9.6.6 Common Problems Encountered in Upper-Air Data Collection

Studies performed to date have indicated that the upper-air measurement systems described in this document can reliably and routinely provide high quality meteorological data. However, these are complicated systems, and like all such systems are subject to sources of interference and other problems that can affect data quality. Users should read the instrument manuals to obtain an understanding of potential shortcomings and limitations of these instruments. If any persistent or recurring problems are experienced, the manufacturer or someone knowledgeable about instrument operations should be consulted.

Radiosonde data are susceptible to several problems, including the following:

• Poor ventilation. Prior to launch, lack of ventilation of the sonde may result in unrepresentative readings of temperature and relative humidity (and thus dew-point temperature) at or near the surface.



• Radio frequency (RF) interference. RF interference may occasionally produce erroneous temperature, dew-point temperature, and relative humidity measurements, which appear as spikes in the data when plotted in a time series or profile plot.



• Uncertainties in the tracking mechanism. Uncertainties in a radio theodolite's tracking mechanism may produce unrealistic changes in the wind speed and direction, especially when the antenna's elevation angle is less than about 1 0 °.



• Tracking problems. Tracking of radiosondes can be problematic within rainshafts or updrafts/downdrafts associated with thunderstorms.



• Icing. When a balloon encounters clouds and precipitation zones where the temperature is below freezing, ice can form on the balloon and cause it to descend. Once the balloon descends below the freezing level, the ice melts and the balloon re-ascends. This causes the balloon to fluctuate up and down around the freezing level, and produces unrepresentative wind and thermodynamic data.



• Poor radio navigation reception. Not all sites have good radio navigation reception. If this technique is used to track the radiosonde, poor reception can produce uncertainties in the wind data. Poor reception will not affect the thermodynamic data.



• Low-level wind problems. Often the first few data points in a radiosonde wind profile tend to have more uncertainty due to initial tracking procedures or difficulties (see Section 9.1 for more details).

Sodar data can be rendered problematic by the following:

• Passive noise sources (also called fixed echo reflections). Passive noise occurs when nearby obstacles reflect the sodar's transmitted pulse. Depending on atmospheric conditions, wind speed, background noise, and signal processing techniques, the fixed echoes may reduce the velocity measured along a beam(s) or result in a velocity of zero. This problem is generally seen in the resultant winds as a rotation in direction and/or a decrease in speed at the affected altitude. Some manufacturers offer systems that have software designed to detect fixed echoes and effectively reject their influence. To further decrease the effect of the fixed echoes, additional acoustic shielding can be added to the system antenna.



• Active noise sources (ambient noise interference). Ambient noise can come from road traffic, fans or air conditioners, animals, insects, strong winds, etc. Loud broad-spectrum noise will decrease the SNR of the sodar and decrease the performance of the system. Careful siting of the instrument will help minimize this problem.



• Unusually consistent winds at higher altitudes. Barring meteorological explanations for this phenomenon, the most common cause is a local noise source that is incorrectly interpreted as a “real” Doppler shift. These winds typically occur near the top of the operating range of the sodar. A good means of identifying this problem is to allow the sodar to operate in a listen-only mode, without a transmit pulse, to see if winds are still reported. In some cases it may be necessary to make noise measurements in the specific operating range of the sodar to identify the noise source.



• Reduced altitude coverage due to debris in the antenna. In some instances, particularly after a precipitation event, the altitude coverage of the sodar may be significantly reduced due to debris in the antennas. In three axis systems, drain holes may become plugged with leaves or dirt and water, snow, or ice may accumulate in the antenna dishes. Similarly, some of the phased-array antenna systems have the transducers oriented vertically and are open to the environment. Blocked drain holes in the bottom of the transducers may prevent water from draining. Regular maintenance can prevent this type of problem.



• Precipitation interference. Precipitation, mostly rain, may affect the data collected by sodars. During rainfall events, the sodar may measure the fall speed of drops, which will produce unrealistic winds. In addition, the sound of the droplets hitting the antenna can increase the ambient noise levels and reduce the altitude coverage.



• Low signal to noise ratio (SNR). Conditions that produce low SNR can degrade the performance of a sodar. These conditions can be produced by high background noise, low turbulence and near neutral lapse rate conditions.

Data from radar wind profiler systems can be affected by several problems, including the following:

• Interference from migrating birds. Migrating birds can contaminate radar wind profiler signals and produce biases in the wind speed and direction measurements [105]. Birds act as large radar “targets,” so that signals from birds overwhelm the weaker atmospheric signals. Consequently, the radar wind profiler measures bird motion instead of, or in addition to, atmospheric motion. Migrating birds have no effect on RASS. Birds generally migrate year-round along preferred flyways, with the peak migrations occurring at night during the spring and fall months [106].



• Precipitation interference. Precipitation can affect the data collected by radar profilers operating at 915 MHZ and higher frequencies. During precipitation, the radar profiler measures the fall speed of rain drops or snow flakes. If the fall speeds are highly variable during the averaging period (e.g., convective rainfall), a vertical velocity correction can produce erroneous data.



• Passive noise sources (ground clutter). Passive noise interference is produced when a transmitted signal is reflected off an object instead of the atmosphere. The types of objects that reflect radar signals are trees, elevated overpasses, cars, buildings, airplanes, etc. Careful siting of the instrument can minimize the effects of ground clutter on the data. Both software and hardware techniques are also used to reduce the effects of ground clutter. However, under some atmospheric conditions (e.g., strong winds) and at some site locations, ground clutter can produce erroneous data. Data contaminated by ground clutter can be detected as a wind shift or a decrease in wind speed at affected altitudes. Additional information is provided in references [107] and [108]. 



• Velocity folding or aliasing. Velocity folding occurs when the magnitude of  the radial component of the true air velocity exceeds the maximum velocity that the instrument is capable of measuring, which is a function of sampling parameters [109]. Folding occurs during very strong winds (>20 m/s) and can be easily identified and flagged by automatic screening checks or during the manual review.

RASS systems are susceptible to several common problems including the following:

• Vertical velocity correction. Vertical motions can affect the RASS virtual temperature measurements. As discussed in Section 9.1, virtual temperature is determined by measuring the vertical speed of an upward-propagating sound pulse, which is a combination of the acoustic velocity and the atmospheric vertical velocity. If the atmospheric vertical velocity is non-zero and no correction is made for the vertical motion, it will bias the temperature measurement. As a rule of thumb, a vertical velocity of 1 ms -1 can alter a virtual temperature observation by 1. 6 °C.



• Potential cold bias. Recent inter-comparisons between RASS systems and radiosonde sounding systems have shown a bias in the lower sampling altitudes [110]. The RASS virtual temperatures are often slightly cooler (-0.5 to -1. 0 °C) than the reference radiosonde data. Work is currently underway to address this issue.

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