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Unformatted text preview: CLOUDS AND SATELLITE IMAGERY SATELLITE MSC 243 Lecture #3 9/8/11 LAB 2, Question 5 Road Map: September Road Today: Homework 1 given Tuesday 13th: Lab on Satellite imagery Tuesday 13th: Homework 1 due Thursday 15th: Radar Tuesday 20th: Hurricane Lab (if there is a hurricane!) Thursday 22nd: Fronts and Forecasting Thursday 22nd: Homework 2 given Monday 26th: We start forecasting! Surface Winds Surface Surface winds spiral into and counter clockwise around a low pressure center http://www.rap.ucar.edu/weather/model/ Surface winds spiral out from and clockwise around a high pressure center Temperature Advection Temperature If Surface winds carry (“advect”) warmer/colder air, the local temperature will increase/decrease Blue circle: area of cold advection Red circle: area of warm advection http://www.rap.ucar.edu/weather/model/ This Lecture This Types of Clouds Satellites • • • • • Geostationary and Polar Radiative Transfer Visible, Infrared and Water Vapor Identifying clouds from satellites Other types of satellite imagery Types of Clouds Types Sorted by height • High (> 7km) Cirrus (ci), Cirrostratus (cs), Cirrocumulus (cc) • Middle (2­7km) Altostratus (as), Altocumulus (ac) • Low (< 2km) stratus (st), cumulus (cu), strato­ cumulus (sc) Types of Clouds Types Sorted by structure • • • • Cirrus – Composed of Ice, ‘wispy’ Cumulus – Detached elements ‘puffy’ Stratus – Uniform Layer, ‘sheetlike’ Nimbo – Precipitation producing Nimbostratus Cumulonimbus Cirrus Clouds Cirrus Ice crystals (frozen super­cooled water droplets). Fair weather, point in direction of air movement at their elevation. Often associated with changing weather. Cumulus Clouds Cumulus As Earth is warmed by the sun, bubbles of warmer air (thermals) rise upward from the surface. Water vapor in a thermal cools and condenses as it rises and mixes with the surrounding air. Stratus Clouds Stratus Uniform gray cloud that covers the entire sky. Sometimes accompanied by a light mist or drizzle. Cumulonimbus Cumulonimbus Thunderstorm clouds, much larger and more vertically developed than fair weather cumulus. Formed by convective updrafts. Water droplets at lower levels, ice crystals at higher levels. Radiation in the Atmosphere Radiation Effect of Clouds on Temperature Effect When forecasting maximum temperatures, remember that thick clouds absorb solar radiation, reducing heating at the surface. When forecasting minimum temperatures, remember that thick clouds inhibit surface cooling. Note that thin cirrus clouds have less of an effect than thick low clouds. Satellite Imagery Satellite 1st weather satellite: TIROS 1 (1960) Satellites observe clouds as well as non­visible radiation (aerosols, ozone) Rest of lecture: Orbits Radiative Transfer Interpretation of images Other types of satellite imagery Geostationary Orbiting Satellites Geostationary Satellite observes same area at all times. Continuous sampling. 36000km from Earth’s surface! Polar Orbiting Satellites Polar Satellite observes globe in “swaths” 700­1700km from Earth’s surface Polar Orbiting Satellites Polar Radiative Transfer Radiative Electromagnetic radiation: alternating electric and magnetic fields. Radiative Transfer Radiative Radiation is specified by wavelength or frequency. Speed = freq x length microwave Radiative Transfer Radiative Electromagnetic radiation can transport energy. Fundamental radiation unit: radiance Radiance = energy per unit time per unit wavelength per unit solid angle crossing a unit area 90o to beam. RADIOMETER measures energy per unit time striking it, at select frequencies (or wavelengths). NASA Satellite Radiometer NASA Radiative Transfer Radiative Each object (including us!) emits radiation with a particular frequency spectrum. Satellite instruments are designed to measure radiances in specific frequency channels, corresponding to the desired frequency that objects / gases emit at a given temperature. Radiative Transfer Radiative Radiative energy depends on wavelength λ and also temperature T. One can estimate temperature from brightness: “brightness temperature”. Total emitted radiation E = σ T4 Radiation reaching a body is then ABSORBED, REFLECTED or SCATTERED OUT by the body. Visible Image Visible VIS imagery indicates the amount of solar radiation reflected from the Earth and clouds, in the 0.4 to 0.7 micron range. A VIS image is an approximation of the albedo (% of sunlight reflected by the Earth’s surface or clouds). Light tones show high reflectivity, darker tones show low reflectivity. Features on the surface of the Earth or in the atmosphere vary in their reflectivity and can be discerned on a VIS image. No data at night. The large, thick clouds appear white since they have a high albedo. Thinner clouds appear light to medium gray. The ocean, with a very low albedo, appears nearly black. The land, characterized by albedos that depend on the nature of the surface, appears as various shades of gray. Infrared Image Infrared The highest (and coldest) cloud tops appear blue (or white on b/w image). Lower clouds appear as orange (lighter shades of gray), and warmer land and water surfaces appear as red (darker shades of gray). The IR sensors on board the polar orbiting and geostationary satellites measure the amount of infrared energy emitted by the Earth and the atmosphere. Because the amount of energy emitted depends on the temperature of the surface, IR imagery is essentially a picture of the surface and cloud top temperatures. This information can be used to observe thermal properties of the Earth and the atmosphere. Water Vapor Image Water In addition to the visible and thermal infrared images, it is possible to record energy absorbed and re­radiated by water vapor. Infrared energy radiated by water vapor at wavelengths of 6.7 and 7.3 microns is measured by the satellite radiometer and converted to “brightness temperature”. The grayness of each pixel in a WV image corresponds to a specific brightness temperature. Water vapor imagery shows the altitude of the highest moist layer in the atmosphere, not just cloud patterns. Meteorologists are able to observe large­ scale circulation patterns even when clouds are not present. The red (darker on b/w) regions are where very little water vapor exists in the middle and upper troposphere, and the purple/blue (lighter on b/w) regions are moist. What is this? What VIS IR What this is What Snow Cirrus Stratus Snow Cirrus Stratus Snow is reflective, but not too cold. Stratus in Mississippi is the same, and the wispy but cold values in Ontario are cirrus http://www.rap.ucar.edu/weather/satellite/ What is this? What VIS IR What this is What Cumulus Cumulo Nimbus Cirrus Cumulus Cumulo Nimbus Cirrus The low (warm) and scattered clouds in Georgia are cumulus. The scattered by higher clouds in central Florida are cumulonuimbus. The wispy clouds southwest of Florida are cirrus clouds What this is What MCN TPA Cumulus Cumulo Nimbus Cirrus MCN TPA Cumulus Cumulo Nimbus Cirrus KMCN 272053Z 28008KT 10SM FEW055 33/19 A2999 RMK AO2 SLP152 T03280194 58023= KTPA 272053Z 01015KT 6SM TS HZ FEW020 BKN040CB BKN080 OVC250 28/23 A3005 RMK AO2 TSB45 SLP175 FRQ LTGICCG TS OHD-N MOV S TCU E VCSH VIS LWR N T02780228 56015= Other Satellite Products Other Satellites observe at many frequencies. The most widely used frequencies are not able to ‘see’ through clouds. Here is a collection of satellite products useful for hurricane analysis and prediction. Winds from Satellite – Upper Level Winds Water Vapor and Infrared. http://tropic.ssec.wisc.edu/ Winds from Satellite – Low Level Winds Tracking the velocity of low­level clouds Wind Shear Wind 200 hPa wind – 850 hPa wind Upper Level Divergence Upper “Spreading out” of wind at upper levels. Low Level Vorticity Low “Local spin” of atmospheric elements Saharan Air Layer Saharan This imagery is created by differencing the 12.0 and 10.7µm infrared channels. This difference is sensitive to the presence of dry and/or dusty air in the lower atmosphere (~600­850 hPa or ~4,500­1,500 m) and is denoted by the yellow to red shading. This imagery is useful for monitoring the position and movement of dry air masses such as the Saharan Air Layer (SAL). Advanced Microwave Sounding Unit Advanced Tropical Rainfall Measuring Mission (TRMM) – Microwave Imager (TRMM) Before eyewall replacement After eyewall replacement Sea Surface Temperature Sea ...
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