修士論文要旨(前田 伸一郎)


前田 伸一郎

梅雨前線に伴う雲域は南北に幅広く発生し、単純な前線構造では説明できない。 近年、中国大陸からの湿潤な気流が梅雨前線帯の形成に重要な役割を果たすこと が分かってきた。しかし、それだけでは説明のつかない雲域が観測されている事 例も存在し、まだまだ未解明な構造が存在すると考えられる。特に、九州・沖縄 地方に降水をもたらす梅雨前線帯の発達は西から流入するため、上流にあたる東 シナ海上での構造が極めて重要であるが、海上という地理的制限のため定常観測 が困難で研究事例が少ない。そこで本研究では、2005年6月23日に航空機を用い た東シナ海上梅雨前線帯の観測結果を行い、この事例における梅雨前線帯の構造 を明らかにした。

観測された梅雨前線帯には、2つの上昇流域X,Yと3つの飽和点(降水域/雲域) が存在した。北側の上昇流域X(28~30N)は広範囲に及ぶ弱い上昇流域で、弱い降 水を伴う飽和点が2つ存在した。南側の上昇流域Y(26.5~27N)では強い対流が存 在し、激しい降水を伴う飽和点が存在した。下層の水蒸気傾度は上昇流域Xの北 側で大きく、上昇流域Yでは小さかった。梅雨前線帯を通して温位傾度は見られ なかった。

各上昇流域の南北を挟む位置に4つのドロップゾンデを投下した。上昇流域Xの 北側では下層に寒気移流がみられ、比較的湿潤で弱い対流不安定度を持つ南側の 大気と収束していた。上昇流域Yの北側では比較的湿潤で弱い対流不安定度を持 つ大気が存在し、下層水蒸気量が多く対流不安定な南側の大気との間で収束して いた。

雲解像モデルCReSSを用いた再現実験を行い、観測により示された梅雨前線帯の 特徴を形成する構造を明らかにした。上昇流域Xは乾燥・寒冷な北東流と、西か ら流入する比較的湿潤な西南西気流の収束により発生していることが分かった。 ここでは西南西気流が北東気流を乗り上げる形の前線構造であったが、西南西気 流のもつ対流不安定が小さかったため活発な対流は発生しなかった。上昇流域Y は台湾の北側から流入した比較的湿潤な南西気流1と下層水蒸気量が多く対流不 安定な南西気流2の収束により発生していた。ここでの収束は強く、持ち上げら れる気塊の対流不安定度も強かったため活発な対流が発生していた。


Detailed Structure of velocity and moisture fields
around the Baiu front over the East China Sea.

Shinichiro MAEDA

The rainfall distribution around the Baiu frontal zone varies in time and space. Many patterns of the Baiu frontal rainfall distribution have been studied. In each case, different features of moisture and wind fields were found. But the Baiu frontal structure remains unknown yet. In particular, there are few observational studies over the East China Sea because of the geographical restriction over the sea. The Hydrostatic Atmospheric Research Center of Nagoya University conducted an aircraft observation over the East China Sea in 23 June 2005 to observe the detailed distributions of precipitation, temperature, moisture and wind fields. In the present study, we will reveal the Baiu frontal structure which determine the rainfall distribution. We will use the aircraft observation data and numerical simulation results.

Observed Baiu front had two ascending areas (X, Y) and three saturated areas (precipitation or cloud area). The northern ascending area X (28~30N) was weak and wide, and two saturated areas with weak rainfall were analyzed. Southern ascending area Y (26.5~27N) had strong convective updraft and one saturated area with heavy convective rainfall was analyzed. At the northern edge of the ascending area X, there was large moisture gradient in lower level, while there was weak moisture gradient around the ascending area Y. Temperature gradient was weak around the Baiu frontal zone.

Drop sonde sounding was conducted at four points to the north and south of each ascending area. Cold advection seen in the lower level to the north of the ascending area X converged with the southerly wind which was characterized by relatively moist middle layer and weak convective instability. The atmosphere to the north of ascending area Y was characterized by relatively moist and weak convective instability, while the atmosphere to the south of the ascending are Y was characterized by moist lower level and intense convective instability.

Numerical simulations using The Cloud Resolving Storm Simulator (CReSS) were performed to reveal the Baiu frontal velocity and moisture fields. The ascending area X was caused by the convergence between the cold and dry northeasterly and the west-southwesterly which had weak convective instability. The west-southwesterly derived from the continent and transferred eastward along the Bau frontal cloud zone. In this area, the northeasterly ascended over the west-southwesterly. The weak convective instability of the ascending northeasterly resulted in weak convection.

The ascending area Y was caused by the convergence between southwesterly 1 which had weak convective instability, and southwesterly 2 which was moist at lower level and convective instable. The southwesterly 1 derived from the north of Taiwan, and transferred eastward along the Baiu frontal cloud zone. The strong convergence and intense convective instability of the ascending southwesterly 2 resulted in strong convection.

In this study, it was found that the westerly along the Baiu front was divided into two airstreams: west-southwesterly and southwesterly 1. The positional relationship among these two airstreams, the northeasterly and southwesterly 2 was an important factor to determine the distribution of the Baiu frontal precipitation.