The climate, which is the synthetic state of Earth's environment, is
maintained by the various interactive processes in the atmosphere and
oceans and also on Earth's surface. The quasi-steady state of the
climate has been sustained, although the climate exhibits large
variations around its quasi-equilibrium state. In order to determine
global-scale climate variability and understand the mechanisms of the
variations, we must study the real features of the phenomena and
processes which are closely related to the variability of climate as
well as the equilibrium state, based on the results of climate
simulations and observational data.
Global-Scale Distribution of Summer Monsoon Precipitation
Figure 1 shows the climatological distribution of observed
precipitation in the summer monsoon season (June, July, and August)
based on the work of Legates and Willmott, 1990, and Spencer, 1993. In
addition to an area of maximum precipitation centered over the Indian
subcontinent and the Indochina Peninsula, two predominate
precipitation zones are seen in association with the Pacific
intertropical convergence zone and the Meiyu/Baiu frontal zone.
Figure 2 presents the distribution of precipitation averaged for the
Asian summer monsoon season as obtained by the CCSR/NIES T21 climate
model. Although the area of maximum precipitation centered over the
Indochina Peninsula is properly simulated by the model, the
precipitation over the western coast of the Indian subcontinent,
Meiyu/Baiu frontal zone, and the Pacific intertropical convergence
zone are not simulated. The model used for this experiment has 21
triangular truncation wave numbers, which correspond to the horizontal
resolution of ~600 km. This horizontal resolution is not enough to
simulate the narrow precipitation zone, such as the intertropical
convergence zone and Meiyu/Baiu frontal zone. In order to simulate the
detailed features of the Asian summer monsoon, we are making a
simulation using the climate model with a higher resolution.
[Figure 1]: Observed climatological precipitation averaged
for June, July, and August.
[Figure 2]: Simulated precipitation by the climate model
(T21 L20) averaged for June, July, and August.
Regional-scale distribution of summer monsoon precipitation
Detailed features of the Asian summer monsoon was studied by utilizing
the 24-hour predicted precipitation by the JMA-GSM T106 numerical
prediction model. Since the 106 triangular truncation wave number
corresponds to a horizontal resolution of ~100 km, the regional
characteristics of the Asian monsoon are very accurately
predicted. Figure 3 shows the 10- day averaged 24-hour predicted
precipitation for July 1-10, 1991, which was the peak period of East
Asian precipitation in 1991. Owing to the high resolution of the
model, the large amount of precipitation concentrated in the narrow
zone over the intertropical convergence zone and the Meiyu/Baiu
frontal zone is reasonably simulated. The accurate simulation of the
precipitation will assure that other meteorological elements such as
wind field, vertical motion, and moisture field are also accurately
simulated. Therefore, it will be possible to study the various
processes of the Asian summer monsoon by utilizing the data predicted
by the model. The following are some examples of the analysis.
[Figure 3]: Precipitation predicted by the numerical prediction
model JMA GSM (T106) averaged for July 1-10, 1991. The blue,
yellow, and red indicate no precipitation, 0-4 mm/day,
and 4-24 mm/day, respectively..
Moisture Flux Convergence and Vertical Stability in the Lower Troposphere
Figure 4 shows the distribution of the vertically integrated moisture
flux convergence, averaged for the same period as for Figure 3. The
areas of moisture flux convergence and divergence correspond to the
moisture sink region and moisture source region, respectively. The
Indian Ocean, the Arabian Sea, the Bay of Bengal, and the Pacific
Ocean under the subtropical anticyclone play an important role as the
moisture source region in sustaining the large amount of monsoon
rainfall.
[Figure 4]: The distribution of the vertically integrated moisture
flux convergence averaged for 1-10 July, 1991. The blue, yellow,
and red areas indicate the areas of moisture flux divergence, weak
moisture flux convergence, and strong moisture flux convergence,
respectively.
Figure 5 presents the vertical gradient of the equivalent potential
temperature in 1000-500 hPa layers in units of K/ (100 hPa). While the
monsoon rainfall areas are characterized by unstable stratification,
the moisture source regions are characterized by stable
stratification. The vertical stability of the atmosphere has a strong
influence on the Asian monsoon precipitation through the control of
cumulus convection and weather systems of various scales.
[Figure 5]: The vertical gradient of the equivalent potential
temperature in 1000-500 hPa layers in units of K/ (100 hPa)
averaged for July 1-10, 1991. The red, yellow, green, and blue
areas indicate areas of strong unstable, weak unstable, weak
stable, and strong stable stratification, respectively.
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