Examining formation mechanism of precipitation patterns during a cold front passes over the Japan islands

  • Haruka KITAHATA,2007: Examining formation mechanism of precipitation patterns during a cold front passes over the Japan islands .

Abstract

Cold fronts are affected by the complex terrain when they across over the Japan islands. That causes various precipitation patterns. In this study, we classified the 36 cold front events into the following four types, from a point of view of the precipitation pattern. And then, we examined the formation mechanism of the precipitation patterns of each type. (i) General type: Precipitation bands move with the cold front and rain falls all over the Japan islands. (ii) Hokuriku type: Only the Hokuriku area has rainfalls. (iii) Jump type: Rain falls over the west side of the Chubu mountains, but the Kanto area has little rain. When the cold front reaches the Pacific Ocean, precipitation bands develop again. (iv) Tokai type: The Tokai area has rainfalls before the cold front arrives at the area. First, we examined the formation mechanism of Hokuriku Type. As a result of the statistical analysis using global analysis data from 2000 to 2006, Hokuriku type often appears in winter, but its trough at 500hPa is relatively shallow. Additionally, we investigated the atmospheric conditions for a typical case. The results showed that the atmosphere above 600hPa is stable and a shallow cold layer exists behind the front. These indicate that the cyclones and the fronts are not so strong for Hokuriku type, and thus the precipitation bands could not go over mountains. Furthermore, we conducted the numerical experiments. Control case represents the precipitation bands well. On the other hand, the experimental case without mountains showed that the precipitation bands disappear soon. The results indicate that the precipitation bands maintain over the Hokuriku area due to the orographic uplift. Second, we examined the formation mechanism of Jump type by the similar approaches. As a result, we revealed that the mountains located in the west side of the Kanto area prevent water vapor from flowing into the area and bring little rain to there. Control case from the numerical model showed that there is no rain over the Kanto area. Contrary to this, the experimental case without mountains showed that the rain falls there. Statistical analysis showed that the cyclones tend to develop rapidly for Jump type. Rapidly developing cyclones cause strong westerly winds. The strong winds flow via the Ise Bay and make a convergence line with wet southwesterly winds when the front reaches the Pacific Ocean. Consequently, they form a precipitation band over the Pacific Ocean.

Abstracts of graduation thesis PDF(Japanese)

The paper was based on a portion of her Guraduation thesis was published in Science Online Letters on the Atmosphere(SOLA)(2009).
Kusaka, H., Kitahata, H., 2009: Synoptic-scale climatology of cold frontal precipitation systems during the passage over central Japan. SOLA, 5, 61-64. 2009.05.01 .
SOLA
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