A large and comprehensive data set taken by the NOAA CO2 Doppler lidar, the NCAR King Air, and rawinsondes on 31 October 1986 during the FIRE (First ISCCP Regional Experiment) field program which took place in Wisconsin are presented. Vertical velocities are determined from the Doppler lidar data, and are compared with velocities derived from the aircraft microphysical data. The data are used for discussion of particle growth and dynamical processes operative within the cloud

Gultepe, Ismail

Heymsfield, Andrew J.

English

NASA Technical Reports Server

VERTICAL VELOCITIES WITHIN A CIRRUS CLOUD FROM 
DOPPLER LIDAR AND AIRCRAFT MEASUREMENTS DURING FIRE, 
IMPLICATIONS FOR PARTICLE GROWTH 
Ismail GULTEPE and Andrew HEYMSFIELD 
National Center for Atmospheric Research' 
P.O. Box 3000 
Boulder, Colorado 80307 
1. INTRODUCTION 
This paper uses a large and comprehensive data set taken by the NOAA C02 Doppler 
lidar, the NCAR King Air, and rawinsondes on 31 October 1986 during the FIRE (First 
International Regional Experiment) field program which took place in Wisconsin. Vertical 
velocities are determined from the Doppler lidar data, and are compared with velocities 
derived from the aircraft microphysical data. The data will be used for discussion of 
particle growth and dynamics1 processes operative within the cloud. 
2. SYNOPTIC DISCUSSIONS 
There WM no large scale disturbances over the experimental area on 31 October 1986. 
The 1800 GMT surface map showed that there WM a cold front 600 km to the west of 
the experimental area. At 1200 GMT, there WM a weak upper level trough at the 200 mb 
level. The jet stream, with maximum wind epeeda of about 45 m s-l, WM situated north 
of Wisconsin. Cirrus cloud formation occurred to the south of the jet stream in a warm 
air zone. There WM not a direct relationship between the cirrus cloud location and the 
jet etream. A sounding from Green Bay at 11 CST (Figure 1) shows a moisture increase 
at  upper levels where later the cirrus cloud formed. The base of the moisture zone is at 
about 450 mb with an isothermal layer just under the base. The cloud studied extended 
over an area of 45x20 km. 
3. DISCUSSION OF AIRCRAFT AND LIDAR MEASUREMENTS 
The NCAR King Air collected data within the cirrus cloud using six horizontal 
penetrations from 7.6 km (-29.a0C), cloud bane, to 9.2 km (-42.6'C), cloud top. The 
King Air sampling period for each penetration WM about 5 minutes, corresponding to 30 
km horizontal legs. The time for total sampling through cloud layer waa approximately 30 
minu tea. 
The principal equipment used for cloud particle spectra measurements were Particle 
Measuring System (PMS) 2D-C and 2D-P probes, although only the size spectra from the 
2D-C probe were used. The 2D-C probe sized in the range 25 pm to 1400 pm. 
The velocity azimuth display (VAD) technique is used to calculate vertical velocities 
from the Doppler lidar measurements. Thia technique was first proposed by LHERMITTE 
The National Center for Atmospheric Research is sponsored by the National Science 
Foundation 
67 
https://ntrs.nasa.gov/search.jsp?R=19910001148 2020-03-19T20:15:08+00:00Z
and ATLAS (lSel), and later developed by BROWNING and WEXLER (1968). In the 
VAD scanning mode, the beam is scanned continously in azimuth angles while the zenith 
angle is held constant. Backscatter power and azimuth angle are digitized in real time and 
stored on magnetic tape. The radial velocities are measured at intervds of 300 m along 
the beam. 
c8AY.645 86/10/31 I100 20.0 n/s ---* 
Figure 1: A skew T-log P diagram from Green Bay at lfOO CST on 31 October 1986. 
The moisture increase is seen approzimately between 400 and 300 mb. 
4. VERTICAL VELOCITY CALCULATIONS FROM AIRCRAFT MEASUREMENTS 
In order to calculate vertical velocities from aircraft measurements, the precipitation 
rate, ice water content and terminal velocity were calculated from 2D-C probe size 
spectra measurements. HEYMSFIELD (1977) presented equations for calculations the 
above parameters. A mean size spectrum was obtained for each sampling pass. Particle 
habits observed in the 2D-C probe and from direct collections on oil coated slides were 
predominantly columns and bullet rosettes although some plates were detected (N. Knight, 
private communication). We then calculated the vertical velocity distribution with 
altitude. 
4.1 STEADY-STATE TECHNIQUE 
The basis of this technique is the conservation equation for total vapor, liquid and 
solid substances in a rising parcel of air (HEYMSFIELD, 1975). The assumption is made 
that ice supersaturation (Si) remains constant with time ( t )  at any given level. Thus, 
68 
the crystal growth and resulting depletion of vapor are balanced due to the updraft. The 
vertical air velocity ia then derived from 
where UA ia the air velocity, and 41, 41 are coefficients (HEYMSFIELD, 1977). %, the 
cumulative growth rate of the particle size spectrum, is not known until the aupersaturation 
with respect to ice (Si) ia obtained (see discussion below). 
4.2 FLUX TECHNIQUE 
The second technique employed in calculating the vertical air velocity from the size 
spectra measurements is the flux method (HEYMSFIELD, 1977). In this technique, the 
vertical velocity was calculated by equating the decrease in moisture between a lower and 
upper sampling levels to the increaae in the precipitation rate between the name levels. 
The velocity is calculated from 
A R  
AIWC + E A p ,  U A  = 
where AR is precipitation rate difference between level 1 and 2, AIWC is ice water content 
difference between the two levels, RH is the mean relative humidity, and Aps is the vapor 
difference between level 2 and 1. Equation8 (1) and (2) can be aolved aimultaneoualy to 
yield UA, RH and S; (HEYMSFIELD, 1977). 
5. VERTICAL VELOCITY CALCULATIONS FROM DOPPLER LIDAR MEASURE- 
MENTS 
Conically scanning Doppler lidar measurements were used to compute the mean 
divergence field, the horizontal wind speed, and direction. The VAD technique is based on 
a least squares technique for obtaining Fourier coefficients. Using the anelastic continuity 
equation, the vertical velocities are calculated at different altitude8 through the cloud 
layer. Assuming that the particle velocity is equal to the terminal velocity, the zeroth 
order Fourier coefficient is given as (SRIVASTAVA et al., 1986) 
- t c o m  - 
a, = DIV-  - Vtsina , 
2 (3) 
- 
DIV is mean divergence, r is the horizontal range, is mean backscatter-weighted 
terminal velocity, and a is the elevation angle. The mean horizontal divergence can 
be calculated from a,, Vt, r and a. According to BROWNING and WEXLER (1968), 
inhomegeneities in the particle fall speed is a primary source of error for the divergence 
calculation. From concurrent DMSP satellite infrared images, it is reasonable to assume 
that the cirrus cloud had formed a homogenous structure. 
Mean backscatter-weighted terminal velocities (6) are estimated from the particle 
size spectrum. is found from: 
- 
where 
which 
1972; 
Ni,j is 
is used 
1975). 
number concentration in size class i with habit j .  The physical diameter (D), 
to calculate Vt, is converted to an equivalent diameter (Deq)  (HEYMSFIELD, 
Then, using the anelastic continuity equation and assuming vertical air 
velocities are zero at cloud top and base, the vertical air velocities are calculated. A 
variational adjustment technique is used to correct the divergence field. Vertical air velocity 
corrections are made following LIN et al. (1986). 
6. RESULTS AND CONCLUSIONS 
The aircraft vertical velocity and temperature measurements showed that the cirrus 
cloud formed in considerably stable atmospheric conditions on 31 October 1986. Maximum 
vertical shear of the horizontal wind was only 1.5 x 8- l  (at about 8.8 km). The cirrus 
cloud may be formed because of a weak wave pattern or large scale lifting. 
m. 
6(0. 
7mo. 
mo. 
66m. 
Moo. 
-8.00 -10.00 0.m IO.00 8.00 
VERTICRL VELOCITY b l / S l  
Figure 2: The calculated uerticd air velocities from aircraft (o), rawinsonde (+) and 
lidar (*) measurements on 31 October 1986. 
The ice crystal habits were predominantly columns and bullet rosettes at altitudes 
between 7.6 (-39.6”C) and 9.2 (-42.6OC) km. The total ice crystal concentration changed 
from 5.5 I - ’  (7.6 km) to a maximum of 22.5 I - ’  (8.8 km) where peak vertical velocities 
70 
are found from the calculations based on aircraft and rawinsonde meaaurementa. The ice 
water content values of to loe2 g m-a through the cloud layer are significantly lower 
than found in convective cirrus, obviously aa a result of weaker vertical motions. 
The peak velocity from the VAD technique with 60 degree elevation angle waa about 
34 cm 8' at about 7.4 km but with 30 and 40 degree elevation anglea waa only about 14 
cm 8-l at 7.3 km ASL (see Figure 2) while vertical velocitiea changed from 9 cm 8"' a t  the 
cloud base to 0 cm s-l at the cloud top. The calculated air velocities from the aircraft in- 
situ measurements showed a peak (10 cm 8 - l )  at about 8.8 km where the highest calculated 
ice crystal growth rates (about 0.2 x lo-' g m-3 s-l), ice supersaturation 42% and relative 
humidity with respect to water of 94% were found. The calculated vertical velocities are 
similar to that are found by HEYMSFIELD (1975), in thin ice clouda, ranging from 2- 
10 cm s-' in frontal overrunning system to 25-50 cm s" in clouds associated with closed 
lows aloft, longitudinal rolls and isolated convective cells. Maximum vertical air velocity 
derived from the triangle technique (BELLANY, 1949) using three rawinsondea along 
baselines of approximately 200 km was found of about 15 cm E-' (at 8.8 km) in the cirrus 
cloud layer (see Figure 2). Differences in derived vertical velocities between the triangle, 
Doppler lidar, and aircraft techniques can be attributed to scale effects. 
REFERENCES 
BELLAMY, J.C., : Objective calculations of divergence, vertical velocity and vorticity. 
BROWNIPJf :. K.A., and WEXLER, R.: The determination of kinematic properties of a 
HEYMSFiELD, A.J.: Ice crystal terminal velocities. J. Atmoa. Sci. 29 (1972) 1348- 
HEYMSFIELD, A.J.: Cirrus uncinus generating cells and the evolution of cirriform 
clouds. Part I: Aircraft meaeurements of the growth of the ice phase. J. Atmos. 
Sci. 32 (1975) 789-808. 
HEYMSFIELD, A.J.: Precipitation development in stratiform ice clouds: A 
microphysical and dynamical study. J. Atmoa. Sei. 34 (1977) 367-381. 
LHERMITTE, R.M., and D. ATLAS: Precipitation motion by pulse Doppler. Prm. 
Ninth Weather Radar Conf., Boston, Amer. Meteor. SOC. (1961) 43, 2302-2327. 
LIN, et al.: Pressure and temperature perturbations within a squall line thunderstorm 
derived from SESAME Dual-Doppler radar data., J. dmos. Sci., 43, 21 (1986) 
Bull. Amcr. Meteor. Soc., 30 (1949) 45-49. 
wind field using Doppler radar. J. Atmoa. Sei., 7 (1968) 105-113. 
1357. 
2302-2327. 
SRIVASTAVA, R.C., T.J. MATEJKA, and T.J. LORELLO: Doppler radar study of the 
trailing anvil region associated with a squall line. J. Atmos. Sei., 43 (1986) 356- 
377. 


Vertical velocities within a Cirrus cloud from Doppler lidar and aircraft measurements during FIRE: Implications for particle growth

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