**Pasquale Contestabile 1,2, Fabio Conversano 3, Luca Centurioni 4, Umberto Mario Golia 1, Luigi Musco 3, Roberto Danovaro 3,5 and Diego Vicinanza 1,3,\***


Received: 24 May 2020; Accepted: 2 July 2020; Published: 7 July 2020

**Abstract:** In this paper, the advantages of shaping a non-conventional triple collocation-based calibration of a wave propagation model is pointed out. Illustrated through a case study in the Bagnoli-Coroglio Bay (central Tyrrhenian Sea, Italy), a multi-comparison between numerical data and direct measurements have been carried out. The nearshore wave propagation model output has been compared with measurements from an acoustic Doppler current profiler (ADCP) and an innovative low-cost drifter-derived GPS-based wave buoy located outside the bay. The triple collocation—buoy, ADCP and virtual numerical point—make possible an implicit validation between instrumentations and between instrumentation and numerical model. The procedure presented here advocates for an alternative "two-step" strategy. Indeed, the triple collocation technique has been used solely to provide a first "rough" calibration of one numerical domain in which the input open boundary has been placed, so that the main wave direction is orthogonally aligned. The need for a fast and sufficiently accurate estimation of wave model parameters (first step) and then an ensemble of five different offshore boundary orientations have been considered, referencing for a more detailed calibration to a short time series of a GPS-buoy installed in the study area (second step). Such a stage involves the introduction of an enhancement factor for the European Centre for Medium-Range Weather Forecasts (ECMWF) dataset, used as input for the model. Finally, validation of the final model's predictions has been carried out by comparing ADCP measurements in the bay. Despite some limitations, the results reveal that the approach is promising and an excellent correlation can be found, especially in terms of significant wave height.

**Keywords:** wave numerical model; directional wave spectra drifter (DWSD); ADCP; GPS wave buoy; triple collocation; Bagnoli-Coroglio Bay

#### **1. Introduction**
