EN324 May 14 ------ Departed Woods Hole 1530. Satellite imagery from May 10 shows complex SST structure (Figure 0514.1). A warm filament and small eddy are present in the study area, possibly under the influence of a ring / streamer / intrusion offshore. Edwin Link reports that their deep drifter south of the front is moving east; assimilation of their ADCP data is reported to capture this behavior to some extent. All these points argue the need for velocity measurements in the stratified region south of the survey track we are to occupy. A course was laid out so that we would enter the model domain at 40 40'N, 69 00'W and proceed east until reaching the 80m isobath, then proceeding ENE along the mean position of the 8m isobath up to 40 57' 67 04', a point just SE of the beginning of the survey track (Figure 0514.2). Forecasting Activities ---------------------- May 14 central forecast includes ADCP data from the Edwin Link, file el9905a2.m3d. Drifters from EN323 and EL9905 were launched in the simulation [EN323pEL9905.ind]. Subsequently an error was discovered in the parameter used to ramp up the atmospheric forcing; this effectively started the simulation with the forcing at full strength. The was corrected in EN324_FC.02, in which the atmospheric forcing was ramped up over a 1-day perid. A sensitivity experiment EN324_FC.03 was conducted with TDBC forcing turned off. We had yet to receive any of the observed drifter trajectories by the close of business on this day, so it was not possible to undertake any quantitative evaluation of the forecasts. However, it was noticed that the RMS residual velocity in these simulations was slightly higher than usual: FC.01 FC.02 FC.03 ----------------- Q4.1 20.0 20.0 17.4 Q4.2 11.7 11.7 11.2 Q4.3 10.4 10.4 10.4 In communications with the Edwin Link, Cisco and Jim indicate they had similar results. They attribute the higher residuals to noise in the ADCP data associated with frequent stops for CTD casts. Close examination of the alongtrack residuals does not reveal any systematic error (Figure 0514.3) It is interesting to note that the inclusion of TDBCS in runs 1 and 2 introduces an additional RMS velocity residual of approximately 2.5 cm/sec to the prior (comparing with run 3 with TDBCS turned off). The assimilation procedure appears to clean this up, with the final results of all three runs indistinguishable in terms of this statistic. May 15 ------ At 0246 local time the ship reached the first waypoint at the western edge of the model domain and slowed to 10knt. Examination of the ADCP data revealed a lot of noise, with only about 25% good returns. At 0530 the ship was slowed to 9knt. The return rate rose to 75%; data collection starts here. VPR trimming began in the late morning, and was completed by 1300. Rendez-vous with the Edwin Link occurred shortly thereafter. Scientific briefings took place, and supplies and data were exchanged. The term "Zodiac-net" was coined to describe the medium over which drifter trajectory data was transmitted between modeling groups on the two ships. VPR survey operations began immediately after rendez-vous. Late in the day, the extremely light winds made for a spectacular sight as we approached the tidal mixing front in the first leg of the survey. Capillary waves were clearly present in the stratified side of the front, while they were almost imperceptible in the well- mixed area. The well-mixed region took on a glassy appearance. This change was undeniably associated with water properties (as opposed to the atmosphere); there was no perceptible change in the ship's anemometer as she passed through the frontal region. The frontal boundary was razor sharp; the transition occured over a distance of meters. Perhaps the change in roughness was due to a surfactant which is present at the sea surface in the well-mixed area, but not in the stratified region. Upon closer examination, the patterns in surface roughness appeared to reveal interleaving of the two water masses. In one instance, the front was interupted by a narrow strip oriented roughly perpendicular to the front, aligned with the NE wind. Small eddies appeared to be associated with this structure, with horizontal scales of 10-20m. There was much excitement and speculation about the mechanisms which may have been responsible for this fantastic display. Forecasting activities ---------------------- The May 15 central forecast (EN324_FC.04) used ADCP data from the Edwin Link, in addition to that collected aboard Endeavor prior to bell time. Due to the fact the the NMFS model weather product did not arrive, we had to proceed with an incomplete heat flux record. The flux file was edited by hand so that zero heat flux was persisted after the end of the data record. Experience from the last leg had shown there were sometimes large differences between the AVN model wind product and the observed winds. The current period was no exception (Figure 0515.1). Sensitivity forecast EN324_FC.05 was forced with a hybrid wind record, composed of the buoy winds available at bell time and AVN model winds thereafter. Processing of the new drifter data was not yet completed by the close of business on the 15th. Quantitative evaluation of the forecasts was therefore postponed until the following day. May 16 ------ VPR survey continued until approximately 1100. A surface drifter was deployed at the front. Endeavor steamed 6km south of the front and released tracer from 1300-1435. Three drifters were deployed in the tracer as the patch was being laid out. An initial survey of the tracer patch was then carried out. Subsequently, Endeavor steamed back to the front to re-assess the distance between the patch and the front. On the way back south toward the tracer, another surface drifter was deployed in between the front and the patch; Endeavor continued through the patch and deployed another drifter seaward of the patch. Survey of the dye patch then recommenced. Forecasting activities ---------------------- The May 16 central forecast (EN324_FC.06) included Endeavor's ADCP data up through bell time. The heat flux forcing record was incomplete because the necessary data file was not received. Two sensitivity experiments were carried out; the first (EN324_FC.07) eliminated TDBC forcing, and the second (EN324_FC.08) included ADCP data from the Edwin Link (several days old at bell time). Evaluation of forecast skill: ----------------------------- The first forecast evaluation was undertaken with runs from May 14 and May 15 (simulations 01 though 05). This comparison was carried out using simulated drifters which were released at the reported locations. Due to delays in between drifter deployment and the beginning of data acquisition, the drifter observations do not begin until a finite time after deployment. In one case (drifter 234) data did not begin until a full tidal cycle after the reported release. It therefore seemed appropriate to carry out a second comparison in which simulated drifters were released at the space/time points of the first entry in each data record. Runs EN324_FC.01 through EN324_FC.05 were re-run as EN324_FC.01_dr through EN324_FC.05_dr with the new release positions. This removed all ambiguity with respect to consistency in the space/time starting points of the simulated and observed drifter trajectories. Fortunately, the results of the second comparison were similar to the first; as expected, the forecast error decreased in the second comparison because of its shorter duration (see below). This suggests that there were no problems with the release points used in the first comparison. As before, forecast error is defined as the mean distance between simulated and observed drifters at the end of the drifter record. Using this metric, forecast 08 has the highest skill; it is the one with the most up-to-date forcing and most complete ADCP data set used in the assimilation. Comparison of forecast 07 with the May 16 central (forecast 06) suggests that the TDBC forcing slightly degrades the skill. Therefore it is possible that a slight improvement in forecast 8 could result by removing the TDBC forcing from that simulation. Forecast Error (km) ------------------- Assessment # 1 2 ---------- EN324_FC.01 May 14 Central; EL ADCP 10.9 9.7 EN324_FC.02 (01) ramping up atmos 10.8 9.6 EN324_FC.03 (02) - TDBCS 10.9 9.8 EN324_FC.04 May 15 Central; EL+EN ADCP 10.8 9.8 EN324_FC.05 (04) + buoy winds 10.1 10.0 EN324_FC.06 May 16 Central; EN ADCP 10.2 EN324_FC.07 (06) - TDBCS 10.1 EN324_FC.08 (06) + EL ADCP 9.4 The data used in these evaluations consists of four drifters from the Edwin Link: two deployed in the well-mixed area at the 50m isobath (drogued at 13 and 33m), and two in the stratified region at the 63m isobath (drogued at 8m and 33m). Simulated and observed trajectories are shown for each forecast experiment in the second comparison are shown in Figures 0516.1(a-h). The results demonstrate very little sensitivity to the inputs which were varied in this set of simulations. The westward motion of the 33m drifter in the well-mixed region is fairly well represented in the model. In the best forecast (8), the distance between the simulated and observed drifter position at the end of the 2.3-day record is 6.3km. This is slightly below the mean 2-day error of 6.8km from the best forecast of the set conducted on the previous leg of the cruise (see EN323 report). The early portion of the 13m drifter record in the well-mixed area closely resembles that of the deeper drifter. About half way through the record, the 13m drifter took a turn to the southwest and accelerated. Neither the turn toward the southwest nor the speedup of the drifter motion was captured in any of the simulations. The simulated trajectory in forecast 8 does have a slightly more southerly component to its movement, but not nearly enough to make it match the observations. Moreover, the southerly component in the model is fairly monotonic; it does not contain the the abrupt change in direction present in the observations. In the stratified region, both drifters move in a mostly southerly direction. Early in the record, there is an eastward component to the motion, which is more pronounced in the shallower drifter than it is in the deeper. Although the southward component of the observed drifter motion is captured in the simulations, the east-west aspect is not. Both shallow and deep simulated drifters move to the southwest. The reason for this discrepancy is not known. However, it is interesting to note that the southwestward movement of the simulated drifters was even more pronounced in the BPE run (Figure 0516.2). Assimilation of ADCP data partially arrested this southwestward motion; the simulated drifters do not go nearly as far to the southwest after assimilation. This result is consistent with that reported by Werner and Manning on the Edwin Link. Operational Products -------------------- Operational products generated on May 16 were based on forecast EN324_FC.08. This forecast had the highest skill, not only in terms of the mean forecast error (see table above), but also the error metric for the drifter phenomenologically closest to the tracer release point (the shallow drifter in the stratified region). Surface current predictions were produced for the tracer release site via a numerical mooring (Figure 0516.3). In addition, a cloud of particles was launched around the release site in the model solutions. The results are best visualized with an animation (see web page); plots of the initial and final particle locations are provided in Figure 0516.4. A time-annotated plot of the drifter trajectory at the center of the cloud (which represents the center of mass of the tracer) is shown in Figure 0516.5. Note that the original cloud release at 0.5m depth (run EN324_FC.09, based on Q4.3 of EN324_FC.08) ran into some problems with particles breaching the surface, even though these were fixed-depth drifters. The simulation was rerun as EN324_FC.10 with particles released at 2.5m depth, and the problem was avoided. 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.