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9.1.2 Radiosonde Sounding System

Radiosonde sounding systems use in situ sensors carried aloft by a small, balloon-borne instrument package (the radiosonde, or simply “sonde”) to measure vertical profiles of atmospheric pressure, temperature, and moisture (relative humidity or wet bulb temperature) as the balloon ascends. In the United States, helium is typically used to inflate weather balloons. Hydrogen is also used. The altitude of the balloon is typically determined using thermodynamic variables or through the use of satellite-based Global Positioning Systems (GPS). Pressure is usually measured by a capacitance aneroid barometer or similar sensor. Temperature is typically measured by a small rod or bead thermistor. Most commercial radiosonde sounding systems use a carbon hygristor or a capacitance sensor to measure relative humidity directly, although a wet-bulb sensor is also used by some systems. With a wet bulb, relative humidity and dewpoint are calculated from psychrometric relationships. Ventilation of the sensors occurs as the balloon rises. The temperature sensor is usually coated to minimize radiational heating effects. The humidity sensor is usually shielded in a ventilated duct inside the sonde's enclosure to minimize exposure to solar radiation.

A radiosonde includes electronic subsystems that sample each sensor at regular intervals (e.g., every 2 to 5 seconds), and transmit the data to a ground-based receiver and data acquisition system. Power for the radiosonde is provided by small dry-cell or wet-cell batteries. Most commercial radiosonde systems operate at 404 MHZ or 1680 MHZ. Once the data are received at the ground station, they are converted to engineering units based on calibrations supplied by the manufacturer. The data acquisition system reduces the data in near-real time, calculates the altitude of the balloon, and computes wind speed and direction aloft based on information obtained by the data systems on the position of the balloon as it is borne along by the wind. Commercial systems available today are relatively compact and easy to operate. The radiosondes are typically smaller than a shoebox and weigh only a few hundred grams. Thus, the previous need to use a parachute to slow the radiosonde's descent after the balloon has burst has greatly diminished, although the manufacturer should still be consulted on this matter. The data systems are either personal computer (PC)-based, or self-contained with standard PC-type computer interfaces for data communications (e.g., RS-232). Data are stored on conventional PC-type hard disks and/or diskettes.

Upper-air winds (horizontal wind speed and direction) are determined during radiosonde ascents by measuring the position of the radiosonde relative to the earth's surface as the balloon ascends. By measuring the position of the balloon with respect to time and altitude, wind vectors can be computed that represent the layer-averaged horizontal wind speed and wind direction for successive layers. The position data have typically been obtained using radio direction finding techniques (RDF) or one of the radio navigation (NAVAID) networks. The use of satellite-based GPS is becoming more common.

RDF systems use a tracking device called a radio theodolite to measure the position of the balloon relative to the ground station. The radio theodolite, which resembles a small tracking radar system, measures the azimuth and elevation angles to the radiosonde relative to the ground station. The radio theodolite automatically follows the motion of the balloon by tracking the primary lobe of the radiosonde's transmitter, making adjustments to the tilt and pointing direction of the antenna as it follows the signal from the sonde. The azimuth, elevation, and altitude information is then used by the data system to compute the length and direction of a vector projected onto the earth's surface that represents the resultant motion of the balloon over some suitable averaging period, typically 30 to 120 seconds.

With NAVAID systems, the radiosonde's position is determined by triangulation relative to the locations of the fixed NAVAID transmitters. The radiosonde and ground station have electronic subsystems to measure the time delay in the transmissions from the NAVAID sites and to convert this information into the relative motion of the radiosonde, from which winds aloft are computed.

GPS is a satellite navigation system, which is funded and controlled by the U.S. Department of Defense. The system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. GPS wind-finding system sondes consist of a 10-channel GPS (Global Positioning System) receiver as well as a platform for temperature, RH and pressure sensors.

The basic steps in performing a sounding involve: preparing the radiosonde (deploying the sensors, connecting the batteries, etc.); activating the data acquisition system and manually or automatically entering the radiosonde calibration information; inflating the balloon and attaching the sonde; releasing the balloon and activating the tracking system; monitoring the data during the sounding; and performing post-sounding procedures as required (e.g., completing sounding documentation, preparing backups of the data, transferring the data to a central data processing facility, etc.). For air quality programs, the entire procedure requires approximately one hour, and one to two operators. Prior to the release of the radiosonde, an accurate measurement must be made of the surface pressure to provide a baseline value for computing altitude from the radiosonde data. This baseline value is used to compute any offsets that are needed for the sonde's pressure measurements. A good quality barometer that is regularly calibrated and audited should be used to make this measurement. Other baseline readings that should be taken include temperature and moisture (wet bulb or relative humidity), and surface winds, although these data are typically not used to offset the sonde measurements.

High quality tracking information is necessary for obtaining high quality wind data within the atmospheric boundary layer. For monitoring programs with a strong emphasis on characterizing low-level boundary layer winds, it is important that the radio theodolite operator get the theodolite to “lock on” to the radiosonde transmission right from the moment of launch. Otherwise, a few minutes of wind data may be lost while the system acquires the signal and begins tracking the radiosonde automatically. Due to this type of delay, for example, typical National Weather Service (NWS) data collection procedures result in a smoothing of the winds within approximately the lowest 300 m. With NAVAID systems, it is important to ensure that position information is being acquired prior to release of the balloon. At some sites, high terrain or other obstacles may block the NAVAID radio signals, so that the balloon must be airborne for a few minutes before accurate position information is available. This, too, can cause a few minutes of wind data to be lost at the beginning of a sounding. Normally autonomous (single receiver) GPS position data are only accurate to about 100 meters due to the use of selective availability by the military to introduce an “uncertainty” into the signal. To compensate for this error, the meteorological sounding systems use the base (receiving) station as a differential GPS location which can increase GPS accuracy to better than 1 meter. The horizontal drift of the radiosonde from the release location may also result in the incomplete characterization of the vertical structure of small (spatial and or temporal) scale features.

Generally speaking, radiosonde soundings made for boundary layer air quality studies do not need to achieve the kind of high altitude coverage required for soundings made by the NWS, where data to the tropopause and to stratospheric levels are needed for weather forecasting. For most air quality studies, the vertical range for radiosonde data will not need to exceed 10,000 m msl (approximately 300 mb), and data coverage to 5000 m msl (approximately 500 mb) will be sufficient. In this case, a smaller weather balloon than that used by the NWS, e.g., a 100-gram balloon as opposed to a 300- to 600-gram balloon, is adequate. Balloon size is stated as weight rather than diameter because the weight relates directly to the amount of free lift needed to achieve the desired ascent rate during a sounding, which in turn influences how much helium must be used and, therefore, the cost per sounding.

In a compromise between adequate ventilation of the temperature and moisture sensors on the sonde and good vertical resolution in the boundary layer, ascent rates used for soundings made during air quality studies (2 to 3 ms -1 ) are also typically less than that used by the NWS (5 to 6 ms -1 ). As noted earlier, these ascent rates are consistent with an elapsed time of approximately one hour. Thus, the vertical resolution of the thermodynamic data is usually 5 to 10 m, depending on the interval at which the data acquisition system samples the signals from the radiosonde and the time response of the sensor. The vertical resolution of the wind data ranges from approximately 45 to 200 m, depending on the type of sounding system used. The data averaging interval for radiosondes is 1 to 2 minutes in the lower part of a sounding (e.g., lowest 3000 m) and approximately 3 to 4 minutes in the upper part of a sounding.

9.1 Fundamentals  
      9.1.1 Upper-Air Meteorological Variables  
     9.1.2 Radiosonde Sounding System  
     9.1.3 Doppler Sodar 
     9.1.4 Radar Wind Profiler 
     9.1.5 RASS  
 9.2 Performance Characteristics  
     9.2.1 Definition of Performance Specifications  
     9.2.2 Performance Characteristics of Radiosonde Sounding Systems 
     9.2.3 Performance Characteristics of Remote Sensing Systems  
 9.3 Monitoring Objectives and Goals  
     9.3.1 Data Quality Objectives  
 9.4 Siting and Exposure
 9.5 Installation and Acceptance Testing 
9.6 Quality Assurance and Quality Control 
     9.6.1 Calibration Methods  
     9.6.2 System and Performance Audits  
     9.6.3 Standard Operating Procedures 
     9.6.4 Operational Checks and Preventive Maintenance  
     9.6.5 Corrective Action and Reporting  
     9.6.6 Common Problems Encountered in Upper-Air Data Collection 
 9.7 Data Processing and Management (DP&M) 
9.7.1 Overview of Data Products  
     9.7.2 Steps in DP&M 
     9.7.3 Data Archiving  
 9.8 Recommendations for Upper-Air Data Collection 

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