In explaining the scattering of VHF radar signals from the mesosphere there are two observational facts that must be accounted for. These are; (1) the aspect sensitivity of the scattered signal and that this aspect sensitivity is largest in the lower part of the mesosphere, and (2) the correlating between the scattered power and the signal correlation time. This behavior is similar to that of the scattering from the troposphere/stratosphere region, and it is suggested that the scattering mechanisms are similar in these three regions. Several different experiments are performed. They all show strong indications of aspect sensitivity and changing correlation between scattered power and correlation time. There is no indication of stratified reflecting layers unless these layers are modulated in space and time to a degree that they cannot be distinguished from turbulence in any other way than that they cause somewhat aspect sensitive scattering

Royrvik, O.

English

NASA Technical Reports Server

136 
2.6h JICAMARCA MESOSPHERIC OBSERVATIONS 
0. Boyrvik 
Department of E lec t r i ca l  Engineering 
University of I l l i n o i s  
Urbana, IL 61801 
In explaining the sca t t e r ing  of VHF radar signals from the mesosphere 
there  are two observational f a c t s  t h a t  must be accounted for.  These are: 
1) the aspect s e n s i t i v i t y  of the scat tered s ignal  and t h a t  t h i s  aspect sensi- 
t i v i t y  i s  l a rges t  i n  the lower part  of the mesosphere; and 2) the  correlat ion 
between the  scat tered power and the s ignal  correlat ion t i m e .  This correlat ion 
tends t o  be pos i t i ve  i n  the lower part of the mesosphere, but changes t o  nega- 
t i v e  i n  the upper par t  of t he  mesosphere. 
This behavior i s  similar t o  t h a t  of the sca t t e r ing  from the troposphere/ 
s t ra tosphere region, and i t  has been suggested t h a t  the sca t t e r ing  mechanisms 
are similar i n  these three regions. I n  pa r t i cu la r ,  it has been suggested t h a t  
a mixture of s ca t t e r ing  from i so t rop ic  i r r e g u l a r i t i e s  and p a r t i a l  r e f l ec t ion  
from s t r a t i f i e d  l aye r s  could explain the observed cha rac t e r i s t i c s  of radar s ig-  
na l  r e tu rns  from the mesosphere. 
Several d i f f e ren t  experiments have been performed a t  the Jicamarca radar 
They a l l  show strong indicat ions of aspect s e n s i t i v i t y  and changing i n  Peru. 
co r re l a t ion  between scat tered power and correlat ion time, The r e s u l t s  from two 
of these experiments w i l l  be considered here. 
I f  as  suggested, the aspect dependence of scat tered power and s ignal  cor- 
r e l a t i o n  time i s  due to  a mixture of s ca t t e r ing  from i so t rop ic  i r r e g u l a r i t i e s  
and r e f l ec t ions  from s t r a t i f i e d  layers ,  one would expect t o  see a c l e a r  differ-  
ence i n  the s ignal  spectra i n  the v e r t i c a l  and off-ver t ical  antennas. 
a near Gaussian d i s t r i b u t i o n  of scat tered power i n  the off-ver t ical  antenna 
representing the turbulent s ca t t e r ,  and a superposit ion of the same Gaussian 
and one or more d i sc re t e  peaks i n  the v e r t i c a l  antenna, representing both the 
scat tered and the r e f l ec t ed  component of the received s ignal .  As can be seen 
from the two examples i n  Figure 2, the  spectra i n  the two antennas are very 
s imilar  and the re  i s  no h i n t  of a r e f l ec t ing  layer i n  the v e r t i c a l  antenna. 
Nevertheless, there  i s  a subs t an t i a l  aspect s ens i t i v i ty  where the scat tered 
power i n  the v e r t i c a l  antenna i s  about 50% larger than t h a t  of the off-ver t ical  
antenna. There is a l so  a s l i g h t  tendency fo r  the co r re l a t ion  t i m e  i n  the 
v e r t i c a l  antenna to be l a rge r  than t h a t  i n  the off-ver t ical  antenna, as one 
would expect from the correlat ion times i n  Figure 1. 
from these data  t h a t  there i s  no indicat ion of s t r a t i f i e d  r e f l e c t i n g  layers  
unless these l aye r s  are modulated i n  space and t i m e  t o  a degree tha t  they 
cannot be distinguished from turbulence i n  any other way than t h a t  they cause 
somewhat aspect s ens i t i ve  scat ter ing.  
Perhaps 
We tentat ively conclude 
An interferometer technique can be used t o  study small-scale horizontal  
va r i a t ion  i n  the amount and Doppler s h i f t  of the scat tered signal.  Data were 
obtained during the spaced antenna d r i f t s  experiment run on October 17, 1981 
(ROYRVIK, 1983). By forming the cross  spectra  between s e t s  of two antenna sec- 
t i ons  w e  can determine the pr incipal  d i r ec t ion  t o  the sca t t e r ing  volume with a 
c e r t a i n  Doppler sh i f t .  
tances a t  a c e r t a i n  a l t i t u d e  range. 
Direction differences transform i n t o  horizontal  dis- 
I n  Figure 3 frequency spectra  a re  presented f o r  f i v e  a l t i t u d e s  having sub- 
s t a n t i a l  scattered. power. The phase of the frequency component has been plot- 
ted only for those frequencies with substant ia l  power i n  order t o  reduce clut-  
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JICAMARCA JUNE 12, 1980 
Figure 1. Signal correlat ion time f o r  June 1 2 ,  1980 fo r  v e r t i c a l  and 
w e s t  antenna sect ions.  
ter i n  the f igures .  It can be seen from the f igu re  t h a t  there  i s  a tendency 
f o r  the d i r ec t ion  of the scat tered s ignal  t o  change systematically with 
changing frequency for  a l l  a l t i t u d e s  except 70 km. From closer  study of the 
data it appears t h a t  the tendency a t  the higher a l t i t u d e s  i s  q u i t e  clear i n  
a l igning these va r i a t ions  i n  the east-west direct ion,  whereas a t  67 km the 
d i r e c t i o n  of alignment appears more random. Also the occurrence of these phase 
changes i s  l imited t o  short  (1-3 min) intermit tent  t i m e  periods. 
suspect these observations t o  be the r e s u l t  of beam width broadening due t o  the 
d i f f e r e n t i a l  line-of-sight ve loc i ty  component of the horizontal  veloci ty .  
However, t h i s  d i f f e r e n t i a l  ve loc i ty  i s  negl igible  i n  an antenna beam of only 1"  
beam width. I n  the present case where the horizontal  ve loc i ty  i s  shown t o  be 
about 10 m / s  (ROYRVIK, 1983) the broadening of the spectra w i l l  be about 0.03 
Ez, o r  only a couple of percent Doppler broadening. 
would r e s u l t  from the observed wind f i e l d  (ROYRVIK, 1983) i s  opposite t o  the 
phase s h i f t  actual ly  observed. 
where individual peaks i n  the frequency spectra represent individual 
s ca t t e r ing  c e l l s  with large-scale organized motions. For example, a turbulent 
c e l l  caused by wind shear would have a large-scale ro t a t ing  motion 
corresponding t o  the outer s ca l e  of turbulence. I n  the simple case where only 
one cel l  i s  present within the antenna beam, one would expect t o  observe 
d i f f e ren t  Doppler s h i f t s  i n  d i f f e ren t  d i r ec t ions  as the one seen a t  79 km i n  
Figure 3. From the examples i n  Figure 3, and other  examples, it i s  estimated 
t h a t  t yp ica l  horizontal  dimensions of these sca t t e r ing  c e l l s  are from 200 t o  
600 meters, with a few occasions where there  are indicat ions of even l a rge r  
horizontal  dimensions. A t  about 67 km the horizontal  dimension seems t o  be 
somewhat smaller, but s t i l l  i n  the range of hundreds.of meters. Also the 
r o t a t i o n a l  veloci ty  i s  smaller a t  lower a l t i t u d e s  r e su l t i ng  i n  narrower 
spectra. 
the spectra  indicate  posi t ive Doppler s h i f t  t o  the east and negative Doppler 
s h i f t  t o  the west. 
r e su l t i ng  from a wind shear where higher westward wind v e l o c i t i e s  occur a t  
higher a l t i t u d e s .  Seen by the radar,  only the v e r t i c a l  v e l o c i t i e s  w i l l  show up 
i n  the spectra  and one would expect the most pos i t i ve  Doppler s h i f t  t o  be t o  
the east  (Figure 4). 
One might 
Also the phase s h i f t  t ha t  
The most l i k e l y  explanation appears t o  be one 
Most of the i r r e g u l a r i t i e s  t h a t  show a d i r ec t iona l  dependence of 
This i s  consis tent  with a ro t a t ing  c e l l  of i r r e g u l a r i t i e s  
138 
West Vertical west Vertical 
1057 LST 15:OO LST 
Figure 2. Frequency spectra for vertical and west antenna sections 
for June 12, 1980. 
DISCUSSION 
- A subs tan t ia l  amount of e f f o r t  has gone in to  the study of the mechanism 
sca t te r ing  EF and VHF rad io  waves from the rnesospherc between 60 and 90 km a l -  
though the  quest ion i s  f a r  from se t t l ed .  S t r a t i f i e d  l aye r s  have been suggested 
a s  a source fo r  t ha t  part of the scat tered s ignal  t h a t  i s  aspect sensi t ive.  It 
has a l so  generally been taken fo r  granted t h a t  turbulence-induced 
i r r e g u l a r i t i e s  i n  the r e f r ac t ive  index are both i so t ropic  and homogeneous, and 
thus accounts for t ha t  par t  of the scat tered power tha t  i s  not aspect 
sensi t ive.  It cannot be concluded from these data tha t  no s t r a t i f i e d  layers  
give r i s e  t o  r e f l ec t ion  from the mesosphere, but i f  s tably s t r a t i f i e d  layers  
exist, they a r e  not s t ab le  and s t r a t i f i e d  enough t o  be eas i ly  distinguished 
139 
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Figure 3. Cross spectra for October 1 7 ,  1981. 
phase has been deleted from those frequencies 
power i s  small. 
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To reduce 
where the 
clutter the 
scattered 
140 
Figure 4 Diagram showing the ro t a t ing  
motion r e su l t i ng  from a wind shear. 
from turbulence scatter occurring i n  the same range gates. 
d i f f i c u l t  t o  believe t h a t  a s t r a t i f i e d  layer  could form and survive f o r  any 
length of t i m e ;  and it  surely must take some t i m e  t o  form i n  the presence of 
shear induced turbulence. 
It i s  a l s o  
Also, i n  the lower pa r t  of the mesosphere around 60 Ian, the temperature 
p r o f i l e  i s  such t h a t  the atmosphere i s  close t o  being convectively unstable, 
whereas a t  a l t i t u d e s  above 60 Ian it becomes more and more s t ab le  with in- 
creasing a l t i t u d e  throughout the mesosphere. 
t h a t  s t ab le  s t r a t i f i e d  l aye r s  should form i n  the lower, but not i n  the  upper, 
pa r t  o t  the mesosphere. 
It thus seeus strange t o  suggest 
It seems more l i k e l y  tha t  the turbulence i s  anisotropic  with larger  hori- 
zon ta l  than v e r t i c a l  dimensions. Such an i so t rop ic  i r r e g u l a r i t i e s  have been re- 
ported i n  boundary layers  observations (STEWART, 1969; MESTAYER et al., 1976 
and GIBSON e t  al . ,  1977) which show anistropy i n  both temperature and ve loc i ty  
va r i a t ions .  To the  degree tha t  temperature i s  a sca l a r  t r ace r  i n  these experi- 
ments, and t h a t  the condition i n  surface boundary layers  and jets apply t o  the 
s i t u a t i o n  of shear layers  i n  the mesosphere, i t  i s  of i n t e r e s t  t o  study these 
r e s u l t s  fu r the r .  
Clearly from these evidences it i s  not s a t i s f ac to ry  t o  consider the turbu- 
lence i n  the mesosphere as i so t rop ic ,  homogeneous and t i m e  s ta t ionary.  
seems much more appropriate t o  discuss  the sca t t e r ing  from the viewpoint of 
generation and decay of Kelvin-Helmholtz i n s t a b i l i t i e s  as presented by WOODS 
(1969); PELTIER e t  al. (1978); SYKES and LEWELLFN (1982) and other. 
It  
It seems reasonable t o  in t e rp re t  the ro t a t ing  c e l l s  as Kelvin-Helmholtz 
vortexes i n  the process of being "rolled-up". Given a typical  observed value 
f o r  the horizontal  dimension of a K-H bil low i n  the mesosphere of 300 m and 
assuming a Richardson number of 0.2 we f i n d  from comparison with the computer 
r e s u l t s  of SYKES and LEWELLFN (1982) t h a t  the bil low layer should be about 
100-200 m thick. 
It i s  a l s o  seen from the computer model study t h a t  i r r e g u l a r i t i e s  are gen- 
erated not only i n  the K-H billows, but a l s o  i n  the braids t h a t  connect the 
billows, and i n  a broader turbulent region r e su l t i ng  from the horizontal  
spreading and collapse of the billows. 
nearly horizontal ly  s t r a t i f i e d .  It may be assumed t h a t  the mean shear t h a t  
causes the large-scale anisotropy 8 b O  causes anisotropy i n  the small-scale 
The large-scale i r r e g u l a r i t i e s  a r e  
141 
i r r e g u l a r i t i e s  responsible fo r  the sca t t e r ing  of the 5O-MHz radio waves. 
Although t h i s  does not necessarily follow from the computer study of SYKES and 
LEWELLPN (19821, strong experimental evidence f o r  t h i s  anisotropy exis ts .  
It appears t h a t  there  should be no problem i n  explaining the  aspect sensi- 
t i v i t y  of scat tered r ad io  s ignals  as r e su l t i ng  from anisotropic  turbulence a t  
the scale  of 3 meters. It does become a problem, however, t o  explain why t h i s  
aspect s e n s i t i v i t y  i s  subs t an t i a l  a t  60-70 lan, but decreases t o  almost zero a t  
higher a l t i tudes.  Perhaps it i s  a r e s u l t  of changing turbulent conditions re- 
su l t i ng  from a change i n  the mean temperature gradient and/or a change i n  the 
v i scos i ty  of the atmosphere. 
There remains the problem of posi t ive co r re l a t ion  between scat tered power 
and s ignal  correlat ion t i m e  below 70 km and the change t o  a negative cor- 
r e l a t i o n  above t h i s  a l t i t ude .  
t o  be r e l a t ed  t o  i so t rop ic  turbulence following the Kolmogorof model. The 
explanation fo r  the pos i t i ve  correlat ion may be found i n  the s p a t i a l  and 
temporal differences i n  the d i s t r i b u t i o n  of turbulent k i n e t i c  energy a f f ec t ing  
the co r re l a t ion  t i m e ,  and the d i s t r i b u t i o n  of electron-density var ia t ions 
representing sca t t e red  power. Comparing Figures 2 and 10 of SPKES and LEWELLEN 
(1982) it i s  seen tha t  the strongest s ca t t e r  ought t o  occur i n  the remnants of 
the braids,  a region tha t  has l i t t l e  turbulent k i n e t i c  energy. 
hand, somewhat l e s s  s ca t t e r ing  w i l l  occur at  the periphery of the K-E billows, 
i n  a region where there  i s  q u i t e  strong turbulent ve loc i ty  f luctuat ions.  Since 
the t i m e  scale  of t h i s  development i s  on the order of 10 minutes, it i s  
reasonable tha t  the posi t ive correlat ion between scat tered power  and s ignal  
correlat ion t i m e  should be observed when the integrat ion period i s  on the order 
of one minute. Furthermore, it appears from the correlat ion between aspect 
s e n s i t i v i t y  and correlat ion t i m e  t h a t  the l a rges t  anisotropy of the sca t t e r ing  
i r r e g u l a r i t i e s  ought t o  occur i n  the braids and not i n  the billows. Again, the 
question t o  be answered is, why does the sca t t e r ing  change from being dominated 
by the large-scale K-H i n s t a b i l i t i e s  a t  a l t i t u d e s  below 75 km t o  be almost 
homogeneous and i so t rop ic  above t h i s  a l t i t u d e .  
both the inner and outer s ca l e  of turbulence increases with increasing height,  
bringing the sca t t e r ing  wavelength fu r the r  and fu r the r  away from the 
horizontal ly  s t r a t i f i e d  region a t  the outer scale, and close t o  what may be an 
i so t rop ic  region a t  the inner scale of turbulence. 
The negative co r re l a t ion  i s  generally believed 
On t h e  other 
One possible answer i s  t h a t  
On the other hand, i f  it i s  assumed t h a t  the received s ignal  r e s u l t s  from 
a mixture of s ca t t e r ing  and p a r t i a l  r e f l ec t ion ,  one must conclude t h a t  the am- 
pli tude of the ref lected s ignal  va r i e s  more than t h a t  of the scat tered s ignal ,  
i n  order t o  explain the pos i t i ve  correlat ion between scat tered power and s ignal  
co r re l a t ion  t i m e .  This can be proven mathematically, however, two short  exam- 
ples  a r e  more instruct ive.  
F i r s t  assume a constant r e f l ec t ing  layer  with correlat ion t i m e  of say 1 
min. 
co r re l a t ion  t i m e .  It i s  c l ea r  t h a t  as the amount of s ca t t e r ed  s ignal ,  and thus 
w i l l  give a negative correlat ion between scat tered power and s ignal  correlat ion 
t i m e .  
Add a continuously increasing amount of scat tered signal with 1 s 
- t o t a l  s igna l ,  increases the t o t a l  s ignal  correlat ion time w i l l  decrease. This 
Secondly, assume a constant amount of scat tered power with co r re l a t ion  
Add a continuously increasing amount of r e f l ec t ed  s ignal  with t i m e  o t  1 s. 
1 min correlat ion t i m e .  A s  the  r e f l ec t ed  s ignal  and the t o t a l  s ignal  increase 
the s ignal  correlat ion time w i l l  increase from 1 s t o  1 min. This gives a 
pos i t i ve  co r re l a t ion  between the signa1 power, and the s ignal  co r re l a t ion  t i m e .  
This l a rge r  va r i a t ion  i n  r e f l ec t ing  layers  than i n  sca t t e r ing  turbulence 
may be hard t o  explain. 
142 
It i s  concluded t h a t  there i s  no strong evidence fo r  a mixture of 
dcattered and r e f l ec t ed  s igna l s  from the mesosphere. 
s c a t t e r  appears t o  be a more l i ke ly  explanation, 
ACKNOWLLM;MENT 
Anisotropic turbulent 
.D 
The work described w a s  supported by the National Science Foundation 
under grant ATM 80-19563. 
REFERFN CES 
Gibson, C. €I., C. A. Freike and S. 0. McConne11 (19771, Structure  of sheared 
turbulent f i e l d s ,  The Physics of Fluids. 20,5156. 
Mestaver. P. G.. C. H. Gibson. M. F. Coantic and A. S. Pate1 (1976). Local 
anisot;opy in-heated and cooled turbulent boundary layers,  The Phis ics  of 
Fluids ,  19, 1279-1287. 
P e l t i e r ,  W. R., J. Bal le  and T. L. Clark (19781, The evolution of f i n i t e  
amplitude Kelvin-Helmholtz billows, Geophvs. Astrophvs. Fluid Dvnamics. 
- 1 0,53 -87 . 
Royrvik, 0. ~ (1983), Spaced antenna d r i f t  a t  Jicamarca, mesospheric measure- 
ments, Radio Sci., 18, 461-476. 
Stewart, R. W. (19691, Turbulence and waves i n  a s t r a t i f i e d  atmosphere, Radio 
Sci.. 4, 1269-1278. 
Sykes, R. I. and W. S. Lewellen (19821, A numerical study of breaking Kelvin- 
Helmholtz bil lows using a Reynolds-Stress turbulence closure model, J- 
Atmos. Sci.,  39, 1506-1520. 
Woods, J. D. (1969), On Richardson's number as a c r i t e r i o n  f o r  laminar- 
turbulent-laminar t r a n s i t i o n  i n  the ocean and atmosphere, Radio Sci.. 4, 
1289-1298 


Jicamarca mesospheric observations

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