Stefan Falke and Rudolf Husar Center for Air Pollution Impact and Trend Analysis

1. Fusion of Satellite Remote Sensing and Elevation Data:    Estimation of Aerosol Layer Height in Rugged Terrain Stefan Falke and Rudolf Husar Center for Air Pollution Impact and Trend Analysis Washington University in St. Louis Three dimensional rendering of the Sahara dust event aids the visualization and analysis of dust transport and influence of topography on transport. The following example illustrates how fusing SeaWiFS imagery and a digital elevation model (DEM) can be applied to generate 3D images of the Sahara dust event. The 3D images are used to estimate the height of the Sahara dust layer over the Canary Islands.

2. Data and Method A SeaWiFS image for February 26, 2000 was downloaded from the SeaWiFS project web site. GTOPO30 was the source of the digital elevation model (DEM) data. It is available at a 30 arc second spatial resolution (~1 km) and was downloaded from the ISGS EROS Data Center - Distributed Active Archive Center. Both the SeaWiFS image and elevation data are georeferenced onto a geographic lat/lon projection. The 3D surface plotting facility in the remote sensing image analysis software package ENVI was used to generate the 3D images by draping the SeaWiFS image onto the 3D DEM surface. ENVI was also used to create the 3D “fly-by” animation.

3. 2D Sahara Dust Image The region used for 3D rendering of the Sahara dust event. The dust is transported off the west coast of Africa and across the Canary Islands.

4. 3D Sahara Dust Image A 3D view of the image indicates that the Fuerteventura and Lanzarote Islands are fully blanketed by the murky yellow colored dust plume. Gran Canria and Tenerife are partly covered by the dust layer but their higher elevations appear to stick out above the dust layer at about 1200m as indicated by their dark green color.

5. 3D “Fly-by” Animation “Click” on the images to view the MPEG animations (~2.5 megabytes each ) (This requires a MPEG plug-in)

6. Summary of Observations • On February 26, 2000, the Sahara dust cloud passed over the Canary Islands.    • The islands of Fuerteventura (peak elev. 800 m) and Lanzarote (peak elev. 650 m) are fully immersed within the dust layer.   • The lower elevations of islands of Tenerife (peak elev. 3700 m) and Gran Canaria (peak elev. 1950 m) are also within the dust layer, but their tops protrude into the dust-free air.    • Hence, the dust layer over the Canary Islands is evidently within the lower stratum of the atmosphere, extending from the surface to about 1000 m.   • As a consequence, the dust flows around rather than over the Tenerife and Gran Canaria islands.     • This illustration suggests that the in rugged terrain, the fusion of satellite remote sensing and elevation data allows the estimation of the aerosol layer height.

7. Limitations • In this method, the SeaWiFS image was simply draped onto a 3D topographic surface so that land, water, dust, and clouds were all projected directly onto the topographic surface. A more realistic and revealing rendering scheme would account for the appropriate heights of each of the atmospheric components. • The elevations were exaggerated in the 3D images to aid the visualization of the dust layer height dependence. • The estimation of the dust layer height was done using only a visual analysis of the SeaWiFS and DEM images. A more rigorous approach using pixel spectral analysis may provide more accurate height estimates. • This example only focused on a limited area and cannot be used for synoptic scale analysis.