Climate & Earth System Science Introduction to Meteorology & Climate MAPH 10050 Peter Lynch Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Chapter 05 Observing the Atmosphere SOME OBSERVING INSTRUMENTS Temperature Thermometer Mercury Resistance Pressure Barometer Humidity psychrometer, dew point hygrometer Wind - Anemometer Precipitation Rain Gauge, pluviometer. Instrument Enclosure Stephenson Screen 1
Thermometers Max-min Thermometers Platinum resistance thermometer Mecury-in in-glass thermometer Temperature Measurements Standard thermometer Platinum resistance thermometer Max-Min Min thermometers Remote measurement by observing thermal IR emissions. HUMIDITY MEASUREMENTS Wet-bulb psychrometer Dew-point hygrometer Remote measurements using microwave spectrometer Wet-bulb psychrometer Wet-bulb psychrometer Two thermometers, one wrapped with a wet cloth. Air passes over both. The wet bulb shows a lower temperature. Relative humidity obtained from pre- calculated tables. 2
Dew-point hygrometer Uses a laser beam and a mirror. The mirror is cooled down, and when dew is formed on the mirror, the laser beam is scattered. Pressure Measurement Mercury barometer long tube sealed at one end and filled with mercury, with open end immersed in a bath of mercury. Closed end has vacuum above mercury column. Height of column measures the pressure. Mercury barometers Aneroid barometer Aneroid without fluid Evacuated cell with thin corrugated wall that distorts under pressure changes Not as accurate as the mercury barometer Pressure Measurement Precision Aneroid Barometer 3
Barograph WIND MEASUREMENT Ground-based: Dines pressure-tube anemometer Spinning cup anemometers Propellers Space-based: Cloud motion vectors Satellite-borne radar Spinning-cup anemometer Spinning-cup anemometer Propeller wind gauge Dines Pressure-tube Anemometer Once the standard wind-measuring instrument Now a museum piece 4
WIND MEASUREMENT FROM SPACE Rain Gauge Conventional Rain Gauge Tipping-bucket Rain Gauge Tipping Rain Gauge Ceilometer 5
Upper Air Observations A radiosonde launch Meteorological balloons are launched regularly from a network of land stations and ships around the world They carry radiosonde equipment aloft. This sends back measurements which describe the vertical profile of the atmosphere. Vaisala radiosonde Radiosonde data coverage STOPPED HERE Valentia Ascent SATELLITE OBSERVATIONS Why do we need them? How do we get them? What can they tell us? Gravity Vs Speed Sun-synchronous polar orbiter, LEO sun-earth plane crosses the poles. orbit ~90 minutes Geosynchronous Two principle orbits are used: around the earth above the equator. 24 hour orbit - geostationary 6
Geosynchronous GOOD: Wide view Always same area Frequent images Sun-synchronous GOOD: High resolution, not dependant on latitude BAD: Low resolution, especially at high latitudes BAD: Only passes over some regions twice a day Types of imagery Radiometers sense radiation at different wavelengths, spectral channels Visible - senses reflected visible radiation from the sun in the 0.4-1.1 1.1μm m band Infra-red red - senses long wave radiation emitted by the earth and clouds in the 10-12 12 μm m band Water-vapour - senses long wave radiation emitted by water vapour in the 6-76 μm m band Visible image, EUMETSAT, 22 Oct 2007 Dark areas low levels of reflected light, e.g. oceans Bright areas snow, thick cloud, still water Better resolution than IR, WV. IR image, EUMETSAT, 22 Oct 2007 Dark areas Warm regions, sea, low cloud Bright areas Cold. High clouds Works day & night. Only works in daylight! WV image, EUMETSAT, 22 Oct 2007 Dark areas Dry air, e.g. descending stratospheric air Bright areas High water content, e.g. rapidly ascending frontal air Some pretty pictures Convection over land Can see storm development, jet streams. Works in areas of no clouds too. 7
Some pretty pictures Some pretty pictures Mountain (lee) waves Sea fog Some pretty pictures Weather Radar Snow over Scotland RADAR OBSERVATIONS RAdio Detection And Ranging Pulse of radio waves is sent out from transmitter. Time of the returned signal tells how far the cloud/precipitation is away. Strength of the scattered signal can be used to tell how much rain is falling. Max range ~ 240km Weather Radar 8
Estimate precipitation amount, track storms Plotted in colour. 20decibels (dbz( dbz) ) = rain. >55dBZ = hail updated every few minutes Doppler Effect Can measure speed of wind in rainy areas. Doppler mode image Cool - towards radar. Warm - away from radar Doppler Radar Fig. 5.21b A Doppler radar pointing skywards to measure wind speed. Detects changes in frequency of returning radar beams. Wind Profiler The horizontal wind is measured by oblique beams in orthogonal directions (e.g. east and north). The beams are tilted 15 to 30 degrees from the zenith, and the Doppler shift of the echoes in each direction are compared to determine the wind speed and direction. The fluctuations of the refractive index are due to turbulence. Turbulence on the scale of half the radar wavelength (Bragg scattering) gives a strong signal. The emitted signal is also sensitive to particles such as hydrometeors (rain, snow, hail...), cloud droplets, insects, birds, and airplanes. 9
Wind profiles Wind Profilers are expensive! High mode has better range, but lower resolution 10