Met Instrumentation Exam 2

What are three types of anemometers?

Cup,Vane, Ultrasonic

What common challenges impact wind measurements?

Physical obstructions, elevation, extreme weather events (ice), high relative humidity, pressure changes

What standards should be followed when installing wind sensors?

Height requirements, following meteorological / WMO guidelines, technical and electrical standards, true calibration (WIND VANES MUST BE ALIGNED TRUE NORTH)

History of Wind Measuring Instruments

1450 - Leon Battista Alberta

1483- Da Vinci

1846 - J.T Romney Robinson

1892 - W. H Dines

1965- P. Minnick

1970 - A.Pfitsch

KNOT

-reference to velocity while the distance is using nautical miles

• was a standard unit of measurement in 1600s using a common log device

• speed of ship was the number of knots counted

• 1knot- 1.15mph-0.514ms^-1

What are typical wind speeds near the surface?

• slight breeze, gusts, sustained winds

• 0-130 its (0-67 ms^-1)

What are typical wind speeds in the mid-troposphere, near the jet stream?

• 200+ its (103 ms^-1)

• wind is a highly non-linear phenomenon

Measuring Wind: Drag Force

• a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid

• can also be described as resistance

Measuring wind: Mechanical Sensors

ex - cup or propeller anemometer, wind vane

• uses: measurement at weather station (official and home)

Measuring wind: In-situ electronic sensors

• ex- hot-wire anemometer

• fast response measurements; turbulence characterization

Measuring wind: Drifting-position sensors

• ex- radiosonde, driftsonde, pibal

• uses: atmospheric sounding

Measuring wind: sensors based on impact pressure

• ex: pitot tube

• use: measuring airspeed of moving vehicles including aircraft

Measuring wind: sensors that use measurements of a time shift

• ex: ultrasonic anemometer → Passive

• Use: modern official weather stations; research that investigates turbulence or fluxes

Measuring wind: sensors that use measurements of a Doppler shift

• ex: doppler radar and lidar; wind profiler

• Use: Remote measurement of wind at a range of positions from the sensor

Does a cup anemometer spin in a different direction based on wind direction?

No, a cup anemometer always spins in the same direction no matter which way the wind is blowing!

• Does not measure direction

Which shape has the larges drag?

• open shapes have larger drag force

Cup Anemometers

• mechanical rotation of the cups and the internal supporting shaft

• rotation of the shaft can be manually determined or determined using an electrical sensors

• Shaft rotation is equal to revolutions per minute (frequency) and is proportional to wind speed

Cup Anemometer - Optical shutter arrangement

• as the shaft rotates, a light beam between the source and electronic detector is interrupted

• Generates a series of digital pulses at a rate which is proportional to the rotation speed

Cup Anemometer - Reed Switch

• as the shaft rotates an internal magnet spins closing and opening the reed switch

• - a brief electrical current is created and measured

Cup Anemometers: Advantages

• simple and robust construction

• Simplicity of speed calculation

• Low maintenance requirements

• Can capture a large measurement range

Cup Anemometers: Disadvantages

• Overspending: records average wind speeds that are too high in gusty winds

- - When the wind speed increases, the cups spin faster, but a decrease in wind of the same magnitude does not cause the cups to slow down as quickly

• reduce the distance between the cups and the rotational axis

• Thresholding: wind speeds greater than 1 ms^-1 are necessary to cause the cup anemometer to turn

Propellar Anemometer

• measures the 3 components of wind (u,v, and w)

• the rotation speed of the propellers is proportional to the wind speed (component) perpendicular to the propeller plane

• often used on buoys and for turbulence studies

Carbon fiber vs. Polystyrene: lighter material breaks faster

Propeller Anemometer: Advantages

• lightweight material have fast time responses

• overestimation of the mean wind speed is less than cup anemometers

Propeller Anemometer:

• V-error: Azimuthal variations in the wind meaning the propeller is not perpendicular (or parallel) to the wind direction - underestimation to the wind speed

• Lightweight propellors can break in high winds

• overspeeding error

Impeller Anemometer

• Wind rotates the impeller, the magnet, and the shaft inducing a current through a coil

-axis of rotation parallel to the direction of the wind

-a magnet located in the impeller along with an inductor

- the rotation of impeller produces an electrical signal proportional to the wind speed

-instrument range: 0.4-35ms^-1

QUICK TIME RESPONSE

Pressure Plate Anemometers

• deflection of a vertically hanging plate that is oriented into the mean wind direction common in 1600s

• The deflection is proportional to the strength of the wind

non-linear response

• A deflection of a small weight is called a telltale

Pressure Tube Anemometers

• measures the pressure differences of static and moving (dynamic) flow

• has directional sensitivity

Ultrasonic Anemometer

• time it takes for ultrasound pulses to travel forward and backwards from fixed transducers

-separation of transducers are usually 10-20 cm

-soundwaves are sent at 40-100 kHz frequency

• Dependent on temperature and humidity

Ultrasonic Anemometer - Advantages

• it is absolute measurement not needing calibration (except to check that the instrument parameters are specified correctly)

• measurements do not exhibit a time lag and can be repeated frequently to resolve high-frequency to resolve high-frequency fluctuations in the wind

• measures all three components of the wind vector and are therefore beneficial for studying turbulent mass fluxes

• the instrument has no moving parts that require maintenance

Ultrasonic Anemometer - Disadvantages

• fairly expensive

• requires more power

• signal loss in heavy rain or wet snow

Wind Profilers

• similar to weather radar, but operates on a wavelength of 0.33m or 0.6m as a vertical doppler radar

-sensitive to the backscattering of turbulent structures in the water-vapor field

-measures the Doppler shift at multiple angles

Wind Vanes

• flat vertical plate attached to a horizontal arm that rotates on a vertical shaft

- the direction of the arm becomes aligned with the wind direction due to drag force and lift force

-reduction of the drag force

-static error: misalignment of vane

Besides visually determining the direction of the vane what methods are there?

• mechanical switch with multiple contacts distributed around the shaft

• optical encoders giving a digital representation of measure angles

• a precision potentiometer

ASOS Wind Measurements

• Prior to 2007: rotating cup anemometers and a wind vane 10 meters in height

• Now: Sonic anemometer

From Satellite

• monitor cloud movements

How is the wind speed/direction at various heights determined?

using a mix of physical instruments, remote sensing technology, and mathematics modeling

Calibration

• wind sensors are place in a wind tunnel with a pitot tube as the reference instrument

• the wind tunnel must be large enough to fit the sensors and provide a uniform wind flow across the tunnel

Exposure - Is the height of wind sensors standardized?

• yes, by the world meteorological organization

• 10 m in height

• good exposure in all directions with 3km

• obstructions should be at least 20 times the height of the obstruction from the sensor

• show accumulation, icing, extreme temperature, high wind gust, lightning induced surges

What are three measurement principles that are utilized for radiometric measurements?

Attenuation (thickness, absorbs, scatters, etc), Resolution, Response Time

What are two of the basic radiation laws?

Stefan-Boltzmann Law & Wien’s Law

What are measured errors that impact radiation sensors?

Exposure, drift, systemic errors, calibration errors

Radiation

pulses of electromagnetic energy traveling as waves and generated as electrons oscillate

Shortwave (SW)

solar radiation

Longwave (LW)

terrestrial

Azimuth

the angle between North, measured clockwise around the observer’s horizon, and a celestial body

Kirchoff’s Law

good absorbers are good emitters and vice versa at peculiar wavelengths

Stefan-Boltzmann Law

the amount of energy emitted by a body is directly proportional to the fourth power of its absolute temperature

• sigmaT^4 = emissivity

• Units: Wm^-2

• (assuming perfect black body)

Wien’s Law

hotter temperatures, shorter wavelengths

• Lamdam = 2898/T. Units [(must)(m)]

Planck’s Law

describes the spectral density of radiation emitted by a black body at each wavelength at a given temperature (blackbody curves)

Intensity Law

the amount of radiant power flux is inversely proportional to distance squared

• Solar constant = 1379 Wm^-2

• E=P/4pid². Units [ Wm^-2)

Sine/Cosine Law

radiant energy striking a surface at an acute angle becomes less concentrated than direct (perpendicular) radiation

Radiant Flux

the amount of radiation coming from a source per unit time (W)

• sun’s radiant flux is 3.9×10^(26) W

Radiance

radiant flux emitted by a unit area of a source or scattered by a unit area of a surface within a given solid angle (wm^-2sr^-1)

• Sun’s radiance is 1×10^7 Wm^-2sr^-1

Watt

Js^-1 (kgm²s^-3) - Power

Radiant Intensity

the radiant flux leaving a point on the source, per unit solid angle of space surrounding the point in W sr^-1

Radiant Flux Density

radiative flux per unit area W m^-2

• flux can be negative or positive

Irradiance

radiant flux incident on a receiving surface from all directions per unit of area surface in W m^-2

• ALWAYS POSITIVE

• all angles combined

Direct Solar Radiation

radiation emitted from the solid angles of the sun, received on a surface perpendicular to the axis of the cone

• comprised of mainly unscattered and unreflected solar radiation in Wm^-2 (1367 Wm^-2)

• MEASURED BY A PYRHELIOMETER

Diffuse Solar Radiation

downward scattered and reflected radiation coming from the whole hemisphere with the exception of the solid angle subtended by the sun’s disc in W m^-2

• measured by a pyranometer in a shadow band

Global solar radiation

solar irradiance received on a horizontal surface in W m ^-2

• measured by a pyrometer

What is the difference between transmitted and emitted?

Transmitted: to pass or send existing energy, light or signal THROUGH a medium

Emitted: to create and give off energy light, heat, etc NOT THROUGH a medium

Reflection

fraction of the incident flux that is reflected by a medium

Absorption

fraction of the incident flux that is absorbed by a medium

Transmission

fraction of the incident flux that is transmitted (passed through) by a medium

Net Radiation (R)

R= SWD-SWU+LWD-LWU [Wm^-2]

^^^ Shortwave down, shortwave up, long wave down, long wave up

Why might the history of radiometer be relatively new?

depending you are referring to radiometer as medical device corporation (founded in 1935) or the radiometer as a scientific instrument

History of the Radiometer

1853- Campbell-Stokes reordered invented

1873- Crookes radiometer

1885 - Jordan Sunshine recorders invented

1893 - pyranometer and pyrheliometer is invented by Angstrom (Knutsson) and anderson

1905 - Angstrom invents the pyrgeometer

Angstrom - a constant and law of absorption (a)

Instruments - Purpose

• Pyrheliometer - direct solar radiation

• Pyranometer - global solar radiation

• Photometer - light in the visible part of the spectrum

• Albedometer - shortwave radiation received by and reflected from a surface

• Pyrradiometer - net radiative flux of shortwave

• Pyrometer - terrestrial radiation

• Pyrometer - longwave radiation emitted from a surface

• Spectro-radioometer - specific wavelength

What does “pyro” and “ano” and “helio” in Greek mean?

• Pyro - fire

• Ano - above

• Helio - sun

Ways radiant energy is detected

• measure the rise of temperature of a small detecting surface

• measure the response of a photoelectric cell

• measure response of photochemical solution

• MET USES FIRST TWO

Photodiode

• LED photodiode in circuit board (I soldered)

• 1) Light (photons) generate electrons are attracted to the K region

• 2) holes are attracted to the A region

Campbell - Stokes Sunshine Recorder

• glass sphere filled with water that focuses the sun’s rays and burns the material behind it

• sphere was altered to be solid glass

• Length of the scorch mark indicates daily sunlight duration

• no scorch mark indicates clouds

• continual scorch marks on same material makes measuring daily insolation difficult

What does a pyranometer measure? Also, why are black and white surfaces used?

measures shortwave radiation (irradiance) (global solar radiation over a hemispheric field of view)

• great reflector and absorbers (large for each)

Pyranometer Theory

• black surface with a thermopile sensor surrounded by a white plate

• the glass bulb shields the body of the instrument from the sunshine.

||» helps isolate the measurement from external thermal interferences and weather - only reacts with pure solar radiation - glass allows solar radiation but doesn’t change anything

• good balance arguments between the black surface and white surface

Pyranometer Theory - Thermopile

• converts thermal energy energy into electrical energy

• composed of thermocouples (usually in series)

• output voltage proportional to a local temperature difference

• range of tens or hundreds of millivolts

Pyranometer Theory

Look at the parts

Pyranometer Theory - S equation

Look at the equation

Eppley Black and White Pyranometer

• measures global solar radiation

• reference instrument in calibration procedures of pyranometers

• the detectors consists of three white and three black wedges representing the cold (white) and hot junctions (black) of an electroplated copper-constantan thermopile

• can be inverted to measure albedo

• (double domes are used to protect the instrument from the weather and a second to isolate the receiving surface from local heating)

Why is albedo important?

Tells us urban heat island, tells us about energy balance

Albedometer

• measures shortwave radiation using a pair of pyrometers

• specifically measures shortwave radiation received by and reflected from a surface

• measurements are poor around sunset and sunrise - sun is at lower angle

Large change in albedo?

• snow, cloud coverage, knocked down forest into parking lot

Which line is which?

Bottom - Su (shortwave up)

Top - Sg (shortwave down)

Pyrheliometer

• measures direct shortwave radiation

• specially, the measurement of direct solar radiation, normal to the solar beam

• has a thermocouple sensor

• a solar tracking instrument is needed- follows the sun in elevation and azimuth

Photometer

• measures shortwave radiation

• specially measures light in the visible part of the spectrum only as energy

• convert light into electricity using a photoresistor, photodiode (resistance down, intensity up), photomultiplier (amplifies the light, very sensitive)

• light might be passed through a filter to measure at a defined wavelength

Diffuse Solar Radiation Measurement

• the pyranometer sensors is shaded from the direct solar beam

• the shade ring must be regularly adjusted

• some of the diffuse radiation is lost by the obstruction of the ring itself

• depends on latitude and time of year

• can be corrected by multiplying the measure value by the k factor

Look at this

“f” is the geometric factor and q is the anisotropy factor (sky cover factor)

• width to radius ration

• hour angle for sunset

• solar declination

SW+LW - Pyrrandiometer Theory

Pyrradiometer

• measures total global radiation (shortwave and long wave)

• glass domes are not used for pyrradiometers

polytene or sillicon domes are transparent to a large range of wavelengths

• sillicon degrades in sunlight becoming brittle, and needs to be replaced monthly

Pyrradiometer vs. pyrometer

• pyrometer doesn’t care about shortwave radiation

Net Pyrradiometer (radiometer)

• measures shortwave and long wave radiation

• specifically measures the net radiative flux of shortwave and long wave radiation

Pyrgeometer

• measures long wave only

• specially, measure the terrestrial radiation usually over a hemisphere, either upward or downward

• measurements are made at night or during the day if the shortwave amount is independently measured and subtracted

Pyrometer - Infrared Thermometer

• measures the long wave radiation emitted in the 7.5-13.5 (mu m)

• radiation received is related to the temperature of the surface

• Emmistitivty = sigmat^4

• a thermopile sensing element, composed of small thermocouples on a sillicon chip, absorb the energy and produce an output signal

ASOS Radiometer

• no radiometers at the ASOS stations

• NYS Mesonet: Li-Cor Pyranometer:

weather resistant and durable in long-term outdoor deployments

measures global solar radiation under unobstructed natural daylight conditions

uniform sensitivity u to 82 degrees incident angle

detachable sensor simplifies installation and removal, making it ideal for platforms with complex cabling

Radiometers and Satellites

• radiometers flown on satellites are NOT cameras

• moving mirror make the radiometer view different regions of the earth

• radiometer scans a scene line-by-line

• data is recorded in a two-dimensional format (line and element) which, when displayed on a computer monitor or television screen, makes up an image

Radiometers on a satellite is a…

Passive sensors

Radiometric resoultion

• the number of bits representing the recorded energy

• 8-bit: 256 values (2^8)

• 4-bit: 16 values (2^4)

• 1-bit: 1 value (0 or 1)

Spectral Resoultion

• the greater the number of bands, the greater the spectral resolution

Spatial Resolution

• the size of a pixel that is recorded in a raster image, typically pixels may correspond to square areas ranging in side length from 1 to 1000m

• very expensive

Temporal Resolution

• geostationary orbit

• near polar-orbit

• sun-synchronous orbit

Measurement Error: Azimuth Error

• change in the sensors output as the sensor is rotated about the normal axis at a particular angle of incident radiation

Measurement Error: Linearity Error

the sensor output is not linearly proportional to the input