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.