Cirrus clouds play an important role in the climate and general circulation because they significantly modulate the radiation properties of the atmosphere. However understanding the processes that govern their presence is made difficult by their high altitude, variable thickness, complex microphysical structure, and relatively little knowledge of the vertical motion field. In the FIRE 2 (First International Satellite Cloud Climatology Regional Experiment) experiment, a 404 MHz wind profiler was set up to provide continuous measurements of clear air wind field at Parsons, Kansas. Simultaneously, the NOAA wind profiler network supplied a wider spacial scale observation. On 26 Nov. 1991, the most significant cirrus cloud phenomena during the experiment with a jet streak at 250 Mb occurred. Analyses of the vertical wind velocity are made by utilizing different methods based on wind profiler data, among them the direct measurements from CSU wind profiler and NOAA network wind profilers, VAD (Velocity Azimuth Display) technique and the kinematic method

Cox, Stephen K.

Song, Ran

English

NASA Technical Reports Server

VERTICAL VELOCITY IN CIRRUS CASE OBTAINED FROM WIND PROFILER
Ran Song and Stephen K. Cox
Department of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
1. INTRODUCTION
Cirrus clouds play an important role in the climate :
and general circulation because they significantly
modulate the radiation properties of the atmosphere.
However understanding the processes that govern
their presence is made difficult by their high altitude,
variable thickness, complex microphysicai structure, t
and relatively little knowledge of the vertical motion : : ..................................................... i
field. _ _-_" .................: .................................'t
=-_:_. _ 12Z ,26 NOV,91
•":'_ ._B JET STREA_ I" _"
=:-   ,LIIIIII.................: -,
:i:.-.::._.......................................'
HBR " k.
HVL • NDS "..
In the FIRE II experiment, a 404MHz wind profiler
was set up to provide continuous measurements of
clear air wind field at Parsons, Kansas. Simulta-
neously, the NOAA wind profiler network supplied
a wider spaciai scale observation. On Nov. 26,
1991, we had the most significant cirrus cloud
phenomena during the experiment with a jet streak at
250 Mb. Analyses of the vertical wind velocity are
made by utilizing different methods based on wind
profiler data, among them the direct measurements
from CSU wind profiler and NOAA network wind
profilers, VAD(Velocity Azimuth Display) technique
and the kinematic method.
On Nov. 26, the Parsons' site was just at the cold
exit region of the jet streak which is shown in Fig. 1.
According to the conceptual model of a jet streak by
Mattocks and Bleck (1986) (MB) shown in Fig.2, the
vertical velocity should have from ascent below to
descent above the height of the maximum horizontal
wind. An agreement between the observations in the
cold exit area and the conceptual model is shown first
time using high resolution wind profiler data.
.................. ' _ _a_sons
I"-- ................-t Tcl . HKt., /._-
....... ! I PRC i // I /
Figure l.Location of CSU wind profiler(square) and
some of the network stations(circle). The 1200Z jet
streak is denoted by shaded area.
! #
•...._,.<._/// -.-,,,__\ ._.,
r,.,'.....
2. METHODS
2.1 Direct Doppler Method:
The CSU wind profiler was operated in a 10-minute
cycle which consists of measurements along five
directions: one pointing vertically with the other four
beams tilted 15 ° from zenith toward east, north,west
Figure 2.Three-dimensional schematic of the
circulation induced by a jet streak. (from Mattocks
and Bleck, 1986)
153
and south. The NOAA network radars had three
directions of operation (one vertical, two tilted away
from zenith toward east and north respectively) with
a 6-minute cycle. The direct Doppler method deduces
vertical velocities from the radial velocities of the
https://ntrs.nasa.gov/search.jsp?R=19940017857 2020-06-16T18:24:54+00:00Z
verticalbeam.In thepast,severalworks(Larsen et
al.,1991a,b; Yoe et a1.,1992) have shown the
uncertainties in direct measurements of vertical
velocity and have also given some comparisons
between these radar direct measurements and other
data sources or other techniques based on radar data.
2.2 VAD Method:
The VAD method was originally derived to retrieve
horizontal wind components from the radial velocities
of a radar azimuth scan (Browning and Wexler 1972;
Wilson and Miller 1972). By applying the mass
continuity equation under assumption that air density
is locally stationary and horizontally strtified, one can
derive the vertical components of wind fields (Larsen
et al. 1991; Doviak and Zrnic 1984). In our
experiments, the CSU wind profiler's five-beam data
was employed in a quasi-VAD technique.
The VAD sampling scheme is shown in Fig.3, U is
the horizontal wind vector, and w is the vertical
velocity. In ideal cases, which have a nearly linear
horizontal wind field within the measurement circle
so that the components u, v and w are well
approximated by the zeroth- and first-order terms of
a Taylor series, the measured radial velocity at
sampling points will fit a sine curve as shown in the
lower panel. Here, the offset C0 at each height is
equal to the average of radial velocities at all
sampling points in one circle at that height and can
also be expressed as equation(l) through a Taylor
expansion (Larsen - et ai., 1991).
co = Ew + z tan 0(%.u)I (z)
2
By assuming the density is stationary in time locally
and that the medium is horizontally stratified, we
write the mass continuity equation:
Vh*p U=pVheU=--_z(p w) (2)
Combining the above equations and integrating the
differential equation from an upper height boundary
z,r by assuming zero w at z,r, we have equation (3):
Z,
. . _
-°z ;
(3)
154
Eq.(3) can be solved numerically for w, the vertical
velocity.
Z
t u
/S/: "
/
0/
\
"\\7
u
rq7/ °
Figure 3.Schematic of the VAD sampling and sine
curve fitted by the sample data.
2.3 Kinematic Method:
The kinematic method was carried out by
Bellamy(1949) as an objective calculation of
divergence and vertical velocity. In this method, we
have an assumption that the wind field is a linear
function of space between the observation points
which are non-collinear located. Then the horizontal
divergence can be caicuiated from the rate of change
of the outflow through the unit vertical sides of the
volume. The vertical velocity in height coordinates
can be computed based on the known horizontal
divergence and mass continuity equation by assuming
incompressible :air m_[um (cf. Beilamy, 1949). In
this study, we use NOAA network stationd NDS,
HBR and LMN to form a triangular area(Fig. 1).
3. RESULTS AND DISCUSSIONS
As described in the above section, three methods are
used to derive vertical velocity. They are the direct
doppler measurement, VAD and kinematic method.
Figs.4, 5 and 6 are the 3-period averaged vertical
velocities from the direct doppler measurements of
the CSU and network wind profiler at NDS, and also
the VAD calculation. The appearance of thin cirrus
clouds was confirmed by lidar from about 1600Z,
and after 1820Z, clouds became much thicker with
[l 26 1991 _'5'L' ',_'P kvera_ea Dir ..v
2
...... ,)0- L2Z
.... i2-24Z
--,)0-24Z
.•• "
i! .
.._."
J
W qlTl _}
Figure 4. Profiles of averaged vertical velocity
measured in the vertical beam of CSU wind profiler
at Parsons on Nov. 26, 1991
17
tL/26/t991NDS (Kansas) Averaged w
v
15
"4
L_
•c -
=
2. -
7
!l
...... O0-12Z
.... L2-2_Z
5
--00-24Z
i
I
4
2
,:i_i '_ 7
w tln/Sl
Figure 5. As same as Fig.4, but from network
profiler at NDS.
cloud tops consistently at about lOkm and bases down
to 8km and even lower• All plots in Figs.4, 5 and 6
present the evidence of a continuously upward motion
in this layer. Since both CSU and network wind
profilers are operating at 404MHz, which
corresponds to a wavelength of 75cm, ice particles
are expected to be relatively transparent to the
155
profiler radars.
Above the cloud, the 12-24Z averaged plots exhibit
a change from upward motion to downward motion
at slightly different locations: 10.7km for CSU direct
measurement, and about 9.95km for both NDS direct
measurement and CSU VAD calculation• In MB
model, this reversal is expected to be close to the
maximum horizontal wind level(Fig.2). A vertical
cross section(from (30°N,II0°W) to (45°N,90°W))
perpendicular to the jet streak indicates the maximum
wind layer is located at 10.5km (Fig.7). The
observations agree extremely well with the model
study.
1[,'26/t991 C._U WP Averaged VAD w
v
1
i
.... [2-24Z
- :_ --O0-'4Z ..._."
I.._'" •.°
"7
:o "'_. P
I
"3
- ...2 "_, |
-.; -,:2 -.i ?0 :3. ,).2
','," ( I11, S )
Figure 6. Vertical velocity profile derived from
VAD. Parsons, Nov. 26, 1991.
Fig.g presents the time-height contouring of the
vertical velocity derived from the kinematic method.
A very thick line which clearly separates the
downward motion and upward motion is evident. The
figure shows this change at cloud level starting from
about 1400Z which was shortly before clouds were
detected by the lidar. This reversal level settled at
ablaut 10kin from 1700Z to 200OZ, which is what is
expected according to the MB model. From 2000Z,
this level dramatically dropped down to lower levels
after the jet streak already passed over the area.
ORIGINAL PAGE IS
OF POOR QUALIT_
\. J 'q
Figure 7. vertical velocity w(mls) derive_l from the
l_nematic method on Nov.26,1991 .Dash line is negative w,
thin solid line is positive w; thick solid line is zero w
HOURLY VERTICAL VELOCITY (M/S) NOV, 26. 199!
t" ") p'.l;[[_}::[]_['t{ '''q [" ['l _ ::_ :'_:
: :i _i! ::.. ....., ! . ": "., ! ," .... " . _.."", -
_,....::: !_ _ i .../lg-,-J / /I_,,%\X'.....J
I
0 I 2 3 4 _ 6 ? 8 9 I0 || |2 13 II IS IO I? 10 19 20 ;_l ,7,2 23
Figure 8.Hodzontal wind speed (rids)contours in a
vertical cross section perpendicular to the jet streak
at |800Z, Nov. 26, 1991.
4.CONCLUSIONS:
The study in this paper shows good consistencies
among vertical velocity measurements-from the direct
doppler method. VAD method and kinematic method.
And the observation results agree very well with the
theoretical model, Upward air m_exist at and
below the cirrus cloud layer with weaker downward
motions above. More confidence is gain for utilizing
wind profiler data m our future researches.
5. ACKNOWLEDGEMENTS
This research has been supported by National
Aeronautics and Space Administration under Grant
NAG 1-1146, The Department of Energy under
Contract No.DE-FG02-90ER60970.
6. REFERENCES
Bellamy, J.C., 1949: Objective calculations of
divergence, vertical velocity and vorticity.
BuL Am. Mrt. Soc., 30, 45-49
Browning, K.A., and R.Wexler, 1972: A
determination of kinematic properties of a
wind field using Doppler radar. J.
Appl. Meteor., 7, 105-113.
Doviak, R.J., and D.S. Zrmc, 1984: Doppler Radar
and Weather Observations. Academic Press,
458 pp.
L.amen, M.F., S. Fukao, O. Aruga, M.D.
Yamanaka, T. Tsuda and S. Kato, 1991a:
A comparison of VHF radar vertical-velocity
measurements by a direct vertical-beam
method and by a VAD technique. J. Atmos.
Oceanic. TechnoL, 8, 766-776
Larsen,M.F.,and J.Rottger, 199 Ib:VHF radar
measurements of m-beam incidence angles
and associated vertical-beam radial velocity
corrections. J. Atmos. Oceanic TechnoL 4,
477-490
Mattocks, C. and R. Bleck, 1986: Jet streak
dynamics and geostrophic adjustment
processes during the initial stages of lee
cyclogenesis. Mon. Wea. Rev., 114, 2033-
2056
Wilson, D.A., and L.J. Miller, 1972: Atmospheric
motion by Doppler radar. Chapter 13,
Remote Sensing of the Troposphere, V. E.
Den', Ed., U.S. Department of Commerce,
NOAA (13)1-(13)34.
Yoe,J.G., M.F.Larsen and E.J. Zipser,1992: VHF
wind-profiler data quality and comparison of
methods for deducing horizontal and vertical
air motions in a _scale convective storm.
J. Atmos. Oceanic TechnoL. 9, 713-727
= 156


Vertical velocity in cirrus case obtained from wind profiler

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