Predictions in the North Sea
247
determined by the locali sed rate of vertical exchange. Table 13.2 indicates the relative contributions of these two processes. Note their roles may be different
depending on the time scale of interest - vertical exchange often being of interest in
the short term (over the tidal cycle for sediments on the seasonal cycle for temperature) but with horizontal circulation tending to predominate in the longer term.
Appendix C describes general features of Residual Horizontal Circulation and
Appendix D of Vertical Mixing.
Finally the table also indicates the primary technique used for monitoring. The
range of capabilities to monitor, whether in real-time or via subsequent processing,
is a major constraint in initialising (including re-initialising via assimilation techniques), providing boundary conditions and in verifying forecasts.
13.2.1 Dynamics
Hydrodynamic models are required as the basis for transporting and mixing contaminants both horizontally and vertically. The dynamical processes involved
occur over time scales of seconds (turbulent motions), to hours (tidal oscillations),
to months and years (seasonal cycle) to inter-annual variability; with corresponding space scales from millimetres to thousands ofkilometres - thus a range of models is required. Appendix B presents the governing equations for dynamics and
mixing.
13.3 Tides, surges, waves and turbulence intensity
While the Proudman-Doodson (1924) co-tidal map provided the basis for predicting tidal levels for the major M 2 constituent, present-day models extend this
capability to upwards of 100 constituents and can include both surge-tide and tidewave interactions (Fig. 13.1). Gravitational attraction within the N.W. shelf se as
can be neglected with these models depending solely on tidal constituents prescribed along the shelf edge. The associated surge-tide models depend primarily
on the prescription of surface wind data together with some smaller reliance on
(dynamical) atmospheric pressure variations and 'externa!' oceanic surges (Heaps,
1967). These systems use conventional shore-based tide gauges both for subsequent verification and, using real-time interrogation, for assimilation purposes.
The tide-surge simulations can be run continuously for decades (subject to relevant data on wind forcing) to indicate seasonal and mean sea level variations (Fig.
13.2) and to calculate long-term statistics. Alternatively tidally-averaged forms of
equations (B 1) to (B3) can be used for more extensive sensitivity tests (Prandle
1984). The seasonal variation in mean sea level is primarily associated with the
seasonality of the associated winds. Perhaps surprisingly, persistent sea level gradients along the Atlantic boundaries do not propagate into the shelf seas, the steepness of this shelf results in a balancing residual circulation confined to the shelf
edge region, Fig.13.3 (Prandle, 1987).
247
determined by the locali sed rate of vertical exchange. Table 13.2 indicates the relative contributions of these two processes. Note their roles may be different
depending on the time scale of interest - vertical exchange often being of interest in
the short term (over the tidal cycle for sediments on the seasonal cycle for temperature) but with horizontal circulation tending to predominate in the longer term.
Appendix C describes general features of Residual Horizontal Circulation and
Appendix D of Vertical Mixing.
Finally the table also indicates the primary technique used for monitoring. The
range of capabilities to monitor, whether in real-time or via subsequent processing,
is a major constraint in initialising (including re-initialising via assimilation techniques), providing boundary conditions and in verifying forecasts.
13.2.1 Dynamics
Hydrodynamic models are required as the basis for transporting and mixing contaminants both horizontally and vertically. The dynamical processes involved
occur over time scales of seconds (turbulent motions), to hours (tidal oscillations),
to months and years (seasonal cycle) to inter-annual variability; with corresponding space scales from millimetres to thousands ofkilometres - thus a range of models is required. Appendix B presents the governing equations for dynamics and
mixing.
13.3 Tides, surges, waves and turbulence intensity
While the Proudman-Doodson (1924) co-tidal map provided the basis for predicting tidal levels for the major M 2 constituent, present-day models extend this
capability to upwards of 100 constituents and can include both surge-tide and tidewave interactions (Fig. 13.1). Gravitational attraction within the N.W. shelf se as
can be neglected with these models depending solely on tidal constituents prescribed along the shelf edge. The associated surge-tide models depend primarily
on the prescription of surface wind data together with some smaller reliance on
(dynamical) atmospheric pressure variations and 'externa!' oceanic surges (Heaps,
1967). These systems use conventional shore-based tide gauges both for subsequent verification and, using real-time interrogation, for assimilation purposes.
The tide-surge simulations can be run continuously for decades (subject to relevant data on wind forcing) to indicate seasonal and mean sea level variations (Fig.
13.2) and to calculate long-term statistics. Alternatively tidally-averaged forms of
equations (B 1) to (B3) can be used for more extensive sensitivity tests (Prandle
1984). The seasonal variation in mean sea level is primarily associated with the
seasonality of the associated winds. Perhaps surprisingly, persistent sea level gradients along the Atlantic boundaries do not propagate into the shelf seas, the steepness of this shelf results in a balancing residual circulation confined to the shelf
edge region, Fig.13.3 (Prandle, 1987).