SUBTERRANEAN ISOBARS By J. M. WALKER Department of Meteorology, Imperial College*

HERE is, especially in text-books on geography, much confused thinking T concerning the nature and function of the Siberian anticyclone. Indeed, in such a mountainous region, one may with some justification question the existence of an anticyclone whose apparent surface pressure may be as high as 1,060 mb and regard the drawing of sea-level isobars as a somewhat uninformative exercise.

Fig. I shows a sounding made at Ulan Bator, Mongolia, on a day when the sea-level synoptic chartst showed this station to be close to the centre of an anticyclone. No cloud was present and with strong radiative cooling a marked inversion had developed. At the station-level. 1,267 m above sea-level, a pressure of 887 mb was reported? and the sea-level equivalent, a t 12 GMT, was quoted as 1,053 mb. Since the pressure-difference between the station level and sea-level depends on the temperature assumed for the fictitious layer of air between the two levels it is obvious that the temperature used in the pressure- reduction is of great significance; some arbitrary system has to be used, based on surface temperatures. The following standard equation, derived from the hydrostatic equation, was employed to ascertain the temperature used in the reduction of Ulan Bator's surface pressure to sea-level:

where R is the gas constant for dry air and T is the virtual temperature, and, in this case, Z, = 0, 2, = 1,267 m, PI = 887 mb and P2 = 1,053 mb. At these low temperatures the virtual and actual temperatures were found to be almost the same, and a value of - 2 1 O C was calculated, corresponding exactly to the reported 12 GMT surface temperature. It appears then that the layer from the surface to sea-level was assumed to be isothermal a t the surface temperature. The maximum surface air-temperature a t Ulan Bator that day was -g"C, which, when substituted in the above equation, yielded a value of 1,045 mb for Pz, whilst the substitution of -226"C, the temperature a t I8 GMT,

yielded a value of 1,057 mb, giving a range of 12 mb in 12 hours! Clearly, it is misleading to construct a pattern of sea-level isobars over high ground and then draw conclusions from it, particularly as the layer of air between the surface

* Now at Hydrological Research Unit, Wallingford, Berks. 7 United States Weather Bureau publications:

World Weather Maps for the Intevnational Geophysical Year 1957-8; Surface and 500-mb

Northern Hemisphere Daily Data Tabulations. charts of the Northern Hemisphere.

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Fig. I . Sounding made at Ulan Bator at 18 GMT on x3 Decembr 1958

and sea-level does not exist. Over the years meteorologists have become accustomed to maps of sea-level isobars, but their slavish use may be a hind- rance to progress, particularly in places like eastern Asia. An examination of the tropospheric processes associated with the clear skies which led to the radiative cooling would appear to be a much more profitable exercise.

During the northern winter (the season when the anticyclone appears over eastern Asia) the subtropical jet-stream lies in a continuous belt around the hemisphere, just below the 200-mb surface, and displays a pattern of three long waves; one trough lies over eastern Asia, with an associated wind-speed maxi- mum in the vicinity of Japan. It is well-known that the ageostrophic flow in the entrance (confluence) region of a jet stream implies a direct cell, with rising warm air on the right and sinking cool air on the left (looking down- stream) and it can be shown that these vertical motions, in the middle and upper troposphere, occur as much as 1,000 km either side of the axis of the subtropical jet-stream. Northern China and Mongolia (where the ‘Siberian’ anticyclone is actually located) are situated to the north of the confluence region of the Japanese subtropical jet-stream, and thus lie in a preferred location for the sinking of cool air, so that here one must expect (a) a general absence of medium and high clouds, and (b) divergence, and hence a region of high pressure, in the lower troposphere. Absence of cloud is the condition required for radiative cooling and the production of a surface inversion such as occurred at Ulan Bator (Fig. I). It is reasonable also to find a region of high pressure in winter, but the intensity of the sea-level anticyclone usually shown is exaggerated by the method of extrapolating surface pressure to sea-level.

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