Results

Now, we examine the effectiveness of the ensemble experiment in noise reduction by increasing the size of ensemble. In the followings, we will show the time-longitude cross sections of precipitation and surface pressure at the equator.

Ensemble Mean Precipitation

Figure 3 (animation) shows the change of the time-longitude cross section of precipitation at the equator by increasing the size of ensemble set.

Figure 3: Time-longitude cross section of precipitation at the equator (W/m²). The change is shown as an animation with increasing the size of ensemble set as 1, 2, 4, 8, 16, 32, 64, 128, and 200. The unit of time axis is day with day 0 as the onset day of the warm SST area. (Please use the reload button of your browser to restart the animation.)

As the increase of the seize of ensemble set, there appear the following characteristics. These characteristics become clearer with the increase of the ensemble size.

1 run

There appears only a tendency of precipitation accumulation over the warm SST area (located at 180E). No feature of east-west asymmetry can be observed.

2 run mean

4 run mean

8 run mean

By carefully examining the ratio between the blue and red colored areas, one can recognize that, from around day 15 after the introduction of the warm SST area, precipitation decreases to the west, while it increases to the east of the warm SST area.

16 run mean

32 run mean

One can recognize a decrease of precipitation to the east of the warm SST area occurring at around day 5 to 10 soon after the switch-on. The increase of precipitation occurs after this decrease. The beginning date of precipitation decrease is delayed as one moves away from the warm SST area.

64 run mean

As the size of ensemble set is increased, the characteristics mentioned above become clearer.

128 run mean

200 run mean

The ensemble mean operation with the size of larger than 32 reproduces the tendency that the precipitation increase (decrease) occurs to the east (west) of the warm SST area. This is the characteristics of precipitation shown by Hosaka et al. (1998) with the long term mean data. The ensemble mean operation revealed that this characteristics establishes about 15 days after the switch-on of the warm SST area.

The ensemble experiment also revealed that, to the east of the warm SST area, the increase of precipitation occurs after it once decreases. The important point is that this temporal decrease of precipitation emerges in the form of an eastward propagating disturbance.