calculating reference evapotranspiration (ETref) and subsequently plant water requirements. Applied and validated under different climatic conditions it generally achieved good results compared to other methods. However, several studies documented deviations between measured and calculated reference evapotranspiration depending on environmental and weather conditions. Therefore, it seems generally advisable to evaluate the model under local environmental conditions. In this study, reference evapotranspiration was determined at a subhumid site in northeastern Austria from 2005 to 2010 using a large weighing lysimeter (ETlys). The measured data were compared with ETref calculations. Daily values differed slightly during a year, at which ETref was generally overestimated at small values, whereas it was rather underestimated when ET was large, which is supported also by other studies. In our case, advection of sensible heat proved to have an impact, but it could not explain the differences exclusively. Obviously, there were also other influences, such as seasonal varying surface resistance or albedo. Generally, the ASCE-EWRI equation for daily time steps performed best at average weather conditions. The outcomes should help to correctly interpret ETref data in the region and in similar environments and improve knowledge on the dynamics of influencing factors causing deviations.

Variations of crop coefficient and its influencing factors in an arid advective cropland of northwest China

Understanding the variation and magnitude of crop coefficient (Kc) is important for accurate determination of crop evapotranspiration and water use. In this study, we calculated Kc in an irrigated maize field with ground mulching by eddy covariance evapotranspiration measurements during the whole growing periods in 2009 and 2010 in an arid region of northwest China. A semi-empirical practical approach for estimating Kc was proposed by introducing the dynamic fraction of canopy cover and incorporating the effect of leaf senescence as a function of days after sowing. The contribution of arid advection of sensible heat resulting from irrigation to Kc and the response of Kc to canopy conductance (Gc) were investigated. The averaged values of daily Kc were lower than typical values obtained previously without mulching due to decreasing effect of mulching on Kc, with 0.82 and 0.80 for the 2 years, respectively. The maximum average Kc occurred at the heading stage, with 1.21 and 1.04 for the 2 years, respectively. The difference of Kc was attributed to the difference of leaf area index. The semi-empirical practical approach could well estimate the variations of Kc, thus could be a robust and useful tool for the practical users and water managers. The contributions to daily Kc from the arid advection were 4.4–28.0% of the measured Kc. The Gc had stronger control on daily Kc at the early and later stages than at the middle stage. When Gc, leaf area index and relative soil extractable water were lower than the respective threshold values of 20 mm s−1, 3.0 m2 m−2 and 0.5, the daily Kc increased significantly with the increase of the three factors, and almost remained constant when the three factors were beyond the threshold values. These results are helpful for quantifying contributions of individual factors to Kc, and subsequently improving water management practices according to Kc. Copyright © 2014 John Wiley & Sons, Ltd.

On Estimating Wet Canopy Evaporation from Deciduous and Coniferous Forests in the Asian Monsoon Climate

Abstract Continuous and direct measurement of evapotranspiration (ET) by the eddy covariance (EC) technique is still a challenge under monsoon climate because of a considerable amount of missing data during the long rainy periods and the consequential gap-filling process. Under such wet canopy conditions, especially in forests, evaporation of the intercepted precipitation (EWC) contributes significantly to the total ET. To quantify the role of EWC, leaf wetness has been measured at multiple levels in the canopy simultaneously with eddy covariance measurements at the KoFlux Gwangneung deciduous and coniferous forests for the entire year from September 2007 to August 2008. In this study, the measured EWC and the controlling mechanism during the wet canopy conditions have been scrutinized. Based on the evaluation of the four different algorithms of EWC estimation, that of the variable infiltration capacity (VIC) land surface model (LSM) has been adopted. All the missing EWC data are then recalculated by using the algorithm of VIC LSM and compared against the traditionally gap-filled EWC data based on the modified lookup table (MLT) method. The latter consistently underestimated EWC on average by 39% in deciduous forest and by 28% in coniferous forest. Major causes of such differences were due to the failure of considering aerodynamic coupling, advection of sensible heat, and heat storage in the MLT-based gap-filling method. Accordingly, a new gap-filling strategy for EWC is proposed that takes proper controlling mechanisms into account.

Micrometeorological processes driving snow ablation in an Alpine catchment

Abstract. Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micro-meteorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological boundary-layer model (Advanced Regional Prediction System) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modeled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances of about 4–6 m. Measurements suggest that mean wind velocities of about 5 m s−1 are required for advective heat transport to increase snow ablation over a long fetch distance of about 20 m. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value of approximately 0.6. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is low. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.

Field estimates of scalar advection across a canopy edge

Measurements of the horizontal and vertical fluxes of energy and mass were taken at a crop canopy edge and downwind of it to examine the effects of inhomogeneity on surface–atmosphere exchange, for the ideal case of flat uniform terrain. Energy balance closure was also examined and the effects of the often-neglected advection terms on the energy budget were evaluated. The results show that neither vertical nor horizontal advection should be neglected for moderate distances downwind from heterogeneous boundaries, with the energy budget closure near the leading edge improved by more than 20% with the inclusion of the horizontal and vertical advection of latent and sensible heat. Significant mean advective horizontal and vertical flux divergences of water vapor and temperature were found even in typical daytime conditions. In stable conditions horizontal advection can be responsible for more than 15% of the scalar (water vapor) transport 136 canopy heights downwind of a change in water vapor source density and surface roughness.

Variability of Sub-Canopy Flow, Temperature, and Horizontal Advection in Moderately Complex Terrain

We examine the space–time structure of the wind and temperature fields, as well as that of the resulting spatial temperature gradients and horizontal advection of sensible heat, in the sub-canopy of a forest with a dense overstorey in moderately complex terrain. Data were collected from a sensor network consisting of ten stations and subject to orthogonal decomposition using the multiresolution basis set and stochastic analyses including two-point correlations, dimensional structure functions, and various other bulk measures for space and time variability. Despite some similarities, fundamental differences were found in the space–time structure of the motions dominating the variability of the sub-canopy wind and temperature fields. The dominating motions occupy similar spatial, but different temporal, scales. A conceptual space–time diagram was constructed based on the stochastic analysis that includes the important end members of the spatial and temporal scales of the observed motions of both variables. Short-lived and small-scale motions govern the variability of the wind, while the diurnal temperature oscillation driven by the surface radiative transfer is the main determinant of the variability in the temperature signal, which occupies much larger time scales. This scale mismatch renders Taylor’s hypothesis for sub-canopy flow invalid and aggravates the computation of meaningful estimates of horizontal advective fluxes without dense spatial information. It may further explain the ambiguous and inconclusive results reported in numerous energy and mass balance and advection studies evaluating the hypothesis that accounting for budget components other than the change in storage term and the vertical turbulent flux improves the budget closure when turbulent diffusion is suppressed in plant canopies. Estimates of spatial temperature gradients and advective fluxes were sensitive to the network geometry and the spatial interpolation method. The assumption of linear spatial temperature gradients was not supported by the results, and leads to increased spatial and temporal variability of inferred spatial gradients and advection estimates. A method is proposed to estimate the appropriate minimum network size of wind and temperature sensors suitable for an evaluation of energy and mass balances by reducing spatial and temporal variability of the spatially sampled signals, which was estimated to be on the order of 200 m at the study site.

Measurement of the Sensible Eddy Heat Flux Based on Spatial Averaging of Continuous Ground-Based Observations

Using the standard eddy-covariance (EC) method to quantify mass and energy exchange at a single location usually results in an underestimation of vertical eddy fluxes at the surface. In order to better understand the reasons for this underestimation, an experimental set-up is presented that is based on spatial averaging of air temperature data from a network of ground-based sensors over agricultural land. For eight days during the 34-day observational period in May and June 2007, additional contributions to the sensible heat flux of more than 50Wm−2 were measured in the lower surface layer by applying the spatial EC method as opposed to the standard temporal EC method. Smaller but still significant additional sensible heat fluxes were detected for four more days. The additional energy is probably transported in organised convective structures resulting in a mean vertical wind velocity unequal to zero at the tower location. The results show that convective transport contributes significantly to the surface energy budget for measurement heights as low as 2–3 m. Since these structures may be quasi-stationary, they can hardly be captured by a single-location measurement. The spatial EC set-up presented here is capable of quantifying contributions to the sensible heat flux from structures up to the scale of our spatial sensor network, which covered an area 3.5 × 3.5 km. For future experiments aiming at closing the energy balance, the spatial EC method should be employed to measure both the sensible and latent heat fluxes. Experimental determination of the horizontal advection of sensible and latent heat should also be considered, since such transport must occur due to convergence and divergence related to convection.

The influence of soil moisture transfer on building heat loss via the ground

In this paper, the influence of soil moisture transfer on building heat loss via the ground is investigated by comparing fully coupled simulations with linear thermal simulations. The observed influences of coupling are (1) the larger amplitude of surface temperature, (2) the variation of thermal conductivity with moisture content, and (3) the advection of sensible heat by liquid transfer. In a parameter study, it is shown that these conclusions hold for a variety of climates, soils and foundation constructions. However, given the current accuracy level of standard methods, the integration of coupling effects in these calculation methods cannot be defended.

Modelling of evaporation in a sparse millet crop using a two-source model including sensible heat advection within the canopy

During two successive growing seasons meteorological measurements were made in a pearl millet field in the Sahel to investigate the evaporation process in relation to crop growth. The evaporation was measured by eddy correlation and simulated using the Shuttleworth Wallace (SW) model [Q. J. R. Meteorol. Soc. 111 (1985) 839–855]. To take sun height and multi-layer scattering into account a radiation balance model was formulated. The model indicates that partitioning of the net radiation between the vegetation and the soil may be estimated ( r 2=0.94) from the fraction of diffuse radiation, the leaf area index and an attenuation coefficient, but that the attenuation coefficient may not be similar in different locations. To solve the SW-model with respect to the soil resistance an iterative solution was employed with the total evaporation estimated from the Bowen-ratio calculated from eddy correlation measurements. The procedure made it possible to derive stable estimates of soil resistance at soil evaporation rates down to 25 W m −2. The soil resistance was found to be in accordance with evaporation through a dry surface layer. The SW-model indicates, that advection of sensible heat from the dry soil to the plants, increases transpiration considerably. This will cause management techniques, such as mulching and dry farming, aimed at reducing soil evaporation to be less effective than might be anticipated. The effects of raising the leaf area index to improve the microclimate is discussed in relation to management of the available water and crop security.

Boundary‐layer integration approach to advection of sensible heat to a patchy snow cover

AbstractIn the calculation of the melting of a patchy snow cover, the energy advected from the adjacent bare soil to the snow surface is an important consideration. The quantity or rate of energy advected depends on the fetches and sizes of snow and bare ground patches. Any successful method to estimate advection will necessarily require the incorporation of relationships describing the same. Complex boundary‐layer methods require detailed spatial knowledge of the patch sizes, wind direction and fetch distances, and are computationally intensive. A physically based approach that can be spatially applied using distributions of snow patch geometry is required.This paper presents a new approach, in which boundary‐layer integration is used to provide a means of calculating the amount of energy removed by the snow patch surface as warmer air moves over it. The method is reduced to a simple parametric form, and the relationships describing the coefficients required for its application are developed.The applicability of this new approach is discussed in light of the fractal nature of snow patches, the relationship between the individual patch length and area, and the distribution of patch sizes as they develop and disappear on the landscape. Copyright © 2002 Crown in the right of Canada. Published by John Wiley &amp; Sons Ltd.