Nonlinear interactions of gravitational waves This work presents the foundations of a new solution technique for the characteristic initial value problem of colliding plane gravitational waves. It has extensive similarities to the approach of Alekseev and Griffiths in 2001 but uses the inverse scattering method with a Riemann Hilbert problem. This allows for a further transformation to a continuous Riemann Hilbert problem with a solution given in terms of an integral equation for a regular unknown function. Ambiguities in the solution of the initial Riemann Hilbert problem lead to the construction of a whole family of exact spacetimes generalising the proper solution of the initial value problem. Therefore the described technique also serves as an interesting solution generating method. The procedure is exemplified by extending the Szekeres class of colliding wave spacetimes with two additional real parameters. The obtained solution features a limiting case of a new type of impulsive waves which are circularly polarised. A semi analytic approximation scheme for the solution to the general initial value problem of colliding plane waves is introduced.