Common atmospheric measurement devices & sampling considerations

Troposphere

Majority of water vapour & atmospheric pollutants commonly associated with anthropogenic contributions to the atmosphere are largely limited to the atmospheric region between the troposphere & stratosphere (known as tropopause).

Atmospheric turbulence

Assume airflow passing over a relatively smooth surface can be idealized as quasi-parallel flowlines where velocities increase within increasing distance away from the ground surface to a given elevation where a relatively uniform velocity field is maintained.

Velocity at ground surface = 0, increase quickly vertically upward & is proportional to resisting shear stress. A turbulent boundary prevails near the ground surface (proportional to height of obstacles → ground effect)

Wind measurement devices

weather vane (±15 degrees), weather sock, wind anenometer & vane,

Weather sock

Wind direction (± 15 degrees) & speed (± 5 km/h up to approx. 30 km/h)

Wind anemometer and vane

Rotation of cups produce an AC sine wave voltage proportional to wind speed frequency of rotation. Rotation frequency is converted to wind speed via the manufacture’s calibration curve.

Wind anemometer and vane accuracy

Dependent on manufactures design. Usually ± 0.1km/h

Wind anemometer and vane limitations

Effective response to strong gusts is not achievable in many makes & models (a ceiling threshold above which a dampener regulates the rotation frequency to minimize damage to the sensor)

Precipitation measurement devices

standard daily rain gauge, tipping bucket rain gauge, geonor (or OTT pluvial) precipitation gauge

Standard daily rain gauge

Used to observe daily precipitation across the world. Historically associated with agricultural practices, the daily rain gauge is accurate to within ±0.5mm (depending upon make and model).

Many of historical records provided by responsible agencies are based upon daily ritual of a person (commonly a farmer) systematically observing & recording precipitation data that has been on (mailed) to responsible agency.

Tipping bucket rain gauge

Rainfall enters into the gauge through the orifice at the top with flows into a small oscillating bucket. When the buck is filled with water, it “tips” and triggers a relay switch, which is recorded by the associated data logger. The volume of each filled buck “tip” is then converted into a per area precipitation rate. Frequency upon which buckets “tip” is employed to calculate the intensity of the precipitation event (rainfall volume over observation time).

Tipping bucket rain gauge - how do you deal with snowfall?

External power source can be used in combination with a heating ring at the top of the orifice to intercept & melt incoming snowfall and convert it to water equivalent (SWE). Works well for “light” snow but decreases in accuracy with increasing` intensity.

Tipping bucket rain gauge limitations

During intense rainstorm events, rain droplets can fall directly through the orifice into the tipping bucket, triggering a false reading = overrepresenting intensity of precipitation events. Diffusers to the orifice can be added to minimize this, but the potential for precipitation to deflect beyond the limits of the orifice (off of the diffusers) = underrepresentation

Geonor (or OTT pluvial) precipitation gauge

Precipitation falls through an orifice into a large collection reservoir. Precipitation that falls into the reservoir is measured via a load cell (programmed recording time intervals — commonly every 15 min-1h) and converted to a per area precipitation rate based upon the orifice area at the top of the precipitation vessel.

Geonor (or OTT pluvial) precipitation gauge accuracy

Depending on make/model but commonly within ±0.1mm

Geonor (or OTT pluvial) precipitation gauge - what is added to prevent water from evaporating?

Thin layer of motor oil on top. Thin layer of glycol added to the bottom to melt receiving snow & frozen forms of precipitation to liquid form.

Wind & precipitation measurement devices - doppler radar

Continuously emit and receive microwave frequencies across a semi-spherical domain. From the relative time lag between a frequency being emitted & received (+ receiving intensity) at radar station (coordinate location), info us sed to calculate intensity & location of precipitation relative to Doppler station.

Solar radiation measurement

short-wave, silicon photocell pyranometers, thermopile pyranometers,

Solar radiation measurement - short-wave solar radiation

Net solar radiation (or solar irradiance) is the transmission of energy from the sun in the form of electromagnetic waves or subatomic particles (W/m²).

Solar irradiance is measured at the Earth’s surface after all atmospheric adsorption & scattering has been accounted for.

Solar radiation measurement - silicon photocell pyranometers

Produce a μA output current similar to how a solar panel converts the sun’s energy into electricity. When current passes through a shunt resistor, it’s converted to a voltage signal (sensitivity in range of μV/W/m²). A plastic diffuser cover is used to provide a uniform cosine response at varying sun angles.

Solar radiation measurement - silicon photocell pyranometers limitations

Only sample a portion of the short-wave radiation & performs well for clear sunny sky weather conditions, but measurement errors are high during cloudy days. The uniformity of daylight spectrum under most sky conditions limits error typically less than ±3% with max errors of ±10%. Error is usually positive under cloud conditions.

Solar radiation measurement - thermopile pyranometers

Using a series of thermoelectric junctions (multiple junctions of 2 dissimilar metals - thermocouple principle) to provide a series of several μV/W/m² proportional to the temp difference between a black absorbing surface & a reference material/colour (may be white reflective surface or internal portion of sensor base). Thermopile pyranometer’s black surface uniformly absorbs solar radiation across the solar spectrum.

Solar radiation measurement - thermopile pyranometer’s black surface uniformly absorbs solar radiation across short-wave solar spectrum from 0.285-2.8μm. The uniform spectral response allows thermopile pyranometers to measure…?

Reflected solar radiation, radiation within canopies or greenhouses, albedo (reflected:incident) when 2 are deployed as an up-facing/down-facing pair.

Common short-wave radiation occurrences & why

Observed outgoing solar radiation at weather station tend to exhibit a damped measured radiation relative to incoming shortwave radiation on some days and others a response that follows the same radiation trends of the incoming radiation.

Days where outgoing radiation exhibits a similar response to incoming radiation are consistent with conditions where snow cover occurs = greater proportion of net radiation is reflected (higher surface albedo). Vice versa