Glossary and Acronyms: D

dd10:

direction of 10-min average wind.

Dangerous semicircle :

the side of a tropical cyclone to the right of the direction of movement of the storm in the Northern Hemisphere and to the left in the Southern Hemisphere, where the winds are stronger because the cyclone's translation speed and rotational wind field are additive.
The opposite side is termed the navigable semicircle. This terminology originated in the days of sailing ships. It occurred naturally since 1) the dangerous semicircle of the storm has the strongest winds and heaviest seas; 2) a sailing ship on this side tends to be carried into the path of the storm; and 3) if the storm recurves, its center is likely to cross the course of a ship running before the wind.

Depression :

region of the atmosphere in which the pressures are lower than those of the surrouding region at the same level. A depression is often associated to an area of disturbed weather. At upper level these low-pressure areas are low-geopotential areas.
A Tropical Depression is a tropical system in which the maximum 10-min average winds are between 52 km/h et 62 km/h or [28 to 33 knots].

Diabatic (process):

thermodynamic transformation during which a system yields heat to its environment or receives heat from it.

Diffluence:

progressive drawing apart of the streamlines in the directon of flow. The diffluence is not associated to a vertical motion, and mainly corresponds to a change in the direction of the wind, without any change in its intensity.

Divergence:

spreading out of an air mass into paths of different directions. In the atmosphere, horizontal divergence predominates, and the word "horizontal" is understood when this term is used. Divergence is linked with the vertical shrinking of the atmosphere, since, by the conservation of matter, an outflow of air must result.
Mathematically, divergence is the expansion of a vector field. Scalar quantity defined in certai conditions, for a vector field V (e.g. wind vector), byt he relationship: div V= δu/ δx + δv/ δy + δw/ δz where u, v and w are the components of the vector along the reegular axes x, y , z. The horizontal divergence is defined by omitting the term δw/δz.
A divergence line is a line along which the horizontal divergence is a maximum.Une ligne de divergence est une ligne selon laquelle la divergence horizontale est maximale.

DMSP:

Defense Meteorological Satellite Program.

Doldrums:

or "equatorial calms": zone of calms or light variable winds, in the lower atmospheric layers, situated near the thermal equator, the annual meridional fluctuation of which it follows with a slight time lag.

Drift current:

or wind-driven current. Motion of a body of water due to the action of wind on its surface.

Dropsonde:

radiosonde launched from an aircraft, with or without a parachute.

DT:

Data T number (acronym used for the analysis of the tropical systems with the Dvorak Technique).

Dvorak Technique:

an analysis procedure, named after Vernon Dvorak, for determining tropical systems intensity from cloud patterns in infrared and visible satellite images. The intensity is on a scale from 1 to 8, and is named Ci (Current intensity).

Empirical relationship between the Dvorak intensity, the maximum wind speed (kt) and the minimum pressure (hPa) for the tropical systems.
Data from the "Global Guide Tropical Cyclone forecasting" (report TCP-31) for the four first columns, and from the Cylone Season 1984/1985 and Cyclone Season 1999/2000.

Dvorak scale used in the South-West Indian Ocean