1. SeaWiFS Highlights July 2001 SeaWiFS Views Eruption of Mt. Etna On July 24, 2001, SeaWiFS viewed a greenish-orange plume resulting from the ongoing eruption of Mt. Etna in Sicily. The plume stretched over 900 kilometers southeastward across the Mediterranean towards Libya. In the first image of this eruption, obtained from SeaWiFS on July 22 (not shown), the ash plume was a yellowish-brown color.

2. SeaWiFS Views Eruption of Mt. Etna July 24, 2001

4. Planned Improvements to the MODIS Snow Algorithm Dorothy K. Hall Vincent V. Salomonson George A. Riggs Nicolo DiGirolamo Jonathan S. Barton The Moderate Resolution Imaging Spectroradiometer (MODIS) snow-mapping algorithm has been shown to be very sensitive to detecting snow cover in North America. Eight MODIS 8-day composite snow maps from the fall of 2000 have been compared with NOAA operational products, and selected Landsat Enhanced Thematic Mapper Plus (ETM+) scenes from the same time period. In some cases, the MODIS snow maps are superior to the operational maps when ETM+ data were available for use as “ground truth.” However, the current MODIS snow algorithm tends to overestimate snow cover in some areas. It also maps some “false snow cover” in areas that are too warm to have snow such as in South America and Africa (and in the southern part of the United States). Some of the false snow cover occurs on coastlines, and may result from urban areas or coastal sand. Improvements in the MODIS snow-mapping algorithm, which are under development, will be implemented for part or all of the upcoming 2001-02 winter. Significant changes to the algorithm include the introduction of a “thermal mask” and a new usage of the MODIS cloud mask. Those changes will improve snow mapping in warm areas, and along coastlines. A scan- angle cut off has also been implemented to preclude mapping of falsely-identified snow cover beyond  45 . The current usage of the cloud mask utilizes the "certain cloud" test of the Wisconsin cloud mask. When this mask is used, much of the snow cover is mapped incorrectly as cloud, and not as snow. The amount of snow cover that is mapped without the cloud mask (at least where we have tested it in North America) is consistent, visually, with actual snow cover observed on inspection of visible-band imagery, and on the NOAA Interactive Multisensor Snow and Ice Mapping System (IMS) operational daily snow-mapping product. Extensive areas of confirmed snow cover are mapped as cloud using the "certain cloud" test. Thus the current algorithm underestimates snow cover in some places due to our usage of the cloud mask. In the revised algorithm, we will run the snow-mapping algorithm without the cloud mask algorithm as input. Then we will overlay a cloud mask that is less conservative in that it does not include thin clouds through which snow can be seen (probably a single or combination of cloud spectral test results from the MODIS cloud mask) onto the snow-cover map. This significantly improves the results in known snow-covered areas as illustrated in Figure 1.

5. (Con’t) To eliminate much of the spurious snow cover in warm areas such as South America and Africa, an additional threshold test that uses MODIS band 31 brightness temperatures has been developed. If the band 31 brightness temperature in a pixel is above 277 K, then it will not be mapped as snow using the new threshold test. Testing has shown that this can eliminate ~98% of the false snow detection in Africa. The current snow-mapping algorithm utilizes the entire MODIS image (+/- 55  scan angles). Previously we had only mapped snow out to +/-45  scan angles, and that code has already been written, and was disabled. Recent tests have indicated that there is a considerable amount of false snow cover that will be eliminated if we use only +/- 45  scan angles. This false snow detection probably occurs because of forward scattering by features on or near (atmospheric aerosols) the surface. Figure 2 illustrates the improvements to the MODIS snow map when the brightness temperature threshold test and scan-angle correction are used. An area on the northeastern coast of South America is shown in Figure 2. Only a small amount of false snow cover is left in the image on the right, after the “thermal mask” and scan-angle corrections are included in the algorithm. The MODIS snow cover algorithm enhancements will be implemented before or during the 2001-02 Northern Hemisphere snow season. While validation of the MODIS snow maps has been ongoing, mainly for North America, there will be an increased emphasis on validation of the snow maps in other parts of the world after the algorithm changes, outlined above, are implemented. Planned Improvements to the MODIS Snow Algorithm Dorothy K. Hall Vincent V. Salomonson George A. Riggs Nicolo DiGirolamo Jonathan S. Barton

6. DarknessSnow Non-snow- covered land Inland waterCloud Figure 1. MODIS snow maps shown with “certain” cloud mask (left), without the cloud mask (center), and with “bit 19” only of the cloud mask overlain on the snow product (right) December 23, 2000 Water

7. Figure 2. MODIS snow maps of the northeastern coast of South America on March 13, 2001. Left - Map derived using the current MODIS snow-mapping algorithm; Right - Map derived using the proposed enhancements to the MODIS snow-mapping algorithm WaterSnowNon-snow- covered land Inland Water