Pulsed Laser Doppler C. DiMarzio, Measurements ol wind

Pulsed Laser Doppler Measurements ol wind Shear

C. DiMarzio,1 C. Harris,1

J. W. Bilbro,2 E. A. Weaver,2

D. C. Burnham,3 and J. N. Hallock3

Abstract A pulsed CO2 laser Doppler system was used to monitor thunderstorm gust fronts. Wind shears associated with the gust fronts were measured and tracked.

1. Introduction Sharp gradients in the wind encountered by aircraft on takeoff or on final approach have caused several serious, in some cases fatal, accidents. The encounter of these gradients creates an abrupt change in the air flow over the wing, causing the aircraft to depart from its intended flight path. This departure is often com-pounded by downdrafts, turbulence, and piloting and therefore results in an accident.

1 Raytheon Company, Wayland, Mass. 01778. 2 NASA Marshall Space Flight Center, Huntsville, Ala.

35812. 3 DOT Transportation Systems Center, Cambridge, Mass.

02142.

0003-0007/79/091061-06$05.50 © 1979 American Meteorological Society

There are three meteorological conditions that are frequently accompanied by regions of severe wind shear: boundary layer shears (low-level temperature inversions and nocturhal jets), fast-moving cold or warm frontal zones, and thunderstorms. Most of the serious shear-related accidents occurred in a thunder-storm-produced outflow or gust front. The gust front is a transitory line or zone extended horizontally, along which the wind shifts abruptly (Goff et al., 1977).

There is a need for a sensor at the airport that can remotely detect, identify, and track wind shears near the airport in order to assure aircraft safety. Lasers have now reached a state of maturity that makes a laser-based remote sensor possible. The detailed requirements for such a ground-based sensor system are described by Kalafus and Hallock (1976) and Kalafus (1978). To determine the viability of a laser wind shear system, the NASA pulsed coherent Doppler CO2 lidar (Jelalian et al., 1972) was installed in a semitrailer van with a rooftop-mounted hemispherical scanner and was used to monitor thunderstorm gust fronts.

Two test sites were employed in a measurement

FIG. 1. The lidar transmitter/receiver unit.

Bulletin American Meteorological Society 1061

Unauthenticated | Downloaded 04/26/22 06:53 PM UTC

1062 Vol. 60, No. 9, September 1979

FIG. 2. The location and scan pattern of the lidar as set up at the Kennedy Space Center. The lidar is at the center of the two concentric rings; the rings indicate the maximum and minimum ranges of the scan pattern.

program ; both sites had a statistically high incidence of thunderstorms. Between 22 April and 16 June 1978, the lidar was deployed in northeast Oklahoma City near the 481 m meteorologically instrumented tower used by the National Severe Storms Laboratory. The time period coincided with the historical peak in the severe storm season. Between 5 July and 4 August 1978, the lidar was deployed near the Vehicle Assembly Building at the Kennedy Space Center (KSC). This test period was part of the Thunderstorm Research International Program (TRIP-78) in which several experimenters performed research related to thunder-storms and lightning. The most significant data were collected at KSC and are addressed herein.

2. System description

The system is shown in Fig. 1. It is a pulsed CO2 laser Doppler velocimeter operating at 10.6 Aim. The pulse length selected for the measurements was 2 /xs (the best compromise for attaining the required spatial resolution and wind velocity accuracy as shown by Kalafus and Hallock (1976) and Kalafus (1978)) with a pulse-repetition frequency of 140 pulses per second. Typical energy output on a per pulse basis was measured at 10 mj.

The van was set up on Universal Camera Site

(UCS) 17. The location of the van and the scan pattern used during the test are shown in Fig. 2. The scan pattern was formed by using a sawtoothed range scan with a continuously varying linear azimuth scan. The range was automatically advanced a distance corre-sponding to half the pulse length (300 m) at the completion of each 25-pulse integration cycle. Conse-quently, in 10 s the range advanced from the minimum of 2 km to a maximum of 8 km and then stepped back to the minimum range. The azimuth angle was sinu-soidally driven at a rate of 2°/s ; the total azimuthal angle coverage was 320°, resulting in an azimuth-scan time of 160 s. To intercept a gust front the elevation angle was maintained as close to the horizon as possible (5°).

Jelalian et al. (1972) give a detailed explanation of the system operation; only a brief description of the signal flow is given below. Figure 3 is a block diagram of the system. The master oscillator outputs a hori-zontally polarized, continuous wave beam. A portion of this beam is used to generate a local oscillator beam offset by 10 MHz from the master oscillator (the offset will allow resolution of the direction of the Doppler shift). The remainder of the master oscillator beam is directed to an electro-optic modulator that chops the beam into 2/zs pulses using a polarization rotation tech-nique. The resultant pulse train then passes through an isolator (which prevents backscattered light from entering the master oscillator) and into a power amplifier array. The plane-polarized, amplified pulse train next passes through a quarter-wave plate where it becomes circularly polarized and is transmitted to the atmosphere via a 30.5 cm modified-Newtonian telescope and a hemispherical scanner.

A pulse reflected by the naturally entrained particles in the atmosphere returns along the same optical path, but having been shifted in frequency by an amount proportional to the velocity of the reflecting particles along the beam. The polarization of the return pulse is rotated 180° by the reflection and, consequently, on passing back through the quarter-wave plate becomes oriented at 90° with respect to the transmitted polariza-tion. As a result the received pulse can be separated from the transmission path with a Brewster window aligned to reflect the polarization of the returned pulse.

The received pulse is imaged on the face of a HgCdTe detector along with the frequency-offset local oscillator beam. The current out of the detector is a function of the quantum efficiency of the detector and the incident flux resulting from the combination of the received pulse and local oscillator fields. The resultant current is composed of several components, among which are a d.c. current produced by the local oscillator and a modulated current whose modulation frequency is the difference in frequency between the Doppler-shifted received pulse and the offset frequency of the local oscillator. The detector output is passed through a 10 MHz IF filter centered at 10 MHz to a bank of twenty 500 kHz wide filters whose outputs are envelope detected, sampled, A / D converted, and stored for



FIG. 3. A simplified block diagram of the lidar.

averaging with the next pulse return from the same range cell. Typically, 25 pulse returns are averaged for each spectrum before the range is advanced to the next range cell. At the completion of the averaging process, the resulting spectrum and its range are passed to an on-line minicomputer (PDP-11/35) where it is recorded on digital tape along with angular position information from the hemispherical scanner.

3. Data processing

A variety of on-line processing can be used to evaluate real-time system operation, but the data to be discussed below are the results of postprocessing of the digital data tapes on an off-line computer (DEC system-10). Line-printer listings and plots were used to evaluate the data and to search for interesting events.

The data were first corrected for anomalies. Several of the spectral filters were corrected for abnormally low outputs. The most positive velocity filter frequently had an anomalous signal and was therefore eliminated from the analysis.

The velocity of the wind measured for a particular range and angle was determined by the spectral filter with the largest amplitude. If the peak amplitude was above a given threshold, the velocity measurement was considered to be valid. A number of algorithms were tried for setting the threshold. The one finally adopted set the threshold at a fixed fraction above the median spectral intensity in a particular spectrum. The same threshold was used throughout each scan in azimuth angle. This procedure was not without problems, since there was some variation in system noise level within a scan.

The velocity data generated by the postprocessing

were displayed in two types of line-printer plots. The first plotted velocity versus azimuth angle and was used for angle scans at fixed range. The second (used to generate the contour plots in Fig. 5) plotted velocity as a symbol (1-9 for approaching velocities, A-I for receding velocities) on the spatial plane being scanned. This mode was essential when both range and angle were scanned simultaneously. A similar plot of intensity, again using symbols to designate levels of peak in-tensity, was used to correlate signal strength with the velocity measurements.

4. Results

During the one-month test period at KSC, three storms that showed the existence of a well-defined gust front with measureable shear were monitored. The data to be discussed were recorded on 29 July 1978.

The data consist of wind flow-field plots obtained from an anemometer network at KSC and laser Doppler velocity plots. The anemometer data were reduced to a wind flow-field plot by plotting wind barbs associated with each 17 m tower on a map of KSC. These barbs were used to construct flow lines and to show the location of fronts when present. The laser Doppler velocity plots indicate mean radial velocity as a function of scan position; in these plots the lidar location is the center of the plot with encompassing rings indicating 2 km range increments. The velocities are coded with 1 through 9 indicating increasing ap-proaching velocity and A through I indicating receding velocity. Each number or letter represents a velocity bin (filter) of 2.5 m/s.

The first wind field plot in Fig. 4 indicates the pene-tration of the lidar scan plane by a sea breeze front at



FIG. 4. Wind flow-field plots derived from ground-based anemometers showing thunderstorm passage. The cross-hatched regions at 1600 and 1630 EDT indicate locations of rainfall based on visual information.



FIG. 5. Wind flow-field plots derived from the lidar showing gust front penetration of the lidar scan area.



approximately 1500 EDT. This resulted in a pre-dominant wind direction from the southeast behind the front and from the southwest preceding the front. By 1530 EDT, the situation shown in the second plot of Fig. 4 existed. A second sea breeze front, with wind from the northeast, has now pushed into the scan plane resulting in convergence at the site—winds approaching from virtually all directions. By approximately 1600 EDT (third plot), a col has formed slightly to the west of the lidar site. At this point the wind is ap-proaching from the west and east, and is receding to the north and south. Simultaneously, a storm is forming over the Indian River to the west and within a half hour has passed over the site and out to sea, as indicated in the final plot of Fig. 4.

The laser Doppler velocity data corresponding to these events are shown in Fig. 5. The first of the plots, generated at 1532 EDT, indicates approaching veloci-ties from all but a small area to the north of the site. By 1542 EDT the wind is flowing toward the north and by the third plot has evolved such that the wind is flowing almost uniformly toward the northwest. In the next plot, 5 min later, a high velocity approaching wind can be observed directly to the west of the lidar system; and by the next scan, a highly turbulent region has been encountered with both approaching and receding velocities in excess of 16 m/s being indicated. By the final plot, the flow has become much more uniform, indicating relatively high winds toward the southeast, which proceed to move the storm out to sea.

5. Conclusions The wind velocity variations associated with gust fronts of thunderstorms were measured with a pulsed lidar on three occasions during TRIP-78. These wind shears were clearly visible in both real-time velocity versus azimuth plots and in postprocessing displays of velocities versus position. The present system is capable of making these measurements to a range of 5 km. A 20 dB increase in transmitter performance would increase this range to approximately 10 km based on an atmospheric attenuation coefficient of 2 dB/km, a value typical of that obtained from tempera-

ture and humidity measurements during the approach of a storm. It appears that a lidar system cannot be used effectively when moderate precipitation exists between the sensor and the region of interest; regions of light rain (5 mm/h path average) were penetrated by the system.

Measurements at a single azimuth angle indicated the presence of a shear along the line of sight, but such measurements suffer from two significant difficulties. First, the approach of a gust from a direction other than a line of sight cannot be observed until after the front has penetrated this line. Second, the front is identified more quickly when some scanning is done to provide reference data from a region unaffected by the gust front. In addition, the convergence data available from a complete scan could be used to provide informa-tion about up- and downdrafts that are normal to the direction of greatest Doppler sensitivity.

Acknowledgments. We would like to acknowledge the contri-butions of the individuals involved in the collection of the data: J. Dunkin, S. Johnson, W. Jones, and P. Marrero of NASA Marshall Space Flight Center; E. Gorzynski, M. Krause, C. Morrow, D. Foley, and W. Richardson of Ray-theon; and R. Kalafus of the DOT Transportation Systems Center.

References

Goff, R. C., J. T. Lee, and E. A. Brandes, 1977: Gust front analytical study. FAA-RD-77-119, final report to Federal Aviation Administration, NOAA National Severe Storms Lab., Norman, Okla.

Jelalian, A. V., W. H. Keene, and C. M. Sonnenschein, 1972: Development of C02 laser Doppler instrumentation for detection of clear air turbulence. NAS8-24742, final report to Marshall Space Flight Center, Raytheon Co., Wayland, Mass.

Kalafus, R. M., 1978: Wind shear requirements and their application to laser systems. FAA-RD-77-123, final report to Federal Aviation Administration, DOT Transporta-tion Systems Center, Cambridge, Mass.

, and J. N. Hallock, 1976: Requirements for an airport ground-based wind shear detection system. Preprints, Seventh Conference on Aerospace and Aeronautical Meteor-ology (Melbourne), AMS, Boston, pp. 182-187. •


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Pulsed Laser Doppler C. DiMarzio, Measurements ol wind