AllScale is a programming environment targeting highly scalable parallel applications, simplifying application development in the ExaScale era by siloing development responsibilities. The front-end AllScale API provides a simple C++ development environment and a suite of parallel constructs denoting tasks to be operated concurrently. Lower level tasks related to the machine and system level are managed by the AllScale toolchain at the core level. We present the development of two large-scale parallel applications within the AllScale API, namely, an advection- diffusion model with data assimilation and a Lagrangian space-weather simulation model based on a particle-in-cell method. Mathematical formulations and implementations are presented and we evaluate parallel constructs developed using the AllScale API. The performance of the applications from the perspective of both parallel scalability and more importantly productivity are assessed. We demonstrate how the AllScale API can greatly improve developer productivity while maintaining parallel performance in two distinct applications. Code complexity metrics demonstrate reduction in application specific implementations of up to 30% while performance tests on three different compute systems demonstrate comparable parallel scalability to an MPI version of the code.