Atmospheric InstrumentationM. D. Eastin Measurement of Wind

Atmospheric InstrumentationM. D. Eastin Outline Measurement of Wind Review of Atmospheric Winds Anemometers Cup / Vane Sonic Pressure Tube Exposure Errors Obstructions Frozen Precipitation

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Wind:A three-dimensional vector describing the speed and direction of total atmospheric air flow where:V = three dimensional wind vector (m s -1 ) u=zonal (east-west) wind component v =meridional (north-south) wind component w= vertical wind component SI unit: meters per second (m s -1 ) or (mps) (for all three components) Meteorology: knots= (mps) mph= (mps) mph= (knots) Instrument: Anemometer Review of Atmospheric Wind x y z u v w V

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Wind:Many meteorological instrumentation systems do not attempt to measure the vertical wind component The two-dimensional horizontal wind is then defined by a speed and angular direction (clockwise from true north) from which air approached the sensor where: V H = horizontal wind vector (m s -1 ) U=horizontal wind speed (m s -1 ) θ =horizontal wind direction (degrees) The speed and direction can converted to the zonal and meridional wind components via where u=zonal (east-west) wind component v =meridional (north-south) wind component Instrument: Anemometer (and Wind Vane) Review of Atmospheric Wind x y U θ

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Mean Wind:Average of all individual wind measurements collected during a 10-minute period → WMO standard for all weather and climate observations Maximum 1-minute Wind Speed:Largest average wind speed obtained from all individual wind measurements during any given 1-minute period within the standard 10-minutes Used by the National Hurricane Center to estimate the maximum intensity of tropical cyclones Maximum 3-second Wind Speed:Largest average wind speed obtained from all individual wind measurements during any given 3-second period within the standard 10-minutes Also called a wind gust Used by wind engineers to calculate the total force exerted on built structures by air flow. Structural failures result when wind gusts are large but also vary in direction and magnitude Review of Atmospheric Wind

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Atmospheric wind speeds can exceed 200 m/s in tornadoes 100 m/s in the polar jet stream 75 m/s in thunderstorm updrafts Typical horizontal wind speeds (at the surface and aloft) range from 0-40 m/s Typical vertical wind speeds range from 0-1 m/s (on the synoptic-scale) and 0-10 m/s (on the mesoscale) Review of Atmospheric Wind

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Most anemometers are designed to directly measure horizontal wind speeds near the surface where winds rarely exceed 50 m/s Upper air winds are measured indirectly by sounding systems, radar, or satellite (we will discuss these later) Surface anemometers should exhibit a dynamic range → 0 m/s to 50 m/s → 0 knots to 100 knots Review of Atmospheric Temperature

Atmospheric InstrumentationM. D. Eastin Cup / Vane Anemometers – Basic Concept: Determines the wind speed my measuring the angular rotation rate of a vertical shaft attached to three hemispherical cups placed equidistant around the shaft. The shaft rotates on bearings arranged to minimize the mechanical friction The output signal is a voltage proportional to a series of electrical pulses generated by an optical or magnetic switch on the shaft where: ω= angular rotation speed (degree s -1 ) k= calibration constant (degree m -1 ) U= wind speed (m s -1 ) U 0 =starting wind speed (m s -1 ) The starting wind speed (U 0 ) for cup anemometers used on standard weather stations is 2 m/s, but it can be as small as 0.5 m/s for research-quality anemometers with good bearings Anemometers

Atmospheric InstrumentationM. D. Eastin Cup / Vane Anemometers – Basic Concept: Determines the wind direction by (1) measuring output voltage along a sliding-scale resistor that varies linearly with angle around the circle This method uses a precision potentiometer Most commonly used since it permits a much finer resolution (~1 degree) Determines the wind direction from (2) an optical encoder giving a digital representation of the measured angle or (3) mechanical switches with multiple contacts distributed regularly around the shaft Less commonly used due to relatively coarse resolution (~5-10 degrees) All methods require the anemometer vane to be installed with a known orientation (true north) Anemometers

Cup / Vane Anemometers – Typical Specifications Cup Accuracy ±1.0 m/s Resolution 0.1 m/s Response Time2-5 s VaneAccuracy ±4.0 degrees Resolution 1.0 degrees Response Time2-5 s Advantages Inexpensive and easily automated Calibration is simple Durable at wind speeds < 50 m/s Disadvantages Insensitive to light winds (< 1 m/s) Instrument drift due to bearing wear Often fail at high wind speeds (> 50 m/s) Can over-estimate mean wind speed (by 1-2 m/s) due to turbulence and a non-zero vertical wind Atmospheric InstrumentationM. D. Eastin Anemometers

Atmospheric InstrumentationM. D. Eastin Sonic Anemometers – Basic Concept: Determines the wind vector my measuring the flight time of sound pulses travelling forward and backward between two fixed transducer / receiver pairs (A and B below) separated by < 20 cm Estimates wind speed component parallel to the path Multiple fixed-angle transducers (and some trigonometry) are used to obtain the two / three dimensional winds Anemometers

Atmospheric InstrumentationM. D. Eastin Sonic Anemometers – Basic Concept: The speed of sound is modified by the wind component parallel to the sound path → the Doppler Effect Pulses are regularly transmitted at Hz giving good accuracy and fast time response Winds can be determined at < 1 s intervals by averaging individual measurements Anemometers where: t A = flight time A to B (s) t B = flight time B to A (s) L=distance between transducers (m) c s =speed of sound (m s -1 ) v=wind speed (m s -1 )

Sonic Anemometers – Typical Specifications Accuracy ±0.05 m/s Resolution 0.01 m/s Response Time<0.1 s Range0-30 m/s Advantages Easily measure 2D / 3D turbulence Durable at wind speeds < 50 m/s Disadvantages Expensive Large power consumption Must manage large volumes of data Require more frequent calibration Can underestimate wind speeds in precipitation due to sonic attenuation by rain and ice Atmospheric InstrumentationM. D. Eastin Anemometers

Atmospheric InstrumentationM. D. Eastin Pressure Tube Anemometers – Basic Concept: Determines the wind speed my measuring the differential pressure between a tube directly facing into the wind (total pressure) and small holes oriented parallel to the wind (static pressure) → Bernouilli’s Principle where: P T = total pressure (Pa) P S = static pressure (Pa) ρ= air density (kg m -3 ) U= wind seed (m s -1 ) Determines wind direction by using a wind vane that keeps the tube facing into the wind Also called pitot tube anemometers Less common than cup anemometers Most often used to calibrate other anemometers and on aircraft (for auto-pilot operation) Anemometers

Atmospheric InstrumentationM. D. Eastin Pressure Tube Anemometers – Typical Specifications Accuracy ±2.0 m/s Resolution 0.5 m/s Response Time1-5 s Advantages Can be inexpensive Calibration is simple Durable at large wind speeds No instrument drift Easy to automate Disadvantages Sensitive to alignment with wind Insensitive to light winds (< 5 m/s) Non-linear response Anemometers

Atmospheric InstrumentationM. D. Eastin Obstructions – Basic Concept: The wind flow (both speed and direction) are easily perturbed by physical obstructions near the surface (buildings, trees, etc.) The WMO requirements for surface wind measurements is for the anemometer to be mounted at 10 m above level ground with open exposure in all directions and at a distance greater than 10 times the height of any nearby obstructions Very difficult to conform to these standards, and most sites are / become compromised to some extent Exposure Errors Charlotte - ASOS

Atmospheric InstrumentationM. D. Eastin Precipitation Errors – Basic Concept: Any cup anemometer sensor wetted by frozen precipitation will either slow down or cease cup rotation al together Such errors can be significant during (1) heavy snow and light winds (2) freezing rain (3) rapid temperature drop below 0°C Exposure Errors

Atmospheric InstrumentationM. D. Eastin Summary Measurement of Wind Review of Atmospheric Winds Anemometers Cup / Vane Sonic Pressure Tube Exposure Errors Obstructions Frozen Precipitation

Atmospheric InstrumentationM. D. Eastin References 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, Coppin, P.A., and K.. Taylor, 1983: A three component sonic anemometer/thermometer system for general meteorological research. Boundary Layer Meteorology, 27, Dilger, H., and P. Thomas, 1975: A cup anemometer testing device for low wind speeds. Journal of Applied Meteorology, 14, Finkelstein, P.J, J.C. Kaimal, J.E. Gaynor, M.E. Graves, and T.J. Lockhart, 1986: Comparison of wind monitoring systems. Part I: In situ sensors. Journal of Atmospheric and Oceanic Technology, 3, Grant, A. L. M., and R. D. Watkins, 1989: Errors in turbulence measurements with a sonic anemometer. Boundary Layer Meteorology, 46, Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp. Hayashi T., 1987: Dynamic response of a cup anemometer. Journal of Atmospheric and Oceanic Technology, 4, Hyson, P., 1972: Cup anemometer response to fluctuating wind speeds. Journal of Applied Meteorology, 11, Kunkel, K.E. and C. W. Bruce, 1983: A sensitive fast-response pressure tube anemometer. Journal of Climate and Applied Meteorology, 22, Snow, J.T., M.E. Akridge, and S. B. Harley, 1989: Basic meteorological observations for schools: surface winds. Bulletin of the American Meteorological Society, 5,