6 Advanced Meteorological Instruments and Measurements

Sunayan Saha

1. Learning outcomes
2. Introduction
3. Advanced sensors / instruments for routine meteorology
3.1. Recording rain gauge
3.2. Digital anemometers
3.3. Radiation instruments
3.4. Temperature sensors
3.5. Relative humidity sensors
3.6. Automatic weather station (AWS)
4.0. Measurements for advanced environmental studies
4.1. Aerosol, ozone and water vapour monitoring
4.2. Cloud measurement
5.0. Instruments for heat and gas flux measurements
5.1. Sonic anemometer
5.2. Large aperture scintillometer (LAS)
5.3. Bowen ratio system
5.4. Eddy covariance system
6. Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• know various electronic automated sensors and their working principle for measurement of routine meteorological parameters
• know various state-of-the-art instruments used in atmospheric science research

2. Introduction

Better theoretical understanding on various atmospheric processes calls for careful analysis of good quality observational datasets on various meteorological parameters. Often the requirement is to generate such data at a considerably high temporal frequency such as at hourly or sub-hourly intervals both during day and night, under difficult atmospheric conditions such as during a storm, and from a very large number of observational sites including inaccessible terrains or deep sea. Meeting this type of requirement with conventional manually operated meteorological instruments is not possible.

Improvement in engineering, particularly in areas of electronics, information technology, computational tools and space technology has made possible collecting meteorological data unbiased, unattended and at high spatiotemporal frequency. With these advances in meteorological instrumentations, scientific community now says with greater conviction about important atmospheric phenomena of recent times such as global warming / deeming and climate change. Advanced instruments also cater to various application needs of the society such as forecasting of weather and climate with greater accuracy. The advance instruments for routine meteorological monitoring and high end atmospheric research are discussed in the following sections.

3. Advanced sensors / instruments for routine meteorology

1. Recording rain gauge

Most rain gauge stations and automatic weather stations in the country have tipping bucket type recording rain gauge. In this type, rain first enters a cylindrical receiver, then funneled through a debris-filtering mesh screen, and collects below in a bucket or compartment. When water accumulated in the bucket equals to a certain amount (generally, 0.25 mm of rain i.e. the increment in which the receiver measures), the bucket tips over and another similar takes its position beneath the funnel. Each time a bucket is emptied it actuates an electrical circuit which is detected by a data logger and/or  telemetry system. By adjusting stainless steel screws under each bucket the instrument can be calibrated. This type of gauge is not suitable for measuring snow.

Fig. 1: Tipping bucket type rain gauge (left: schematic drawing; right: a commercial model)

2.  Digital anemometers

Anemometers consisting of horizontally rotating cups are the simplest and conventional technique for measuring wind speed. Nowadays many small compact electronic devices have come up. Two examples are described below:

Vane anemometers: These work in a similar fashion like that of a cup anemometer. However, instead of horizontal rotation, there is a propeller which spins vertically like in a windmill. As the rotation is vertical, the propeller needs to face the direction from which the wind is coming. Vane anemometers (Fig. 2) are electronically operated and the propeller spins are recorded and translated into wind speed.

Hot-wire anemometers: The main parts of of a hot-wire anemometer are conducting wires and a wheat stone (resistance) bridge (Fig. 3). The basic principle of its operation is when an electrically heated wire is placed in a flowing stream of gas, heat is transferred from the wire to the gas resulting in a decrease of temperature of the wire and a corresponding change in the resistance of the wire. By calibrating the relationship between the flow rates and the changes in resistance of the wire, wind speed is measured. There are two methods of measuring flow rate using an anemometer bridge combination, viz., constant current method and constant temperature method. This type of anemometer provides accurate readings, often comes attached with data logging devices to store data and hence highly recommended for professional and industrial use.

3.3. Radiation instruments

Principle of radiation measurement: The measurement of radiation includes measuring the quantity (intensity or flux density), quality (wavelength range) and duration. Generally, such instruments are termed as radiometers. The principle of operation is the same for all variants of radiometers i.e. a signal (electric current/voltage) is produced because of differential absorption of solar radiation by two different surfaces like between a black and white surface, two black surfaces, or a blackened surface and the base of the instrument. The sensors/detectors convert the irradiation into a different form of energy as shown in table 1.

Thermal sensors are generally used for non-selective measurements of radiant energy over broad wavelength ranges. Photoelectric sensors are more useful for spectral measurements i.e. measurement over narrow wavelength bands. For meteorological purposes, thermal sensors are used normally. Mostly, these are based on the principles of thermocouples and resistance thermometers.Different instruments that are used to study various aspects of radiation are described below:

Pyrheliometer: Also known as tube solarimeter, pyrheliometer is used to measure direct component of solar irradiation at normal incidence, in the spectral range of 0.3-15 µ and for intensities 10-200 mw cm-2. The sensor is mounted at one end of a long metallic tube provided with a series view limiting diaphragms. The front aperture is usually sealed with optical glass or quartz. A sun tracking unit helps in aligning the sensor with the direct beam of the sun so that radiation always reaches the sensor element at normal incidence angle. The viewing diaphragm limits the aperture to 5o. Thermoelectric pyrheliometers use thermopile sensors, mostly made of copper-constantan. Another type known as Angstroms compensation pyrheliometer use two blackened strips as sensor.

Pyranometers: A pyranometer (Fig. 4) can measure solar radiation in the spectral range 0.3-4 µ. Multijunction thermopile is generally used as the sensor. The sensor is coated with optical black and is mounted on a chromium plated brass case having concentric pair of glass domes. When exposed, the blackened surface rises in temperature and produces thermal e.m.f which is measured on a recording millivolt meter or fed to a datalogger. When a pyranometer is facing up i.e. towards the sky, it gives a measure of global radiation i.e. direct solar radiation plus diffuse/sky radiation (radiation scattered by gas molecules, water vapour, dust, aerosols etc.). To measure diffuse radiation alone, a semi-circular shadow ring can be attached over the pyranometer along with the sun tracker. This arrangement prevents the direct solar beam from falling onto the sensor. Such variant is called as shaded ring pyranometer. There is another type, in which two pyranometers are kept in one assembly, one facing the sky (for measuring the incoming shortwave radiation) and the other facing the ground (for measuring the reflected shortwave radiation). This type is called albedometer or reflectometer as albedo/shortwave reflectivity of a surface can be measured from the above pairs of data measured simultaneously.

Quantum sensors: Quantum sensor is used to measure the PPFD i.e. photosynthetic photon flux density which is the part of the shortwave irradiance known as photosynthetically active radiation, PAR (0.3-0.7 µ). Silicon photodiode with an enhanced response in the visible wavelength (0.4-0.7 µ) is used as the sensor. A point quantum sensor is used to measure PPFD over an uniform location such as on top of a canopy whereas for spatially non-homogeneous region like within a crop canopy, line quantum sensor is used.

Pyrgeometer: A pyrgeometer (Fig. 5) can measure the terrestrial radiation (atmospheric and earth radiation). Like in pyranometers it measures radiation on a horizontal plane surface from a hemispherical sky or 2Π solid angle but instead of shortwave it measures the long wave or infrared radiations (4.5 – 100 µ). A pyrgeometer has a thermopile sensor (about 250 copper-constantan junctions) covered with a silicon dome or window with a solar blind filter coating that has a transmittance between 4.5 µm and 50 µm to exclude shortwave radiation. There is a sun shield to minimize heating of the instrument due to solar radiation and an internal body temperature sensor near the cold junctions of the thermopile. The thermopile detects the net radiation balance between the incoming long wave radiation received by it and long wave radiation flux emitted from it, and subsequently converts the difference into voltage.

An Angstrom pyrgeometer consists of four manganin strips, of which two are blackened and two are gold plated. The blackened strips are allowed to radiate to the atmosphere while the gold plated strips are shielded and remain at air temperature. The difference in temperature is sensed by the copper-constantan thermocouple attached at the back of each strip. The strips which are at lower temperature are heated by passing current and the electrical power required to equalize the temperature of the four strips is taken as a measure of the terrestrial radiation.

Pyrradiometer: This measures total radiant energy in the wavelength region 0.3 – 100 µ either in upward or downward direction. Thus it can measure SW i.e. shortwave (0.3-3 µ) as well as LW i.e. longwave (3-100 µ) terrestrial radiation. Net pyrradiometer is one type which is used to measure the difference between the total downward radiation (sum of solar and terrestrial) and the total upward radiation. The net pyrradiometers are uniformly painted black on the two sides – the surface facing up receives downwelling energy and the one facing down receives the upwelling energy. By proper electrical connections, the outputs of the two surfaces can be measured individually as well as the differential outputs between the two surfaces.

     3.4. Temperature sensors

Soil and air temperature measurements can be done by both contact probes and remote devices.The sensors of these instruments / thermometers could be thermocouples, resistance temperature detectors (RTDs), thermistors, semiconductor and infrared sensors. Thermocouple and thermistor type thermometers are most widely used. Platinum resistance thermometers are highly accurate and stable in performance, although the costs are relatively high.

Functioning of the non-contact temperature probes, also known as infrared thermometers (Fig. 6), is based on the black body radiation principle. The instrument works out the temperature of the object, as per Stefan-Boltzmann law (eq. 1), by detecting the radiation energy emitted from the object. Most infrared thermometers are responsive in the 8-14 µ atmospheric window region where there is least attenuation of infrared signal due to water vapour absorption. Such thermometers are often used to detect stress by measuring canopy-air temperature differences and for crop water management.

Based on the parameter which is used for measuring humidity, these sensors can be classified into the following categories:

Capacitive humidity sensors: The function is based on the changes in electrical permittivity of a dielectric material with change in humidity. The common method of constructing a capacitive RH sensor is to use a hygroscopic polymer film as dielectric and embedding two layers of electrodes on the either side (Fig. 7)

Resistive humidity sensors (electrical conductivity sensors): These are usually made up of materials with relatively low resistivity and this resistivity changes significantly with changes in humidity. The low resistivity material is deposited on top of two electrodes. The resistivity between the electrodes changes when the top layer absorbs water and this change can be measured with the help of a simple electric circuit. Some of the commonly used low resistivity materials are salt, specially treated substrates, solid polyelectrolytes and conductive polymers. The electrodes in the sensor are usually made of noble metals like gold, silver or platinum.

Thermal conductivity humidity sensors: This type of sensor (Fig. 7) measures the absolute humidity. The sensors measure the thermal conductivity of both dry air as well as moist air. The difference between the individual thermal conductivities can be related to absolute humidity. Often, two tiny thermistors with negative temperature coefficient are used to form a bridge circuit. In that, one thermistor is kept in an air-tight chamber filled with dry nitrogen while the other is exposed to outside environment through small vent holes. When the circuit is powered on, the resistance of the two thermistors are calculated and the difference between those two values is directly proportional to absolute humidity.

3.6.        Automatic weather station (AWS)

An AWS (Fig. 8) is essentially comprised of some electronic sensors to record meteorological parameters and data logger for storage of data. Sensors for wind speed (cup anemometer with interruptor), wind direction (potentiometer), humidity (capacitive sensor), air temperature (PRTD thin film), rainfall (tipping bucket), barometer (transducer), insolation (pyranometer), soil temperature (thermocouple or thermistors) are commonly used. Often special observations like on net radiation, leaf/surface wetness, UV (A & B) radiation, soil moisture, soil heat flux, air quality (SOX, NOX, COX concentrations) and sea water temperature are also recorded using appropriate sensors. A hardware framework, generally in the form of a tower or tripod, to mount the sensors at different heights, battery, solar panels and suitable enclosures are also parts of a weather station.

Data logger is like a combination of a micro-computer, clock, multimeter, calibrator, scanner, frequency counter, controller and signal generator all in one small box. It has multiple channels for connecting various types of sensors. Inside the datalogger there are software(s) installed which is programmed in such a way so as to instruct the sensors the sequence and interval of data collection, its storage in memory and during data retrieval or remote communications through radio, telephone or satellite links. Often it is programmed to compute and display values of parameters that are not directly recorded such as evapo-transpiration. Nowadays, portable automatic weather stations are also in use.

4. Measurements for advanced environmental studies

• 1. Aerosol, ozone and water vapour monitoring

The concentration of some selected gases and aerosol in earth’s atmosphere can be studied using a sun photometer (Fig. 9). This is just like any other photometer but designed to measure direct beam of sun radiance. The instrument is equipped with accurately aligned optical collimators, internal baffles to eliminate internal reflections, spectral filters, narrow-band interference filters associated with each filter, a photodiode suitable for the particular wavelength range and a data acquiring system. A very important part is also the sun tracker (either manual or automated). When the image of the sun is centered in the bull’s-eye of the sun target, all optical channels are oriented directly at the solar disk. A small amount of circumsolar radiation is also captured, but it makes little contribution to the signal. Radiation captured by the collimator and band pass filters radiate onto the photodiodes, producing an electrical current that is proportional to the radiant power intercepted by the photodiodes. These signals are first amplified and then converted to a digital signal by a high resolution A/D converter.

As the extra terrestrial radiation propagates through the earth’s atmosphere from top of the atmosphere (TOA), it gets attenuated due to absorption and scattering by various atmospheric gases and aerosols. By employing Beer’s law and Langley extrapolation the atmospheric effect can be known and removed and thus extraterrestrial radiance can be measured with ground-based sun photometer. If the signal at two or more suitably selected spectral intervals is measured, one can derive necessary information for computations of the vertically integrated concentrations of aerosols (Aerosol Optical Depth, AOD) and selected atmospheric gases such as ozone (Total Columnar Ozone), water vapour (Total Precipitable Water, TPW) etc. Example of commercially available models in the market includes 540 Microtops II™ sunphotometer, Dobson ozone spectrophotometer etc. Some wavelengths (nm) that are used by sunphotometers for turbidity studies are 340, 380, 440, 500, 675, 870, 936 and 1020.

4.2. Cloud measurement

Measurement of cloud base height is required for applications like aviation. Cloud base can be assessed using Tephi gram method but it needs temperature measurements with heights. However, Cloud base can be readily measured to a reasonable level of accuracy using a “Cloud base Recorder”. The type of cloud base recorder which is used widely at synoptic observing stations is shown in Fig.   It employs a pulsed diode laser LIDAR (light detection and ranging) technology whereby short laser pulses (eye safe) are sent out in a vertical or near vertical direction. The backscatter caused by reflection from the surface of cloud, precipitation or other particles is analyzed to determine the height of the cloud base. There are many such modern cloud base recorders which are designed to detect even up to three cloud layers simultaneously.

A sonic anemometer uses sound to measure wind speed. It sends a sound signal from a fixed transmitter to a fixed receiver, and by measuring the time the sound takes to arrive, the instrument can compute the speed of sound (Fig. 11). Wind speed will increase or decrease the speed of sound depending on whether it is a tail wind or a head wind. By measuring the speed of sound in both directions and from the difference of the two measurements the wind speed along that axis can be calculated. Different forms of sonic anemometers (1-D to 3-D and with different designs) are nowadays available. A 3-D sonic anemometer uses three pairs of orthogonally oriented, ultrasonic transmit/receive transducers to measure the transit time of sound signals. A pair of measurements is made along each axis ten times per second. The wind speed along each axis is determined from the difference in transit times. The sonic temperature is computed from the speed of sound, determined from the average transit time along the vertical axis. The sonic temperature in turn is an input for sensible heat flux computation.

5.2. Large aperture scintillometer (LAS)

The Large Aperture Scintillometer (LAS) system (Fig. 12) is used to obtain area integrated measurement of sensible heat flux (energy flow that increases the temperature of a body) throughout the diurnal period at desired time interval. When a signal, say a beam of electromagnetic (EM) radiation, passes through the atmosphere it gets attenuated. Addition or removal of sensible heat into or out of an air parcel causes fluctuations in its temperature and humidity leading to changes in the density and refractive index of the air as well. The changing refractive index is the main mechanism that leads to fluctuations in the beam signal intensity, a phenomena known as scintillations. A LAS works on the principle of measuring these scintillations and through a set of mathematical steps arrive at an estimation of sensible heat flux.

Bowen ratio (BR) system

Bowen ratio (ß), which is the ratio of sensible (H) to latent heat (LE) energy fluxes, is a mathematical method to calculate heat lost or gained by a surface using the concept of surface energy balance (SEB). The Bowen ratio technique required simultaneous measurements of air temperature and water vapour pressure in atleast two vertical points (separated by a distance of about 1 m) above the canopy but within the constant flux layer, and also requires measurements on net radiation and ground heat flux to compute any of the two turbulent fluxes (H & LE). The form of SEB and formulae for H & LE computation are given below:

5.4.        Eddy covariance (EC) system

The majority of transport between the soil or vegetation surface and atmosphere takes place mainly through the process of turbulence. An eddy covariance (EC) system (Fig. 14) essentially consists of fast response sensors, viz., a 3-D sonic anemometer and a gas analyzer capable of measuring at a very high frequency (generally 10 Hz or more). The system measures the statistical covariance between the vertical wind speed and an atmospheric property of the turbulent rotating eddies (e.g. temperature, humidity and partial gas concentration) whose flux (the quantity of an entity moving through a unit area per unit time) is to be determined within the atmospheric boundary layer. The mean vertical flux density of a gas (Fz) is approximately equal to mean air density (α) multiplied by the mean covariance between the instantaneous deviations of vertical wind speed (w) and mixing ratio (c) of the gas under consideration (i.e. ratio of the molecular weight of the gas divided by the weight of dry air) for a given time period such as 30 minutes or 1hour (Baldocchi, 2003).

In the above equation, the over bars denote time averaging and the primes represent fluctuations from average value. In a similar manner, other turbulent fluxes such as sensible and latent heat can also be computed. For example, in calculation of sensible heat flux, temperature and for latent heat flux, water vapour density have to be considered instead of mixing ratio.

While the sonic anemometer measures the wind speed vector at a very high frequency (10 Hz or more), the infrared gas analyzer measures water vapor density and carbon dioxide density by detecting the absorption of infrared radiation by water vapor or carbon dioxide in the light path. Two infrared wavelength bands are used, centered on strong water vapor or carbon dioxide absorption bands.

To obtain best measurements using EC technique proper site selection & positioning of the flux tower, appropriate height and orientation of sensors, fast response of sensors (10 Hz or above) and robust dataloggers are must. Flux data must be screened and or error corrected (e.g. for spikes and noise, frequency response error, sensor responses mismatch, density fluctuations error etc.) and gap filled (e.g. using techniques like mean diurnal variation, multiple regression, look-up table, non-linear regressions, multiple imputation, artificial neural networks and process based models) to correctly interpret the results.

6. Summary

The module can be summarized as follows:

• Advanced meteorological studies require good quality observational data sets at high temporal frequency and from high network density. Electronic sensors are free from human bias, capable to provide unattended measurement and log data for future analysis.
• Sensors that are mostly used in advanced radiation  instruments include thermopiles, blackened silver   disc,   bimetallic    strip,   silicon   photodiode  and   silicon   solar   cells.   For    electronic thermometers, thermocouples, thermistors, RTDs, semiconductors and for humidity, capacitive, resistive or thermal conductivity sensors are used.
• Automatic weather station (AWS) network data are nowadays extensively used for weather and environmental monitoring and forecasting services.
• For advanced studies such as energy and greenhouse gas exchanges between the earth surface and atmosphere, state-of-the art instruments like sonic anemometer, LAS, BR and EC systems are used. Sunphotometers are used for atmospheric turbidity monitoring.

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