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Section D D index 121-129 of 573 terms
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delta Eddington—An approximate technique for handling the complexity of shortwave radiative transfer in anisotropically scattering atmospheres. The approximation is fairly accurate for transmitted and reflected irradiances provided the optical depth of the medium is greater than ≈ 3 and the absorption is weak. It is used mainly to speed up shortwave radiation computations in cloudy atmospheres.
delta region—1. A region of diffluence in the atmosphere. 2. A region aloft beneath which low-level cyclogenesis is likely to occur. Compare exit region.
delta rule—Method of training a neural network using backpropagation. The error in the output of the neural network (e.g., a prediction of minimum nighttime temperature) is a function of the values of the input parameters (cloud cover, 6 temperature, etc.) and the weights assigned to them. The weights are adjusted to minimize the error. Application of the delta rule results in the most rapid error reduction (learning rate). The learning rate can be adjusted as necessary to avoid being trapped in a local error minimum.
dendrite—Same as dendritic crystal.
dendritic crystal—(Or dendrite.) A crystal, particularly a planar ice crystal, with its macroscopic form (crystal habit) characterized by intricate branching structures of a treelike nature. Dendritic ice crystals possess hexagonal symmetry, and tend to develop when a crystal grows by vapor deposition at temperatures within a few degrees of −15°C, providing saturation is close to supercooled water. Similar forms occur by ice growth into supercooled liquid water at temperatures down to −10°C. Spatial dendrites grow in three dimensions from a central frozen drop.
dendrochore—A treed region. See biochore.
dendrochronology—The analysis of the annual growth rings of trees, leading to the calculation of significant indices of climate and general chronology of the past. The width of a tree ring was determined by the temperature and/or the moisture that prevailed during the year of its formation. Since stress from temperature and/or moisture variations reduces the width of the seasonal growth of a tree ring, dendrochronology has important application in the study of long-term climatic variations.
dendroclimatology—See dendrochronology.
dendrohydrology—The inference of the long-term statistics of various hydrological phenomena in a region using tree rings.
denitrification—1. The conversion of nitrite or nitrate to gaseous end products NO, N2O, and N2 by denitrifying bacteria. Molecular nitrogen, N2, is the most abundant end product. The oxidized forms of N (nitrate, nitrite, nitric acid, nitric oxide, and nitrous oxide) serve as alternative electron acceptors in the absence of sufficient oxygen; thus, denitrification is primarily an anaerobic process. 2. The removal of active nitrogen from the atmosphere through uptake into particles. The term is particularly used in context with the irreversible uptake of nitric acid into ice clouds (polar stratospheric clouds) in the antarctic stratospheric vortex. This reduction in the level of active nitrogen allows ozone depletion by chlorine oxides to occur unchecked.
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