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100°
110°
120°
130°
140°E
Axis of jet stream
Wind speed (m s -1 )
50
40°N
60
Rainfall (cm)
20 - 40
70
40 - 60
> 60
40
70
30
60
50
30°
40
30
20
20°
Figure 11.21 The mean winter jet stream axis at 12km over the Far East and the mean winter precipitation over
China (in cm).
Source: After Mohri and Yeh; from Trewartha (1958). By permission of the University of Wisconsin Press.
jet strengthens and begins to extend across central
China and into Japan, the southerly branch
remains positioned south of Tibet, although
weakening in intensity.
In April there is weak convection over India,
where the circulation is dominated by subsiding
air originating along the convective ITCZ trough
centered over the equator and following the
overhead sun northward over the warm Indian
Ocean (see Figure 11.16 ). The weather over
northern India becomes hot, dry and squally in
response to the greater solar radiation heating.
Mean temperatures in Delhi rise from 23°C in
March to 33
north with 'westerly disturbances', particularly
towards the Ganges delta, where the low-level
inflow of warm, humid air is overrun by dry,
potentially cold air, triggering squall-lines known
as nor'westers. In the northwest, where less
moisture is available, the convection generates
violent squalls and dust-storms termed andhis.
The mechanism of these storms is not fully
known, although high-level divergence in the
waves of the subtropical westerly jet stream
appears to be essential. The early onset of summer
rains in Bengal, Bangladesh, Assam and Burma
(e.g., Chittagong: Figure 11.20 ) is favored by an
orographically produced trough in the upper
westerlies, which is located at about 85-90
C in May. The thermal low pressure
cell (see Chapter 9H.2) now reaches its maximum
intensity, but although onshore coastal winds
develop, the onset of the monsoon is still a month
away and other mechanisms cause only limited
precipitation. Some precipitation occurs in the
°
E in
May. Low-level convergence of maritime air from
the Bay of Bengal, combined with the upper-level
divergence ahead of the 300mb trough, generates
thunder squalls. Tropical disturbances in the Bay
°
 
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