top4m is the 30 second GTOPO topography interpolated to 4 minutes resolution. Similarly, top8m is the one interpolated to 8 minutes. Original GTOPO30 topography is divided into several files and inconvenient to use. top4m and top8m are the globally merged and interpolated (area averaged) topography from GTOPO30 files. For T62 model resolution, we recommend using 4 minutes topography. The resolution of the topography file affects model topography and variance of topography used in gravity wave drag parameterization. I expect the difference in model topography itself from top4m and top8m is small but the difference in topography variance can be large.

Note that for high resolution regional model (grid distance less than 30km), it is recommended to use original GTOPO30 topography, which can be obtained by using the installation script:

cvs co Install
inst rsm_latest hi

There is no way to get GTOPO30 topography files if you are running GSM.

(contribution from mkanamitsu)