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A Positive Feedback Process Related to the Rapid Development of an Extratropical Cyclone over the Kuroshio/Kuroshio Extension

The active roles of sensible heat supply from the Kuroshio/Kuroshio Extension in the rapid development of an extratropical cyclone, which occurred in the middle of January 2013, were examined by using a regional cloud-resolving model. In this study, a control experiment and three sensitivity experiments without sensible and latent heat fluxes from the warm currents were conducted.
When the cyclone intensified, sensible heat fluxes from these currents become prominent around the cold conveyor belt (CCB) in the control run. Comparisons among the four runs revealed that the sensible heat supply facilitates deepening of the cyclone’s central pressure, CCB development, and enhanced latent heating over the bent-back front. The sensible heat supply enhances convectively unstable conditions within the atmospheric boundary layer along the CCB. The increased convective instability is released by the forced ascent associated with frontogenesis around the bent-back front, eventually promoting updraft and resultant latent heating. Additionally, the sensible heating leads to an increase in the water vapor content of the saturated air related to the CCB through an increase in the saturation mixing ratio. This increased water vapor content reinforces the moisture flux convergence at the bent-back front, contributing to the activation of latent heating. Previous research has proposed a positive feedback process between the CCB and latent heating over the bent-back front in terms of moisture supply from warm currents. Considering the above two effects of the sensible heat supply, this study revises the positive feedback process.

 2013年1月中旬に急発達した温帯低気圧事例に注目して、黒潮・黒潮続流からの顕熱供給が低気圧急発達に果たす能動的な役割を 領域雲解像モデルを用いて調査した。本研究では再現実験(コントロールラン)の他に、暖流からの顕熱除去・潜熱除去の3種類の 感度実験も行った。
 コントロールランにおいて、低気圧の強化時には黒潮・黒潮続流からの顕熱フラックスが寒冷コンベアベルト (CCB)周辺で極端に増加していた。4種類の数値実験の比較結果から、その顕熱供給は低気圧中心の気圧低下、CCBの発達、 そして後屈前線上の潜熱加熱の強化を手助けをしていることが確認された。顕熱供給はCCBに沿った大気境界層内の対流不安定を 強める働きをする。強化された対流不安定の状態は後屈前線付近のfrontogenesisに関係した強制上昇によって解消されることで、 結果的に上昇流および潜熱加熱を促進している。また付加的な働きとして、顕熱加熱は飽和混合比の増加を通して CCBと関連した飽和空気の水蒸気量の増加をもたらす。水蒸気量の増加によって後屈前線での水蒸気フラックス収束は強められ、 潜熱加熱の増大に寄与している。
 先行研究では暖流からの水蒸気供給の観点で、後屈前線上の潜熱加熱(LH)とCCBとの正のフィードバック過程が提案された。 本研究では上述の顕熱供給の2種類の効果を考慮して、CCB-LHフィードバック仮説の改訂版を提出した。  

*Please refer to the following manuscript.
Hirata, H., R. Kawamura, M. Kato, and T. Shinoda (2018): A positive feedback process related to the rapid development of an extratropical cyclone over the Kuroshio/Kuroshio Extension. Mon. Wea. Rev., 146, 417-433.


Fig. 6. (a)–(d) Latitude–height cross-sectional maps of latent heating rate owing to condensation (shading), the horizontal moisture flux convergence (contours), and meridional and vertical winds (vectors) at 1400 UTC 14 January 2013 along lines A–Aʹ, B–Bʹ, C–Cʹ, and D–Dʹ, as illustrated in Fig. 5. Latent heating rates less than 10 K h-1 are suppressed. The contoured interval is 5 × 10-6 kg kg-1 s-1. Convergence of less than 5 × 10-6 kg kg-1 s-1 is suppressed. The reference arrows for the meridional and vertical winds are 40 m s-1 and 1 m s-1, respectively. Meridional winds of less than 25 m s-1 are suppressed. (e)–(h) Same as (a)–(d) but for vertical velocity (shading) and the convergence of horizontal wind (contours). Vertical velocity less than 1 m s-1 is suppressed. The contour interval is 5 × 10-4 s-1. Convergence of less than 5 × 10-4 s-1 is suppressed.