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Data Resources Weather Imaging

OVERVIEW

The ALERT Flood Detection System is supported by a subscription-based weather information/graphics display system consisting of a computer workstation running MxVision Weather Sentry software. The system receives NWS forecast products, weather maps, GOES weather satellite images, and both single site and composite NEXRAD weather radar images through continuous broadcasts from a commercial communication satellite. Broadcasts are received through the internet and a one-meter satellite dish located at the Glassell Street facility.

GOES weather satellite images are automatically updated every hour, NEXRAD composite images every 15 minutes, and local radar images every 6 minutes. The computer software is configured to display enhancements to the last image in display loop including: 1) watch boxes, 2) icons indicating the issuance of a NWS watches and warnings, and 3) arrows indicating the distance and direction of storm cell movement. Storm movement is also visualized by running sequences of weather satellite/radar images.

GOES WEATHER SATELLITE IMAGES

The Geostationary Operational Environmental Satellites (GOES) provide visible and infrared satellite images of the earth's surface. There are currently two GOES weather satellites in geostationary orbit about 22,000 miles above the equator, GOES-West (currently GOES-15) and GOES-East.

Visible Images

GOES WEATHER Satellite Image

Visible satellite images record visible light from the sun that is reflected back to the satellite by cloud tops, land surfaces and ocean surfaces. These black-and-white images show what the naked eye would see from space (if color blind) and hence represent nothing fancier than ordinary black-and-white photographs of the earth from space. The brightness of any feature on a visible satellite images depends on (1) how directly light from the sun strikes it, and (2) how reflective the feature is. Cloud tops and snow and ice surfaces tend to reflect visible light best, so they tend to be the brightest (i.e., whitest) features on a visible satellite image. Ocean surfaces tend to reflect the least visible light, so they tend to be the darkest features. Visible images are unavailable during the night when the sun is down.

Infrared Images

GOES Satellite Infrared Image

Infrared satellite images record invisible infrared radiation emitted directly by cloud tops, land surfaces or ocean surfaces. The warmer an object is, the more intensely it emits radiation, so the intensity with which a feature on earth emits infrared radiation tells us about that feature's temperature. Unlike visible images, infrared images are available during the night when the sun is down.

Using computer software, the intensities of infrared radiation can be translated into different shades of gray and/or different colors, and overlaid onto maps.

On the color-enhanced infrared images, colors other than gray are assigned to some of the coldest temperatures. Far from the polar regions, the coldest features invariably comprise the tops of clouds in the upper troposphere or lower stratosphere and are often associated with thunderstorms, hurricanes or mid-latitude cyclones. Hence, color-enhanced infrared images help highlight storms of various types.

NEXRAD WEATHER RADAR IMAGES

NEXRAD Composite Radar Image

In July of 1996, the last US Next Generation Weather Radar (NEXRAD) WSR-88D Doppler radar system was built in Santa Ana Mountains. The system was accepted by the NWS in February 1997. This local NEXRAD system (KSOX) provides storm intensity monitoring over Orange County and the Upper Santa Ana River Watershed. The addition of the local system to the existing NEXRAD systems located in Santa Barbara (Vandenberg AFB), Ventura (Sulfur Mountain), and San Diego Counties provide coverage for metropolitan Southern California. The color images can be displayed, printed and saved. Storm rainfall intensity and movement is monitored by running sequences of mosaic images. Mosaic images consist of imaging data from multiple radar sites overlaid onto a regional map. The storm maps and images are displayed in multiple colors representing radar reflectivity from water droplets in the atmosphere.