LIDAR Summary

WET Labs is evaluating the feasibility of using space-based lidar to detect the depth of the oceanic mixed layer. Measurement and monitoring of the oceanic mixed layer depth on a global scale has important applications to understanding and predicting climate change and changes in marine ecosystems, global primary productivity, and air-sea fluxes.

Our objectives are to:

1.    Determine the relationship of the vertical structure of optical scattering and absorption and the mixed layer depth using our global data base of optical properties as well as other sources such as SeaBASS by examining the distribution of the appropriate backscattering to absorption ratios (bb/a) in various regimes such as central gyres, tropical regimes, subpolar seas, continental shelves, large semi-enclosed seas (e.g., Gulf of California).

2.  Further develop radiative transfer programs to calculate the time-dependent photon return as a function of lidar system parameters (footprint size, pulse size, wavelength, etc.) using known or estimated vertical distributions of bb/a obtained under objective 1.

3.  Examine the possibility of obtaining the vertical structure of the backscattering and absorption coefficients (and hence particle, chlorophyll, and CDOM concentration), from the signal if fluorescence, multiple wavelengths, and a passive sensor on the same spacecraft are taken into account.

4. Provide NASA with calculations of signal strength as lidar systems design parameters mature.

Our scientific approach is to examine existing data bases to obtain global overview of the global distribution of the vertical distribution of the backscattering and absorption coefficients. Using these vertical distributions we will determine the fraction of power returned as a function of depth from a space based lidar system. In order to do so we will expand a Modulation Transfer Function based radiative transfer program to include three dimensional effects and time dependence. The program addresses limited beam widths and does not use the plane-parallel assumption of other commonly used radiative transfer programs. This program will be able to determine the effects due to varying system parameters such as footprint-size, pulse length, receiver aperture, etc., as well as the effect of environmental parameters such as surface waves, vertical stratification of optical properties, bottoms, etc.

Secondarily we will calculate the return from lidar-stimulated chlorophyll fluorescence and Raman scattering. We will also examine the possibility of determining the vertical profiles of backscattering and absorption coefficients separately from such measurements.

The value of the results to the Oceanography Program will be an assessment of where and when the mixed layer depth can be determined using space-based lidar. The dependence of the accuracy of the mixed layer depth determination on system parameters will also be calculated. This study is a critical link in establishing the utility of space-based lidar systems for oceanographic measurements.