9.6.6 Common Problems Encountered in Upper-Air Data Collection
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.
data are susceptible to several problems, including the following:
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.
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.
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 °.
problems. Tracking of radiosondes can be problematic within
rainshafts or updrafts/downdrafts associated with thunderstorms.
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.
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.
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
data can be rendered problematic by the following:
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.
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.
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
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
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.
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.
from radar wind profiler systems can be affected by several problems,
including the following:
from migrating birds. Migrating birds can contaminate radar wind
profiler signals and produce biases in the wind speed and direction
measurements . 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 .
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.
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  and .
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 . 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.
systems are susceptible to several common problems including the following:
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.
cold bias. Recent inter-comparisons between RASS systems and
radiosonde sounding systems have shown a bias in the lower sampling
altitudes . 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
9.1.1 Upper-Air Meteorological Variables
9.1.2 Radiosonde Sounding System
9.1.3 Doppler Sodar
9.1.4 Radar Wind Profiler
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
Terrain Data &
Leading Air Dispersion Modeling & Risk Assessment Software
Advanced Air Dispersion Model