East Asia and Western Pacific METEOROLOGY AND CLIMATE

time to derive the radar estimated rainfall for the same 5~minute
periods as the raingauge. The CAPPI element corresponging to each
gauge is highlighted also in Figure 1. Data pairs with gauge value
less than ,5 mm and radar value of less than .05 mm were excluded.
For each gauge site, the correlation coefficient (c) between
radar and gauge data, the sum of radar estimated rainfall (Ra) and the
sum of gauge rainfall (Rg) over the duration of the storm as well as
the ratio of radar to gauge total (Ra/Rg) were computed. The mean
(Ra/Rg) and standard deviation (s.d.) of Ra/Rg over all available
gauges for a rain case were then calculated. Assuming that extreme
values of Ra/Rg were likely to result from bad radar or gauge data,
Ra/Rg from a gauge site which was more than two s.d. from the mean was
discarded. The analysis was repeated in each rain case with 1 km CAPPI
data to study the differences in the radar estimates in the vertical.
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RATIO OF RADAR TO RAINGAUGE TOTAL
Table 1 summaries the comparison of radar to gauge rainfall total
for the rain cases investigated in this study. The first case in which
hail was reported will be discussed separately because of large spatial
variation of Ra/Rg (a s.d. of .98 vs a mean of .97). The average of
Ra/Rg (3 km) over the remaining 18 cases is .48, indicating that the
radar estimated rainfall is only about half of that recorded by
raingauges. The average relative dispersion about the mean (s.d.
/mean) for the 18 cases is 34 % showing considerable in-storm spatial
variation in Ra/Rg.
In the 6 cases of tropical cyclones, Ra/Rg ranges from .12 to
.52, which are on the low side. Except in the last two cases involving
Typhoon Warren, the centre of the tropical cylcones were more than 200
km from Hong Kong. This suggests that the radar tends to underestimate
surface rainfall more seriously in the peripheral of tropical cyclones.
In the 9 trough cases, Ra/Rg ranges from *30 to .92 and tends to be
higher for troughs in the months of April and May but lower for those
in the months of June and July. The ratio Ra/Rg tends to be more
variable spatially for tropical cyclone cases (average relative
dispersion of 41%) than for trough cases (average relative dispersion