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Snow ablation modeling at the stand scale in a boreal jack pine forest

The purpose of this study is to predict spatial distributions of snow properties important to the hydrology and the remote sensing signatures of the boreal ecosystem. This study is part of the Boreal Ecosystems Atmosphere Study (BOREAS) of central Saskatchewan and northern Manitoba. Forested environments provide unique problems for snow cover process modeling due to the complex interactions among snow, energy transfer, and trees. These problems are approached by coupling a modified snow process model with a model of radiative interactions with forest canopies. Additionally, a tree well model describes the influence of individual trees on snow distribution on the ground. The snow process and energy budget model calculates energy exchange at the snow surface, in-pack snow processes, melting and liquid water flow, heat conduction, and vapor diffusion. The surface radiation model provides input on the radiation receipt at the snow surface for model runs in the jack pine forest. Field data consisted of measured meteorological parameters above and within the canopy, spatial variability of snow properties, and variations of incoming solar irradiance beneath the forest canopy. Results show that the area beneath tree canopies accumulated 60% of the snow accumulated in forest openings. Peak solar irradiance on the snow cover was less than one half that measured above the canopy. Model runs are compared between the open and the forested sites and show the open area ablating four days before areas beneath the canopy and eight days before forest openings and compare favorably with measured data. Physically based modeling of snow ablation was successful at the forested site and nearby open area.

Transmission of solar radiation in boreal conifer forests: Measurements and models

A combined geometric-optical and radiative transfer (GORT) model allows incorporation of multiple scales of clustering in conifer canopies on the estimation of radiation transmission. Consideration of clustering of branches into whorls is the latest addition to this model. Modification of the GORT model to include whorl orientation improves the ability to model the observed patterns of solar radiation transmission as a function of solar zenith angle and height in the canopy. Whorl orientation distributions are derived from multidirectional measurements using a geometric optical mutual shadowing model. For BOREAS test stands, model estimates and vertical measurements of photosynthetically active radiation transmittance within the canopy show (1) general decreases in transmission as solar zenith angles increase in the range of solar zenith angles dominated by beam irradiance, (2) increases in PAR transmission at very high solar zenith angles where diffuse skylight is dominant, (3) maximum scattering and absorption occur in the middle of the canopy. Model estimates match measurements from the forest floor, indicating the value of the model for providing radiation inputs to snowmelt models in forested landscapes.

Variation of snow cover ablation in the boreal forest: A sensitivity study on the effects of conifer canopy

The duration and meteorological history of winter and thaw periods in the boreal forest affect carbon exchange during the growing season. Characteristics of conifer canopies exert important control on the energy exchange at the forest floor, which in turn controls snow cover processes such as melting. This analysis investigated the role of the conifer tree characteristics, including height and canopy density. Canopy and snow models estimated radiation incoming to the snow surface, the net energy budget of the snow, and melting rates of snow cover under conifer forests with different canopy density and tree height. This analysis assumed that canopy effects dominated snow surface energy exchange under conifers in the boreal forest. We used data layers of forest characteristics from the Boreal Ecosystem-Atmosphere Study (BOREAS) modeling subareas in Saskatchewan and Manitoba to guide the choice of modeled tree height and canopy density. Modeled stand characteristics assumed random location of trees and used a uniform tree height within a stand and regular crown geometry scaled to tree height. Measurements during winter and thaw in 1994 of incoming solar and longwave radiation, humidity, and wind speed above the forest canopy provided input to the models, along with air temperature measured in the canopy. Results showed the importance of canopy density and tree height as the first-order controls on cumulative incoming solar radiation at the forest floor for the range of these variables in the BOREAS test area. The combined canopy and snow models showed a large range of snow ablation within conifers, which showed the trade-offs between canopy density and tree height. Solar fluxes dominated the net transfer of energy to the snow in the north, while sensible heat exchange, net solar, and net longwave radiation played important roles in the south.

An analytical hybrid GORT model for bidirectional reflectance over discontinuous plant canopies

The geometric optical (GO) bidirectional reflectance model, combined with a new component spectral signature submodel, can be used to estimate the bidirectional reflectance distribution function (BRDF) of discontinuous canopies. This approach retains the GO approach of incorporating the effect of shadows cast by crowns on the background. The newly developed submodel uses an analytical approximation of the radiative transfer (RT) within the plant canopies to model the spectral properties of each scene component. A multiple scale-hotspot function that incorporates effects for smaller canopy objects like branches, stems and leaves was also well modeled. Comparison of model results with field measurements (ASAS, POLDER and PARABOLA) over an old black spruce forest in central Canada demonstrated that the model ran predict the basic features of the BRDF, i.e., bowl shape and the hotspot. The benefits of the model presented are simplicity, improved treatment of multiple scattering and a new method of estimating the component signatures.

Snow ablation modelling in a mature aspen stand of the boreal forest

Snow ablation modelling at the stand scale must account for the variability in snow cover and the large variations of components of energy transfer at the forest floor. Our previous work successfully predicted snow ablation in a mature jack pine stand by using a one-dimensional snow process model and models predicting radiation below forest canopies. This work represents a second test of our basic modelling scenario by predicting snow ablation in a leafless, deciduous aspen stand and verifying the results with field data. New modifications to the snow model accounted for decreased albedo owing to radiation penetration through optically thin snowpacks. A provisional equation estimates litter fall on the snowpack, thereby reducing the areal averaged albedo. We showed that subcanopy radiation measurements can be used with a canopy model to estimate a branch area index for defoliated aspen as an analogue to the foliage area index used for conifers. Modelled incoming solar and long-wave radiation showed a strong correlation with measurements, with r 2 a 0.96 and 0.91 for solar and long-wave radiation, respectively. Model results demonstrate that net radiation overwhelms turbulent exchanges as the most significant driving force for snowmelt in aspen forests. Predicted snow ablation in the aspen stand compared very favourably with available data on snow depth. #1998 John Wiley & Sons, Ltd.

Effect of canopy structure and the presence of snow on the albedo of boreal conifer forests

A Geometric-Optical and Radiative Transfer (GORT) approach for modeling the radiation regime within plant canopies is capable of predicting temporal variation in the albedo of boreal conifer forests. Model predictions of daily surface albedo patterns and reflected solar radiation during the winter and summer seasons were validated using field measurements from two forest stands in the northern study area of BOReal Ecosystem-Atmosphere Study (BOREAS) in 1995. The model is able to predict the “W” shape for the daily albedo over the sparse old jack pine forest stand during the snow season and the “bowl” shape of daily albedo during clear days in the summer. Results immediately following new snow and at the end of the snowmelt season indicate the sensitivity of overall forest albedos to the albedo of snow. Incorporation of time-varying values for snow albedo may improve future efforts to estimate forest albedos in the winter. Forest albedos are a complicated function of the canopy structure, the presence or absence of snow on the ground and the angular distribution of irradiance. These effects differ for the visible, near-infrared and midinfrared portions of the solar spectrum. Forest albedos vary dramatically as a function of canopy cover when snow covers the ground, but very little when snow is not present. It is found that for tree cover over about 70%, the presence of snow has little effect on albedo.

Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models

Net solar radiation (Q * ) and the partitioning of Q * between vegetation (Q veg * ) and the substrate (Q soil * ) are important quantities computed by soil-vegetation-atmosphere-transfer (SVAT) models. Commonly, two-stream models of canopy radiative transfer are used for this purpose. We examine the validity of this approach for nonuniform canopies and compare estimates of Q * , Q veg * , and Q soil * computed using a two-stream model with estimates computed using a model that accounts for the effects of three-dimensional (3-D) structure in vegetation on radiative transfer. To accomplish these goals, a sensitivity analysis is conducted for the key input parameters to two-stream canopy radiative transfer models. The parameters examined include leaf area index (L), leaf optical properties, solar zenith angle, leaf orientation, and background albedo. Sensitivity analyses are also conducted using a Geometric-Optical Radiative Transfer (GORT) model. The GORT model treats vegetation canopies as being composed of 3-D crowns and allows for both vertical variation in leaf area and horizontal variation in stem density. Results show that Q * computed by the two-stream model varies by up to 10% relative to the GORT model. Further, the partitioning of Q * between the vegetation canopy and substrate computed by the two-stream model can vary by up to 30% relative to the more realistic GORT model, even for relatively dense canopies (L=6). These differences arise because two-stream models use parameterizations for gap probabilities that are not realistic in discontinuous canopies. Based on this conclusion, a parameterization is proposed to include the effects of canopy heterogeneity in two-stream models. Results from the original two-stream model, a parameterized two-stream model, and the GORT model are compared using data from the Boreal Ecosystem-Atmosphere Study. These results show that the proposed parameterization captures the effect of 3-D structure in vegetation on radiation regimes and is therefore suitable for inclusion in SVAT models. Inclusion of improved treatment for radiative fluxes in land surface parameterizations should improve modeled estimates for other flux quantities computed by SVAT models.

Variance in bidirectional reflectance over discontinuous plant canopies

Abstract An exploratory study of the variance of the bidirectional reflectance over discontinuous plant canopies indicates that the patterns in variance can be related to the properties of the plant canopies. The spatial variance of the bidirectional reflectance calculated from ASAS images shows peak values at the hotspot and near nadir. This behavior can be explained by the geometric effect of discontinuous tree crowns and the regularization effect. Validation of Jupp and Woodcock’s two-component geometric optical (GO) model (1992) shows that it captures the basic features of the spatial variance of the bidirectional reflectance over discontinuous plant canopies. Their two-component GO model is modified to account for the spatial interactions of four scene components. Validation shows that the modified GO model improves predictions. This exploratory study will benefit future use of directional imagery to recover surface parameters by helping characterize the distributional properties of directional imagery.

Spatial variance in directional remote sensing imagery‐recent developments and future perspectives*

High resolution images using a range of different technologies and sensed from air and space platforms are becoming increasingly available. Spatial statistics may be used to analyze the texture of such images to infer many types of information – including plant canopy structure. This paper summarizes a line of work which has sought to interpret the spatial variation of directional images of vegetation canopies and vegetation‐covered surfaces using a common framework of geo‐optical models. The early work on statistical analysis of hemispherical photographs by Nilson is described as well as Li and Strahlers variance model for crown size estimation. Then, the texture based work by Jupp and Woodcock is described. This involved analysis of aerial photos and airborne scanner data in open forests. Finally, some recently presented work using airborne ASAS images in dense conifer forest by the present authors is described. Previously unpublished work on the inversion of a simple spatial variance model, combined with considerations of BRDF, points to how interpreting the spatial variance of directional remote images through geo‐optical models can contribute the retrieval of canopy parameters such as tree sizes and canopy gaps through the integration off all of these stands. This paper aims to benefit future use of directional imagery to recover surface parameters by bringing these various pieces of work together. They collectively show how characterizing the distributional properties of directional imagery and interpreting them through geo‐optical models can provide canopy information and suggest how interpretation of various data sources ‐ including from canopy Lidars – can also benefit from the outcomes of model‐based spatial and directional variance research.

Modeling solar radiation transmission in boreal conifer forests

A hybrid geometric-optical and radiative transfer (GORT) model developed for the radiation regime in discontinuous vegetation canopies, is modified to include branch orientation properties. This new version of the model was used to estimate solar radiation transmission in old jack pine (OJP) and old black spruce (OBS) forests in Canada. Radiation measurements collected in OJP and OBS forests in Canada show that radiation transmission as a function of solar zenith angle and height in the canopy deviates from what is predicted by Beers law. By considering gap probabilities between crowns in addition to within individual crowns, GORT estimates more accurately transmission distribution in the canopy. Modification of the GORT model to include branch orientation improves the ability to model the observed patterns of solar radiation transmission as a function of solar zenith angle and height in the canopy in the SOJP site. Branch orientation distributions are derived from PARABOLA measurements by the geometric-optical mutual shadowing (GOMS) model.