Scotti Lab Research

• Nonlinear internal waves

Nonlinear internal waves are widespread in coastal waters. The combination of nonlinearity and short wavelength makes their observation and modeling a challenging subject. Scotti has collaborated with several colleagues to develop observational and modeling tools needed to overcome these difficulties. From the observational point of view, nonlinear internal waves are unusual in that they are coherent phenomena with a horizontal wavelength that can be as small as few tens of meters, and with frequencies of the order of minutes. In collaboration with R. Beardsley (WHOI) and B. Butman (USGS), we showed that the standard approach to interpreting ADCP output fails when the wavelength of the nonlinear waves approaches the beam separation. What apparently represents a liability of the instrument can be turned into an asset, because the spreading beams can be used as a directional antenna to measure direction and speed of propagation. With the aid of a modified post-processing algorithm, we can then recover the “true” velocity signal from the instrument output. We used this technique to study the energetics of internal waves in Massachusetts Bay. With J. Pineda (WHOI), we used the same technique to provide for the first time a high-resolution “radiography” of a train of internal waves of elevation with recirculating cores propagating along the bottom in the shallow reach of Massachusetts Bay (see Figure 3.31).

The short horizontal scales associated to nonlinear waves represent a challenge from the modeling point of view as well. In this respect, we are taking an innovative approach to numerical modeling by adapting techniques originally developed in the context of gas-dynamics. The idea is to take advantage of the fact that at any given time, waves occupy only a modest fraction of the domain, and to use numerical grids that self adjust in real time to provide high resolution only where waves are present. This work is in its early stage of development; it is a collaboration with M. Minion and S. Mitran, (Mathematics Department, UNC), with funding from ONR.

Scotti is also involved in the modeling of small-scale sediment transport induced by surface gravity waves. The work, funded by NSF, is in collaboration with the experimental group led by K. Kiger (Univ. Maryland-College Park). We are developing models of particle-bottom interaction to be included in state-of-the-art Large Eddy Simulation models. Scotti also collaborates with colleagues in the math department (R. Camassa, R. MacLaughlin) on problems of large scale mixing in the ocean.