Dow

nslo

pe W

inds

torm

s D

owns

lope

Win

dsto

rms

and

Rot

ors

and

Rot

ors

Jona

than

Vig

hJo

nath

an V

igh

AT7

07A

T707

21,2

3,25

Feb

ruar

y 20

0521

,23,

25 F

ebru

ary

2005

Asp

ects

of f

low

ove

r bar

riers

Asp

ects

of f

low

ove

r bar

riers

•2D

mou

ntai

n w

aves

•Fl

ow o

ver/a

roun

d is

olat

ed h

ills

•B

lock

ing

by la

rge

ampl

itude

mou

ntai

ns–

Gap

flow

s, fu

nnel

ing

–C

old

air d

amm

ing,

bar

rier j

ets

•Fö

hn a

nd B

ora

win

ds•

Dow

nslo

pe w

inds

torm

s•

Ther

mal

ly-d

riven

circ

ulat

ions

(slo

pe w

inds

, mou

ntai

n w

inds

, val

ley

win

ds),

kata

batic

win

ds•

Oro

grap

hic

cont

rol o

f pre

cipi

tatio

n•

Qua

si-g

eost

roph

ic fl

ow o

ver a

mou

ntai

n•

Lee

cycl

ogen

esis

•G

ravi

ty w

ave

drag

Nam

ing

conv

entio

ns a

nd

Nam

ing

conv

entio

ns a

nd

term

inol

ogy

term

inol

ogy

Wha

t are

the

diffe

rent

type

s of

dow

nslo

pe w

inds

and

whe

re d

o th

ey

occu

r?Fö

hnw

inds

(war

min

g w

inds

)Fö

hn(A

lps)

, Chi

nook

(Roc

kies

), zo

nda

(eas

tern

And

es),

pulc

he(w

este

rn A

ndes

), S

anta

Ana

(sou

ther

n C

alifo

rnia

), su

ndow

ner(

San

ta

Bar

bara

, CA

), be

rg(S

outh

Afri

ca),

kosc

hava

and

ljuka

(Cro

atia

), ge

rmic

h(S

W C

aspi

an S

ea),

afga

neta

nd ib

e(C

entra

l Asi

a), k

achc

han

(Sri

Lank

a), C

ante

rbur

y no

rth-w

este

r(N

ew Z

eala

nd)

Bor

aw

inds

(coo

ling

win

ds)

bora

(Cro

atia

), m

istra

l(Fr

ance

)G

lenn

(196

1) c

oncl

uded

that

ther

e w

as n

o sa

tisfa

ctor

y de

finiti

onfo

r a

Föhn

flow

–Y

oshi

no (1

975)

sai

d th

at th

e de

finiti

on s

houl

d si

mpl

y be

bas

ed o

n w

heth

er th

e te

mpe

ratu

re o

f the

dow

nslo

pe fl

owin

g ai

r was

war

mer

or

cool

er th

an th

e ai

r it w

as d

ispl

acin

g–

Chi

nook

s an

d bo

ras

do n

ot a

lway

s ha

ve s

ever

e w

inds

It is

bes

t to

just

cal

l thi

s ph

enom

enon

“dow

nslo

pe w

inds

torm

”or

“sev

ere

dow

nslo

pe w

inds

torm

”

Föhn

and

bor

a w

inds

of th

e W

este

rn U

.S.

Gap

flow

s be

twee

n m

ount

ains

of E

urop

e

Figu

res

from

Whi

tem

an (2

000)

A fe

w n

otes

on

the

Chi

nook

A fe

w n

otes

on

the

Chi

nook

Why

are

Chi

nook

s w

arm

?Th

e de

scen

ding

air

can

be w

arm

er fo

r one

or m

ore

reas

ons:

•C

ompr

essi

onal

hea

ting

as a

ir de

scen

ds th

e le

e si

de o

f th

e m

ount

ain

(air

war

ms

acco

rdin

g to

the

dry

adia

batic

la

pse

rate

: 9.8

°C

per

km

of d

esce

nt)

•La

tent

hea

t of c

onde

nsat

ion

(Hah

n ef

fect

): ai

r asc

ends

m

oist

adi

abat

ical

ly (c

oolin

g ~5

°C p

er k

m) o

n w

indw

ard

side

of m

ount

ain

whi

le p

reci

pita

tion

is p

rodu

ced,

then

de

scen

ds d

ry a

diab

atic

ally

on

the

leew

ard

side

•B

lock

ing

on th

e w

indw

ard

side

of t

he m

ount

ain

can

caus

e ai

r to

desc

end

from

a h

ighe

r lev

el (h

ighe

r θ)

•D

ispl

acem

ent o

f col

d ai

r by

war

mer

air

•N

ight

time

mix

ing

Oth

er p

ecul

iarit

ies

can

occu

r if a

col

d po

ol is

pre

sent

–W

aves

can

pro

paga

te o

n th

e in

vers

ion

surfa

ce (s

ee

Bla

ck H

ills p

aper

)–

Mira

ges

can

mak

e th

e m

ount

ains

look

hig

her

Figu

res

from

Whi

tem

an (2

000)

Not

able

Bou

lder

Win

dsto

rms

Not

able

Bou

lder

Win

dsto

rms

7-8

Janu

ary

1969

–13

0 m

ph a

t NC

AR, 9

6 m

ph d

ownt

own

–1

deat

h23

Jan

uary

197

1–

147

mph

at N

CAR

11 J

anua

ry 1

972

–w

ind

gust

s to

97

mph

–40

trai

lers

dam

aged

–$3

milli

on d

amag

e4

Dec

embe

r 197

8–

148

mph

at ?

–1

deat

h17

Jan

uary

198

2–

137

mph

at N

CAR

–20

gus

ts a

bove

120

mph

in 4

5 m

in!

–At

leas

t 15

inju

ries

–$2

0 m

illion

dam

age,

40%

of B

ould

er b

uild

ings

dam

aged

!24

Jan

uary

198

2–

140

mph

at W

onde

rvu

24 S

epte

mbe

r 198

6–

131

mph

at ?

–$1

00,0

00 d

amag

e24

Jan

uary

199

2•

143

mph

(kt?

) nor

th B

ould

er2-

3 Fe

brua

ry 1

999

–12

7 m

ph a

t Sug

arlo

af, 1

20 m

p in

Laf

ayet

te, 1

19 m

ph in

Won

derv

u,

100

mph

in L

ongm

ont,

98 m

ph in

Bou

lder

–$3

milli

on d

amag

e8-

10 A

pril

1999

–12

0 m

ph a

t Sug

arlo

af, n

ear 1

00 m

ph in

sou

th B

ould

er, 9

0 m

ph in

Lo

ngm

ont

–$2

0 m

illion

dam

age

from

Pue

blo

to F

ort C

ollin

s

From

196

7-19

95, w

inds

> 1

00 m

ph o

ccur

red

35 ti

mes

Dam

age

in S

outh

Bou

lder

from

the

8 Ja

nuar

y 19

69

win

dsto

rm (E

d Zi

pser

)

Pho

tos

cour

tesy

of U

CA

R

Sou

rce:

http

://w

ww

.co.

boul

der.c

o.us

/she

riff/p

df/o

em/h

ighw

ind.

pdfa

nd h

ttp://

ww

w.c

dc.n

oaa.

gov/

Bou

lder

/win

d.ht

ml

Win

dsto

rm D

amag

eW

inds

torm

Dam

age

•A

ccor

ding

to R

MIIA

, Col

orad

o's

top

five

mos

t co

stly

win

dsto

rms

are:

–$2

0 m

illio

n in

insu

red

dam

age

occu

rred

alo

ng th

e Fr

ont R

ange

on

Apr

il 8-

10, 1

999.

–

$20

mill

ion

in in

sure

d da

mag

e oc

curr

ed in

Bou

lder

C

ount

y on

Jan

. 17,

198

2.

–$1

0 m

illio

n in

insu

red

dam

age

occu

rred

alo

ng th

e Fr

ont R

ange

on

Jan.

28-

29, 1

987.

–

$5.2

mill

ion

in in

sure

d da

mag

e oc

curr

ed a

long

the

Fron

t Ran

ge o

n O

ct. 2

9, 1

996.

–

$3 m

illio

n in

insu

red

dam

age

occu

rred

alo

ng th

e Fr

ont

Ran

ge o

n Fe

b. 2

-3, 1

999.

Obs

erva

tions

from

196

8 Fi

eld

Pro

ject

Obs

erva

tions

from

196

8 Fi

eld

Pro

ject

Figu

re fr

om K

uettn

er a

nd L

illy

(196

8)

Figu

re fr

om

Kue

ttner

an

d Li

lly

(196

8)

Dat

a fro

m a

com

posi

te s

tudy

Dat

a fro

m a

com

posi

te s

tudy

Figu

res

from

Brin

kman

n (1

974)

Com

posi

te s

ound

ings

at o

nset

of

Com

posi

te s

ound

ings

at o

nset

of

Bou

lder

win

dsto

rms

Bou

lder

win

dsto

rms

Not

e th

at b

ase

of is

othe

rmal

leve

l is

575

in th

e up

stre

am s

ound

ing,

but

is lo

wer

(650

mb)

in th

e do

wns

tream

sou

ndin

g.A

lso,

dow

nstre

am s

ound

ing

has

stee

per l

apse

rate

, hig

her θ

at lo

wer

leve

lsFi

gure

s fro

m B

rinkm

ann

(197

4)

Com

posi

te D

enve

r sou

ndin

gs fo

r C

ompo

site

Den

ver s

ound

ings

for

diffe

rent

type

s of

win

dsto

rms

diffe

rent

type

s of

win

dsto

rms

•D

owns

tream

so

undi

ng

com

posi

tes

–In

vers

ion

leve

l av

erag

es ~

675

mb

for B

ould

er

win

d ca

ses

–~6

25 m

b fo

r slo

pe

win

d ca

ses

Figu

res

from

Brin

kman

n (1

974)

Obs

erva

tions

from

the

11 J

anua

ry

Obs

erva

tions

from

the

11 J

anua

ry

1972

Win

dsto

rm19

72 W

inds

torm

Figu

re fr

om

Lilly

(197

8)

Figu

re fr

om L

illy

(197

8)

Figu

res

from

Lill

y (1

978)

•P

ress

ure

trace

s du

ring

a w

inds

torm

•N

ote

in

parti

cula

r the

su

dden

pr

essu

re d

rop

at

Bou

lder

dur

ing

the

stor

m•

Is th

is c

ausi

ng

the

high

win

ds?

Figu

re fr

om L

illy

and

Zips

er (1

972)

Dow

nslo

pe W

inds

torm

D

owns

lope

Win

dsto

rm

Mec

hani

sms

Mec

hani

sms

Ther

e ar

e 3

prop

osed

m

echa

nism

s fo

r dow

nslo

pe

win

dsto

rms:

1.D

evel

op w

hen

the

flow

ove

r a

mou

ntai

n tra

nsiti

ons

from

su

bcrit

ical

-> s

uper

criti

cal

over

the

mou

ntai

n,

anal

ogou

s to

a h

ydra

ulic

ju

mp

2.La

rge-

ampl

itude

ver

tical

ly-

prop

agat

ing

mou

ntai

n w

aves

that

und

ergo

par

tial

refle

ctio

n at

inte

rface

s –

prop

erly

‘tun

ed’ w

aves

re

sona

te w

ith in

crea

sing

am

plitu

de3.

Wav

e br

eaki

ng a

nd w

ave-

indu

ced

criti

cal l

ayer

s

Ver

tical

ly p

ropa

gatin

g w

aves

with

larg

e am

plitu

deTr

appe

d le

e w

aves

un

derg

oing

refle

ctio

n

Figu

res

from

Whi

tefie

ld (2

000)

Long

’s H

ydra

ulic

Jum

p (1

953a

)Lo

ng’s

Hyd

raul

ic J

ump

(195

3a)

•H

omog

eneo

us fl

uid

flow

ing

over

rid

ge-li

ke o

bsta

cle.

Ass

ume

flow

is

in h

ydro

stat

ic b

alan

ce a

nd

boun

ded

by fr

ee s

urfa

ce.

•C

onsi

der y

-inde

pend

ent m

otio

ns•

Ass

ume

stea

dy-s

tate

flow

.

0=

∂∂+

∂∂+

∂∂xh

gxD

gxu

u Whe

re D

is th

e th

ickn

ess

of th

e flu

id

and

his

the

obst

acle

hei

ght.

Usi

ng th

e co

ntin

uity

equ

atio

n

gDuFr

xhxh

DFr

22

2 w

here

)(

)1

1(

=

∂∂=

∂+∂

−0

)(

=∂

∂xuD

Mos

t peo

ple

Inte

rpre

t Fr2

as th

e Fr

oude

#. H

ere

it is

a ra

tio o

f the

flui

d sp

eed

to

the

prop

agat

ion

spee

d of

sh

allo

w li

near

gra

vity

wav

es

we

get:

So

the

free

surfa

ce c

an e

ither

rise

or

fall

depe

ndin

g on

the

mag

nitu

de o

f Fr2

Sup

ercr

itica

l cas

eS

uper

criti

cal c

ase

Flui

d th

icke

ns a

nd s

low

s as

it c

ross

es to

p of

ob

stac

le, m

inim

um s

peed

at c

rest

Sch

emat

ic fr

om D

urra

n (1

990)

Sub

criti

cal c

ase

Sub

criti

cal c

ase

Flui

d th

ins

and

acce

lera

tes

as it

cro

sses

top

of

obst

acle

, rea

ches

max

imum

spe

ed a

t cre

st

Sch

emat

ic fr

om D

urra

n (1

990)

Hyd

raul

ic ju

mp

case

Hyd

raul

ic ju

mp

case

Flow

tran

sitio

ns fr

om

subc

ritic

al to

su

perc

ritic

al a

t top

of

obst

acle

, pot

entia

l en

ergy

is c

onve

rted

to

kine

tic e

nerg

y ov

er

the

entir

e ba

rrie

rFi

gure

from

Lon

g (1

953a

)

Sch

emat

ic fr

om D

urra

n (1

990)

Fore

cast

ing

Fore

cast

ing

Ther

e ap

pear

to b

e at

leas

t thr

ee m

echa

nism

s w

hich

can

ca

use

the

flow

to u

nder

go tr

ansi

tion

from

sub

criti

cal t

o su

perc

ritic

al:

1. W

ave

brea

king

-in

an

atm

osph

ere

with

con

stan

t N a

nd u

0; m

ount

ain

larg

e en

ough

to c

ause

bre

akin

g w

aves

(Cla

rk a

nd

Pel

tier 1

977)

2.

Sco

rer-p

aram

eter

laye

ring

–in

an

atm

osph

ere

with

con

stan

t u0

and

two

laye

rs o

f N: m

ount

ain

too

smal

l to

caus

e br

eaki

ng

wav

es (D

urra

n 19

86a)

3. C

appi

ng b

y a

mea

n-st

ate

criti

cal l

ayer

: in

an a

tmos

pher

e w

ith

cons

tant

N a

nd u

0be

low

a c

ritic

al la

yer,

whe

re in

the

abse

nce

of

the

criti

cal l

ayer

, the

mou

ntai

n is

too

smal

l to

caus

e br

eaki

ng

wav

esO

ther

mec

hani

sms:

wav

e-in

duce

d cr

itica

l lay

ers?

Gen

eral

dev

elop

men

t G

ener

al d

evel

opm

ent

char

acte

ristic

sch

arac

teris

tics

•Fo

r the

cas

e of

dee

p cr

oss-

mou

ntai

n flo

w a

nd n

o m

ean-

stat

e cr

itica

l lay

er, o

bser

vatio

ns s

ugge

st a

win

dsto

rm w

ill

occu

r whe

n:–

Win

d is

dire

cted

acr

oss

mou

ntai

n (w

ithin

30°

of p

erpe

ndic

ular

to

ridge

line)

and

win

d at

mou

ntai

ntop

leve

l exc

eeds

a te

rrai

n de

pend

ent v

alue

of 7

to 1

5 m

s-1

–U

pstre

am te

mpe

ratu

re p

rofil

e ex

hibi

ts a

n in

vers

ion

or a

laye

r of

stro

ng s

tabi

lity

near

mou

ntai

ntop

leve

l (C

olso

n 19

54; B

rinkm

ann

1974

)Th

ese

cond

ition

s fa

vor d

evel

opm

ent o

f a d

owns

lope

sto

rm

by c

reat

ing

cond

ition

s si

mila

r to

Sco

rer p

aram

eter

la

yerin

g. T

hey

also

pro

mot

e th

e de

velo

pmen

t of l

arge

r am

plitu

de m

ount

ain

wav

es, i

ncre

asin

g th

e ch

ance

s fo

r br

eaki

ng w

aves

. Bre

akin

g w

aves

are

favo

red

whe

n th

e up

per t

ropo

sphe

ric w

inds

are

not

too

stro

ng.

The

effe

ct o

f a m

ount

aint

op

The

effe

ct o

f a m

ount

aint

op

inve

rsio

nin

vers

ion

Figu

re fr

om D

urra

n (1

990)

Oth

er fo

reca

st fa

ctor

sO

ther

fore

cast

fact

ors

•Id

eal t

erra

in fo

r win

dsto

rms

are

long

ridg

es w

ith g

entle

win

dwar

d sl

opes

and

st

eep

lee

slop

es (e

ffect

ive

terr

ain

shap

e ca

n be

mod

ified

by

upst

ream

bl

ocki

ng)

•Lo

w h

umid

ity is

bet

ter (

moi

stur

e se

ems

to re

duce

am

plitu

de)

•N

ight

time

or e

arly

mor

ning

mor

e lik

ely

(sta

bilit

y ch

angi

ng d

urin

g th

is ti

me?

)•

Kle

mp

and

Lilly

(197

5) fo

und

that

the

stro

nges

t dow

nslo

pe e

vent

soc

cur i

n B

ould

er w

hen

a on

e-ha

lf w

avel

engt

h ph

ase

shift

was

pre

sent

bet

wee

n gr

ound

and

trop

opau

se (p

artia

l ref

lect

ion

mec

hani

sm o

f lin

ear t

heor

y)•

Dur

ran

(198

6a) r

an s

imul

atio

ns o

f the

11

Janu

ary

1972

eve

nt –

this

co

nditi

on a

ppea

rs n

eces

sary

, but

not

suf

ficie

nt fo

r stro

nges

t win

dsto

rms

•E

leva

ted

inve

rsio

ns m

ight

be

requ

ired

for b

reak

ing

wav

es to

form

•Le

e et

al (

1989

) fou

nd th

at th

e pr

esen

ce o

f col

d po

ols

in th

e le

e of

the

mou

ntai

n co

uld

have

a s

trong

det

erm

inan

t in

whe

ther

dow

nslo

pe w

inds

w

ould

mak

e it

to th

e m

ount

ain

base

–it

also

alte

red

the

over

all s

truct

ure

of

the

mou

ntai

n w

ave

in s

imul

atio

ns (d

iffer

ent l

ower

bou

ndar

y, c

hang

e in

sc

ale?

)•

In s

ome

case

s, p

reci

pita

tion

effe

cts

coul

d pl

ay a

role

? (e

.g. t

he 3

Jul

y 19

93

Fort

Col

lins

win

dsto

rm c

ase)

25 O

ctob

er 1

997

Blo

wdo

wn

Eve

nt

25 O

ctob

er 1

997

Blo

wdo

wn

Eve

nt

Wes

t of t

he P

ark

Ran

geW

est o

f the

Par

k R

ange

13,0

00 a

cres

of o

ld g

row

th tr

ees

blow

n do

wn

in M

ount

Zirk

elW

ilder

ness

Are

a/R

outt

Nat

iona

l For

est,

trees

sta

cked

30

feet

hig

h, h

unte

rs tr

appe

d fo

r 2 d

ays;

stro

nges

t win

ds

last

ed ~

30 m

inW

ind

gust

s ex

ceed

ed 1

00 m

ph fo

r 7

hrs

at A

rapa

hoe

Bas

in S

ki

Are

a (e

l. 12

,500

ft; p

eak

gust

11

4 m

ph o

ut o

f the

eas

t, w

indc

hill

to -6

0°F)

Fact

ors:

stro

ng s

ynop

tical

ly-d

riven

flo

w fr

om e

ast t

o w

est (

bliz

zard

of

’97)

, an

unus

ually

col

d an

d st

able

laye

r on

the

win

dwar

d si

de o

f the

mou

ntai

ns

Pic

ture

cou

rtesy

U.S

. For

est S

ervi

ce

For m

ore,

see

Mey

ers

et a

l (20

03)

RA

MS

Mod

el S

imul

atio

nFi

gure

s fro

m F

SL

Foru

m, F

eb. 1

999

(Win

d sp

eed

in k

nots

, hei

ght i

n m

eter

s)

His

toric

al F

ort C

ollin

s W

inds

torm

sH

isto

rical

For

t Col

lins

Win

dsto

rms

Fort

Col

lins

Win

dsto

rms

note

d in

con

junc

tion

with

Bou

lder

Win

dsto

rms

(as

repo

rted

in th

e B

ould

er D

aily

Cam

era)

•D

ec. 4

-5, 1

880

•M

ar. 1

8, 1

920

•M

ay 2

1, 1

925

also

incl

udin

g E

aton

, Gre

eley

, Win

dsor

, and

Pla

ttevi

lle

•Ja

n. 1

5, 1

943

incl

udin

g Lo

vela

nd, F

t. M

orga

n•

Dec

. 20,

194

8 av

erag

e w

ind

of 4

1 m

ph, g

ust t

o 96

mph

•D

ec. 6

, 196

3

Fort

Col

lins,

Gre

eley

, Ste

rling

•Ja

n 27

-28,

196

5 g

ust t

o 73

mph

•Fe

b. 1

3-14

, 196

7

•

Dec

. 6, 1

967

gu

st to

66

mph

•D

ec. 2

1-22

, 196

9 L

arim

er C

ount

y •

Nov

30-

Dec

1, 1

970

•

Mar

31,

197

1

Ft C

ollin

s an

d La

porte

, 40-

50 m

ph, g

usts

to 7

2 m

ph, o

ne fa

talit

y an

d se

vera

l inj

urie

s in

For

t Col

lins

•Ja

n 11

-12,

197

2 F

ort C

ollin

s Lo

vela

nd a

nd ju

st a

bout

eve

ryw

here

•N

ov 2

5-26

, 197

2 F

ort C

ollin

s

Dat

a fro

m W

hite

man

and

Whi

tem

an (1

973)

Fort

Col

lins

Win

dsto

rm C

limat

olog

yFo

rt C

ollin

s W

inds

torm

Clim

atol

ogy

•P

eak

win

dsto

rm s

easo

n is

dur

ing

the

win

ter m

onth

s•

Win

dsto

rms

ofte

n oc

cur i

n ‘s

treak

s’; J

an. 1

977

had

7 se

para

te e

vent

s!•

Sum

mer

eve

nts

are

rare

, but

not

unp

rece

dent

ed (J

une

1973

, Jul

y 19

93)

Dat

a co

urte

sy o

fJo

hn W

eave

r (s

ee L

ee e

t al 1

989;

W

eave

r and

Phi

llips

1990

)

Fort

Col

lins

Win

dsto

rms,

197

7-19

89

02468101214161820 Janu

aryFeb

ruary

March

April

May

June

July

Augus

t Septem

ber

Octobe

r Novembe

r Decembe

r

Mon

th

Number of windstorm events

> 54

kt

> 70

kt

Mos

t or a

ll of

the

data

is fr

om o

ld

Foot

hills

cam

pus

stat

ion

•Fo

r com

paris

on, t

he

seas

onal

cyc

le o

f B

ould

er w

inds

Sou

rce:

http

://w

ww

.bcn

a.or

g/w

inds

.htm

l

FC W

inds

torm

Clim

atol

ogy,

con

t’dFC

Win

dsto

rm C

limat

olog

y, c

ont’d

•65

win

dsto

rms

with

max

gus

ts >

54 k

t(~5

per

yea

r)•

12 w

inds

torm

s w

ith m

ax g

usts

>70

kt(

~1 p

er y

ear)

Dat

a co

urte

sy o

fJo

hn W

eave

r (s

ee L

ee e

t al 1

989;

W

eave

r and

Phi

llips

1990

)

Fort

Col

lins

Win

dsto

rms

024681012 1977

/7819

78/79

1979

/8019

80/81

1981

/8219

82/83

1983

/8419

84/85

1985

/8619

86/87

1987

/8819

88/89

Year

Number of windstorm events

> 54

kt

> 70

kt

5055606570758085

06

1218

24

Max

gus

ts in

kt (

y-ax

is) v

ersu

s tim

e of

day

occ

urs

(x)

5055606570758085

01

23

45

67

89

1011

12

Max

gus

t in

kts

vers

us M

onth

of y

ear

Not

e th

at th

ere

is n

opr

efer

entia

l tim

e of

yea

r fo

r max

imum

gus

ts

Max

gus

ts o

ccur

dur

ing

all p

arts

of t

he d

ay, w

ith

a w

eak

peak

aro

und

mid

day

Ave

rage

win

dsto

rm d

urat

ion

is 8

hrs

(min

of 1

.25

h,

max

of 2

0 h)

Dat

a co

urte

sy o

fJo

hn W

eave

r (s

ee L

ee e

t al 1

989;

W

eave

r and

Phi

llips

1990

)

Win

d tra

ce o

f win

dsto

rm a

t For

t Col

lins

Not

e th

at w

inds

are

muc

h st

eadi

er th

an B

ould

er s

torm

s

Trac

e co

urte

sy o

f Ric

hard

Joh

nson

Not

able

For

t Col

lins

Win

dsto

rms

Not

able

For

t Col

lins

Win

dsto

rms

823

Jul 1

993

7722

Dec

198

8

7918

Sep

198

8

732

Dec

198

7

7328

/29

Jan

1987

784/

5 M

ay 1

986

7315

Feb

198

6

7230

Mar

198

3

782

Apr

198

2

7429

/30

Mar

198

2

8324

Jan

198

2

743

Jan

1982

7126

Nov

197

7

7917

/18

Jun

1973

Stre

ngth

of m

ax g

ust

(kt)

Dat

eM

ost o

r all

of th

e da

ta is

from

the

old

Foot

hills

ca

mpu

s st

atio

n,

data

afte

r 198

8 is

fro

m C

hris

tman

Fi

eld

stat

ion

Dat

a co

urte

sy o

fJo

hn W

eave

r (s

ee L

ee e

t al 1

989;

W

eave

r and

Phi

llips

1990

)

Rot

ors

Rot

ors

Rot

or: a

regi

on o

f rev

erse

d flo

w

Figu

res

from

Doy

le a

nd D

urra

n (2

004)

A U

niqu

e O

bser

vatio

nA

Uni

que

Obs

erva

tion

“On

anot

her o

ccas

ion

(25

Apr

il 19

55)

a un

ique

obs

erva

tion

of th

e ro

tor

circ

ulat

ion

was

mad

e w

hen

the

Sie

rra

Wav

e P

roje

ct s

ailp

lane

bro

ke a

part

inse

vere

turb

ulen

ce n

ear t

he u

pwin

d ed

geof

the

roll

clou

d an

d th

e pi

lot,

Mr.

Larr

yE

dgar

, des

cend

ed th

roug

h th

is re

gion

by p

arac

hute

. Afte

r bei

ng c

arrie

d ra

pidl

ydo

wn

the

dire

ctio

n of

the

mai

n st

ream

east

war

d ac

ross

the

Val

ley

belo

w th

e ro

ll cl

oud,

he

enco

unte

red

a la

yer o

f cal

mai

r at a

bout

2,5

00 m

(1,3

00 m

abo

ve th

egr

ound

) bel

ow w

hich

he

drift

ed w

estw

ard

in a

win

d es

timat

ed a

t 25

knot

s. H

e fin

ally

land

ed o

n th

e w

ests

ide

of th

eO

wen

s V

alle

y be

low

the

lead

ing

edge

of

the

roll

clou

d.”

--Q

uote

from

Sco

rer a

nd K

liefo

rth (1

958)

Figu

re fr

om G

rubi

ši a

nd L

ewis

(200

4)

Rot

or o

bser

vatio

ns fr

om S

ierr

a R

otor

obs

erva

tions

from

Sie

rra

Wav

e P

roje

ctW

ave

Pro

ject

Figu

re fr

om G

rubi

ši a

nd L

ewis

(200

4)

Figu

re fr

om G

rubi

ši a

nd L

ewis

(200

4)

Figu

re fr

om

Gru

biši

and

Lew

is (2

004)

Rot

or F

orm

atio

nR

otor

For

mat

ion

•Q

uene

y (1

955)

pr

opos

ed a

sim

ple

“cat

’s e

ye”

form

atio

n m

echa

nism

for

roto

rs–

Tran

sfor

mat

ion

of a

st

atio

nary

wav

e m

otio

n in

to a

sy

stem

of v

ortic

es

in th

e vi

cini

ty o

f a

leve

l whe

re th

e ba

sic

win

d ve

loci

ty

is v

anis

hing

Figu

re fr

om Q

uene

y (1

955)

Rot

or fo

rmat

ion,

con

t’dR

otor

form

atio

n, c

ont’d

•D

oyle

and

Dur

ran

(200

2) h

ave

a re

cent

pap

er o

n ro

tor d

ynam

ics:

–K

inem

atic

con

side

ratio

ns s

ugge

st th

at b

ound

ary

laye

r sep

arat

ion

is a

pr

ereq

uisi

te fo

r rot

or fo

rmat

ion.

–N

umer

ical

sim

ulat

ions

sug

gest

that

bou

ndar

y la

yer s

epar

atio

n is

gre

atly

fa

cilit

ated

by

the

adve

rse

pres

sure

gra

dien

ts a

ssoc

iate

d w

ith tr

appe

d m

ount

ain

lee

wav

es a

nd th

at b

ound

ary

laye

r pro

cess

es a

nd le

e-w

ave

indu

ced

pres

sure

gra

dien

ts in

tera

ct s

yner

gist

ical

ly to

pro

duce

low

leve

l ro

tors

.–

Mec

hani

cal s

hear

in th

e pl

anet

ary

boun

dary

laye

r is

the

prim

ary

sour

ce

of a

she

et o

f hor

izon

tal v

ortic

ity th

at is

lifte

d ve

rtica

lly in

to th

e le

e w

ave

at th

e se

para

tion

poin

t, an

d pa

rtly

carr

ied

into

the

roto

r.–

Rea

listic

roto

rs a

ppea

r to

only

dev

elop

in th

e pr

esen

ce o

f sur

face

fri

ctio

n.–

Sur

face

hea

t flu

x ab

ove

the

lee

slop

e in

crea

ses

the

verti

cal e

xten

t of t

he

roto

r circ

ulat

ion

and

the

stre

ngth

of t

he tu

rbul

ence

but

dec

reas

es th

e m

agni

tude

of t

he re

vers

ed ro

tor f

low

.

Figu

re fr

om D

oyle

and

Dur

ran

(200

4)

Figu

re fr

om D

oyle

and

Dur

ran

(200

4)

Ref

eren

ces

Blie

r, W

., 19

98: T

he S

undo

wne

r win

ds o

f San

ta B

arba

ra, C

alifo

rnia

. Wea

. For

ecas

ting,

13

, 702

--71

6.B

rinkm

ann,

W. A

. R.,

1971

: Wha

t is

a fo

ehn?

Wea

ther

wis

e, 2

6, 2

30--

239.

Brin

kman

n, W

. A. R

., 19

74: S

trong

dow

nslo

pe w

inds

at B

ould

er, C

olor

ado.

Mon

. Wea

. R

ev.,

102,

592

--60

2.C

otto

n, W

. R.,

J. F

. Wea

ver,

and

B. A

. Bei

tler,

1995

:An

unus

ual s

umm

ertim

e do

wns

lope

w

ind

even

t in

Fort

Col

lins,

on

3 Ju

ly 1

993.

Wea

. For

ecas

ting,

10,

786

--797

.D

oyle

, J. D

., an

d D

. R. D

urra

n, 2

002:

The

dyn

amic

s of

mou

ntai

n-w

ave-

indu

ced

roto

rs. J

. A

tmos

. Sci

., 59

, 186

--20

1.D

oyle

, J. D

., an

d D

. R. D

urra

n, 2

004:

Rec

ent d

evel

opm

ents

in th

eth

eory

of a

tmos

pher

ic

roto

rs. B

ull.

Am

er. M

eteo

r. S

oc.,

85, 3

37--

342.

Dur

ran,

D. R

., 19

86a:

Ano

ther

look

at d

owns

lope

win

dsto

rms.

Par

tI: T

he d

evel

opm

ent

of a

nalo

gs to

sup

ercr

itica

l flo

w in

an

infin

itely

dee

p, c

ontin

uous

ly s

tratif

ied

fluid

. J.

Atm

os. S

ci.,

43, 2

527-

-254

3.D

urra

n, D

. R.,

1986

b: M

ount

ain

wav

es. M

esos

cale

Met

eoro

logy

and

For

ecas

ting,

Ray

, P

. S.,

Ed.

, Am

er. M

eteo

r. S

oc.,

472-

-492

.D

urra

n, D

. R.,

1990

: Mou

ntai

n w

aves

and

dow

nslo

pe w

inds

torm

s. A

tmos

pher

ic

Pro

cess

es o

ver C

ompl

ex T

erra

in, M

eteo

r. M

onog

r., N

o. 4

5, A

mer

. Met

eor.

Soc

., 59

--81

.G

afn,

D. M

., 20

02: U

nexp

ecte

d w

arm

ing

indu

ced

by fo

ehn

win

ds in

the

lee

of th

e S

mok

y M

ount

ains

. Wea

. For

ecas

ting,

111

, 907

--91

5.G

lenn

, C. L

., 19

61: T

he C

hino

ok. W

eath

erw

ise,

14,

175

--18

2.

Ref

eren

ces,

Con

t’d

Ham

ann,

R. R

., 19

43: T

he re

mar

kabl

e te

mpe

ratu

re fl

uctu

atio

ns in

the

Bla

ck H

ills

regi

on,

Janu

ary

1943

. Mon

. Wea

. Rev

., 71

, 29-

-32.

Her

tens

tein

, R. F

., an

d J.

Kue

ttner

, 200

2: S

imul

atio

ns o

f rot

ors

usin

g st

eep

lee-

side

to

pogr

aphy

. 10t

h C

onf.

on M

ount

ain

Met

eoro

logy

, Am

er. M

eteo

r. S

oc.,

Par

k C

ity, U

T,

330-

-333

.H

ugh

D. C

obb,

III,

2004

: The

Gul

f of T

ehua

ntep

ec h

urric

ane

forc

ew

ind

even

t of 3

0-31

M

arch

200

3. M

ar. W

ea. L

og, 4

8, 7

--9.

Jone

s, C

. N.,

J. D

. Col

ton,

R. L

. McA

nelly

, and

M. P

. Mey

ers,

200

2: A

n ex

amin

atio

n of

a

seve

re d

owns

lope

win

dsto

rm w

est o

f the

Col

orad

o P

ark

Ran

ge. N

atl.

Wea

ther

Dig

., 26

, 73-

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