I am trying to understand how the sublimation process can cause a positive mass balance in general, intuitively as highlighted in the text taken from W. S. B. Paterson,The Physics of Glaciers: 3rd Edition(Pergamon, New York, 1994). Thank you.
Strictly speaking sublimation is the phase change from solid to gas without an intermediate liquid phase. So the way the term sublimation has been used in the text is partly wrong. Ice and snow on a glacier can sublimate so that mass is lost directly to the atmosphere as water vapour.
Water vapour in the atmosphere an also be deposited on the glacier as a solid in a reverse process called deposition or desublimation. This is where the text example contains an error in terminology.
The term in the equation conerning phase changes from solid to gas or gas to solid is of course correct and it may be either positive or negative depending on which phase change direction dominates. As is stated losses from solid to gas is likely larger in most if not all cases.