WIND PATTERNS FROM RIME DEPOSITS

By D. M . ELSOM

Geography Section, Oxford Polytechnic

EAR the ground deposits of rime are formed by supercooled fog droplets freezing on N impact on the windward side of objects (Fig. I ) . The rime deposit accumulates at quite a slow rate, rarely growing faster than 10 mm a day even in mountainous districts of Britain (Meaden 1976). Since the rime accumulates on the windward side as a banner-like projection, leaving the leeward side of the object relatively or completely rime-free, the deposit provides a very useful indicator of the direction of the light winds prevailing during its formation. Given that such light wind conditions, produced during an anticyclonic synoptic situation, are also those during which local- or topo-climates are most pronounced, a technique to examine local wind patterns is readily available. The frequency of such occasions of supercooled fogs is limited, but the numerous photographs and letters concerning rime deposits in past issues of Wearher suggest that during most winters these conditions are experienced on one or more occasions.

Fig. I . Rime deposit near Radcliffe Observatory, Oxford

An example of an occaslon of a supercooled fog producing rime deposits occurred on 17-18 December 1978 in the Oxford area. During the evening of the 17 December a ridge of high pressure in mid-Atlantic had moved towards the British Isles. By 0600 G M T on 18 December a high pressure centre had developed within the extensive ridge near the west Midlands (Fig. 2) . On the morning of 18 December a survey of the rime deposits around the periphery of the urban area was undertaken to examine wind patterns. The direction of the rime projection was measured to the nearest 10 degrees on tree branches

approximately 3-5 mm in diameter a t about 2 m height (Fig. 3). The inferred wind directions varied between 340"-050" with a pattern being evident of consistent 030"-040" directions at the north-eastern periphery of Oxford, 360"-020" to the south-east and 03O0-O4O0 to the west. These inferred winds can be compared with the hourly records taken at the Radcliffe Observatory where freezing fog was recorded between 2300 G M T ~ ~ the 17th and the time of the rime survey at 1000 GMT the following day. Throughout the period the Radcliffe Observatory recorded wind speeds at 50 m above ground level of 1-4 kt with gusts of 6-8 kt. Wind speeds at ground level might be expected to have been significantly less. Wind directions varied between 340"-020". During the period of freezing fog the wind directions of 340" (4 hours) and 360" ( 5 hours) dominated.

Fig. 2. Surface synoptic chart for 0600 GMT on 18 December 1978

Under anticyclonic synoptic conditions, two local factors which might have been expected to affect wind directions in some areas are urban-induced wind flows (country breezes) and katabatic flows. The inducement of a wind flow convergent towards the urban area should occur when heat-island intensities exceed 2 or 3°C (Padmanabhamurty and Hirt 1974). Air minimum temperatures a t the Radcliffe Observatory (altitude 63 m) and Oxford Polytechnic (altitude 98 m) were +O.I"C and -5 1°C respectively. The hypothesised country breeze may have caused the 030"-040" wind directions to the north-east of the urban periphery but elsewhere no evidence of a convergent wind flow was found. This contrasts with the study of rime deposits around Asahikawa City, Japan, by Okita (1960) where a similar heat-island intensity was shown to induce a country breeze. The absence of a clear convergent wind pattern in Oxford may suggest that although heat-islands are experienced they are associated with localised areas of high densities of buildings. The high proportion of open grassland areas (college quadrangles) within the college-dominated city of Oxford precludes the formation of a cliff-like thermal gradient which is evident a t the periphery of many urban areas such as London and which favours the inducement of a country breeze (Chandler 1965).

No evidence of katabatic flow was found from the rime survey such as between the city centre and Oxford Polytechnic where the ground rises by 30 m. The wind speed within the supercooled fog may have been too high to allow katabatic flow. An alternative explanation for the absence of evidence from the rime deposits for katabatic flow, and

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indeed, even tor tlie country breeze, may relate to the depth of the freezing fog. Although fog was presrnt at the Observatory from 2300 GMTon 17 December to 1000 GM1on 18 December i t rnight not have existed for as long at the Polytechnic, a site 35 m higher than the Observatory. Production of the low Polytechnic minimum temperature may not have occurred therefore during the freezing fog conditions which formed the rime deposits in this area.

Fig, 3. Wind directions in Oxford inferred fi-om rime deposits

'l'he thickncss of the rime deposits depends upon wind speed, ambient teniperaturc. tlioplrt diameter and liquid water concentration as ell as on the diameter of the tree branch on which i t accumulates ( I . acy 1977; Macklin 1962). Okita (1960) measured the thickriess of the rime as an index of wind speed but found no significant regional differencrx. In the Oxtord study rime thickness was approximately 5 mm o n branches 3-5 inn1 in diameter with little regional variation. 'The noticeable regional differences were not o f thickness but of the nature of the ice deposit. On the high& ground near the Polytechnic the ice deposit was o f a more crystalline appearance (Fig. 4) because the lower temperatures in this area also favoured the formation of hoar frost. Hoar frost is formed by sublimation o f saturated air directly into ice. The ice deposits in this area were therefore a mixture of both rime and hoar frost with the heaviest accumulation being on the windward side of the object.

Although the Oxford study did not find that local tactors had significantly modified the regional wind flow, this paper has outlined and illustrated the technique of using rime deposits to invehtigate any such modifications. Rime formation is not frequent but the rclative lack of evidence in this country of urban-induced wind flows suggests that attcmpts should be made in other urban areas t o use this technique to document such local wind circulations. Such opportunities need t o be taken since Findlay and Hirt ( 1969) have noted that 'case documentation ofthis phenomenon is relatively rare'. For example, in the United Kingdom only the early work by Chandler (1960, 1961) in his studies of temperature patterns in London and L,etcester has substantiated the existence of country

breezes. Our need to understand this aspect of urban climates has been highlighted in relation to the possible role of country breezes in redistributing air pollutants within urban areas (Elsom 1978). During air pollution episodes produced under anticyclonic synoptic conditions urban-induced wind flows may play an important role in transferring suburban pollutants into the centre of the urban area. Neglect of this process could be why present meteorological models to predict air pollution concentrations in urban areas underestimate peak concentrations reached during episodes.

Fig. 4. Rime and hoar frost near Oxford Polytechnic

REFERENCES

CHANDLER. T. J. 1960

1961

CHANDLER, T. J. 1965 ELSOM, D. M. 1978

FINDLAY, B. F. and 1969

LACY. R. E. 1977 HIRT, M. S.

MACKLIN, W. C. 1962

MEADEN, G. T. 1976

OKITA, T. 1960

PADMANABHAMURTY, B. 1974 and HIRT, M. S.

Wind as a factor of urban temperatures - a survey in north-east London. Weather, 15, pp. 204-213

Surface breeze effects of Leicester’s heat-island. East Midland Geographer, 2, pp. 32-38

The Climate of London. London: Hutchinson, 292 pp. Meteorological aspects of air pollution episodes in

urban areas. Discussion Paper 5 , Oxford Poly- technic, 52pp.

An urban induced meso-circulation. Atmospheric En- vironment, 3, pp. 537-542

Climate and Building in Britain. London: HMSO,

The density and structure of ice formed by accretion. Quart. J. R. Met. Soc., 88, pp. 30-50

Rapid precipitation from supercooled fog. J. of Met. (Trowbridge), 1, p. 167

Estimation of direction of airflow from observation of rime ice. J. Met. Soc. Japan, 38, pp. 207-209

The Toronto heat-island and pollution distribution. Water, Air and Soil Pollution, 3, pp.81-89

pp. 91-95

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