R. G. Prabhudesai

Integrated Coastal Observation Network (ICON) for real-time monitoring of sea-level, sea-state, and surface-meteorological data

National Institute of Oceanography (NIO) has established an Integrated Coastal Observation Network (ICON) of in-house designed and developed Internet-accessible real/near-real time reporting cellular based sea-level, sea-state, and surface meteorological (Met) stations at several locations on the Indian coasts & Islands (http://inet.nio.org). Subsurface pressure sensors and downward-looking microwave radars are incorporated in the sea-level station network. Sea-level, Met, and surface wave parameters are acquired using dedicated Linux based data loggers and uploaded to an Internet server at 5-, 10- and 30-min intervals, respectively with the use of GPRS cellular modems. The sensors and data loggers are powered from sealed lead acid batteries, which are charged through solar panels. The ICON provides graphical presentation of sea-level information (observed sea-level, predicted tide, residual sea-level); significant wave height and wave direction; and Met information (vector-averaged wind speed & direction, barometric pressure, atmospheric temperature, solar radiation, relative humidity, and rainfall). Installation of sea-level sensors free from the influence of stilling-wells and long narrow tubes renders the measurements ideal for tsunami and storm-surge studies by preventing waveform distortion and non-linearity of largeamplitude short-period signals. The network maintains accurate time-stamp of the dataset through Internet-time synchronization using network time protocol (NTP). Real-time reporting capability of ICON yields several benefits, such as (i) remote monitoring of proper working condition of individual stations; (ii) implementation of repair/maintenance in the shortest possible time, thereby minimizing break in the time-series data stream; (iii) periodic arrival of data stream from all stations at a single central server, thus yielding backup for the data from all the stations; (iv) access to the latest in-situ information; (v) allows possible use of data with automated real-time running numerical models for operational forecast. In contrast to the limited bandwidth provided by INSAT transmitters, coastal observations at high bandwidth at significantly low cost have become realizable using cellular GPRS network. The NIO-network allows, Internet based real/near-real time tracking and monitoring of sea-level, sea-state, and meteorological conditions along the Indian coasts and islands and from almost anywhere - an issue of considerable practical significance during natural disasters such as storm, storm-surge, and tsunami.

Comparison of sea-level measurements between microwave radar and subsurface pressure gauge deployed at select locations along the coast of India

Abstract Sea-level data are obtained from several remote and coastal locations using absolute pressure gauges deployed at known level, known as chart datum. However, to yield correct sea-level measurements from absolute pressure measurements, it is necessary to take into account the atmospheric pressure and water density at the measurement locations. We used data collected from microwave radar and an absolute pressure gauge deployed at Verem, Goa (January 2009 to May 2010), Tuticorin, and Mandapam, Tamil Nadu (June 2010 to March 2011) to carry out comparative studies. The root-mean-square difference between the estimated sea level from radar and pressure gauge (incorporating atmospheric pressure correction) is ∼ 2.69 , 2.73, and 1.46 cm at Verem, Tuticorin, and Mandapam, respectively. Harmonic analysis of the two time-series of sea-level data at Verem produces similar residuals and tidal constituents. Our results indicate the importance of concurrent measurement of atmospheric pressure along with subsurface absolute pressure gauge measurements. Internet-based real-/near-real-time tracking and monitoring of sea level, sea state, and surface-meteorological conditions from a network of several island and coastal stations provides considerable information to disaster managers and local administrators during episodic events such as storms, storm surges, and tsunamis.

On the usability of "compensated temperature" output of Honeywell PPTR sensor for coastal oceanographic and limnological studies

Usability of the temperature output, primarily meant for temperature-compensation of the pressure output of Honeywell silicon piezoresistive precision pressure transducer (PPTR) for sea-level measurements, is presented. Laboratory calibration in the range 2-50degC against a platinum resistance thermometer showed linear response (typical example: y = 1.02X - 0.40 with R = 1). It was found that beyond 15degC, linearity and accuracy are better than 2% and plusmn1.5% respectively of the sensors temperature output. However, the sensors performance degrades below 15degC. Field inter-comparison measurements from an offshore platform over a period of 36 days (3456 samples) between the temperature outputs of the PPTR and those of Aanderraa current meters Fenwall GB 32JM19 thermistor probe and NIO water level recorders YSI thermistor probes were encouraging. However, while the Fenwall GB 32JM19 thermistor probe and the YSI thermistor probes detects changes in temperature with fine resolution, the PPTRs observed temperature resolution is O.1degC only. As precautionary measures are undertaken as a technical requirement to prevent corrosion and bio-fouling effects at the water inlet of the sensor, the PPTR could be a useful tool for acquisition of long-term temperature measurements with a resolution of O.1degC, for limited applications at least, from tropical regions where the temperature is in a range in which the sensor has a linear response.

Detection of 12 th September 2007 Sumatra Tsunami at Goa and Kavaratti Island

Subsurface pressure based real-time reporting and Internet-accessible coastal sea-level stations designed and established by the Indian National Institute of Oceanography (NIO) reported the 12th September 2007 Sumatra tsunami waves from Goa (west coast of India) and Kavaratti Island (Lakshadweep archipelago) in the Arabian Sea. The sea-level stations sampled and transmitted the subsurface pressure data to the Internet server (IS) via GPRS cellular network at 5-minutes intervals. The subsurface pressure data received at the IS are converted to sea-level data and then the observed, predicted, and residual sea-levels in graphical format are displayed in real time on Internet. The graphical display from our sea-level station at Goa provided an indication of the presence of a distinct signal of tsunami periodicity. Spectral analysis of this data clearly reveals a dominant period of about 43 minutes. The tsunami wave arrived at the Goa site at ~00 hr: 45 minutes 1ST on 13 September 2007, after traveling for ~8 hr from the tsunami source region (Sumatra). The first wave was negative (trough). The observed and the maximum trough-to-crest wave height was 29 cm. The tsunami wave which arrived at Kavarathi Island at 22 hr 1ST on 12th September 2007 was less prominent (trough-to-crest wave height ~5 cm) relative to that observed in Goa. Thus, tsunami signal at Goa was ~ 6-fold larger than that at Kavarastti Island. Arrival of a detectably large tsunami signal first at Kavaratti Island and 2 hr: 45 minutes later at the shallower Goa coastal region of the mainland shows the importance of having real-time monitoring and Internet-accessible sea-level stations on Indias island locations for effective tsunami warning purposes.

GPRS based Real-Time Reporting and Internet Accessible Sea Level Gauge for Monitoring Storm Surge and Tsunami

Development and implementation of a real-time reporting and Internet-accessible sea level gauge is described. The paper addresses the application of mobile communication technology and particularly the use of General Packet Radio Service (GPRS) for accessing data from remote sea level gauge. By using a microcontroller and existing mobile phone network with a GPRS support, a continuous connection to the Internet is implemented for real time update of sea level data on a web server. The system provides a graphical illustration of the predicted fair-weather sea level, the current sea level, and the residual sea level (i.e., measured minus predicted fair-weather sea level). Thus, a cost-effective and easily maintainable platform is realized for real-time reporting of sea level and providing the requisite input for efficient implementation of any alert and warning mechanism in the event of oceanogenic natural disasters, particularly storm surges and tsunamis. The system described herein is operational in Goa, India since 24th September 2005.

November 2009 tropical cyclone Phyan in the eastern Arabian Sea: Oceanic response along west India coast and Kavaratti lagoon

Spatial and temporal response of the coastal waters of eastern Arabian Sea (AS) and Kavaratti lagoon to the tropical cyclonic storm ‘Phyan’, which developed in the southeastern AS and swept northward along the eastern AS during 9–12 November 2009 and finally made landfall at the northwest coast of India, is examined based on time-series measurements of sea-surface wind (U10), gust, gust factor, barometric pressure, precipitation, atmospheric temperature, SST, and significant wave height from satellite-derived and in-situ measurements. The maximum wind-speed (U10) of ∼16 m/s occurred at Kavaratti Island region followed by ∼8 m/s at Dwarka in Gujarat, where the cyclone landfall occurred, and ∼7 m/s at Diu located just south of Dwarka as well as two southwest Indian coastal locations at Mangalore and Malpe. All other west India coastal locations recorded maximum wind speed of ∼5–6 m/s. Gust factor during peak storm event was highly variable with respect to topography, with steep hilly stations and proximate thick and tall vegetation exhibiting the largest value whereas coastal planes and Island stations exhibiting the least. Rainfall in association with Phyan was temporally scattered, with the highest 24-h accumulated precipitation (∼60 mm) at Karwar and ∼45 mm at several other locations. Impact of Phyan on the west India coastal waters was manifested in terms of intensified significant wave height (∼3 m at Karwar, Panaji, and Ratnagiri), sea surface cooling (∼5°C at Calicut), and surge flooding (∼80 cm at Verem). Several factors such as (i) water piling up at the coast supported by seaward flow of the excess water in the rivers due to heavy rains and westerly cross-shore wind, (ii) water piling down at the coast supported by the northerly alongshore wind (by virtue of Coriolis effect) and upstream penetration of seawater into the rivers, and (iii) possible interaction of upstream flow with river runoff, together resulted in the observed surge flooding at the west India coast. Despite the intense wind forcing, Kavaratti Island lagoon experienced insignificantly weak surge (∼7 cm) because of lack of river influx and absence of a sufficiently large land boundary required for the sustenance of wave/wind-driven water mass which tends to pile up at the land-sea interface.

Role of "effective density" in improving the accuracy of bottom-pressure based sea level measurements - a case study from gulf of guinea

Seasonal observations of a set of bottom pressures and concurrent tide-staff measurements from Takoradi harbour located in the Gulf of Guinea indicated that the effective depth-mean density value, peff of this clear water body is less than (= 0.25% - 1.84%) its bulk density, pb obtained from precision density measurements made on discrete water samples. Thus, use of pb to estimate sea level elevation yields an underestimation of this order in sea level measurements. Although, this effect was observed in association with both wind and rainfall, the comparative reduction of peff value was more prominent during the period when rainfall was strong. Presence of microbubbles introduced into the upper layers of the sea water, as a result of turbulence induced by wind forcing and impact of rain drops, is suspected to be responsible for the observed effective reduction in the in situ density of this shallow water body. The accuracy of bottom-pressure based sea level measurements can be improved by introducing peff value into sea level estimation. It is shown that peff value can be estimated by using a statistically derived simple linear model, which is constructed from a set of bottom pressures and concurrent tide-staff measurements.

Meteorologically Induced Modulation in Sea Level off Tikkavanipalem Coast—Central East Coast of India

Abstract This article examines the seasonal variability of nontidal contributions of the sea level oscillations off Tikkavanipalem coast in Andhra Pradesh (India), which is located on the western boundary of the Bay of Bengal. The analysis is based on simultaneous observations of tidal and surface meteorological parameters in four temporal segments of 1-month duration each during a 1-year period in 1997–98. Sea level oscillations along the Tikkavanipalem segment of the central east coast of India contain contributions from tidal and set-up/set-down motions. Local wind pattern, coastal current, and air pressure variations particularly influence the residual sea level elevation (difference of the measured and astronomically induced daily mean sea level) in this region. Observed response of the sea level elevation to the barometric pressure represents an amplified/delayed inverted barometer effect. The relative importance of tidal and nontidal contributions to the sea level is found to be seasonal, as the coastal current and the meteorological forcing suffer large seasonal variations.

Identification of Thermohaline Structure of a Tropical Estuary and its Sensitivity to Meteorological Disturbance through Temperature, Salinity, and Surface Meteorological Measurements

Thermohaline structure of Kochi backwaters (KB) in India and its sensitivity to meteorological disturbances has been identified through measurements of temperature, salinity, and surface meteorological parameters. Silicon p-n junction semiconductor sensors have been used for measurements of subsurface temperature. Linearity and accuracy based on laboratory calibration of the sensors are found to be better than 0.4% and -0.8% respectively beyond 20degC. Induction type conductivity cell was used for salinity measurement. Linearity and accuracy of salinity measurements are found to be better than plusmn3% and plusmn5% respectively beyond 20 psu. In-situ measurements show that the thermohaline structure of KB exhibits fortnightly spring-neap variability in which thermal and haline variability bear an inverse relationship, with cooling and enhanced salinity during spring tide and vice versa during neap tide. The diurnal variability in temperature is controlled by day/night cyclicity rather than tidal, whereas this feature is absent in haline variability. Horizontal thermohaline structure of the KB is inhomogeneous, where the upstream boundary region is warmer and less saline than the mouth region. Changes in the meteorology disturb the thermohaline structure of KB. Rainfall and associated river influx cause large haline stratification in the mouth region, in which a ap1-4 m thick layer of low salinity water (ap0-2 psu) floats on the surface.