Multiple scattering by aerosols as seen from CALIPSO - a Monte-Carlo modelling study.

Michael Kahnert,Ronald Scheirer

doi:10.1364/oe.27.033683

Abstract

The impact of multiple scattering (MS) by aerosols on satellite-borne lidar measurements is studied by Monte-Carlo radiative transfer simulations. A total of 48 aerosol scenarios are considered. We find that the frequently used MS correction factor can be parameterized as a function of aerosol size and aerosol optical depth. Its dependencies on vertical distribution and total optical depth can be treated as a random error. We illustrate the use of our parameterization by considering an episode of high sea salt concentrations over the ocean. Neglecting MS, or using a constant value of the MS correction factor, can introduce a negative bias in the computed backscattered power that exceeds the random error in our approach.

Highlights

Multiple scattering poses a challenge to the interpretation of Lidar [1] and Radar measurements [2]
It was found that (i) irregular multiple scattering can be neglected; its fractional contribution to the returned energy was always less than 0.005. (ii) The contribution of regular multiple scattering to the detected signal mostly depends on the effective radius of the aerosols and the optical depth
We proposed a parameterization of the multiple-scattering correction factor η as a function of effective radius and optical depth

Summary

Introduction

Multiple scattering poses a challenge to the interpretation of Lidar [1] and Radar measurements [2]. The return signals are commonly interpreted by use of the lidar equation, which is an approximate description of the radiative transfer problem based on neglecting the effect of multiple scattering. The validity of this approximation depends on the geometry of the observing system, the density of the scattering medium, and the size of the scatterers. The field of view (FOV) of the instrument’s telescope and the beam divergence need to be sufficiently small Even when these conditions are met, for space-borne lidar instruments, the beam footprint often has a diameter comparable to the photon mean free path. In such case multiple scattering cannot be neglected (e.g. [3])

Methods

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Discussion

Conclusion