July 25, 2006
2006 Hurricane Season Generates Interest in APL Images
For it its August cover, National Geographic magazine chose a hurricane image developed by APL. As the National Oceanographic and Atmospheric Administration's (NOAA) polar-orbiting weather satellites pass overhead, APL's Tracking Station receives broadcast data from an onboard instrument. The Advanced Very High Resolution Radiometer (AVHRR) detects electromagnetic radiation of selected wavelengths (both visible and infrared) emitted or reflected from the Earth's surface (which can be cloud tops, smoke, haze, water, ice, or land). AVHRR's host satellite orbits at an altitude of about 833 kilometers, making roughly two passes daily over a particular spot on the Earth. With a surface resolution of about 1 kilometers, the AVHRR scans the surface, making strip maps from pole to pole. (APL receives only part of the strip when the satellite is at our horizon).
APL also receives hurricane information (location, etc.) from official National Hurricane Center (NHC) reports, based on NHC satellite observations, as well as Hurricane Hunter aircraft reports and other information. An APL meteorologist monitors these NHC reports for signs of tropical depressions, storms, and hurricanes in the Atlantic Ocean. He creates a track file with the time, location, wind speed, and central pressure that NHC provides. When a storm comes within range of APL's ground station, the track file is entered in Dynatrack to create specific images centered on hurricanes. When the satellite coverage is exceptional, a high-resolution image may be created, such as the one used by National Geographic.
The APL Satellite Communications Facility sends the received signal via fiber-optic cable to the Remote Sensing Group. There the data are processed by Terascan software, and Interactive Data Language (IDL) software is used to create two types of images:
1. Sea Surface Temperature (SST) Images. Because hurricanes require warm water to develop and intensify, overlaying the tracks of these storms on SST images helps the NHC predict how the strength of a storm may change. APL produces SST images daily, but because clouds interfere with obtaining surface data, the Laboratory also makes 3-day and 7-day composite images to mitigate cloud effects (because clouds don't often stay that long in one spot).
2. Cloud Cover (or Storm) Images. In contrast to the SST images, we want to see the clouds more than the surface. For daytime images, these are created by blending data from different wavelengths to create a false-color image in which warmer (lower) clouds are more yellow and cooler (higher) clouds are whiter. More intense storms build towers of clouds that reach such high altitudes that they are quite cold at their tops, helping to indicate storm intensity. Nighttime images use only emitted infrared and not visible light, so those images are black and white.
Two APL staff members began enhancing hurricane track plots more than 10 years ago. After taking a hurricane track with latitude, longitude, and wind speed and refining the plot, they developed a webpage of hurricane track maps. They then began making hurricane images from NOAA satellite data. Later, one of them wrote software to use track files to make a window that follows each hurricane and automatically makes images. The images, with captions added, are then placed into the hurricane page at http://fermi.jhuapl.edu/hurr/index.html. When satellite coverage is exceptional, they may create a high-resolution version of the image for the hurricane pages (used by both CNN and National Geographic).
APL's Ocean Remote Sensing Group supports the following webpages:
- Overviews. Near real-time graphical representations of area coverage and gray-scale versions of recently acquired AVHRR imagery.
- Dynatrack. Near real-time color-composite imagery over various land areas of the United States generated from AVHRR imagery.
- Hurricanes. Hurricane imagery generated from AVHRR data. This site is very active during hurricane season. Archive imagery is also available.
- Image Gallery. Selected AVHRR imagery.