Plans for model-data comparison and experiment design

Model-Data Comparison

Preliminary LES model development has been guided by earlier development of the 1-D LTC model. While this has allowed some preliminary simulations, comparisons between the modified LES and observations of turbulence below the ice are necessary to verify the thermodynamic parameterizations of ice-ocean interaction. Data for verification will come from past observations of both ANZFLUX and SHEBA experiments. Comparisons will include vertical fluxes and dissipation rates as well as the evolution hydrographic profiles. Figure 3 demonstrates the equivalence of LES subgrid TKE dissipation and dissipation observations from Tim Stanton's Turbulence Instrument Cluster (TIC) during the ANZFLUX experiment (n.b. two different time frames).

figure 3

Figure 3: Time series of measured dissipation profiles (left plot) from the two final days of observation East of Maud Rise, along with LES subgrid dissipation (right plot) from the simulation of the subsequent two days, initialized from observed hydrographic profiles. Model resolution is 6.25m.

The two quantities are equivalent above the instrument noise level because the LES resolution scale is the same order as the length scale characterizing the TIC response, and because most of the dissipation is at much smaller length scales. We anticipate three verification simulations, two based on ANZFLUX observations and one based on SHEBA results, each about one week in duration. Ice-ocean parameterizations will be tuned to give the best agreement in these comparisons.

Modeling for Experiment Design

Modeling for predictions of upper ocean instabilities will focus on what may have happened in the area East of Maud Rise after the end of ANZFLUX ocean observations. This region is of interest because hydrographic analysis indicates a weak barrier to thermobaric instability if the upper ocean layer, and because satellite observations indicate significant areas of open water (Drinkwater, 1997) in 1994, and large polynyas in other years. The goal of this modeling is to use the dynamics of the LES predict more accurately the point at which instabilities occur and the form they will take. Furthermore, timeseries from ensembles of virtual measurements embedded in the LES will predict the shape of observed thermobaric plumes and other features of the upper ocean, and assess the random and systematic uncertainties associated with different observing systems. Embedded measurements will include Lagrangian, isobaric, and tethered Lagrangian drifter ensembles as well as Eulerian point and volume-averaged timeseries representing ice-fixed measurements.

In short, these embedded measurements will be used to assess the effectiveness of an array of experimental strategies, thus aiding in the design of future field expeditions. These virtual measurement ensembles will simulate observations in about six LES cases of one to two weeks in duration each, initialized from different 1994 ANZFLUX CTD profiles from East of Maud Rise, and forced with observed time-dependent surface forcing also derived from observations.