Building a Global Hotspot Ecology. with Triana Data. S. A. W. Gersti

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1 Building a Global Hotspot Ecology with Triana Data S. A. W. Gersti Los Alamos National Laboratoiy, NIS-DO MS-C323, Los Alamos, N.M , USA. Tel. (505) , sig1anl.gov (In 1999: Visiting Scientist at Joint European Research Centre, Ispra/Italy, ABSTRACT Triana is an Earth remote sensing satellite to be located at the distant Langrange Point L-1, the gravity-neutral point between the Earth and the Sun. It will provide continuous full disk images ofthe entire sunlit side of the Earth with 8 km pixel resolution. The primary remote sensing instrument on Triana is a calibrated multispectral iniager with 10 spectral channels in the UV, VIS, and MR between 3 17 and 870 nm (reflected solar radiation). Due to its unique location at the Lagrange L-1 point, in the direct line-of-sight between Earth and Sun, Triana will view the Earth always in and near the solar retro-reflection direction which is also known as the hotspot direction. The canopy hotspot effect has rich information content for vegetation characterization, especially indications of canopy structure and vegetation health and stress situations. Primary vegetation-related data are the hotspot angular width W, and a hotspot factor C that quantifies the magnitude of the hotspot effect. Both quantities are related to the structural parameters of canopy height, foliage size, shape, and leafarea index (LAT). The continuous observations by Triana will allow us to establish time-series of these ecological parameters for all land biomes by longitude, latitude, and wavelength, that form the basis data set for a new global hotspot land vegetation ecology. The hotspot factor C will allow the determination of the enhanced radiant flux reflected from the Earth into space due to the hotspot effect. The hotspot flux enhancement due to the vegetation hotspot effect is estimated to account for about 1 % ofthe entire Earth radiative energy balance. Keywords: ecology, land surface reflectance, Triana satellite, hotspot, BRDF, earth radiation budget. THE TRIANA SATELLITE AND IMAGER Triana is a scientific satellite mission for Earth observations from the distant Lagrange L-1 point, and the latest planned in NASA's small explorer Earth Probes. The mission is scheduled to be launched in early 2001 from the Space Shuttle cargo bay to this neutral gravity point (the Lagrange libration point L-l) between the Earth and the Sun (Fig. 1) and is expected to deliver continuous full-disk multispectral images of the Earth's globe by Part of the EUROPTO Conference on Remote Sensing for Earth Science Applications 184 Florence, Italy September 1999 SPIE Vol X/99/$1 0.00

2 the middle of 2001 through the Internet. A Triana mission overview, its science objectives, and mission design parameters are described in Ref. 1. We address here ony those science aspects that are relevant to land surface observations and the proposed new global land vegetation ecology. FIG. 1: The Triana satellite location at the Lagrange Point L-l relative to Low-Earth- Orbiting (LEO) satellites, Geostationary (GEO) satellites. and the moon. Fig. 1, which is drawn approximately to scale (except for the Earth-Sun distance). shows two of the unique aspects of the Triana mission. namely the 1.5 million kilometer distance oftriana at L-1 from the Earth and its location in the line-of-sight between Earth and Sun. From this unusually large distance. the full disc of the Earth subtends only 0.48 degrees. approximately the same field-of-view as that of the moon or the sun when viewed from Earth. Also, the fact that L- 1 is a semi-stable libration point with a saddle potential necessitates that the Triana spacecraft orbit around L- 1. This elliptical orbit results in a Lissajous (or halo) orbit with 6-month period when viewed from the Earth. The Lissajous orbit configuration allows for Triana to observe the Earth not only in the precise hotspot (solar retro-reflection) direction. but also up to approximately 15 degrees away from the direct line-of-sight between the Earth and Sun. 185

3 The primary instrument on Triana is a calibrated multispectral imager, the Earth Polychromatic Imaging Camera (EPIC) with 10 spectral channels in the ultraviolet, visible and near infrared wavelength regions. Additional broad-band radiometers will also measure the Earth's reflected solar radiation from the entire globe continuously. These radiometers will be useful for monitoring global change on time scales from minutes up to the mission liefetime of 5 years. They will contribute unique new data to help solve the outstanding solar radiation/climate problem of our time: how much solar energy is absorbed in the atmosphere? (cf. Ref. 1) What Does TRIANA Measure? Spectral Radiance LW/rn2 tm sr.] 11 spectral channels UV, VIS, IR 2000x2000 IMAGE / / Pixel size 8 km 7 7 5yr.sciencedataandRGB images to WWWevery 15 mm. R (X, x, y, ê,tp, t) Hotspot angular signature 2Q to Li S U N Fig. 2: Radiance Measurements by Triana through the hotspot angular signature BRDF. Full-disk images from the EPIC instrument will allow retrievals of surface, cloud, and aerosol properties at 8 km spatial resolution due to its 2000x2000 pixel focal plane, allowing cloud cover, vegetation, snow/ice cover, ozone, and atmospheric pollution episodes to be monitored globally as the Earth rotates under Triana. The uniquelocation of Triana in space, at L-1, offers unique scientific advantages because all quantities are retrieved instantly across the entire illuminated globe in and near the solar retro-reflection direction, Fig. 2. This observation direction will show the hotspot effect of vegetation canopies [1] continuously and accross the entire globe. Monitoring vegetated land 186

4 surfaces under clear sky conditions will provide the data to develop a new global land vegetation ecology with these unique Triana data. THE HOTSPOT EFFECT In recent years it has been widely recognized that Earth's surfaces reflect solar radiation anisotropically, especially vegetated land surfaces, a fact that must be considered when remote sensing of surface characteristics is performed, e.g. [2, 3]. Such effects as the sunglint over water and varying amounts of shadows over structured surfaces create the anisotropy and necessitate the consideration of the non-lambertian nature of the reflected angular distribution. Clearly, these anisotropy effects are correlated with physical scattering and absorption events which allows sometimes the retrieval of interesting surface parameters from the remotely sensed angular distribution of the reflected radiation. The canopy hotspot is such an anisotropic reflection effect that may be used to derive bio- and geophysical parameters for the vegetation canopy that is remotely sensed. The hotspot effect is a retro-reflection enhancement due to the absence of mutual shading within three-dimensional structured surfaces when viewed in the illumination direction. It has also been called the "Heiligenschein" in atmospheric optics and the "Opposition Effect" in planetary physics, and is a special case of a surface's Bidirectional Reflectance Distribution Function (BRDF) that is interpretable as an angular signature in remote sensing [4]. In satellite remote sensing the hotspot effect is only seldomly observed because most observation geometries are designed such that the hotspot effect is missed since it happens only within about 10 degrees of the exact retroreflection direction. For example, most of today's satellite remote sensing instruments operate as cross-track scanners, and they would have to fly in an orbit that is exactly perpendicular to the principal plane (the orbit plane that contains the solar illumination direction) to be able to scan into the hotspot. This is usually not the case, and is even avoided by design, because in that case the view directions of the scanner would also include the sun-glint direction which could upset the instrument's calibration. Figure 3 shows the measured hotspot angular distribution over the "Great Dismal Swamp" area, a deciduous forest near Norfolk, Virginia. The data have been acquired by the MODIS Airborne simulator (MAS), see Refs. [5, 6], from 20 km altitude which results in a pixel size of 50 m. The magnitude of this HS-effect shows a hotspot peak in the radiance, or equivalent reflectance factor as plotted in Ref. [6], of up to a factor of 2 above the background radiance, which is consistent with model calculations, e. g. Ref. [7]. 187

5 MAS Data from a Deciduous Forest O.6O', I p ' I y 0.50 MOOS uiotor Data (2.5 mrod at 20 km) ' '. 75nm.._. C., a U C 04-, U : : 2152nm o.1o..' s. :' $47iim" ".... ;... : : : : : : '... :.. : : : : : : i : : 0.00 i!!.. i I I BKWD View Zenith Angle ( ) FWD Fig. 3 : Measured canopy hotspot angular distribution over a deciduous forest in visible and near infrared wavelengths from the MODIS Airborne Simulator (1995), see [5,6] at solar zenith angle of 3 1 deg. The potential significance of the hotspot as an angular signature and vegetation identifier has first been pointed out in Ref. [8] and is based on the shadow-hiding nature of the hotspot effect. Assume a simplified vegetation canopy that consists only of horizontal leaves modeled as circular discs with constant radius. Take a random distribution of these leaves within a 1 meter high canopy to make a leaf area index of LAI = 3 and perform a simple ray tracing through such a model canopy for mono-directional solar illumination rays. The probability of observing an illuminated pixel from any point above the canopy is shown in Fig. 4 for varying view angles and leaf sizes. 188

6 1.0 Variation in leaf sizs (cm) Solar z.nlth angi. 45 LA I -- I - I P View Angle (Degree) 100 Fig. 4: Simplified model calculation that correlates the hotspot angular size, see Ref. [81. width with leaf When the view direction equals the solar illumination angle (45 degrees in this case) the probability approaches 1 because all canopyinternal shadows are hidden, and reduces to 0.5 far away from this retro-reflection direction. This model shows the correlation of hotspot width with leaf size and the optimum hotspot magnitude of the HS-peak to be twice the non-hotspot value, as also demonstrated in the measured data of Fig. 3. In more realistic vegetation canopies the shape of the hotspot angular distribution function will also depend on the leaf angle distribution and the overall structural characteristics of the canopy with many different structural elements that together will make the canopy 189

7 architecture. Many, more elaborate and detailed 3D model calculations have demonstrated these correlations, see Refs. [7, 8, 9], but an equal abundance of field measurements are still lacking. A NEW GLOBAL HOTSPOT ECOLOGY WITH TRIANA DATA Due to its location near the Lagrange L-1 point in the direct line-of-sight between Earth and Sun, Triana will view the Earth always in and near the solar retroreflection direction, i. e., the hotspot direction, see Fig. 1. Many scientific investigations within the last 10 years have shown that the canopy hotspot effect has rich information content for vegetation characterization, especially indications of canopy structure and vegetation health and stress situations. These canopy structure data, like canopy height and leave/phytoelement size and shape, are not obtainable by classical remote sensing measurements that primarily rely on spectral signatures, like vegetation indices, e.g. Ref Therefore the angular signatures from Triana canopy hotspot measurements promise to be an ideal complement to the existing spectral index characterizations of vegetation cover. The Triana data will thus provide the opportunity to build a new global hotspot ecology for vegetated land surfaces. As illustrated in Fig. 2, the spectral radiance measured by the EPIC instrument scans through the hospot angular signature, and can provide width and height of that angular distribution function. Because the Earth subtends only 0.48 deg. FOV, all Earth locations reflect the solar radiation with a hotspot enhancement into the Triana sensors. While the EPIC imager "stares" at the Earth's full disk, the globe rotates (east to west) and a pixel near athe eastern rim will show the hotspot effect at near 90 deg. local solar zenith angle. During the course of the day the local solar zenith (and Triana observation) angle changes through the zenith (near the equator, e.g.) to -90 deg. zenith angle near the western rim at sunset. Therefore each pixel in the Triana (EPIC) image will be illuminated and observed for all local solar zenith angles during 1 day, but always in the hotspot condition. These are unusual observation conditions for the near Earth satellite remote sensors and are also seldom realized. Thus, Triana will have a great potential for new discoveries because of its unique operating regime in angle space. In and near the solar retroreflection direction several unusual radiative effects are expected: in the surface reflectances the hotspot enhancement, in the atmospheric reflectances from aerosols the backscattering enhancement due to Mie scattering, from droplets the higherorder rainbows, and from ice crystals the halos. Firstorder estimates in how these effects influence the remotely sensed radiation field are given in Ref. 10. Under cloud-free conditions we expect that Triana will measure the hotspot refelectance (spectral radiance) from vegetated areas in its 1 0 spectral channels resulting in similar hotspot angular distribution functions as shown in Fig. 3. For each pixel many such data (for all solar zenith angles, lattitudes and longitudes) will be acquired and composed during a 3 month observation cycle, while the Triana satellite completes a half-cycle of its Lissajous orbit. These hotspot angular signatures can then be compiled by biomes, 190

8 vegetation types and seasons. However, due to the much larger pixel size of Triana (8 km) compared to that from the MAS instrument shown in Fig. 3 (50 m), we expect a large number of 'mixed pixels', where the vegetation cover is a mixture of several or many biomes per pixel. That would likely broaden the HS angular signatures compared to Fig. 3. From every 3-month Triana observation cycle one would obtain many HS signatures from all vegetated land areas across the globe that may be characterized by their angular width W and height or other magnitude measure, like the hotspot parameter C shown in Fig. 5. Continuous observations with Triana will allow us to establish time-series of these ecological parameters {W, C} for all biomes by longitude, latitude, wavelength and season, which form the basis data set for a new global hotspot land vegetation ecology. Radiance R() Non-HS L71'7'7/ Li C = JJR cos ) d/ if HS peak Li Hotspot / Hemisphere Li + 10 cose) d e Non-HS W = Hotspot Width (FWHM) (full width at half maximum) Fig. 5: Hotspot angular signature parametenzation with width W and magnitude C as basis for the proposed global hotspot land vegetation ecology. 191

9 HOTSPOT EFFECT ON EARTH RADIATION BALANCE Since Triana "sees" the entire globe from its distant observation location at L1,-it is also applied to measure the total radiant energy flux leaving the Earth. For this purpose Triana has a separate instrument package on board (in addition to the EPIC imager) consisting of three cavity radiometers to measure the Earth reflected radiant power (energy flux) in the direction toward Triana to an unprecedented high accuracy of 0. 1 %, see Ref. 1.Like the data from the EPIC imager, these full-disk radiometer data wifi also show the hotspot effect as well as the enhanced backscattering effects from atmospheric constituents discussed earlier. Therefore it is important to separate out the radiant flux enhancements measured in the Triana direction from the more isotropically emitted radiation flux into all other directions outside the hotspot angular region. The hotspot factor C defined in Fig. 5 is suited to estimate this fractional radiant flux enhancement. In the following we estimate the hotspot flux (radiant power) enhancement that can globally be expected due to the vegetation hotspot effect. We assume a simplified radiance distribution function for the entire angular hemisphere above a pixel, approximating the measured angular distributions shown in Fig. 3: symmetry around Li with angular width of 10 degrees (to each side) and a constant radiance outside the hotspot region, equivalent to assuming an isotropic radiance R10. If we further assume that the hotspot peak is 2R0, faffing off linearly to the isotropic value, the necessary angular integrations to compute the radiant flux F =$$R.cos d2 can easily be carried out. Since the hotspot effect (and the other retro-reflection enhancements) always add to the totally emitted flux, we can write it as a flux enhancement term FHSE due to the hotspot effect: 'otal = ;50 + FHSE = i[1 + Cl where the hotspot fractional flux enhancement C is defmed as C FHSE/PSO which is explicitly written out on Fig. 5, where the angular integration in the nominator is limited to the hotspot peak with an (arbitrary) cut-off at Li and Li + 10, excluding the isotropic component. The integration in the denominator is carried out over the entire angular range of the hemisphere with constant Rj. Numerically we obtain for this hotspot flux enhancement under the assumptions explained above a value of C =0.0447, or approximately 5 %. 192

10 We must consider the fractional vegetation cover across the globe to obtain a first estimate of the global radiant flux enhancement of the Earth-leaving radiant power due to the vegetation hotspot effect. Allowing for 2/3 water cover of the Earth and an approximate 50 % vegetation cover fraction over land, we obtain only 1/6 of the Earth's surface which might be creating this hotspot enhancement. This results in approximately 1 % as the contribution to the global Earth radiation balance due to the vegetation hotspot flux enhancement. The additional retroreflection enhancement effects stemming from the atmospheric effects discussed earlier could amount to about the same or even twice of that due to the vegetation hotspot. A quantitative estimate has yet to be performed. SUMMARY Triana, from its unique Earth observation location at Li can provide unique canopy hotspot angular signature data for all vegetated land covers across the globe. Since the hotspot parameters {W,C} are related to biophysical vegetation canopy characteristics indicative of canopy structure and plant architecture, a global data base of these parameters lends itself as the basis for a new land vegetation ecology. Integrating the Triana structural canopy data with the spectral canopy data from near-earth orbiting satellite instruments, like MISR, MODIS, POLDER, or VEGETATION, will enhance and facilitate the interpretation of vegetation canopy observations. These data will be useful to quantify global ecological parameters, like land cover and land use change, seasonal variations in vegetation biomass and vegetation cover, regional agricultural stand assessments, and environmental health. In addition, new data will be provided to the study of global ecology, aerosol climatology, cloud4op physics and global scale cloud dynamics, cloud radiative forcing, earth radiation budget and planetary albedo calculations. REFERENCES [1] F.P.J. Valero, J. Herman, P. Minnis, W. Wiscombe, S.A.W. Gerstl and K. Ogilvie, "Triana Mission to L-1, Science Objectives", University of California Scripps Institute report to NASA, April 1999, published at the Triana Website hup://triana.gsfc.nasa.gov/home/. [2] C. Simmer and S.A.W. Gersti, "Remote Sensing of Angular Characteristics of Canopy Reflectances", IEEE Trans. Geosci. & Remote Sensing, GE-23, No. 5, (1985). [3] S.A.W. Gerstl and C. Simmer, "Radiation Physics and Modelling for Off-Nadir Satellite-Sensing of Non-Lambertian Surfaces", Remote Sensing of Environment, 2Q, 1-29 (1986). 193

11 [4] S.A.W. Gersti, "The Angular Reflectance Signature of the Canopy Hotspot in the Optical Regime", 4-th Intl. Colloqium on Spectral Signatures of Objects in Remote Sensing, Aussois, France, Jan , also ESA Report SP-287, 129(1988). [51 MODIS Airborne Simulator (MAS) data from SCAR/A measurement campaign July 1993, see the MODIS WWW homepage bin/texisfmodis/search [6] J.L. Privette and E. Vermote, Proceedings of SPIE Symposium on Remote Sensing, Vo. 2311, p. 135 (1995) [7] W. Qin and N.S. Goel, "An Evaluation of Hotspot Models for Vegetation Canopies", Remote Sens. Reviews, Vol. 13, (1995). [8] S.A.W. Gersti, C. Simmer and B.J. Powers, "The Canopy Hotspot as Crop Identifier", Proc. mt. Symp. on Remote Sensing for Resources Development and Environmental Management, Enschede, The Netherlands, M.C.J. Damen et al. (Eds.), (1986). [9] W. Qin, N.S. Goel and B. Wang, "Estimation of Leaf Size from Hotspot Observations", Proc. Intl. Goesci. and Remote Sensing Symp. IGARSS'96, Lincoln Neb., May 1966, Vol. III, pp (1996). [10] J. laquinta and B. Pinty, "Radiation field in multilayered geophysical medium: Icewater-aerosol-vegetation-soil (I WAVES) model", J. Geophys. Res., Vol. 12, pp (1997) [11] R.B. Myneni, S. Maggion, J. laquinta, J.L. Privette, N. Gobron, B. Pinty, D.S. Kimes, M.M. Verstraete, and D.L. Williams, "Optical Remote Sensing of Vegetation: Modelling, Caveats, and Algorithms", Remote Sensing of Environment, Vol 51, pp (1995). 194