A study of cold fronts over the British Isles

Autor: H. W. Sansom
Rok vydání: 1952
Předmět:
Zdroj: Quarterly Journal of the Royal Meteorological Society. 78:275-279
ISSN: 1477-870X
0035-9009
DOI: 10.1002/qj.49707833619
Popis: Fifty cold fronts are classified by means of two simultaneous aerological soundings. Two general categories are found: (1)Those in which the warm air is descending, usually forming a subsidence inversion. The name Katafront (following Bergeron) is applied to such fronts. (2)Fronts at which the warm air is ascending. These, which are named Anafronts, usually show no sharp discontinuity on a tephigram. The two types of cold front give rise to quite different weather conditions. The changes in wind and temperature are generally much more abrupt at an anafront, where there is also much more precipitation. A katafront frequently brings a rapid clearance followed by fine weather. The variation of wind with height through the front is markedly different at the two types, the chief characteristics being: Anafront: Rapid backing of wind with height; the component normal to the front decreases. The upper winds are inclined at a small angle (about 15°) to the line of the front. Katafront: Slight backing with height; the normal component increases considerably. The upper winds are inclined at a large angle (about 40°) to the front. Consideration of the horizontal wind changes across the front leads to the expectation of troughs with strong cyclonic shear at anafronts, and the more rapid occlusion of katafronts. Anafronts and katafronts are generally associated with different stages in the history of a cold front; an anafront is normally the initial state, but develops into a katafront later as the depression becomes more occluded. Katafronts are the more common type in this country, especially in summer. Calculations of the mean slope above the friction layer give values of about 1/70 for anafronts, and 1/300 for katafronts. The vertical velocity of the warm air at an anafront is of the order of 10 cm/sec upwards, while at a katafront it is about 2.5 cm/sec downwards. Vertical cross-sections of wet-bulb potential temperature show that there is often latent or convective instability at the nose of a katafront. This may be the explanation of the large ascending velocities found in this region by sailplane pilots.
Databáze: OpenAIRE
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