I. Introduction
Shared distributed infrastructures such as the Teragrid [1], the Open Science Grid [2], and others can provide the software and hardware resources for cross-organizational collaborative work. These types of collaborations have proven to be of value across a wide range of scientific fields such as astronomy [3], high energy physics [4], earthquake science [5], etc. Such collaborations, also called virtual organizations (VO) [6] require resources from multiple providers for executing large-scale scientific applications. Furthermore, emerging classes of deadline-driven scientific applications such as severe weather modeling [7] require simultaneous access to multiple resources and predictable completion times.