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Hurricane Awareness Week
The National Weather Service (NWS) has several tools to monitor hurricanes. While they are still far at sea indirect measurements using satellites are the main tool, although ships and buoys also provide observations. Once the storms come closer to land, more direct measurements (reconnaissance aircraft, radiosondes, and Automated Surface Observing Stations) are also used. Within 200 miles of the coast, radar provides important information about the storm as well.
Computer models used to forecast storm intensity and movement require a great deal of data about the atmosphere. Lack of observations (especially over the ocean) and errors and inconsistencies in the data are major sources of forecast errors.
The primary observing systems in the tropics are the Geostationary Operational Environmental Satellites (GOES). With satellite images, forecasters can estimate the location, size, movement, and intensity of a storm, and analyze its surrounding environment. The instruments on board the satellites measure emitted and reflected radiation from which atmospheric temperature, winds, moisture, and cloud cover can be derived.
Satellite images provide:
- Basic day/night cloud imagery
- Observations of land surface temperature data
- Sea surface temperature data
- Winds from cloud motions at several levels
- Hourly cloud-top heights and amounts
- Rainfall estimates for flash flood warnings
SHIPS & BUOYS
Ships and buoys provide additional information about the wind speed and direction, pressure, air and sea temperature, and wave conditions within the tropical cyclone. Ships and buoys are the only routine source of measured waves in areas unobstructed by land and are often the only way to take direct measurements when a tropical storm is still at sea. As a result, they provide observational ground truth for indirect measurements (such as satellite and radar) in the marine environment.
Reconnaissance aircraft can be used to take direct measurements of the storm far away from land. These measurements provide a snapshot of parts of the hurricane. This information is critical in analyzing the characteristics needed to forecast the future behavior of the storm.
The U.S. Air Force Reserve uses specially equipped C-130 aircraft to conduct most operational reconnaissance. Pilots fly the aircraft into the hurricane center to measure winds, pressure, temperature, and humidity, as well as to provide an accurate location of the eye.
The National Oceanic and Atmospheric Administration (NOAA) also flies aircraft into hurricanes to aid scientists in better understanding these storms and to improve forecast capabilities. The P3 Orion aircraft and state-of-the-art high altitude jet aircraft complement the U.S. Air Force Reserve's hurricane reconnaissance function.
A radiosonde is a small instrument package and radio transmitter that are attached to a large balloon. As the balloon rises through the atmosphere, the radiosonde instruments measure air temperature, humidity and pressure. This data is relayed back to a computer at the surface.
The data provide an important vertical profile of the hurricane's environment, which is critical for forecast models. Radiosondes are generally released over land, leaving a large data gap over ocean areas.
Dropsondes are a variation on the radiosonde. Instead of being carried aloft by a balloon, dropsondes, which are attached to a small parachute, are dropped into the hurricane from reconnaissance aircraft. These instruments are helping forecasters make great strides in understanding and predicting hurricane behavior.
When a hurricane nears the coast, it is monitored by land-based weather radar. Radars provide detailed information on hurricane wind fields, rain intensity, and storm movement. As a result, local NWS offices are able to provide short-term warnings for floods, tornadoes, and high winds for specific areas.
Sophisticated mathematical calculations give forecasters important information derived from the radar data, such as estimates of rainfall amounts. A limitation of radars is that they cannot "see" farther than about 200 miles from the coast, and hurricane watches and warnings must be issued long before the storm comes into range.
||Radial Wind Velocity
The images above show two important radar products used by forecasters. The reflectivity images (above left) are the ones frequently shown on TV. In these pictures, the forecaster can pick out details about storm features (such as the locations of the eye and rainbands), storm motion, and intensity. The radial wind velocity product (above right) gives forecasters important information about wind speed and direction. These tools allow forecasters to provide much more timely and accurate warnings than were possible only a few years ago.
Automated Surface Observation Systems (ASOS) are located in more than 850 locations throughout the country. These monitoring systems provide forecasters with surface weather observations around the clock. These systems are land-based and ASOS data is mainly useful once the hurricane has come close to shore or after it has made landfall. This information is invaluable in post-analysis.