| For the first time in the
Mediterranean Sea various temperature sampling strategies are studied and
compared to each other by means of the Observing System Simulation Experiment
technique. Their usefulness in the framework of the Mediterranean Forecasting
System (MFS) is assessed by quantifying their impact in a Mediterranean General
Circulation Model in numerical twin experiments via univariate data
assimilation of temperature profiles in summer and winter conditions. Data
assimilation is performed by means of the optimal interpolation algorithm
implemented in the SOFA (System for Ocean Forecasting and Analysis) code. The
sampling strategies studied here include various combinations of eXpendable
BathyThermograph (XBT) profiles collected along Volunteer Observing Ship (VOS)
tracks, Airborne XBTs (AXBTs) and sea surface temperatures. The actual sampling
strategy adopted in the MFS Pilot Project during the Targeted Operational
Period (TOP, winter-spring 2000) is also studied.
The data impact is quantified by the error reduction relative
to the free run. The most effective sampling strategies determine 25–40%
error reduction, depending on the season, the geographic area and the depth
range. A qualitative relationship can be recognized in terms of the spread of
information from the data positions, between basin circulation features and
spatial patterns of the error reduction fields, as a function of different
spatial and seasonal characteristics of the dynamics. The largest error
reductions are observed when samplings are characterized by extensive spatial
coverages, as in the cases of AXBTs and the combination of XBTs and surface
temperatures. The sampling strategy adopted during the TOP is characterized by
little impact, as a consequence of a sampling frequency that is too low.
Key words. Oceanography: general
(marginal and semi-enclosed seas; numerical modelling) |