Climate Calibration Observatory in Orbit: Using COSMIC RO Data to Calibrate and Validate the AIRS and AMSU Troposphere and Stratosphere Temperature Climate Data Records

At 0140 UTC on 15 April 2006, the joint Taiwan-U.S. COSMIC/FORMOSAT-3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite mission #3; hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit from Vandenberg Air Force Base in California. Using on-board propulsion these satellites are being deployed to their final orbits at 800 km with 30 degrees of separation. The final orbital configuration gives global coverage of approximately 2500 soundings per day distributed nearly uniformly in local solar time. COSMIC data are shown to be useful in validating weather prediction and ionospheric models. In addition, because its fundamental measurement is traceable to the international system of units (SI) (SI traceability), COSMIC RO is also a self-calibrated observing technique from space.

The purpose of this study is to demonstrate the usefulness of COSMIC RO data to serve as a climate calibration observatory in orbit to calibrate the current most used satellite sounders, the Advanced Microwave Sounding Unit (AMSU) and the hyper-spectral infrared sounding from Atmospheric Infrared Sounder (AIRS). Combined with AMSU measurements, the AIRS is the first of a new generation of operational remote sensors for upwelling atmospheric emission that provide excellent temperature and water vapor retrievals at middle atmosphere, which has significant impacts on short-term numerical weather forecasts. However, both AIRS and AMSU also exhibit biases in retrieving atmospheric temperatures and moistures when compared with in situ measurements. These retrieval biases have diverse and complex dependencies on the temperature/moisture being measured, the season and geographical location, surface conditions, and sensor temperature, which is difficult to quantify. In this study, we use COSMIC RO data to simulate AMSU and AIRS brightness temperatures for the lower stratosphere (TLS) and compare them to AMSU TLS and those of AIRS brightness temperatures at the same height. Our analysis shows that because RO data do not contain mission-dependent biases and orbit drift errors, and are not affected by on-orbit heating and cooling of the satellite component, they are very useful to identify the AMSU time/location dependent biases for different NOAA missions and long term drift of the AIRS retrieved temperatures. The potential usefulness of GPS RO as a climate benchmark to calibrate measurements from microwave sounders and high spectral resolution infrared sounders is presented.