WMOD00430

• Title: A Diagnostic Technique for Targeting During Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada
• Date: 7/7/1988
• Weather Modification - Doc Type: Report

814

JOURNAL OF APPLIED METEOROLOGY

7

elevation site (1600 m), Blue Canyon; and 3) numerical
simulations of the cross-barrier airflow using the two-
dimensional primitive equation airflow model for
Sierra Nevada terrain developed by Parish (1982) and
Waight (1984).
Surface D and the terrain below define a large chan-
nel of air flowing between Sheridan and Kingvale.
Within this channel, mass flux was assumed to be con-
served. This large channel of air was then subdivided
into seven smaller channels of equal pressure depth
(50 mb at the inflow location). The u-component in
each individual channel was then determined by as-
suming mass flux conservation within each smaller
channel, such that

Ui,j = Ui.l(LlPi,I)(Mi,jrI (3)

where Ui,l = u-component in flow channel i at inflow
boundary, Ui,j = u-component in flow channel i at
downward grid point} (10 km spacing), ti.Pi,I = depth
of flow channel i at inflow boundary (50 mb), and ti.Pi,j
= depth of flow channel i at downwind grid point}.
On occasion, a blocked surface layer (u < 2.5 m S-I)
was present within the Sacramento Valley. When such
blocked flow conditions existed in the lowest levels, the
integration (Eq. I) began at the top of the blocked layer
rather than at Psh' The profile of the terrain surface
was then adjusted to incorporate the region of blocked
flow (Fig. 3b).
Vertical motion was determined by computing

t;CC.-

4 =','.

i.:

5:~ -.~ ...

55."' ."

'DbO:: .

.5 (',(J ___
W
0::
~ 7::C.
(f)
w
~ .'5-:

... -. ..

. .
.

~~ :)..;

5~:)

9~'_~ T~_:~:!~_...-..- ....--:,...
~,~'~ -n ;~:::d~!>..../
I..;-~:.: p r -~ - I - I -. r I I
o 20 40 60 80
DISTANCE (km)

Valley
I
100 120
7
13

40\

451

501
, ....../;
551 -

~:"',....j-
~--- .,~ ~~

:.~ : \ i:
/ ~

_601
.c
.5 651

w
~ 701
(f)
(f)
~ 751
<L

801

85!

900.- ..'

950'i~ci?~~~'---
1000-l~ ,
I 3 5

II

,
13

,
9

I
7

HORIZON TAL GRI D POINT

FIG. 3. Flow channel configuration used to determine wind com-
ponents without (top) and with (bottom) the presence of a blocked
surface layer. Regions A-C and surface D are discussed in the text.
The coordinate system was aligned with the Sierra Nevada crestline
and chosen so that the Sierra Crest was located 100 km from the
origin. Sheridan was located 7 km east of the origin and Lincoln at
the origin. Both soundings were assumed to apply at the origin since
they were located well within the Sacramento Valley.

These two points, P650 and PI' were then connected
by a surface, defined in Fig. 3 as the surface D. The
profile of this surface was assumed to have a peak 40
km upwind of the crest. This profile was chosen instead
of a linear profile to characterize the upstream prop-
agation of the primary mountain wave predicted by
theories of mountain airflow (e.g., Smith 1979). The
mean position of the airflow crest during meteorolog-
ical conditions typical of fixed target experiments was
determined by examining I) aircraft measurements of
equivalent potential temperature (Be) between the base
of the Sierra Nevada and the crestline; 2) Be analyses
constructed from data collected by simultaneously re-
leased rawinsondes at Sheridan, Kingvale, and a middle

VOLUME 27

5

5

4
E
""

. 3 l-
I
. <.0
w
I

5

w
I

;~

w = u(ti.h/ D.x)

(4)

rl

where ti.h/ D.x is the slope of the flow channel between
grid points. Typical vertical velocities varied from 0.1
m S-I over the lower foothills to 0.5 m S-I near the
crestline. The barrier-parallel (v) component was de-
termined by direct interpolation along the flow chan-
nels of the v-component measured at Sheridan and
Kingvale. Derivatives of all variables in the y (barrier
parallel) direction over the research area were assumed
to be zero.
Region B on Fig. 3, which included the Lake Tahoe
valley downwind of the crest, had no measurement
support to calculate wind fields. Winds in this region
were estimated by assuming a profile of surface D which
corresponded approximately to the terrain shape. The
profile of surface D downwind of the crestline was also
adjusted to account for the upstream vertical propa-
gation of the mountain wave associated with the
mountain range east of Lake Tahoe (see Fig. 3). The
u- and w-components of the wind in region B were
then determined by the methods described above. The
v-component was conserved in each flow channel
downwind of the crest.
Winds in region C were estimated by assuming mass
conservation between surface D and the 400 mb sur-
face. Since the 400 mb surface was assumed to be hor-
izontal, this assumption artificially accelerated winds
near the top of region C in the vicinity of the mountain