East Asia and Western Pacific METEOROLOGY AND CLIMATE

mean vertical speed (W^ - K NM ) between 110-120°E at 24 and 48
hr integration, respectively. From Pig.4 one can see that up- and
downdraft of the local Hadley cell shown in ALL are stronger than
in NM, which suggests greater contribution of the vigorous surge to
the excitation of the cell. Further, the cold front lifting is more
intense in the presence of a large-scale orography than without.
4. DISCUSSIONS AND CONCLUSIONS
During the cold surge episode the leading edge differs from the
vigorous NE wind center in the southward march as shown in ALL
(see Fig.2). The former moves on average at about 51 m / s ar *d "the
latter at 31 and 20 before and after arriving at 20 N, respectively,
and close to zero when reaching 10 N. Subsequently, the NE wind
center moves westward along the south side of the Plateau* On the
other hand, the edge of the surge marches down to the south at approximately
51 m / s when no such orography is available. For this
reason, we think that cold air forced to move down along the east
side of the Plateau may excite two types of waves. One is the fasttravelling
gravity waves that bear no relation to the topography
and so is probably excited by the cold front itself. It goes southward
ahead of the front, thus causing the strengthening of the
northerly wind at the leading edge of the surge to be prior to temperature
drop. The other has characteristics of Kelvin waves, travels
at a slow pace, depends on the orography for its existence
and becomes non-existent apart from the Plateau. The second type
occurring at the periphery of the topography with the wind direction
nearly parallel to it is identified as the vigorous NE wind
region behind the front.
From the foregoing discussions the following points are summarized
j
(1) The Qinghai-Tibetan Plateau has no significant thermal effect
on the propagation of eastern-Asian cold surges;
(2) The Plateau has, however, very pronounced dynamic effect on
the surge. It steers cold air to move southward along the east side