Transcript Slide 1
Micrometeorology Instrumentation Micrometeorology is the study of the atmosphere near the surface and its associated turbulence and transport of scalars. It deals with the small horizontal and short temporal scales of motion in meteorology. Typically, less than 1 km and 1 hr. The atmospheric boundary layer is formed as a consequence of the interactions between the atmosphere and the surface of earth. Micrometeorologists study these interactions. A variety of instrumentation is used to measure the atmospheric boundary layer and related turbulence. Boundary Layer Defined • A boundary layer is the layer next to a physical boundary such as the inside wall of a pipe. In our case the earth’s surface. • Transport processes modify the lowest 100 to 3000 m AGL (above ground level) of the troposphere creating the boundary layer. • The atmosphere above the boundary layer is called the free atmosphere, typically. • The boundary layer air separates the atmosphere from the earth’s surface. Actually, the atmosphere really doesn’t know that the earth is below it. Planetary Boundary Layer The layer of air influenced by surface friction is called the planetary boundary layer (PBL). Define the boundary layer as that part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a timescale of ~ 1 hr or less. Background: definitions What is turbulence? •Simply defined as perturbation from the mean. u (u u) How do we measure u′? u (u u) A sonic anemometer measures at very high sampling rates. This is typically at 10 or 20 Hz. Data from a high temporal resolution time series is used to calculate the mean of the time series and subsequently a perturbation from the mean. Once the perturbations are calculated, turbulent statistics can be calculated. u u u Turbulent Flux? • Transport of a quantity by eddies or swirls. • The covariance of a velocity component and any quantity. 1 N 1 1 N cov( w ) (W W ) ( ) wii w N i 0 N i 1 z Eddy Mixes some air down And some air up Turbulent Sensible Heat Flux ´= + w´= neg. ´= neg. 0 w´= + Net upward heat flux Variance One statistical measure of the dispersion of data about the mean is the biased variance N 1 1 2 ( Ai A ) 2 N i 0 It is a good measure of the dispersion of a sample of BL observations. However, recall that a A A Substituting this into the biased definition of variance: N 1 1 2 (ai) 2 a2 N i 0 Turbulence Kinetic Energy (TKE) During Fire • is a measure of the intensity of turbulence • simply the summed velocity variances 1 2 TKE e u v 2 w 2 2 Friction Velocity Friction velocity is the surface stress imposed on the flow when turbulence is generated by wind shear near the ground. The friction velocity is defined as 2 u* uw vw u w v w 2 1 4 are the u and v components of momentum fluxes. Surface Energy Budget The energy budget at the earth’s surface can be thought of as energy transferred through a layer at the surface. This includes turbulent energy transfer with the air above it, radiative transfer through the top of it, and molecular energy transfer into the soil below it. -Q* = QH + QE – QG + ΔQs -Q* = net upward radiation at surface QH = upward sensible heat flux QE = upward latent heat flux QG = upward molecular heat flux into the bottom ΔQS = storage/residual QH sun QE -Q* -QG Radiation Definitions Radiation flux: is the amount of radiation coming from a source per unit time in W. Radiant intensity: is the radiant flux leaving a point on the source, per unit solid angle of space surrounding the point. [W/steradian] Radiance: is the radiant flux emitted by a unit area of a source or scattered by a unit area of a surface.[W m-2 sr-1] Irradiance: is the radiant flux incident on a receiving surface from all directions, per unit area W m-2. Absorptance, reflectance, transmittance: fractions of the incident flux that are absorbed, reflected, or transmitted by a medium. Global Solar radiation: is the solar irradiance received on a horizontal surface [W m-2]. This is the sum of direct solar beam plus the diffuse component of skylight, and is the physical quantity measured by a pyranometer. Radiation Definitions Direct solar radiation: is the radiation emitted from the solid angle of the sun’s disc, received on a surface perpendicular to the axis of this cone, comprising mainly unscattered and unreflected solar radiation. At the top of the atmosphere this is usually: 1367 W m-2 . The direct solar radiation at the earth’s surface is the physical quantity measured by a pyrheliometer. Diffuse Solar Radiation: (Sky radiation) is the downward scattered and reflected radiation coming from the whole hemisphere. Diffuse radiation can be measured by a pyranometer mounted in a shadow band. Radiation Definitions Visible Radiation: is the spectral range of the standard observer. Most of the visible radiation lies between 400 nm and 730 nm. Ultraviolet Radiation: is the radiation with wavelengths in the range 100 to 400 nm. It is subdivided into three ranges: UVA is 315-400 nm, UVB is 280-315 nm, and UVC is 100-280 nm. Infrared Radiation: is the radiation with wavelengths longer than 730 nm. Radiation Definitions Photosynthetically Active Radiation (PAR): is the band of solar radiation between 400-700 nm that plants use in the photosynthesis process. PAR is usually expressed in moles of photons, a mole being Avogadro’s number of photons, 6.022 x 1023 photons. Albedo: is the fraction of incoming solar radiation to reflected solar radiation. α = SWreflect / SWincoming Methods of Measurement Two primary methods in the measurement of radiation: Thermal detection: response to heat gain or loss due to absorption of incoming or emission of outgoing radiation. Photovoltaic detectors: convert absorbed radiation to a voltage. Shortwave or solar radiation is defined to be 0.3 μm to 4 μm BUT since high-quality glass windows are transparent from 0.3 to 3 μm, an upper limit of 3 μm is generally accepted. Longwave radiation sensors have windows transparent to radiation from 3 or 4 μm to at least 50 μm . Types of instruments Radiation measuring instruments can be classified according to their use. The generic term for all radiation measuring instruments is the radiometer. A pyranometer is used to measure global solar radiation, so it must respond to both the direct solar beam and to diffuse sky radiation from the whole hemisphere. The sensing element must be a horizontal flat surface. A pyrgeometer is used to measure global, long-wave earth radiation. An instrument that measures the difference between incoming and outgoing radiation is called a pyrradiometer or net radiometer. Pyranometers Thermal detectors measure the temperature change induced by the heat gain (loss) due to absorption (emission) of radiation by a black surface. The temperature change is measured relative to a white surface or to the shell of the instrument. A thermopile is a nested array of thermocouples, usually 10 to 100. The advantage of a thermopile over a single thermocouple is that the number of thermocouple pairs in the pile multiplies the output voltage. The measurand is the incident radiation, irradiance. The sensor is the thermal plate and the raw output is the temperature difference between the absorbing plate instrument shell. Pyranometers Typical sensitivity of a thermopile pyranometer is 4 μV W-1 m2 and the time constant 5 s. Pyrgeometers Measure only earth or longwave radiation and therefore glass domes cannot be used. Typically, a silicon window (flat, not domed) is used to measure from 3 to 50 μm and the field of view is limited to 150 degrees. Some sensors incorporate an electrical heater to prevent dew or frost formation. Silicon windows are more stable and reliable than polyethylene. These instruments require temperature correction. The simplest form involves a calibration with two terms: E=a1V1 +a2Vt, where a1 and a2 are constants. V1 is the raw sensor voltage output, and Vt, is the voltage proportional to sensor temperatures. Turbulent Heat Fluxes from Tower at UH Coastal Center (10 min Ave of Covariances) 1000 LE_irga Hs Qg_2_Avg Rn_CNR1_Avg 800 600 400 200 0 -200 07/08/06 07/10/06 07/12/06 07/14/06 07/16/06 07/18/06 Surface Energy Balance: Q*=QH+QL+QG 07/20/06 07/22/06