Radio Science, Volume 30, Number 1, Pages 75-88, January-February 1995

A new frequency-modulated continuous wave radar for studying planetary boundary layer morphology

Frank D. Eaton, Scott A. McLaughlin, and John R. Hines U.S. Army Research Laboratory, White Sands Missile Range, New Mexico

Abstract. This paper describes a new generation frequency-modulated continuous wave (FMCW) radar using state-of-the-art electronics and computerized data processing that greatly enhance the use of the radar as a practical tool for atmospheric research. The system senses at high resolution ( ~2-m range and 12 s for obtaining each profile), has ultrasensitivity ( < -165 dBm), and is accurately calibrated for the refractive index structure parameter (C;). The authors present salient features, discuss the calibration procedure, and present and discuss examples of various types of fine detail wave and frontal activity, boundary layer convection, and a light winter storm sensed by the radar over the last 2 years. The authors also show associated data from radiosonde and tower-mounted sensors that are relevant to the episodal events shown from the FMCW radar observations. Since the FMCW radar technique can resolve and sense individual insects, these point targets are shown to act as tracers and enhance flow visualization.

1. Introduction Many planetary boundary layer (PBL) studies in

the atmosphere require a detailed knowledge of the refractive index structure parameter (C;). This re­fractive-turbulent information is also essential to explain acoustic, optical, and radio propagation effects and performance of various remote sensors as well as weather phenomena. Such fine C; struc­ture was seen after Richter [1969] introduced the frequency-modulated continuous wave (FMCW) technique for PBL research, and many wave mo­tions associated with a maritime environment were revealed in detail. Observations from this system stimulated research involving elevated layers [Gos­sard and Richter, 1970] and Kelvin-Helmholtz (KH) instabilities [Gossard et al., 1971]. Excellent views of gravity waves and convective plumes were found, often resolving individual insects acting as tracers in the airflow. Atlas et al. [1970], utilizing the high-range resolution of the FMCW radar tech­nique, developed a non-Doppler method for deter­mining the horizontal speeds of point targets (in­sects) crossing a circular radar beam. The early FMCW radar measurements clarified the fact that atmospheric layers are often a few meters thick

This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union.

Paper number 94RS01937.

75

instead of tens of meters thick as had been con­cluded previously from the Wallops Island multi­wavelength radar experiment [Hardy et al ., 1966] .

A second FMCW radar system was built in the early 1970s at the Wave Propagation Laboratory (WPL) of the National Oceanic and Atmospheric Administration (NOAA). Doppler capability for de­riving winds, as developed by Strauch et al. [1976] and Chadwick et al. [1976], was added. This was previously thought impossible, since Doppler infor­mation was thought to be ambiguous in FMCW processing. The system successfully detected air­craft wingtip vortices (R. B. Chadwick et al., un­published manuscript, 1983) and was used in a field experiment for design of a wind shear detection radar for airports [Chadwick et al., 1979]. As part of this evaluation the WPL radar measured C; for a year, providing diurnal as well as long-term statis­tics [Chadwick and Moran, 1980]. Studies involving over-the-horizon propagation affected by elevated refractive layers involved data taken with the same system [Gossard et al., 1984a]. The fine structure sensed remotely was verified by in situ measure­ments. Among other research involving the WPL system was a study examining fluxes, gradients, and structure parameters in elevated layers [Gos­sard et al., 1982]. The study showed that fluxes may be estimated by using only ground-based radar measurements and a local radiosonde observation (RAOB).

76 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

Another FMCW radar system is used at the Delft University of Technology in the Netherlands [Ligthart, 1980] for sensing clear air layered struc­ture and the reflectivity levels of hydrometeors. Bauer and Peters [1993] have applied the radio acoustic sounding system (RASS) technique to an FMCW radar for obtaining temperature profiles.

The new FMCW radar presented here was de­signed for studying the PBL and cloud physics research for the U.S. Army Research Laboratory. State-of-the-art technology was incorporated throughout, enhancing the temporal and spatial resolution and allowing a more precise calibration than was possible with previous atmospheric sys­tems. The highly linear FM sweep is generated with a frequency-synthesized source instead of a yttrium iron garnet-tuned transistor oscillator as used in earlier generation systems. All data collection, sys­tem calibration, and processing is performed in real time. The characteristics of this new FMCW radar (high resolution, ultra sensitivity, accurate calibra­tion, and sensing near the ground) are crucial for modern experimental programs in the PBL.

2. Site The FMCW radar used in this study is one of four

different radar systems (a 50-MHz MST, a 404-MHz wind profiler, a 924-MHz boundary layer profiler, and the FMCW radar) operating at the Atmospheric Profiler Research Facility (APRF) of the U.S. Army Research Laboratory. Hines et al. [1993] describe the facility, which has a complete radiation station, sodar, optical devices, instrumented short and tall towers, and many specialized developmental mi­crometeorological sensors. Examples of research using the suite of instrumentation are found in work by Eaton et al. [1988]. Nastrom and Eaton [1993a] studied the coupling of gravity waves and turbu­lence using the VHF system. The APRF is located at 32°24'N, 106°21 'W, at 1220 m mean sea level (msl), about 13 km east of the headquarters area at White Sands Missile Range (WSMR), New Mexico. This site is in the Tularosa Basin, about 25 km east of the Organ Mountains. The immediate local area is characterized by flat terrain covered with low brush (predominantly mesquite) that stabilizes hill­ocks, associated desert grasses, and herbaceous plants indigenous to the Chihuahuan Desert.

The general climate is continental arid, with an annual precipitation at WSMR of 7 to 11 inches

(17.8 to 27.9 cm). Seventy percent of the annual rainfall occurs from June to October, due to moist air advection. Nastrom and Eaton [1993b] pre­sented a case study of the onset of this "summer monsoon" over WSMR and the associated change in c; as seen by the White Sands VHF radar.

3. FMCW Radar System Description The FMCW radar is a portable system consisting

of two trailers, one for the data processing and radio frequency electronics equipment, and the other for the two antennas. As shown in Plate 1, two identical 10-foot-diameter parabolic antennas, one for trans­mitting and one for receiving, are positioned on a fully steerable mount. Slow rate azimuth scans can be made for vertical azimuth display (VAD) wind profiling, or the antennas can be directed vertically (or lower, for example, for slant path measure­ments) for high-resolution backscatter · profiling. Wind measurements utilizing this scanning capabil­ity will be presented in a future paper.

The radar uses a phased-locked-loop digital fre­quency synthesizer to obtain a highly linear, very low noise 200-MHz bandwidth sawtooth 50-ms sweep centered at 2.9 GHz. The final amplifier is a traveling wave tube (TWT), with a continuous 220-W output. The received signal is homodyned, amplified and filtered, and then sampled with a 16-bit analog-to-digital converter. The digitized data is then sent to an array processor for integration and fast Fourier transform (FFT). A true real-time com­puter controls the hardware and data acquisition cycle as well as the data flow. The calibrated data is written to disk and/or tape and finally to a high resolution color printer in a continuous time-height display of c; profiles. Table 1 displays some of the key specifications and operating parameters used in this study.

A thorough calibration is performed to relate backscattered power to radar C;. The basic clear air meteorological radar equation is used and can be expressed as

where

PrRa(DIR)G 2A. 28 2.6..YJ P,. = --2-9-ln_2_7r_2R_2 __

P,. received power; Pr transmitted power; G antenna gain;

(1)

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 77

Plate 1. The White Sands frequency-modulated continuous wave (FMCW) radar side-by-side 10-foot-diameter parabolic antennas mounted on a steerable mount.

.A radar wavelength; () antenna 3-dB half-power beam width;

Ll radar range resolution; R radar range;

Table 1. Key Specifications of the U.S. Army Research Laboratory FMCW Radar

FMCW Radar Characteristics

Operating frequency Spatial resolution Height coverage Temporal resolution Number of range gates Antenna type Antenna 3-dB beam width Transmitter type Transmitter power Minimum detectable signal

Value

2.9 GHz ± 100 MHz Typically - 2-4 m Typically surface to 2-4 km 6 to 12 s per profile 1024 Parabolic 2.7° Traveling wave tube 220 W continuous Less than -165 dBm

RG(D/R) antenna autocorrelation function; Y/ radar cross section per unit volume.

After Ottersten's [1969] relationship

YJ = 0.38C~.A - i t3 (2)

is incorporated and system parameters are simpli­fied, c,~ can be derived from

P,.R2 C 2 =K----

n P 1RG(DIR) (3)

where K is a constant depending on operating parameters (for example, range and gate settings). An additional term, not included here, is measured and used to account for the digital signal processing (DSP) gains. Similar calibrations are applied to cor­rectly look at the radar reflectivity from hydrometeors (Z units) and for point targets (a radar cross section).

78 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

Range Weighting Function for ARL FM-CW Antennas 1.0 ,---,--,-----,--:::r::==+====F==r==-i

c: 0 u c:

0.8

~ 0.6 <1l c: c:

~ ~ 0.4 Q) C> c:

~ 0.2

300 600 900 1200 Range (m)

Figure 1. Range weighting function due to the overlap of the two side-by-side FMCW radar antennas calculated from the measured antenna patterns. The antennas are aligned with the center axes parallel to each other.

An important term, particularly for calibrating short-range measurements, is the antenna autocor­relation term Rc(DIR). This term accounts for the ''overlap'' or correlation of the two codirected antenna beam patterns. Because the antennas and feeds are identical, the autocorrelation function is used with the beam patterns to correct for the diminishing overlap at short ranges. Figure 1 shows the calculated weighting function for the measured antenna patterns of this radar, and its importance at short ranges is evident. The weighting function approaches zero at ranges less than 75 m, but it doesn't apply here anyway because this short range is in the near field. An ad hoc empirical correction is currently used for the near field regime ( ~ < 180 m) with planned follow-on comparisons utilizing an acoustic sounder, a tall tower, and tethersonde system.

The actual electronic calibration procedure used to determine receiver forward (including DSP) gains/losses is now briefly described. With the final transmitter on but radiating into a dummy load, an attenuated sample of the final amplifier drive signal is delayed in an electroacoustic delay line and coupled back into the receiver along with the noise from the antenna-mounted receiver preamplifier. The final response from this calibrated signal (in­cluding neighboring FFT points resultant from DSP signal spreading) is measured and entered back into the radar equation. All other gains and losses, as

well as the transmitted power, are measured in the standard way. Figure 2 is a block diagram showing the RF/data flow path and the calibration signal.

4. Experimental Results and Discussion Results shown were taken from the FMCW radar

with the system set up for vertical sensing of the boundary layer in the Tularosa Basin. Four types of atmospheric features are displayed and discussed: wave activity and its influence on boundary layer dynamics, fine structure of weather fronts, mor­phology of the convective boundary layer, and the morphology of a weak winter storm. The color assignments in Plates 2-9 show equal decibel steps, selected individually for each displayed figure, in calibrated c; (returned power). Colors range in a ''rainbow'' fashion from black and darker blues through greens, yellows, and finally red, with white representing the highest calibrated return power. Occasional extreme C; values are included in the white and black color bins.

4.1. Wave Activity and Its Influence on Boundary Layer Dynamics

The FMCW radar technique, with its ultrasensi­tivity and unequalled range resolution, is ideal for examining fine detail of wave activity in the atmo­spheric boundary layer.

Plate 2 shows an example of Kelvin-Helmholtz (KH) instability observed with the FMCW radar from 0854 to 0936 UTC on April 3, 1992, from the surface to 2200 m. The total range of c'j; in this example is from 1.0 x 10- 13 m - 213 to 6.3 x 10- 16

m - 213 . A theoretical explanation of the braided ''cat's-eye'' patterns found in KH instabilities was

TRANSMIT ANTENNA

r------ --------~ ~----------~

: :CALIBRATION : J(HTEST SIGNAL : : :PATH : I L-------------' I I I I

I I L ______________ _J

ANTI-ALIAS AID ARRAY FILTER CONVERTER PROCESSOR COMPUTER OUTPUT

Figure 2. Block diagram displaying the RF/data flow path and method of calibration for the FMCW radar.

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 79

en (i)

1596

1303

11011

....J (9 <(

.E 720 .. O>

'Q) I

428

135 8.9 9.1 9.2 9.4 9.5

Time UTC (hours)

Plate 2. FMCW radar record showing the braided pattern of a Kelvin-Helmholtz instability on April 3, 1992. A color bar chart showing the order of colors used is included.

~ Q)

Q) _§, ....J (9 <(

.E O>

'Q) I

1310

1056

805

551

301

47 2.8 3 3.2 3.4

Time UTC (hours)

Plate 3. FMCW radar record showing low-level waves on October 20, 1992.

3.6

80 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

3 April 1992 3000 ---------- - ----,----

2000

I _J

~ :E OJ 'Qi I

1000

Air

Dew Point

0 -20.0 -10.0 0.0 10.0

Temperature (°C)

Figure 3. Air and dew point temperature profiles ob­tained from radiosonde data on April 3, 1992. The height of the detailed structure in these profiles corresponds to the same height increment as the wave activity that is observed in Plate 2.

first described by K elvin [1880] as a pattern of stream lines resulting from disturbances in a shear layer. Gossard and Hooke [1975] review the histor­ical development of KH wave theory and describe some rigorous mathematical solutions to related phenomena, including the observed transition from cat's-eye to "cockeyed cat's-eye" patterns.

The example (Plate 2) shows asymmetry in reflec­tivity of the crest-trough patterns commonly seen in radar and sodar measurements of KH waves. Maslowe [1973] explained this effect as zones of reduced Richardson' s number resulting in enhanced turbulence at the edge of the cat's-eye patterns where discontinuities in velocity and temperature occur. Richardson's number, R i, is defined as

where

(g!T)(ao/az) Ri=---­

jaV/azJ2

g acceleration due to gravity; T local temperature; () mean potential temperature ;

V wind velocity ; z the vertical coordinate.

(4)

Delisi and Coreas [1973] suggested that the large temperature gradients associated with the isotherm density pattern correspond to the reflectivity pat­tern.

The KH instability, which appears to be super­imposed on a long-wavelength wave, shows rapid decay and enhanced turbulence at 0920 UTC near 1100 m AGL. The pattern centered near 650 m AGL displays sloping structures, presumably caused by a low-level jet between the two wave patterns, slop­ing in the direction opposite from that seen in the KH instability feature above.

The operational surface weather map (not shown) indicated WSMR under a high-pressure system on April 3, with the center of the system located about 500 km northwest of the APRF. Clear sky condi­tions and surface winds less than 2 m s - l were reported at the WSMR weather station. Air and dew point temperature profiles obtained from an RAOB released at 0957 UTC at Oasis Site are shown in Figure 3. In addition to a strong surface inversion, considerable structure is shown in the temperature profile in the height region where the wave activity occurred. The wind speed profile (not shown) displays variations from a "smooth" profile at the same heights where similar deviations in temperature are seen. Wind direction shows a rapid spiral, contributing to wind shear, from 191° azi­muth near the surface to 0°N near 0. 7 km AGL and 65° azimuth at 1.6 km.

Occasionally at the APRF, wave activity is ob­served in the lower few hundred meters of the atmosphere by the FMCW radar. Plate 3 shows a time-height display of waves occurring from near the surface to about 1 km AGL on October 20, 1992, from 0148 to 0336 UTC , with a range in c,7 from 3.2 x 10- 14 to 1.0 x 10 - 16 m-213 . The point echoes displayed are from insects; the vertical streaks are due to birds and bats. Three features are seen in these measurements: (1) multiple layers, (2) a rapid growth in amplitude with height of each wave, and (3) pronounced KH instabilities on the upper-level waves. These thin zones of strong backscatter are commonly associated with sharp gradients of tem­perature and humidity [Gossard et al., 1984b]. This effect was verified by comparing radar and sodar results to high-resolution measurements taken by sensors mounted on a slowly moving carriage on the 305-m Boulder Atmospheric Observatory tower.

Surface weather maps (not shown) indicated a low-pressure system was over the APRF on Octo-

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 81

VJ 03

1642

l 1238

_J

(.'.) <(

.E 832 O'>

'(j) I

VJ 03 a> E _J

(.'.) <(

.E O'>

'Ci) I

428

22 15 15.5 15.9 16.4

Time UTC (hours)

Plate 4. FMCW radar record showing wave activity on January 28, 1993, under overcast conditions. The feature in the upper left corner is virga.

1303

983

664

344

24 21.2 21.4 21.6 21.8

Time UTC (hours)

Plate 5. Fine structure of dry frontal passage observed with the FMCW radar on January 15, 1992. The layered feature near 1600 m is commonly observed over the Tularosa Basin. Associated meteorological data are presented in Figures 4a and 4b.

16.9

22

82 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

ber 20, 1992. An RAOB released at Holloman Air Force Base (54 km north-northeast of the APRF) at 1058 UTC showed a strong surface inversion with a second inversion near 700 m, corresponding to the zone of wave activity shown in Plate 3. Altocumu­lus and standing lenticular clouds, indicating waves aloft, were reported at the WSMR weather station for a 7-hour period before the surface wave activity. Towering cumulus clouds were observed shortly before 2400 UTC, while lightning was reported at 0357 UTC, near the time that the observed surface wave activity dissipated.

Boundary layer waves are often seen under strong surface inversion and wind shear conditions, as shown in Plates 2 and 3. Strong radiation inver­sions develop at night under clear or nearly clear sky conditions, such as those that occurred for these two examples. Occasionally complex and multilayered wave patterns are found, as shown in Plate 4 under overcast conditions from 1500 to 1654 UTC on January 28, 1993. The patterns are inter­twined and overlapping and display considerable instability until 1558 UTC when several thin, inter­mediate, and nearly equally spaced waves develop with troughs nearly in phase at 1609 UTC. The range of C ~ for this example is from 1. 0 x 10 - 13 to 1. 0 x 1 O - 16 m - 213 • The feature that saturated the receiver in the upper-left corner of Plate 4 is virga. It appears that a rainshaft may have fallen as low as 300 m AGL at 1514 UTC. At these times overcast conditions and virga were reported in all quadrants at the WSMR weather station. Sunrise, under cloudy conditions, was less than an hour before the beginning of the results shown in Plate 4.

The early morning surface inversion was about one half as strong as that found in the previously shown two examples (Plates 2 and 3) of wave activity, as determined from an RAOB released at 1131 UTC at Holloman Air Force Base. A second inversion was found near 600 m AGL. The sounding also displayed strong wind shear in the lower few hundred meters. At this time the APRF was under a low-pressure system.

4.2. Fine Structure of Weather Fronts Since weather fronts generally exhibit high spa­

tial and temporal variability in the lower part of the atmosphere, the FMCW radar technique is ideal for detecting fine structure during these events.

Operational weather maps indicate that a dry front passed over the APRF on January 15, 1992.

15-16 January 1992

-8 -12 12:00 15:00 18:00 21 :00 0:00 3:00 6:00

GMT (hours)

Figure 4a. Air temperature and dew point temperature (2 m above ground level (AGL)) sensed at the APRF on January 15 and 16, 1992. The pulses in dew point after the leading edge of the front correspond to the features seen in Plate 5.

The leading edge of the frontal passage was ob­served at 2125 UTC by the FMCW radar, as shown in Plate 5, with successive pulses at about 2138 and 2144 UTC. The observed effect is contained by a layered feature near 1600 m AGL, a turbulent layer over the Tularosa Basin at an altitude near the top of the surrounding mountains. c; ranged from 3.2 x 10- 14 to 5.6 x 10- 16 m-213 in this example. Corresponding surface measurements of meteoro­logical parameters are shown in Figures 4a and 4b. An abrupt decrease of about 2°C in dew point temperature occurred at the time of the frontal passage and was followed by a few sharp pulses that correspond to the pattern seen in the radar obser­vations. The wind direction displays a veering from a northeast flow to a south-southeast flow with a rapid change in direction at the actual frontal pas­sage time. These results demonstrate the need for high-resolution measurements when examining boundary layer properties during frontal passages.

A low-pressure system, centered near Yuma, Arizona, on October 18, 1992, moved eastward, causing a weak front to pass over the APRF early the following day. A few altocumulus and standing lenticular clouds, an abrupt rise in dew point (a 9.4°C increase), and a drop in visibility (from 64 to 32 km) were reported at 0100 UTC at the WSMR

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 83

weather station. Figure 5 shows the sudden rise in dew point temperature measured at the APRF, and Plate 6 displays FMCW radar measurements of the frontal passage on October 19, 1992, with C~ rang­ing from 7.9 x 10 - 14 to 3.2 x 10- 16 m-21 . The sensed frontal activity exceeded 800 m AGL, and the leading edge was followed by a wake of wave perturbations. A KH instability suddenly appeared at 0113 UTC in the persistent undulating layer near 2500 m. Although the KH wave lasted only about 15 min, its time of onset being so soon after the frontal leading edge was sensed suggests a vertical flux of energy from this traveling frontal disturbance.

Plate 7 shows a time-height display of FMCW radar data obtained during a gust front episode from the surface to 1100 m AGL with c; ranging from 3.2 x 10- 13 to 2.5 x 10 - 15 m - 213 . The frontal leading edge, occurring at 0102 UTC on August 20, 1992, is followed by three distinct sloping, turbulent wave patterns. A. J. Bedard and S. J. Caplan (unpub­lished manuscript, 1987) have described the nose of a frontal interface as a giant rotor. This example shows many insects (point targets) dispersed throughout the entire measuring volume. Rain com­menced at 0156 UTC and saturated the radar re­ceiver in its normal clear air mode. At this time, cumulonimbus clouds, rainshowers, thunder, and cloud-to-ground lightning were reported at the WSMR weather station. Figure 6 shows wind speed

15-16 January 1992 8

6 en ], ] 4

c.. Cl)

2

0 270

0

180 Oi" Sl 5· ::J

c: Cl>

90 <O Cil Cl> ~

12:00 15:00 18:00 21 :00 0:00 3:00

GMT (hours)

Figure 4b. Wind speed and wind direction (10 m AGL) sensed at the Atmospheric Profiler Research Facility (APRF) on January 15 and 16, 1992.

30

0

18:00

18-19 October 1992

Air

Dew Point

21:00 0:00

GMT (hours) 3:00 6:00

Figure 5. Air temperature and dew point temperature measurements (2 m AGL) taken at the APRF during the period of the front shown in Plate 6.

measurements taken at the APRF. The wind speed was light (around 3 m s - l) for several hours before the gust front episode and then increased rapidly at all measured heights.

4.3. Morphology of the Convective Boundary Layer

Plate 8 shows a summertime midafternoon exam­ple of boundary layer convection sensed from 2054 to 2154 UTC on August 26, 1992. c; ranged from 3.2 x 10- 13 to 3.2 x 10- 15 m-213 for this example. Strong surface heating and light variable winds occurred during this time. Cumulonimbus clouds were reported to the northwest of the APRF, and by late afternoon there were rain showers and lightning north of the site. Surface measurements taken at 6-min intervals showed air temperature often varied to about a degree Celsius and dew point tempera­ture fluctuated several tenths of a degree. Wind speed (10 m AGL) varied between about 0.5 to 2.5 m s -l during this time.

Typically, within the core of a convective cell or thermal is a superadiabatic region (ae/az < 0). This example shows a mature convective regime where several thermals are capped by an elevated capping inversion reaching heights in excess of 2 km. Sev­eral insects are seen and act as tracers after being entrained into the convective updrafts or descend­ing in the subsidence regions. The cell boundaries

84 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

2933

2354

F (])

~ 1775

_J

0 ~

:g, 1200 'Q) I

Ci)" (i) (ii E _J

0 ~

1: O')

.<ii I

622

1.3 1.5 1.8 Time UTC (hours)

Plate 6. FMCW radar record of a front on October 19, 1992. A Kelvin-Helmholtz instability developed soon after the appearance of the frontal leading edge.

1100

885

670

457

242

27 0.9 1.2 1.4 1.7

Time UTC (hours)

Plate 7. Gust front episode observed by the FMCW radar on August 20, 1992. The point targets are insects.

2

1.9

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 85

I "'C Q) Q) a.

Cl)

"'C c ~

30

20

10

23:00

19-20 August 1992

7.6m

15.3m

38.1 m

91.4 m

1:00

GMT (hours) 3:00 5:00

Figure 6. Wind speed measured at four heights at the APRF on August 19 and 20, 1992, during the gust front episode shown in Plate 7.

are precisely delineated since the low wind speed during this time allows the same insect to be sensed in several successive measured profiles, taken at 12-s intervals.

This example also shows pronounced turbulent overturning of the stable layer that caps the thermal activity. Associated with this process are breaking billows, seen at the crests of the hummocks. The C; values seen in this upper layer are about 2 orders of magnitude higher than regions above and below the feature. The overturning process transports parcels of this high turbulent atmosphere to midboundary layer levels.

4.4. Morphology of a Weak Winter Storm On January 21, 1992, a light winter storm depos­

ited snow on the nearby Organ Mountains and rain in the Tularosa Basin. The returned backscatter sensed by the FMCW radar from 1230 to 1330 UTC is shown in Plate 9.

The color assignments are also in equal decibel increments and vary depending on the highest and lowest values selected for a given example. The returns from hydrometeors (frozen and melted) are much higher than from c; under clear conditions. The nearly horizontal band of strong echo, the "bright band," is about 100 m thick, with the top of

the band located about 550 m AGL. Although the characteristics of these bands can be explained by several factors as proposed by Austin and Bemis [1950] and Wexler [1955], the greatest contribution is the increase in the dielectric constant as the snow melts. Gossard et al. [1992] discussed how modern radars will provide additional information on this ice-water transition level in addition to several other cloud and precipitation issues.

The 0°C isotherm was found to be about 570 m AGL from an RAOB released from Holloman Air Force Base at 1351 UTC and it agrees well with the top of the radar-detected melting layer. Intermittent light rain showers were reported at the WSMR weather station during this event under light wind conditions.

5. Summary and Conclusions The White Sands FMCW radar, a new generation

radar using the FMCW technique, uses state-of-the­art technology throughout the system. A new de­sign phased locked loop digital frequency synthe­sizer replaces previously used YIG-tuned transistor oscillators for controlling the required "sawtooth" sweep pattern. The enhanced sensitivity and cali­bration procedure for the new system have proven to be essential for providing accurate C; fine struc­ture measurements in the PBL. These quantified values are required for many propagation and atmo­spheric studies as well as for evaluation of other remote sensors.

From the results presented in this paper as well as from other observed similar examples, five main points emerge regarding the PBL in the Tularosa Basin as sensed by the FMCW radar:

I. Wave activity was found under both clear or nearly clear sky conditions and overcast conditions in the presence of virga. Smface inversions were about twice as strong during clear conditions as they were during the overcast period.

2. Standing lenticular clouds, indicating wave activity aloft, were occasionally present for several hours before and during the times that waves in the boundary layer were observed.

3. A nearly permanent observed feature is a layer near the height of the mountain tops surround­ing the Tularosa Basin. In one case (Plate 6) a KH instability appeared at this layer simultaneously with the passage of a low-level front.

4. The high spatial resolution allows individual

86 EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR

en Q)

2202

1772

484

54 -20.9 21.1 21.4 21.6

Time UTC (hours)

Plate 8. Midday boundary layer convection sensed by the FMCW radar on August 26, 1992. Insects act as tracers delineating the cell boundaries and providing flow visualization.

2202

1776

~ 1353

_J

~ <{

.E 927 O>

·a; I

503

77 12.5 12.7 13 13.3

Time UTC (hours)

Plate 9. A light winter storm observed on January 21, 1992, with the FMCW radar. Changes in the "bright band" with time are clearly seen.

21.9

13.5

EATON ET AL.: A NEW FREQUENCY-MODULATED CONTINUOUS WAVE RADAR 87

insects to be sensed, and under low wind speeds these point targets can act as tracers of airflow. These tracers, although a contaminant for c; mea­surements, were found to be valuable in providing detail in the flow during wave, frontal, and convec­tive condition episodes. Research is on-going at the APRF concerning aerobiological climatology and radar cross sections of insects and birds.

5. The "bright band" during a light winter storm was observed in great detail concerning both change in thickness and calibrated returns. The FMCW radar has the capability to define several cloud and storm features.

In summary, the White Sands FMCW radar is the remote sensing ''tool of choice'' for many PBL and cloud physics studies. Phenomena such as atmo­spheric waves, fronts, boundary layer evolution, midday convection, and storms display great spatial and temporal variability that can be observed with the FMCW technique. Observations taken with this system are showing results often not predicted or understood by existing theory. Results obtained from measurements using the FMCW radar on various PBL research programs are expected to contribute to the development of new understand­ings of atmospheric processes and mechanisms.

Acknowledgments. Special appreciation is expressed to R. B. Chadwick; J. R. Jordan, E. E. Gossard, and R. G. Strauch for their many helpful comments concerning the design and use of the FMCW radar.

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F. D. Eaton, J. R. Hines, and S. A. McLaughlin, U.S. Army Research Laboratory, White Sands Missile Range, NM 88002-5501. (e-mail: featon @arl.army.mil; jhines@ ad.army .mil; smclaugh©arl.army .mil)

(Received March 10, 1994; revised July 22, 1994; accepted July 25 , 1994.)