Workflow applications are frequently used in many production Grids. There is a natural need to run the same workflow with many different parameter sets. Unfortunately current Grid portals either do not support this kind of applications or give only specialized support and hence users are obliged to do all the tedious work needed to manage such parameter study applications. P-GRADE portal has been providing a high-level, graphical workflow development and execution environment for various Grids (EGEE, UK NGS, GIN VO, OSG, TeraGrid, etc.) built on second and third generation Grid technologies (GT2, LCG-2, GT4, gLite). Feedback from the user communities of the portal showed that parameter study support is highly needed and hence the recent release of the portal supports the workflow-level parameter study applications. The current paper describes the semantics and implementation principles of managing and executing workflows as parameter studies. Two algorithms are described in detail. The black box algorithm optimizes the usage of storage resources while the PS-labeling algorithm minimizes the load of Grid processing resources. Special emphasis is on the concurrent management of large number of files and jobs in the portal and in the Grids as well as providing a user-friendly, easy-to-use graphical environment to define the workflows and monitor their parametric study execution.