Atmospheric InstrumentationM. D. Eastin Measurement of Wind

Atmospheric Instrumentation M. D. Eastin

Measurement of Wind


Outline

Measurement of Wind

• Review of Atmospheric Winds

• Anemometers• Cup / Vane• Sonic• Pressure Tube

• Exposure Errors• Obstructions• Frozen Precipitation


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 componentv = meridional (north-south) wind componentw = vertical wind component

SI unit: meters per second (m s-1) or (mps)(for all three components)

Meteorology: knots = 0.5144 (mps)mph = 0.4470 (mps)mph = 0.8689 (knots)

Instrument: Anemometer

Review of Atmospheric Wind

wvuV ,,

x

y

z

u

v

w

V



Wind: Many meteorological instrumentation systems do notattempt 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: VH = 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 zonaland meridional wind components via

where u = zonal (east-west) wind componentv = meridional (north-south) wind component

Instrument: Anemometer (and Wind Vane)


x

y

U

,UVH θ

sinUu cosUv



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 allindividual wind measurements during any given1-minute period within the standard 10-minutes

Used by the National Hurricane Center to estimatethe maximum intensity of tropical cyclones

Maximum 3-second Wind Speed: Largest average wind speed obtained from allindividual wind measurements during any given3-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. Structuralfailures result when wind gusts are large but alsovary in direction and magnitude




• 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 thesurface and aloft) range from 0-40 m/s

• Typical vertical wind speeds range from0-1 m/s (on the synoptic-scale) and 0-10 m/s (on the mesoscale)




• 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 bysounding systems, radar, or satellite (we will discuss these later)

• Surface anemometers should exhibit adynamic range → 0 m/s to 50 m/s

→ 0 knots to 100 knots

Review of Atmospheric Temperature


Cup / Vane Anemometers – Basic Concept:

•Determines the wind speed my measuring the angular rotation rate of a vertical shaft

attached to three hemispherical cups placedequidistant around the shaft.

•The shaft rotates on bearings arranged tominimize the mechanical friction

•The output signal is a voltage proportional toa 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)

U0 = starting wind speed (m s-1)

•The starting wind speed (U0) for cup anemometersused on standard weather stations is 2 m/s, butit can be as small as 0.5 m/s for research-qualityanemometers with good bearings

Anemometers

0UUk


Cup / Vane Anemometers – Basic Concept:

•Determines the wind direction by (1) measuringoutput voltage along a sliding-scale resistorthat 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 themeasured angle or (3) mechanical switcheswith multiple contacts distributed regularly around the shaft

•Less commonly used due to relatively coarseresolution (~5-10 degrees)

•All methods require the anemometer vane to beinstalled with a known orientation (true north)

Anemometers

Cup / Vane Anemometers – Typical Specifications

Cup Accuracy ±1.0 m/sResolution 0.1 m/sResponse Time 2-5 s

Vane Accuracy ±4.0 degreesResolution 1.0 degreesResponse Time 2-5 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 turbulenceand a non-zero vertical wind


Anemometers


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


Sonic Anemometers – Basic Concept:

•The speed of sound is modified by the wind component parallel to the soundpath → the Doppler Effect

•Pulses are regularly transmitted at 5-100 Hz giving good accuracy and fast time response

•Winds can be determined at < 1 s intervals by averaging 10-20 individual measurements

Anemometers

where: tA = flight time A to B (s) tB = flight time B to A (s) L = distance between

transducers (m)cs = speed of sound (m s-1)v = wind speed (m s-1)

vc

Lt

sA

vc

Lt

sB

BA tt

Lv

11

2

Sonic Anemometers – Typical Specifications

Accuracy ±0.05 m/sResolution 0.01 m/sResponse Time <0.1 sRange 0-30 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 sonicattenuation by rain and ice


Anemometers


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: PT = total pressure (Pa) PS = static pressure (Pa)

ρ = air density (kg m-3)U = wind seed (m s-1)

•Determines wind direction by using a wind vanethat 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

2

2

1UPPP ST


Pressure Tube Anemometers – Typical Specifications

Accuracy ±2.0 m/sResolution 0.5 m/sResponse Time 1-5 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


Obstructions – Basic Concept:

•The wind flow (both speed and direction) areeasily perturbed by physical obstructionsnear the surface (buildings, trees, etc.)

•The WMO requirements for surface windmeasurements is for the anemometer to be mounted at 10 m above level groundwith open exposure in all directions andat 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


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


Summary

Measurement of Wind

• Review of Atmospheric Winds

• Anemometers• Cup / Vane• Sonic• Pressure Tube

• Exposure Errors• Obstructions• Frozen Precipitation


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, 5-19.

Coppin, P.A., and K. . Taylor, 1983: A three component sonic anemometer/thermometer system for general meteorological

research. Boundary Layer Meteorology, 27, 27-42.

Dilger, H., and P. Thomas, 1975: A cup anemometer testing device for low wind speeds. Journal of Applied Meteorology, 14, 414-415.

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, 583-593.

Grant, A. L. M., and R. D. Watkins, 1989: Errors in turbulence measurements with a sonic anemometer. Boundary Layer Meteorology, 46, 181-194.

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, 281-287.

Hyson, P., 1972: Cup anemometer response to fluctuating wind speeds. Journal of Applied Meteorology, 11, 843-848.

Kunkel, K.E. and C. W. Bruce, 1983: A sensitive fast-response pressure tube anemometer. Journal of Climate and Applied Meteorology, 22, 1942-1947.

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, 493-508.

Documents

Atmospheric InstrumentationM. D. Eastin Measurement of Wind