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CMeteorological Observing Systems for
Tracking and Modeling C/B/N Plumes
Summary of a presentation by Walter F. Dabberdt, Vaisala zinc.
Meteorological observations play a critically important role in tracking and predicting the
dispersion of gases and particles in the atmosphere. Depending on which variables are characterized
(e.g., transport, diffusion, stability, deposition, plume rise), a wide range of meteorological parameters
must be quantified. These can include wind speed and direction, temperature, humidity, precipitation
type and intensity, mixing height, turbulence, and energy fluxes. Table C.1 summarizes the measurement
requirements according to dispersion and meteorological variables. The specific variables that must be
measured are a function of the algorithms and parameterizations used in the dispersion model. Because
of their variability with height in the bounty layer, vertical profiles are important in addition to the
more common practice of making meteorological measurements at or near the ground surface (see
Lenschow, 1986, for a comprehensive discussion of atmospheric measurements in the planetary
boundary layer). In the same way spatial variability ofthe dispersion variables maY necessitate multiple
~ ~ , - , ~ ~ ~ ~ ~
, . . , , . . . , . . , . . ~ , , , . _
observing sites, model parameter~zat~ons, orJud~c~ous combinations ot~measurements and modeling. the
following is a brief overview of the types of instrumentation that can be used to obtain the various
meteorological observations. The primary focus is on measurement devices that are readily available
from commercial sources, but some of the more promising research systems and concepts also are
discussed.
IN SITU MEASUREMENTS
Meteorological towers (typically 6 to 10 m tall) are used widely as platforms for collecting
in situ "surface" observations of wind, turbulence, temperature, and humidity. Mechanical wind
sensors (bivanes, propeller vanes, etc.) have been used for decades, and their performance has
improved steadily over this time. Sonic anemometers have come into widespread operational use
over the past few years, having overcome earlier limitations, such as water-sensitive transducers,
exposure characteristics, and price. Temperature can be measured to acceptable accuracy and
precision by any of several different methods (e.g., resistance, capacitance), provided the probe is
well shielded from solar insolation and properly ventilated. The vertical temperature gradient over
the height of the tower is an important measurement for determining atmospheric stability and
estimating turbulence. Typically, temperature gradients are measured using thermocouples or
platinum resistance thermometers. The humidity or water vapor mixing ratio is a more difficult
measurement, but it still can be made with acceptable accuracy and precision. The two most
common methods are thin-film capacitance sensors and dewpoint measuring devices. Though less
72
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APPENDIX C
TABLE C. 1 Candidate Meteorological Observing Systems.
73
Dispersion Variables
Meteorological Variables
(not all required; algorithm
dependent)
Candidate
Measurement Systems
Pro fi l e s Do p p l e r weathe r radar
Three-d~mens~onal fields of '
Transport . . . . RAOBs mesonets aircraft
wind speed and wind direction ' ' .
tethersonde; Doppler l~dar
Turbulence; wind speed 3D sonic anemometers; cup and
variance; wind direction vane anemometers; RAOBs;
Diffusion variance; stability; lapse rate; profiles; RASS; scanning
mixing height; surface microwave radiometer (maybe);
roughness tethersonde
Towers, ceilometers, profiler-
Temperature gradient; heat {lux; MASSE RAOBs aircraft
Stability cloud coyer ~nsolat~onor net ' '
' tethersonde net radiometers
radiation . ' .
pyranometers; pyrgeometers
. . Precipitation rate; phase; size Weather radar (polarimetric);
Deposition wet
' distribution cloud radar; profilers
Deposition, dry
Turbulence; surface roughness See turbulence
Wind speed; temperature Profilers/RASS; RAOBs; lidar;
Plume rise profile; mixing height; stability ceilometer; tethersonde; aircraft
aRAOB stands for radiosonde observation.
BRASS stands for radio acoustic sounding system
common' meteorological towers can also be instrumented to measure heat and radiative fluxes and
a number of other relevant meteorological and chemical variables.
For in situ upper-air measurements' balloon-borne radiosondes commonly are used on an
operational basis. Radiosondes have in situ sensors that measure temperature' humidity' and
Pressure while winds are measured using either of two general methods.
~ ~ ~ ~ . . . ~ .
one wlnd-llndmg
method uses an onward navigation aid receiver to measure the movement or change in location of
the sonde. The second method tracks the flight of the radiosonde from the ground using radar or
radio direction-finding equipment. Radiosondes are launched twice daily from 100 locations in
the United States (992 locations worldwide in 1999~. The typical ascent rate is 5 ms~~ and raw
data are obtained every 1-6' seconds depending on the radiosonde type and manufacturer.
REMOTE SENSING
Remote sensing techniques are finding increasing use as an operational method to obtain
vertical (and horizontal) profiles in the troposphere.
Radar wind profilers transmit short pulses of radio-frequency energy' which are scattered
by clear-air atmospheric inhomogeneities and also by hydrometeors to produce a spectrum of
Doppler velocities.
. ~
There are numerous types of radar wind profilers available' and they can
provlcte coverage ranging from near the surface to the lower troposphere to the lower stratosphere
(depending on their radio frequency). The most commonly used measurement principle is
Doppler beam swinging' which involves alternating the radar beam direction and measuring the
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74
APPENDIX C
Doppler shift as a function of range (height) in each of several directions (pointing angles). The
ambient vector velocity is then retrieved from the radial velocities along each pointing angle.
Another method, called spaced-antenna profiling, transmits a single vertically directed radar beam
and measures the phase relationships of the returned signal at multiple, adjacent antenna locations
to retrieve the vector wind profile. Radar wind profilers provide the benefits of continuous
unattended operation with high temporal resolution (5 minutes for UHF systems). Height
resolution is 60-75 m with minimum heights of about 150 m; maximum height depends on
atmospheric humidity and turbulence, and is typically 3-5 km for commercial UHF profilers. The
lack of a dedicated UHF profiler frequency in the United States and growing commercial pressure
by telecommunications providers for access to the commonly used profiler bands are concerns that
require immediate attention.
Profiling of the lowest 150 m of the boundary layer is important, especially during noc-
turnal periods when the mixed-layer depth may be 50 m or less. So-called minisodars (profilers
that use sound waves rather than radio waves) can provide the minimum range and resolution
required, and they are a particularly useful complement to radar wind profilers. Unfortunately,
sodars are inherently noisy (an audible signal is transmitted every few seconds) and, thus,
encounter significant human resistance, especially in urban areas. Conversely, ambient noise can
also impact sodar performance.
Meteorological radars and lidar (light detection and ranging) are two additional remote
sensing systems useful for wind and other measurements important for dispersion and deposition.
Operational Doppler meteorological radars transmit at wavelengths of 3, 5, and 10 cm; all three
wavelengths can measure the radial velocity of hydrometeors, while the longer-wavelength
systems can also measure clear-air velocities out to a few kilometers. Meteorological radars are
especially valuable for quantifying wet deposition because of their ability to detect precipitation
and estimate rain rates with reasonable accuracy over a wide area. Wind profiling radars also can
detect and identify precipitation, but they yield only a single vertical profile, whereas
meteorological radars can provide volumetric distributions over wide areas. Multiparameter
radars transmit and measure returned signals from both horizontally and vertically polarized
beams, enabling them to differentiate precipitation type (e.g., rain, snow, hail) and, thus, better
estimate precipitation rates. The National Weather Service has plans to upgrade its WSR-88D
weather radars to include this capability beginning around 2005. The Next Generation Weather
Radar system (NEXRAD; see NRC, 1995) comprises approximately 160 WSR-88D sites through-
out the United States and selected overseas locations. Figure C.1 shows NEXRAD coverage
above 3 km for the contiguous United States. A limitation of NEXRAD for dispersion
applications is its limited area of coverage in the lower troposphere due to Earth's curvature,
blockage by obstacles, and the 0.5-degree minimum elevation angle. Networks of smaller but
more densely spaced radars are being considered to complement NEXRAD and overcome these
limitations (NRC, 2002~.
Lidar systems emit pulses of energy at wavelengths that can vary from ultraviolet to visible
to near-IA depending on the particular device. Light is scattered back from the atmosphere by
particulate matter (and hydrometeors), which can serve as tracers of atmospheric mixing in the
boundary layer. This enables simple backscatter lidars to estimate mixing depth, especially during
unstable atmospheric conditions when there is turbulent mixing and the particulates are well
mixed below the capping inversion layer. Nocturnal estimates by lidar provide higher signal-to-
noise ratios but are less definitive because of uncertainties associated with "residual" particulates
aloft the result of earlier convective mixing. Ceilometers are backscatter lidars that have been
demonstrated to be useful for measuring clear-air particulate profiles in and above both the
daytime and the nocturnal boundary layer with 15-m height resolution and 15-m minimum range;
the minimum sampling period is 15 seconds. Doppler lidars measure the range-resolved radial
velocity with high resolution. For existing commercial systems, wind resolution is 0.5 my over
range intervals of 5-50 minutes. Maximum range is a function of averaging time and can extend
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APPENDIX C
75
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N' Y~EY + ~ ~ + CEaDrr ~ | ~ + ~ a ~ :+ CITY r) EVA*~E LOlilStillZ ~ ~ ~7 N~F~imM~D
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~ \, OCiESSA SAN CENTRAL TEXAS ~ x ~ ~ 4~ ~ ~_~ x ~+~~ ~ ~
+ NWS W~-8~ SITE
x DoD WSR-8BD SITE
AREAs NOT COvERED BELOW
10,000 tt ABOvE ~lE IEVEL
[23 REDUCED COvERACE BY NEXRAO
0 100 NAVnCAL killES (nml)
~1
0 185 KiLOUE1ERS (hm)
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:-~;7t + ~ ~NEW ORLEANSi FLORIDA ~ ~l \,
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{N ~RlEn ~
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KEY r/
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FIGURE C.1 Composite WSR-88D coverage at 3 km above site level for the contiguous United States
and the locations of the NWS and DOD radar sites. Courtesy of SRI International (2003~.
to 16 km in clear air with 10-minute averaging (10 km with 5-minute averaging). All lidars, how-
ever, are range limited in the presence of intervening clouds.
To summarize, operational radio-, acoustic- and optical-frequency profilers provide critical
atmospheric measurements needed to support dispersion and deposition modeling. Each can
provide vertical profiles of wind speed, wind direction, and turbulence (derived from spectral
width data), and they also are able to estimate the depth of the mixed layer~s). A comprehensive
intercomparison study by Seibert et al. (2000) showed positive results at estimating mixing height
from radar wind profiler, sodar, and lidar data against in situ sounding data. Bianco and Wilczak
(2002) have explored the simultaneous use of data from multiple profilers using a fuzzy logic
analysis scheme.
Research lidar systems offer capabilities beyond those currently available from commercial
suppliers, although they tend to be more expensive and require significant human expertise to
operate. However, both limitations could be minimized or eliminated in the presence of
significant demand for operational systems. The research community operates two types of
Doppler lidars. One is a "long-range" instrument that can sense out beyond 20 km in dry
conditions and, typically, to about 15 km with higher ambient humidities. These systems are ideal
for measuring flow in complex terrain, such as canyon outflows, downslope winds, and flow
around barriers. The other type of research Doppler lidar is a "high-resolution," boundary layer
focused lidar. These lidars have much lower pulse energy and higher pulse rates, and they are
designed to probe fine-scale structure in the planetary boundary layer. These systems are typically
operated in either a vertically pointing mode (for probing the convective boundary layer) or a
scanning mode (stratified boundary layer), and they measure vertical velocity, vertical velocity
variance, high-resolution horizontal wind profiles, and horizontal velocity variance to identify
turbulent layers.
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76
APPENDIX C
In addition to measuring winds, research lidar technology also makes it possible to obtain
profiles of atmospheric properties (e.g., temperature and density) and constituents (e.g., H2O, 03,
SOL. Lidar sensing methods employ a wide variety of optical phenomena including elastic
scattering from molecules (Rayleigh scattering) and particles (Mie scattering) where the
transmitted wavelength does not change; inelastic molecular (Reman) scattering or fluorescence
where the wavelength is shifted according to the type of molecule; and differential absorption
where molecules absorb differentially at slightly different transmitted wavelengths.
Profiling temperature in the boundary layer and through the troposphere is also very
important, especially when turbulence profiles are unavailable. Techniques for obtaining high-
resolution, time-continuous temperature profiles are less well developed than those for winds and
mixing height. Radiosondes are an important source of profile data for temperature but have the
disadvantage of being instantaneous measurements that are available only infrequently. Radar
wind profilers can measure the vertical profile of virtual temperature when configured to operate
as a RASS. An acoustic source is used in RASS systems to emit intermittent sound pulses whose
speed through the atmosphere is tracked by the radar wind profiler; the temperature is retrieved
from the speed-of-sound measurements, which are proportional to virtual temperature. The
maximum height resolution of RASS temperature profiles is 60 m and maximum range is typically
1-2 km. As with sodar, noise is a nuisance factor that limits RASS deployment in populated
areas. Passive multiple-frequency, microwave radiometers have been used in research as a means
to retrieve temperature profiles over deep layers of the atmosphere. Their height resolution is
limited and decreases rapidly with height above the ground (Mariner et al., 1992~. More recently,
passive single-frequency scanning microwave radiometers have been introduced; they scan in
elevation and use inversion techniques to retrieve temperature profiles in the lowest 600-1000 m
of the atmosphere, with a reported height resolution of 50 m. Early results are encouraging but not
yet definitive.
RAPID RESPONSE MEASUREMENTS
In the context of a terrorist attack, the time, location, and nature of the source term are not
known in advance and may not be known with great specificity in the minutes to hours after an
attack. As a consequence, fixed meteorological observing systems that characterize dispersion in
numerical models may need to be supplemented with a rapid-response deployable meteorological
observing facility. There are a number of promising commercial measurement options for mobile
and transportable systems. Candidates include the following:
· Low-altitude rocketsondes currently provide lower tropospheric soundings of tem-
perature, pressure, and humidity; winds and other measurements could be added to these sondes.
· Tethered meteorological balloon systems can provide high-resolution fixed-level
observations and profiles through the boundary layer
· Unmanned aerial vehicles represent a rapidly advancing airborne platform that could be
adapted to measure all necessary meteorological variables as well as chemical, biological, and
nuclear contaminants.
SERENDIPITOUS MEASUREMENTS
An equally important consideration is the status and availability of measurements from the
many disparate surface meteorological observing stations already in operation. Numerous
meteorological observations are made by local and regional networks that currently are not
available to the National Weather Service or the broader scientific community. These systems
primarily are surface weather stations that could provide the backbone of a surface mesonet
capability for emergency response. They should be evaluated to ensure proper siting and per-
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APPENDIX C
77
formance specifications, and it is important that they be quality controlled. Incremental stations
then could be added to optimize these mesonets as needed. Plans to evaluate, access, and use
these data should be developed well in advance of an emergency event.
SUMMARY
In summary, providing meteorological observations to support response to C/B/N releases
involves the following broad challenges:
determining what measurements are essential and/or desirable;
designing integrated observing and modeling systems and taking maximum advantage
of synergies with other day-to-day applications (e.g., air pollution, mesoscale weather, hydrology,
aviation);
areas; and
.
establishing dedicated, comprehensive meteorological observing systems near sensitive
developing rapid-response meteorological (and chemical) observing systems.
REFERENCES
Bianco, L. and J.M. Wilczak. 2002. Convective boundary-layer depth: Improved measurement by Doppler
radar wind profiler using fuzzy logic methods. J. Atmos. Oceanic Technol.
Lenschow, D.H. 1986. Probing the Atmospheric Boundary Layer, American Meteorological Society,
Boston.
Mariner, B.E., D.P. Wuertz, B.B. Stankey, R.G. Strauch, E.R. Westwater, K.S. Gage, W.L. Ecklund, C.L.
Martin, and W.F. Dabberdt. 1992. An evaluation of wind profiler, RASS, and microwave radiometer
performance. Bull. Amer. Meteor. Soc. 74:599-613.
NRC (National Research Council). 1995. Toward a New National Weather Service Assessment of
NOMAD Coverage and Associated Weather Services. National Academy Press, Washington, D.C.
NRC. 2002. Weather Radar Technology Beyond NE^AD. National Academy Press, Washington, D.C.
Seibert, P., F. Beyrich, S.E. Gryning, S. Joffre, A. Rasmussen, and P. Tercier. 2000. Review and
intercomparison of operational methods for the determination of mixing height. Atmos. Environ.
34:1001-1028.
Representative terms from entire chapter:
radar wind
