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J. M. Magalhaes et al.
14.1 Introduction
Internal Waves (IWs) are an important factor in several processes within geophysical
fluid dynamics. They are ubiquitous features in the oceans and in the atmosphere,
which can exist in a wide variety of time and space scales, and owe their existence
to the vertical density stratification that characterizes both mediums.
In the ocean, tidal flow over irregular bottom topography can force vertical motions at the tidal frequency and generate IWs of tidal period, which can then propagate
along the thermocline and are often referred to as Internal Tides (ITs). ITs can steepen
and generate IWs of shorter period that are usually termed Internal Solitary Waves
(ISWs). The term ‘solitary’ is used since they tend to occur in individual packets
(usually trapped in the troughs of the IT), and have often been identified with the
soliton solutions of nonlinear wave theory. In the atmosphere, these short-scale waves
that are usually highly nonlinear, can also be observed to propagate in the lower troposphere, and are often accompanied by roll-cloud formations that resemble the
packet-like structure of their oceanic counter-part. Their origin can be attributed to
various generation mechanisms, often involving convection, wind-shear, gravity currents and different air mass collisions. In summary, any atmospheric process leading
to the displacement of density surfaces can be a potential source of atmospheric IWs,
providing that suitable propagation conditions are previously met.
During the last few decades remote sensing has played a key role, in providing the
scientific community with an increasing number of IW observations (both of oceanic
and atmospheric nature). Imaging sensors such as Synthetic Aperture Radars (SARs),
as well as visible and near-infrared spectrometers, have proved to be particularly useful in providing new and valuable insights into these waves’ physical characteristics.
These new measuring capabilities boosted the number of observations, and led to
several studies that indicated that the phenomenon is in fact much more frequent than
previously thought (see e.g. Jackson 2007; Jackson et al. 2012; da Silva and Magalhaes 2009; Magalhaes et al. 2011). SARs in particular have proved very useful in
revealing the two dimensional structure of IWs, as well as their spatial and temporal
distributions. It is well known that both oceanic and atmospheric IWs are capable
of producing roughness patterns on the ocean surface. Therefore, they can generate
recognizable signatures in radar images. The oceanic roughness patterns are a result
of either hydrodynamic modulation of Bragg waves by surface current gradients or
wave damping due to surface films (Alpers 1985; da Silva et al. 1998). Similarly,
the atmospheric IW signatures are a consequence of wind stress variability introduced by the disturbed wind velocity field associated with wave propagation (Alpers
and Stilke 1996). SAR imagery has also been playing a key role in identifying and
characterizing several IW hotspots, as well as in providing valuable insights into the
generation mechanisms (see e.g. New and da Silva 2002; Vlasenko and Alpers 2005;
Nash and Moum 2005; da Silva and Helfrich 2008; da Silva and Magalhaes 2009;
da Silva et al. 2009, 2011; Magalhaes et al. 2011).
IWs can play an important role in several dynamic aspects and in a wide variety
of geophysical processes. As already mentioned above, a more special case of these
J. M. Magalhaes et al.
14.1 Introduction
Internal Waves (IWs) are an important factor in several processes within geophysical
fluid dynamics. They are ubiquitous features in the oceans and in the atmosphere,
which can exist in a wide variety of time and space scales, and owe their existence
to the vertical density stratification that characterizes both mediums.
In the ocean, tidal flow over irregular bottom topography can force vertical motions at the tidal frequency and generate IWs of tidal period, which can then propagate
along the thermocline and are often referred to as Internal Tides (ITs). ITs can steepen
and generate IWs of shorter period that are usually termed Internal Solitary Waves
(ISWs). The term ‘solitary’ is used since they tend to occur in individual packets
(usually trapped in the troughs of the IT), and have often been identified with the
soliton solutions of nonlinear wave theory. In the atmosphere, these short-scale waves
that are usually highly nonlinear, can also be observed to propagate in the lower troposphere, and are often accompanied by roll-cloud formations that resemble the
packet-like structure of their oceanic counter-part. Their origin can be attributed to
various generation mechanisms, often involving convection, wind-shear, gravity currents and different air mass collisions. In summary, any atmospheric process leading
to the displacement of density surfaces can be a potential source of atmospheric IWs,
providing that suitable propagation conditions are previously met.
During the last few decades remote sensing has played a key role, in providing the
scientific community with an increasing number of IW observations (both of oceanic
and atmospheric nature). Imaging sensors such as Synthetic Aperture Radars (SARs),
as well as visible and near-infrared spectrometers, have proved to be particularly useful in providing new and valuable insights into these waves’ physical characteristics.
These new measuring capabilities boosted the number of observations, and led to
several studies that indicated that the phenomenon is in fact much more frequent than
previously thought (see e.g. Jackson 2007; Jackson et al. 2012; da Silva and Magalhaes 2009; Magalhaes et al. 2011). SARs in particular have proved very useful in
revealing the two dimensional structure of IWs, as well as their spatial and temporal
distributions. It is well known that both oceanic and atmospheric IWs are capable
of producing roughness patterns on the ocean surface. Therefore, they can generate
recognizable signatures in radar images. The oceanic roughness patterns are a result
of either hydrodynamic modulation of Bragg waves by surface current gradients or
wave damping due to surface films (Alpers 1985; da Silva et al. 1998). Similarly,
the atmospheric IW signatures are a consequence of wind stress variability introduced by the disturbed wind velocity field associated with wave propagation (Alpers
and Stilke 1996). SAR imagery has also been playing a key role in identifying and
characterizing several IW hotspots, as well as in providing valuable insights into the
generation mechanisms (see e.g. New and da Silva 2002; Vlasenko and Alpers 2005;
Nash and Moum 2005; da Silva and Helfrich 2008; da Silva and Magalhaes 2009;
da Silva et al. 2009, 2011; Magalhaes et al. 2011).
IWs can play an important role in several dynamic aspects and in a wide variety
of geophysical processes. As already mentioned above, a more special case of these