About the Different Retrieval Systems

Geophysical quantities (Level 2 and Level 3 data) retrieved from AIRS radiances (Level 1 data) are produced by three different retrieval systems. These are Version 7 (V7) of the AIRS-team retrieval, CLIMCAPS-Aqua (Version 2), and JoSFRA. Brief descriptions of these systems are given below but further descriptions with references to more details are given in the Overview of the AIRS Mission and Algorithm Theoretical Basis Documents (available from the AIRS Documentation Page). Data from the AIRS-team V7 and CLIMCAPS-Aqua retrievals cover the full duration and coverage of the AIRS mission, but JoSFRA retrievals are currently provided for a limited subset (see here).

AIRS-team V7 and CLIMCAPS-Aqua

AIRS-team V7 and CLIMCAPS-Aqua have a common heritage: CLIMCAPS shares a common history with the AIRS-team Version 5 but deviates substantially in its error quantification, uncertainty propagation, a-priori definition (e.g., MERRA2 reanalysis for temperature, water vapor and ozone), and diagnostic reporting (e.g., full averaging kernel matrices). AIRS-team V7 changed considerably since Version 5, including a neural network initial state. For these and many other reasons the two data records have structural differences despite a common heritage.

Development of the AIRS-team Level 2 algorithm began prior to launch, and continued through the release of the V7 data set. The NASA-supported CLIMCAPS Level 2 algorithm was developed to process data from instruments on multiple satellites. Thus, CLIMCAPS is intended to provide a complete and consistent record from hyperspectral infrared instruments on three separate satellites, while the AIRS-team algorithm is applied to AIRS alone. An overview and comparison of the different instrument configurations of CLIMCAPS is presented here. The CLIMCAPS Science Application Guide applies to all the configurations and Smith and Barnet (2020) present the AIRS/AMSU version of the CLIMCAPS data set. These data are currently available at the NASA GES/DISC for the Suomi-NPP, JPSS-1, and Aqua satellites. Additional retrieval algorithms have been developed for AIRS, and associated publications are available on the AIRS website.

The AIRS-team retrieval system includes separate software to invert AIRS-Only, AIRS/AMSU and AIRS/AMSU/HSB radiances into estimates of geophysical state, referred to as Level 2 products. These separate retrieval algorithm paths accommodate the loss of HSB and the degradation of some AMSU channels. CLIMCAPS produces AIRS-only and AIRS/AMSU records, but no AIRS/AMSU/HSB record.

The AIRS-team retrieval algorithm is described in greater detail in Susskind et al. (2003; 2011), and in the Algorithm Theoretical Basis Document (AIRS L2 ATBD), with most recent updates in the latest testing report (AIRS V7 L2 Performance Test and Validation Report). Details of the CLIMCAPS retrieval algorithm are described in Smith & Barnet (2019; 2020). Results of analyses performed by the JPL AIRS Project to assess the general quality of various core products produced by the CLIMCAPS-Aqua retrieval system are presented in the CLIMCAPS-Aqua V2 L2 Retrieval Product Performance Test Report. This includes comparisons between CLIMCAPS-Aqua and AIRS-team V7 products.

A compact comparison between the retrieval choices, CLIMCAPS and the AIRS-team retrievals, is shown in the table below.

V7-CLIMCAPS comparison table


Technical details of the Level-2 processing steps can be found in the Algorithm Theoretical Basis Document (ATBD) [Irion et al., 2023]. Briefly, JoSFRA retrieves atmospheric and surface properties from the thermal infrared spectra at AIRS’ native horizontal footprint. Clouds are used in the forward radiative transfer model without resorting to a cloud-cleared state (as in the AIRS Version 7 or CLIMCAPS retrieval algorithms). The algorithm uses an optimal-estimation scheme (Rogers et al., 2000). Forward modeling is accomplished by using a Delta-4-Stream (D4S) parameterization for cloud transmissivities (Ou et al., 2013) incorporated into the operational AIRS forward model, the Standalone AIRS Radiative Transfer Algorithm (SARTA; Strow et al., 2003). For a priori parameters, National Center for Environmental Prediction (NCEP) forecast products are used for temperature, skin temperature, water vapor, and surface pressure. A priori cloud-top temperature, optical depth (estimated from cloud emissivity) and daytime effective particle radius are derived from co-located Moderate Resolution Infrared Sounder (MODIS) retrievals (Platnick et al., 2017) averaged over the AIRS spatial footprint.

With an improved spatial resolution compared to AIRS Version 7 and CLIMCAPS-Aqua results, and with information content quantification, JoSFRA retrievals are well-suited for process studies. Use cases for JoSFRA retrievals include instances when high spatial resolution (finer than 45 km) is needed or is beneficial, and where there are strong horizontal gradients in water vapor. Use of JoSFRA retrievals is recommended under medium to low cloud amount.