Hardy B. Granberg

Effects of shadowing types on ground-measured visible and near-infrared shadow reflectances

Abstract We analyzed the effect on the visible and near-infrared reflectance of shadows cast by a building, a conifer, and a hardwood on three different surfaces (grass, bare soil, and asphalt). Ground-measured radiances were collected with a radiometer measuring the SPOT-HRV bands and with a spectroradiometer measuring in the SPOT-HRV and LANDSAT-TM bands as well as giving the whole spectrum from 400 nm to 900 nm. We considered two types of reflectances: the “sunlit reflectance” and the “shadow reflectance,” which is calculated, using as irradiance, the radiance of the sunlit reference panel and of the shadowed reference panel, respectively. An analysis of variance showed that visible and near-infrared reflectances of shadows are significantly affected by the surface type and the shadow type. F-test values are higher for sunlit reflectances than for shadow reflectances as well as for near-infrared reflectances than for visible reflectances. Reflectances of hardwood shadows are generally higher and more variable than the ones of conifer shadows. Building shadows generally have lower reflectances than hardwood shadows and similar reflectances as conifer shadows. These reflectances were used to calculated two vegetation indices (NDVI and ARVI). They are also affected by the surface and shadow types. Whatever the object casting the shadows, spectra measured in the shadow are typical of the surface type (grass, bare soil, or asphalt). Further studies are needed on multiple shadows and penumbra effects, which are particularly important in the case of forest canopies.

Physical properties of the seasonal snow cover in Dronning Maud Land, East Antarctica

Abstract Snow stratigraphy was analyzed in the Maudheimvidda area of western Dronning Maud Land, East Antarctica, during austral summer 1999/2000 as a part of the Finnish Antarctic Research Programme (FINNARP). Measurements were made in shallow (1–2m) snow pits along a 350 km transect from the coast to the polar plateau, covering at least one annual cycle and an elevation range from sea level to about 2500 m. The aim of the study is to document spatial and temporal variations in snow-cover properties, with the further aim of relating these variations to environmental factors and to patterns observable by remote sensing. The measurements suggest five principal snow zones: (i) sea ice, (ii) the seaward edge zone of the ice shelf, (iii) the inner parts of the ice shelf, (iv) the snow cover above the grounding line and (v) the local topographic highs. Local topographic highs such as ice domes and ice rises differ from other snow environments: the snow is less densely packed, possibly an indication of locally reduced speed of the katabatic outflow. Fewer and thinner crusts on the topographic highs are consistent with RADARSAT backscatter variations.

A forest shading model using bit-mapped graphics

Abstract A digital model has been developed for evaluation of the percentage fraction of the ground surface that is shaded by trees or other terrain elements. The model uses the bit-mapped graphics on a Hewlett-Packard 9816 computer to estimate the relative portion of the ground that is shaded at different solar elevations and azimuths. Different tree types and spacings can be built easily into the model, which also may be employed to evaluate the shade cast by high-rise and other building structures.

Radiative and aerodynamic effects of an open pine shelterwood on calm, clear nights

Abstract Net radiation represents a disequilibrium between incoming and outgoing radiative fluxes at a surface. This disequilibrium is caused by the non-radiative, i.e. the convective, conductive and chemical (mainly photosynthetic and respiratory) heat fluxes. Contrary to the prevailing common sense regarding net radiation in shelterwoods, a more negative (negative in this context refers to any energy flux directed away from, i.e. cooling, the surface) net radiation was observed beneath the canopy of an open pine shelterwood than in a nearby clearcut area. These observations were made during comparative net radiation surveys at Vindeln, Sweden, using 22 net radiometers at each site. In the evening the net radiation was initially more negative in the clearcut, but later became more negative in the shelterwood by about 10 W m −2 . This was a consistent pattern on all calm, clear nights during the survey. On windy nights the net radiation was slightly more negative in the clearcut than in the shelterwood. Since non-zero net radiation can only be caused by the non-radiative fluxes, and the nocturnal chemical heat fluxes are small, this difference is caused either by the ground heat flux or the convective fluxes or both. The temporal variations in measured ground temperatures and net radiation suggest that convective rather than conductive heat fluxes are responsible for the net radiation difference we observed on calm nights. On such nights, light winds interacting with the upper parts of the tree crowns induce slight movements of the air in the near-surface layers of the shelterwood. In the clearcut there are no such aerodynamic effects. Thus, in calm conditions there is, in addition to the radiative shelterwood effect, also an aerodynamic effect of the open shelterwood.

A semi‐empirical model to estimate the biomass production of forest canopies from spectral variables Part 1: Relationship between spectral variables and light interception efficiency

Abstract Empirical studies using remotely sensed data to estimate forest stand parameters have shown that the biological variable which is most closely functionally linked to the spectral response of the canopy is the leaf area index (LAI). The LAI is highly important for the photosynthetic functioning of the canopy since it has a dominant influence on the light interception capability or efficiency e(i). This efficiency depends on the LAI and on the optical properties of the canopy elements and it is also functionally linked to the canopy reflectance. Furthermore, this efficiency is linked to the biomass production of forest canopies since this latter is the result of photosynthesis, through which a fraction of the intercepted incident solar energy is converted into biomass. We propose a semi‐empirical model between ei and canopy reflectance (in terms of normalized difference vegetation index (NDVI)) which results from coupling semi‐empirical models of reflectance and light interception which are based o...

Application of Target Decomposition Theorems Over Snow-Covered Forested Areas

This paper compares two well-known polarimetric decomposition theorems, Cloude-Pottier and Freeman-Durden, applied to L- and C-band Airborne Polarimetric Synthetic Aperture Radar (AIRSAR-POLSAR) data acquired during the Cold-Land Processes Field Experiments. Three field campaigns were carried out in February 2002, March 2002, and March 2003 over a snow-covered open terrain, a sparse coniferous forest, and a dense coniferous forest. The analysis evaluates the ability of the two target decomposition methods for the identification and understanding of the main scattering mechanisms.

Mapping heat loss zones for permafrost prediction at the northern/alpine limit of the boreal forest using high-resolution C-band SAR

Abstract Lack of thermal insulation of the ground by snow cover is the prime factor causing permafrost to occur in the alpine tundra areas near Schefferville (55°N, 67°W). Lack of protection by seasonal snow cover is also the prime factor suppressing the growth of brush vegetation that is omnipresent elsewhere. The sensitivity of C-band synthetic aperture radar (SAR) to slight variations in an already sparse vegetation cover permits areas of shallow snow cover, and thus strong wintertime heat loss, to be identified. The map of such heat loss zones produced from a high-resolution HH-polarized C-band SAR-image imaged by the Canada Centre for Remote Sensing in early June 1991, compares favorably with previous permafrost maps produced by the Iron Ore Company of Canada.

Observations of sea ice ridging in the Weddell Sea

Sea ice surface topography data were obtained by helicopter-borne laser profiling during the First Finnish Antarctic Expedition (FINNARP-89). The measurements were made near the ice margin at about 73°S, 27°W in the eastern Weddell Sea on December 31, 1989, and January 1, 1990. Five transects, ranging in length from 127 to 163 km and covering a total length of 724 km, are analyzed. With a lower cutoff of 0.91 m the overall ridge frequency was 8.4 ridges/km and the average ridge height was 1.32 m. The spatial variations in ridging were large; for 36 individual 20-km segments the frequencies were 2–16 ridges/km and the mean heights were 1.16–1.56 m. The frequencies and mean heights were weakly correlated. The distributions of the ridge heights followed the exponential distribution; the spacings did not pass tests for either the exponential or the lognormal distribution, but the latter was much closer. In the 20-km segments the areally averaged thickness of ridged ice was 0.51±0.28 m, ranging from 0.10 to 1.15 m. The observed ridge size and frequency are greater than those known for the Ross Sea. Compared with the central Arctic, the Weddell Sea ridging frequencies are similar but the ridge heights are smaller, possibly as a result of differences in snow accumulation.

Shadowing effects on SPOT-HRV and high spectral resolution reflectances in Christmas tree plantations

Abstract Shadowing is one of the factors that most affects the relation between canopy reflectance and forest light interception efficiency. For Christmas tree plantations, a decreasing exponential function can assess the shadow proportion from canopy SPOT-HRV red or green reflectances. Near-infrared reflectances as well as the wavelength of the inflexion point in the red-edge region (λi) are not affected by shadowing in the canopy. The linear correlation coefficients calculated between the shadow proportion and three common vegetation indices (Normalized Difference Vegetation Index, Perpendicular Vegetation Index and the Transformed Soil Adjusted Vegetation Index) estimated with the SPOT-HRV reflectances are non-significant in the case of PVI and TSAVI (if it is calculated with green reflectances). The limits and possible improvements of such a study are discussed.

A snow sensor experiment in Dronning Maud Land, Antarctica

In January 2000 nine snow sensors were deployed in Dronning Maud Land, Antarctica, along a 355 km transect from Kvitkuven near the shelf edge via the Finnish research station, Aboa, to the Amundsenisen plateau. The purpose was to test a sensor system for spatio-temporal variations in temperature across the snow-air interface and snow accumulation/ablation, which includes atmos- pheric net balance and migrating snow dunes. In the dry snow conditions, environmental static electricity interfered with data transfer; several sensors were disabled early, while the longest record reached 6 months. Along the main transect, the year 2000 mass balance ranged from 52 to 221 mm w.e., largely following spatial patterns seen by other researchers. The level increased toward the edge of the ice sheet; unloading of drifting snow as the slope flattens, rather than increased snowfall as previously thought, may be responsible for this. At the Hogisen ice dome site the mass balance was 897 mm w.e., possibly due to unloading of wind-blown snow, as katabatic winds are locally forced uphill. This mechanism is important to maintain such topographic features along the Antarctic ice sheet margin. Major precipitation events occurred at 3-5 week intervals and much of the precipitation fell before mid- June. The daily signal in temperature disappeared after the autumn equinox, then the spectrum displayed a broad peak at synoptic frequencies. Potential temperature decreased towards the shelf edge, displaying a pattern consistent with strong inversions and suggesting that strong evaporative cooling is associated with the katabatic winds.