Topic16 of j-kawamura

HOME > Our Laboratory

Influential role of moisture supply from the Kuroshio Current/Kuroshio Extension in the rapid development of an extratropical cyclone

 This study has focused on an explosive cyclone migrating along the southern periphery of the Kuroshio Current/Kuroshio Extension in the middle of January 2013 and examined how those warm currents played an active role in the rapid development of the cyclone using a high-resolution coupled atmosphere-ocean regional model.
 The evolutions of surface fronts of the simulated cyclone resemble the Shapiro-Keyser model. At the time of the maximum deepening rate, strong mesoscale diabatic heating areas appear over the bent-back front and the warm front east of the cyclone. Diabatic heating over the bent-back front and the eastern warm front are mainly induced by the condensation of moisture imported by the cold conveyor belt (CCB) and the warm conveyor belt (WCB), respectively. The dry air parcels transported by the CCB can receive large amounts of moisture from the warm currents, whereas the very humid air parcels transported by the WCB can hardly be modified by those currents.
 The well-organized nature of the CCB plays a key role not only in enhancing surface evaporation from the warm currents but also in importing the evaporated vapor into the bent-back front. The imported vapor converges at the bent-back front, leading to latent heat release. The latent heating facilitates the development of cyclone, through the production of positive potential vorticity in the lower troposphere. Its deepening can, in turn, reinforce the CCB. In the presence of a favorable synoptic scale environment, such a positive feedback process can lead to the rapid intensification of a cyclone over warm currents.

 2013年1月中旬に暖流(黒潮・黒潮続流)の南縁に沿って急発達した温帯低気圧(爆弾低気圧)に焦点を当て、高解像度領域大気海洋結合モデルを用いて 低気圧の急発達に暖流がどのように積極的な役割を果たしているのかを調査した。
 再現された低気圧の前線構造の時間発展はシャピロ・カイザー・モデルに類似している。最大発達率に達した時に、 後屈前線と低気圧中心東側の温暖前線上にメソスケールの強い非断熱加熱域が出現していた。後屈前線の非断熱加熱は 主に寒冷コンベアベルト(CCB)によって流入した水蒸気の凝結によって生じており、一方、東側の温暖前線上の非断熱加熱は 温暖コンベアベルト(WCB)によって輸送された水蒸気の凝結によるものであった(下図参照)。CCBによって輸送される 乾燥空気塊は暖流からの多量の水蒸気供給によって強い変質を受けるが、WCBに沿って流入する暖湿空気塊はとても湿潤なため、暖流による 変質をほとんど受けない。
 CCBの組織化と発達は暖流上の海面蒸発を強化するのみならず、その蒸発した多量の水蒸気を後屈前線へ流入させる 重要な役割を担っている。流入した水蒸気は後屈前線で水平収束し潜熱を放出する。潜熱は、対流圏下層の正の渦位の生成を 通して、低気圧の発達を促進させる。そして、低気圧の中心気圧の低下は逆にCCBを強化させることになる。総観スケールの好適な環境条件下で、 上記の正のフィードバック過程は暖流上の温帯低気圧の急発達をもたらすことができる。  

*Please refer to the following manuscript.
Hirata, H., R. Kawamura, M. Kato, and T. Shinoda (2015): Influential role of moisture supply from the Kuroshio/Kuroshio Extension in the rapid development of an extratropical cyclone. Monthly Weather Review, 143, 4126-4144. doi:10.1175/MWR-D-15-0016.1


Fig. 8. Trajectories of the air parcels that are initially located to the (a) west and (b) east of 146°E, which are calculated by a backward trajectory analysis. The color lines denote the heights (unit: m) of the air parcels. See text for details. The SLP distribution at 1500 UTC 14 Jan 2013 is also represented. The contoured interval is 4 hPa.