New CAM topography generation software

A new algorithm and associated software (to be released with NCAR's Community Atmosphere Model (CAM), version 5.2) to generate surface height and associated sub-grid-scale orographic variances is presented. The sub-grid-scale variance is separated into scales smaller than approximately 3km (referred to as SGH30), that is used for turbulent mountain stress parameterizations, and scales larger than approximately 3km and less than the model grid scale (referred to as SGH) used for gravity wave drag parameterizations. The raw topographic data is binned to an intermediate gnomonic cubed-sphere grid which, contrary to the older versions of CAM topography generation software, results in a quasi-isotropic separation of scales over the entire sphere for SGH30 and SGH. The cubed-sphere data is thereafter rigorously remapped using a volume conserving method to any target model grid. The algorithm supports structured and unstructured meshes; even meshes with non-convex control volumes. The new consistent specification of sub-grid-scale variances result in several improvements in `AMIP'-style climate simulations using CAM.