- Acronym for Research on Antarctic Coastal Ecosystem Rates, a
JGOFS program designed to test several
hypotheses regarding the interaction of biological and physical
processes in antarctic coastal regions in general, and the importance
of the study area as nursery ground for antarctic krill in particular.
The principal objective of this 1986-1987 program was the study
of the physical and biological processes causing the high productivity
in the coastal waters of the Antarctic Peninsula.
RACER was a comprehensive, 4-month field study conducted in a
25,000 km region of the western Bransfield Strait during the 1986-1987
Some of the significant results of the RACER program were:
See Huntley et al. (1991).
- the documentation of an extensive phytoplankton bloom in the
northern Gerlache Strait with biomass estimates 750 mg Chl a
m and primary production rates in excess of
4 g C m day, the initiation, continuation, and demise
of which was controlled largely by the physical conditions of the
water column and, specifically, by the depth of the surface mixed
- the partial pressure of CO was reduced to 100 atm
in the regions of extensive bloom formation, thereby creating a
potentially large sink for atmospheric CO;
- investigations of the population dynamics, distribution, abundance and
growth of the krill species Euphausia superba identified at least
two year classes of immature and adult populations in the study area,
with three biogeographic zones identified;
- comprehensive hydrographic surveys confirmed the presence of several
different water masses, two major frontal structures, and a flow from
the southwest to the northeast across the study area called the
Bransfield Current; and
- a second RACER field experiment was designed for November-December
1989, which would focus more closely on the initial stages of the spring
bloom phenomenon in a smaller geographic area.
- An acronym for radio detection and ranging, the use of
reflected electromagnetic radiation to obtain information about
distance objects. The wavelength used in normally in the
radio frequency spectrum between 30 m and 3 mm.
- An earth observation satellite developed by Canada to provide
information for researchers in such fields as agriculture,
cartography, hydrology, forestry, oceanography, ice studies, and
coastal monitoring. The satellite, launched on Nov. 4, 1995 by the
Canadian Space Agency (CSA),
carries a C-band SAR capable of imaging
a ground swath 500 km wide at 100 meter resolution. The
expected lifetime of RADARSAT is five years.
RADARSAT-1 circles the Earth at an altitude of 798 km and an inclination
of 98.6 deg. to the equatorial plane. It has a sun-synchronous orbit,
making its overpasses always at the same local mean time.
The satellite's SAR can shape and steer its beam
from an incidence angle of 10 to 60 degrees, in swaths from 45 to
500 km in width, with resolutions ranging from 8 to 100 m.
It covers the Arctic daily and most of Canada every three days, with
data downlinked in real time or stored onboard until the satellite is
within range of a receiving station.
A RADARSAT-2 is in the planning stages.
- radar altimeter
- An instrument that uses radar to determine a
vehicle's (e.g. a satellite) height above the surface and for measuring
the height of small objects (e.g. waves, hills) on a planetary
surface. In oceanography, the former capability is used to
obtain the absolute sea surface height in relation to the
geoid, and the latter to gather information
about oceanic wave fields.
An altimeter works by transmitting an electronic pulse in the microwave
frequency to the Earth's surface. The pulse reflects off the surface
and returns to the sensor, with altitude determined from the pulse travel
time and from the waveform of the returned pulse.
- The radiation energy per unit time coming from a specific direction
and passing through a unit area perpendicular to the direction.
- radiant flux density
- See irradiance.
- radiation stress
- A mechanism whereby waves can exert a stress on the fluid in which
they propagate. This stress tensor was discovered and named by
Longuet-Higgins and Stewart (1964) and defined as the excess flux
of momentum due to the presence of waves. Gradients in this
quantity therefore correspond to a net addition of loss of
momentum to a water column, i.e. a net force, arising from
the processes of wave shoaling and breaking. The theoretical
work was prompted by laboratory experiments with breaking waves
that showed a mild depression or set-down in sea level in the vicinity
of the wave breaking point and a larger elevation or set-up
throughout the rest of the surf zone.
If longshore uniformity is assumed, then the
flux of x-directed momentum
is given (correct to second
where is the wavenumber, the wavelength, the
depth below still water, and the wave energy density
where is the fluid density, the acceleration due
to gravity, and the wave height. This will given, for
equilibrium conditions, a momentum balance of the form
is the adjustment of the sea level away
from still water level, i.e. the sea level will adjust
until the radiation stress gradients are everywhere balanced by the
sloping sea level.
See Holman (1990),
- radio altimeter
- See radar altimeter.
- See carbon-14.
- radiocarbon dating
- See carbon dating.
- radioisotopic dating methods
- Dating methods that take advantage of the fact that
unstable atoms called radioactive isotopes
undergo spontaneous radioactive decay by the loss
of nuclear particles and may transmute into a new element.
If the decay rate is invariable a given amount
of a radioactive isotope will decay to its daughter product
in a known interval of time, creating a geological clock
by which large time intervals can be measured.
Measuring the present isotope concentration indicates the
amount of time that has passed since the sample was emplaced
and the clock, i.e. the decay process, started. An important
factor is the time it takes for the material to decay to half its
original amount, i.e. its half-life, an indicator of the
length of the time interval over which it can be used.
A radioisotope's usefulness for dating is dependent
on whether it or its daughter products occur in
measurable quantities and can be distinguished from
other isotopes or have a measureable decay rate.
It must also have a half-life appropriate to the period
being dated, a known initial concentration, and some connection
between the event being dated and the start of the radioactive
Radioisotopic dating methods can be divided into three major
See Bradley (1985).
- See Racki and Cordey (2000).
- radiolarian ooze
- A deep-sea sediment composed of at least 30% of the remains of
siliceous radiolarians. These sediments
occur in the equatorial Pacific and Indian ocean regions where
the depth exceeds the
carbon compensation depth
and therefore aren't overwhelmed
by calcareous ooze.
These form deep deposits covering 1-2% of the ocean floor, and are a type
of siliceous ooze along with
See Tchernia (1980).
- A device that uses a photocell to measure the power of a
specific light field.
- The use of a radiometer to
quantitatively describe the power from a specific light
field. The description can be made in terms of several
properties including magnitude, geometrical distribution
(or direction), spectral distribution, state of polarization,
and time variability.
Before the advent of satellite oceanography, the primary use of radiometry
was to sample the radiant power in
the vicinity of an organism to obtain quantitative information
about how it reacts to light.
Now the use of radiometers in instruments aboard
satellites to measure various properties of incident,
reflected and emitted radiation
is nearly ubiquitous, with new types of radiometers
seemingly developed for each new mission.
See Tyler (1973) for a discussion of the physics of
radiometry and its appliation to studying the responses of
organisms to light.
- An isotope of radium that is useful as a tracer in ocean studies.
It is the 5.75 year half-life daughter of thorium-232.
Thorium, a highly insoluble substance, is delivered to shelf
and deep ocean
sediments chiefly in detritus of
continental origin. This decays into radium which dissolves
off the particles and diffuses into the water column where it
is mixed by diffusion and advection.
This leads to a generic
profile with a relative maxima at the surface and near bottom
with the surface concentration decreasing with increasing
distance from the shore (and the near-surface shelf sediment
See Sarmiento (1988) and
Broecker and Peng (1982).
- radius of deformation
Rossby radius of
- A subsurface float introduced by Thomas Rossby in 1985 that listens
to acoustic signals instead of transmitting (like the
earlier SOFAR float). At the end of its
mission it surfaces by dropping a weight and uploads to the
Argos satellite all the information it collected at depth,
including the Times of Arrovals (TOAs) of pulses sent by sources
at known geographical positions.
See Rossby et al. (1986).
- random variable
- A function (or mapping) from the sample space of possible outcomes
of a random experiment to the real line, the complex plane, or
some other such mappable entity. Basically, it's a variable
denoting and containing the outcome of a random experiment,
families of which comprise a
- Abbreviation for Real Aperture Radar.
- Acronnym for Regional Association for Research on the Gulf of
Maine, an association of institutions which have active research
interests in the Gulf of Maine and its watershed. It was founded
in 1991 and is housed at Dartmouth College. The missions of
the association are to advocate and facilitate a coherent program
of regional research, to promote scientific quality, and to provide
a communication vehicle among scientists and the public.
RARGOM Web site.
- Ras al Hadd Jet
- An intense offshore jet that forms at the easternmost tip of Oman as
the East Arabian Current (EAC) separates
from the coast at the eastern tip of the Arabian Peninsula.
The RAH Jet is found at the northern edge of EAC during the southwest
monsoon, and may be considered as its offshore extension.
As the wind regime reverses and the EAC weakens, the RAH Jet becomes
the southern edge of the warm circulation in the Gulf of Oman.
See Böhm et al. (1999).
- Acronym for Research on Antarctic Shallow and Littoral Systems.
- Rayleigh-Bénard convection
- See Bodenschatz et al. (2000).
- Rayleigh number
- A dimensionless number used
for describing unstable stratified flows.
It expresses a balance between thermal expansion, temperature, thermal
diffusivity, viscosity, and the thickness of a convecting layer, with
the most significant parameters being the depth and viscosity of the
The Rayleigh number can be defined as:
where is gravitational acceleration,
the thermal expansion coefficient,
the kinematic viscosity,
the thermal diffusivity, and
a width scale.
This is equivalent to:
where is the
Grashof number and
It expresses the competition between overturning due to top-heavy
density due to temperature expansion and viscous and diffusive smearing
of the buoyancy.
Convection begins at a Rayleigh number of around 2000, with irregular
chaotic convection being near . The higher the number, the more
mixing occurs in the substance being convected.
It is around in the ocean thermocline and
in the atmosphere boundary layer.
This is the natural convection equivalent of the
Peclet number used in forced convection.
- recirculating current
- See recirculating gyre.
- recirculating gyre
- Strong opposing flow elements adjacent to western boundary currents,
e.g. the Gulf Stream in the upper ocean and the deep western boundary
current in the deep water of the North Atlantic. These are a
subbasin-scale component to the large-scale gyre flow, and can
dominate the distribution of transport in the basin interior.
See Schmitz and McCartney (1993),
Hogg and Johns (1995) and
- Alfred Redfield (1890-1983)
- Discover of the Redfield ratios.
- Redfield ratios
- These represent the relatively constant proportions maintained
between the elements C, N, P and O taken up during the synthesis and
released by subsequent remineralization of organic matter by
marine organisms. It was originally suggested that during organic
matter cycling, carbon, nitrogen, phosphorous and oxygen are cycled
in the ratio C:N:P:O2 = 106:16:1:138, i.e. for every phosphate ion
taken up during photosynthesis, 16 nitrate ions and 106 molecules
are taken up and 138 molecules of oxygen are produced.
More recent studies have modified the ratios to 140:16:1:172.
See Redfield et al. (1963) and
Takahashi et al. (1985).
- red noise
- Noise with relatively enhanced low frequency power that results
simply from serial correlation. The resulting power spectrum
will have a negative slope.
This is usually a good model
for the noise component in a variety of climatic time series
including proxy records, historical sea
and air surface temperatures, and precipitation records.
This type of noise can be explained in terms of the slow-response
components of the climate system, such as the thermal inertia
of the oceans, providing a memory that effectively integrates
the forcing of such fast-reponse and more white noise-like
components such as the weather. The produces a temporal persistence
that leads to great noise energy at lower frequencies.
Contrast with white noise.
- Red Sea
- A long, narrow marginal sea centered at about
38 E and 22 N which separates the African
and Asian continents.
Its total length is 1932 km and the average width 280 km, with
a maximum width of 306 km and a minimum width of 26 km.
The area is about 450,000 km and the volume around 50,000 km.
The average depth is about 491 m with the greatest depths over
2500 m in the trough between 19 and 22 N.
The Sinai peninsula divides the northern part into the shallow
Gulf of Suez to the west and the deep Gulf of Aquaba to the east.
The southern limit, which separates it from the Gulf of Aden,
is a line joining Husn Murad and Ras Siyan.
The circulation in the Red Sea is summarized in
RSMAS (2000) as:
The Red Sea is similar to the Arabian Gulf in that it acts as an
inverted estuary, with dense, salty water
formed by evaporation and deep convection in the northern
Red Sea flowing out into the Gulf of Aden
underneath a fresher inflowing layer from the Gulf of Aden
(Fig. 3b). Unlike the Arabian Gulf, however, the
exchange is known to be highly seasonal, with maximum
exchange occurring in winter. Indirect estimates of
the transport of Red Sea water through the Bab el Mandeb
Strait suggest an annual mean transport of 0.33 Sv
(Siedler, 1968), varying from approximately 0.6 Sv in
winter to nearly zero in late summer (Patzert (1974)). The
winter period (November-May) is characterized by a classical
two-layer exchange flow (Siedler, 1968).
However, in summer the northwesterly winds apparently
drive a three-layer exchange, consisting of a thin
surface outflow from the Red Sea, an inflowing layer of
Gulf of Aden thermocline water, and a weak
outflowing deep layer (Maillard and Soliman (1986)).
Estimates of the annually averaged rate of Red Sea deep water formation
range from 0.06 Sv
to 0.16 Sv (Cember (1988)). This water forms in the northern
Red Sea predominantly during winter, and fills the
deep basin below the Bab el Mandeb Strait sill depth (approximately
160 m) with a nearly homogeneous water mass of temperature 21.7 C
and salinity 40.6 psu (Neumann and McGill (1962)). A second
source of somewhat less dense Red Sea water, or Red Sea
"intermediate" water, is believed to be formed also predominantly
in winter by an open sea convection process in the northern Red Sea
that remains poorly understood (Morcos (1970)). This process
appears to be distinct from the Red Sea deep water formation process
that occurs in the northern gulfs of the Red Sea (Gulf of Suez
and Gulf of Aqaba) and that fills most of the deep volume of the Red Sea.
Another class of intermediate waters may be formed on shallow
shelves in the southern Red Sea. Volumetrically, the
rate of intermediate water formation appears to be greater than the
rate of deep water formation, and is thought to supply the main
contribution to the lower layer outflow from the Red Sea through
Bab el Mandeb.
The seasonal cycle of the Red Sea exchange through the Bab el
Mandeb is driven primarily by the seasonal change in winds over the
southern Red Sea and Gulf of Aden (Fig. 1). In winter the
southeasterly winds act to reinforce the thermohaline circulation of upper
layer inflow and deep outflow. Conversely, in summer the
northwesterly winds act in opposition to the thermohaline forcing and this may
partly explain the reversal to outflow in the surface layer of the
strait in summer. Upwelling in the western Gulf of Aden during summer
is also believed to play a role in forcing the surface current reversal
in the Strait and thermocline layer intrusion into the Red Sea, by
changing the stratification and sea level in the western gulf and hence
affecting the alongstrait pressure
gradient (Patzert (1974)). The
relative importance of these two wind-forced effects, one direct
and one indirect, is not yet clear. Seasonal changes in surface
buoyancy forcing may also affect the seasonality of the exchange, but
this possibility has yet to be investigated.
The Red Sea water exits the Bab el Mandeb strait with a sill depth
of 160 m and spills down the topography of the western Gulf of
Aden where it entrains resident Gulf waters and sinks to an average
depth of about 600 m. Hydraulic control of the outflow is much
debated and there is as yet no consensus on the exact nature of
hydraulic controls that may govern the exchange. The overflow
character of the outflowing Red Sea water is suggestive of hydraulic
control. However, there is no evidence of a Gibraltar-like internal
bore, a feature that would serve as indirect evidence for hydraulic
control. Recently, a three-layer
hydraulic model that reproduces the gross characteristics of
the stratification and exchange in both summer and winter has
been constructed. However the
critical conditions required in the summer and winter solutions
differ considerably from direct wave speed calculations
based on data collected by
at the sill and narrows.
The horizontal circulation of the Red Sea appears to consist
of a number of gyres or eddies distributed along the length of the Sea (Fig.
3b), of which some may be semi-permanent (Quadfasel and Baudner (1993)).
There is little detailed information on this circulation as
most studies have tended to treat the Red Sea as a two-dimensional
basin. Most oceanographic measurements are therefore confined
to its central axis. In the northern Red Sea, drifter trajectories
point to a cyclonic gyre at least in
winter (Clifford et al. (1997)). This gyre
may be linked to the aforementioned intermediate water formation
process in the northern Red Sea, and could possibly serve in a
preconditioning role for the intermediate water formation. In the
central Red Sea the circulation appears to be dominated by anticyclones
that occur most regularly near 23-24 N and 18-19 N. These locations
may be tied to coastline and topography
variations (Quadfasel and Baudner (1993)).
Both cyclonic and anticyclonic features are
found in the southern Red Sea but no persistent gyre pattern seems to
exist there. When present, these gyres usually span most of the
width of the Red Sea and can have horizontal velocities of 0.5 m/s or
more. Thus they are energetic compared to the 0.1 m/s mean flows in
the surface layer associated with the large scale thermohaline
circulation of the Red Sea.
Coastal boundary currents may exist both in the southern Red Sea
off Yemen and in on both sides of the northern Red Sea (Eshel and Haik (1997))
Little direct evidence is available for these currents,
however. Particularly in the northern Red Sea, the opposing influences
of the wind and thermohaline forcing throughout the year make it
unclear what sense should be expected for these boundary currents.
The central axial zone of the Red Sea contains a series of
0.02-60 km basins between 1500 and 2800 m deep.
These are filled with anoxic, dense and hot brines whose temperatures
range from 23.25-44.60C and salinities from 144 to 270 ppt.
The transition zone between brines and overlying seawater is
marked by strong gradients, and therefore extremely stable, i.e.
the transfer of properties across it is controlled mostly by
See Neumann and McGill (1962),
Maillard and Soliman (1986),
Quadfasel and Baudner (1993),
Tomczak and Godfrey (1994),
Clifford et al. (1997)
Eshel and Haik (1997) and
Bower et al. (2000).
- red tide
- More later.
- redox discontinuity layer
- A zone of rapid transition between areas of aerobic and
anaerobic decomposition in oceanic sediments. Its depth within
the sediment depends on the quantity of organic matter available
for decomposition and the rate at which oxygen can diffuse down
from the overlying water. For example, in organic muds,
relatively impermeable to oxygen-carrying water, the upper
aerobic layer may only be a couple of millimeters deep, while
in permeable sands with a low rate of organic input aerobic
conditions can extend for tens of centimeters.
See Barnes and Hughes (1988).
- reduced gravity
- In oceanography, a term that arises when the
is made where variations in density are neglected when they affect
inertia but retained when they affect buoyancy, i.e. when they
occur in the combination
where is the reduced gravity, the normal
a density perturbation, and a standard reference
See Turner (1973).
- More later.
See coral reef.
- A global database on coral reefs and their resources.
This is available on CD-ROM from
ReefBase Web site.
- reference level
- A depth, pressure or density level at which the horizontal
current field is either known from direct measurements or
indirectly estimated. This may be zero velocity surface
or one with non-zero horizontal velocities.
This reference level is combined with the relative velocity
fields obtained via the
to obtain fields of absolute
The techniques of
satellite altimetry have
provided another possibility for a reference level, i.e. the
ocean surface. If the vertical departure of the ocean surface
from the local geoid can be measured
with sufficiently accuracy then it can be used as a reference
This is also known variously as the level of no motion, the
level of known motion, the zero velocity surface, etc.
- In radiation transfer, the fraction of incoming radiation that
is reflected from a medium.
The sum of this, the transmittance,
and the absorptance must equal unity.
- regenerated production
- The uptake of ammonium by phytoplankton in the
It is so-called because ammonium is a product of internal processes
within the euphotic zone and it is therefore recycled or
See Najjar (1991).
- regional modeling
- In climate modeling this is defined as simulating the climate
over a limited area or region rather than over the entire globe
using Regional Circulation Models (RegCM).
The boundary conditions needed to drive these models are supplied
either from GCM output via a procedure called
nested modeling or from
analyses of observations. The RegCMs perform consistently
better when driven by observations than by GCM output. This
is largely due to the lack of regional scale geographical
features (e.g. coastlines, lakes, etc.) and their concomitant
climate effects in the output of GCMs, effects which are
implicitly included in observations. Increased GCM resolution
is found to improve RegCM simulations. This is a felicitous
result since a lack of adequately dense observational
data is the major limitation of using observations to drive
See Houghton and Filho (1995).
- regional sea
- A body of water smaller than the main sections of the world ocean
that is bound by geographic and/or hydrographic regions.
Regional seas whose names can be encountered in the
literature include the
Australasian Mediterranean Sea,
Sea of Azov,
East China Sea,
East Siberian Sea,
South China Sea,
White Sea, and the
- relative humidity
- The ratio of the observed mixing ratio
in a sample of moist air to the
saturation mixing ratio
with respect to water at the
same temperature. It is given by
where is the specific humidity
and the saturation specific humidity.
- relative vorticity
- The vorticity imparted to a parcel
or column of fluid by fluid motion. It is a characteristic of
the kinematics of the fluid flow which expresses the tendency
for portions of the fluid to rotate. Technically speaking, this
is the curl of the fluid velocity vector, although in oceanography
and meteorology it is usually only the vertical component of
the curl of the horizontal
velocity vector since all other components are usually negligible.
- Rennell, James (1742-1830)
- See Peterson et al. (1996), p. 47.
- Rennell's Current
- ``A relatively strong (1.0 to 1.5 knots) nonpermanent current that
sets northward across the western approaches to the English Channel.
The current appears to be independent of the North Atlantic Drift
or local winds and occurs most frequently during winter.''
From Baker, Jr. (1966).
- research submersibles
- More later.
- research vessels
- See Estok and Boykin (1976),
Treadwell et al. (1988),
Wust (1964) and the
oceanography history section
for further details.
- Research Vessel Technical Enhancement Committee (RVTEC)
- An organization of technical support personnel associated with
the university oceanographic Research Vessel fleet of the U.S.
RVTEC is charted by UNOLS and
publishes a newsletter called ``INTERFACE.''
RVTEC Web site.
- In numerical modeling, the distance between
contiguous points in the computational grid.
This can refer to either temporal or spatial resolution, with the
two being dependent in procedures using both.
- resonance angle
- The angle at which the component of the wind speed acting in the
direction of a wave field is equal to the wave speed.
From Baker, Jr. (1966).
- A general class of phenomena where, after a
storm surge, the water level falls,
rises, falls again, rises again, and so on for many hours after the
passage of a hurricane. This has been variously explained as
being due to oscillating long waves, edge waves, Kelvin waves
or some combination thereof.
See Wiegel (1964).
- See daily retardation.
- In oceanography, this refers to a geographical looping of a current
away from its original direction to a substantially different
See Schmitz and McCartney (1993).
- Revelle, Roger (1909-1991)
- More later.
- Revelle factor
- See buffer factor.
- reversed tide
- A tide completely out of phase with the
apparent motions of the principal attracting body, i.e. the
lowest heights are directly under the body on opposite sides
of the earth. See also direct tide.
From Baker, Jr. (1966).
- reversing current
- See Baker, Jr. (1966).
- reversing thermometer
- Reynolds equations
- An equation set for turbulent flow wherein the instantaneous
values of the dependent variables
in the equations of motion are split into mean and fluctuating
parts, e.g. u = U + u where
is the mean and the turbulent or
fluctuating part. These are substituted into the equations of motion and
an average is taken over a suitable period of time (where ``suitable''
means an averaging interval large compared to the timescale of the
turbulent fluctuations yet small compared to the timscale of the
change of the mean flow) to obtain the
Reynolds equations. These have the same form as the original motion
equations - with mean quantities replacing total quantities - except
for new terms involving velocity fluctuations that arise from the
nonlinear terms in the original equations. These terms represent
the effect of velocity fluctuations or turbulence on the mean
flow, and are called
Reynolds stresses since the
turbulence has an effect equivalent to stress on the mean flow.
The Reynolds equations can be expressed as:
where and are the mean and fluctuating velocity
are the spatial components,
is the alternating, third-order tensor,
is the vertical component of the rotation vector (i.e. the
is a constant reference density,
is the pressure,
is the mean part of the second-order, symmetric
viscous stress tensor defined as:
where is the kinematic viscosity,
is the density,
The Reynolds equations give rise to what is known as the
closure problem, where the averaging procedure results in
new unknowns in the form of the fluctuating quantities
obtained from the nonlinear terms. Specific expressions for
these fluctuating quantities can be obtained but at the price
of generating yet more unknowns, ad infinitum. At some point
a closure assumption must be made and the fluctuating quantities
parameterized in terms of known quantities like the mean
flow. The use of the
eddy viscosity concept is the simplest
way of obtaining closure.
This is ultimately a problem of flow resolution. If we could
explicitly model the flow at a sufficiently high resolution (i.e.
on a sufficiently small grid) then we wouldn't need to use an
eddy viscosity since the molecular viscosity would suffice.
Unfortunately, the length scale required for this is on the
order of a millimeter or less, rendering it infeasible to
explicitly model flow in a pipe (much less atmospheric or oceanic
flow) without parameterizing the turbulent, i.e. unresolved,
portion of the flow in terms of the mean, i.e. resolved, portion
of the flow.
- Reynolds stresses
- Stress terms obtained by transforming the equations of motion into
the Reynolds equations.
They are so-called in analogy to the terms in the original
motion equations involving the molecular viscosity, and to
further the analogy the concept of an
eddy viscosity is used to
perform closure on the Reynolds equations and render them
The forces that give rise to the stresses are due to the fact that
in a turbulent flow there are rapidly fluctuating as well as
mean components. The fluctuating components oppose the mean
motion and redistribute energy and other properties
via a physical effect analogous to molecular friction, i.e. turbulent
friction. This causes a more rapid distribution of momentum, heat
and salt than would occur solely via molecular processes, and the
analogous stresses are called Reynolds stresses.
- Reynolds stress tensor
- A quantity arising in the development of the
Reynolds equations defined as
where is a constant reference density
is a matrix of the time average of the
products of the turbulent
velocity components. The instantaneous velocity
has been decomposed into average and fluctuating quantities, i.e.
and the overbar indicates a
- Reynolds number
- A dimensionless number
expressing the ratio of
viscous to inertial forces.
It is expressed by
where is the kinematic viscosity, an appropriate velocity scale,
and a horizontal length scale.
If this is at least one order larger than unity then viscosity cannot
significantly affect the motion; if it is much less than unity then
molecular viscosity plays a significant role.
See Kraus and Businger (1994), p. 29.
- Acronym for RADARSAT Geophysical
Processor System, a computer system that takes RADARSAT SAR
images of Arctic sea ice for input and creates geophysical
data products for output. These include sea ice motion,
the thickness distribution of new ice, and the backscatter
history of the ice.
RGPS Web site.
- Abbreviation for relative humidity.
- Rhodes Gyre
- See Milliff and Robinson (1992).
- Richardson, Lewis Fry
- More later.
- Richardson number
- A ratio of buoyancy to inertial forces which measures the
stability of a fluid layer.
There are several different definitions of this for various
and flux Richardson numbers.
See Turner (1973).
- Acronym for Ridge Inter-Disciplinary Global Environments
Initiative, a coordinated program aimed at understanding the
geology, physics, chemistry and biology of processes occurring
along the global mid-ocean ridge system.
RIDGE Web site.
- rigid lid approximation
- A filtering approximation incorporated
into oceanographic models to increase their computational efficiency.
This approximation filters out the fast barotropic
gravity waves by setting the time variation of the surface elevation
in the equations of motion equal to zero. A computational price is
paid for this approximation since it requires that a prognostic Poissonlike
elliptical equation be solved for the barotropic stream function
(or surface pressure) at each model time step. This can be a problem
as the condition number increases faster
than linearly with the resolution of the computational grid, causing
the equations to become increasingly difficult to solve.
This approximation also has dynamical effects that can be
non-negligible. For example, although a surface elevation can
be calculated from the prognostic surface pressure solution,
it is strictly applicable only in the limit of a steady-state and
as such the surface height cannot be accurately computed for
transient and nonequilibrated flow. Additionally, this approximation
effectively makes the phase speed of all barotropic
infinite and equilibrates them at all scales. This is a reasonable
approximation at mid- and high-latitudes where Poincare waves exist
at high frequencies, but not so good near the equator where they
evolve on a time scale equivalent to the
Finally, this approximation affects the phase speed of Rossby waves
with wavelengths greater than the
Rossby radius of deformation.
See Dukowicz and Smith (1994) and Thacker and Raghunath (1994).
- Rim Current
- A permanent, strong current system encircling the Black Sea basin
cyclonically over the continental slope zone.
It is accompanied by a series of anticyclonic mesoscale eddies as
well as transient waves with an embedded train of mesoscale eddies
propagating cyclonically around the basin.
According to Oguz and Besiktepe (1999):
The Rim Current is identified as a well-defined meandering
jet stream confined over the
steepest topographic slope and associated cyclonic-anticyclonic
eddy pairs located on both its sides. It has a form of highly energetic
and unstable flow system, which, as it propagates cyclonically
along the periphery of the basin, is modified in character. It possesses a
two-layer vertical structure with uniform upper layer speed in
excess of 50 cm/s (maximum value 100 cm/s), followed by a relatively
sharp change across the pycnocline (between 100 and 200 m)
and the uniform sub-pycnocline currents of 20 cm/s (maximum value
40 cm/s) observed up to the depth of 350 dbar, being
the approximate limit of ADCP measurements. The cross-stream velocity
structure exhibits a narrow core region (30 km), flanked
by a narrow zone of anticyclonic shear on its coastal side and a broader region
of cyclonic shear on its offshore side.
See Oguz and Besiktepe (1999).
- Rio de la Plata Estuary
- See Guerrero et al. (1997).
- rip current
- A narrow seaward return flow caused by waves breaking in the
surf zone and piling up water against the coast. This establishes
a hydraulic head which, combined with bathymetric irregularities
along the coast, causes the narrow seaward flow.
See Komar (1976).
- rip feeder current
- A current that flows parallel to the shore before converging and
forming the neck of a rip current.
- Abbreviation for Ross Ice Shelf Program or Project, a New
- An instance of the
phenomenon in the harbor of Ciutadella on the Island of Menorca in
the Balearic Islands.
See Monserrat et al. (1991).