AOSN-II August 2003

ESSE Uncertainty Forecasts

ESSE uncertainty initialization and forecast procedure

Uncertainties were initialized for August 11, GMT00, using the ESSE initialization approach. The background ocean field on August 11, GMT00 is based on a HOPS forecast simulation which assimilates all calibrated data up to 1PM on August 9.

The observed field variables are mostly temperature (T) and salinity (S), and the T and S data collected from August 2 to August 9 are used to initialize uncertainties. The differences between these data and the background simulation are utilized to compute multivariate T and S vertical empirical functions (EOFs). To obtain three-dimensional EOFs for T and S, the vertical EOFs are combined with horizontal eigenmodes. These horizontal eigenmodes were computed based on a "Mexican hat" (second derivative of a Gaussian) covariance function, whose parameters were set to 5km decorrelation (decay) scale (km) and 12.5km zero crossing. To compute the non-observed velocity uncertainties, the background field is pertubed with each three-dimensional EOFs for T and S, and the momentum equations are then numerical integrated until adjustment. The result is an ensemble of adjusted velocity anomalies. The singular value decomposition of normalized T, S, u and v anomalies is then computed to lead a complete error subspace (dominant error singular vectors/EOFS). Presently, 500 three-dimensional, multivariate singular vectors are retained to determine the initial error subspace.

To forecast uncertainties for August 12, the fields on August 11 are perturbed using a random combination of the 200 dominant T, S, u and v initial singular vectors, weighted by their eigenvalues. A white noise of an amplitude proportional to the estimated absolute and relative errors in the observations is added to this random combination, in part to represent the errors truncated by the error subspace. An ensemble of 1-day forecast simulations, each forced by forecast COAMPS atmospheric fluxes, is then carried out. Presently, 52 of such forecast simulations are utilized to forecast the uncertainties on August 12.

Error Subspace Statistical Estimation (ESSE) Results
Full Domain
Temperature Salinity U velocity component V velocity component
Monterey Bay Zoom
Temperature Salinity U velocity component V velocity component
Barotropic Streamfunction (including Monterey Bay)

Analyses of ESSE Results

Focusing on the Monterey Bay zoom, there are some instabilities forecast near the mouth of the Bay.

  • On the northern side (towards Ano Nuevo), there is a Monterey Bay cyclone feeding a weak northward flow. This flow "collides" with a locally weak offshore southward flow. This variability has mainly a temperature signal, thus showing in the temperature uncertainty forecast.

  • On the southern side of the mouth of the Bay, there is a strong southward flow, driven by locally favorable upwelling wind forecasts. This has a stronger signature in salinity than in temperature. It also has a barotropic variability (see the high spot in streamfunction, which is logical, given a strong flow and steep topography). These effects are visible in the different error STD forecasts.

  • At the center of the mouth, the southward flow splits in two (bifurcates), the weaker portion of the upwelled jet feeds the cyclone in Monterey Bay (mentioned above). The location of that bifurcation is uncertain, see temperature error field at 0m, 10m and 30m, and salinity at 30m.

  • The barotropic transport uncertainty is higher all around Monterey Bay, in accord with the forecast cyclone (weak transport in the center of the Bay). It is highest on the southwestern corner, where the edge of a strong shouthward current (forced by the local wind forcings and corresponding upwelling) is forecast.

    Focusing on the full domain, the East-West temperature and salinity fronts, and the corresponding meandering of the plumes, are the reasons for the local high temperature and salinity error patterns.

    Adaptive sampling recommendations

  • send the adaptive gliders of WHOI to the inflow, center and outflow points of the mouth of the small MBay domain. Send the gliders slightly upstream of these three locations, using the circulation forecast for August 12 to define upstream.

  • determine if the tracks of some of the spray gliders could be useful to observe these locations. Observing the strong upwelling south of PS (southeast corner of the model domain) would be beneficial.

     

     

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