1. Atmospheric InstrumentationM. D. Eastin Principles of Measurement and Instrumentation

2. Atmospheric InstrumentationM. D. Eastin Outline Principles of Measurement and Instrumentation Basics about Sensors and Instruments Instrument Performance Characteristics Measurement Standards Interpretation / Reporting of Measurements Preliminary Data Analysis

3. Atmospheric InstrumentationM. D. Eastin Components of a Measurement System: Instrument:A physical device or system, used to measure or monitor a measurand, that contains a sensor, an energy transfer device, an amplifier, a data display, and a storage device Parameter:Physical quantity to be measured (ex: pressure). Also called the measurand Sensor:Device which responds directly to changes in the measurand and produces an output signal. Typical output signals include: (1) mechanical deflection, (2) a rotation rate, (3) a voltage, (4) a resistance, or (5) a frequency Primary input: Desired (ex: barometer measures pressure) Secondary input: Undesired / unavoidable (ex: barometer sensitive to wind) Transducer:Device that converts an output signal to a more useful signal for convenient signal processing, display, transmission, and/or recording Analog signal:Sensor output that fluctuates continuously with the measurand Digital signal:Sensor output that fluctuates in discrete steps for a given small change in the measurand Basics about Sensors and Instruments

4. Atmospheric InstrumentationM. D. Eastin Components of a Measurement System: Instrument:A physical device or system, used to measure or monitor a measurand, that contains a sensor, an energy transfer device, an amplifier, a data display, and a storage device Amplifier:Device that magnifies a small output signal into a larger signal, that extends overa greater range, to better discern small changes in the measurand [ These devices are common in most instruments due to a number] [ of electrical engineering considerations (see Chapter 3)] Meter:Displays the “final” (converted and amplified) output signal - digital or analog Often converts the electrical signal (ex: voltage) into physical units (ex: °C) Recorder:Device employing a retrieval method whereby successive meter readings can be preserved (e.g., paper, film, computer hard drive) Transmitter:Device that transfers any electronically recorded data to another site via wired or wireless communication systems (ex: phone lines or satellites) Basics about Sensors and Instruments

5. Atmospheric InstrumentationM. D. Eastin Instrument Performance Characteristics

6. Atmospheric InstrumentationM. D. Eastin Instrument Performance Characteristics Terms and Definitions: Static Characteristics:Those that do not change in time. Examples include: (1) bias, (2) accuracy, (3) precision, (4) resolution, (5) sensitivity, and (6) dynamic range → all of these will be defined soon! Specific values determined through careful calibration Dynamic Characteristics:Those that involve changes with time. Examples include: (1) response time, and (2) drift Specific values determined through careful calibration Exposure Characteristics:Those resulting from unique local conditions at the site where an instrument is located. Examples include: (1) conduction (2) radiation effect, (3) wind effects, (4) wetting effects, and (5) obstructions Often dynamic in nature since they are related to changes in local weather (rain -- sunshine) and obstructions (trees grow) Cannot be determined through careful calibration – Must be accounted for by instrument design and careful siting

7. Atmospheric InstrumentationM. D. Eastin Instrument Performance Characteristics Terms and Definitions: Calibration:A series of performance tests between a new instrument and a high-quality instrument used to determine performance and uncertainty characteristics These tests can be conducted in a controlled laboratory setting or in the field through a series of inter-comparisons (or “buddy checks). Laboratory Calibration Field Calibration

8. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Systematic Error:A consistent / repeated offset in a measurement as a result of a fixed or regular discrepancy in the instrument response Also called a “bias” – can be accounted for or removed Random Error:Variations in measurement due to statistical fluctuations in the quantity sensed, the internal operation of the instrument, or some combination of the two – cannot be removed Instrument Performance Characteristics

9. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Accuracy:A measure of the overall uncertainty in the value of the measured parameter, when compared against an external standard – determined by the combination of random and systematic errors Precision:An instrument is precise if, in repeated trials, it is able to give the same output response for the same input parameter – determined by systematic errors Note: A precise instrument can still be inaccurate Instrument Performance Characteristics

10. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Resolution:The smallest change in a parameter measureable by an instrument Sensitivity:Ratio of the instrument response to a unit change in the parameter sensed Linear response:Accuracy is the same regardless of parameter magnitude Response is characterized by a straight line Non-Linear response:Accuracy varies with parameter magnitude Response is characterized by a polynomial function Linear Non- Linear Instrument Performance Characteristics

11. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Dynamic Range:The full range of parameter values over which an instrument can detect a measureable response Instruments unable to measure the full range of possible values will “saturate” at a maximum / minimum measureable value and report no further variation An instrument with insufficient dynamic range “Saturation” occurred at maximum measurable value Instrument Performance Characteristics

12. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Response Time ( τ ):How long an instrument takes to respond by a required amount to a sudden / step change in the parameter being sensed. Required amount Sudden change Full Response Time (3 τ ) “Long” “Short” “Average” Instrument Performance Characteristics

13. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Response Time ( τ ):How long an instrument takes to respond by some fractional amount to a typical oscillation in the parameter being sensed. “Long” “Small Fraction” “Short” “Large Fraction” “Average” Instrument Performance Characteristics

14. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Drift Error: Due to physical changes that occur in the sensor with time. These errors are difficult to detect during initial calibration, and are often compensated through frequent calibrations ASOS instruments are calibrated at least once per year to account for any drift Days Instrument Performance Characteristics

15. Atmospheric InstrumentationM. D. Eastin Examples: Davis Instrument Vantage Pro-2 surface weather station See Pages 3-8 in Davis-Vantage-Pro2-Specfication.PDF on course website (Page 6 for Temperature Sensor performance shown below) Resolution Dynamic Range Response Time Accuracy ??? +2 slides Instrument Performance Characteristics

16. Atmospheric InstrumentationM. D. Eastin Examples: Davis Instrument Vantage Pro-2 surface weather station Which of the following sensors exhibit→ linear sensitivity? → non-linear sensitivity? → How do you know? Pressure Humidity Temperature Reported (Expected) AccuracyParameter Instrument Performance Characteristics

17. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Exposure Error:Due to imperfect coupling between the sensor and the desired parameter to be measured – site specific – cannot be accounted for in calibration Example: A thermometer designed to measure air temperature will never exactly measure the true air temperature due to a number of errors introduced by thermometer mounting and exposure to other parameters that can influence the measured temperature Stagnation heating (at low wind speeds) (prevents a regular air flow) (that provides “offset cooling”) (of any conduction heating) Solar heating (in direct sunlight) Evaporative Cooling (when wetted) (by rain or dew) Conduction heating (due to solar heating ) (of mounting brackets) Infrared heating (close to a “heated”) (building) Instrument Performance Characteristics

18. Atmospheric InstrumentationM. D. Eastin Terms and Definitions: Exposure Error:Example: Reduced by placing instruments away from large buildings and in well-designed fan-aspirated shelters Rain shield Sensor Concentric Air Intakes Fan Fan Aspiration (maintains regular) (flow of ambient air) (prevents) (stagnation heating) Rain Shield (prevents sensor wetting) (and evaporative cooling) Bright White Shield (Limits both ) (solar heating) (and) (conduction heating) (by) (mounting hardware) “Open” exposure (Placement away from) (buildings) (limits) (IR heating)

19. Atmospheric InstrumentationM. D. Eastin Measurement Standards

20. Atmospheric InstrumentationM. D. Eastin Measurement Standards Calibration Standards: In the United States, the “calibration standards” (or high-quality instruments) are maintained by the National Institute of Standards and technology (NIST) Each calibration laboratory must acquire their “standards” from NIST Each country has its own “standards” institute Performance Standards: A common set of methods used to determine instrument specifications, such as (1) sensitivity, (2) resolution, (3) dynamic range, (4) accuracy, and (5) time constant In the United States, the American Society of Testing and Materials (ASTM) establish and maintain these standards Procedural Standards: A common set of methods and documentation, such as (1) averaging periods, (2) wireless communication frequencies, and (3) station description “metadata” that documents each station’s characteristics (location, land cover, exposure, instruments, calibration dates) World Meteorological Organization (WMO) sets averaging periods (wind speed → 10 min) International agreements set communication frequencies (wx balloons → 400-405 MHz) National Climate Data Center (NCDC) maintains station metadata files

21. Atmospheric InstrumentationM. D. Eastin Measurement Standards Exposure Standards: Surface Stations World Meteorological Organization (WMO) sets exposure requirements for each parameter Wind Instruments → Mounted at 10-m above ground level in open terrain, whereby the shortest distance between the anemometer and an obstruction (buildings, trees, etc.) must be at least ten times the height of the obstruction (e.g., at least 100-m away from a 10-m tall tree). No rooftop mounting! PTH Instruments →Mounted inside a radiation / rainfall shield, with or without forced ventilation, at a height of 1.5 to 2.0-m above ground level in open terrain, whereby the site is not located on a steep slope or in a depression where thermal conditions might not be representative of the larger scale. No rooftop mounting! Precipitation →All gauges should be situated in open terrain with a gauge- mounted wind screen to minimize the effects of blowing wind on “gauge catch” – especially in areas expecting snowfall. Any frozen precipitation must be melted to obtain a liquid equivalent

22. Atmospheric InstrumentationM. D. Eastin Measurement Standards NOAA ASOS Surface Station (operational) Conforms to WMO Standards Most are located at airports Precipitation (heated) (tipping bucket) (with) (wind screen) Wind Speed / Direction (cup anemometer) (and wind vane) (at 10 m) Temperature and Humidity (fan-aspirated) (at 1.5 m) Pressure (at 1.5 m) Note the open and level terrain away from obstructions

23. Atmospheric InstrumentationM. D. Eastin Interpretation and Reporting of Measurements

24. Atmospheric InstrumentationM. D. Eastin Interpretation and Reporting of Measurements What does an instrument REALLY measure and then RECORD? Most automated (electronic) instruments collect measurements (every 5-10 seconds) Surface weather stations are required to report once per hour (usually at 5 min to the hour) What happens to the 600+ individual measurements made each hour by the sensor?

25. Atmospheric InstrumentationM. D. Eastin What does an instrument REALLY measure and then REPORT? Most automated (electronic) instruments collect measurements (every 5-10 seconds) Surface weather stations are required to report once per hour (usually at 5 min to the hour) What happens to the 600+ individual measurements made each hour by the sensor? These individual observations are NOT reported Many are used to estimate a sample mean (X ̅ ) over some time interval where: X i =one individual observation N = total observations collected in specified time interval The instrument records this sample mean! Interpretation and Reporting of Measurements

26. Atmospheric InstrumentationM. D. Eastin How should YOU report the observation? Since we now know that an instrument’s “measurement” is really an average of many individual observations, it is important to include the expected variability in the reported value of that parameter If the instrument only reports a sample mean (X ̅ ), how do we know the variability (σ) associated with that sample mean? where:μ = true mean X ̅ =sample mean σ=expected variability Interpretation and Reporting of Measurements

27. Atmospheric InstrumentationM. D. Eastin How should YOU report the observation? Since we now know that an instrument’s “measurement” is really an average of many individual observations, it is important to include the expected variability in the reported value of that parameter If the instrument only reports a sample mean (X ̅ ), how do we know the variability (σ) associated with that sample mean? → We don’t! However, we do know the expected variability associated with any sample mean observed by that instrument → the reported accuracy obtained via calibration where:μ = true mean X ̅ =sample mean σ=expected variability Interpretation and Reporting of Measurements

28. Atmospheric InstrumentationM. D. Eastin How should YOU report the observation? Suppose the temperature sensor on a Davis Instruments Vantage Pro-2 reports the following: T ̅ = 18.753°C We know from the specifications manual: σ = ±0.5°C→sensor accuracy (see page 6) Thus, after accounting for significant figures, the reported observation should be: 18.8 ± 0.5 °C Note: See Chapter 2 in the text for how to account for expected variability when multiple observed parameters are used to calculate another parameter Example: Vantage Pro-2 stations calculate dewpoint temperature from the observed temperature and relative humidity where:μ = true mean X ̅ =sample mean σ=expected variability Interpretation and Reporting of Measurements

29. Atmospheric InstrumentationM. D. Eastin How should YOU report the observation? Such variability should also be reported in graphical displays using error bars where:μ = true mean X ̅ =sample mean σ=expected variability Incorrect (without error bars) Temperature (°C) Correct (with error bars) Interpretation and Reporting of Measurements

30. Atmospheric InstrumentationM. D. Eastin Preliminary Data Analysis

31. Atmospheric InstrumentationM. D. Eastin Preliminary Data Analysis A very important step! All reported / recorded observations should be checked: Do the observations fall within the expected range Are there any unphysical outliers that need to be removed. Calibration procedures are not perfect! Exposure errors can never be completely removed! This step is often called quality control and can be automated, but automation will only identify common errors, so a visual inspection is always wise Step 1: Convert output signal to physical units This step is accomplished by applying transfer equations obtained during calibration (see the next slide) Most instruments and/or instrument display software come pre-programmed with the transfer equations obtained during the original calibration Additional transfer equations may need to be applied to account for any drift error or exposure error determined during subsequent field calibrations

32. Atmospheric InstrumentationM. D. Eastin Preliminary Data Analysis Step 1: Convert output signal to physical units

33. Atmospheric InstrumentationM. D. Eastin Preliminary Data Analysis Step 2: Check for outliers and out-of-range values This step is accomplished by plotting ALL observations as a function of time using the provided display software or custom software, and then methodically removing the suspicious observations

34. Atmospheric InstrumentationM. D. Eastin Summary Principles of Measurement and Instrumentation Components of Instrument (terms and definitions) Instrument Performance Characteristics Static / Dynamic / Exposure Calibration Bias / Accuracy / Precision Resolution / Sensitivity / Dynamic Range Response Time / Drift Exposure errors (types and reduction methods Measurement Standards Winds / PTH / Precipitation Interpretation / Reporting of Measurements What does an instrument really record? Preliminary Data Analysis Methods and Steps

35. Atmospheric InstrumentationM. D. Eastin References Bradley, J.T., K. Kraus, and T. Townsend, 1991: Federal siting criteria for automated weather observations, Preprints 7 th Symposium on Meteorological observations and Instrumentation, New Orleans, LA, American Meteorological Society, pp 207-210. Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp. Brock, F. V., K. C. Crawford, R. L. Elliot, G. W. Cuperus, S. J. Stadler, H. L. Johnston, M.D. Eilts, 1993: The Oklahoma Mesonet - A technical overview. Journal of Atmospheric and Oceanic Technology, 12, 5-19. Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp. Helmes, L., and R. Jaenicke, 1985: Hidden information within series of measurement – four examples from atmospheric science, Journal of Atmospheric Chemistry, 3, 171-185 Huffman, G. J., and J. N. Cooper, 1989: Design issues in nearly real-time meteorological systems and sites. Journal of Atmospheric and Oceanic Technology, 6, 353-358. Parker, D.E., 1994: Effects of changing exposure of thermometers at land stations, International Journal of Climatology, 14, 1-31. Stokes, G.M., and S. E. Schwartz, 1994: The Atmospheric Radiation Measurement (ARM) Program: Programmatic background and design of the cloud and radiation test bed. Bulletin of the American Meteorological Society, 75, 1201-1221.