Introduction

The Optimal Retrieval of Aerosol and Cloud (ORAC) is an optimal estimation scheme that determines aerosol and cloud properties from multispectral imagery. There are several components to the software:

1. The pre-processor takes a level 1B image from a satellite (an orbit or granule) along with other geophysical data to save a set of geolocated fields into several NetCDF files.
2. Those files are the inputs for the main processor, which performs the retrieval given some set of microphysical properties for a type of aerosol or cloud particle. This is called repeatedly, once for each type of particle you wish to evaluate. Various cloud, aerosol and ash particles are currently supported, as well as layers of one above another.
3. The post-processor merges multiple processed outputs (e.g. liquid and ice phase cloud) into the final product.
4. An optional broadband flux processor derives the top/bottom of atmosphere fluxes corresponding to the derived properties.

It can be installed directly as a binary or compiled from source. For the latter, you will require git, a version control package and a Fortran 2003 compiler. The code has been tested on the Intel, GNU and PGI compilers on Ubuntu 12.04-18.04 systems.

Each ORAC executable takes a single argument - a path to a driver file. A driver file specifies which inputs should be evaluated and specifies various parameters and settings. Python scripts have been written to automate much of that process and are the recommended method to run ORAC.

Contributions to the development of ORAC-CC4CL are welcome, and can be made using GitHub pull requests. For substantial or frequent contributions you can be added as a developer by emailing Don Grainger ([email protected]). Approval may take a few days. It is advised you join the ORAC developer's mailing list. To do so, create an account with http://www.jiscmail.ac.uk, go to http://www.jiscmail.ac.uk/devorac, click "Subscribe or Unsubscribe", select your desired preferences, click "Subscribe", and click the link in the confirmation email that you will eventually receive.

The main GitHub page for ORAC-CC4CL contains a number of folders. Broadly, the folder structure is:

• docs) Documentation, largely similar to that shown in the wiki.
• tool) Python scripts that assist in running the ORAC code and reading its outputs.
• config) Configuration files that define the compilers, flags, and libraries used to compile ORAC.
• patches) Local patches for the libraries used by ORAC.
• feedstocks) Scripts used to compile ORAC's dependencies.
• common) Routines which are common to all three components.
• pre_processing) The ORAC pre-processor source code. The main program is called orac_preproc.F90 and it's is executable orac_preproc.
• src) The ORAC main processor source code. The main program is called orac.F90 and it's executable is orac.
• post_processing) The ORAC post-processor source code. The main program is called orac_postproc.F90 and it's executable is orac_postproc.
• derived_products) Programs that evaluate the final ORAC product and produce subsidiary products from the ORAC output.