Reporter: Prudence Chien Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood Reporter: Prudence Chien Reference: Schumacher, R. S., and R. H. Johnson, 2008: Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood. Mon. Wea. Rev., 136, 3964-3986.

Introduction In eastern US, 74% warm-season extreme rainfall events were associated with MCSs. Back-building/ quasi-stationary MCS Schumacher and Johnson (2005, 2006)

Introduction Case period: 6-7 May 2002 Southwest of St. Louis, Missouri metropolitan area A cluster of quasi-stationary convection produced over 300mm of rain in 9h. Rain gauge obs. 1200UTC 6 May – 1200UTC 7 May

Description of the event Data: Rapid Update Cycle (RUC) analyses Horizontal grid space = 40km 40 vertical levels NOWrad : WSI radar mosaic product Horizontal grid space = 2km Time interval = 15 mininutes

Description of the event - MCV Shaded: η MCV 探空：1) 西南方空氣潮濕、易飽和 2) 深MAUL (低層多雲，有一些層狀降水) 3) 風向隨高度反轉 東南->西北 MAUL PVmax MAUL: moist absolutely unstable layer

Description of the event - MCV CAPE +1000 (from 00UTC to 06UTC) LLJ(1) 提供暖濕空氣 (2) 幫助空氣等(火商)抬升 / 絕熱抬升 Shaded: CAPE (J/kg) Short barb: 850-hPa wind Solid line: isotachs (m/s)

Description of the event Radar obs. Mature stage 合併 之後對流繼續往東南移 1500UTC 消散

Description of the event Surface obs. Solid: pressure adjusted to sea level Dashed line: sfc temperature Weak cold dome Pressure ridge Deep convection developed & intensified => Sfc cool pool was limited

Model configuration WRF-ARW v2.2 Period: 0000UTC 7 May - 0000UTC 8 May 2002 1KM / 3KM (NOLATENT & NOEVAP)

Results Overall structure of convection and precipitation in convection-permitting simulations Initial of convection Organization and maintenance of convective line Illustration of low-level waves in idealized simulations Development of surface low pressure and reintensification of MCV

Preci.max = 309mm (obs.) Preci.max = 261mm (1KM-sim.) 模式的MCS走得比較慢，位置偏西 模式MCS消散時間點 after 1500UTC 和觀測相符 => 夜間發生的LLJ，到了早上就減弱

Results Overall structure of convection and precipitation in convection-permitting simulations Initial of convection Organization and maintenance of convective line Illustration of low-level waves in idealized simulations Development of surface low pressure and reintensification of MCV

Potential instability 風的切變向量，隨高度反轉 低層南 -> 高層北 等(火商)面圖：有跨越等壓面的氣流，(LLJ)中低層的空氣上滑+濕平流，往MCV的中心方向移動。MCV南側的lifting有助於對流的發展及維持。 整層空氣被MCV-related lifting 達到飽和 =>MAUL層 在MAUL裡面發展出分散的對流，之後逐漸構成MCS *在MAUL中，任何的小擾動都可能產生一個新的對流胞。 *空氣塊在MAUL裡面不會快速上升，但是他們會持續的上升，並且還有低層飽和空氣的供給。所以有最小的蒸發量，並且上升運動能一直維持一段很長的時間。 *這些對流胞的生命期 was unusual:有些對流胞維持了2小時，並移動50-75公里，之後結合成MCS。 這樣的環境對於發展中的對流胞提供一個較好的環境(slightly favorable)，使他們能維持並且能增強，最後結合成一個成熟的系統。

Results Overall structure of convection and precipitation in convection-permitting simulations Initial of convection Organization and maintenance of convective line Illustration of low-level waves in idealized simulations Development of surface low pressure and reintensification of MCV

Two mechanisms for the initiation of upstream convection: A long-distance mechanism: Upstream parcels are lifted to their LFC by MCV-related lifting and then slowly approach the ongoing deep convection. A “short-distance” mechanism: New cells are initiated much more quickly in close proximity to the mature convective system. Shaded: w Solid line: Div. Dashed line: Conv.

Contour: 7-km vertical velocity Shaded: 0.75-km vertocal velocity

Dot line: Wind speed in the direction of wave propagation TROF: surface pressure trough Shaded: Div. Surface pressure and the wind in the direction of wave propagation vary exactly in phase.

Results Overall structure of convection and precipitation in convection-permitting simulations Initial of convection Organization and maintenance of convective line Illustration of low-level waves in idealized simulations Development of surface low pressure and reintensification of MCV

單一對流的發生是由於buoyant perturbation 低層的波是由於非絕熱加熱造成, 非絕熱加熱是在LFC生成雲的位置附近 c) 最下面會產生小範圍的旋轉，加強前側上升氣流 => d)新胞 右上) 拿掉latent cooling。重力波特徵相似，因此重力波可說是latent heating的代表 *)用不同的探空(較乾的環境)，因為有冷池的產生，造成low-level wave很快消散

Results Overall structure of convection and precipitation in convection-permitting simulations Initial of convection Organization and maintenance of convective line Illustration of low-level waves in idealized simulations Development of surface low pressure and reintensification of MCV

氣壓梯度在NOLATENT的模擬中與real不同，且有較廣的氣旋式環流。PV(nolatent)分布不規則。P(nolatent)和實際情況不符。 In a) 氣壓梯度和地面低壓維持中尺度的上升運動，使新對流胞持續發生，因此低層的重力波是對流線(convective line)發生的主要機制。 Convergence associated with the gravity wave was most responsible for the linear organization of the convection.

The diabatic heating associated with the convective system also served to reintensify the MCV. NOLATENT: MCS只是重分配PV並往東移，持續減弱 3KM: MCV維持在原地較久，並且結構較紮實

Conclusions Midlevel MCV

Thanks for your listening. & Questions?

Description of the event Cutoff Low -> MCV Shaded: η

Results (a.) – radar sim. Obs.

Results (c.) – FIG14 Simulated rolls occur in moist absolutely unstable environment outside of the leading line/trailing stratiform squall-line archetype.