9.4 Siting and Exposure

Siting and exposure issues related to radiosonde sounding systems, sodar, radar wind profiler, and RASS meteorological measurement systems are addressed in this section.

Careful planning should accompany the siting of upper-air measurement systems, since siting and exposure directly affect the quality of the data. The complexities of ground based remote sensing devices provide a challenge for the user to balance the conditions favorable for the technology with the availability of sites and the overall data collection goals of the program. Site selection may benefit from the experience of vendors or users of the type of instrument to be installed. Additional information on siting can be found in reference [2]. Listed below are some key issues to consider in siting upper-air systems.

• Representative location. Sites should be located where upper-air data are needed to characterize the meteorological features important to meeting the program objectives.Panoramic photographs should be taken of the site to aid in the evaluation of the data and preparation of the monitoring plan. Data collected at sites in regions with local geographic features such as canyons, deep valleys, etc., may be unrepresentative of the surrounding area and should be avoided, unless such data are needed to resolve the local meteorological conditions. Measurements made in complex terrain may be representative of a much smaller geographical area than those made in simple homogeneous terrain. See reference [101] for a discussion of the influence of terrain on siting and exposure of meteorological instrumentation.





• Site logistics. 





• Adequate power should be available for the instrument system as well as an environmentally controlled shelter that houses system electronics, and data storage and communication devices.





• The site should be in a safe, well lit, secure area with level terrain, sufficient drainage, and clear of obstacles. The site should allow adequate room for additional equipment that may be required for calibrations, audits, or supplementary measurements.





• A fence should be installed around the equipment and shelter to provide security, and appropriate warning signs should be posted as needed to alert people to the presence of the equipment.





• A remote data communications link (e.g., dedicated leased line, standard dial-up modem line, or a cellular telephone link) should be installed at the monitoring site. It is recommended that a 9600 baud or higher line be established to facilitate rapid data transfer and uploading and downloading of information. A site in a remote location with no communication capabilities may collect valid data, but if the system goes down it may not be discovered until the next time the site is visited.




• Collocation with surface meteorological measurements. Several advantages can be gained by locating an upper-air site with or near an existing meteorological monitoring station. For instance, collocated data can be used for data validation purposes and for performing reasonableness checks (e.g., do surface winds roughly agree with near-surface upper-air winds, surface temperatures with near-surface RASS measurements). Existing shelter, power, and personnel could also be used for operating the upper-air instrument. Additional surface meteorological measurements of wind speed, wind direction, temperature and humidity are recommended. The height of the wind sensors will depend on the terrain. In homogeneous terrain, wind data collected at a height of 10 m may be sufficient.



• Instrument noise. Sodar and RASS generate noise that can disturb nearby neighbors. Depending on the type of sodar or RASS instrument, power level, frequency, acoustic shielding around the system, and atmospheric conditions, the transmitted pulse can be heard from tens of meters up to a kilometer away. An optimum site is one that is isolated from acoustically sensitive receptors [102].



• Passive interference/noise sources. Objects such as stands of trees, buildings or tall stacks, power lines, towers, guy wires, vehicles, birds, or aircraft can reflect sodar or radar transmit pulses and contaminate the data. Not all sites can be free of such objects, but an optimum site should be selected to minimize the effects of such obstacles. If potential reflective “targets” are present at an otherwise acceptable site, the beams of the instrument should be aimed away from the reflective objects. In the case of sodars, locating the antennas so that there are no direct reflections from objects will help minimize potential contamination. In the case of the radar profiler, it is best to aim the antennas away from the object and orient a phased array antenna's corners so they are pointing toward the objects. As a rule of thumb, sites with numerous objects taller than about 15° above the horizon should be avoided. The manufacturers of the remote sensing equipment should be contacted regarding software that may be available to identify and minimize the effects of these passive noise sources.



• Active interference/noise sources. For sodars, noise sources such as air conditioners, roadways, industrial facilities, animals, and insects will degrade the performance of sodar systems [102]. If proximity to such sources cannot be avoided, then additional acoustic shielding may help minimize the potentially adverse effects on the data. In general, noise levels below 50 decibels (dBA) are considered to be representative of a quiet site, while levels above 60 dBA are characteristic of a noisy site. For radar wind profilers and RASS, radio frequency (RF) sources such as radio communications equipment and cellular telephones may have an adverse effect on performance.



• Licenses and Ordinances. Before operating a remote sensor it is recommended that all applicable requirements for operation of equipment be addressed. For example, to operate a radar wind profiler or a RASS, a Federal Communications Commission (FCC) license is required. For radiosonde sounding systems (or other balloon-borne systems), a Federal Aviation Administration (FAA) waiver may be required. Local noise ordinances may limit the operation of sodar or RASS instruments. Some of these requirements may take several months to address and complete.



• Surveying Candidate Locations. Prior to final site selection, a survey is recommended to identify audio sources [103] and RF sources that may degrade system performance. Additionally, panoramic photographs should be taken to aid in the evaluation of the candidate site and for the preparation of the monitoring plan. As part of the survey, appropriate topographic and other maps should be used to identify other potential sources of interference, such as roadways and airports.

9. UPPER-AIR MONITORING 
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