East Asia and Western Pacific METEOROLOGY AND CLIMATE

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O.Qg-m" 3 , which is probably due to an increase of the
efficiency of the particles' collision-coalescence process
as melting snowflakes turn wet; (2) an obvious shift of
the horizontal wind (Fig. 2); (3) a transition toward
mesoscale descent motion below the melting layer. These
findings support the idea that the pressure perturbation
induced by the cooling of melting snow causes a decoupling
of dynamics above and below the melting layer (Johnson and
Young, 1983)
Analyses of the probe-imaging data show that the
significant increase of particle terminal velocity from
melting snow to liquid droplet results in a sharp decrease
of the particle number .density by an order of magnitude
(Fig. 3). Lo and Liu (1989) have applied a theoretical
approach to quantify such relationship.
Meanwhile, a steady increase in the percentage of
large particles (diameter 2500-5000jLim for 2D-C and
2000-lOOOOjLtm for 2D-P) immediately above the melting
layer followed by a sudden decrease are noticed in both
probes' data (Figs. 4 and 5), These phenomena stress that
snowflake aggregation generates large particles, in
particular in the 0°C isothermal layer where wet
snowflakes have a much higher chance to collect other
particles. Willis and Heymsfield (1989) have claimed that
the radar reflectivity maximum (bright band) is due to
these relatively few, very large aggregates that survive
to warmer temperatures. Before reaching the bottom of the
melting layer, either the collision-breakup or the
spontaneous breakup process causes a rapid disappearance
of these large, wet particles, since the breakup
efficiency increases with particle size.
Evolution of the size spectra (Fig. 6) also reveals
the physical processes of aggregation, melting and
breakup. From -1.7°C (Fig. 6a) to 0.2°C (Fig. 6b),
snowflakes aggregate to form a few particles larger than
0.7cm diameter at the expense of smaller particles and
therefore the total particle concentration. From 0.2°C
(Fig. 6b) to 1.4°C (Fig. 6c), both the melting and breakup
processes help to eliminate larger particles, while the
increase in fallspeed caused by the transition from ice to
water phase results in a significant decrease in total
particle concentration.
3 r THE WARM-RAIN REGION
In this section, our focus is on analyzing the
evolution of spectra along the vertical direction below
the melting layer. The liquid water content (LWC)
measured by the Johnson-Williams device shows a constant