Sridhara, I'm checking your tip about the value of abtm.
Graziano, I created ICBC using data from ERA-Interim for 2009. They are in
http://meteoro.cefet-rj.br/felipe/REGCM/input/ (AS_ICBC-ERA. .... nc). In all cases I'm using ROTMER projection. And I think it's because of this that there are missing values at the top of the field and also on the sides and bottom. Look at the variable snowam in file AS_DOMAIN000.nc. In file AS_ICBC-ERA....nc you can see that this domain is the same in any sigma level.
Felipe.