Cloud Lidar System

1. General Information

The CLS instrument was deployed under the direction of Principal Investigator Dr. James Spinhirne of NASA/Goddard Space Flight Center, to acquire atmospheric measurments during the intensive observation period of the Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The NASA component of the TOGA COARE Campaign, while contributing to overall COARE objectives, emphasized the scientific objectives associated with the Tropical Rainfall Measuring Mission (TRMM) and NASA's cloud and radiation research programs.

TOGA COARE is an international research program aimed at investigating the interactions between the atmosphere and ocean in the warm pool region of the Western Pacific. The field experiment phase of the program took place from November, 1992 through February, 1993 and involved the deployment of research vessels and buoys, ship and land based Doppler radars, multiple low and high level aircraft equipped with many different types of airborne sensors, as well as a variety of surface based instruments.

The TOGA COARE Field Campaign covered a 4-month period from November 1, 1992 through February 28, 1993. The NASA aircraft participated during January and February of 1993.

The TOGA COARE region covered roughly 140E --&GT 180E and 10N --&GT 10S, however the NASA aircraft flew several missions as far as 20S to investigate hurricane Oliver.

2. Instrument Information

The Cloud Lidar System (CLS), which was operated from the left ER2 superpod, measured the backscatter cross-sections of cloud and aerosol particles at 1.064 and 0.532 microns. It was used in the TOGA/COARE experiment primarily to profile clouds below the flight level of the aircraft which was typically 18.0-20.5 km. In cases where the cloud optical thicknesses were small, boundary layer aerosol backscatter signals were detectable. The 0.532 micron lidar return was split into signals that were parallel and perpendicular to the outgoing laser radiation, thereby providing polarization sensitive data principally for cloud particle phase state detection. The cloud lidar provided information on the internal vertical structure of optically thin clouds which aids in the determination of the overall influence of such clouds on the radiative balance in both the shortwave and longwave portions of the spectrum. Another principal application that was planned for the ER2 CLS was the study of radiative heating and cooling rates for tropical cirrus. The CLS provided a detailed picture of internal cloud structure which aided in the interpretation of visible, infrared, and microwave radiometric data when they are applied to the determination of radiative fluxes and forcing. The lidar signal was useful up to a maximum optical thickness of 3 to 4. It provided the locations of cloud layer boundaries, both vertically and horizontally, which was useful in most types of cloud studies.

The CLS can provide information on cloud particle characteristics. Depolarization of the laser pulses detected at the green wavelenghts can reveal water phase state of the particles. CLS backscatter information, when combined with multispectral radiometric observations, can be used in the determination particle size and concentration within the clouds.

The lidar also gathered data on boundary layer heights and aerosol backscatter cross-sections. From the distribution of cloud base heights in and around the marine boundary layer, an estimate of the LCL (lifting condensation level) can be obtained. When combined with the sea surface temperature this could provide a measure of the moisture content. From the intensity of the lidar return, estimates of cloud liquid water are possible.

The ER2/CLS system was one of several instruments flown aboad the ER2 during the TOGA/COARE field campaign. It was operated by the ER2 pilot, the sole occupant of the aircraft, whose only control during the data acquisition was system power and two major component on/off switches. The laser used in the CLS system was a Nd:YAG which produces the primary 1.064 um(infrared) pulse. A doubling crystal was used to produce another signal at 0.532um(green). The parallel and perpendicular polarized components of the 0.532um signal were detected and stored in separately. Thus, each laser pulse produced three profiles of atmospheric backscatter. The profiles were very close to nadir and extended from the aircraft to the earth's surface. The laser operated at 10 Hz which gave a 20 meter minimum horizontal. The minimum vertical resolution was 7.5m. The system employed incoherent signal detection using solid state detectors. It used logarithmic amplifiers and 8 bit A/D converters which allowed signals with a dynamic range of about five orders of magnitude to be recorded. The data system acquired navigation data (DADS) from the ER2 in real time and prepended it along with engineering data to each lidar return data record as it was written to 8 mm tape.

Some processing was done to prepare the CLS profiles for public archiving. First, the 8 bit logarithmic digital data are converted back into signal proportional values. The empirically determined constant offset component(background) of the signal was computed and subtracted from the total signal for each sample. The values were then multiplied by the square of the range and divided by the laser output energy which yielded a value proportional to the atmospheric scattering backscatter at the point which generated the signal. In order to increase the signal to noise ratio and reduce the quantity of data to be stored, pairs of contiguous samples in a normalized profile were averaged and ten reduced resolution profiles were averaged to produce one CLS profile record for storage. The resulting profile record was referred to as the CLS normalized averaged profiles and each profile represented one second of time.

Conversion of the normalized CLS signals to physical backscatter units can be accomplished by calculating profiles of the ratio of the normaized signal to computed Rayleigh backscatter coefficient in the clean regions of atmosphere within the region 5Km below the ER2. Average values of these ratios,referrd to as calibration profiles, one each for the green and IR signals, would be calculated from selected regions in which the aerosol content is found to be small and not a signifcant contributer to the backscatter signal. The average values are referred to as the backscatter calibration coefficients. The backscatter coefficient profiles would then be computed by dividing the values in the normalized averaged profiles by the empirically determined calibration constants.

Calibration profiles were useful for finding horizontal cloud surface boundaries. Ideally, in a perfectly aerosol free atmosphere, the calibration profile values should be constant from the aircraft's level to the earth's surface. Therefore, some threshold value was determined by inspection which indicated the presence of clouds to some arbitrary tolerance. By searching each calibration profile for values which exceeded the threshold, the altitudes of the demarcations between clouds and clear air were determined and stored. The location of the earth's surface in the calibration profile was found in a similar manner.

3. Data Organization

The ER2 TOGA/COARE field operations took place generally over the ocean to the north and east of the northeastern part of Australia. The time of day of most of the flights along with the time zones of this region resulted in ambiguity when time was used as an identifier of data acquisition flights. Therefore, the ER2 sortie was selected as a unique identifier in data file names. The table below shows the dates and times (in UTC) of the ER2 sorties. This table should be used as a reference to convert the data file names based on sortie number to a day and time point of view.

3.1 General Characteristics

The CLS data currently comprise two types of file:

a) ASCII Data files such as 93053.cloudbounds which contain the cloud boundary information. They have an approximate volume of 20Mb and file size approach 3 Mb.

b) Associated GIF Images such as 93053.gif of backscatter cross-section. They have an approximate volume of 2Mb and range up to 200Kb.

3.2 ASCII Data Format

In 93066.cloudbounds, 93066 refers to the ER2 sortie number, a single unambiguous identifier of the each CLS data flight which occurred during Toga Coare. The time and navigation data are contained in the file itself.

 hh.hhhh    lat     lon   h(km)   roll n g  ctop1  cbot1  ctop2  cbot2

 19.4600  -17.52  147.96  17.31   0.04 3 1 16.425 15.990 15.210 14.820

 19.4603  -17.52  147.96  17.31   0.12 3 1 16.442 15.992 15.167 14.822




              Section beyond column 69

    ctop3  cbot3  ctop4  cbot4  ctop5  cbot5   h-surf

    5.670  5.490 -3.000 -3.000 -3.000 -3.000   -0.390

    5.687  5.477 -3.000 -3.000 -3.000 -3.000   -0.388



hh.hhhh.....universal time in decimal hour
    lat.....latitude in degrees
    lon.....longitude in degrees
      h.....ER2 pressure altitude in km
   roll.....ER2 roll in degrees
      n.....number of cloud layers detected
      g.....earths' surface detection flag
            0-> no surface detected;1-> surface detected
  ctop1.....top of cloud layer 1, in km
  cbot1.....bottom of cloud layer 1, in km
  ctop2.....top of cloud layer 2, in km
  cbot2.....bottom of cloud layer 2, in km
  ctop3.....top of cloud layer 3, in km
  cbot3.....bottom of cloud layer 3, in km
  ctop4.....top of cloud layer 4, in km
  cbot4.....bottom of cloud layer 4, in km
  ctop5.....top of cloud layer 5, in km
  cbot5.....bottom of cloud layer 5, in km
 h-surf.....location of earth's surface,
            based upon ER2 pressure altitude and
            CLS ranging

Notes: a)A maximum of five cloud layer boundary pairs
         are given even if more layers were detected.
       b)All vertical locations are base upon ER2
         pressure altitude and CLS ranging.

3.3 Image file format

The image file 93053.gif displays a cross-section image of the lidar data. The 93053 refers to the ER2 sortie number. The legend at the top of each image describes the data type, location and the duration. It is a good idea to use the image in conjunction with the cloud boundary file. This is especially important when there are thick clouds present. This makes identification of the lower cloud layer boundaries more difficult.

4. Data Access

4.1 Access to Cloud height files and associated gif images

The following table details start and end record information for various CLS data files from the ER2 and associated GIF images showing backscatter cross-section. The second to final column specifies the name of the ascii file and the final column the corresponding gif image. The ascii file format is explained in Section 3.2.

5. Quality Assessment

5.1 Data Validation

The ascii data files were validated using an interactive computer display on which the derived cloud boundaries were displayed simultaneously with cross sectional images of the lidar signal. Any inconsistencies became apparent by using this display method. The agreement was very good in most cases with the lidar penetrating multiple cloud layers in many cases. Sometimes the very high persistent tropical tropopause layer was not detected because its backscatter signal was below the detection threshold. Some false cloud layers were indicated, usually in the middle levels because of instrument noise. These false layers were characterized by a lack of horizontal and vertical extent. Layers underneath higher layers with a vertical cumulative optical thickness of 3 to 4 could not be detected. Finally, the earth's surface signal would not be detected under opticallly thick conditions.

6. Points of Contact

For information about or assistance in using any NASA/TOGA COARE data, contact

GES DISC User Services

Code 610.2

NASA Goddard Space Flight Center

Greenbelt, Maryland 20771

GES DISC User Services

301 614-5224

The Principal Investigator for the CLS instrument is:

Dr. James Spinhirne

Code 613.1

NASA Goddard Space Flight Center

Greenbelt, MD 20771, USA

Internet: James.Spinhirne@nasa.gov

(301) 286-9099 (voice)

Please send technical inquiries to Co-Investigator

William Hart

Code 613.1

NASA Goddard Space Flight Center

Greenbelt, MD 20771, USA

Internet: William.D.Hart@nasa.gov

(301) 286-4936 (Voice)

(301)-286-1762 (Fax)

7. References

TOGA COARE Mission Summary Reports,November 1993

Spinhirne, J.D. and W.D. Hart, Cirrus Structure and Radiative Parameters from Airborne Lidar and Spectral Radiometer Observations: The 28th October 1986 FIRE Study, Monthly Weather Review, Vol. 118, No. 11, 2329-2343, 1990.

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