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 University2 University of California, Davis3 North Carolina State University

Outline

o Introduction

o Numerical Simulations

and

Results

o Summary

Upstream Propagating Convective System

Introduction

Jou, 1997Jou, 1997

Quasi-stationary Convective System

Introduction

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, 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

hNU

Fw

w

zg

N v

vw

2

• 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

21 a/)xx(

hh

osfc

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.

Nw = .942 ~ 1.01 x 10-2 s-1

LFC

U = 2.5, 5, 10, 15, 20, 30

m/s for

F1 – F6, respectively

h = 2 km

a = 30 km

hNU

Fw

w

Fw

CAPE(J/kg)

0.131 0.262 0.524 0.786 1.048 1.572

487

1372

1895

2438

3000

3578

CP0F1

CP1F1

CP2F1

CP3F1

CP4F1

CP5F1

CP0F2

CP1F2

CP2F2

CP3F2

CP4F2

CP5F2

CP0F3

CP1F3

CP2F3

CP3F3

CP4F3

CP5F3

CP0F4

CP1F4

CP2F4

CP3F4

CP4F4

CP5F4

CP0F5

CP1F5

CP2F5

CP3F5

CP4F5

CP5F5

CP0F6

CP1F6

CP2F6

CP3F6

CP4F6

CP5F6

Fw

CAPE(J/kg)

0.131 0.262 0.524 0.786 1.048 1.572

487

1372

1895

2438

3000

3578

CP0F1

CP1F1

CP2F1

CP3F1

CP4F1

CP5F1

CP0F2

CP1F2

CP2F2

CP3F2

CP4F2

CP5F2

CP0F3

CP1F3

CP2F3

CP3F3

CP4F3

CP5F3

CP0F4

CP1F4

CP2F4

CP3F4

CP4F4

CP5F4

CP0F5

CP1F5

CP2F5

CP3F5

CP4F5

CP5F5

CP0F6

CP1F6

CP2F6

CP3F6

CP4F6

CP5F6

Experiment Design (U and CAPE)

Shading – rainfall Contours – w at 3.6km

Shading – w Contours – θ

7h

CP4F1Fw=0.131

(2.5m/s)

CP4F3Fw=0.524

(10 m/s)

I

II

Flow Regimes(CAPE = 3000 J/kg)

(CAPE = 3000 J/kg)

CP4F4Fw=0.786

(15 m/s)

CP4F6Fw=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

CAPE

ZZ~Toro LFCEL

2

Total Accumulated Rainfall (10h)

Small Fw

Large Fw

When Fw increases,

the flow shifts to a higher number

Flow regime.

II

IIIII

IVIV

Small CAPE

Large CAPE

IIII

III

I

When CAPE increases,

the flow shifts to a lower number

Flow regime.

Total Accumulated Rainfall (10h)

Orographic Rainfall

Small Fw

Large Fw

II

Orographic Rainfall(Large Fw, Strong Wind)

Small CAPE

Large CAPE

2D Flow Regime Diagram

(Fw and CAPE)

Bifurcation?

II IIIIII

IVIVIIII

Flow Regime Table(Fw and h/a)

hNU

Fw

w

A1(2/7.5)(7.5km

)

A2(1/7.5

)(15km

)

A3(1/7.5)(22.5k

m)

A4(1/15)(30km

)

A5(2/45)(45km)

F1 (0.131)(2.5m/s)

I I I I I

F2 (0.262)(5m/s)

I I I I I

F3 (0.393)(7.5m/s)

I I I I I

F4 (0.524)(10m/s)

II I I I I

F2 (0.786)(15m/s)

III III II II II

F2 (1.048)(20m/s)

III III III III III

F2 (1.572)(30m/s)

IV III III III III

Fw

h/a

h = 2 km, CAPE=3000 J/kg,

Accumulated Rainfall(vary mountain width)

U = 7.5 m/s

0

4

8

12

16

tota

l rai

n (x

1000

mm

)

7.5km 15km 22.5km30km 45km

Time (h)

2 4 6 8 10

U = 30 m/s

0

4

8

12

16

20

24

28

tota

l rai

n (x

1000

mm

)

7.5km 15km 22.5km30km 45km

Time (h)

2 4 6 8 10

a = 7.5

km

a = 22.5

km

a =45 km

IV III III

Accumulated Rainfall(vary mountain width)

Accumulated Total Rainfall

a = 22.5 km

0

4

8

12

16

20

24

tota

l rai

n (x

1000

mm

)

2.5 m/s 5 m/s 7.5 m/s10 m/s 15 m/s 20 m/s30 m/s

Time (h)2 4 6 8 10

a = 7.5 km

0

4

8

12

16

20

tota

l rai

n (x

1000

mm

)

2.5 m/s 5 m/s 7.5 m/s10 m/s 15 m/s 20 m/s30 m/s

Time (h)2 4 6 8 10

IV

Summary

FW FD

Regime I

C

FW FD

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.

FW FD

Regime III

S/C C/S

FW FD

Regime IV

SC/S/N

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.

Summary