The Xnl inter-comparison project provides an objective method to inter-compare different methods for the computation on non-linear four-wave interactions for application in discrete spectral wave prediction methods. Xnl is an acronym for eXact Non-Linear, which indicates that the source term Snl4 is numerically solved as exactly as possible.
The theory behind the Snl4 source term is described by Hasselmann (1962) or equivalently by Zakharov (1968). It consists of a six-fold integral in wave-number space and includes 2 delta-function to effectively reduces to integration space to a three-dimensional manifold. These properties make it rather difficult so solve this integral.
In literature (at least) 3 methods have been proposed that aim to solve the same equation. However, in practise it turns out that minor and surprising differences in results occur. In this project, a set of discrete test spectra has been defined to objectively inter-compare these exact methods, as well as newly proposed (approximate) methods.
The test data can be obtained by sending me a request by mail. An in-depth description of this project is provided in Van Vledder and Hashimoto (2013).