Retrievals

The retrieval, or inversion process requires three main components:

  1. A measurement of the system under investigation. In this case, an ALI measurements of the atmosphere, typically a series of images at different wavelengths and polarization states stored as list of RadianceSpectralImage.

  2. A forward model capable of simulating the measurement of the system. For this work, this is a radiative transfer model coupled to an instrument Simulator.

  3. A method to update the state of the forward model. The ALI retrieval uses the Rodgers optimal estimation technique.

The forward model uses the state of the system to generate a simulated measurement. The state is then updated by comparing the measured and modelled results. This processes is repeated until the measurement and model are sufficiently close. The ALI retrieval uses the skretrieval package to implement the base elements of the retrieval.

Inputs

The ALI retrieval package implements two aerosol retrievals to perform these operations, with the ExtinctionRetrieval and LognormalRadiusRetrieval classes. As input, these classes take three important inputs:

  1. Level 1 Measurement A list of ALI measurements in the form of RadianceSpectralImage s. These can be any combination of wavelengths and polarization states.

  2. Instrument A list of ALIDualChannelSensor objects. Each instrument should map to a Level 1 Measurement and are used to generate the

  3. Geometry A list of OpticalGeometry objects that describes the measurement geometry of the Measurement objects.

Outputs

The output of these retrievals is a NetCDF4 file containing information on the atmospheric state and retrievals. Information is broken into 5 categories:

  1. level1 Input level 1 measurements including wavelength, polarization, exposure time, geometry, etc.

  2. optical_geometry

  3. retrieval Detailed information on the retrieval status and iterations:

    • state The state of the system including Jacobian, gain and covariance matrices.

    • vectors Information on the measurement vectors used to retrieve the atmospheric state.

  4. retrieved The retrieved state of the atmosphere, including aerosol, cloud and BRDF information.

  5. true The true state of the atmosphere, if known. This is present only when synthetic retrievals are being performed.

Retrieval Software Overview

These retrieval classes are wrappers around a series of classes that transform the measurements and atmospheric state Retrieval Components

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