Lecture 1: Meteorological Instruments and Methods of Observation PDF

LECTURE 01 Principles of Instrumentation and Measurement  Principles of instrumentation and measurement Meteorological Observation Meteorological (and related environmental and geophysical) observations are made for a variety of reasons. Examples:...

LECTURE 01 Principles of Instrumentation and Measurement  Principles of instrumentation and measurement Meteorological Observation Meteorological (and related environmental and geophysical) observations are made for a variety of reasons. Examples: 1. used for the real-time preparation of weather analyses, forecasts and severe weather warnings 2. for the study of climate 3. for local weather dependent operations (for example, local aerodrome fling operations, construction work on land and at sea), 4. for hydrology and agricultural meteorology, and 5. for research in meteorology and climatology.  Principles of instrumentation and measurement Meteorological Observation Knowledge of the principles of instrumentation and the characteristics of the particular instruments can help make sure that the measurements are meeting scientific goals. many research projects today are to validate satellite or radar retrieval algorithms and to develop better algorithms to ensure the relationship between the instrument response and the natural signal is robust. An instrumented tower measuring several atmospheric variables sits high above treeline in the Rocky Mountains measuring several atmospheric variables.  Principles of instrumentation and measurement Meteorological Observation Before measurements are made, careful evaluation of the instrumentation, its characteristics, and its uncertainty are very important to ensure that the observations are accurate and precise. Selecting a suitable instrument based on its characteristics is critical because it is vital that those characteristics are able to actually represent the phenomenon that is intended to be measured. Understanding instrument characteristics and applying them in appropriate ways plays a vital role in accurately observing changes in the physical environment.  Principles of instrumentation and measurement Types of Observation Measurements aim to determine the state of a system through direct and/or indirect measurements. Direct measurements are those that measure mass, length, and time directly. example, a balance measures the mass of an object.  Principles of instrumentation and measurement Types of Observation Indirect measurement is a technique that employs a device to make a measurement which is then interpreted. Examples are (1) an anemometer used to measure wind speed or (2) using the height of a column of alcohol in a liquid-in-glass thermometer to infer temperature. Nearly all meteorological measurements are indirect and are based on how the atmospheric variable of interest influences a sensor.  Principles of instrumentation and measurement Types of Observation For each of the following observation types, select whether the measurements is direct or indirect. 1. Instrument aboard Earth-orbiting satellite 2. Basic rain gauge with inner and outer tube 3. A weather radar system  Principles of instrumentation and measurement Types of Observation In-situ measurements are obtained through direct contact between the sensing device and the medium, material, or object. A mobile balloon-borne radiosonde is an example of an in-situ sensor.  Principles of instrumentation and measurement Types of Observation Remote sensing observations are obtained without direct contact between the sensing device and the substance. Remote sensing can be used to yield atmospheric profiles, scans, and images at a variety of wavelengths to extract information about the atmosphere that cannot be obtained by in-situ methods. The S-band and Ka-band S-PolKa Radar deployed in the Maldive Islands for a study of the Madden-Julian Oscillation, a large-scale intraseasonal variability initiated in the Indian Ocean.  Principles of instrumentation and measurement Types of Observation Remote sensing is a measurement of electromagnetic radiation that has interacted with the atmospheric variable being observed which can be then used to infer information about the atmospheric variable.  Principles of instrumentation and measurement Types of Observation Active remote sensing devices are electromagnetic and acoustic devices that rely on a transmitted pulse and a backscattered return signal, which is converted to a meteorological variable during signal conditioning and processing (e.g., radar, lidar and sodar).  Principles of instrumentation and measurement Types of Observation The NCAR High Spectral Resolution Lidar actively sends and receives a lidar signal used to detect the presence of scatterers in the atmosphere and determine the full (Mueller) backscatter phase matrix.  Principles of instrumentation and measurement Types of Observation Passive remote sensing does not transmit a signal, but instead relies on the emission or reflection of electromagnetic radiation from a source, which is detected by a sensor, processed, and displayed. Passive remote sensors facilitate measurement from a distance without direct contact with the environment to be measured. An example of passive sensing is an infrared temperature probe (e.g., infrared thermometer).  Principles of instrumentation and measurement Types of Observation  Principles of instrumentation and measurement Types of Observation A dropsonde is an instrument package that is launched out of an aircraft to measure a profile of temperature (green), relative humidity (blue), and wind speed (red) as a function of pressure [hPa]. Data are transmitted back to the aircraft in real-time.  Principles of instrumentation and measurement Types of Observation For the following sensors for measuring temperature identify the appropriate category (In Situ vs. Remote). 1. Liquid-in glass thermometer 2. Sonic devices 3. Thermocouple 4. Radiometric thermometer Many remote sensing devices are designed with a sensitivity to a particular spectral bandwidth so that the selective absorption characteristics can be used to provide further information about the environmental condition.  Principles of instrumentation and measurement Types of Meteorological Observing Systems Meteorological observing system – a set of instruments assembled for simultaneous use, either to extend the spatial coverage of measurements or to collect measurements of more comprehensive groups of atmospheric properties for use together. Examples include arrays of similar sensors located over large areas to characterize low-level wind, temperature, and pressure fields, or research aircraft where groups of many different instruments are included for comprehensive studies of air chemistry. Instrument makes a single measurement whereas an observing system collects a set of related measurements.  Principles of instrumentation and measurement The World Meteorological Organization coordinates global atmospheric and ocean observations from a range of different stations and types of sensors.  Principles of instrumentation and measurement Types of Meteorological Observing Systems Broad groups of observing systems include: a. surface networks consisting primarily of in-situ sensors; b. profiling networks consisting of remote sensors of wind, temperature, and (emerging) water vapor; c. balloon-borne sensors measuring temperature, pressure, humidity and wind as they ascend after release at many locations simultaneously or sensors (dropsondes) that fall through the atmosphere after release by an aircraft; d. radar networks with overlapping coverage to measure wind and radar reflectivity over large areas; e. research aircraft with combinations of in-situ and remote sensors, increasingly supplemented by unmanned aircraft systems (UAS) that can be deployed in tandem or for extended coverage; f. satellite systems of remote sensors showing various properties of the atmosphere and clouds on near-global scales  Principles of instrumentation and measurement Surface-based Networks Surface-based networks can be operated manually or automated, and located on land or offshore on ships and buoys. Observing stations can measure the wind, pressure, temperature, humidity and precipitation near the surface. In some cases, these observations are complemented by measurements of radiative fluxes, soil characteristics, or other variables. A ground-based Integrated Surface Flux System set up at the base of the Sierra Nevada mountains in California.  Principles of instrumentation and measurement Surface observing stations that report regularly at fixed times and provide the observations from which surface weather maps are constructed.  Principles of instrumentation and measurement Profiling Networks Many systems exist to measure vertical profiles of the atmosphere. Examples are wind profilers that continuously measure wind direction and speed, rawinsonde facilities that can be deployed for special research purposes, light aircraft and drones that can climb and descend to measure profiles, and towers and tethersondes. A technician launches a rawinsonde from a ship in the Indian Ocean to collect temperature, pressure, humidity and wind data to learn more about the Madden-Julian Oscillation.  Principles of instrumentation and measurement Profiling Networks Profiling system have the longest history, during the 1930s the expansion of economically important government weather forecasting services and their increasing need for data motivated many nations to begin regular radiosonde (or rawinsonde) observation programs Rawinsondes collect observations of temperature, humidity, and wind as functions of pressure and/or height in the atmosphere in twice-daily “synoptic” soundings at 00 and 12 Coordinated Universal Time (UTC).  Principles of instrumentation and measurement Soundings provide the essential data for weather maps and forecasts, either via analysis of the patterns or through their input to computer models of the atmosphere.  Principles of instrumentation and measurement Radar Networks Radar (radio detection and ranging) is one of the most valuable tools available to the weather forecaster. Radars transmits electromagnetic pulses of energy to the target the it will be reflected and scattered off by particles (target) in the atmosphere, including precipitation, clouds, airplanes, birds and insects. Some of the energy returns to the radar and is received by the antenna and receiver which are processed to determine information about the scatterers in the atmosphere. Scanning Doppler weather radars can measure radial velocities, the flow of wind and precipitation toward or away from the radar and measures also the reflectivity which is typically converted into precipitation rate and rainfall accumulation fields which directly impact society. Principles of instrumentation and measurement  Principles of instrumentation and measurement Radar Networks Radar networks have been established around the world. A radar network database is maintained by the WMO and is important to assist with the international exchange of radar data and to protect radio-frequency spectrum allocation.  Principles of instrumentation and measurement Aircraft Observing Systems A lot of research aircraft now carry comprehensive sets of instruments and provide the ability to install new instruments for specialized studies. Many commercial aircraft also report weather observations during flight using systems called ACARS (Aircraft Communication Addressing and Reporting System) in the U.S. or AMDAR (Aircraft Meteorological Data Relay) internationally.  Principles of instrumentation and measurement Satellites Satellite-borne sensors have the unique vantage of observing Earth and its atmosphere from orbit using remote sensing techniques. Low-Earth Orbiting (LEO) satellites have the capability of in-situ sampling of the exosphere but can only probe all other layers of the atmosphere and Earth’s surface via remote sensing techniques. Geostationary satellites use passive sensing to make measurements of electromagnetic energy emission from the ground or atmospheric layers, while LEO satellites use both passive sensors and active sensors.  Principles of instrumentation and measurement Polar and Geostationary Satellites  Principles of instrumentation and measurement Satellites Satellite sensors require in-situ measurements from rawinsondes and other ground- based or aircraft systems for validation, calibration and algorithm development to convert the signal to meaningful atmospheric variables.  Principles of instrumentation and measurement Instrument and Measurement Considerations It is also equally important to consider to where and when the measurements will be made. Foremost in the selection of a site for an observing system is knowledge of the requirements needed for measurement representativeness. Representativeness of an observation is the degree to which it accurately describes the value of the variable needed for a specific purpose.  Principles of instrumentation and measurement Surface stations representative of the research environment set up near the North Pole during the Arctic Ocean Expedition.  Principles of instrumentation and measurement Worldwide Guidelines Listed here are basic considerations for site selection and instrument exposure as suggested by the WMO. 1. Surface ground cover should be representative of the location 2. Outdoor instruments should be installed on level ground 3. Sharp topographical gradients in the vicinity should be avoided 4. Sites should be well away from obstructions such as buildings, walls, and trees to avoid interference 5. For most sites, the preference is for openness and homogeneity but not for every instrument; for example, rainfall amount can be significantly affected when rain gauges are exposed to wind of even moderate speed  Principles of instrumentation and measurement Worldwide Guidelines 6. Instruments used for observing sunshine, radiation, clouds, cloud coverage, sky condition, and visibility should be as open as possible and command the widest possible view of the sky and the surrounding area 7. External lighting should be avoided or dimmed for nighttime observations such as clouds or visibility 8. Estimation of wind speed and direction are best performed outside of an enclosure 9. Site locations must be physically, politically, and economically accessible and have adequate infrastructure (e.g., roads, power, communication, staff accommodations) 10. Sites that will be used for global climate monitoring must be a sufficient distance from urban environments, for example, as not to be affected by the heat island effect Principles of instrumentation and measurement Layout of an observing station in the northern hemisphere showing minimum distances between installations  Principles of instrumentation and measurement Measurand Measurand is the measured property of the atmosphere. The desired output from the instrument represents this property, which may be a. some atmospheric state property like temperature or pressure, b. a more specialized characteristic like the size distribution of cloud droplets or some measure of the energy in radiation passing through the atmosphere, or c. a property derived from compound properties (like radar reflectivity).  Principles of instrumentation and measurement Measurands can be any of a variety of measured properties of the atmosphere including radiative energy from the Sun or reflected from Earth's surface, temperature, pressure, or cloud thermodynamics.  Principles of instrumentation and measurement Key Components of an Instrument Instruments used in studies of the atmosphere can be characterized by what they measure and by the technological components they incorporate. A broad characterization of an instrument needs to include three basic components: 1. Sensor 2. Data acquisition/signal conditioning 3. Display/recording system  Principles of instrumentation and measurement Key Components of an Instrument Sensor provides an output, usually an electrical property like voltage or resistance, that has a known relationship to the atmospheric property being measured. A transducer is a special form of sensor that converts from one form of energy to another, like a photoelectric cell. Usually the output of a sensor is voltage and the input is a property of the atmosphere.  Principles of instrumentation and measurement Key Components of an Instrument Sensor usually includes these components: 1. Sensing element. This is the component of the sensor that has some measurable property dependent on the atmospheric property being measured. 2. Exciter. If necessary, this component produces a voltage or current dependent on the sensitive property of the sensing element. 3. Signal processor. It is often desirable to reduce noise in the electrical signal, amplify to levels suitable for recording, and process the signal to provide a linear relationship to the atmospheric property being measured.  Principles of instrumentation and measurement Key Components of an Instrument A display and recording system usually consists of two components: 1. Data display. Usually an indication of the property being measured is displayed by a meter or digital display, although sometimes this is not available except through the recording system. 2. Data recorder. While recording can be simply an observer writing down the displayed output, more common and useful is a recording system that automates this process. 3. In most modern cases, the recording system converts the sensor output to a digital number through use of an analog-to-digital converter (frequently abbreviated ADC) and then transmits that digital number to a computer where it is recorded along with a time reference to give a recorded history of the measurement.  Principles of instrumentation and measurement Key Components of an Instrument Write each label to its correct place on the block diagram of an instrument. data display sensor signal processor exciter data recorder  Principles of instrumentation and measurement Performance Characteristics of Instruments It is desirable to incorporate some assessment of the quality of the measurement, perhaps by tests against expected limits, noise levels, or comparison to other instruments. When using instruments, it is also important to understand A software engineer checks the data how the instrument is logging system on a meteorological calibrated and the uncertainty station deployed as part of an Integrated associated with the Sounding System. measurement.  Principles of instrumentation and measurement Performance Characteristics of Instruments Important requirements for meteorological instruments deployed for autonomous operation are (WMO-No. 8): 1. an instrument uncertainty that remains relatively low and uniform over long periods between calibrations; 2. reliability in that the instrument can be expected to stay within its operational parameters, 3. stability or that the instrument does not drift much between calibrations; 4. convenience of operation, calibration and maintenance; 5. simplicity of design; 6. durability over a full range of possible environmental conditions; 7. reasonable cost of the instrument and consumables, and availability of spare parts  Principles of instrumentation and measurement Error and Uncertainty An error in measurement is the difference between the measurement and the value of the measurand. Measurement: A set of operations having the objective of determining the value of a quantity. Result of a measurement: Value attributed to a measurand (the physical quantity that is being measured), obtained by measurement.  Principles of instrumentation and measurement Error and Uncertainty Error is usually unknown; instead the "uncertainty" of a measurement characterizes the dispersion of the values reasonably attributed to a measurand on the basis of the measurement and characteristics of the instrument. A measurement may by chance have small error even when the uncertainty is large, but errors are expected to lie within the dispersion specified by the uncertainty.  Principles of instrumentation and measurement Types of Error Error contributions associated with an instrument or measurement system can be classified as systematic or random. Systematic error is the mean error that would result from averaging a very large number of measurements of the same measurand carried out under the same conditions. The value of this systematic error is also called the “bias” of an instrument and it can be reduced by calibration, which may indicate a correction to be applied to the measurement.  Principles of instrumentation and measurement Types of Error Random error is thee repeated set of measurements leading to the mean will themselves have some scatter about the mean. The error in a given measurement is the sum of the systematic and the random error, and the latter can be estimated, for example, from the standard deviation of a set of repeated measurements. 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 𝑒𝑟𝑟𝑜𝑟 = 𝑠𝑦𝑠𝑡𝑒𝑚𝑎𝑡𝑖𝑐 𝑒𝑟𝑟𝑜𝑟 (𝑜𝑟 𝑏𝑖𝑎𝑠) + 𝑟𝑎𝑛𝑑𝑜𝑚 𝑒𝑟𝑟𝑜𝑟  Principles of instrumentation and measurement Types of Error “Operator errors” can be systematic (e.g., an observer consistently but incorrectly reading the top of a meniscus) or illegitimate (e.g., recording an incorrect value) and can be the most costly of all errors because they are so hard to identify or characterize.  Principles of instrumentation and measurement Types of Error Precision characterizes random error but should be used with an explanation of the cited quantity. A measurement can have low scatter about the mean, as described by precision, and yet have large error arising from a systematic error, so a precise measurement does not necessarily have small error.  Principles of instrumentation and measurement Sources of Error Errors may arise from: 1. incorrect calibration (a static error), 2. from a time lag or hysteresis in the measurement (a dynamic error), 3. from drift (change in the calibration with time), or 4. from exposure (where a sensor may not be coupled properly to the atmospheric property being measured). Drift is a time-dependent error caused by, for example, physical change in a sensor over time and must be corrected by repeated calibration. Instrument stability provides an estimated limit to the expected drift; the lesser the drift, the more stable the instrument.  Principles of instrumentation and measurement Calibration Calibration is the process of correlating an observation from one instrument with an observation from another instrument of known quality; these are called standards. Calibration is essential to understanding an instrument and observation and is one of the most important aspects of operating instruments. A well-designed research project must incorporate calibration before, during, and after gathering data. Principles of instrumentation and measurement The weather parameters measured by the AWS was compared to the weather parameters measured from the nearby Synoptic and Agrometerological Weather Station of the Philippine Atmospheric, Geophysical and Astronomical Services (PAGASA) in Central Luzon State University (CLSU).  Principles of instrumentation and measurement Meteorological Variables “State variables” - basic parameters that specify the properties of a system, in the case of meteorology, a parcel of air. The required number of state variables depends on the components to be considered in the parcel. For example at the simplest level, the “intensive” state variables (those not dependent on volume) for a parcel consisting only of “air” (i.e., a standard mixture of nitrogen and oxygen) can be specified fully by two state variables: pressure and temperature.  Principles of instrumentation and measurement Meteorological Variables Other quantities like air density are dependent functions of these two state variables (pressure and temperature), based on the ideal gas law. 𝑝 = 𝜌𝑅𝑑 𝑇 𝑝 – pressure 𝜌 – air density 𝑅𝑑 - specific gas constant for dry air 𝑇 – temperature  Principles of instrumentation and measurement Meteorological Variables The subset of the state variables to be measured will depend on the nature of the problem being studied. For example, if water vapor is also considered, another state variable is required, and if the water can exist in liquid, gaseous, and solid forms then two more state variables are needed. Additional state variables may be needed to describe trace-gas constituents of the atmosphere, the electromagnetic radiation entering or leaving the parcel, and the motion of the air parcel relative to Earth.  Principles of instrumentation and measurement Meteorological Variables All these may vary with location, so a full specification of the atmospheric state would require specifying these state variables as a function of position and time. Temperature varies with time and location. This map shows the annual mean daily temperature range across the U.S.  Principles of instrumentation and measurement Meteorological Variables Also, the atmosphere is a dynamic fluid, always in motion thus, the evolution of the state variables depends on the mean, or steady state, and turbulent, or fluctuating, exchanges of mass, energy and momentum that force local changes in these parameters. These continual motions and exchanges are why measurements over time are so necessary for studying the atmosphere. END OF TOPIC