Extension of the constrained ratio approach to aerosol retrievals from elastic-scatter and high spectral resolution lidars

Christopher J Mcpherson,John A Reagan

doi:10.1117/1.jrs.10.036019

Abstract

A methodology is presented, by which atmospheric aerosol retrievals from a standard, elastic-scatter, lidar can be constrained by using information from coincident measurements from a high spectral resolution lidar (HSRL) or Raman lidar at a different wavelength. As high spectral resolution or inelastic-scattering lidars are now being incorporated coaxially into instruments with traditional, elastic-scatter channels at different wavelengths, a standard approach is needed to incorporate or fuse the diversity of spectral information so as to make maximal use of the aerosol measurements made from the elastic-scatter channel or channels. The approach is evaluated through simulation and with data from the NASA Langley Research Center Airborne HSRL instrument. The generality and extensibility of the method is also explored and discussed in the context of aerosol modeling.

Highlights

The problem of atmospheric aerosol retrieval from elastic-scatter lidar measurements has been approached in a number of different ways, each incorporating or imposing various assumptions and constraints to the underdetermined nature of the problem
Given the all-important lidar ratio, various mathematical solutions exist to the underlying differential equation to determine the aerosol backscatter and extinction based upon different boundary constraints that may apply in different contexts.[1,2,3,4]
Independent measurements of aerosol spectral optical depth can be used to complement lidar measurements and constrain the lidar solution to be consistent with the measured optical depth, in the context of an assumed spatially fixed lidar ratio at a given wavelength.[5]

Summary

Introduction

The problem of atmospheric aerosol retrieval from elastic-scatter lidar measurements has been approached in a number of different ways, each incorporating or imposing various assumptions and constraints to the underdetermined nature of the problem. The NASA Langley Research Center (LaRC) Airborne HSRL13 incorporates a high spectral resolution, constantly tuned 532-nm channel together with a conventional 1064-nm elasticscattering channel into a robust, reliable airborne system that has to date flown many hundreds of hours across the North American continent, offshore, and over the Caribbean, in numerous measurement campaigns, accompanied at times by a host of validating sensors.[14,15,16,17,18,19,20] We propose a retrieval scheme, basically an extension or modification of the CRAM technique (called E-CRAM), that attempts to take advantage of the added information available from HSRL at 532 nm in the dual-wavelength retrieval.[21] Since range-resolved profiles of aerosol backscatter and extinction are available at 532 nm via HSRL, the retrieval component of E-CRAM applies just to the 1064-nm elastic-scatter data. The technique is conceptually somewhat similar to the dual-wavelength retrieval approach taken by Sasano and Browell,[22] except that the 532-nm aerosol extinction and backscatter profiles are known, reducing the number of degrees of freedom in the retrieval to one from two, resulting in more stable and reliable solutions

Retrieval Method

Proof-of-Concept Simulations

Findings

Conclusions