Dale M. Simonich¹, Barclay R. Clemesha¹

¹Aeronomy Department, Instituto Nacional de Pesquisas Espaciais – INPE

Two things that can be done with the atmospheric density derived from the backscattered light from a LIDAR are:

1. To determine the atmospheric temperature profile by assuming hydrostatic equilibrium and the perfect gas law for the atmosphere.
2. Assuming a standard atmosphere one can derive the scattering ratio profile (ratio of actual scattering from the atmosphere to that which would result from the assumed atmosphere) resulting from Mie scattering by aerosols suspended in the upper atmosphere.

 

The problem is that with aerosols present one cannot derive the atmospheric temperature through the aerosol region and below unless you know the aerosol distribution and properties which you can’t (unless you have an independent measurement) because you have one measurement and two unknowns. We will describe a system for obtaining an independent measurement of the atmospheric scattering from the region of aerosols which does not include the Mie scattering contribution. The technique uses the radiation reflected from the interference filter for the elastic scattering channel. This radiation contains the Raman scattering from the molecular constituents of the atmosphere which do not contain any Mie scattered radiation. The strongest of these scattering bands is the pure rotational Raman scattering band. Its only problem is that the Stokes and anti-Stokes bands are very close to the elastic scattered radiation which requires an interference filter with a very sharp cutoff in the direction of the elastic wavelength. The rotational Raman band consists of very many lines from oxygen and nitrogen whose intensity in the band varies with the temperature of the atmosphere. While it is possible to select a filter bandwidth which would have a very small signal variation over the temperature range of the atmosphere, the cost of such a filter is prohibitively high. We chose a filter bandwidth which gave a reasonable cost and developed an iterative scheme starting from an assumed temperature profile for the region, calculating the received signal using the Raman scattering from the atmosphere at that temperature and the filter transmission supplied by the manufacture and calculating the temperature profile using the first item above which is then used as the starting point for the next cycle. A simulation of the results is presented starting from the measured temperature profile obtained from a meteorological sounding and an assumed ideal scattering profile.