Ra-Rm

RACER

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$^2$ region of the western Bransfield Strait during the 1986-1987 austral summer.

Some of the significant results of the RACER program were:

• the documentation of an extensive phytoplankton bloom in the northern Gerlache Strait with biomass estimates $>$750 mg Chl a m$^{-2}$ and primary production rates in excess of 4 g C m$^{-2}$ day$^{-1}$, 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 layer;
• the partial pressure of CO$_2$ was reduced to $\le$100 $\mu$atm in the regions of extensive bloom formation, thereby creating a potentially large sink for atmospheric CO$_2$;
• 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.

See Huntley et al. (1991).

[http://hahana.soest.hawaii.edu/racer/racer.html]

radar

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.

RADARSAT

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.

[http://www.space.gc.ca/csa_sectors/earth_environment/radarsat/]

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.

radiance

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 The x-directed flux of x-directed momentum is given (correct to second order) by

$${S_{xx}}\,=\,E\left({{2kh}\over{\sinh 2kh}}\,+\, {1\over 2}\right)$$

where $k$ is the wavenumber, $L$ the wavelength, $h$ the depth below still water, and $E$ the wave energy density given by

$$E\,=\,{1\over 8}\rho g {H^2}$$

where $\rho$ is the fluid density, $g$ the acceleration due to gravity, and $H$ the wave height. This will given, for equilibrium conditions, a momentum balance of the form

$${ {d{S_{xx}}} \over {dx} }\,+\,\rho g ({\bar\eta}\,+\,h) { {d{\bar\eta}} \over {dx} }\,=\,0$$

where ${\bar\eta}(x)$ 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.

radiocarbon

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 decay process.

Radioisotopic dating methods can be divided into three major groups:

• those that entail the direct measurement of radioisotopes or decay products, e.g. carbon-14 dating and potassium-argon dating;
• those that measure the degree to which members of a chain of radioactive decay are restored to equilibrium following some initial external perturbation, e.g. uranium-series dating; and
• those that measure the effect of some local radioactive process on the sample materials compared to the environmental flux, e.g. fission-track dating and thermoluminescence dating.

See Bradley (1985).

radiolaria

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 diatom ooze. See Tchernia (1980).

radiometer

A device that uses a photocell to measure the power of a specific light field.

radiometry

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.

radium-228

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 sources). See Sarmiento (1988) and Broecker and Peng (1982).

radius of deformation

See Rossby radius of deformation.

RAFOS

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 stochastic process.

RAR

Abbreviation for Real Aperture Radar.

RARGOM

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. See the 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).

RASCALS

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 layer. The Rayleigh number can be defined as:

$${R_a}\,=\,{ {g\alpha\Delta T {d^3}} \over {\nu\kappa} }$$

where $g$ is gravitational acceleration, $\alpha$ the thermal expansion coefficient, $\nu$ the kinematic viscosity, $\kappa$ the thermal diffusivity, and $d$ a width scale. This is equivalent to:

$${R_a}\,=\,{G_r}{P_r}$$

where ${G_r}$ is the Grashof number and ${P_r}$ is the Prandtl number. 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 $10^6$. The higher the number, the more mixing occurs in the substance being convected. It is around $10^{16}$ in the ocean thermocline and $10^{17}$ 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 McCartney (1992).

Alfred Redfield (1890-1983)

Discover of the Redfield ratios.

[http://www.nap.edu/readingroom/books/biomems/aredfield.html]

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$^\circ$ E and 22$^\circ$ 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$^2$ and the volume around 50,000 km$^3$. The average depth is about 491 m with the greatest depths over 2500 m in the trough between 19 and 22$^\circ$ 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 $\sim$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$^2$ basins between 1500 and 2800 m deep. These are filled with anoxic, dense and hot brines whose temperatures range from 23.25-44.60$^\circ$C 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 molecular diffusion. See Neumann and McGill (1962), Siedler (1969), Morcos (1970), Patzert (1974), Maillard and Soliman (1986), Cember (1988), Quadfasel and Baudner (1993), Tomczak and Godfrey (1994), Clifford et al. (1997) Eshel and Haik (1997) and Bower et al. (2000).

[http://mpo.rsmas.miami.edu/~zantopp/AMSG-report.html]

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 Boussinesq approximation 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

$$g'\,=\,{g\rho '\over{\rho_0}}$$

where $g'$ is the reduced gravity, $g$ the normal gravitational acceleration, $\rho '$ a density perturbation, and $\rho_0$ a standard reference density. See Turner (1973).

reef

More later. See coral reef.

ReefBase

A global database on coral reefs and their resources. This is available on CD-ROM from ICLARM. See the 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 geostrophic method to obtain fields of absolute geostrophic velocitiesa 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 level. This is also known variously as the level of no motion, the level of known motion, the zero velocity surface, etc.

reflectance

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 euphotic zone. It is so-called because ammonium is a product of internal processes within the euphotic zone and it is therefore recycled or regenerated nitrogen. 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 RegCM simulations. 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 Adriatic Sea, Aegean Sea, Aland Sea, Alboran Sea, Amundsen Sea, Andaman Sea, Arabian Sea, Arafura Sea, Aral Sea, Australasian Mediterranean Sea, Sea of Azov, Balearic Sea, Bali Sea, Baltic Sea, Banda Sea, Barents Sea, Beaufort Sea, Bellingshausen Sea, Belt Sea, Bering Sea, Bismarck Sea, Black Sea, Bohol Sea, Bothnian Sea, Burma Sea, Camotes Sea, Caribbean Sea, Caspian Sea, Catalan Sea, Celebes Sea, Celtic Sea, Ceram Sea, Chukchi Sea, Chukotsk Sea, Coral Sea, Cretan Sea, East China Sea, East Siberian Sea, Flores Sea, GIN Sea, Greenland Sea, Halmahera Sea, Iceland Sea, Ionian Sea, Irish Sea, Irminger Sea, Japan Sea, Java Sea, Jawa Sea, Kara Sea, Labrador Sea, Laptev Sea, Levantine Sea, Ligurian Sea, Lincoln Sea, Maluku Sea, Marmara Sea, Mediterranean Sea, Mindanao Sea, Molucca Sea, Nordenskjold Sea, Nordic Seas, North Sea, Norwegian Sea, Okhotsk Sea, Red Sea, Ross Sea, Samar Sea, Sargasso Sea, Savu Sea, Sawu Sea, Scotia Sea, Seram Sea, Sibuyan Sea, Solomon Sea, South China Sea, Sulawesi Sea, Sulu Sea, Sunda Sea, Tasman Sea, Tethys Sea, Timor Sea, Tyrrhenian Sea, Visayan Sea, Weddell Sea, White Sea, and the Yellow Sea.

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

$$U\,=\,{ {q(1\,-\,{q_w})} \over {{q_w}(1\,-\,q)} }$$

where $q$ is the specific humidity and $q_w$ 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.

[http://itri.loyola.edu/subsea/toc.htm]

research vessels

See Estok and Boykin (1976), Guberlet (1964), Nelson (1971), Rice (1986a), Treadwell et al. (1988), Wust (1964) and the oceanography history section for further details.

[http://scilib.ucsd.edu/sio/archives/histoceanogr/mills-handlist.html]

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.'' See the RVTEC Web site.

resolution

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).

resurgence

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).

retardation

See daily retardation.

retroflection

In oceanography, this refers to a geographical looping of a current away from its original direction to a substantially different direction. See Schmitz and McCartney (1993).

Revelle, Roger (1909-1991)

More later.

[http://scilib.ucsd.edu/sio/archives/siohstry/revelle-biog.html]
[http://www.nap.edu/readingroom/books/biomems/rrevelle.html]

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 $U$ is the mean and $u$ 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:

$${ {\partial {U_i}} \over {\partial {x_i}} }\, = \,0$$

$${ {\partial {U_j}} \over {\partial t} }\,+\, { {\partial} \over {\partial {x_k}} }({U_k}{U_j})\,+\, {\varepsilon_{jkl}}{f_k}{U_l}\, = \, -{1\over{\rho_0}}{ {\partial P} \over {\partial {x_j}} }\,+\, ... ...o\,-\, { {\partial} \over {\partial {x_k}} } \left(\overline{{u_k}{u_j}}\right)$$

$${ {\partial {\rho}} \over {\partial t} }\,+\, { {\partial} \over {\partial {x_k}} }({U_k}\rho )\, = \, { {\partial} \over {\partial {x_k}} } \left({k_T}{ {\partial\r... ...t)\,-\, { {\partial} \over {\partial {x_k}} } \left(\overline{{u_k}\rho}\right)$$

where $U_i$ and $u_i$ are the mean and fluctuating velocity components, respectively, $x_i$ are the spatial components, ${\varepsilon_{ijk}}$ is the alternating, third-order tensor, $f_k\,=\,{\delta_{k3}}2\Omega\sin\theta$ is the vertical component of the rotation vector (i.e. the Coriolis force), $\rho_0$ is a constant reference density, $P$ is the pressure, $\Sigma_{ij}$ is the mean part of the second-order, symmetric viscous stress tensor defined as:

$${\Sigma_{ij}}\,=\,2\nu\,{1\over 2} \left({ {\partial {U_i}} \over {\partial {x_j}} }\,+\, { {\partial {U_j}} \over {\partial {x_i}} }\right)$$

where $\nu$ is the kinematic viscosity, $g_j$ is gravity, $\rho$ 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 soluble.

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

$${\tau_{ij}}\,=\,-{\rho_0}\,\overline{{u_i}{u_j}}$$

where $\rho_0$ is a constant reference density and $\overline{{u_i}{u_j}}$ is a matrix of the time average of the products of the turbulent velocity components. The instantaneous velocity ${\tilde{u}_i}$ has been decomposed into average and fluctuating quantities, i.e. ${{\tilde u}_i}\,=\,{U_i}\,+\,{u_i}$ and the overbar indicates a time average.

Reynolds number

A dimensionless number expressing the ratio of viscous to inertial forces. It is expressed by

$$Re\,=\,{UL\over{\nu}}$$

where $\nu$ is the kinematic viscosity, $U$ an appropriate velocity scale, and $L$ 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.

RGPS

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. See the RGPS Web site.

RH

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 situations,including the overall, gradient, and flux Richardson numbers. See Turner (1973).

RIDGE

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. See the 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 Poincare waves 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 Rossby waves. 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 $\approx$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 $\approx$40 cm/s) observed up to the depth of $\approx$350 dbar, being the approximate limit of ADCP measurements. The cross-stream velocity structure exhibits a narrow core region ($\approx$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.

RISP

Abbreviation for Ross Ice Shelf Program or Project, a New Zealand project.

Rissaga

An instance of the meteorological tsunami phenomenon in the harbor of Ciutadella on the Island of Menorca in the Balearic Islands. See Monserrat et al. (1991).