Observing-System Research and Ensemble Data Assimilation at JAMSTEC - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 510

Fig. 21.5 As in Fig. 21.4 but near Japan

strong precipitation. The surrounding regions have weak precipitation because not
all the members necessarily precipitate. Another likely state is given by the control
run or the forecast from the analysis ensemble mean (Fig. 21.4 c). The control run
has less weak precipitation. The excess of weak precipitation in the ensemble mean
can also be understood statistically. If the probability distribution of precipitation is
lognormal, the mean is larger than the mode.
The overestimation of weak precipitation in ALERA2 is also evident in the
histogram (Fig. 21.6 ). The almost clear mmd 1 / bin holds 68 % in GPCP

but only 38 % in ALERA2. In the control run, the first bin holds 47 %, and the
percentages in the bins between 2 and 16 mm d 1 are reduced by 1-3 %. The control
run indicates that weak precipitation is partly caused by statistics. A local minimum
at 1-2 mm d 1 bin of GPCP might imply the limited performance of detecting weak
precipitation.

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