CAMEX-3 Scanning Raman Lidar (SRL)

Table of Contents

Introduction

The Scanning Raman Lidar (Light Detection and Ranging) is an instrument designed to determine the composition and vertical distribution of several atmospheric constituents, specifically water vapor and aerosols. SRL is generally used as an atmospheric profiler, where the lidar is aimed upwards returning a vertical cross-section of the atmosphere.

The SRL was located on Andros Island during the CAMEX3 campaign, and operated nearly every night (exceptions noted) throughout the period 6 Aug-20 Sep 98.

Scattering: Anyone who has driven at night in the fog has experienced firsthand the effect that particles have in scattering a light source. In this case, the source is the headlight, and the particles are the water droplets making up the fog. As the light hits the fog, it reflects and bounces off in all directions, but some is redirected back at the driver. Because the droplets in this example are very large with respect to the wavelength of light, the cause for the scattering is due to the optical geometry of the droplets, that is , the droplets act as little lenses actually reflecting the light. Another type of scattering is called Rayleigh scattering, and it's cause is due to an interaction of the electromagnetic properties of the light and the atmospheric molecules. Rayleigh scattering occurs when the particle size is an order of magnitude less than the wavelength of light incident upon it. Blue sky is due to the Raleigh scattering of sunlight by the molecules in the atmosphere. If the particle size is on the order of the incident light (+ about an order of magnitude), Mie scattering is the result. Found in this regime is scattering of light by smoke, haze, smog and dust.

In the 1920s, C.V. Ramen, an Indian physicist, discovered what would become known as Raman scattering. He noted that light incident upon polyatomic molecules was inelastically scattered, as is the case with either Rayleigh or Mie scattering. Furthermore, the amount of scattering is constant for each particular molecule, and the shift in wavelength between the incident light and the Ramen scattered light depends on that type of particular molecule. Thus, if the incident wavelength is known, then the Raman scattered light is also known. If one then measures the backscattered Raman light, one can determine the amount of that particular molecule present as the two are directly related.

With the SRL setup, the lasers have been selected to provide Raman scattering from oxygen, nitrogen and water vapor. The water vapor mixing ratio is computed from the ratio of backscatter from water vapor to nitrogen.

Aerosol scattering and extinction profiles are also produced for the troposphere and lower stratosphere. The ratio of laser return (backscatter) to the nitrogen return is indicative of (Rayleigh + Mie) over (Rayleigh) scattering. A ratio of 1.0 indicates pure molecular scattering-- no aerosols. The aerosol extinction profile is derived from the derivative of the log of the nitrogen signal.

The SRL consists of two lasers providing the light source. The XeF excimer laser produces light at 351nm, and the Nd;YAG lases at 355nm. As seen in the diagram, light is produced (in this case) by the XeF laser. The light is then sent into the atmosphere via a scanning mirror. The scattered signal (whether it be Mie, Raleigh, or Raman) is collected at the mirror, concentrated in the telescope and then sent to the detectors. The detectors are photomultiplier tubes which enhance the weak backscatter signal from the aerosols and molecules.

All of this equipment is located inside of one environmentally controlled trailer. This trailer is then connected via cables to a second environmentally controlled trailer (see image above) containing data acquisition and analysis equipment.This allows the SRL to be readily transportable.

There are three kinds of files produced by the SRL. Two of these are data files, one each for aerosols and the other for water vapor. The third is an image (.gif) file.

Water vapor data files are named:

 98218.srl-wv-v1.txt

where

98218 is in the format yyddd, srl indicates the dataset, wv for water vapor, v1 means version 1 of the reduction software, and txt indicates the file type.

Similarly, aerosol files are named:

98218.srl-ae-v1.txt

where the only difference is ae indicating aerosol instead of watervapor.

Image files are time series representations of data files. The file 98218.srl-wv-v1.gif represents all data from the 98218.srl-wv-v1.txt file.

The format as indicated by the file name is in ascii. The example below shows the first dozen lines of the file 98218.srl-wv-v1.txt.

160 = number of files

    0     3.2044 = file,time

    2   106 = number of header, data lines

dummy line

           Range(km)    height(km)        w (g/kg)       err (g/kg)       res (km)

    0     1.0000e-002 1.0000e-002   1.8280e+001 9.9990e+003 1.0000e-002

    1     2.3115e-001 2.3115e-001   2.0423e+001 4.0566e-002 2.2115e-001

    2     2.9198e-001 2.9198e-001   2.0498e+001 7.2385e-002 6.0830e-002

    3     3.5281e-001 3.5281e-001   2.0803e+001 1.1927e-001 6.0830e-002

    4     4.1364e-001 4.1364e-001   1.9992e+001 8.4092e-002 6.0830e-002

    5     4.7447e-001 4.7447e-001   1.9985e+001 7.4280e-002 6.0830e-002

    6     5.3530e-001 5.3530e-001   1.9392e+001 1.4833e-001 6.0830e-002

Each of these data files are made up of all of the profiles which were captured during that observation period. The number of profiles created (called files) is shown on the first line of the data file. Thus, on this date (98218 = 6 August 98) there were 160 profiles made. The second line of data indicates the particular profile number (in this case 0 ) and the time in UTC of this profile. In this case, the time is given in digital hours, and 3.2044 converts to 03:12:26 UTC.  Line three gives the number of header lines following, as well as how many data lines follow the header lines- here it indicates two header lines follow, the dummy line and the actual header line followed by 106 data lines. These correspond to the number of layers into which the vertical structure is stratified.

Data follows in this set for 160 lines and are organized into six columns. 1- bin number and has no header. 2- range from lidar in km (1.0000e-002= 0.001km). 3- altitude from lidar in km (equals "2" if lidar is pointed vertically). 4-water vapor mixing ratio in g/kg. 5- error in the water vapor mixing ratio. 6- vertical resolution of the data in km.

Data from 98218.srl-ae-v1.txt is identical for the first four lines, and begins to vary with the headings for the columnar data. Headers for the aerosols are as follows: 1- bin number and has no header. 2- range from lidar in km (1.0000e-002= 0.001km). 3- altitude from lidar in km (equals "2" if lidar is pointed vertically). 4- aerosol scattering ratio. 5- error in the scattering ratio. 6- vertical resolution of the data in km.

Errors are calculated using Poisson statistics (error equals the square root of the counts) and is propogated through the equations to obtain the given error.

IMAGERY: Images are in .gif format, and as mentioned, represent a time series of the entire data series for that date. There are three images, and they are named in a simililar format. 98218.srl-ae-v1.gif is an image of aerosol data, and 98218.srl-wv-v1.gif  is an image of the water vapor data. The third in the series is named 98218.srl-co-v1.gif which is a comparison of lidar data taken at a specific time to radiosonde data taken at the same time. An example of which is shown.

To order these data or for further information, please contact:

Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/