LaRC Lidar Data User's Guide
Langley Research Center Cloud Lidar
FIRE IFO 1991 -- Parsons, Kansas
Updated: April 1994

For questions, requests, and comments please contact:

Jose M. Alvarez                 Mark A. Vaughan
Mail Stop 417                   Mail Stop 417
Langley Research Center   or    Langley Research Center
Hampton, VA 23681-0001          Hampton, VA 23681-0001
(804) 864-2677 Phn               (804) 864-5331 Ph
(804) 864-7711 Fax              (804) 864-7711 Fax

------------------------ Section 0 : The Introduction --------------------------

     This document contains a complete description of the Langley Research
Center Cloud Lidar (LaRC-CL), its participation in the 1991 FIRE Intensive
Field Observations (IFO), and the resulting archived data product.  A more
concise overview of much of this same information can be found in the file
LARCINFO.4U, also present in this archive.

     In the following seven sections we address these topics:

     Section 1 : The Langley Research Center Cloud Lidar
          The physical specifications of the lidar are listed and the field
          site location is described.

     Section 2 : 1991 FIRE IFO Operating Hours
          Dates and times of lidar operation during the experiment are given,
          and listing of graphic image files corresponding to each data
          acquisition period is provided. A brief description of the
          predominant cloud cover is also included.

     Section 3 : The LaRC-CL Data File Format
          The format of the raw data files is presented.  We give an overview
          of the structure of the data and describe the individual fields of
          the record headers.

     Section 4 : Calibrating the LaRC-CL
          We describe the procedure used to calibrate the lidar.

     Section 5 : Calculating Total Signal Profiles and Depolarization Ratios
          Our method for obtaining several derived measurements is explained.

     Section 6 : Mission Notes and Comments
          A synopsis of the mission logbook entries.

     Section 7 : References and Other Documents
          A listing of all files submitted by the Langley Research Center Cloud
          Lidar Group to the 1991 FIRE IFO archive.  A short selection of
          pertinent references is given.


------------- Section 1 : The Langley Research Center Cloud Lidar --------------

     The Langley Research Center Cloud Lidar is a dual-channel, polarization
sensitive lidar using a frequency doubled Nd:YAG laser as a linearly polarized
transmitter and an eight inch Cassagranian telescope as a receiver.
Backscattered laser light collected by the receiver is collimated, directed
through a half wave plate, and then passed through polarizing optics which
decompose the signal into two components, one parallel and the other
perpendicular to the polarization plane of the transmitted beam.  Separate
amplification and digitization paths are employed for each component, resulting
in two arrays of backscatter data for each measured laser pulse.  The LaRC-CL
is designed for optimum cloud monitoring operations at altitudes between
approximately 3 and 18 kilometers.  To prevent saturation of the detectors at
lower altitudes, a gating circuit is used to delay the activation of the first
dynode in the photomultiplier (PMT).  The PMT is brought to full sensitivity
only after this delay time has elapsed.

     The physical specifications of the LaRC-CL are as follows:

                         -- Transmitter --

           Laser                      Continuum Surelite Nd:YAG
           Wavelength                 532 nm
           Energy                     150 mj @ 532 nm
           Pulse width                4-6 ns
           Divergence                 0.6 mrad
           Beam diameter                6 mm
           Repetition rate            10 Hz


                          -- Receiver --

           Primary diameter           20 cm
           Field of view              2 mrad
           Filter transmittance       50% at 1 nm bandwidth centered at 532 nm
           Detector                   Photomultipliers (2), EMI 9658
           Quantum efficiency         15%
           Calibration                Zero order half-wave plate mounted on a
                                      rotatable assembly


                    -- Data Acquisition System --

           DSP Technology Model 2012s Transient Recorders (2)
              resolution              12 bit
              bandwidth               10 MHz (3 dB, full scale)
              data samples            variable, 64 to 8192 samples per waveform
              sampling frequency      variable, 0.2 to 20 MHz

           DSP Technology Model 4101 Signal Averaging Memory modules (2)
              24 bit capacity

           Everex 386/20 desktop computer
              4 Mb RAM
              108 Mb hard disk
              200 Mb optical drive
              SVGA card and monitor


                 -- Nominal Acquisition Parameters --

           Altitude resolution        15 meters
           Temporal resolution        15 seconds (150 lidar pulses are
                                      integrated)
           PMT gate delay             1 to 4 km


     During the entire 1991 FIRE IFO the LaRC-CL was located at a field site
(37 deg. 18 min. north, 95 deg. 07 min. west, 264 meters above MSL) near
Parsons, Kansas.  A specially modified recreational vehicle housed the lidar
and its auxiliary equipment.




------------------ Section 2 : 1991 FIRE IFO Operating Hours -------------------


     Below we list the days and approximate times during which the LaRC-CL was
operational.  Data acquisition may not be continuous during any session; there
were sporadic interruptions during which we performed tasks associated with
instrument operation and maintenance.

     The "Cloud Cover" column gives a qualitative assessment of the cloud cover
-- i.e., the number of cloud layers and their approximate location(s) --
detected during the measurement period.  Layers are reported only for that part
of the return occurring after the first dynode of the photomultiplier has been
gated on.  Pre-gate features are NOT reported.  (For example, see 11-28-91.  We
report cirrus layers from 8 to 10 km.  Also present were intermittent cumulus
layers from 1 to 2 km, which we do not report.  Users interested in data from
lower altitudes can obtain the Colorado State University ceilometer data, also
taken at the Parsons site.)  Cloud layers are indicated by giving the base
height to the nearest kilometer.

     We have also submitted color modulated time history plots of our data to
this archive.  These are .GIF files, rendered according to the FIRE lidar
graphics guidelines.  The "Image File" column lists the filenames of all images
associated with each data acquisition period.  The naming convention used in
constructing individual filenames is "LxMMDDTT.GIF".  The "L" identifies the
file as being Langley Research Center FIRE lidar data; the "x" is either S for
attenuated [S]cattering ratios, or D for [D]epolarization ratios (these
quantities explained in Section 5); "MM" indicates the month; "DD" indicates
the day; and "TT" gives the starting time, in GMT.  Each plot covers a six hour
time period, so "TT" will be either 00, 06, 12, or 18.

     All times are given in Greenwich Mean Time (GMT).

  Date              Time                Cloud Cover              Image Files
-----------------------------------------------------------------------------
11-13-91       12:44 to 16:52      1 layer, 9 km                 Lx111312.GIF
               19:09 to 23:45      2 layers, 6 km and 9 km       Lx111318.GIF

11-14-91       00:59 to 01:19      2 layers, 3 km and 8 km       Lx111400.GIF

11-17-91       16:44 to 20:08      1 layer, 6 to 7 km            Lx111712.GIF
               22:33 to 23:32      clear                         Lx111718.GIF

11-18-91       00:09 to 01:30      clear                         Lx111800.GIF
               12:13 to 22:44      clear                         Lx111812.GIF
                                                                 Lx111818.GIF

11-19-91       13:07 to 16:31      multiple layers beginning     Lx111912.GIF
                                   at 3km

11-20-91       15:39 to 16:45      1 layer, 3 km                 Lx112012.GIF

11-21-91       12:28 to 13:56      clear                         Lx112112.GIF
               15:39 to 22:45      clear                         Lx112118.GIF
               22:45 to 23:29      1 layer, intermittent,
                                   9 km

11-22-91       00:02 to 07:25      1 layer, variable,            Lx112200.GIF
                                   7 to 9 km                     Lx112206.GIF
               07:29 to 09:26      2 layers, 6 and 8 km          Lx112212.GIF
               09:33 to 10:57      clear
               11:18 to 15:12      1 layer, descending,
                                   5.5 to 4 km

11-24-91       22:20 to 23:52      1 layer, 7 km                 Lx112418.GIF

11-25-91       00:03 to 03:11      1 layer, descending,          Lx112500.GIF
                                   6 to 2 km                     Lx112506.GIF
               11:41 to 14:36      1 layer, 2 km                 Lx112512.GIF
               21:10 to 23:50      clear above 4.5 km            Lx112518.GIF

11-26-91       10:47 to 12:26      1 layer, 7 km                 Lx112606.GIF
               13:21 to 22:14      1 layer, descending,          Lx112612.GIF
                                   10 to 5 km                    Lx112618.GIF

11-28-91       14:54 to 16:08      1 layer, descending,          Lx112812.GIF
                                   10 to 8 km                    Lx112818.GIF
               18:42 to 23:38      1 layer, 8 km

11-29-91       00:19 to 00:34      1 layer, 7 km                 Lx112900.GIF

11-30-91       14:22 to 15:26      1 layer, 4 km                 Lx113012.GIF

12-03-91       13:28 to 14:00      1 layer, 2 km                 Lx120312.GIF
               15:35 to 16:08      clear

12-04-91       03:32 to 04:43      clear                         Lx120400.GIF
               14:19 to 17:25      1 layer, intermittent,        Lx120412.GIF
                                   10 km                         Lx120418.GIF
               17:37 to 21:50      1 layer, intermittent,
                                   12 km
               21:50 to 23:52      clear

12-05-91       00:45 to 05:15      1 layer, 11 km                Lx120500.GIF
               05:15 to 10:32      2 layers, 8 and 11 km         Lx120506.GIF
               17:03 to 23:59      1 layer, descending,          Lx120512.GIF
                                   11 to 9 km                    Lx120518.GIF

12-06-91       00:00 to 01:25      1 layer, 9 km                 Lx120600.GIF
               01:35 to 07:30      2 layers, 12 and              Lx120606.GIF
                                   9 descending to 8 km          Lx120612.GIF
               07:30 to 12:40      intermittent and              Lx120618.GIF
                                   variable, 7 to 10 km
               12:40 to 19:50      1 layer, 7 km
               19:50 to 21:16      clear

12-07-91       19:37 to 21:16      clear                         Lx120718.GIF


------------------ Section 3 : The LaRC-CL Data File Format --------------------

     The lidar data in the archive, collected during the FIRE IFO 2 at
Parsons, Kansas are named MMDDYY_CI2_LRC_LIDAR.BIN and
MMDDYY_CI2_LRC_LIDAR.ASC where MMDDYY are the month, day, and year the data
were acquired.  Each file contains all of the data collected on that day.
The first set of archive files were written in PC binary, and should be
easily read using any PC-based programming language(Borland Pascal 7.0,
Borland C/C++ 3.1, and IDL for Windows have been tested successfully).
The second set of archive files are stored in standard asce format and should
be readable by a wide variety of platforma.

Each file consists of a series of data records acquired during
a single GMT day.  For each data record there is a header section (140 bytes)
followed by two simultaneously acquired arrays of digitized lidar backscatter
data (9340 bytes each).  The first array contains parallel channel data, and
the second contains perpendicular channel data.  All data have been corrected
for background; that is, background shape and level have been subtracted on a
record-by-record basis, treating each channel independently.  The sampling
altitudes associated with each data point in the arrays can be calculated
using

               Z[n] = 0.5 * n * C * SampleRate * cos(T)                 (1)

where C is the speed of light, SampleRate is the elapsed time between
successive digitizer samples, the one half accounts for the two-way travel
between the lidar and the sampled volume, T is the lidar pointing angle, and n
is an index ranging from 1 to 2335.  For a zero degree pointing angle and a
SampleRate of 100 nanoseconds, the minimum altitude is Z[1] = 15 meters, and
the maximum altitude is Z[2335] = 35 kilometers.  Digitization of the signal
begins immediately after the laser pulse is emitted.  Since the PMTs are not
immediately brought to full sensitivity, reliable science data is not obtained
for some (variable) time delay after the pulse is initiated, and hence after the
beginning of the data array (see Section 1, and the "DelayTime" parameter
below.)  The file LARC_DOC.GIF contains an actual lidar return showing the
effects of this variable time delay.

     Each record header contains a number of individual data fields describing
various characteristics of the subsequent data arrays.  Those fields for which
information does not exist are filled with a constant (-999) representing an
"unassigned field".  The header format for the binary files is presented as
a guide for those wanting to store this data in a more economical binary
format.  As an aside, on a pc, the asce data files take up 1.2-1.3 times as
much space as the binary data files.  The binary format for the header is as
follows:

      Field         Data  Size in
    Identifier      Type   Bytes                   Explanation
  ------------------------------------------------------------------------------
    DataPoints    integer    2    Number of values in the data arrays.  For the
                                  entire FIRE IFO 1991 data set, this entry will
                                  be 2335, indicating a data array size of 2335
                                  long integer values.  (array size in bytes is
                                  4 x 2335 = 9340 bytes.  there are two arrays,
                                  so each data RECORD will contain 140 + (2 x
                                  9340) = 18820 bytes.)

          Date   long int    4    date of data acquisition in YYYYMMDD format
                                  (ex. November 28, 1991 = 19911128)

     StartTime   long int    4    data record starting time, in seconds elapsed
                                  since midnight GMT

       EndTime   long int    4    data record ending time, in seconds elapsed
                                  since midnight GMT -- typically this is 15
                                  seconds greater than the "StartTime" value

     RecNumber    integer    2    data records are numbered sequentially
                                  throughout the day, beginning with #1.  The
                                  occasional missing data records are noted,
                                  with explanation, in Section 6.

      Latitude                    Latitude of the Parsons, Kansas lidar site,
       degrees    integer    2    in degrees (37) and minutes (18).  The seconds
       minutes    integer    2    field is left unassigned (-999).
       seconds    integer    2

     Longitude                    Longitude of the Parsons, Kansas lidar site,
       degrees    integer    2    in degrees (95) and minutes (7).  The seconds
       minutes    integer    2    field is left unassigned (-999).
       seconds    integer    2

        Z_Zero    integer    2    Height above sea level, in meters, of the data
                                  acquisition site.  This should always be 264
                                  meters.

        System    integer    2    In-house identifier for the lidar system used

    WaveLength    integer    2    laser wavelength, in nanometers.  This field
                                  should always contain the value 532.

   FieldOfView    integer    2    telescope field of view, in tens of
                                  microradians.  This is another constant value
                                  field, and should always contain 200
                                  (telescope field of view = 2 milliradians)

    SampleRate    integer    2    digitizer sampling rate, in nanoseconds;
                                  i.e., we acquire one data point in a time
                                  equal to "SampleRate".  The FIRE IFO data was
                                  collected at a uniform sample rate of 100
                                  nanoseconds, giving an altitude resolution of
                                  approximately 15 meters.

     GainRatio      float    4    calibration constants for retrieving total
   OffsetAngle      float    4    signal and depolarization measurements.  The
      CalAngle      float    4    significance and utility of these constants
                                  and the quantities derived from them is
                                  discussed below in Section 4, "Calibrating
                                  the LaRC-CL", and Section 5, "Calculating
                                  Total Signal Profiles and Depolarization
                                  Ratios".  Angles are in radians.

    Tilt_Angle      float    4    lidar pointing angle, in degrees from zenith.
                                  In general the pointing angle should be either
                                  0 degrees (the most frequent data taking
                                  mode) or 5 degrees.

  P_Background   long int    4    a composite number representing the sum of
  S_Background   long int    4    i) the intensity of the ambient (i.e.,
                                  background) light at the time of acquisition,
                                  and ii) the total DC offset of the
                                  amplification stage.  P_BACKGROUND refers to
                                  the parallel data, and S_BACKGROUND refers to
                                  the perpendicular data.

    Shots_Avgd    integer    2    number of laser pulses comprising each data
                                  record.  Each pulse is digitized at 12 bit
                                  resolution using a DSP 2012S 10 MHz Transient
                                  Recorder, and successive pulses are
                                  accumulated in a DSP 4101 Averaging Memory
                                  module.  This number should always be 150,
                                  giving a uniform temporal resolution of one
                                  record every 15 seconds (150 shots @ 10 Hz
                                  = 15 seconds).

    P_detector[1]   A series of five integer values describing various characteristics
		    of the parallel detector package.  The binary data files
		    have room for 3 detectors.  In translating to asce, we
		    stripped the other two detectors since we used only one
		    detector for FIRE IFO 2.

     BandWidth    integer    2    Bandwidth of the amplification stage of the
                                  detector package.  units are tens of kiloHertz
                                  (e.g. 5 MHz would be stored as 500)

     DelayTime    integer    2    setting of the delay generator which controls
                                  the activation of the first dynode of the
                                  photomultipliers; units are tenths of
                                  microseconds.  The delay between the
                                  initiation of the laser pulse and the gating
                                  of the PMTs is APPROXIMATELY (DelayTime - 85).
                                  With reference to the data array, the index
                                  of the point at which the PMT is gated on is
                                  APPROXIMATELY
                                      ((DelayTime - 85) / SampleRate) * 10;

            P1    integer    2    photomultiplier voltage, in volts

            P2    integer    2    gain associated with amplification stage.  for
                                  the 1991 FIRE IFO data set this has a uniform
                                  value of 100.

            P3    integer    2    an unassigned field (P3 = -999) for this lidar
                                  system.

 S_detector[1]   A series of five integer values describing various
                 characteristics of the perpendicular channel detector package.

     BandWidth    integer    2    the S_detector sub-fields serve the same
     DelayTime    integer    2    functions for the perpendicular channel
            P1    integer    2    detector package as those described earlier
            P2    integer    2    did for the parallel channel detector package.
            P3    integer    2

  DataArray[RecNumber]  A list of long integers(4 bytes) comprising the lidar
			data.  The parallel data comes first  and the the
                        perpendicular data follows on the same line.  There
                        are DataPoints such lines.  The numbers consist of the
                        accumulation of Shots_Avgd laser firings minus the
			appropriate background counts as described earlier.


--------------------- Section 4 : Calibrating the LaRC-CL ----------------------

     The single scattering lidar equation assumed here is given by

                                C * PWR
                        P(Z) =  -------- * B(Z) * T2(Z)                 (2)
                                    2
                                   Z

where PWR is the transmitted laser power, C is a system constant which includes
terms describing the effective receiver area and the laser pulse width, B(z) is
the volume backscattering coefficient, and T2(z), the two way transmittance, is
the exponential of -2 times the integral with respect to range of the extinction
coefficient, X(r), from z=0, at the lidar transmitter, to some range z.
Alternately, for a zenith pointing lidar,

          T2(z) = EXP{ -2 * INTEGRATE[ X(R), LoLim=0, UpLim=z, dR ] }   (3)

     The signal received and stored by the data acquisition sub-system is
proportional to the optical power, P, at the receiver; that is

                                 S = G * P                              (4)

where G is the electro-optic gain, which includes transmission optics losses,
detector electronics gain, electro-optic conversion factors such as photo-
multiplier quantum efficiencies, and the electronic gain of any preamplifiers.
Note that the received parallel and perpendicular signals share certain factors
in common -- in particular the laser output, the receiver geometry prior to the
polarizing cube, and the optical depth of the atmosphere.  However, upon
entering the polarizing cube the signals are routed through separate optical
and electric pathways that will almost certainly have very different gains, if
for no other reason than that the perpendicular signal in clear air requires
much more amplification than the parallel signal.

     The linear nature of the process of converting light into a series of
digitized signals may be written in more detail as

                   S[p,s] = G[p,s] * C' * PWR * T2 * B[p,s]             (5)

where S[p,s] represents the signal stored in the parallel (p) and perpendicular
(s) channels of the data acquisition sub-system.  C' accounts for geometric and
optical effects encountered BEFORE the signal is transmitted to the polarizing
optics (i.e., C' = C / Z^2), T2 represents extinction effects, and PWR is once
again the transmitted power.  B[p,s] is the volume backscattering coefficient
in the parallel and perpendicular channel, and the G[p,s] term quantifies the
electro-optic signal gain for each channel incurred AFTER the beam passes
through the polarizer.

     Assuming perfect alignment of the optical components, we can write the
ratio of the signals received in the two channels as

               S[s]   C' PWR T2 G[s] B[s]   G[s]   B[s]
           m = ---- = ------------------- = ---- * ---- = GR * D        (6)
               S[p]   C' PWR T2 G[p] B[p]   G[p]   B[p]

where GR is the gain ratio, and D the depolarization ratio.  In this case, all
we need to retrieve the depolarization ratios from the measured ratios is an
accurate estimate of the gain ratio.

     Suppose, however, that the optical axis of the polarizing cube is somewhat
misaligned with that of the transmitted beam, leading to a apparent depolari-
zation ratio which is artificially (and incorrectly) low.  To eliminate these
sorts of measurement errors we inserted a half-wave plate into the optical path
of the LaRC-CL.  If the polarization plane of the transmitted beam exactly
matches that of the principle axis of the half-wave plate, each component of
the light is transmitted unchanged to the polarizing optics.  However, if the
half-wave plate is misaligned, or offset, by some angle, Q, the polarization
plane of the transmitted light is rotated in the same direction by a factor of
2*Q.  Consequently, after transmission, the electric field vectors of the
parallel and perpendicular components are given by

               E[p,t] = E[p,i] cos(2Q) + E[s,i] sin(2Q)                 (7a)
               E[s,t] = E[p,i] sin(2Q) - E[s,i] cos(2Q)                 (7b)

where E[?,t] represents the field transmitted through the half-wave plate, and
E[?,i] represents the incident field.  Since the electric fields produced by the
scattering process are assumed to uncorrelated, the powers associated with the
transmitted fields are

                              2    2             2    2
           P[p] = 0.5*( E[p,i]  cos (2Q) + E[s,i]  sin (2Q) )           (8a)

                              2    2             2    2
           P[s] = 0.5*( E[p,i]  sin (2Q) + E[s,i]  cos (2Q) )           (8b)

Recognizing that
                     2
                    E [?,i] = K * C' * PWR * T2 * B[?]                  (9)

where K is a constant of proportionality, we can substitute (8a) and (8b) into
(5) and write our expressions for the signals received in the parallel and
perpendicular channels as

                                         2               2
     S[p] = 0.5 K C' G[p] PWR T2 { B[p]*cos (2Q) + B[s]*sin (2Q) }      (10a)

                                         2               2
     S[s] = 0.5 K C' G[s] PWR T2 { B[p]*sin (2Q) + B[s]*cos (2Q) }     (10b)

If we now recompute the measured ratio given in (8) we have

                    2               2                        2
     G[s] { B[p]*sin (2Q) + B[s]*cos (2Q) }         { D + tan (2Q) }
 m = -------------------------------------- = GR * ------------------   (11)
                    2               2                       2
     G[p] { B[p]*cos (2Q) + B[p]*sin (2Q) }        { 1 + D*tan (2Q) }

     Our calibration scheme uses equation 11 to simultaneously determine both
the gain ratio, GR, and the offset angle, Q, due to any misalignment of the
optical components.  We do this by rotating the half- wave plate through a
series of three or more accurately measured calibration angles, acquiring some
number of data records at each angle.  For the "j"th calibration angle, A[j],
our depolarization equation becomes

                                    2
                             D + tan (2Q - 2A[j])
                m[j] = GR * ----------------------                      (12)
                                     2
                            1 + D*tan (2Q - 2A[j])

After averaging the data for each angle, we apply a nonlinear least squares
algorithm to solve the resulting system of equations in a "clear air" region of
the data.  In addition to calculating the system calibration constants (i.e.,
"GainRatio" and "OffsetAngle", as in Section 3), this procedure also determines
the mean depolarization ratio for the selected altitude range.


--- Section 5 : Calculating Total Signal Profiles and Depolarization Ratios ----

     The LaRC-CL is designed to make two simultaneous measurements: a total
backscatter (i.e., total signal) profile, and a depolarization ratio profile.
Both of these are derived quantities, obtained from the raw parallel (p) and
perpendicular (s) channel backscatter data by using the calibration constants
contained in the record header.  The total signal profile (TS), which
is the sum of the parallel and perpendicular contributions, is calculated as

                                         S[s,Z]
                      TS[Z] = S[p,Z] + ----------                       (13)
                                       Gain Ratio

In calculating the total attenuated backscatter we in essence create a signal
return proportional to what we would have obtained using a lidar that was NOT
equipped to make depolarization measurements.

     To calculate the depolarization ratio we first compute a measured ratio, m,
which is defined as

                                     S[s,Z]
                              m[Z] = ------                             (14)
                                     S[p,Z]

We can then retrieve the depolarization ratio, D, using

                                        2
                                 G * TAN (Q) - m
                             D = ---------------                        (15)
                                        2
                                 m * TAN (Q) - G

where G = GainRatio, and the angle Q = 2 * (OffsetAngle - CalAngle).  For each
data record, the appropriate values for GainRatio, OffsetAngle, and CalAngle are
contained in the header fields of the same name (i.e., the calibration constants
referred to in Section 3).

      We also routinely compute a third quantity, the attenuated scattering
ratio, obtained by dividing our total signal profile by a model "clear air"
profile -- that is, the profile we would expect if we were operating our lidar
in a purely molecular atmosphere.  In practice this is done by calculating a
constant of proportionality, D, between the lidar data and the model over some
altitude range of the measured profile judged to be "clear air".  (If at all
possible, this clear air region is chosen to occur below any clouds or aerosol
layers.) Then for each sampled altitude we compute the attenuated scattering
ratio, R[i], using

                                  TotalSignal[i]
                           R[i] = --------------                        (16)
                                   D * model[i]

In terms of the lidar equation (equation 2 above), the attenuated scattering
ratio is given by the second equation displayed in the file LARCINFO.GIF.

     For any given data session a clear air model can be constructed using the
lidar equation, atmospheric densities derived from local rawinsonde data and
theoretical values for the Rayleigh backscattering and extinction coefficients
[Penndorf, 1957].



------------------- Section 6 : Mission Notes and Comments ---------------------

     During the course of the experiment we maintained a lidar operator's log
book.  In addition to a written record of the various instrument parameters,
this document contains notes concerning

     o  visual observations of current sky conditions;
    o  best-guess estimates of the World Meteorological Organization (WMO)
        cloud code;
     o  reasons for certain changes in settings; and
     o  anomalies in the data and/or features of particular interest.

All observer/operator comments are reproduced below, separated by day, and time
tagged.  All times are given in Greenwich Mean Time.

11-13-91
     12:43 -- small, strongly depolarizing feature @ 9+ km; when it intensifies
          we get low spikes in the S channel data, so an amplifier bandwidth
          change to 5 MHz may be necessary
     13:12 -- almost completely light now; feature @ 9 km remains constant
          WMO code : H=1
     13:51 -- sky beginning to fill up with discrete, visible clouds
     13:58 -- reduced PMT voltages to keep greatly intensified cirrus returns
           on scale; base approximately 9 km, top about 10 km
          WMO code : H=4
     14:56 -- aborted calibration, records 486-493; problems with the half-wave
           plate rotator
     15:07 -- calibration
          WMO code : H=1
     15:58 -- two distinct layers, 9 and 11 km
     19:03 -- ran rotator to -114.2 counts, reset to zero [ Note: 161.6 rotator
          counts = 1 degree ]; now using 5 MHz on amplifiers due to abrupt,
          very intense, intermittent base, 6.5 to 7 km
     19:09 -- calibration
     19:38 -- dangerously close to saturating the P channel
          WMO code : H=9? this could also be a combination of altocumulus and
          cirrostratus
     20:12...21:59 -- intermittent P saturation in peak 6 to 7 km
     21:59 -- reduced both PMTs, everything now on scale
          WMO code : M=8, H=8
     23:24 -- calibration
          WMO code : H=7

11-14-91
     01:06 -- serious saturation @ 3.5 km

11-15-91
     intermittent rain all day . . . no observations attempted

11-16-91
     steady rain all day long

11-17-91
     16:47 -- cirrus(?) 5 to 8 km
          WMO code : M=7
     17:46 -- record #240, break to copy data to optical disk
     18:04 -- base @ 7 km, top @ 9 km
          WMO code : H=7?  are these cirrus or alto clouds?  same cloud bank,
          but base has varied from almost 4 km to about 7 km
     18:37 -- PMT voltages turned up some, hoping to get unambiguous clear air
          return above top of cloud; clouds 7 to 10 km
          WMO code : H=7
     19:11 -- cirrus, 8 to 10 km; stratus stuff has moved away
          WMO code : H=4
     22:20 -- laser energy monitored; records 715-757 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     22:35 -- calibration
          WMO code : clear
     22:58 -- S channel signal looks wrong; behaves similar to, if not exactly
          the same as, the records taken with the telescope covered.  what's
          going on here!!!
     23:33 -- tuned doubling crystal; got a good increase in the P signal, but
          the S channel is still a mess

11-18-91
     00:09 -- clear; P signal out to 30 km, S not believable
     00:53 -- opened up the rotator case, found rotator at approximately 260
          degrees [ Note:  desired rotator position = 0 ]; ran rotator FORWARD
          14616 counts (just over 90 degrees), did a 5 angle calibration, then
          moved rotator FORWARD another 487.2 counts
     01:13 -- calibration; clear air depolarization ratios now in the
          neighborhood of 4.5%, as opposed to around 1% previously
     12:13 -- clear
     12:47 -- turned PMT voltages down, no apparent change in the returns
     20:09 -- calibration; angle #7 is no good, got a flat S channel return for
          one of the four records
     20:43 -- continued very clear

11-19-91
     13:07 -- intense peak @ 3.5 km saturates both channels; serious ringing in
          the S channel; puffy altocumulus clouds; see records #20, #21 - when
          the intense stuff subsides, we've got a cloud deck from 3 to at least
          6 km
          WMO code : M=5
     13:18 -- reduced voltages to eliminate saturation; both channels ring hard
          off intense peak at 3.5 km; background levels look confused,
          difficult to calculate when the system takes a heavy cloud hit
     13:38 -- altocumulus are gone, leaving "rain-o-stratus" in their place;
          black altostratus, base @ 3 km, out of energy by 4 km; no saturation,
          heavy ringing in the S channel, some also in the P channel;
          background subtraction seems like a bad guess
     13:50 -- laser energy monitored; records 131-160 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     14:01 -- resumed data taking; base now @ 2 km, occasionally drifting lower
     14:47 -- huge peak, both channels, 4 km
          WMO code : L=7
     14:50 -- brutally spiked right at PMT gate by low level cloud; looks like
          rain soon
     15:05 -- a hint of a 12 km feature in a very brief gap in the lower level
          clouds
     15:32 -- we may have a break in the low clouds coming our way; voltages
          turned up as much as I dare in the hunt for cirrus
     15:33 -- cirrus peak @ 8km, alto clouds, 4.5 to 6 km
     15:43 -- return to previous settings

11-20-91
     15:33 -- laser energy monitored; records 1-20 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     15:39 -- altocumulus at 3 km
          WMO code : M=5
     16:32 -- clearing
     16:45 -- clear

11-21-91
     12:28 -- calibration
     12:55 -- clear
     14:28 -- laser energy monitored; records 305-324 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     14:37 -- calibration; angles 8 and 9 contain bad S channel data and must
          not be used in the calibration inversion
     15:02 -- calibration
     15:38 -- clear
     16:18 -- record #568 (about), non-visible cirrus at 8.2 km
     18:22 -- severe clear continues
     21:36 -- plenty of cirrus to the west, but overhead the severe clear
          continues
     22:49 -- cirrus at 10 km; switched amps to 5 MHz

11-22-91
     00:01 -- cirrus at 8 km
          WMO code : H=? (dark)
     00:57 -- reduced PMT voltage to eliminate P channel saturation
     01:20 -- persistent feature, 7.8 to 10 km
     04:06 -- cirrus produces spikes in the return at 8 and 10 km
     06:25 -- intermittent cirrus of varying intensity, mostly pretty weak
     07:29 -- big dinger @ 8.5 km inspires sudden reduction in PMT voltages
     07:32 -- now getting a very thin, brief peak at 6 km
     07:51 -- 6 km spike makes a quick return, then leaves almost immediately;
          8 km feature remains, but reduced in intensity
     08:22 -- getting crushed at 6 km; PMTs turned way down
          WMO code : M=5; this is visible in the moonlight
     09:10 -- alto cumulus bank breaking up, moving out of the lidar field of
          view; should be clear soon
     09:33 -- clear
     10:39 -- calibration; clear
     11:19 -- alto spikes at 6 km (hence lower PMT voltages), cirrus spikes at
          10 km
          WMO code : M=5; plainly visible by moonlight
     11:57 -- alto spike at 6 km; 2nd layer 6.5 to 8.5 km
          WMO code : M=5
     12:05 -- PMT voltages reduced to eliminate P channel saturation; cloud now
          6 to 9 km
     12:19 -- had to reduce voltages again; cloud rapidly moving lower, base
          now at 5 km, top about 8km; lots of structure, multiple peaks
     12:21 -- PMT voltages down again . . .
     12:28 -- and again . . .
     12:32 -- and AGAIN!!!
     12:51 -- an enormous altostratus bank appears to be settling in for a
          while
          WMO code : M=1
     13:10 -- base = 4 km, top = 7 km
     13:46 -- PMT voltages reduced to eliminate P channel saturation; base = 4
          km, top = 6 km
     14:07 -- increasing intensity of return at 5.5 km leads to a further
          reduction of the P PMT voltage
          WMO code : M=7
     15:15 -- P channel return has subsided a bit, PMT voltage up slightly
     15:21 -- cloud re-intensifies, P PMT voltage returned to previous level;
          altostratus 4 to 6 km with intense embedded peak @ 5 km
     15:38 -- base = 3 km, top between 5.5 and 6 km
          WMO code : M=2
     15:45 -- sky is completely overcast

11-23-91
     an official standing down day; no acquisition attempted; sky is mostly
     clear, with scattered to occasionally heavy "cotton-ball" cumulus

11-24-91
     22:20 -- reports of subvisual cirrus from Coffeyville, but clear overhead
          here, with lots of visible cirrus to the west
     23:00 -- momentary blip at 9 km, subvisible for sure
     23:16 -- visible cirrus at 8 km

11-25-91
     00:03 -- cirrus or altostratus?  Unknown, can't see it (dark); base just
          over 6 km, top about 7.5 km
     00:29 -- voltage reduced to prevent P-saturation at 6 km
     00:30 -- PMT voltage down again; base now 5 km, beak between 5.5 and 6 km,
          top at 8 km
     00:40 -- another voltage reduction as the cloud return continues to
          intensify
     00:57 -- yet another PMT voltage reduction
     01:15 -- base = 3.5 km, top = 8 km
     01:36 -- strong P peak at 4 km coupled with weak S channel response
     01:46 -- lidar pointing angle = 5 degrees; returns appear to have almost
          identical shapes, whereas they were often wildly different pointing
          at 0 degrees
     01:55 -- returns continue to rise and fall in concert
     02:02 -- back to 0 degree pointing angle; getting some P saturation
     02:05 -- PMT voltages reduced; base = 3km, top = 6.5 km
     02:09 -- 4 km P channel spike is back, with no corresponding feature in
          the S channel
     02:16 -- voltages reduced; base = 3 km
     02:41 -- voltages reduced; base now just above 2 km
     02:52 -- pointing angle = 5 degrees
     02:58 -- pointing angle = 0 degrees; P channel is hugely saturated; cloud
          now 2 to 4 km
     11:41 -- base = 2 km; cloud appears to be altostratus translucidus (can
          see the moon shining through . . . )
          WMO code : M=1
     13:06 -- WMO code : M=1
     14:36 -- still socked in by stratus; base = 2.5 km
     21:10 -- PMTs gated on about 4 km, above a very intense altocumulus layer
          @ 3 km
     21:45 -- weak, intermittent cirrus layer at 9 km; alto layer is breaking
          up, passing over
     22:53 -- heavy alto layer returns, destroys all signal above PMT gate

11-26-91
     10:47 -- strong cirrus return, 6.5 to 8 km; huge ring around the moon;
          visible cirrus patches are scatter all about; lots of stars also
          visible
     11:04 -- clearing
     11:53 -- clear
     12:26 -- laser energy monitored; records 377-396 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     12:44 -- calibration; clear
     13:21 -- calibration; clear
     13:53 -- big contrail spike, 9.5 km; amps switched to 5 MHz
     15:55 -- clear, and has been for a while; lots of cirrus to the northwest,
          which should get here eventually . . .
     16:58 -- cirrus at 10 km
     19:21 -- cirrus signal steady since 16:58; base slowly moving lower, now @
          8 km, cloud just over 2 km thick
     19:40 -- solid cloud bank, 7 to 10 km
     19:48 -- voltages reduced; base = 7 km, peak = 7.5 km, top around 10 km
     20:33 -- base = 6 km, peak = 6.5 km, both channels threaten to saturate
     20:40 -- getting tremendous variations in cirrus ceiling height, base
          varies between 5 and 7 km
     20:49 -- getting harassed by a thin, intense peak in the pre-PMT region,
          about 1+ km
     21:13 -- base just over 5 km, apparent cloud top at 9 km
          WMO code : M=7
     22:04 -- altostratus, 5 to 8 km, with light but noticeable pre-PMT cumulus
          at 1 km

11-27-91
     13:20 -- turned on for a moment to check conditions, as this is supposed
          to be an ops period.  Ceiling less than 1 km, with thick gray
          overcast.  No data recorded . . .

11-28-91
     14:54 -- its Thanksgiving Day, and only us turkeys are working . . . we've
          got cirrus, 10.5 to 11.5 km; we've also got a roaring breeze, and a
          covering of low lying cumulus that makes cirrus acquisition
          incredibly difficult
          WMO code : L=2, H=4(?)
     15:02 -- cirrus moving lower, base now 9.5 km, top just under 12 km;
          cirrus acquisition is intermittent at best
     15:30 -- cirrus spikes at 9 and 11 km, clear in between; cumulus rapidly
          increasing in coverage and intensity
     18:41 -- cirrus, 10 to 11.5 km; contrails overhead too
          WMO code : L=2, H=1,5
     19:10 -- PMT voltages up for increased resolution, everything comfortably
          on scale; cirrus 8 to 11 km
          WMO code : H=5
     19:34 -- quick voltage reduction due to saturation in both channels at 9
          km; cirrus steady 8 to 11 km, still besieged by the occasional cotton
          ball of cumulus
          WMO code : L=1, H=7
     21:06 -- could be trouble soon, as cumulus, 1 to 2 km, are invading the
          area
          WMO code : L=1, H=1
     21:22 -- two distinct layers, 7.5 and 10 km
     21:30 -- 8 to 11 km cirrus
     22:16 -- another wave of cumulus has come to interfere with the cirrus
          signal; 3 distinct cirrus peaks now, centered at 8, 9.5 and 11 km
     22:58 -- cirrus 8 to 11 km, with tons of structure; truly awesome cirrus
          returns
     23:01 -- resurgent cumulus crushes all signal after PMT gate

11-29-91
     00:01 -- cirrus, 7 to 11 km; tremendous interference from low, strong
          cumulus
     13:10 -- scanned with the lidar for 12 to 15 minutes, no data recorded; a
          huge bank of stratocumulus (base just over 1 km) is overhead, and
          there's no way to penetrate it

11-30-91
     14:22 -- mean, low, black stuff @ 1 km; altocumulus, base 3.5 to 4 km
          WMO code : L=5, M=8
     14:31 -- both channels ringing off the back of the 4 km feature
     14:51 -- altocumulus base = 3.5 km
          WMO code : L=8, M=8

12-01-91
     17:40 -- an official standing down day . . . good thing, too . . . we've
          got sleet and/or freezing rain coming down

12-02-91
     A repeat of yesterday, more or less; freezing rain throughout the morning,
     heavy overcast all afternoon; no data taking even attempted . . .

12-03-91
     03:14 -- testing of half-wave plate rotator, records 1-160 [ Note: these
           records are NOT part of the archive ]
     03:14 -- calibration; clear
     03:32 -- clear

12-04-91
     14:19 -- faint cirrus at 12 km
          WMO code : H=1
     14:58 -- amplifier bandwidth switched to 5 MHz; cirrus thickening, 9 to 10
          km
     15:10 -- a mixed bag of cirrus, fibratus and uncinus
     16:18 -- big cirrus spike at 9.5 km
     17:29 -- clear
     18:00 -- cirrus, near subvisible, 12 km
     19:14 -- continuing small, thin peak at 11 km
     19:43 -- clear
     20:12 -- subvisual cirrus intermittent at 13 km; very faint return,
          extremely thin
     21:12 -- persistent subvisual cirrus, 12.5 to 13 km
     22:04 -- clear, has been for a while, though subvisual cirrus still makes
          brief, sporadic appearances
     22:54 -- clear

12-05-91
     00:45 -- thin cirrus at 11 km
          WMO code : H=1
     01:30 -- clear
     02:42 -- clear
     02:48 -- cirrus, 11 km
     02:50 -- amps to 5 MHz
     03:27 -- reduced P PMT voltage to avoid saturation in cloud peak; cirrus
          steady, 10.5 to 12 km; S channel rings exiting cloud top
          WMO code : H=1
     05:03 -- PMT voltages increased; no saturation; cirrus steady, 10.5 to 12+
          WMO code : H=1
     05:34 -- cirrus 9.5 to 12.5 km with peak at 10.5; very active cloud, lots
          of structure
     05:50 -- cirrus 9 to 12.5 with huge peaks, 9 to 10 km, both channels
     05:56 -- lidar pointing angle = 5 degrees; two layers now; sharp spike at
          9 km, second layer 11 to 12.5 km; neither feature is very strong
     06:13 -- lidar pointing angle = 0 degrees; peaks at 9+ and 10+ km, with a
          solid mass extending 11 to 12.5 km; weakly scattering, a very puny
          bunch of clouds
     07:01 -- 2 layers, 8.5 to 9.5 km and 11 to 12 km
     07:37 -- 2 layers, 8.5 to 9.5 km and 10.5 to 12.5 km
     08:22 -- voltages reduced; cirrus 8 to 19 km, with a tiny bump at 11 km
     08:27 -- cirrus 8 to 12 km, with 3 distinct peaks
     08:38 -- lidar pointing angle = 5 degrees; the wind is howling!!
     08:42 -- cirrus 8 to 9.5 km and 10+ to 12 km
     08:48 -- 10 km layer has vanished
     09:54 -- clear
     10:17 -- back to vertical pointing, lidar pointing angle = 0 degrees;
          still clear
     17:03 -- cirrus 11 to 12 km
     17:53 -- still getting cirrus in the 10 to 12 km range, but the cloud
          return is very weak
     18:07 -- layers at 10 and 11 km, each about 1/2 km thick
     18:19 -- solid cirrus layer, 10 to 12 km
          WMO code : H=4
     18:57 -- looks like we've got a small break-away base moving lower, around
          9 km; main cloud mass is 11 to 12 km
     19:46 -- continuous cloud mass, 10.2 to 12.2 km
     20:48 -- cloud bank moving lower, thickening and intensifying, base = 9.5
          km, top = 13 km
     21:18 -- laser energy monitored; records 3207-3226 acquired with receiver
          and/or transmitter off-line, no signal recorded [ Note: these records
          are NOT part of the archive ]
     21:33 -- calibration
          WMO code : H=4
     21:50 -- cirrus overcast; cirrostratus designation may be a bit premature
          for the patch of sky directly overhead, but it does accurately
          reflect general conditions; cloud base is just over 9 km, top at
          about 12.5 km
          WMO code : H=7
     22:09 -- had to reduce PMT voltages due to P saturation at 9.75 km; cirrus
          remains 9 to 12.5 km
     23:20 -- base = 9 km, top almost 13 km
          WMO code : H=7

12-06-91
     00:00 -- base just under 9 km, top about 13 km; cloud intensity has
          subsided considerably since late afternoon
     00:54 -- cirrus 8.5 to 13 km
     01:10 -- lidar pointing angle = 5 degrees; data looks like just tilting
          the lidar gives us an incredible drop in laser output . . .
     01:35 -- must have banged up the second harmonic generator going into tilt
          mode; tried a little angle tuning, made a substantial improvement in
          signal levels; base remains @ 8.5 km, top @ 13 km
     01:50 -- halt in operations to off-load data from hard disk to optical
          disk
     02:42 -- ops resume; single layer 8.8 to 12 km
     03:41 -- main cloud 8.8 to 11 km; also a small, thin peak at 12 km; stars
          are visible through the clouds
     05:00 -- 1st base down to 7.9 km, 1st top about 11 km; 2nd base at 11.5 km
          with top at 12.4 km
     06:11 -- 2 layers, 7.5 to 10 km and 11 to 12.5 km; stars are plainly
          visible
     07:24 -- 2 layers still, but upper feature has been steadily fading
     07:39 -- single layer, 7 to 9 km
     09:00 -- PMT voltages increased; cloud intensity considerably weaker,
          still located 7 to maybe 10 km
     09:07 -- now only a single small peak @ 8 km, mostly in the S channel
     09:12 -- clear
     09:20 -- slight cloud return, 8.5 to 9.5
     09:43 -- clear
     10:11 -- cirrus spike at 9.5 km
     10:33 -- major voltage reduction due to big spikes, both channels, 9.5 and
          10.5 km
     10:48 -- spikes gone, all that's left is a tiny tit at 8 km
     10:55 -- 10 km spike returns
     11:00 -- clear
     11:23 -- cirrus, 9 to 10 km
     11:58 -- we've got P channel peaks close to saturation at 9 km . . . and
          you can still see tons of stars
     12:16 -- clear again
     12:46 -- intense peaks at 7.5 and 8.5 km
          WMO code : H=4
     13:00 -- possible P channel ringing exiting the cloud; definite S channel
          ringing
     13:07 -- heavy cirrostratus, 7 to 9 km
          WMO code : H=7
     14:29 -- cirrostratus, 6.5 to 9 km
          WMO code : H=7
     14:53 -- 2 layers, 1st at 6 to almost 8 km, 2nd at 9 km
     15:42 -- base occasionally drifts down to 5 km, generally stays at about 6
          km; apparent top varies between 8 and 9 km
     15:54 -- base at 5 km, top at 8+ km
     16:12 -- halt in operations to off-load data from hard disk to optical
          disk
     17:28 -- ops resume; altocumulus layer @ 7 km
          WMO code : L=1, M=5
     18:01 -- mostly clear, with only a tiny tit at 6.5 km
     19:15 -- altocumulus at 7 km, approximately 0.3 km thick; mackeral sky
          WMO code : M=5
     20:00 -- clear

12-07-91
     19:37 -- lots of scattered "cotton-ball" cumulus, but clear overhead
          WMO code : L=1




----------------- Section 7 : References and Other Documents -------------------


As of July 1994, the Langley Research Center Cloud Lidar group has submitted
the following files to the FIRE archive:

    filename                                       description
  --------------------------------------------------------------------------
   LARCINFO.4U          An ASCE file containing a synopsis of the FIRE IFO 2
			 LIDAR activity at Parsons, Kansas.  This file
			 accompanied the GIF lidar data catalog images to
			 explain the measurements presented.
   LARCINFO.GIF         This is an image file which contains a graph of the
			 time that the Parsons lidar was operating and two
                         equations referred to in the data explanation
                         contained in LARCINFO.4U .
   LSxxxxxx.GIF         The Attenuated Scattering Ratio for all lidar data
                         collected at Parsons during FIRE IFO 2.  File naming
			 is explained in Section 2.
   LDxxxxxx.GIF         The Depolarization Scattering Ratio for all lidar data
                         collected at Parsons during FIRE IFO 2.  File naming
			 convention is explained in Section 2.
   LARCINFO.DOC         The users guide corresponding to the binary data set.
   LARC_DOC.GIF         An annotated image file showing a lidar return which
			 explains the effect of the variable time delay used
			 in bringing the PMT's to full sensitivity.
   MMDDYY_CI2_LRC_       All of the lidar data collected at
	LIDAR.BIN        Parsons, Kansas during FIRE IFO 2 by
			 Langley Research Center personnel.  Data is pc
			 binary.  Naming convention is MMDDYY-> Month,
			 Day, Year data were acquired, CI2-> FIRE
			 Cirrus IFO 2 , LRC-> Langley Research Center,
			 LIDAR, BINary data.  The data file name is
			 twenty characters long with the date coming
			 first.
   LARCINFO.ASC         The users guide corresponding to the asce data set,
			i.e. this document
   MMDDYY_CI2_LRC_       All of the lidar data collected at
	LIDAR.ASC        Parsons, Kansas during FIRE IFO 2 by
			 Langley Research Center personnel.  Data is standard
			 asce.  Naming convention is MMDDYY-> Month,
			 Day, Year data were acquired, CI2-> FIRE
			 Cirrus IFO 2 , LRC-> Langley Research Center,
			 LIDAR, ASCe data.  The data file name is
			 twenty characters long with the date coming
			 first.


REFERENCES
   Penndorf, Rudolf:"Tables of the Refractive Index for Standard Air and the
    Rayleigh Scattering Coefficient for the Spectral Region between 0.2 and
    20.0 microns and Their Application to Atmospheric Optics", Jour. Opt.
    Soc. Amer., vol. 47, no2, Feb.1957, pp.176-182.

A description of the use of the attenuated scattering ratio to obtain cloud
optical extinction may be found in:
   Alvarez, J.M and M. A. Vaughan:"Numerical Calculation of Cloud Optical
    Extinction from LIDAR", OSA Proc. on Inaugural Forum for the Research
    Center for Optical Physics, Sept. 22-23, 1993, A.P. Maclin editor,
    vol 19, pp 90-95.

A description of the lab experiments linking the lidar depolarization ratio
to characteristics of artificial cloud particles is found in:
   Sassen, Kenneth:"Depolarization of Laser Light Backscattered by Artificial
    Clouds", Jour. Appl. Met., vol 13, Dec. 1974, pp 923-934.