A dropsonde is an expendable weather reconnaissance device created by the National Center for Atmospheric Research (NCAR), designed to be dropped from an aircraft at altitude over water to measure (and therefore track) storm conditions as the device falls to the surface. The sonde contains a GPS receiver, along with pressure, temperature, and humidity (PTH) sensors to capture atmospheric profiles and thermodynamic data. It typically relays these data to a computer in the aircraft by radio transmission.
Since the early 1970s, hurricane hunters have employed dropsondes while flying over the ocean to obtain meteorological data on the structure of hurricanes deemed to be of possible concern to land locations in the northern Atlantic and northeastern Pacific oceans. Dropsonde instruments are typically the only way to measurement the wind and pressure near the sea surface within the core of such cyclones, allowing meteorologists to reliably establish the storm's intensity and size. The data obtained is typically fed into supercomputers for numerical weather prediction, enabling forecasters to better track and predict what will happen to the hurricane. During a typical hurricane season, the 53d Weather Reconnaissance Squadron Hurricane Hunters deploys 1000 to 1500 sondes on training and storm missions.
Aircraft reconnaissance missions are also sometimes requested to investigate the broader atmospheric structure over the ocean when cyclones may pose a significant threat to the United States. These interests include not only potential hurricanes, but also possible snow events (like nor'easters) or significant tornado outbreaks. The dropsondes are used to supplement the large gaps over oceans within the global network of daily radiosonde launches. Typically satellite data provides an estimate of conditions in such areas, but the increased precision of sondes can improve forecasts, particularly of the storm path.
Dropsondes may also be employed during meteorological research projects.
The sonde is a lightweight system designed to be operated by one person and is launched through a chute installed in the measuring aircraft. The device's descent is slowed and stabilized by a small square-cone parachute, allowing for more readings to be taken before it reaches the ocean surface. The parachute is designed to immediately deploy after release so as to reduce or eliminate any pendulum effect, and the device typically drops for three to five minutes. The sonde has a casing of stiff cardboard to protect electronics and form a more stable aerodynamic profile.
To obtain data in a tropical cyclone, an aircraft (in the US, operated either by NOAA or the U.S. Air Force) flies into the system. A series of dropsondes are typically released as the plane passes through the storm, typically launched with greatest frequency near the center of the storm, including into the eyewall and eye eye (center), if one exists. Most drops are performed at a flight level of around 10,000 feet (approx. 3,000 meters).
The dropsonde sends back coded data, which includes:
- The date and time of the drop. Time is always in UTC.
- Location of the drop, indicated by the latitude, longitude, and Marsden square.
- The height, temperature, dewpoint depression, wind speed, and wind direction recorded at any standard isobaric surfaces encountered as the dropsonde descends, which are from the set of: 1000, 925, 850, 700, 500, 400, 300, 250 hectopascals (hPa), and at the sea surface.
- The temperature and dewpoint depression at all other atmospheric pressure deemed significant due to important changes or values in the atmospheric conditions found
- Air pressure, temperature, dewpoint depression, wind speed and wind direction of the tropopause.
Also included in the report is information on the aircraft, the mission, the dropsonde itself, and other remarks.
A driftsonde is a high altitude, durable weather balloon holding a transmitter and a bank (35 in the first models) of miniature dropsonde capsules which can then be dropped at automatic intervals or remotely. The water-bottle-sized transmitters in the dropsondes have enough power to send information to the balloon during their parachute-controlled fall. The balloon carries a larger transmitter powerful enough to relay readings to a satellite. The single-use sensor packages cost US$300 to $400 each.
After being introduced in April 2007, around a thousand a year are expected to be used to track winds in hurricane breeding grounds off of West Africa, which are outside the operating region of Hurricane Hunter planes.
The Sounding Group in ISF operates the AVAPSTM (Airborne Vertical Atmospheric Profiling System) also known as the AVAPS Dropsonde System.
The AVAPS Dropsonde system, has dramatically extended the envelope of atmospheric profiling capabilities. Since its debut in 1997, it has flown on numerous missions in support of operational weather forecasting and atmospheric research, with impressive results.
The AVAPS Dropsonde System is a key atmospheric instrument that measures high resolution vertical profiles of ambient temperature, pressure, humidity, wind speed and wind direction. Measurements are taken by a parachuted GPS dropsonde that is launched from the aircraft and descend to the surface. In-situ data collected from the sonde’s sensors are transmitted back in real time to an onboard aircraft data system via radio link.
Atmospheric soundings from dropsondes provide the ability for targeted observations over remote areas such as the oceans, Polar Regions and land masses; they also provide a means to obtain soundings in and around severe weather systems, such as hurricanes. Atmospheric soundings obtained from dropsondes during hurricane reconnaissance flights have improved the accuracy of forecasts of hurricane landfall by about 20 percent over the decade of the 1990’s. The use of aircraft released dropsondes have had a dramatic impact on the forecast track of hurricanes.
The sonde is a small electronic device which contains atmospheric sensors: pressure, temperature, humidity and a GPS receiver to derive winds. The sonde is launched from an aircraft where a parachute is deployed. As the sonde descends to the Earth’s surface it continuously measures the state of the atmosphere and telemetries this information to the research aircraft. The aircraft is equipped with dedicated hardware and software to process the signal from the sonde in real-time to display and archive the data. Once the sonde has reached the surface, all data collected during the descent is sent via satellite to atmospheric research centers or the National Hurricane Center or the World Meteorological Organization. The mini Sonde and aircraft is the equivalent of a standard radiosonde or weather balloon launched by the National Weather Service launched twice a day from over 100 locations in the U.S.
The Dropsonde is composed of a small electronic circuit board, sensors and a battery housed in a cardboard tube with a parachute. The total weight of the sonde is less than 6 ounces with dimensions of a 1.75” diameter tube 12 inches long. The inner electronic components of the dropsonde consist of precision temperature, pressure and humidity sensors, low powered telemetry transmitter, GPS receiver and a microprocessor. As the sonde descends it continuously measures the atmosphere from the release altitude to the Earth’s surface. Measurements are made every half second which provides a precise detailed profile of the atmosphere. The parachute deploys from the top of the sonde within seconds of being released from the aircraft. The parachute is a specially designed for high reliability and a very stable descent. As the sonde descends the GPS receiver tracks the position and velocity of the sondes, this change in motion corresponds to the atmospheric winds. The sensor data, GPS receiver 3D position and 3D velocity along with engineering health of the sonde is all wirelessly sent via radio waves to the aircraft with a low powered transmitter operating in the 400-406 MHz Meteorological band.
The dropsonde incorporates a pressure, temperature, humidity sensor module (RSS903) designed by Vaisala, Inc., for their RS92 radiosonde and a GPS receiver module.
USERS OF THE NCAR GPS DROPSONDE SYSTEM (AVAPS)
SSSF has built a "double" 8-channel AVAPS system plus spares for the NOAA G-IV and two additional data systems plus spares for the two NOAA P-3's. These systems were delivered to NOAA in August 1996 for testing and use, and have since been used in numerous research programs as well as operational forecasting of tropical and winter storms.
An AVAPS system has also been built for DLR in their Falcon research aircraft.
USAF Reserve Hurricane Hunters
ISF has also built 10 AVAPS systems plus spares for the 53rd Weather Reconnaissance Squadron (aka Hurricane Hunters) at Keesler Air Force Base in Biloxi, Mississippi. These systems are operational in their WC-130J aircraft. During a typical hurricane season, the 53rd deploys 1000 to 1500 sondes on training and storm missions.
SSSF has also built and installed an AVAPS system in Atmospheric Environmental Services (AES) Canada-s Convair 580 research aircraft.
The Met. Office (UK)
UCAR/Intellectual property and EOL/SSSF has also licensed Vaisala to sell the AVAPS system to their customers worldwide. One such system has been installed in the United Kingdom Meteorological Office's BAE-146, operated by the Meteorological Research Flight (MRF) division.
Vaisala has sold another system to the National Institute for Polar Research (NIPR) in Japan.
NCAR has one AVAPS system, which can be used in NSF/NCAR's C-130 or G-V, or by the scientific research community in other aircraft, such as the NASA DC-8.