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Abstract: The spatial and temporal patterns of the forest background optical properties are critically important in retrieving the biophysical parameters of the forest canopy (overstory) and in ecosystem modeling. In this paper we carry out background reflectivity mapping over conterminous United States, Canada, Mexico, and the Caribbean land mass using Multi-angle Imaging SpectroRadiometer (MISR) data at 1.1 km resolution. The refined methodology uses the nadir and 45° forward directions of the MISR camera images. The background reflectivity is shown to vary between coniferous and deciduous stands, particularly in the near-infrared band, and with the overall amount of overstory vegetation. The largest seasonal differences were observed over a boreal region. The main drawback is a high amount of missing MISR data due to the presence of clouds and other atmospheric effects. The paper also contains a demonstration of the effect on LAI estimates when the dynamic background reflectivity information is inserted into a global LAI algorithm. Multi-angular remote sensing is thus shown to enable us to effectively map yet another forest structure parameter over large areas, which was not possible using mono-angle data. [Copyright &y& Elsevier]

Satellite altimetry is currently one of the most widely used techniques for monitoring lake water levels. Raw altimetry measurements are generally delivered in an ellipsoid height system, while lake heights are finally required in a local level height system to ensure the lake surface being strictly flat under the physical constraints of both gravity and non-gravity factors. Therefore, there is always a need for height system transformation of altimetry data. Many of the previous studies have used orthometric heights in an approximate manner in the altimetric data processing. The geoid errors, together with other error sources, can introduce significant biases into water level estimations. In this paper, we propose a spatio-temporal regression method to calculate orthometric height to level height (O2L) differences, which can be used to yield accurate lake levels from satellite altimetry data. In the experiments conducted in this study, we chose ICESat-2 data to train the O2L bias correction model, and evaluated the algorithm performance on two large lakes in central Asian (Issyk-Kul and Baikal lakes) and one lake in North America (Lake Michigan). The experimental results indicate that the spatio-temporal modeling method performs better than the traditional spatial modeling method in terms of regression accuracy, and the water levels derived from the new method are much closer to in-situ gauging measurements. Influenced by O2L bias, there is often a jitter phenomenon in the time series of lake levels, and the results of different satellites are, accordingly, not mutually consistent. A visual analysis reveals that the spatio-temporal modeling method can greatly reduce the high-frequency bias. • Improper choice of the vertical datum can introduce serious errors into lake level computation. • A spatio-temporal regression method is proposed for calculating orthometric height to level height (O2L) differences. • High-frequency errors in lake level time series are significantly reduced after O2L bias correction. • NASA's ICESat-2 satellite provides high-precision altimetric measurements for O2L bias modeling. [ABSTRACT FROM AUTHOR]

The launch of NASA's Ice, Cloud, And Elevation Satellite-2 (ICESat-2) in September 2018 provides the scientific community an opportunity to observe high-resolution and three-dimensional surface elevations with global coverage. ICESat-2's Land and Vegetation Height (ATL08) data product focuses on the along-track terrain and canopy heights observations at a 100 m × 11 m spatial resolution. This work expands on past ATL08 validation studies to assess a higher spatial resolution (30 m × 11 m) version of ATL08's height product. This new dataset enables higher resolution mapping and fusion with Landsat data, but has not previously been validated across large geographic extents. In this paper, we examine the accuracy of multi-resolution ICESat-2 ATL08 across North America boreal forests using Land, Vegetation, and Ice Sensor (LVIS), an airborne laser ranging system as reference datasets. Overall, strong agreements of terrain elevation and canopy height were found between ATL08 and LVIS at both 100 m × 11 m (RMSE terrain = 2.35 m; bias terrain = −0.17 m; RMSE canopy = 4.17 m; bias canopy = 0.08 m) and 30 m × 11 m (RMSE terrain = 3.19 m; bias terrain = 0.49; RMSE canopy = 4.75 m; bias canopy = 0.88 m) spatial resolutions. We found the accuracy of high-resolution terrain and canopy height measurements were constrained by sensor and external conditions during the time of data acquisition with lower uncertainties observed from samples along high-intensity ground tracks and with low topography/slope variabilities. Through this work, we provide insight into the use of multi-resolution ICESat-2 ATL08 for terrain and canopy heights characterization in northern forests. The results found in our study serve as a benchmark for end users to select high-quality ATL08 for a variety of scientific applications. • ICESat-2 ATL08 terrain and canopy heights were assessed across boreal forests. • High-resolution ATL08 performed comparably to original datasets. • High-resolution ATL08 data quality was more susceptible to slope variability. • On average ICESat-2 over-estimated short trees and under-estimated tall trees. [ABSTRACT FROM AUTHOR]

A new set of recently developed leaf area index (LAI) algorithms has been employed for producing a global LAI dataset at 1 km resolution and in time-steps of 10 days, using data from the Satellite pour l''observation de la terre (SPOT) VEGETATION (VGT) sensor. In this paper, this new LAI product is compared with the global MODIS Collection 4 LAI product over four validation sites in North America. The accuracy of both LAI products is assessed against seven high resolution ETM+ LAI maps derived from field measurements in 2000, 2001, and 2003. Both products were closely matched outside growing season. The MODIS product tended to be more variable than the VGT product during the summer period when the LAI was maximum. VGT and ETM+ LAI maps agreed well at three out of the four sites. The median relative absolute error of the VGT LAI product varied from 24% to 75% at 1 km scale and it ranged from 34% to 88% for the MODIS LAI product. The importance of correcting field measurements for the clumping effect is illustrated at the deciduous broadleaf forest site (HARV). Inclusion of the sub-pixel land cover information improved the quality of LAI estimates for the prairie grassland KONZ site. Further improvement of the global VGT LAI product is suggested by production and inclusion of pixel-specific global foliage clumping index and forest background reflectance maps that would serve as an input into the VGT LAI algorithms. [Copyright &y& Elsevier]

Observations in the visible and infrared spectral bands from the Imager instrument onboard Geostationary Operational Environmental Satellite (GOES) have been used to derive snow depth. The technique makes use of correlation between depth of the snow pack and satellite-derived subpixel fractional snow cover. Previous efforts to infer snow depth from satellite data with this technique were focused on grasslands and croplands, where the snow depth/snow fraction relationship is most pronounced. In this paper we improve the retrieval algorithm to extend snow depth estimates to forested areas. The enhanced algorithm accounts for the tree cover fraction and for the type of forest, deciduous or coniferous. The developed technique was used to derive maps of snow depth over mid-latitude areas of North America during winter seasons of 2003–2004 and 2004–2005. Satellite-based snow depth maps were produced daily at 4 km spatial resolution. To validate the retrievals we compared them with surface observations of snow depth and with the snow depth analysis prepared at the NOAA National Operational Hydrological Remote Sensing Center (NOHRSC). The estimated retrieval error was about 30% for snow depths below 30 cm and increased to 50% for snow depths ranging from 30 to 50 cm. Snow depth retrievals were limited to scenes with less than 80% deciduous forest cover fraction and less than 50% needle leaf forest cover. [Copyright &y& Elsevier]

Satellites provide unique perspectives on aerosol global and regional spatial and temporal distributions, and offer compelling evidence that visibility and air quality are affected by particulate matter transported over long distances. The heights at which emissions are injected into the atmosphere are major factors governing downwind dispersal. In order to better understand the environmental factors determining injection heights of smoke plumes from wildfires, we have developed a prototype system for automatically searching through several years of MISR and MODIS data to locate fires and the associated smoke plumes and to retrieve injection heights and other relevant measurements from them. We are refining this system and assembling a statistical database, aimed at understanding how injection height relates to the fire severity and local weather conditions. In this paper we focus on our working proof-of-concept system that demonstrates how machine-learning and data mining methods aid in processing of massive volumes of satellite data. Automated algorithms for distinguishing smoke from clouds and other aerosols, identifying plumes, and extracting height data are described. Preliminary results are presented from application to MISR and MODIS data collected over North America during the summer of 2004. [Copyright &y& Elsevier]

Fire disturbance is a significant component of the climate system. Analysis of satellite-derived burned areas has allowed the identification of fire patches and their morphology as a new resource for tracking fire spread to improve fire models used to assess the impact of fires on climate and the carbon cycle. A critical parameter of the flood-fill algorithm used to create fire patches is the cut-off (in days) below which it aggregates two contiguous burned pixels to the same fire patch. However, the current level of validation is insufficient to understand the effect of the cut-off values and sensor resolutions on the subsequent fire-patch morphology. The FRY v1.0 database of functional fire-patch traits (e.g., size, elongation, and direction) emanates from the analyses of two global burned-area products derived from MODIS and MERIS sensors with different spatial and temporal resolutions and with cut-off values of 3, 5, 9, and 14 days. To evaluate whether the FRY products are accurately identifying the spatial features of fire patches and what are the most realistic cut-off values to use in different sub-regions of North America, we propose a new functional diversity trait-based approach, which compares the satellite-derived fire patches to forest service perimeters as reference data. This paper shows the accuracy of the FRY fire patches ≥300 ha in North America during 2005–2011. Our analysis demonstrates that fire patches with a high cut-off of 14 days and those derived from the MODIS sensor, with their high temporal resolution, better identify the fire diversity in North America. In conclusion, our statistical framework can be used for assessing satellite-derived fire patches. Furthermore, the temporal resolution of satellite sensors is the most important factor in identifying fire patches — thus space agencies should consider it when planning the future development of cost-effective climate observation systems. Unlabelled Image • The trait-based method can be used for assessing satellite-derived fire patches. • Functional–structure relationships are complemented by functional dissimilarities. • High temporal resolution is most important for fire-patch identification. • Fire patches with a cut-off of 14 days are better identified in North America. [ABSTRACT FROM AUTHOR]