The role of jet streams in the development of severe storms was analyzed. Local variations of flow resulting from rapid changes of geostrophic velocity along the air trajectory or inertial instability can produce cold advection. An analysis of meso-network data for 1968 and 1969 was performed. Although inertial instability was present in the network, local deviations in wind direction and cold advection were not detected

Goldstein, M. G.

English

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NEH YORK UNIVERSITY
School of Engineering and Science
Department of Meteorology and Oceanography
Annual Report
SEVERE STORHS
Grant E22-l0l-70(G)
National Oceanic and Atmospheriy.Administration
By
Melvin G. Goldstein
Research Assistant
Principal Investigator
James E. Hiller
December 1970
N72-18619 'l
UncIas
17336
Annual
Univ.)
CSCL 04B
(CATEGORY)
New York; N. Y.
~ (NASA CR OR TMX OR AD NUMBER)
(NASA-CR-125614) SEVERE STOR~S
Report M.G. Goldstein (New York
Dec. 1970 11 P ~.~)
J) (j Itt- \1''1 f \ ,/
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https://ntrs.nasa.gov/search.jsp?R=19720010969 2020-03-11T18:39:56+00:00Z
CONTENTS
2
Abstract • • . . . . • • • • • • • 0 • • • • e· • • .• •
Page
3
Introduction • • . . . . . . . . . . . . . . . . . . . 4
Local Deviations of Air Flow . . . . . . . . . . . •. . 4
Deviations arising from rapid changes of
geostrophic wind speed •••••• • • • • • •• 5
Deviations produced by inertial instability • •• 6
Location of Severe Storms'with Respect to Jet HClxima. 8
Observations of Storms on the NSSL Meso-net,,,ork • • • 8
Conclusions. . . . . . . . . . . . . . . . . . . . . . 10
Bibliography • • • • • • • • • • • • • • • • • • • •• 10
ABSTRACT
The role of jet streams in the development of severe
storms "las analyzed. Local variations of flou resulting from
rapid changes of geostrophic velocity along the air trajec-
tory or inertial instability can produce cold advection.
The cold advection occurs in 'regions which frequently have
severe storms. An analysis of the National Severe Storms
Laboratory meso-net''lork data for 1968 and 1969 ,..ras performed.
r
Although inertial instability was present in the network,
local deviations in wind direction and cold advection were
not detected.
3
4
INTRODUCTION
The role of jet streams in the development of severe weather is not clearly
defined. One theory on severe Heather states that strong vertical wind shear
present in the vicinity of jet streams produces high-level outflow which enhances
the development of convective clouds (Newton, 1962). Another theory (Beebe and·
Bates, 1955) suggests that vorticity advection in jet streams produces patterns
of high-level divergence '~lich increase vertical motions and lead to severe
,~eather. However, this theory requires that the jet maximum be stationary. This
requirement is not justified by observations (Reiter, 1961). Also, some recent
studies indicate that patterns of high-level divergence and outflow are not
necessary for severe storm development (Barnes, .1970).
Cold advection at high levels is another mechanism ''1hich could lead to severe
storms. Because both temperature gradients and~ind speeds are large in jet
streams, the potential for strong advection is high. Hm.,ever, in large scale
analyses of jets, the air flow and temperature gradient are apparently perpendicu-
lar. Consequently, no advection would be anticipated. Hmlever, deviations from
the main air flow may develop in local regions which escape detection in large
scale analyses. Such deviations may result in cold advection. The purpose of
the present study was first to see whether local deviations from a main air cur-
rent have a theoretical basis, and secondly to determine the patterns of advection
which could develop.
LOCAL DEVIATIONS OF AIR FLC1.oJ*
Two mechanisms have been studied that could lead to local deviations of flow
from a main current. The first results from large variations of horizontal vel-
ocity along air trajectories observed in the vicinity of jet maxima, and the
second involves deviations resulting from inertial instability. Both mechanisms
produce deviatio"ns which could cause cold advection dm-mstream and to the right of
ie! maxima.
*Supported in part by NASA Grant NGL-33-0l6-0l3.
5Deviations arisin~from rapid changes
of geostrophic wind speed
Large variations of horizontal velocity are observed in the vicinity of jet
maxima. If a parcel moves through this region without immediately adjusting its
speed to the changing field, the parcel will deviate from its original path,
The first and second equations of motion for a frictionless flow can be
written as follows:
du
- c:f (v-v)
at g
dv
- c: f (u -u)
at g
(1)
(2)
where f is the Coriolis parameter. When a pareel with velocity components u
and v moves into a field ·of geostrophic velocities
r
its original path will develop from the accelerations
u and v , deviationsg g
du dv;t and ;t.
from
In order to determine the importance of these parcel accelerations, trajec-
tories of parcels in a hypothetical jet stream were computed.
A suitable schematic description of a jet's geostrophic velocity field can
be· given as follows:
ug (3)
l-lhere x and yare orthogonal and x is along the jet axi$. .x = y = 0 at the
center of the jet maximum. The constant A is the magnitude of the maximum u •g
The constants 13 and C determine the shape of the velocity distribution.
Because the wind is geostrophic, the height, Z , of a pressure surface is given
by,
z == - (4)
where g is gravity, and f(x) is set to zero, so that z = 0 when y = o.
When"y > 0, Z will be negative, and ,·,hen y < 0, Z will be positive. The
y-component of the geostrophic velocity is given by
2 sy
v ==.& az = + 2 ABx e-13x
g f ax
o
2
-cy dy
e (5)
6The constants Band C were chosen such that the isotachs of the velocity dis-
-1tribution would be elliptical in shape. At x = y = 0, u = 100 m sec ; atg
-1 -1Y = 0, x = 500 km, u = 40 m sec , and, at x = 0, y = 300 km, u = 40 m sec •g . g
After the geostrophic fields were derived, trajectories of parcels were com-
puted from equations (1) and (2). The time step was 100 sec.
The deviations were small. A parcel starting at x' = y = 0 llloves downstr~am
and slightly to the right (looking do,~stream). Within 350 km from the origin,
the parcel obtains a transverse velocity of a magnitude slightly greater than
-11 m sec
• This small deviation may be significant • Looking downstream from a
jet maximum, one can usually observe cold air to the left of the axis and warm air
to the right. Consequently, the computed trajectory would produce some cold ad-
vection.
...Deviations produced by inert1al instability
Inertial instability is another factor which can contribute to transverse
displacements in the vicinity of jet maxima. If the wind shear in the transverse
direction on an adiabatic surface exceeds the Coriolis acceleration, the flow is
inertia11y unstable. According to Van Meighem (1951), if a parcel in this field
is given an initial transverse velocity v , the parcel will accelerate in the
o
transverse direction and will follow a path given by:
x = v If (1 - cosh Ivl t)o .
y = v Iv sinh I Y I t
o
where t is the time, and v is given by
y 2 = f (f _ au)
ay
(6)
(7)
(8)
The transverse gradient of velocity can approach -2f to the right of jet
maxima (Reiter, 1961). Figure 1 shows trajectory calculations ''lith initial trans-
-1
verse velocities of -1, -5, and -10 m sec From relatively small transverse
velocities, large displacements can result. Because colder air is located
Y
I
km
X
I
km
10
0
20
0
30
0
40
0
50
0
O
l.
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=
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8closer to the axis, cold advection is a strong possibility do~mstream and to the
right of the jet maximum.
LOCATION OF SEVERE STaID-IS WITH RESPECT TO JET NAXIMA
Both inertial instability and ageostrophic accelerations can produce cold
advection in the same region. A study was made to see whether severe weather
occurs more frequently in that region.
The study was performed on severe weather l.,hich occurred lJithin 4 hr of
1200 Q'lT during the period of January 1970 to April 1970. The distance and angu-
lar displacement of the severe ~~eather from the jet maxima were noted. The angu-
lar displacement was measured clockwis~ from the axis. Table I shows the results.
All storms occurred within 90 0 of the jet maximum. This is the same region in
which cold advection is predicted by local deviations of floH.
J.'"
OBSERVATIONS OF STORHS ON THE NSSL MESO-NETHOPJZ
The most direct test of any theo~ related to small scale storms requires
the examination of data on a similar scale. In recent years, the Nati('mal Severe
Storms Laboratory (NSSL) has provided useful data on severe storms occurring in
central Oklahoma during the months of April, Hay, and June. The data collected
by the NSSL meso-network during 1968 and 1969 was analyzed. Three storms in each
year were analyzed for the presence of inertial instability, local deviations of
Hind direction" and mid-level cooling.
The occurrence of anticyclonic shear which would be sufficiently large for
the development of inertial instability is rarely revealed in large scale analyses.
For instance, the last column in- Table I lists the shear measured on the synoptic
scale at 500 mb for cases of severe storms occurring near jets. In these analyses
the shear was always less than the Coriolis parameter. However, an examination
of the meso-network data revealed that very large anticyclonic shear may frequent-
ly occur on the small scale in the vicinity of local storms. Hithin one hour of
the storm's passage through the netl.;rork, inertial instability became a distinct
Table I
Severe l\;
. Tornado
i
Date Weather
0930~1230 i C.A1a.
I •
I
1245 IHiami,Fla. I
1310 II Lubbock, Tex. I
!
0245-0945.' Tex.panhandle :~I i1230 I W.Missouri
I
0900-1230 i C. La.
-0.:39
-0.14
-0.16
-0.12
-0.16
-0.16
-0.10
-0.25
-0.13
-0.18
-0.32
-0.15
-0.11
-0.20
Shear
(sec-Ix 10-1
-0.16
090 0
090 0
017 0
050 0
. 025 0
025 0 •
070 0
030 0
015 0
025 0
0900
023 0
031 0
0900
Direction
from Max.-
0400
600
SOO
560
240
400
SOO
110
150
450
150
: Dist. from
Location IHax. (N.M.)
Fla.Panhandle I 330
F · 1 ,'Max.overt.P~erce,F a. .
reg~on
N.E. Texas I 180
S.F1a. j 390
C.Texas 350
Fla. Panhandle I 200.
E.Texas- iMax. over
S. La. I region
I
S.w. Ill.
N. Miss.
S. Car.
H. Ala.1200
1200
1330
1100
Time(GHT) !
1505-1635 I,:
1200
1330-1530
1230-1615
1200-0215
1540-1732
0745-1430
II
"
"
II
"
"
II
Tornadoes
Severe l\"
Tornadoes
1.5" Hail
1.75" Hail
Funnel aloft
Funnel cloud
Tornadoes
1/6/70
1/15/70
2/1/70
2/3/70
3/3/70
3/5/70
3/17/70
3/19/70
3/19/70
3/20/70
3/29/70
4/1/70 I
4/5/70 I
4/16/70
4/1S/70
4/1S/70
4/19/70
-0.11
-0.14
-0.21
-0.13
. -0.21i
I
I
shear was
and on Hay 31,
ISO
145
240
320
600
All measurements were made near the 500 rob
i
I
1000-1400 i N.Kentucky-IH. Va•.
0920-1230 i Tenn.,Ky.,!Hiss.,Ala •
1100-1300 IN.E.Texas
l43~~Abilene'Tex.
0700-1015 Ok1. ,Ark.,
Kan. ,Mo.
--- ----........-------'-------'-------
22, 1968, the magnitude of the anticyclonic
-4 -11968 the shear was 4.4 x 10 sec,
-1
sec
Tornadoes
Tornadoes
3/4" Hail
4/23/70 Severe l\, &
. ; Tornadoes
I
4/24/70 I Severe ~ &
.Tornadoes
4/25/70
4/29/70
4/30/70
possibility. On April
~4 -13.4 x 10 sec • On May 13,
i96S the shear was 3.5 x 10-4
level. Jets ",ere present on the large scale during April 22 and Hay 13; hmvever,
no jet was in the vicinity on May 31. It is interesting that local inertial in-
stzbi1ity can develop not only. ,.,hen a jet is present on the large scale but also
when it is absent •.
Although inertial instability was revealed in the small scale, no local devia-
lions of llind direction from the large scale· were detected. The storms analyzed
J.U
appeared to originate outside the meso-network. Deviations from a main current
are thought to be most important in the initiation of the severe weather. It is
quite possible that deviations of flow occurred outside the meso-network at the
time of storm development. Considering the magnitude of the anticyclonic shear,
the expectation of these deviations is reasonable.
A detailed analysis of temperature and moisture was .also performed. In four
of the six cases analyzed, cooling between 450 mband 750 mb occurred in portions
of the netHorl~ 1 hr prior to the precipitation. This cooling ,1185 instrumental to
the destabilization of the air. Table II shows these results. The origin of the
cooling may have been due to evaporation rather than cold advection, because rela-
tive humidities were quite low prior to the cooling. In Table II, the stability
is expressed in terms of the average temperature difference bet,~een the environment
and a parcel lifted moist adiabatically from the Lifting Condensation Level (tCL).
f"
The LCL was determined from the mean mixing ratio and temperature of the lowest
kilometer.
CONCLUSIONS
Theoretically, local deviations of wind direction can develop from either
large accelerations of flow or inertial instability. These deviations would pro-
duce cold advection in a region which frequently contains severe storms.
Although cold advection and local variations of wind direction were not found
in the meso-scale analysis, large anticyclonic shear was measured. This shear pro-
.duces inertial-instability in the region. The results of the present study are
not necessarily inconsistent with the theory. The storms did not appear to be
gznerated within the network.' Consequently, local deviations of flow may have
~cr.urred outside the analyzeQ region, and in that region the storm development Has
:i.nitiated.•
J..1.
Table II
Average ,-
Initial /Layer of / .' Average
Date Station Period Cooling (mb) Temp.diff"/Fo 1 T Initial, ~na emp •
, ./ difference Re1.J::lum.
.\
\
5/31/68 Rush 1500- 750- -1.6 /3.3 41.9Springs 1600 CST 450
4/26'169 Minco 1400- 650- -0.8ILi.l 61. 9'1550 CST 450
Express- I 1240- 750-
-1.6/3.3 .5/6/69 \'lay ARPT. I 1410 CST 450 41.4
I
I 1903- 750- +0.8/_1.9 48.55/7/69 Chickasha I
j 2015 CST 550
I
BIBLIOGRAPHY
" Barnes, S.L.) 1979: Personal communication
\
\
Beebe, R.G.) and F~C. Bates, 1955: A mechanism for assisting in the release of
convective instability. Monthly Weather Review, vol. 83, pp. 1-10•.
Newton, C.H., 1962: Qynamics of severe convective storms. National Severe Storms
Project, report nQ. 9, 44 pp.
\
Reiter,E.R., 1961: ~t\stream meteorology. The University of Chicago Press,
Chicago and London, 515 pp.
\
U. S. Department of Commerc~, 1970: Storm data. Environmental Science Services
Administration. " I
Van Meighem, J.M., 1951: Hydrodynamic instability. Compendium of Meteorology,
American Meteorological Society, pp. 434-453.
\
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