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 iversity, 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'e r 1998; revised received 1 February 1999: accepted 19 March 1999 Studies on mi crowave signal propagation provide an opportunity to in vestigate the variable nature of th e atmosphere. The present study deal s with the effect of sea breeze on the propagation characteristics observed over a lin e-of -sight (LOS) microwave link operating at 6 GHz and situated in th e south-east coast of Indi a. It has been found that the amplitude of the received mi crowave s ign al suffers appreciably during the onset of sea breeze. The correlation between th e cas es of on set of sea breeze observed from microwave amplitude measurements and those from meteorological observations has been found to be good . The large chan ge in refractivity has been observed during the onset of sea breeze. Such chan ge in refrac ti 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 sub- refractive conditions which are caused by alteration in the dielectric properties of the atmosphere near the surface of the earth I. Recent st udies indicate that the atmospheric circu lation of lake-land breeze is responsible for strong horizontal and vertical inhomogeneiti es of moisture distribution affecting the refractivity in the neighbourhood of a microwave LOS link 2 . A land or sea breeze may modify the refractive conditions in ways depending upon the distribution of mois ture in the lower layers. These breezes are of lo ca l nature and generally extend only to a few kilometres around the coastal . 3 regIOn s . In the past, climatol og ical studies of sea breezes have been attempted in India by many researchers 4 - lo . These are mostly confined to study the onset of sea breeze in different parts of the country. Sodar observations of sea 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 1 3: Circulations of sea breeze under diff erenti al thermal heating co nditions 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 limitations in the spatial separation of India Meteorological Department (IMD) observatories, th e changes in the refractive ind ex for this circulation are often missing. The sta ti sti cs deduced from these studies form inputs to the models of ITU-R and al so help, to some extent, in the design of digital LOS microwave links operating near the coastal sites which are subjected to various com plex propagation phenomena like advection, sea breeze effec t s, super- refraction and ducting. The above facts have made the in vestigators to undertake the proposed study. In thi s paper, to understand the local atmospheric circu- lation, an attempt has been made to st udy the effect of sea breeze on fading characteristics. Amplitude measurements of microwa ve 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) a nd 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 Ne ll ore
Transcript
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 distribution affecting the refractivity in the neighbourhood of a microwave LOS link2
. 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 have been attempted in India by many researchers4
-lo
.
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 heating conditions of sea- land interface areas are found to be of great concern for the LOS links. situated along the coastal paths in tropics \3. Due to the limitations in the spatial separation of India Meteorological Department (IMD) observatories, the changes in the refractive index for thi s c irculat ion are often mi ssi ng. The stati stics deduced from these studies form inputs to the model s of ITU-R and also 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 propagation phenomena like advection, sea breeze effects, superrefraction and ducting. The above facts have made the investigators to undertake the proposed study. In this 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 Nellore
114 IND IAN J RADIO & SPACE PHYS, JUNE 1999
and Chittedu is 44 km long. The link is located along the sea coast. The receiving 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 the above link are given in Table I.
The terrain profile is shown in Fig. I. It is shown that a major portion of the propagation path fall s over the cultivated land and irrigation tanks, whi ch cou ld be the source for the observed multipath fading . Fadings associated with number of sea breeze cases were identified in different seasons, viz. pre-monsoon (March-May), ·monsoon (June-September), post-monsoon (October-November) 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 the period June 1997-May 1998, were obtained from (MD, as it is the nearest rad iosonde station from the 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
10 W 80 m !WIll 40 dB 40 dB Parabolic Paraho li c Il orl! 14 1 dn
~ Res ident ial area
E ..J VI ~
UJ
~ ~ III :;: <t
UJ
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 requisite. A direct thermally 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 moist air mass moves over the land during the circulation of a sea breeze, a change in temperature and humidity takes place and the 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 abrupt increase of humidi ty and decrease in temperature are taken as a sign of onset of sea breeze9
. Typica l microwave fad in gs observed on 4 and 5 April 1998 du ring the onset of sea breeze are shown in Figs 2 and 3, respectively. Simul taneous observations of temperature and humidity recorded at the receiving site using thermograph and hygrograph during the onset of sea breeze are also shown. In Figs 2 and 3, at around 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, respectively. It can also be seen from Figs 2 and 3 that the onset of sea breeze effect observed from microwave fie ld strength record s a lmost co incides with the onset of sea breeze ded uced fro m meteorologica l hygrograph and thermograph
~ Palm trlZlZS i. ~ Cul tivated land
g 20 t---__ Canal Car.al Sur veoa lli [r rig ation Irr igation tank r- ~~\.----l~e'5rV~tat~f---------'l , ,-----...... ---S 10 II II I I I I I <t I I II I I
NELLORE 0 I
10 30 1.'0 CHITTEDU 20 DI ST AN C E , k m
Fig. I-Terrain profile urt he Nc ll ore-Chittedu LOS microwave link
RAM A RAO et al.: SEA BREEZE ON PROPAGATION CHARACfERISTICS OYER LOS LINK 115
Fig. 2-Typical ' record of microwave fad ing ohserved on 4 Apr. 1998 during the onset of sea breeze along \vith hygrograph 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 l4
. 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 months.
The sea breeze effect is predominant during premonsoon period due to the most intense heati ng. Inversions are maximum during pre-monsoon followed by winter. and minimum during monsoon and post-monsoon seasons" ,12. During the premonsoon period (March-May) the land is subj ected to intensive heating and also, because of the absence of any monsoon system, the sea breeze is usually powerful. The monsoon months (June-September) 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 unfavourable prevailing winds. Thus, sea breeze is not only 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 monsoon with occasional catastrophic cyclones. The impact of sea breeze, again in these months, is low. In fact, the monsoon rains break the formation of well defined
_A_ cases from microwave records ~. '., ••••.. . cases from meteorological data " ~
.:d · ":;1\;1 10 Iii , i I I
JUNE JULY AUG. SEP. OCT. NOV. DEC . JAN. FEB MAR. APR . MAY
MONTHS
Fig. 4-{:orrcl ation 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'!. Then 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 phenomena over the east-coast of Indi a are dominated by monsoon patterns. From December to February (winter) the weather over inland is usually dry and there is no dominating preva iling wind due to the absence of any active mon soon system. This results in a fairly systematic onset of sea breeze bringing, over the land , re latively cooler and humid air that, in turn , will not only cause a temperature inversion , but also a steep humiditY. gradient. This causes super-refracting conditions, sometimes ducting. A typical 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 conditions. Super-refraction may occur in a limited height interval in the troposphere where (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
...
RAM A RAO et al.: SEA BREEZE ON PROPAGATION CHARACTERISTICS OVER LOS LINK 11 7
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 the high refractiv ity is reached in pre-monsoon month s and low in winter months. Surface refractivity
3~,------------------------------.
2500 4 Apr. 1998 : 1200 hrs GMT
E 2~
~ 6 1500
iii I1~
500
o . 200 250 300 350 400
REFRACTIVITY, N units
Fig. 5--Typical refrac ti vity pro fik observed on 4 Apr. 1998 over
Ind ian coastal station
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 the microwave amplitude measurements that the observed average fade depth and fade rate are around 5 dB and 40-50 fades/h, respectively. Table 2 gives the monthly
Table 2- Monthly occurrence percentage o f sea breeze and average med ian fi eld strength during the onset pf sea breeze
Month
June July Aug. Sep . Oct. Nov. Dec . .I an. Feb . Ma r. Apr. May
Occurrence percentage of sea breeze from IMD
Average medi an fi eld strength (MFS) (dBm)
data o bserved during sea breeze ( orma l MFS is - 23 dBm )
JUNE JULY AUG. SEP. OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY
MONTHS
Fig. 6--{'lln tou r plots of monthly 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.
4 Conclusions 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.
Acknowledgements
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 &
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.
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