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 3 North Carolina State University

Outline o Introduction o Numerical Simulations and Results o Summary

Upstream Propagating Convective System Introduction Jou, 1997

Quasi-stationary Convective System Introduction

Downstream Propagating Convective System

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. Introduction

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

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 Model Configuration

Experiment Design (CAPE) 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. N w =.942 ~ 1.01 x s -1 LFC

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

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

CP4F4 F w =0.786 (15 m/s) CP4F6 F w =1.572 (30 m/s) Mixed convective and stratiform clouds Stratiform clouds III IV Flow Regimes

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

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

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

The vertical velocity might be induced by orography and/or environment (e.g. Alpert 1986; Lin et al. 2001) W = Woro + Wenv Woro ~ U dh/dx Wenv ~ CAPE or synoptic scale W (Lin et al. 2001) Total Accumulated Rainfall (10h)

Orographic Rainfall Small F w Large F w II

Orographic Rainfall (Large F w, Strong Wind) Small CAPE Large CAPE

Orographic Rainfall (Small F w, Weak Wind) System may propagate upstream and block the inflow. Therefore, it is hard to do comparison for orographic rainfall..

2D Flow Regime Diagram ( F w and CAPE) Bifurcation? IIII IV II

Flow Regime Table ( F w and h/a ) A1 (2/7.5) (7.5km) A2 (1/7.5) (15km) A3 (1/7.5) (22.5km) A4 (1/15) (30km) A5 (2/45) (45km) F1 (0.131) (2.5m/s) IIIII F2 (0.262) (5m/s) IIIII F3 (0.393) (7.5m/s) IIIII F4 (0.524) (10m/s) IIIIII F2 (0.786) (15m/s) III II F2 (1.048) (20m/s) III F2 (1.572) (30m/s) IVIII FwFw h/a h = 2 km, CAPE=3000 J/kg,

Accumulated Rainfall (vary mountain width) Time (h)

Time (h) a = 7.5 km a = 22.5 km a =45 km IV III Accumulated Rainfall (vary mountain width)

Accumulated Total Rainfall Time (h) Time (h) IV

Summary FWFW FDFD Regime I C FWFW FDFD Regime II C 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

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. FWFW FDFD Regime III S/C C/S FWFW FDFD Regime IV S C/S/N Summary

 When the F w (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 F w.  When the CAPE is small, the orographic rainfall amount is strongly dependent on the F w.  The domain integrated rainfall amount is sensitive to F w, 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. Summary