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differences in the emitted energy. This difference is called ZDR and can be as much

as several decibels. Even though hailstones are often very irregular, they tum ble

randomly and the sum of the ZDR returns from thousands of hailstones is very close

to zero (Fig. 17). Ice particles, however, can exhibit preferred orientations just like

raindrops, and can produce ZDR. A good rule of thumb is that high Z with high

ZDR ¼heavy rain while high Z with low ZDR ¼hail.

New techniques are in development to make use of other meas urements possible

with multiple polarization radars, such as the linear depolarization ratio (LDR),

which measures how much radiation from the horizontal beam is reemitted from

drops with vertical polarization, and speciﬁc differential phase, F

, which measures

the differences in the phase of the horizontally and vertically polarized returned

energy. Some of these hold promise to reﬁne the identiﬁcation of particle types

(rain, hail, large hail, ice, etc.) and rain rate. F

holds particular promise since

changes in F

are thought to be proportional to D

, the volume of water in a

resolution volume, and therefore more directly related to rain rate than Z. LDR is

a very difﬁcult measurement that requires a very precisely manufactured, there fore

expensive, antenna.

Polarization radars are now primarily used in research but will probably be used

operationally by forecasters in coming decades.

Mobile Radars

No matter which choices are made for radar wavelength and antenna size, the radar

beam will always spread out with distance. Few radars have beams that are much

narrower than 1



. This means that at 60-km range the beams are 1 km wide, and at

120 km they are 2 km wide, much too large to resolve small phenomena such as

tornadoes, microbursts, etc. The best solution found to this problem so far has been

Figure 17 Large raindrops (left ) are oriented similarly, so the ZDR effect from individual

drops adds constructively to produce large ZDR values. Hailstones (right) tumble and are

therefore oriented randomly so individual ZDR values tend to cancel out producing ZDR ¼0.

See ftp site for color image.

916

RADAR TECHNOLOGIES IN SUPPORT OF FORECASTING AND RESEARCH

to put the radars on vehicles: ground based, in the air, and on the o cean. The vehicles

are then deployed near the weather to be measured.

These systems tend to be very specialized and, with the exception of the hurricane

hunter aircraft, are used mostly for research. Their mobi le nature makes design and

operation difﬁcult.

Ground-Based Mobile Radars A leading example of the mobile radar concept

is the Doppler On Wheels (DOW) research radars (Fig. 18). These radars have

obtained unprecedented high-resolution three-dimensional data in tornadoes, hurri-

cane boundary layers, etc., at scales as small as 3 to 60 m. Typically two or more

DOWs are deployed in a mobile multiple-Doppler network to retrieve high-resolu-

tion vector wind ﬁelds. The DOWs use 3-cm radiation and 2.44-m antennas to

produce 0.93



beam widths, which are comparable to the WSR-88Ds (but 3-cm

radiation suffers more from attenuation in heavy precipitation). Fast scanning and

very short pulses and gate lengths are combined for ﬁne spatial and temporal scale

observations. Other mobile radars using energy with wavelengths ranging from

3 mm to 10 cm have also been used by researchers to study similar phenomena.

None are currently in use for forecas ting. But the idea of using DOW-type radars

to augment the stationary radar network, part of a concept called adaptive observa-

tions, is being explored. In the future, forecasters who need information in speciﬁc

areas, say a hurricane landfall, severe weather outbreak, ﬂood, or the Olympics, may

be able to request high-resolution tailored multiple-Doppler radar measurements.

Figure 18 DOW2 mobile radar. The DOW2 uses a 2.44-m antenna transmitting 3-cm

radiation with a 0.93



beam. It is used to intercept tornadoes, hurricanes, and is deployed in

mountain valleys or wherever an easily movable system is needed. See ftp site for color image.

6 NEW AND NONCONVENTIONAL TECHN OLOGIES 917

Airborne and Ship-Based Radars Since many areas are inaccessible to

DOW-type tr ucks, particularly oceanic areas that cover 70% of Earth including

hurricane spawning grounds, and because trucks are limited to highways speeds of

30 m=s, limiting deployment ranges, researchers and operational meteorologists use

radars mounted on aircraft (Fig. 19) and ships to get closer to the weather of interest.

These aircraft regularly ﬂy into hurricanes before they make landfall to aid in

predictions. They have been used to study meteorology as diverse as tornadoes in the

Midwest and tropical climates in the western Paciﬁc.

Bistatic Radar Networks

These collect stray radiation emitted in many directions by raindrops to measure

vector wind ﬁelds and were discussed above.

Rapid Scan Radars

Military radars have long been able to move their beams electronically rather than

having to mechanically move an a ntenna. Since the beam can be moved almost

instantaneously, these hold the promise of extremely rapid scanning of weather.

Instead of requiring 6 min to survey the entire sky, it might be sampled in only 10

to 30 s. A type of antenna called a phased array is used. Very few exist outside

military applications, in part due to the high cost of construction. There are efforts

being made to adapt this military technology to meteorological use.

A new type of rapid scan radar is under development and holds promise as a

research and operational tool primarily due to its low cost, compared to phased array

systems. These radars transmit multiple frequencies from an unusual antenna

designed to split the various frequencies into simultaneous multiple beams (Fig. 20).

Figure 19 ELDORA airborne radar can be deployed quickly to almost any point in the

world, even over oceans. It has a sophisticated radar in the protuberance extending beyond

the normal tail. Similar systems are used operationally to intercept hurricanes. (Courtesy

NCAR=ATD.) See ftp site for color image.

918

RADAR TECHNOLOGIES IN SUPPORT OF FORECASTING AND RESEARCH

Figure 20 A ﬂat panel, slotted waveguide array with 100 individual slotted waveguide

antennas produces beams that emanate at different elevation angles depending on the

frequency of the transmitted radiation. Therefore, nearly simultaneous transmissions at

multiple frequencies, can be simultaneously received from several elevation angles at once.

Ten or more elevation angles can be surveyed in 10 s, providing truly rapid scanning with a

nonphased array system.

Figure 21 NCAR ISS in Kapingamarangi, Pohnpei State, Federated States of Micronesia. A

wind proﬁler antenna is pointed vertically and shrouded by a clutter fence visible on top of the

shelter. Aluminum cylinders shield a portion of a RASS radio acoustic sounding system. See

ftp site for color image.

6 NEW AND NONCONVENTIONAL TECHN OLOGIES 919

A single mechanical sweep of this antenna produces multiple beams, as many as 10

or more, permitting a typical volumetric scan to occur in just 2 sweeps, in as little as

10 s.

Wind Proﬁlers

There is another class of radars called wind proﬁlers that usually point vertically, do

not scan, and sample vertical cross sections of wind and temperature. The principles

of operation share some similarities to conventional weather radars, but there are

substantial differences. They usually collect just one vertical sounding, averaged

over about 30 min. Many can collect wind velocity data in clear air, in the absence

of precipitation, through substantial depths of the lower troposphere. There is a

network of proﬁle rs in the Midwest that augments the balloon sounding network

by providing half hourly measurements at intermediate locations. Data from wind

proﬁlers is valuable for the forecasting of severe weather outbreaks in the Midwest.

Deployable wind proﬁlers are used by researchers to provide vertical soundings of

wind between balloon launches (Fig. 21). An instrument called a RASS, or radio

acoustic sounding system, uses an ingenious method whereby emitted sound waves

disturb the atmosphere in a manner that can be measured by the proﬁler radar to

measure the temperature of the atmosphere in the vertical column.

920 RADAR TECHNOLOGIES IN SUPPORT OF FORECASTING AND RESEARCH

CHAPTER 49

BASIC RESEARCH FOR MILITARY

APPLICATIONS

W. D. BACH, Jr.

If you know the enemy and know yourself, your victory will never be endangered; if

you know Heaven and know Earth, you may make your victory complete

—Sun Tzu, The Art of War X, 31 circa 500 BC

The ancient Chinese general succinctly summari zes necessary ingredients for

success in war. Understanding that ‘‘Heaven’’ represents the atmospheric environ-

ment, the need to know the weather is deeply rooted in military preparation and

tactics. History is replete with examples of commanders using weather as an ally or

suffering its bad effects. Even with today’s modern techno logy, the dream of an

all-weather military has not become a reality. Thus the military continues to seek

ways to use the atmosphere as a ‘‘combat multiplier’’ in the order of battle.

1 MILITARY PERSPECTIVE

The military must understand the atmosphere in which it operates. That understand-

ing will ultimately depend on scientiﬁc understanding of the atmospheric processes,

an ability to use all of the information available, and an ability to synthesize and

display that information in a quickly understandable fashion.

The military’s needs for understanding the atmosphere’s behavior has changed as

a result of geopolitics and advancing technology. On the battleﬁeld, future enemies

are more apt to resort to chemical or biological attacks as a preferred method of mass

destruction. The atmospheric boundary layer is the pathway of the attack. Increased

Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Systems, and Measurements,

Edited by Thomas D. Potter and Bradley R. Colman.

ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.

921

reliance on ‘‘smart’’ weapons means that turbulent and turbid atmospheric effects

will inﬂuence electromagnetic and acoustic signals propagating through the bound-

ary layer. Intelligence preparation of the battlespace, a key to successful strategies,

depends on full knowledge of the enemy, weather, and terrain. It requires an ability

to estimate atmospheric details at speciﬁc locations and future times to maximize

strategic advantages that weather presents a commander. It also avoids hazards and

strategic disadvantages.

A convergent theme of basic research in the atmosphere is the interdependence of

research progress in acoustic and electromagnetic propagation with the measurement

and modeling of the dynamical boundary layer. The scattering of propagating energy

occurs becau se of ﬁne structure changes in the index of refraction in the turbulent

atmosphere. The connection between the two—the refractive index structure func-

tion parameter, C

—is proportional to the dissipation rate of the turbulent kinetic

energy.

Military operations are often in time (seconds to hours) and space (millimeters to

tens of kilomete rs) scales that are not addressed by conventional weather forecasting

techniques. Atmospheric information is required for meteorological data-denied

areas. Fur thermore the real conditions are inhomogeneous at various scales. Signiﬁ-

cant adverse effects may have short lifetimes at high resolution. Such breadths of

scales make accurate quantiﬁcation of important boundary layer processes very

difﬁcult. Carefully planned and executed experiments in the uncontrolled laboratory

of the atmosphere are needed for almost every phase of the research. The combined

range of atmospheric conditions and potential propagation types and frequencies that

are of interest to the military is too broad to condense into a few nomograms. The

propagation studies require intensive and appropriate meteorological data to under-

stand the atmospheric effects. Understanding of heterogeneous atmospheric ﬁelds

arising from inhomogeneous conditions requires measurements achievable only by

remote sensors.

Chemical and biological agents constitute major threats to militar y ﬁeld units as

well as to civil populations. Electromagnetic propagation and scattering are the

principal means of remote or in-place detection and identiﬁcation of agents. Current

models of atmospheric transport and diffusion of the agents over stable, neutral, and

convective conditions are marginally useful and based on 40-year-old relationships.

Signiﬁcantly improved models of the transport and diffusion are needed to estimate

concentrations and ﬂuctuations at various time and space scales for simulated

training, actual combat, and for environmental air quality.

2 RESEARCH ISSUES

Basic research for the military in the atmospheric sciences comes under two broad,

interdependent scientiﬁc efforts:

Atmospheric wave propagation, which is concerned about the effects of the

atmosphere, as a propagating medium, on the transmission of electroma gnetic (EM)

or acoustic energy from a source to a receptor, and

922 BASIC RESEARCH FOR MILITARY APPLICATIONS

Atmospheric dynamics, which is concerned with quantifying the present and

future state of the atmosphere.

Atmospheric Wave Propagation

Electromagnetic and acoustic propagation and scattering is a large subject that

addresses problems in both characterizing the batt lespace and in detecting targets

for engagement and situational awareness. Although there has been much previous

research in the general area of wave propagation and scattering, there remains a

pressing need for research that speciﬁcally address issues related to propagation and

scattering in realistic atmospheric environments.

Electro-optical and infrared propagation in the atmosphere is limited by turbu-

lence and by molecular and aerosol extinction by both absorption and scattering. The

relative importance of turbulence and extinction is dependent on the wavelength of

the radiation and the atmospheric conditions. Modern, high-resolution imaging

systems are often turbulence limited, rather than diffraction limited, impairing

long-range, high-resolution target acquisition, recognition, and identiﬁcation.

Laser-based systems are limited in their ability to retain spatio-temporal coherence

and effectively focus at tactical ranges. Like acoustic signals, electromagnetic signals

are primarily useful for target detection or ranging and for remote sensing of the

atmosphere itself. Although much research has been done in the area of electro-

magnetic extinction, and current models are generally good, applications of these

models for remote-sensing purposes is an area of active research. A large body of

research exists in the area of atmospheric turbulence effects for electromagnetic

propagation, but with less conclusive results. Hence understanding of atmospheric

turbulence for optical and infrared propagation is still an active area of research.

Understanding of the interaction between propagation and turbulence is rather

straightforward in the theoretical sense. Practically, the time scales of the EM propa-

gation interactions with the air are much shorter, O(10

9

s) along a path, than the

capability to locally sample the atmospheric turbulence or density ﬂuctuations,

O(10

1

s) and to characterize the turbulence spectra, O(10

þ3

s). Acou stic

interactions also occur on short time scale s. Second, the turbulent structure of the

atmosphere between the source and detector is largely unknown, so the scale of

the disturbance is undetermined. Furthermore, with models, grid volume samples are

realized at O(10

s) in large eddy simulation models and at O(10

s) in ﬁne

mesoscale models. To make the connection between propagation effects, turbulence

characteristics, and atmospher ic models, new measurement techniques are needed.

In careful ﬁeld campaigns to relate propagation to atmospheric conditions, the

biggest problem is independent g round truth.

Atmospheric Dynamics

The military emphasis on global mobility requires weather forecasting support at all

scales from global to engagement scales. In recent years, the explosion in computer

capacity enabled highly complex numerical weather prediction models on most of

2 RESEARCH ISSUES 923

these scales. Model accuracy has improved even as complexities are added. Model

results have become more accepted as a representation of the atmospheric environ-

ment. As computation power increases, ﬁner and ﬁner scales o f motion are repre-

sented in smaller grid volumes. Lately, large eddy simulations represent dynamics at

grid sizes of a few meters in domains of 5 km

by 1 km deep.

As the models go to ﬁner scales, the variability imposed by large-scale synoptic

and mesoscale inﬂuences the Atmospheric Boundary Layer (ABL) is modiﬁed by

both the cyclical nature of solar radiation and metamorphosis due to the stochastic

behavior of clouds and other natural processes as well as anthropogenic causes. The

resulting (stable and unstable) boundary layers are sufﬁciently different that current

models of one state do not adequately capture the essential physics of the other or the

transition from one state to the other. Accurate predictions become more difﬁcult,

principally because the atmosphere is poorly represented. Data are lacking at the

scales of the model resolutions. Parameterizations required to close the set of equa-

tions are inadequate. Observations are lacking to describe the four-dimensional

ﬁelds of the forecast variables at the model resol ution. The models are unlikely to

improve until better theories of small-scale behavior are implemented and observa-

tions capable of testing the theories are available.

For the military, techniques to represent the inhomogeneous boundary layer in all

environments—urban, forest, mount ains, marine, desert—is absolutely essential for

mission performance. In most cases, this must be done with limited meteorological

data. Furthermore, the militar y is more frequently interested in the effects—visibility,

trafﬁcability (following rain) , ceiling—than in the ‘‘weather’’ itself. Nevertheless,

without high-quality, dependable models that represent the real atmosphere in time

and space, the effects will not be representative.

Basic research attempts to improve modeling capability by increasing the know-

ledge base of the processes of the atmosphere. Until new measurement capabilities

are developed and tested, our ability to characterize the turbulent environment—

affecting propagation and dispersion of material s—will be severely limited.

3 PROTOTYPE MEASUREMENT SYSTEMS

Military basic research has participated in several new techniques to measure winds

and turbulence effects in the atm ospheric boundary layer. These techniques co ncen-

trate on sampling a volume of the atmospher e on the time scales of (at least) the

signiﬁcant energy containing eddies of the atmosphere.

The Turbulent Eddy Proﬁler, developed at the University of Massachusetts,

Amherst, is a 90-element phased-array receiving antenna operating with a 915-MHz

25-kW transmitter. It is designed to measure clear air echoes from refractive index

ﬂuctuations. The received signal at each element is saved for postprocessing. These

signals are combi ned to form 40 individual beams simultaneously pointing in differ-

ent directions every 2 s. In each 30-m range gate of each beam, the intensity of the

return, the radial wind speed is computed from the Doppler shift, and the spread of

the Doppler spectrum is calculated. These data are displayed to show a four-

924 BASIC RESEARCH FOR MILITARY APPLICATIONS

dimensional evolution of refractive index structures in the boundary layer. Further

processing gives estimates of the three components of the wind velocity. Combining

the wind vectors with refractive index structures has shown signiﬁcant horizontal

convergence occurring with strong refractive index structures. At 500 m above the

ground the volume represented is approximately a 30-m cube.

The Volume Imaging Lidar at the University of Wisconsin measures backscatter

from atmospheric aerosols in three dimensions at 7.5-m range resolutions. The

evolution of boundary layer structures can be seen in a variety of experiments.

Capabilities to estimate horizontal winds at selected altitudes have been developed

and demonstrated.

An eye-safe, scanning Doppler lidar operating at 2 mm has been jointly developed

with NOAA=ERL=ETL to measure radial wind speeds at 30-m resolution. The error

of the measurement is < 0.2 m=s. Various scanning approac hes have shown several

different evolutions of the morning transition and breakdown of a low-level jet.

Another lidar system has been developed at the University of Iowa to measure the

horizontal wind in the boundary layer at about 5-m increments every minute.

Teamed with existing FMCW radars, having comparable vertical resolution,

should add to meteorological understanding of layers of high refractive index.

4 CURRENT MEASUREMENT CAPABILITIES

Reliable measures of atmospheric temperature and moisture at high resolution in

space and time are still lacking. Some progress has been made but does not yet

achieve the resolutions of the wind measurements.

Prototype and existing wind=wind ﬁeld data do not yet measure the turbulence.

Although the Doppler spectrum width is an indicator of the eddy dissipation rate,

few researchers or equipment developers report the variable or its spatial variability.

To date, none have done so operationally.

5 CONCLUSIONS

Some progress is being made by the military to develop necessary instrumentation to

measured atmospheric ﬁelds at high time and spatial resolution. This is motivated by

the need to improve the theory and subsequently the models of atmospheric motions

at small scales to provide the armed services with reliable models on which to gain

superiority over the adversary and successfully complete the mission.

5 CONCLUSIONS 925