7 Assimilation of Satellite Altimetry in
Ocean Models
KEITH HAINES
University of Edinburgh, Department of Meteorology
7.1 Introduction
In this chapter we willlook at some ofthe problems, both conceptual and practical, of assimilating one of the most widely available ocean data sets, that from satellite altimeters. The first sections will deal with conceptual issues. We consider
how the ocean and atmospheric assimilation problems differ in some respects. We
will look at the 4DVar (Four Dimensional Variational Assimilation) and 3D
Sequential assimilation techniques to consider their merits from a physical perspective. Results are then shown from twin experiments of a 3D sequential assimilation approach which tries to account for the ocean physics when assimilating
altimeter data. In the final sections we will show preliminary results from a study
designed to assimilate TOPEX altimeter data into a global ocean model. Many new
problems arise when real satellite data are used, some of which have been solved,
and some not. We will summarise by describing the present state of the art and
speculate on the likely developments over the next few years. 1
7.2 Physical relationships between sea level and hydrography
One of the most important concepts we have in leaming about ocean circulation
is the concept of a water mass. The literature of physical oceanography contains
many descriptions of water mass properties (temperature, salinity, and more
recently, CFCs, nutrients, etc.) and their spatial distributions. The reason why
water masses are so useful is because many of the properties which de fine them
change only on very long timescales. They are the physical embodiment of the
ocean's memory of past climate conditions at the surf ace where most water properties are determined at the time of formation. The air mass concept is also sometimes used in synoptic meteorological descriptions but air masses can mix with
each other very rapidly, especially vertically during tropospheric convection
events. In the ocean, such strong, deep convective mixing only occurs in a few
places such as the Labrador, Greenland and Weddell seas. The result is that the
1. For up-to-date results an implementing the methods discussed here see, Fox et al.
(2000a,b).
Ocean Models
KEITH HAINES
University of Edinburgh, Department of Meteorology
7.1 Introduction
In this chapter we willlook at some ofthe problems, both conceptual and practical, of assimilating one of the most widely available ocean data sets, that from satellite altimeters. The first sections will deal with conceptual issues. We consider
how the ocean and atmospheric assimilation problems differ in some respects. We
will look at the 4DVar (Four Dimensional Variational Assimilation) and 3D
Sequential assimilation techniques to consider their merits from a physical perspective. Results are then shown from twin experiments of a 3D sequential assimilation approach which tries to account for the ocean physics when assimilating
altimeter data. In the final sections we will show preliminary results from a study
designed to assimilate TOPEX altimeter data into a global ocean model. Many new
problems arise when real satellite data are used, some of which have been solved,
and some not. We will summarise by describing the present state of the art and
speculate on the likely developments over the next few years. 1
7.2 Physical relationships between sea level and hydrography
One of the most important concepts we have in leaming about ocean circulation
is the concept of a water mass. The literature of physical oceanography contains
many descriptions of water mass properties (temperature, salinity, and more
recently, CFCs, nutrients, etc.) and their spatial distributions. The reason why
water masses are so useful is because many of the properties which de fine them
change only on very long timescales. They are the physical embodiment of the
ocean's memory of past climate conditions at the surf ace where most water properties are determined at the time of formation. The air mass concept is also sometimes used in synoptic meteorological descriptions but air masses can mix with
each other very rapidly, especially vertically during tropospheric convection
events. In the ocean, such strong, deep convective mixing only occurs in a few
places such as the Labrador, Greenland and Weddell seas. The result is that the
1. For up-to-date results an implementing the methods discussed here see, Fox et al.
(2000a,b).