Prakash Mehra

Observational Evidence of Summer Shamal Swells along the West Coast of India

Wave data collected off Ratnagiri, which is on the west coast of India, in 2010 and 2011 are used to examine the presence of the summer shamal swells. This study also aims to understand variations in wave characteristics and associated modifications in wind sea propagation at Ratnagiri. Wind data collected using an autonomous weather station (AWS), along with Advanced Scatterometer (ASCAT) and NCEP data, are used to identify the presence of summer shamal winds along the west coast of the Indian subcontinent and on the Arabian Peninsula. NCEP and ASCAT data indicate the presence of summer shamal winds over the Arabian Peninsula and northwesterly winds at Ratnagiri. This study identifies the presence of swells from the northwest that originate from the summer shamal winds in the Persian Gulf and that reach Ratnagiri during 30% of the summer shamal period. AWS data show the presence of northwest winds during May and southwest winds during the strong southwest monsoon period (June‐August). Another important factor identified at Ratnagiri that is associated with the summer shamal events is the direction of wind sea waves. During the onset of the southwest monsoon (May), the sea direction is in the direction of swell propagation (northwest);however,during the southwestmonsoon (June‐August), a major part of the windsea direction is from the southwest. The average occurrence of summer shamal swells is approximately 22% during the southwest monsoon period. An increase in wave height is observed during June and July at Ratnagiri due to the strong summer shamal event.

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.

A one year comparison of radar and pressure tide gauge at Goa, west coast of India

At many locations absolute pressure gauges are used to obtain sea level data. Therefore, knowledge of the other physical processes which effect the sea level measurements at the coast is very important to insert the required corrections. Data from the radar and an absolute pressure gauge are obtained over a period of one year from the test site at Verem, Goa, India. The variance of difference between the radar and absolute pressure gauge is 15.9 cm2, which reduces to 5.7 and 4.0 cm2 respectively, when atmospheric pressure and water density variations are introduced for obtaining sea level from an absolute pressure gauge. The regression analysis indicates that atmospheric pressure, water density and rainfall can account for the difference individually up to 71%, 33% and 27% respectively. The aim of the present study is to quantify the effects of atmospheric and oceanographic variables while measuring sea level using pressure gauges and to highlight the advantages of the radar gauge over other type of gauges with regard to easy installation, maintenance and could be given precedence in future applications.

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.

Surfzone wave characteristics during flood tide on the central west coast of India

Surfzone wave characteristics, measured using a wave and tide gauge (WTG) during a flood tide, were studied at three different beaches having different nearshore slopes. The spectral wave characteristics viz., wave-height and mean wave period were estimated considering different sample sizes. Inter-comparisons of wave climate between each of three beaches for a similar tide level are presented. The wave-height and mean wave period values obtained from the wave analysis for each of the record using waves by wave method show that surfeone wave-height increased with time during a flood tide and the mean wave period decreased with time up to mid tide and then increased during the rest of the measurement period at Keri and Miramar beaches. At Candolim, the reversing trend of mean wave period increase is observed to occur much before reaching the mid tide level. The surfzone waves during a flood tide indicated that the wave-heights increased with time while the mean wave period showed a decreasing trend in general. The variation of breaker index showed a decreasing trend from low tide to high tide ranging between 0.45 and 0.23. The surf similarity parameter estimated corroborates with the visual observations.

Comparison of sea-level measurements using microwave radar and subsurface pressure gauge deployed in Mandovi estuary in Goa, Central West Coast of India

Data from the radar and an absolute pressure gauge collected from Verem, Goa over a period of one year- January, 2009 to May, 2010 is used to carry out the comparative studies. The root mean square difference between the estimated sea level using radar and pressure gauge with atmospheric pressure correction is ~ 2.6 cm. The harmonic analysis over the two time series produces similar residuals and tidal constituents. The results from the study indicate the importance of concurrent measurement of atmospheric pressure along with sub-bottom absolute pressure gauge. The radar gauge has advantages over other type of gauges with regard to easy installation, maintenance and also sea level measurements are absolute and could be given precedence in future applications.

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.