Application of theory to observed cases of orographically forced convective rainfall*

R. Rotunno

NCAR**

Boulder, CO

*Miglietta & Rotunno (Mon Wea Rev, 2012)

M. M. Miglietta

CNR/ISAC

Italy

**NCAR sponsered by

Dependence on external parameters of rainfall intensity/location over a simple 2D ridge*

a

CAPE, CIN

⇒U

th

LFCLCL

mhDCAPE

2N0=f

Bryan 3D Cloud Model (Bryan and Fritsch 2002 MWR)

*Miglietta and Rotunno (2009 JAS)

Dependence on external parameters of rainfall intensity/location over a simple 2D ridge*

a

CAPE, CIN

⇒U

th

LFCLCL

mhDCAPE

2N0=f

22,,,,,,,

U

CAPE

U

Nh

CIN

CAPE

U

DCAPE

LCL

h

LFC

h

a

h

h

h mmmm

t

m

Bryan 3D Cloud Model (Bryan and Fritsch 2002 MWR)

*Miglietta and Rotunno (2009 JAS)

Three nondimensional parameters emerge as most important:

1. Triggering:

2. Orographic forcing:

3. Ratio of advective to convective time scale:

hm

LFC

hm

a

τ adv

τ conv

=a U

ht CAPE

hm

LFC

hm

a

ht

CAPE

U

a

t =0

tgrow

tmature

train

τ conv ≈ tgrow + tmature + train

*Miglietta and Rotunno (2009 JAS)

For steady orographic rain:

/ certain combinations of

hm

LFC,hm

a⎛⎝⎜

⎞⎠⎟

No steady orographic rain if: Ridge too short

Wind weak, strong instability

hm

LFC<<1

τ adv

τ conv

=a CAPE

htU>> 1

Summary

τ adv

τ conv

=a CAPE

htU~ O(1) − O(10)

*Miglietta and Rotunno (2009 JAS)

Big Thompson Flood Colorado, 1976

N

Big Thompson Flood Colorado, 1976

low-level flow

N

Big Thompson Flood Colorado, 1976

170mm in several hoursCaracena et al. (1979)

Big Thompson Flood Simulation (2D)

30 km

00

10h

5h

x

t

mm/h

τ adv

τ conv

=14.4 ,hmLFC

=1.33 ,hma

=0.07

10m/s

U

Big Thompson Flood Simulation (2D)

30 km

ConstantWind

Low-LevelWind Only

00

10h

5h

x

t

0

10h

5h

t

mm/h

τ adv

τ conv

=14.4 ,hmLFC

=1.33 ,hma

=0.07

10m/s 17m/s

U

U

Oahu Flood, Hawaii 1974

Oahu Flood, Hawaii 1974

low-level flow

Schroeder (1977)

Oahu Flood, Hawaii 1974

250mm in several hours

Oahu Flood Simulations (2D)

30 km

00

10h

5h

x

t

mm/h

τ adv

τ conv

=3.4 ,hmLFC

=1.25 ,hma

=0.10

10m/s

U

Oahu Flood Simulations (2D)

ConstantWind

Low-LevelWind Only

30 km

00

10h

5h

x

t

0

10h

5h

t

mm/h

τ adv

τ conv

=3.4 ,hmLFC

=1.25 ,hma

=0.10

10m/s

U

U

Conclusions

•Case-study simulations consistent / MR09 idealized analysis

•However…low-level wind decreasing with height doubles the rainrate and brings it closer to obs

UU

A HyMex Objective: High-Precipitation Events

Ducrocq et al. (2008)

Aude

Gard

Cevennes

For steady orographic rain:

/ certain combinations of (no upstream cold pool)

hm

LFC,hm

a⎛⎝⎜

⎞⎠⎟

No steady orographic rain if: Ridge too short

Wind weak, strong instability

hm

LFC<<1

τ adv

τ conv

=a CAPE

htU>> 1

Summary

τ adv

τ conv

=a CAPE

htU~ O(1) − O(10)

(upstream cold pool)

But…low-CAPE is a special caseRainrate ~ independent of

hm

LFC,hm

a⎛⎝⎜

⎞⎠⎟

DCAPE

U≈2.5 and

NLFCU

≈ 2.5

Miglietta and Rotunno (2010)