Effect of ice on the generation of a generalized potential vorticity

In this work (using Hauf and Holler's entropy temperature) a potential vorticity is defined that generalizes the moist potential vorticity. This 'generalized' potential vorticity is used to analyze the possible effect of ice on changes of potential vorticity. The authors find that the existence of spatial gradients of ice concentration supplies a mechanism to generate 'generalized' potential vorticity. An estimate of this 'ice solenoid' term shows that there are cases in which this new term and the classic solenoid term could have the same order of magnitude.