Sound Science System Description
The observing system is deployed on the P.T. Barnum, the newest vessel of the Bridgeport – Port Jefferson Steamboat Company’s fleet of three ferries. The schematic diagram above shows the general placement of the different environmental sensors. In general, the system consists of one set of instruments to measure near-surface meteorological properties (upper diagram), one set of instruments to measure surface water properties and current profiles below the ship (lower diagram), and additional equipment to log, display and transmit the data. Measurements of the near-surface water properties are based on sampling water from a sea-water intake system. Measured quantities include sea surface temperature (SST), salinity, chlorophyll-a, and dissolved oxygen. In general, the meteorological sensors are placed on either the roof of the upper deck or on masts fixed to the two bridge wings. In this case, measured quantities include wind speed and direction, temperature, relative humidity, pressure, incoming solar and infrared radiation, and precipitation. The system also includes a Global Positioning System (GPS) to provide position information for the data samples. In addition, a compass provides orientation information for the vessel. This, along with the GPS position, provides the information necessary for correcting the wind observations for the movement and orientation of the vessel.
Apart from providing basic meteorological and hydrographic information for the central Long Island Sound, the above set of meteorological measurements, in conjunction with the SST data, provide the necessary information to compute robust estimates of the exchanges of heat, momentum and freshwater across the air-sea interaface (see Objective 2). Specifically, this involves the four main components of the surface heat flux (i.e., net solar and infrared radiative fluxes, latent and sensible fluxes), the surface wind stress, and the surface freshwater flux.
The instrument packages relay their measurements to a computer housed on the vessel. This computer stores the data internally as well as transfers the data in “real-time” (about every 15 minutes) to the Marine Sciences Research Center at Stony Brook University where it is displayed on the world wide web, disseminated to the local National Weather Service office as well as analyzed in the context of the project’s Scientific Objectives. The computer also displays the data on board the vessel itself to provide the observed information to the passengers and crew.
The P.T. Barnum traverses the central Long Island Sound approximately eleven times every day of the year during the hours of about 7am to 9pm, except for short intervals (~days) when it is dockside for periodic maintenance. For the most part, this sampling scheme will provide the means to measure and document seasonal variations, synoptic weather variations, and some aspects of the diurnal cycle as well as the cross-Sound variability that might be present in the quantities discussed above.
Near-surface atmospheric wind speed and direction are measured by RM Young Wind Monitors mounted atop mass attached to each bridge wing. Having two sensors provide redundancy in case of instrument failure. In addtion, during most periods when both wind sensors are operational, the measurment from the upwind sensor is the one that is reported and used in subsequent analysis since it is the one least influenced by the vessel. The wind speed sensor is a four blade helicoid propeller that turns a multi-pole magnet. Propeller rotation induces a variable frequency signal in a stationary coil, which is measured and converted to wind speed. The wind direction sensor is based on the unit’s molded weather vane. The vane angle is detected by an optical encoder. The range of the wind speed sensor is 0-60 m/s (134 mph), with gust survival rated to 100 rn/s (220 mph). The wind speed sensor has a resolution of 0.1 m/s and an accuracy of ±0.3 m/s (0.6 mph). The range of the wind direction sensor is 0-360 degrees, and it has a resolution of 1 degree and an accuracy of ±2 degrees.
The pyrgeometer provides high accuracy infrared radiation measurements for meteorological purposes in a wavelength range of 450 to 4200 nm. The way the instrument works is similar to the pyranometer. The voltage output signal is proportional to the amount of incident infrared radiation which is afterwards converted to W/m². Like the pyranometer, in addition to the main thermopile sensing device, a body thermistor provides temperature compensations. The outer surface of the single ellipse-shaped solar blind silicone dome, has been protected with a hard carbon coating against oxygenation and scratching. The dome ring was designed to drive away the heat load of the dome for accurate day time measurements. This pyrgeometer works under temperatures ranging from –40°C to 80°C, and with a thermopile output between –250 W/m²-250 W/m².
In the ferry application, the pyranometer is used to measure total downward solar radiation, while the reflected upward component will be estimated using observed values of albedo over the ocean. The model CM 21 is an ISO-class instrument, which means it meets one of the categories outlined in the ISO 9060 standard. For instance, the cosine directional response and the temperature dependence are verified criteria. The incoming radiation is received by the sensing element housed under two concentric glass domes, and coated with a stable carbon based non-organic, which allows a good spectral absorption and a long term stability. The role of the inner hemisphere is to filter wavelength of 305 to 2800 nm. In addition to the black unit, a second built-in sensor detects and compensates any change in body temperature, so that the response is independent of ambient temperature. Thus, the temperature sensed is nearly linear with the flux density of incident radiation, with less than 0.25% of reported non-linearity for incoming shortwave under 1000 W/m². The use of this pyranometer in combination with Kipp & Zonen CV2 Ventilation system reduces instrument maintenance, and offers better measurement performance.
Surface pressure is measured by a Honeywell Precision Barometer (HPB) that uses silicon sensor technology with microprocessor-based signal compensation, the latter of which eliminates the need to insulate or temperature regulate the instrument. The HPB provides accurate and stable measurements in a pressure range of 500-1200hPa (=500-1200 mb). When operating under temperature ranging between –40°C – 85°C, its accuracy is 0.4hPA (=0.4mb). The long-term stability of this instrument is estimated to be 0.25hPa (=0.25mb) per year. This barometer is well adapted to environmental conditions with negligible sensitivity to humidity, acceleration, and a good resistance to vibrations, mechanical and temp shocks. The HPB is attached to the communications mast above the main bridge, in an area shielded from the wind. This location, along with the use of the the use of a RM Young pressure port, minimizes the dynamic pressure due to wind that can adversely affect the barometer measurements.
Precipitation measurements are made with an RM Young 50202 precipitation gauge. Precipitation is collected in a funnel whose cross sectional area is 100 cm2. After capture, it is drained from the funnel to a 20 cm2 cross-section measuring tube. A capacitive transducer located in the center of the tube senses water column height, while an integrated electronic circuit converts the capacitance value to a calibrated voltage proportional to collected precipitation. When the water column becomes full, additional precipitation starts the siphon process which empties the siphon in approximatively 30 seconds. Thus, the water level in the tube returns to 0 mm and the output voltage goes to 0 V. Then the cycle is repeated, and so on. The evaporation rate is assumed to be negligible between two siphoning events. The water column height, and thus precipitation amount collected at any given time, ranges between 0 to 50mm precipitation, with a detected accuracy of 1mm. A self-contained thermostatically controlled heater allows the gauge to operate at temperatures as low as -20°C. Therefore, when snow is encountered, it is melted so that an equivalent amount of rainfall is measured. The precipitation gauge is installed on the roof of the main bridge, on a small mast.
Position information for the vessel, and thus all the environmental measurements, is provided by a Garmin, Int. (GPS 17-N) Global Positioning System (GPS) sensor. The is primarily designed for marine applications. It is well adapted to harsh environments with the sensor’s antenna housed in a rugged waterproof design. It has a 12-channel receiver that allows for continuous tracking of all visible satellites. On average, the accuracy of the position given by this GPS is less than 3 meters. In addition, to providing position information for the vessel, this unit also provides a “differential” GPS capability which provides an absolute measure of the vessel’s speed. Having such information is necessary in order to correct the wind speed observations for the movement of the vessel itself. The GPS sensor is installed onto a small mast on the roof of the main bridge.
Heading information for the vessel is provided by a KVH C100 fluxgate compass. The sensor can operate under temperatures ranging between -40° and +65° with a resolution of 0.1° and a maintained accuracy of 0.5°. Heading information for the vessel is necessary in order to correct the wind speed observations for the movement of the vessel itself. The compass is installed onto a small mast on the roof of the main bridge.
Air Temperature and Humidity
Near-surface atmospheric temperature and relative humidity are measured by sensors that are combined into a single unit, made by Rainwise, attached to each bridge wing. Having two separate units provide redundancy in case of instrument failure. In addition, during most periods when both units sensors are operational, the measurements from the upwind sensors are the ones that are reported and used in subsequent analysis since those are the ones least influenced by the vessel. Both sensors along with a fan are mounted in the unit’s white PVC housing. The temperature sensor is a combination of thermistors and thermocouples. Temperature measurements have a resolution of 0.1°C and accuracy of 0.3 °C for a range between –50°C and +50°C. Humidity is measured using a thin film polymer capacitor mounted within a porous plastic filter. The measurements have an expected accuracy of 2% over the range 0 to 100 % and in temperatures ranging between –40°C and +85°C.
The crew display provides current atmospheric conditions for the crew of the PT Barnum. Data includes temperature, relative humidity, pressure, precipitation and wind. Furthermore, they will be given access to the latest watches and warnings from the National Weather Service, as well as a current doppler radar in order to foresee potentially hazardous weather.
This chlorophyll fluorometer can provide measurement of the chlorophyll fluorescence within natural waters at a depth rated to 600 meters. The fluorometer comes in an all plastic, corrosion-resistance housing well adapted to marine environement. Knowing that chlorophyll fluorescence is an indicator of phytoplankton concentrations, the use of the fluorometer gives an estimation of biological activity in the water column. The WetStar was designed to be particularly robust to light fluctuations. This instrument operates under an excitation wavelength of 470 nm and an emission wavelength of 685 nm. Its optical sensitivity is 0.03 g/l, in a dynamic range of 0.03 to 75 g/l. It will be mounted on seawater intake system.
The salinity sensor provides high accuracy conductivity, temperature, salinity and sound velocity measurements with direct digital output. The sensor utilizes FSI’s inductive conductivity sensor in conjunction with their new patented EXCELLÒ circuit technology to offer a high-accuracy in addition to low-power sensor requirements.
The main processing and communications computer is located inside the passenger’s deck to the left of the ticket counter.
The passenger display presents a repeating 6 minute movie that describes the motivation for the project, the sensors and their placement, provides some example data and a description of why such data is useful, and highlights and thanks the project’s sponsorship.
The passenger display is located inside the passenger deck to the left of the bursars’ widow. The picture above shows the location before the installation of the passenger display.
The passenger display has three panels within the viewing area. The left panel shows updated information from the observations (e.g., position, temperature, wind speed). The upper right panel shows the movie described above. The lower right panel shows the position of the vessel on a map of the Long Island Sound.
Sea Surface Temperature
Surface temperature is monitored in two different ways. In one case, the temperature is measured directly from the sea water intake (see left and middle photos below) just after it enters the vessel. Thus this value measures water at a depth of approximately 2 meters below the surface. In the second case, the temperature of the hull near the water line is measured with a temperature sensor pressed against the port side of the hull (see left and right photos below).
Sea Water Intake
Surface water properties are obtained by pumping near-surface (~2m) water from below the hull, through a small (~2in) sea chest, up to an instrument package on the car deck that includes a debubbler, salinity, dissolved oxygen and chlorophyll sensors. Installation of sea water intake was performed during the P.T. Barnum’s dry dock period.
Acoustic Doppler Current Profiler
ADCPs measure water motion by transmitting sound at fixed frequency. The instrument measures the Doppler-shifted echoes backscattered from scatterers in the water column (e.g., plankton and sediment) and converts the echoes to along (acoustic-) beam velocity components. The ADCP then converts the along beam velocities to north/south, east/west, and vertical velocity components. Velocity profiles are determined by range gating the echoes so that velocities are determined at preset distances along the acoustic path (called bins).
The instrument used in this application, the RD Instruments Workhorse Mariner, is specifically designed for underway current profiling from inshore hydrographic survey vessels. The head of the transducer, which is made of molded composite plastic consists of four beams at 20° from vertical in a convex configuration. The electronics of the instrument also integrate temperature sensors, a fluxgate compass and pitch and roll sensors. It works at a nominal frequency of 600kHz to give a water velocity profile to depths of 70 meters with a vertical resolution of about 4 meters. The ADCP is equipped for bottom tracking capability to measure the ADCP speed and direction over ground. It operates under temperatures ranging from –5°C to 45°C to measure velocities in a range between 5mm/s and 20m/s with an accuracy of 2.5 mm/s.