WMOD00466

• Title: Dynamical and Microphysical Characteristics of Winter Storms over the Sierra Nevada
• Weather Modification - Doc Type: Report

.~ ...

field for an extended period of time a ground
station was developed for rapid reduction of
flight data upon landing. The in-depth analysis
began upon landing with an Hewlett-Packard RTE-
III computer system. With the RTE-III system the
aircraft tapes were stripped and the reduced data
placed in files on a storage disk. From the disk
a variety of subroutines were available to pro-
duce digital listings of data, flight tracks,
soundings, analog traces of derived parameters
and 2-D images. The typical procedure was to
have a quick first-look at all the data on the
same day as the flight. Non-flying days were
spent conducting more in-depth data analysis.

"

To compliment the airborne cloud
physics data, a mobile ground cloud physics ob-
server collected ice crystal and other data along
1-80. The ground cloud physics data were espec-
iallyimportant because even when flying at MOCA
the aircraft is still approximately 1 km AGL.

Even though there was time to conduct
a significant amount of in-depth analysis for
each storm, there llimply was not time to corre-
late all the data in the form of graphs and fig-
ures for presentation in this manuscript. We
have reached a number of significant preliminary
conclusions which we will attempt to justify with
data at the conference.

3.

STORM DYNAMICS

California is unique in that it is
completely surrounded by mountains except at San
Francisco where the. water from the whole of the
California Valley converges and drains into the
Pacific Ocean. In the winter season the net
radiation over the valley is negative. Being
surrounded by mountains a strong, persistent pool
of quiesent stably stratified air forms in the
California Valley. It is not at all uncommon
for this pool of air to cool down to the dewpoint
and for the resultant fog and low stratus to
persist for days.

With the approach of a cyclonic storm,
a significant amount of upper level divergence is
associated with the central and left exit zone of
the approaching jet stream. The upper level di-
vergence induces low level convergence in the
stably stratified pool of air in the California
Valley. This pool of air responds by surging
northward and is channeled by the Sierra Nevadas
on the east and by the Coastal Range on the west.
The net result is a low level jet at 1 to 2 km
and upslope and convergence in the northern end
of the valley.

An approaching storm is first noted
with the approach of high level cirrus. The first
moisture to advect over the Sierras arrives at
2 to 3 km altitude because that air has the
shortest travel time of moist air from off the
Pacific Ocean. Consequently the cloud develop-
ment sequence is advection of cirrus followed by
mid-level stratiform clouds which either advect
into or are formed over the Sierras. These
clouds are nearly always stable.

Prior to the arrival of the cold
front, it is typical for a number of pre-frontal
convective cloud bands to move across the valley

from off the Pacific Ocean. The origin of these
convective cloud bands is not known. . When the
low level stable layer is present the "roots" of
the convection are well above the surface. The
convective cloud bands dissipate over the foot-
hills of the Sierras after they have overturned
the convectively unstable air above the stably
stratified air mass trapped in the valley. The
convective instabilities develop due to differen-
tial advection. The flight during the morning of
l4 December was through the southern tip of a
weak convective cloud band. Ahead of convective
cloud bands the low level jet is typically paral-
lel to the Sierras. Behind convective cloud bands
the wind speeds decrease and the directions veer
to a more westerly direction.

With the approach of the main jet
core at 300 mb before dawn on 15 December a low
level jet with speeds of approximately 24 mps de-
veloped (Moore, 78). On a number of cases we
were able to takeoff ahead of an approaching con-
vective cloud band. On these occasions the sur-
face winds were approximately lO mps with gusts
to approximately IS mps and the direction was
from the SSE, i.e. parallel to the Sierras. Mod-
erate to severe turbulence was experienced below
the low level jet regardless of the stabi.lity.
This implies that the turbulence was priDmrily
mechanically induced from vertical wind shear.
MUch more dramatic decreases in wind speed and
veering of direction were documented on other con-
vective cloud bands than were observed on 14 and
15 December. On the few occasions when we were
able to transverse most of the valley and inter-
cept either a convective cloud band or cold front
near the Coastal Range, the preliminary indica-
tion is that the low level jet was uniform across
the valley rather than confined to the Sacramento
or foothills area.

,i,
~ {-

H
t
l.
i'::
r
i'
,
I

The passage of a cold front in the
California Valley is typically denoted by a well
defined convective cloud band, rising barometer
and more often than not a slight increase in the
surface temperature. Rarely does the surface
winds turn to the west or northwest. The explana-
tion for these phenomena seems to be that the
front rides above the low level stable air, the
convection occurs just above the low level stable
air and the warming occurs because of downward
flux of sensible heat due to turbulent mixing.

If there is still some convective
instability remaining following the passage of a
cold front and if there are mechanisms to initiate
this convection over the Coastal Range, then a
series of post-frontal convective cloud bands will
develop and move across the California Valley.

As the convective cloud bands and
frontal convective bands move over the foothills
of the Sierras, they first seem to invigorate
from the additional mechanical lifting then ra-
pidly decrease in intensity having overturned and
processed the available high energy air ahead of
each band. It is not clear at this ~ime whether
or not low level air is injested from the back-
side of moving convective cloud bands.

It is quite typical for a post-frontal
convective cloud wall to form over the foothills
following the passage of a cold front. This was

150