Measurements of high resolution atmospheric water-vaporprofiles by use of a solar blind Raman lidar

John Cooney, Kenneth Petri, and Alfred Salik

Presented here are preliminary results of measurements of atmospheric water-vapor profiles which were ob-tained by use of a solar blind Raman lidar. Interesting new features of the data gathered include high spa-tial resolution during daylight hours along with associated measurement errors.

The concept of the solar blind phenomenon as wellas its use for a potential lidar system, although discussedinformally since the early days of lidar, has recentlyacquired more practical importance with the advent ofusable ultraviolet (UV) laser sources.1' 2 The first reportof measurements of water-vapor profiles using the solarblind lidar concept has demonstrated its practical ap-plicability.3

The solar blind Raman lidar takes advantage of thesolar blind phenomenon which, by virtue of the ab-sorption of the solar radiation in the UV region of thespectrum (230.0-320.0 nm) due to the presence of ozonein the atmosphere, permits a lidar to operate duringdaytime with an effective ambient background fluxapproximately equivalent to that which would be ex-perienced at night in the visible portion of the spectrum.Because of this phenomenon, solar blind Raman lidarbackscatter signals, in contrast with Raman lidar day-time operation in the visible, have signal levels wellabove ambient flux levels. Use of the solar blind con-cept preserves the great simplicity of design inherentin Raman lidars while permitting full daylight opera-tion. It should be noted that a Raman lidar could bemade to operate during the daytime in the visible ifequipped with a very narrow output bandwidth laserand receiver bandwidth and very small angular field ofview typical of differential absorption lidar (DIAL)systems. Such systems are costly to build and maintainhowever, hence the selection of the solar blind con-cept.

As noted above, the advent of pulsed laser systemsin the UV with high power and high energy output havemade practicable the solar blind Raman lidar. There

J. Cooney is with Drexel University, Physics & Atmospherics De-partment, Philadelphia, Pennsylvania 19104; the other authors arewith U.S. Naval Air Development Center, Warminster, Pennsylvania18974.

Received 4 June 1984.

are presently several lasers variously suited for use ina solar blind lidar. The frequency doubled dye laser canbe tuned over significant portions of the UV. As hasbeen shown, the tunability provides distinct advantageswhen confronted with variable 03 atmospheric loading.2

The flash-pumped version of the dye laser usually as-sociated with high pulse energy applications fails toprovide good pulse energies on the nanosecond timescale. A second system is the quadrupled YAG systemwhich provides great reliability but relatively low pulsepower. The tripled alexandrite system appears to havegreat potential but is almost without field experience.Finally, the rare gas halide laser provides high pulseenergies on a nanosecond time scale. Thus, a KrFsystem operating at 248.5 nm provides a pulse of 0.75J in 20.0-nsec (full width at half-maximum). Althoughnot itself fully tunable, when equipped with a Ramanshifter cell it can reach all the useful UV range with littleadded system complexity and with moderate powerdegradation. For this and other operating reasons, theKrF system was chosen for the lidar.

The present lidar is limited to two profiles per secondbecause of the current data acquisition system. A muchhigher rate can be implemented when needed and thislatter point is now thought to have significant conse-quences with respect to effective atmospheric statisticalstationary time intervals. The lidar receiver system hasa 0.75-m diam primary and employs two channels ineach of which is mounted an appropriate narrowbandfilter and a solar blind photomultiplier. The data ac-quisition system consists of a Tektronix 7912 A-Dconverter and a PDP-11 data processing system.

The ability of the lidar to acquire water-vapor profilesin the first few kilometers of the troposphere is by nowwell established. However, only recently, by use of thesolar blind concept, has Raman lidar been shown to beable to operate in full daylight.3 In this work, althoughdaytime water-vapor profiles were acquired routinelyduring a sequence of episodes as a part of the first phaseof the investigation, it is not the overall purpose of this

104 APPLIED OPTICS / Vol. 24, No. 1 / 1 January 1985

program to acquire profiles as such. In the longerrange, an important aspect of this program is to attemptto determine practicable limits for the lidar in its abilityto determine the shortness of time interval and small-ness of spatial resolution of the water-vapor field in theneighborhood of a few kilometers with concentrationaccuracies typical of current radiosondes or better.Thus, spatial resolution in intervals of 5-10 m and timescales (<5.0 min) with measurement errors less than-10% are among the goals of this program. A primaryreason for this level of measurement comes about dueto a propensity of the water-vapor field to create hori-zontal channels called wetouts and dryouts. These arenarrow channels of anomalously high or low water-vaporcontent usually 100-200 in thick and most often locatedwell within the first kilometer. Such ducts are instru-mental in causing local redirection of microwave ra-diation beams. These ducts or layers of overly moistor overly dry air, arranging themselves in randomfashion as they do, cause random variations in thepropagation path of electromagnetic radiation in themicrowave region of the spectrum. Equipped with aknowledge of the spatiotemporal distribution andwater-vapor content of these ducts as well as the sur-rounding atmosphere, one can compute the nature andextent of the variation imposed on the propagationpaths of the microwave radiation. It is suggested thatlidar designed to monitor the changing disposition ofthe local water-vapor field can provide the needed in-formation to compute corrections and so ascertain thenature and extent of the atmospheric redirections of themicrowave radiation.

A study of a representative distribution of atmo-spheric ducting based on the deep Asheville data base4

reveals that ducts divide themselves for present pur-poses into surface and elevated ducts. This study re-veals that 55% of all ducts are <200 m deep. Indeed,27% are <100 m deep. In addition, ducts were foundto be present 60.0% of the time during sampling. Thearea surveyed is the ocean environment off the NorthCarolina coast in the fall of 1979. The so-called M-deficit is a convenient term used to define changes in theoptical refractive index of the atmosphere at microwavefrequencies due to the presence of anomalous changesof water vapor such as the ducts mentioned above. Itis designed to indicate ray trapping of microwaves whichoccurs when the M-deficit takes on a negative slope withaltitude. For the water-vapor profiles included in theAsheville data noted above, the median M-deficit was16.0. For a case chosen as most typical, this particulardeficit was equivalent to a change of water-vapor con-tent of from 7.8 g/kg at 510.0 in to 2.82 g/kg at 663.0 m.This corresponds to a reduction by 64.0% in an-150.0-m altitude interval. In addition, multiple we-touts and dryouts have been observed locally on theradiosonde traces. Hence, in sum these ducts occurquite often and are frequently accompanied by multiplelayering within the geoplanetary boundary layer.

In addition, temperature anomalies as well aswater-vapor anomalies play a meaningful role in mi-crowave ray path variation. Hence, a lidar acquiring

both water-vapor and temperature profiles in theboundary layer on a timely basis can do much to miti-gate the uncertainties due to ray path anomalies. Highresolution temperature profiles will be dealt with in alater phase of this lidar investigation.

For historical perspective it should be noted that thefirst use of lidar for acquisition of water-vapor profilesoccurred in 1965 and was performed by Schotland atNew York University using an early variant of what isnow familiarly known as DIAL.5 The first water-vaporprofiles acquired using Raman backscatter were re-ported in 1969 by the NCAR group which incidentallyalso initiated the practice of simultaneous radiosondesas a method of independent verification.6 The firstdaytime measurements of water-vapor profiles usingthe solar blind Raman procedure were accomplished bythe French Meteorological Service in 1979.3 Currently,water-vapor profiles by use of the solar blind Ramanlidar method being reported in this paper have highspatial resolution as one of its primary characteristics.Spatial resolution of 5.0 m has been achieved.

In this first measurement phase of the current pro-gram, lidar profiles of water-vapor mixing ratio wereacquired during each of six episodes which occurredduring September 1982 at the Naval Air DevelopmentCenter at Warminster, Pa. Most of the profiles so ac-quired were of a routine nature in that good correlationof values was had between the lidar profile and the ac-companying radiosonde profiles. On the other hand,two of the profiles taken on separate days showed quiteunusual variations and as a result have been the subjectof considerable analysis.

The two profiles which engendered so much attentionshowed, in the midrange of profile height from -350 to700 in, a very pronounced alternation of moist and drylayers. Initially it was concluded that the data werecontaminated by an instrumental artifact. Nonethe-less, after very extensive investigation of the experi-mental evidence, no case can be made for the instru-mental origins of these regularities. On the other hand,the evidence is by no means deep enough statisticallyto support a positive assertion that these regular vari-ations truly represent water-vapor variations. Whatlittle light can be shed on the matter by the presence ofthe concurrently launched radiosondes supports thecase for the reality of these variations. Analysis of thedata tapes and the noise tapes is still proceeding. Asimplied above, subsidiary evidence was present such asvery erratic behavior of the accompanying radiosondetraces oscillating back and forth to high humidity valuessuggesting that in the same approximate altitude in-terval the sonde was swamped. The sonde subse-quently recovered at higher altitudes to give veryplausible readings thereafter. In addition, the Fouriertransform of the accompanying noise traces, whileshowing some activity in the appropriate spectral in-terval, was nonetheless of significantly smaller magni-tude and not attributable to some agency external to theatmosphere as an obvious causative agent. In view ofthe fact that extremely sharp changes in refractive index(50 N/m) have been measured by Lane7 and also that

1 January 1985 / Vol. 24, No. 1 / APPLIED OPTICS 105

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Fig. 2. Same data as in Fig. 1, however, with a very high spatialresolution of 5.0 m in contrast with 120.0-m spatial resolution of therange averaged data exemplified in Fig. 1. Due to background noiseas the limiting noise source, the uncertainty is approximately equal

to data in Fig. 1.

M-deficits of 60 and more have been reported for ele-vated ducts in more than 10% of the cases,4 it seemspremature to dismiss these profile regularities out ofhand. Nonetheless, further instrument checks as wellas additional episodes of this nature are needed beforea reasonable level of credibility can be assigned to suchunusual atmospheric behavior.

Figure 1 shows a moderately high resolution water-vapor profile in the spatial resolution of 120.0 m. Thisis a 300-pulse profile and is background noise limited.The profile on the right is the statistical uncertainty inpercent. The same profile is shown in Fig. 2 with amuch finer spatial resolution of 3.0 m. The uncertaintyis essentially the same except for minor variations andarises as a result of the fact that the data are backgroundnoise limited. Greater precaution in this area in futuretests provides the potential to get down to shot noise-limited uncertainty in the neighborhood of 5.0%.

The profiles which have occasioned extensive anal-yses are depicted with moderate resolution by the typ-ical case shown in Fig. 3. Figure 4 shows the identicaldata in very high resolution. As noted above, initiallyit was quite an attractive hypothesis to 'attribute theoscillatory behavior between 400 and 700 in as due tosome external source. Aside from our inability, afterextensive efforts, to identify a source of the corre-sponding frequency by use of highly sensitive receivers,there are additional problems with such an explanation.The most difficult problem with invoking an externalsource as an explanation is that it occurs only in themiddle portion of the profile. In addition, because theprofile represents an average of 300 profiles it had tooccur over and over again on a synchronized basis.Such a hypothesis just is not credible. Spurious cwsources would have imposed themselves over the entiretrace.

A further set of difficulties attends an explanationwhich hypothesized that the source was somehow in-ternal to the lidar such as might emanate from a laserpower supply. The chief difficulty with this explana-tion is that it occurred only twice. The two episodes

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106 APPLIED OPTICS Vol. 24, No. I January 1985

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Fig. 6. Unlike the data in Fig. 5, this frequency distribution does notgather around a specific value. This frequency distribution variesover a wide span of values of water-vapor content. It represents dataat 250-m altitude from a 3.0-m interval. These contrasting frequencydistributions demonstrate the wide variation in atmospheric boundary

layer dynamics within an altitude change of 300 m.

involved were seen to be in no way different from theothers. The power spectral density curves of the noiseprofiles themselves do not exhibit evidence of thepresence of frequency components in the appropriaterange. Such would have to be the case were the oscil-latory portions of the profile generated by the lidar.Finally, it should noted that data with significantstratified regularity have already been published.8

A novel method of lidar data presentation, exempli-fied in Figs. 5 and 6, has been used to help visualize someof the details of how photomultiplier current counts fora given range bin distribute themselves. The figuresare a series of so-called point curves (i.e., only the integervalues of the abscissa have meaning). Each curverepresents how this current (photoelectron counts) froma series of 25 successive individual pulses is distributed.A specific count number is associated with each indi-vidual pulse. After each 25-pulse interval the recordedcount distribution for that interval is added to the prior

distribution to give a running (in time) account of howthe water-vapor content of a given bin is changing overthe 300-pulse interval. Here the ordinate is relative andindicates the relative frequency of counts. The abscissais also relative and each integer point represents (isproportional to) a given number of counts. Thus, eachsucceeding curve combines the count distribution fromall previous pulses with the count distribution of thecounts of the 25 subsequent pulses and thus representsa cumulative or running distribution of all previouscounts for a given range bin. The top curve representsthe count distribution of all 300 pulses in a given pro-file.

Figure 5 depicts the accrual of current counts over avery narrow distribution indicative of a quiescent layer.On the other hand, Fig. 6 depicts a wide spread in thedistribution which is more indicative of a layer ofchanging water-vapor content. The temporal changesover the duration of the 300 pulses of the profile are nodoubt suggestive of air motion which at this point canbe little more than qualitatively indicated. It doesimpinge however on the question of the meaning,meteorologically, of what is being measured. Longduration averaging may be relatively meaningless de-pending on the use to which the data are put. In thecase of random displacement of microwave radiationaway from rectilinear propagation, long duration av-erages of the water-vapor content take on less and lesssignificance as this duration is lengthened. Such ap-plications put a premium on obtaining measurementsof a given accuracy of a very timely nature and over arelatively short duration average.

This latter form of observation has only begun andfurther work will be required to determine how to bestrender these data more operationally meaningful.What immediately suggests itself for the microwaveducting problem is the need for temporal weighting ofthe individual pulses going into making up a profile; forexample, an exponential weighting in time with the neartime events getting more weighting than the earlierones. One thing does seem clear and that is that suchatmospheric conditions clearly mandate, at least for theapplications to the microwave propagation problem,short averaging times. This, in turn, would indicate theneed for higher power higher pulse energy lasers for thelidar. Further study will be needed to ascertain theimportance of these problems.

In summary, two basic forms of data have been pre-sented. Water-vapor profiles with extremely highspatial resolution of 5.0 m have been acquired. Thisresolution will be vital to allow a sufficiently refinedspatial delineation of such common occurrences as ductswhich in turn will permit appropriate corrections toradar propagation errors arising from the presence ofsuch ducts. Second, the frequency of occurrence curvessuggest that some altitude bins experience large changesin water-vapor content over the duration of a pulse av-eraging. This, in turn, calls into question the meaningof long duration averaging of data. Finally, some un-usual profiles have been acquired whose stratified fea-tures need to be verified further.

1 January 1985 / Vol. 24, No. 1 / APPLIED OPTICS 107

This work was funded by the Naval EnvironmentalPrediction Research Facility, Monterey, Calif. ProjectWF59 553.

References1. J. Cooney and K. Petri, "A Solar Blind Raman Lidar," in Pro-

ceedings, Ninth International Laser Radar Conference (AmericanMeteorological Society, Boston, 1979).

2. K. Petri, A. Salik, and J. A. Cooney, "Variable-WavelengthSolar-Blind Raman Lidar for Remote Measurement of Atmo-spheric Water-Vapor Concentration and Temperature," Appl. Opt.21, 1212 (1982).

3. D. Renaut, J. C. Pourny, and R. Capitini, "Daytime Raman-LidarMeasurements of Water Vapor," Opt. Lett. 5, 233 (1980).

4. M. Werst, "Anomalous RF Propagation Effects in an Ocean En-vironment," Naval Air Development Center Report NADC-79087-30 (12 Jan. 1980).

5. R. M. Schotland, in Proceedings, Third Symposium on RemoteSensing (Environmental Research Institute of Michigan, AnnArbor, 1964).

6. J. Cooney, "Remote Measurement of Atmospheric Water VaporProfiles Using the Raman Component of Laser Backscatter," J.Appl. Meteorol. 9, 182 (1970).

7. J. A. Lane, "Small Scale Variations of Radio Refractive Index inthe Troposphere," Proc. IEE 115, No. 9 (Sept. 1968).

8. J. C. Pourny, D. Renaut, and A. G. Orszag, "Raman-Lidar Hu-midity Sounding of the Atmospheric Boundary-Layer," Appl. Opt.18, 1141 (1979).

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108 APPLIED OPTICS / Vol. 24, No. 1 / 1 January 1985