Monitoring Agricultural Drought in South Korea - Monitoring and Predicting Agricultural Drought: A Global Study - page 389

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Low Precipitation during the Spring Season The period from April 1 to
May 15 is the spring rainy season (Tian and Yasunari, 1998; Byun and
Lee, 2002). The warm and humid southerly airflow in the layer below
the 700 hPa level combines with the upper-level, cold northerly current
(that is still dominant in the layer over 500 hPa) and causes rains with
intensified hydrostatic instability. However, sometimes, for reasons not
adequately understood at present, the precipitation is low and evaporation
is high in spite of the gradual rise in temperature and humidity. Recently
Byun and Lee (2003) found that the low precipitation is caused partly
due to the decrease in the meridional gradient of the adjacent sea-surface
temperature. In March, the groundwater level rises due to the melting of
snow and ice. However, this has only a small effect on the replenishment
of water resources. As a result, drought occurs and hampers the process of
germination and sprouting of seeds, thus damaging the crops.

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Late Onset of the Summer Rainy Season Influenced by the Asian mon-
soon, the summer rainy season in Korea begins in the second half of June
and continues until the end of July. The precipitation during this period
makes up more than 50% of the annual precipitation. Therefore, if the
summer rainy season begins late, nonirrigated land is affected first, but oc-
casionally irrigated farming also suffers. The main reason for a late onset of
the summer rainy season is that the marine tropical air mass (mT; Oliver,
1987) called the Northern Pacific High becomes weak and develops late
due to low heating of the Asian continent. Hence, the warm southerly flow
does not reach Korea. For example, when snow is widespread on the Ti-
betan Plateau, it reflects more solar rays and retards the heating and melting
of snow. Further, if there is a significant amount of ice in the Okhotsk Sea,
a marine polar air mass (mP; Oliver, 1987) develops due to snow thawing
in May and June. When the mP is strong, the frontal system that forms the
rainy season is obstructed from proceeding northward, and there is a delay
in onset of the rainy season (Byun et al., 1992a, 1992b).

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Low Precipitation in Summer Rainy Season Low precipitation during
summer seasons is attributed not only to the absence or infrequence of low-
pressure systems, but also to lower temperature, a lower moisture gradient,
and the persistence of upper level high-pressure systems (Byun and Han,
1994; Byun, 1996).

Low Precipitation during Winter Kaul-Changma (the early fall rainy sea-
son in Korea) begins about the middle of August and ends in the middle
of September. This phenomenon is dominant in the southern half of the
Korean Peninsula, and the precipitation during this period is stored in
reservoirs or dams for irrigating crop in the following year. In this case,
the precipitation is supplied mainly by the lows that pass near the Korean
Peninsula. At times, the weakened tropical cyclones pass the peninsula and
cause heavy rains. Also, an early-winter downpour occurs between late