Richard A. Fournier

Improved general circulation models of the Martian atmosphere from the surface to above 80 km

We describe a set of two “new generation” general circulation models of the Martian atmosphere derived from the models we originally developed in the early 1990s. The two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations. The vertical resolution near the surface has been refined, and the vertical domain has been extended to above 80 km. These changes are accompanied by the inclusion of state-of-the-art parameterizations to better simulate the dynamical and physical processes near the surface (boundary layer scheme, subgrid-scale topography parameterization, etc.) and at high altitude (gravity wave drag). In addition, radiative transfer calculations and the representation of polar processes have been significantly improved. We present some examples of zonal-mean fields from simulations using the model at several seasons. One relatively novel aspect, previously introduced by Wilson [1997], is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi-global, angular-momentum conserving Hadley cell which has no terrestrial equivalent. Within such a cell the Coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km. This winter polar warming had been observed but not properly modeled until recently. In fact, thermal inversions are generally predicted above one, and often both, poles around 60–70 km. However, the Mars middle atmosphere above 40 km is found to be very model-sensitive and thus difficult to simulate accurately in the absence of observations.

A climate database for Mars

A database of statistics which describe the climate and surface environment of Mars has been constructed directly on the basis of output from multiannual integrations of two general circulation models developed jointly at Laboratoire de Meteorologie Dynamique du Centre National de la Recherche Scientifique, France, and the University of Oxford, United Kingdom, with support from the European Space Agency. The models have been developed and validated to reproduce the main features of the meteorology of Mars, as observed by past spacecraft missions. As well as the more standard statistical measures for mission design studies, the Mars Climate Database includes a novel representation of large-scale variability, using empirical eigenfunctions derived from an analysis of the full simulations, and small-scale variability using parameterizations of processes such as gravity wave propagation. The database may be used as a tool for mission planning and also provides a valuable resource for scientific studies of the Martian atmosphere. The database is described and critically compared with a representative range of currently available observations.

A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission

Due to the scarcity and high cost of conventional film-based hemispherical photographic systems, some forest scientists are now using multi-purpose, consumer-grade digital cameras for the analysis of forest canopy structure and gap light transmission. Although the low cost of digital cameras and direct capture of digital images appear to offer significant advantages over film camera systems, relatively little is known about their technical differences from an applications perspective. In this study, we compared the performance of a popular digital camera (Nikon Coolpix 950 with FC-E8 fisheye) with a conventional film camera (Nikon F with Nikkor 8 mm fisheye) under different stand structures and sky conditions. Our findings show that the Nikon Coolpix 950 digital camera produced hemispherical canopy photos with substantial color blurring towards the periphery of the exposure. We believe that chromatic aberration associated with the camera’s lens optics may be the source of this phenomenon; however, other factors may have also contributed to the diminished image sharpness. Color blur influenced (i) the size, shape, and distribution of canopy gaps; (ii) the accuracy of edge detection and the binary division of pixels into sky and canopy elements, and (iii) the magnitude, range, and replication of canopy openness, leaf area, and transmitted global radiation results. The Nikon Coolpix 950 produced canopy openness measures that were 1.4 times greater than film estimates in 22 of the 36 photo pairs. Cloud cover and sky brightness also influenced the spectral characteristics of the lateral chromatic aberration (halos), and thus had an added and unpredictable effect on canopy openness. Setting the Nikon Coolpix 950 to record in black and white, and shooting only under uniformly overcast skies will help to minimize the unpredictable effects of chromatic aberration. Nevertheless, we recommend a cautious approach when undertaking canopy measurements with the Nikon Coolpix 950, particularly when stands are dense and canopy openness falls below 10%. High-quality (1:4) JPEG compression had no significant influence on mean canopy openness; however, lower XGA and VGA image resolutions

Remote sensing and the measurement of geographical entities in a forested environment. 2. The optimal spatial resolution

Abstract The prime objective of this study was to propose and test a method to identify the optimal spatial resolutions for detection and discrimination of coniferous classes in a temperate forested environment. The approach is based on the paradigm that there is an intricate relationship between the definition and the measurement of geographical entities and implies the following steps: 1) a priori define the geographical entities under investigation, 2)determine an optimization criterion for the choice of a sampling system, 3) progressively aggregate data acquired from a fine spatial sampling grid, 4) apply the optimization criterion on the series of spatially aggregated data, and 5) verify the validity of the results obtained in relation to the goal of the study. Airborne MEIS-II data, acquired at 0.5 m in eight spectral bands of the visible spectrum, were used for the study. Fourteen forest classes, at the stand level, were defined on the basis of four attributes: species, density, height, and organization of the trees. Representative sites for each forest class were selected. From the center of each site, the spatial resolution of the original data was degraded to 29.5 m, with an increment of 1 m, using an averaging window algorithm. The intraclass variance was calculated for each forest class, at every spatial resolution and for the eight spectral bands. The minimal variance was used as the indicator of the optimal spatial resolution. To evaluate the importance of the optimal resolution for class discrimination, a bivariate test of variance was performed for each pair of forest class considered at their optimal spatial resolution. Profiles of spectral separability were also established in relation to the whole series of spatial resolutions. The results show that, for all coniferous classes and for the eight spectral bands considered in the study, there is a minimal value in intraclass variance that indicates the optimal spatial resolution for each class, varying between 2.5 m and 21.5 m. The optimal spatial resolution is primarily affected by the spatial and structural parameters of the forest stands. The analysis of variance between each pair of forest classes considered at their respective optimal spatial resolution reveals that all classes are significantly different in at least two spectral bands, except for 10 pairs. The spectral separability of the forest classes is at a maximum at, or very close to, their optimal spatial resolution. The study confirms the validity of the concept of optimal spatial resolution and proposes an original solution to the problem of the adequate scale of measurement for geographical entities.

An architectural model of trees to estimate forest structural attributes using terrestrial LiDAR

Terrestrial lidar (TLiDAR) has been used increasingly over recent years to assess tree architecture and to extract metrics of forest canopies. Analysis of TLiDAR data remains a difficult task mainly due to the effects of object occlusion and wind on the quality of the retrieved results. We propose to link TLiDAR and tree structure attributes by means of an architectural model. The proposed methodology uses TLiDAR scans combined with allometric relationships to define the total amount of foliage in the crown and to build the tree branching structure. It uses the range (distance) and intensity information of the TLiDAR scans (i) to extract the stem and main branches of the tree, (ii) to reconstruct the fine branching structure at locations where the presence of foliage is very likely, and (iii) to use the availability of light as a criterion to add foliage in the center of the crown where TLiDAR information is sparse or absent due to occlusion effects. An optimization algorithm guides the model towards a realistic tree structure that fits the information gathered from TLiDAR scans and field inventory. The robustness and validity of the proposed model is assessed on five trees belonging to four different conifer species from natural forest environments. This approach addresses the data limitation of TLiDAR scans and aims to extract forest architectural metrics at different structural levels.

Seasonal change in understory reflectance of boreal forests and influence on canopy vegetation indices

One objective of the Boreal Ecosystem-Atmospheric Study (BOREAS) is to increase our understanding of the nature of canopy spectral bidirectional reflectance in the visible/near-infrared regimes for open canopies typical of boreal forest stands. For such stands, the need to characterize the reflectance of the sunlit and shaded vegetated understory is critical. These variables are subject to temporal variability due to differences in species phenology and foliar display as well as diurnal and seasonal changes in solar illumination through a seasonally varying upper canopy foliar area. To provide for this need, a multiteam field effort was mounted to measure the nadir midday understory reflectance for the flux tower sites during 1994 BOREAS field campaigns between February and October, specifically during the winter focused field campaign (FFC-W), the spring thaw focused field campaign (FFC-T), and the three intensive field campaigns (IFC-1, IFC-2, and IFC-3) between June and September, which sample vegetation phenological change. This was accomplished by measuring at near-solar noon the sunlit and shaded nadir reflectance of the understory along a surveyed leaf area index (LAI) transect line at each flux tower site. Site-to-site comparisons of understory reflectance spectra reveal stand differences that become more significant as the season progresses. Mean midday understory reflectance spectra were observed to be remarkably consistent over the season for young jack pine stands, followed by somewhat increased variability for mature jack pine, and significant seasonal variability for black spruce stands. Derived vegetation indices for understories are generally consistent with extrapolations of previous relationships of canopy spectral vegetation indices (VIs) versus leaf area index to zero LAI. Inclusion of these “zeroLAI” understory-derived indices significantly enhance the correlation in the linear VI-LAI relationships.

Mapping canopy height using a combination of digital stereo-photogrammetry and lidar

Ranging techniques such as lidar (LIght Detection And Ranging) and digital stereo‐photogrammetry show great promise for mapping forest canopy height. In this study, we combine these techniques to create hybrid photo‐lidar canopy height models (CHMs). First, photogrammetric digital surface models (DSMs) created using automated stereo‐matching were registered to corresponding lidar digital terrain models (DTMs). Photo‐lidar CHMs were then produced by subtracting the lidar DTM from the photogrammetric DSM. This approach opens up the possibility of retrospective mapping of forest structure using archived aerial photographs. The main objective of the study was to evaluate the accuracy of photo‐lidar CHMs by comparing them to reference lidar CHMs. The assessment revealed that stereo‐matching parameters and left–right image dissimilarities caused by sunlight and viewing geometry have a significant influence on the quality of the photo DSMs. Our study showed that photo‐lidar CHMs are well correlated to their lidar counterparts on a pixel‐wise basis (r up to 0.89 in the best stereo‐matching conditions), but have a lower resolution and accuracy. It also demonstrated that plot metrics extracted from the lidar and photo‐lidar CHMs, such as height at the 95th percentile of 20 m×20 m windows, are highly correlated (r up to 0.95 in general matching conditions).

Integrating clumping effects in forest canopy structure: an assessment through hemispherical photographs

Methods for analysing foliage nonrandomness in forest canopies by means of hemispherical photographs are assessed. These methods involve calculation of the canopy element clumping factor, at scales coarser than that of the shoot, to adjust for clumping effects on leaf area index (LAI) estimates derived from gap fraction measurements. Two approaches are presented. The first is based on a gap size accumulation method (the Chen and Cihlar clumping index), whereas the second relies on a gap size distribution method (the Pielou coefficient of segregation). Both methods take advantage of hemispherical photographs, allowing measurement of gap size and gap fraction from sequences of black (foliage) and white (sky) pixels along circular transects over the whole range of angles. Clumping factors generated by hemispherical photographs have been analysed and compared using (i) hemispherical photographs of simulated forest stands with fixed LAI (2, 4, and 6) and three-dimensional spatial distribution of foliage, ranging from complete randomness to full clumping; and (ii) in situ hemispherical photographs from forest canopies with known architecture. The following conclusions are drawn based on our experimental data set: (i) overall clumping factors can be obtained by integrating values over the same range of angles as that used to derive LAI, therefore ensuring consistency between estimations of LAI and clumping factors; (ii) the Chen and Cihlar clumping index tends to underestimate clumping in highly clumped canopies, whereas the Pielou coefficient of segregation tends to overestimate clumping in poorly clumped canopies; and (iii) hemispherical photographs provide an efficient tool for describing the degree of canopy nonrandomness in all directions of the hemisphere and to adjust clumping effects on LAI estimates derived from gap fraction analysis. However, clumping factors derived from hemispherical photographs need to be further tested in real canopies and compared with other methods to define their merits and limitations.

An object-based method to map wetland using RADARSAT-1 and Landsat ETM images: test case on two sites in Quebec, Canada

The Canadian Wildlife Service, Quebec region, of Environment Canada tested a multiscale object-based classification method on two test sites using satellite images to map wetlands in the context of the Canadian Wetland Inventory (CWI). The objective of this study was to assess the method most adapted for the Canadian inventory program to map five wetland classes (bog, fen, swamp, marsh, and shallow water), for a minimal geographical unit of 1 ha, from RADARSAT-1 and Landsat-7 enhanced thematic mapper (ETM) data. The top-down object-based classification selected was based on the Canadian Wetland Classification System and identifies quickly and precisely the ecologically meaningful polygons of wetlands. Validation was done on two levels: (i) between “other” versus “wetland” and (ii) between each class of wetland. Global accuracy values for the first level are greater than 80% for both test sites and about 76% and 67%, respectively, for the two sites for the second level. This approach is well adapted to wetland mapping on both the thematic and the spatial level.

Modelling light obstruction in three conifer forests using hemispherical photography and fine tree architecture.

A catalogue of hemispherical photographs was collected from conifer forest plantations in order to assess light obstruction by tree crowns. The three conifer species of interest were red pine (Pinus resinosa), jack pine (Pinus banksiana Lamb.), and white spruce (Picea glauca). A tree reconstruction technique called the vectorization method provided the means by which to reproduce fine 3-dimensional spatial distributions of the foliage and support structures of forest trees. More specifically, the detailed geometry of three selected canopies was simulated using the data set produced from the vectorization method and the in situ site characterisation. Moreover, a ray tracing model based on material density provided by the vectorization was developed to replicate hemispherical photos taken looking up from beneath the canopy. The comparison of in situ with simulated hemispherical photographs met three objectives: 1. 1. to estimate the elementary scale driving the obstruction patterns, 2. 2. to validate the vectorization method, and 3. 3. to assess the degree to which the trunks and support structures obstruct light.