A Case study of AIRS Level 2 Temperature and Water Vapor Products using different Cloud Clearing Methods

The Atmospheric Infrared Sounder (AIRS) was launched aboard the AQUA satellite to provide measurements of temperature, humidity, and various trace gases in support of climate research and weather prediction. Only clear sky measurements of the outgoing radiance are used in the AIRS physical retrieval of temperature, water vapor, and certain trace gases. To overcome cloud contamination the clear sky radiance is estimated using a procedure that combines microwave measurements from the AMSU-A instrument also aboard the AQUA satellite along with a three by three grid of AIRS measurements. One drawback of this method is that the AMSU-A's microwave measurement provides an indirect starting point for the clear sky measurement, requiring a forward model to calculate the outgoing radiance based on crude estimates of temperature, humidity, and assumptions on further geophysical parameters. The radiance error estimate, a component critical to the AIRS physical retrieval, must include contributions from all assumed parameters input to the forward model on top of instrument noise and amplification from cloud clearing. When the error estimate is too large the AIRS physical retrieval becomes over-constrained to the first guess profile. There is a new cloud clearing method using the broadband IR instrument MODIS also aboard AQUA to provide a direct estimate of the clear sky radiance. This method removes the dependence on a forward model and reduces the number of components needed to calculate the radiance error estimation. We examine AIRS Level 2 retrieved temperature and water vapor that are compared to radiosondes for accuracy. We present global plots of lower tropospheric water vapor and contrast it against the first guess to determine which cloud clearing method has greater dynamic range. Averaging kernels for temperature and water vapor are examined to understand the vertical sensitivity for the physical retrieval using different cloud clearing methods.