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Shu-Hua Chen 12 , Yuh-Lang Lin 3 , Zhan Zhao 2 , and Heather Reeves 3

Parameters Controlling Precipitation Associated with a Conditionally Unsaturated, Unstable Flow over a Two-Dimensional Mesoscale Mountain. Shu-Hua Chen 12 , Yuh-Lang Lin 3 , Zhan Zhao 2 , and Heather Reeves 3. 1 National Central University 2 University of California, Davis

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Shu-Hua Chen 12 , Yuh-Lang Lin 3 , Zhan Zhao 2 , and Heather Reeves 3

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  1. Parameters Controlling Precipitation Associated with a Conditionally Unsaturated, Unstable Flow over a Two-Dimensional Mesoscale Mountain Shu-Hua Chen12, Yuh-Lang Lin3, Zhan Zhao2, and Heather Reeves3 1 National Central University 2 University of California, Davis 3 North Carolina State University

  2. Outline • Introduction • Numerical Simulations and • Results • Summary

  3. Introduction Upstream Propagating Convective System Jou, 1997

  4. Introduction Quasi-stationary Convective System

  5. Introduction Downstream Propagating Convective System

  6. Introduction Objective Perform idealized simulations for a conditionally unstable flow over a 2D mountain ridge to investigate the propagation and types of cloud precipitation systems controlled by three control parameters.

  7. Control Parameters ► Moist Froude number, Fw U Basic flow speed (m/s) h Mountain height (m) Nw Moist Brunt-Vaisala frequency ► CAPE ► Orographic aspect ratio (h/a) a half mountain width

  8. Model Configuration • Weather Research and Forecasting (WRF) model • 2D simulations • Domain = 1000 km x 20 km, • ∆x = 1km, and vertical grid interval stretched • Purdue-Lin microphysics scheme • Bell-shaped mountain with a half width, a, and a mountain ridge, h. • Integration time = 10 hours

  9. Experiment Design (CAPE) LFC Long-dashed lines from right to left are soundings for CP0, CP1, CP2, CP3, CP3,CP4, and CP5, respectively. The CAPE values for them are 487, 1372, 1895, 2438, 3000, and 3578 J/kg, respectively. Nw= .942 ~ 1.01 x 10-2 s-1

  10. Experiment Design (U and CAPE) U = 2.5, 5, 10, 15, 20, 30 m/s for F1 – F6, respectively h = 2 km a = 30 km

  11. Flow Regimes (CAPE = 3000 J/kg) 7h CP4F1 Fw=0.131 I (2.5m/s) CP4F3 Fw=0.524 II (10 m/s) Shading – rainfall Contours – w at 3.6km Shading – w Contours – θ

  12. Flow Regimes (CAPE = 3000 J/kg) Mixed convective and stratiform clouds CP4F4 Fw=0.786 III (15 m/s) CP4F6 Fw=1.572 IV (30 m/s) Stratiform clouds

  13. What makes a straitform cloud develop over a mountain ? (Large CAPE) 3 time scales are relevant: 1) Advection time: Tadv ~ a/U 2) Cloud growth time: Tc (controlled by microphys processes) 3) Orographic perturbation time: Assume Tc=20 min, a = 30 km: For U = 15 m/s, Tadv = 20 min ~ Tc => convective cloud may develop For U = 30 m/s: Tadv = 10 min > Tc => not enough time for a convective cloud to develop Toro ~ 4 – 5 min

  14. When Fw increases,the flow shifts to a higher number Flow regime. Total Accumulated Rainfall (10h) Small Fw I I II III IV IV Large Fw

  15. When CAPE increases, the flow shifts to a lower number Flow regime. Total Accumulated Rainfall (10h) Small CAPE II II II I I Large CAPE

  16. Orographic Rainfall Large Fw Small Fw II

  17. Orographic Rainfall (Large Fw, Strong Wind) Large CAPE Small CAPE

  18. I III IV II Bifurcation? 2D Flow Regime Diagram (Fw and CAPE)

  19. Flow Regime Table (Fw and h/a) h = 2 km, CAPE=3000 J/kg, h/a Fw

  20. 4 8 10 2 6 Time (h) Accumulated Rainfall (vary mountain width)

  21. a =45 km a = 7.5 km a = 22.5 km IV III III 4 8 10 2 6 Time (h) Accumulated Rainfall (vary mountain width)

  22. 4 8 10 2 6 Time (h) 4 8 10 2 6 Time (h) Accumulated Total Rainfall IV

  23. Regime I Regime II C C FW FD FW FD Summary Regime I: Flow with an upstream propagating convective system Regime II: Flow with a long-lasting orographic convective system over the mountain peak, upslope or downslope

  24. Regime III Regime IV S S/C C/S C/S/N FW FD FW FD Summary Regime III: Flow with a long-lasting orographic convective or mixed convective and stratiform precipitation system over the mountain peak and a downstream propagating convective system ; and Regime IV: Flow with a long-lasting orographic stratiform precipitation system over the mountain and possibly a downstream propagating cloud system.

  25. Summary • When the Fw(or basic wind speed) increases, the flow tends to shift to a higher number flow regime. • When the CAPE increases, the flow shifts to a lower number regime. • When h/a increases, the flow shifts to a higher number flow regime. • When the CAPE is large, the orographic rainfall amount is not very sensitive to the Fw. • When the CAPE is small, the orographic rainfall amount is strongly dependent on the Fw. • The domain integrated rainfall amount is sensitive to Fw, but not to the aspect ratio, in particular for flow Regimes I and II. • Local orographic rainfall amount from straitiform precipitation systems can be as heavy as that from convective or mixed type precipitation systems.

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