4. The Reanalyis and Real Time Streams

When in 2004 the monthly forecasting system became operational at ECMWF, it was necessary to produce near-real-time ocean initial conditions, since the typical 11 days delay of the reanalysis product was not adequate for the monthly system. To this end, an early delivery ocean analysis system was introduced in May 2004. Details of the timing of the near real time S2 ocean analysis are given in Balmaseda 2005. In what follows, we denote the Near Real Time stream as NRT, and for BRT we refer to the reanalysis, which runs 11 days Behind Real Time. The BRT and NRT analysis systems are almost identical: they only differ in the details of the assimilation cycle and in the earlier cut-off of the assimilation windows. The BRT product is a continuation of the historical ocean reanalysis, while the NRT only exists for the recent period

In S3, the NRT ocean analysis is an integral part of the operational ocean analysis system. To avoid degradation of the NRT product, the NRT analysis always starts from the most recent BRT analysis and is then brought forward to real time every day. In principle, the design of the S3 ocean analysis system allows the paradigm of a seamless prediction system, since the forecasts for the different time scales (medium range, monthly and seasonal) could all be initialized using the NRT product. Equally, the BRT product would provide initial conditions for the hindcasts needed for the calibration (or re-forecasting) of the seamless products.

The figure below shows schematically the schedule followed in the production of the NRT and BRT ocean analyses in S3 (which is sligthly diffrent from that described in Balmaseda 2005). Every day, the BRT ocean analysis is advanced by 1 day, starting from the BRT analysis from the previous day (D0-12 in fig) to produce the BRT analysis at day D0-11. If the 10-day assimilation cycle is due on that day (D0-12), the OI analysis is performed using a 10-day centered window,

Assimilation cycle in the daily production of the behind-real-time (BRT) and near-real-time (NRT) ocean analyses in S3.

Similarly, the NRT ocean analysis is also produced daily. Starting from BRT (D0-12), the model is integrated forward 12 days, so bringing it up to real time. All the available observations during that period are used. During the 12 days there are always two assimilation cycles. The first assimilation takes place at (D0-12), using a 10-day window centred at D0-12 (W1), and the assimilation increment using the incremental update method is computed . This increment is only applied for the next 7 days, at which time the second assimilation W2 is performed (i.e. at D0-5), using observations from the 10-day window as shown in fig 5. The corresponding increment is then computed , and applied during the next 5 days of the integration. Effectively, W2 only has 9 days worth of data, since the daily averages for day D0 are not available at the time of the operational production. The assimilation windows W1 and W2 overlap for 3 days. The observations within this overlapping period, although used in two different analysis cycles are not given extra weight, since the increment of the first assimilation cycle is not applied to completion.