Expendable instruments are routinely launched from the National Oceanic and Atmospheric Administration (NOAA), Aircraft Operations Center’s (AOC) WP-3D Lockheed Orions and the Gulfstream IV-SP aircraft in various environments, from benign weather to the strongest hurricane eye walls, to gather vertical profiles of the atmosphere or upper regions of the ocean.

In recent years, the primary expendable probe has been the GPS dropwindsonde, a meteorological instrument that descends from the aircraft to the sea surface, transmitting information back to the aircraft while in the air. The GPS dropwindsondes measure air temperature, dewpoint, atmospheric pressure, and use the GPS positioning to detect horizontal and vertical winds. These data are measured and transmitted twice per second while the probe is in the air. NOAA’s Gulfstream IV-SP primary mission during hurricane season is to fly in the environment surrounding a hurricane, launching GPS dropwindsondes at predetermined locations in data sparse regions to gather vital information regarding the steering currents that will determine a hurricane's future track and likely landfall position.

AVAPS Installation in 19 inch rack

Dropsonde Sensor

The WP-3D Lockheed Orions aircraft not only augment these Gulfstream IV-SP surveillance missions, but also deliver GPS dropwindsondes into the most extremely dynamic wind, turbulence and precipitation areas of tropical cyclones. Recent findings regarding the strength and location of low altitude wind maxima, gained from GPS dropwindsondes, are helping redefine rules of thumb forecasters rely upon for hurricane wind warnings.

Other aircraft-launched expendable probes include Aircraft Expendable BathyThermographs (AXBTs), Aircraft Expendable Current Probes (AXCPs) and Airborne Expendable Conductivity Temperature Depth (AXCTD) probes. The AXBTs transmit sea surface temperature versus depth as the thermistor sinks through the water column. AXBTs provide key information about ocean temperature (an important factor in determining the possible strength a hurricane may reach). AXCPs measure horizontal ocean current velocity with depth, an important tool for understanding heat transport and ocean/atmosphere dynamics. The AXCTDs measure the ocean salinity, or saltiness (proportional to conductivity), and temperature, which are necessary 1) for computing ocean density, stability and buoyancy, and 2) for identifying different ocean water masses. GPS DropSonde

More information about the GPS dropwindsonde can be found at the University Corporation for Atmospheric Research web site.