Indi an Journal of R<Jdio & Space Physics
Vol. 28 . June 1999 . pp. 11 3- 118
Effect of sea breeze on propagation characteristics over a LOS microwave link
located in Indian south-east coast
T Rama Rao & S Vijaya Bhaskara Rao
Department of Physics, S V Un iversi ty, Tirupati 517 502
and
M V S N Prasad, S K Sarkar & D R Lakshmi
Radio & Atmospheric Sciences Division, National Physical Laboratory. New Delhi 11 00 12
Received 21 Septemb'er 1998; revised received 1 February 1999: accepted 19 March 1999
Studies on microwave signal propagation provide an opportunity to investigate the variable nature of the atmosphere. The
present study deals with the effect of sea breeze on the propagation characteristics observed over a line-of-sight (LOS)
microwave link operating at 6 GHz and situated in the south-east coast of India. It has been found that the amplitude of the
received microwave signal suffers appreciably during the onset of sea breeze. The correlation between the cases of onset of sea
breeze observed from microwave amplitude measurements and those from meteorological observations has been found to be
good . The large change in refractivity has been observed during the onset of sea breeze. Such change in refracti vity profile
causes fading in microwave radio signals.
1 Introduction
Microwave line-of-sight (LOS) links are frequently
affected by severe fading, resulting from defocusing
of electromagnetic waves in ducts, multi path and subrefractive conditions which are caused by alteration
in the dielectric properties of the atmosphere near the
surface of the earth I . Recent studies indicate that the
atmospheric c ircu lation of lake-land breeze is
responsible for strong horizontal and vertical
inhomogeneities of moisture di stribution affecting the
refractivity in the neighbourhood of a microwave
LOS link 2 . A land or sea breeze may modify the
refractive conditions in dim~ rent ways depending
upon the distribution of moisture in the lower layers.
These breezes are of local nature and generally
extend only to a few kilometres around the coastal
.
3
regIOns .
In the past, climatological studies of sea breezes
4 lo
have been attempted in India by many researchers - .
These are mostly confined to study the onset of sea
breeze in different parts of the country. Sodar
observations of sea breez~ front reveal that, over land,
a modified thermal internal boundary layer is
characterized by a convectively turbulent layer'" 12.
Attempts were also made to investigate the influence
of sea breeze on LOS microwave link in tropical
coastal areas 13:
C irculations of sea breeze under differenti al
thermal heatin g condition s of sea-land interface areas
are found to be of great concern for the LOS lin ks.
situated along the coastal paths in tropics \3. Due to
the limitatio ns in th e spatial separation of India
Meteorological Department (IMD) observatories, th e
changes in the refractive index for thi s c irculat io n are
often mi ssi ng. The stati stics deduced from these
studies form inputs to the model s of IT U-R and al so
help, to some extent, in the design of digital LOS
microwave links operating near the coastal sites
which are subj ected to various com plex propagati on
phenomena like advection, sea breeze effects, superrefraction and ducting. The above facts have made the
investigators to undertake the proposed study. In thi s
paper, to understand the loca l atmospheric circulation, an attempt has been made to study the effect of
sea breeze on fading characteristics . Amplitude
measurements of microwave signal at 6 GHz were
made on 24-h basis from June 1997 to May 1998,
using an operational communicational link between
Nellore (lat., 14° 26' 34" N ; long., 78° 58' 58" E) and
Chittedu (Iat., 14° 02' 43 " N ; long., 80° 0' 58" E) located
in the Indian south-east coast.
2
Data base and system characteristics
The LOS microwave link situated between Nell ore
114
IND IAN J RADIO & SPACE PHYS , JUNE 1999
and Chittedu is 44 km long. The link is located along
the sea coast. The recei ving end is situated at Nellore.
Measurements in the variation of amplitude of
microwave signal have been made by a strip chart
recorder. The system characteristics of th e above link
are given in Table I.
3
Results and discussion
Sea breeze over the coast is due to thermally driven
mesosca le atmospheric circulation, for which
differential heating is the essential req uisite. A direct
th ermall y driven circulation such as sea breeze occurs
more frequently and more regularly in the tropics than
in middle and high lat itudes 10 . Since a moi st air mass
moves over the land during the circulation of a sea
breeze, a chan ge in temperature and humidity takes
place and th e wind direction changes from sea to
land .
Whenever the sea breeze sets in, there is an abrupt
change in wind direction, an increase in relative
humidity and a decrease in temperature. The ab rupt
increase of humidi ty and decrease in temperature are
taken as a sign of onset of sea breeze 9 . Typica l
microwave fad in gs observed on 4 and 5 April 1998
du rin g the onset of sea breeze are shown in Figs 2 and
3, respectively. Simul ta neous observations of temperature and humidity recorded at the receiving site
using therm ograph and hygrograph durin g the onset
of sea breeze are also shown. In Figs 2 and 3, at
aro und 1340 and 1420 hrs [ST, respectively, there is
sudden increase . in hu midity and decrease in
temperature indicating the onset of sea breeze. At the
same time, in th e microwave field strength records
shown in top panel of Figs. 2 and 3, a sudden
decrease can be observed from \ 340 and 1420 hrs
1ST onward s, respectivel y. It can also be seen from
Figs 2 and 3 that th e onset of sea breeze effect
observed from microwave fie ld strength rec ord s
almost co incides with the onset of sea breeze ded uced
fro m meteorologica l hygrograph and th ermograph
The terrain profile is shown in Fig. I. It is shown
that a major portion of th e propagation path fall s over
the cultivated land and irrigation tanks, whi ch cou ld
be the so urce for the observed multipath fading .
Fadi ngs associated with number of sea breeze cases
were identified in different seasons, viz. pre-monsoon
(Ma rch-May), ·monsoon (June-September), post-monsoo n (October-N ovember) and winter (DecemberFebruary) and are used to find out the depth of fading.
The surface meteorological data over Nellore and
radio onde observations over Madras, pertain ing to
th e period June 1997-May 1998, were obtained from
(MD, as it is the nearest rad iosonde station from th e
microwave link .
Table I-System charac teristi cs of Ne ll orc-ChitteJ u
microwave link
Power transmitted
Height of the transmitting antenna above the grollnd
Height of the receiving antenna above the ground
Transmitting antenna gai n
Receiving antenna gain
Type of transmitting antenna
Type of receiving antenna
reed :vpe
Free space loss
~ Res ident ial
10 W
80 m
!WIll
40 dB
40 dB
Parabolic
Paraho li c
Il orl!
14 1 dn
area
~ Palm trlZlZS i.
~ Cul tiv ated land
E
..J
VI
~
UJ
~
III
~
:;:
<t
UJ
g
20
t---__
r-
S
10
<t
NELLORE 0
Canal
Car.al
II
II
II
II
Sur v eoa lli
[r rig ati on
Irr igation tank
~~\.----l~e'5rV~tat~f---------'l
I
I
I
I
I
I
, ,-----.--...
I
I
10
20
30
DI ST AN C E , k m
Fig. I-Terrain profile urt he Nc ll ore-Chittedu LOS microwave link
1.'0 CHITTEDU
115
RAM A RAO et al.: SEA BREEZE ON PROPAGATION CHARACfERISTICS OYER LOS LINK
- - - --- - -- - -Q- - --- -- .----,- - - - - - - - - -
4-4-1998
:I:
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l!>
Z
W
0: t-
OJ)
-30 - ------'-'--'----
..J
.q
z.
- - - -- ----
.,
- - - ----- --- - - - - - .-- - - -- - -..
- - - - - - --
---.~~
.. - .
l!>
OJ) -
- - 40 '
I
__
. _ _ __ _ _1400 _ _ _ _ _ _ _ _ _ __ 1430
__ _ _ _ _ _ _ _ _ _ 1500
TlME , hrs 1ST
THERMOGRAPH C HART
HYGROGRAPH CHART
-'---i. .
10
TIME, hrs 1ST
11... 1). : .J.Ll5
,16_ _~ : . ~ .
11
19
20
21
: ". r:-·-: .'
..
!: : : : - --
-q,
bO
•
. - - ?- .
--
... ,. _.
- ~-
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=t='iI¥
•
-
-..= : : ~ . ; .-
~ . :';
=-=-~ , .:.=-:: :
:.
. .
_ .
: i
:c.::·
Fig. 2-Typical ' record of microwave fad ing ohserved on 4 Apr. 1998 during the onset of sea
breeze along \vith hygrograp h and thermograph observati ons
observations. The onset of sea breeze observed from
the microwave field strength records is mostly
between 1200 and 1500 hrs 1ST. It has been observed
that the daytime convective conditions do not affect
the microwave signals, and the field strength remains
steady during afternoon hours l 4 . However, in this case
the fading is observed during the afternoon hours
which could be due to the onset of sea breeze.
Another facet of the observations is the
considerable decrease of the median field strength by
6-7 dB during the onset of sea breeze. Figure 4 shows
the number of cases of onset of sea breeze
occurrences derived from microwave field strength
measurements and from surface meteorological
observatIons. Figure 4 shows a good correlation
(correlation coefficient is 0.86) between the number
of cases of onset of sea breeze deduced from the
observed
fading
characteristics
and
surface
meteorological parameters. Further, it can also be
observed from Fig. 4 that the occurrences of sea
breeze are greater during pre-monsoon month s
(March-May) than during other month s.
The sea breeze effect is predominant durin g premonsoon period due to the most inten se heati ng.
Inversions are maximum during pre-monsoon
followed by winter. and minimum during monsoon
and post-mon soon seasons" ,12. During the premon soon period (March-May) the land is subj ected
to intensive heating and also, because of th e absence
of any monsoon system, the sea breeze is usually
powerful. The monsoon months (June-Septem ber)
bring in the south-west monsoon with the
accompanying cooling of the land surface as well as
higher humidity over the land. In addition, the sea
breeze has to contend with the unfavoura ble
prevailing winds. Thus, sea breeze is not onl y
truncated but also relatively ineffective in modifying
the humidity and temperature patterns over land
because of the already existing humid and cool
conditions. The months of October and November
(post-monsoon) are dominated by north-east monsoo n
with occasional catastrophic cyclones. The impact of
sea breeze, again in these month s, is low. In fact, the
monsoon rains break the formation of well defined
116
INDIAN J RADIO & SPACE PHYS, JUNE 1999
~NELLORE
CH ITTEDU
LOS 6 GHz
~~-i~~E~iE
-
~ i ~ ~I
•
-- _..-- .--- - -- ---.- - ------
--
-
- - . --~
i
~ §
~-. - . ----- - . ----------- - -.~ . -- .
----. -- ..• ---:-..:.=..:.-:.:=....:....- -::..-_ .• "
-
...
--_ ..
.
--
1500
1430
TIME, hrs 1ST
TlME,hrs 1ST
.. __.
=-- 2().. · ··- -_ 1()"-.
HYGROGRAPH
~ o ·-
- - _ ._.
~~~_ . THERMOGRAPH ~~
~~------
':"'-"
----~-
-:_.::ro- -:-
__:.:~:c-=::=-...:..c=-=_= _--- =-:f- =
Fig. 3-Samc as in Fig. 2, but for 5 Apr. 1998
2~
~.
cases from microwave records
••••.. . cases from meteorological data
_A_
UJ
U
Z
UJ
::J
u
g
LL
o
ci
z
" ~
.:d ·
(/)
a::
a::
'.,
":;1\;1
10
I i i
JUNE JULY
AUG . SEP.
OCT.
NOV. DEC . JAN.
,
FEB
i
I
MAR. APR .
I
MAY
MONTH S
Fig. 4-{:orrcl ati on of sea breeze occurrences observcd
from microwave fi eld strength measurements ar.d surface
meteorological observations
sea breeze and, during post-monsoon period also, the
sea breeze effect is found to be minimum'!. Th en the
water over the land does not allow a severe
convective activity, resulting in the suppression of the
formation of sea breeze . The sea breeze phe nomena
over the east-coast of Indi a are do minated by
monsoon pattern s. From Dece mber to Fe bruary
(winter) the weather over inl and is usually dry and
there is no dominatin g preva iling w ind due to the
a bsence of any active mon soo n system. This results in
a fairly systematic onset of sea breeze brin g in g, over
the land , re latively cooler and humid air that, in turn ,
will not only cause a temperature in version , but al so a
steep humiditY. gradient. This causes super-refracting
conditions, sometimes ducting. A ty pical refractivity
profile observed on 4 April 1998 during sea breeze is
shown in Fig. 5. A steep decrease in refractivity can
be observed indicating the super-refractive condition s. Super-refraction may occur in a limited height
interval in the troposphere wh ere (dN/dh) < - 0. \57
m- I . With super-refraction ; the refractive index
.decreases at a rate which is fo ur times the lapse rate
of standard refraction 3. As the present LOS link is
s ituated along the sea coast, the large changes in
11 7
RAM A RAO et al.: SEA BREEZE ON PROPAGATION CHARACTERISTICS OVER LOS LINK
refractivity associated with the sea breeze produce
fading during the intrusion of sea breezes.
Surface meteorological data collected at Nellore is
used to study the change in the surface refractivity. A
contour plot of monthly mean surface radio
refractivity is illustrated in Fig. 6. It is observed that
th e hi gh re fracti v ity is reached in pre-monsoon
month s and low in winter month s. Surface refracti vity
is high during nighttime and early morning hours and
low at daytime as the temperature rises. At the time of
onset of sea ·breeze it has been found from th e
microwave amplitude measurements that the observed
average fade depth and fade rate are around 5 dB and
40-50 fad es/ h, respectively. Table 2 gives the monthl y
Ta ble 2- Mo nthl y occ urrence percentage o f sea breeze and
average med ian fi eld strength durin g th e o nset pf sea breeze
3~,------------------------------.
2500
E
Mo nth
4 Apr. 1998 : 1200 hrs GMT
2~
June
July
Aug.
Sep .
Oct.
Nov.
Dec .
.I an.
Feb .
Ma r.
Apr.
May
~
6 1500
...
iii
I1~
500
o.
200
250
300
350
400
REFRACTIVITY, N units
Fig. 5--Typical refr ac ti vity pro fik observed o n 4 Apr. 1998 over
Ind ian coastal station
JUNE JULY
AUG.
SEP.
OCT.
Average medi an fi eld
Occ urrence percentage
strength (M FS) (dBm)
of sea breeze fro m IMD
data
o bse rved during sea breeze
( o rma l M FS is - 23 dBm )
NO V.
DEC.
53.3
4 1. 9
48 .3
53 .3
4 1. 9
46 .6
54 .8
58
67 .8
77.4
83 .3
77 .4
JAN.
FEB.
MAR.
- 25 .5
- 26
- 27 .5
- 28
- 31.5
- 32
- 30.5
- 30
- 32 .5
- 33
- 32 .5
- 33
APR.
MAY
MONTHS
Fig. 6--{'lln tou r plots of mon thly mean surface rad io refracti vi ty ovcr the rece ivi ng sitc.
\
cllon:
118
INDIAN J RADIO & SPACE PHYS. JUNE 1999
occurrence percentage of sea breeze calculated from
IMD data and monthly averaged median field strength
observed during the onset of sea breeze. It is observed
that the median field strength decreases considerably
in pre-monsoon months as compared to other months.
Technology, Govt. of India, for providing
meteorological data and financial assistance,
respectively, to carry out this work. The authors are
also thankful to Shri G V Rama, SHAR Centre,
Sriharikota, for helpful discussion on sea breeze.
4 Conclusions
References
An attempt is made to study the fading phenomena
during the onset of sea breeze observed over a coastal
LOS microwave link operating at 6 GHz. The onset
of sea breeze manifests itself by bringing about some
distinct characteristic changes in the LOS microwave
propagation phenomena. Correlation of meteorological parameters and the LOS recordings indicates that
the observed fading in the afternoon hours is mostly
due to the effect of sea breeze. Local atmospheric
circulation is capable of promoting the advection of
water vapour' around the propagation path. The
advection of water vapour changes its refractivity
environment. Such change of refractivity causes
fading in radio signals.
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Acknowledgements
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The authors are grateful to the General Manager,
Maintenance, Southem Telecomm Region, for
granting permission to record the microwave signals
and wish to thank Shri K V Subba Rao, Assistant
Engineer, Microwave station, Nellore, for providing
the facilities to record the microwave signal. The ·
authors are thankful to the India Meteorological
Department and to the Department of Science &