SARTA - the Stand-alone AIRS Radiative Transfer Algorithm

Introduction

The SARTA code provides a means to quickly calculate simulated AIRS radiances for a specified atmospheric profile, surface, and instrument viewing parameters. The complete SARTA package consists of three pieces: the SARTA source code, the SARTA database, and the RTP input/output library.

The RTP interface is explained in the user guide included with the RTP library package. The user guide is also available here:

GES DISC Version5 Processing Equivalent

The AIRS "version 5" processing at the GES DISC is making use of our AIRS-RTA fast forward model. The v5 processing corresponds to the GES DISC's "collection 005" AIRS products. The v5 RTA is an update of our v4 RTA with some new features and improvements. A stand-alone implementation of the v5 RTA is available here for download.

We made a small bug fix and added a new feature, but it is still very similar to the official version5 equivalent. New bug fix 31 Jan 2008.

GES DISC Version6 Processing Equivalent

The SARTA v108 code for AIRS version 6 processing is now available. It is still quite new, so it is possible there may be minor updates released in the coming months. For v6 processing, the RTA database has been expanded to handle the change to the entrance filter fringes in the SRFs pre-vs-post the November 2003 shutdown/restart of AIRS. Also, the RTA now has the ability to track to small drift in AIRS channel frequency over orbits, seasons, long term, and Doppler shift. These are all relatively small effects of a few ppm in frequency.

Note: the source code and datebase for 10 Feb 2012 are unchanged from the 23 Nov 2011 source and 21 Dec 2011 database except for the use of an updated database A/B weight table and corresponding minor changes to the source include files.

A new A/B weight table for the AIRS detectors was uploaded to the spacecraft on 21 January 2012, which changed the A/B weight of 133 of the 2378 channels. Most AIRS channels have two separate detectors (named A and B), and the A/B weight refers to which of those detectors are used to measure the radiance. The choices are A, B, or A+B. The A/B state of a channel strongly affects the noise properties, and also very slightly the channel center frequency. SARTA v1.08 uses a lookup table to track changes to the A/B weight of some channels versus time (for channel center frequency purposes only). However, the v6 PGE RTA currently ignores this effect.

Over the years we have developed numerous variants of the SARTA code and database to add other capabilities (clouds/aerosols/gases) and/or other instruments (CrIS, IASI, MODIS-IR, and others). These variants are not currently available to the general public.

Related Packages

The klayers package converts atmospheric profiles at discreet point levels into integrated slab layer profiles of the type used by SARTA. klayers uses the RTP I/O format, making it very convenient for use with SARTA.

klayers v2.05

MATLAB users who will be working with RTP files will definitely want to pick up our basic RTP I/O toolkit. Our older codes use RTP v105, while our AIRS v6 processing use RTP v201.

Tabulated AIRS Spectral Response Functions (SRFs) are available here. The file "srftables_051118v4.hdf" is the basic SRFs used for the AIRS v4 and v5 processing. The "srftables*Nov2003.hdf" files are used in the v6 processing to account for drift in the channel center frequencies as a function of time, and also for the shift in the relative positions of interference fringes pre/post the November 2003 shutdown of AIRS.

The kCARTA package provides a relatively fast way (a few minutes) to compute quasi-monochromatic transmittances, radiances, and jacobians.

Compiler considerations

Publications related to AIRS-RTA & SARTA

"An Overview of the AIRS Radiative Transfer Model", L. Larrabee Strow, Scott E. Hannon, Sergio De Souza-Machado, Howard E. Motteler, and David Tobin. IEEE Transactions On Geoscience And Remote Sensing, vol.41 No.2, February 2003.

"Validation of the Atmospheric Infrared Sounder radiative transfer algorithm", L. L. Strow, S. E. Hannon, S. De Souza-Machado, H. E. Motteler, and D. C. Tobin. Journal Of Geophysical Research, vol 111, D09S06, doi:10.1029/2005JD006146, 2006.

"Fast Forward Radiative Transfer Modeling of 4.3 um Non-Local Thermodynamic Equilibrium effects for the Aqua/AIRS Infrared Temperature Sounder", S. De Souza-Machado, L. L. Strow, S. E. Hannon, H. Motteler, M. Lopez-Puertas, B. Funke and D. P. Edwards. Geophysical Review Letters, 34, L01802 (2006), doi:10.1029/2006GL026684.

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