(The following is an excerpt from the introduction of the ECMWF-AUX - Description and Interface Control Document. The complete document can be found at the CloudSat Standard Data Product Interface Control Document links page)
The ECMWF-AUX data set is an intermediate product that contains the set of ancillary ECMWF state variable data interpolated to each CloudSat cloud profiling radar (CPR) bin. These data are required for input to the 2B-GEOPROF, 2B-CLDCLASS, 2B-TAU, and 2B-FLXHR algorithms. The ECMWF -AUX product is created by the Generic-AUX Interpolate-to-Reference algorithm. The input data is obtained from the AN-ECMWF dataset provided by the European Center for Medium-Range Weather Forecasts.
Also see the CloudSat Data I/O Interface Specifications