Hello All, Summaries from the last few days follow. I'm not sure if you received those from the 17th and 18th given our recent email troubles, so I include them along with the new material on the 19th. Rick and Brian: we are a few days behind with respect to the UNC forecast products. The last files we received were the hindcast for the 17th and the forecast for the 18th. Cheers, Dennis May 17 ------ Dye survey operations continued until 1610. The ROV was tested later that afternoon. A larger scale dye survey was embarked upon at 1900 to define the extent of the tracer patch. Forecasting ----------- The May 17 Central forecast (EN324_FC.13) showed a slight improvement in forecast skill (9.3km mean distance) over the best forecast from the prior day (EN324_FC.08, 9.4km mean distance). Qualitatively, however, there was very little difference in the simulated drifter trajectories. One particularly striking aspect of the observations is the shallowness of the pycnocline in the stratified region. Surface stratification is very strong, and confined to a thin layer only 6m depth in some cases. The possibility that this vertical structure is important to the fate of the dye patch cannot be ruled out. It is therefore relevant to assess the extent to which this structure is present in the model. Examination of vertical sections of temperature extracted from the simulations reveal a comparatively diffuse thermocline (Figure 0517.1a). It was speculated that the diffuse thermocline could be a result of surface mixing that was too vigorous. Experiment EN324_FC.08h was conducted to see if a sharp pycnocline could be formed by increasing the surface heat flux to five times the climatological mean for this season. Clearly, this is an unrealistically large heat flux, but the intent of the experiment was to qualitatively assess the phenomenological impact on the solution. In fact, a much sharper pycnocline forms (Figure 0517.1b). However, this increase in surface heat flux is incompatible with the present boundary conditions on temperature, which are clamped to the observed values. This results in the spinup of an anomalous baroclinic jet around the bank, making the simulated circulation very unrealistic. It is clear that realistic treatment of this strongly stratified upper layer will require some effort involving several aspects, including the turbulence model, surface fluxes, and horizontal boundary conditions. As it turns out, the observed surface stratification described above may have strong lateral gradients which are not associated with the tidal mixing front. The satellite image from May 15 (which arrived just after the numerical stratification experiment was analyzed) shows a discrete patch of warm water straddling the Schlitz mooring line, oriented northeast-southwest adjacent to the 60m isobath (Figure 0517.2). Analysis of the alongtrack hydrography from the VPR surveys suggests this feature is associated with a salinity anomaly that is approximately 0.5 psu fresher than the surrounding water. There was much speculation about the origin of this feature. Operational Products: --------------------- Surface current predictions from the numerical mooring at the tracer release site were updated (Figure 0517.3) and provided to the chief scientist. May 18 ------ Survey of the tracer patch continued. A weak dye signal remained at the apparent center of mass. Several things appear to have happened: (1) dye in the very thin upper layer was driven by the easterly wind toward the front and into the well-mixed region where it was fairly uniformly distributed in the vertical; (2) dye which penetrated below the thin surface layer in the stratified area was mixed rapidly toward the bottom; (3) dye in both the well-mixed and stratified areas was dispersed in the along-isobath direction so that the patch expanded to about 10km in size; (4) the patch translated with the subtidal flow at a rate of approximately 10km per day. Forecasting Activities: ----------------------- Given that the dye had been mixed vertically both offshore and onshore of the front, the question arose as to how that might impact advection of the tracer by the subtidal flow. The chief scientist asked that we compare our forecast trajectory for the cloud of particles released at the surface with a cloud released at depth. Figure 0518.1 compares the trajectories of a surface particle at the center of the cloud (solid line) with one at 30m depth (dashed line). Westward translation of the deeper particle is not as rapid as that of the surface particle. A separation of 5-10km develops over the 1.76 days of forecast simulation time presented here. The decrease in forecast error that was noted in the May 17 central forecast was interesting. This improvement could have resulted from any of the following: (1) better atmospheric forcing (i.e. hindcast wind/heat fluxes), (2) more velocity observations, or (3) more recent velocity observations. In an attempt to distinguish between (2) and (3), a sensitivity experiment to the previous day's central forecast (EN324_FC.13) was conducted in which the older velocity data from the Edwin Link survey was not assimilated. The forecast error of run EN324_FC.14 increased from 9.3 to 10.1km, although the qualitative characteristics of the drifter trajectories remained the same (Figure 0517.2). Thus, it appears that more velocity data is better in this case. Given that, we chose to assimilate as much data as possible into the May 18 central forecast. Updated ADCP data from the Edwin Link received via email was merged and sorted with all available EN324 data. The mean forecast error was reduced to 9.1km from the 9.3km result of the May 17 central forecast. Once again, there was little change in the character of the drifter trajectories (Figure 0517.3). A note on the atmospheric forcing used in the May 18 central forecast: there was a period of several days in which the model forecast heat fluxes were not received from NMFS due to an email problem. That problem was corrected and transmissions were resumed in time to be incorporated in this day's central forecast. However, there was a gap in the heat flux record between the end of the May 14 transmission (day 136.50) and the beginning of the May 18 transmission (day 137.12). This gap was filled manually by copying the heat fluxes from day 137 into the those needed to complete the record for day 136. May 19 ------ The final survey of the tracer patch was concluded in the wee hours of the morning. A VPR section out to the 200m isobath and back was then occupied. We appeared to have reached a warm core ring at the most offshore extent of the track. Upon return to the study site, the six drifters which had been deployed prior to, during, and just after the tracer release were recovered. ROV operations were then undertaken, lasting until approximately 2300. A VPR survey of the study site was then begun in preparation for the second tracer release experiment, to take place in the pycnocline. Forecasting Activities ---------------------- A disk failure on the shore-based mail server prevented any email from reaching the ship. Therefore, we did not have the atmospheric weather or far-field ocean model forecasts needed to produce our operational products. We could have proceeded without the far-field ocean model, and forced the forecast system with wind predictions based on NOAA weather radio reports. However, it was not necessary to do so because shipboard operations planned for the following day (continuation of the VPR survey of the study area) did not require any input from the modeling team. The arrival of new drifter data from both Endeavor and Edwin Link sources provided additional opportunity for forecast evaluation. Observed drifter trajectories were compared with model results from the most up-to-date forecast, run EN324_FC.16. This simulation was re-run as EN324_FC.20 with the new drifter release points, shown in Figure 0519.1. The discussion below is split between surface drifters and those with subsurface drogues because there is a dramatic difference in our ability to forecast the motion of these two groups. Deep Drifters: In the well-mixed region, both the 16m and 26m drifters move southwest (Figure 0519.2). The shallower drogue moves slightly slower (approximately 10 km/day) than the deeper (approximately 5 km/day), indicating the presence of vertical shear even though water mass properties appear well-mixed. Simulated trajectories are quite similar to the observations, with separations at the end of the two-day record of only 1.6 and 3.7km for the shallow and deep drifters, respectively. The flow in the stratified area is more sluggish. Both the 12m and 36m drifters move west at speeds of 3 km/day or less. The simulated drifter trajectory at 36m agrees quite well with the observed trajectory, with only 3.9km separation developing by the end of the record. A larger separation (6.8 km) develops in the shallower case because the model drifter moves in a more southerly direction than is observed. Mean forecast error for the deep drifters is 4.0 km. Surface Drifters: Whereas the trend in the deeper drifters was movement toward the west and southwest, the surface drifters have a pronounced northwest component to their motion (Figure 0519.3). This aspect of the observed drifter motion is not captured by the simulations, resulting in a mean forecast error (11.9km) that is much higher than that for the deeper drogues. It is possible that this discrepancy may be associated with windage on the surface structure of the instruments; the directionality of the additional displacement is not inconsistent with what would be expected under the easterly wind conditions present during the deployment. END