Dn-Dz

 

Dobson units

See total ozone.

 

DOC

Abbreviation for dissolved organic carbon. This consists of three pools: (1) a refractory pool not readily usable by biological organisms with a turnover time on the order of ocean mixing cycles, (2) a usable labile pool with turnover time from hours to a day, and (3) a semi-labile pool with a turnover time of months to season.

 

DODS

Acronym for the Distributed Ocean Data System. See the DODS Web site.

 

Dole effect

Defined as the difference between the of atmospheric in air and the of contemporaneous seawater. This effect mainly reflects the isotopic composition of produced by marine and terrestrial photosynthesis, as well as the extent to which the heavy isotope is discriminated against during respiration. Secondary factors include changes in terrestrial and marine fertility, varying isotope fractionation associated with the hydrologic cycle, and changes in respiratory isotope effects on either a species or community level. See Bender et al. (1994).

 

DOM

Abbreviation for Dissolved Organic Matter. This includes colloidal as well as purely dissolved material and is operationally defined as all organic matter that will pass through a filter with a fine mesh size, typically between about 0.1 and 1 m. See Najjar (1991).

 

DON

Abbreviation for Dissolved Organic Nitrogen.

 

Doppler broadening

In atmospheric radiative transfer, a process by which the broadening of absorption lines results from the movement of molecules relative to a photon, which can cause the frequency of radiation to be Doppler-shifted. This allows a broader range of frequencies of radiation to effect a particular transition, and is the dominant broadening mechanism at high altitudes where the collisions responsible for pressure broadening are less frequent.

 

DORIS

Acronym for Doppler Orbitography and Radio-positioning Integrated by Satellite.

 

double diffusion

In physical oceanography, this refers to the difference in the molecular diffusion rates of heat and salt in sea water, the molecular diffusion rate of heat being about 100 times that of salt. If two water masses with the same density but different combinations of temperature and salinity are in contact, the double (or differential) diffusion can give rise to density changes that render the layers unstable. Two phenomena that are possible consequences of this are double diffusive instability and layering.

 

double diffusive convection

See double diffusive instability.

 

double diffusive instability

In physical oceanography, this is a consequence of the double diffusion phenomena. If a layer of warmer, saltier water overlies a layer of cooler, fresher water such that the density of the upper layer is equal to or less than the density of the lower layer, the saltier water at the interface will lose heat to the cooler water below faster than it will lose salt because of the differences in molecular diffusivities. This may cause the water immediately above/below the interface to become denser/lighter which will cause it to sink/rise to the lower/upper layer. These fallings and risings occur in thin columns and the process is referred to as salt fingering, and the process of the water actually moving as double diffusive convection (also thermohaline or thermosolutal convection).

 

double Kelvin wave

To be completed.

 

downburst

In meteorology, a phenomenon occurring in the downdraft of thunderstorms where the rain-filled air of the storm mixes with neighboring drier air. The resulting mixture, not being saturated, allows evaporation from the raindrops and consequent cooling of the air due to the absorption of latent heat. This increase the negative buoyancy of the downdraft and causes it to accelerate into a downdraft called a downburst. Downbursts of 0.5 to 5 km in diameter and with speeds of almost 10 m/s 100 m above the ground have been observed.

 

Drake Passage

A narrow constriction between South America and Antarctica between 56 and 63 deg. S through which the Antarctic Circumpolar Current (ACC) must accelerate and squeeze in its trip around the globe. It is about 780 km wide at a depth of 500 m. Direct current and bottom pressure measurements in the Passage have led to estimates of ACC flow of around 110-130 Sv through it.

 

dry adiabatic lapse rate

The temperature lapse rate of dry air which is subjected to adiabatic ascent or descent. Its magnitude is 9.76 deg. C per km. (This rate also applies to unsaturated moist air.)

 

dry-bulb temperature

The temperature of the ambient air and water vapor mixture measured by a thermometer where the thermal element is dry and shield from radiation.

 

dry line

A boundary between warm humid and warm dry air. There is little temperature contrast across dry lines so they cannot be labeled as fronts. Convective clouds are often triggered along dry lines from midday through the afternoon, and can on occasion grow into organized thunderstorm squall lines that propagate to the east of the dry line. They tend to move eastward during the morning and return westward during the evening.

 

dry snow zone

One of five glacier zones classified on the basis of ice temperature and amount of melting. The dry snow zone in one in which no melting occurs, even in summer. The boundary between this and the next underlying type of zone, the percolation zone, is called the dry snow line. The only dry snow zones are found in the interiors of Greenland and Antarctica.

 

DSDP

Abbreviation for the Deep Sea Drilling Project, the predecessor to the ODP.

 

DSP

Abbreviation for digital signal processing.

 

DSRT

Abbreviation for Deep-Sea Reversing Thermometer.

 

DTR

Abbreviation for Diurnal Temperature Range, the range of temperature over a 24-hour cycle.

 

dust veil index

A measure devised by H. H. Lamb to assess the possible influence of volcanic dust on climate. There are three forms of the index which use, in various combinations, the following variables: the greatest percentage depletion in monthly direct solar radiation in the hemisphere in which the eruption occurred; the greatest proportion of the Earth affected by the dust veil; the estimated lowering of average temperatures for the most affected year over the middle latitude zone of the hemisphere in which the eruption occurred; and the total time between the eruption and the last observation of the dust veil or its effect upon monthly temperatures or radiation in middle latitudes. The reference dust veil index is 1000, based on the Krakatoa eruption of 1883. See Robock (1991) for a review.

 

DVI

See dust veil index.

 

DWBC

Abbreviation for Deep Western Boundary Current.

 

dwell sounding

A mode of operation for detection devices on satellites with several detectors and/or filters, e.g. VAS. The same line is scanned many times by the same detector through the same filter to improve the SNR of the measurement. The number of scans per channel is called the spin budget.

 

dynamic depth

See dynamic height.

 

dynamic distance

See dynamic height.

 

dynamic height

In oceanography, this refers to the pressure associated with a column of water. Horizontal variations of this (due to horizontal variations in temperature and salinity) are mapped to determine what is called the dynamic topography and its corresponding geostrophic flow field in the ocean. The dynamic height is measured in dynamic meters and is defined by

where and are two reference pressure levels, the specific volume anomaly, T the temperate, S the salinity, and p the pressure. This is analogous to a meteorologist's use of a pressure chart, with the direction of flow aligned with the contours and the intensity of flow inversely proportional to the contour spacing. Dynamic heights are preferred over geometric heights in oceanography and meteorology because energy is generally lost or gained when a parcel of fluid moves along a surface of equal geometric height but not when it moves along a surface of equal dynamic height. This quantity has also been called dynamic thickness, dynamic distance, geopotential height, geopotential thickness, and geopotential distance.

 

dynamic meter

In oceanography, a unit of gravity potential used to express the amount of work that is performed or gained in moving a unit mass from one level to another. A dynamic meter represents the work performed in lifting a unit mass nearly 1 m and is defined as 10 dyn-cm/gm or 10 J/kg. The depth in dynamic meters is related to the depth in geometric meters via , where g is the gravitational acceleration and h the geometric depth, i.e. 1 dynamic meter corresponds roughly to 1.02 geometric meters. This is used instead of the geometric meter since gravity is the most important of the acting forces and as such a coordinate system based on gravity is advantageous.

 

dynamic method

In oceanography, a method for determining the relative geostrophic flow field in the ocean from the distribution of density in the ocean. An absolute geostrophic flow field can additionally be found with the additional assumption of a level of no motion.

 

dynamic thickness

See dynamic height.

 

dynamic topography

In oceanography, a field of horizontally varying dynamic heights in the ocean, analogous to, for example, a topography field on land. This is also called geopotential topography.

 

dynamic velocity

See friction velocity.