Downslope Windstorms and Rotors - RAL...Downslope Windstorms and Rotors. Jonathan Vigh. AT707....
47
Downslope Windstorms Downslope Windstorms and Rotors and Rotors Jonathan Vigh Jonathan Vigh AT707 AT707 21,23,25 February 2005 21,23,25 February 2005
Transcript of Downslope Windstorms and Rotors - RAL...Downslope Windstorms and Rotors. Jonathan Vigh. AT707....
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
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