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2 Active Earth Remote Sensing for Atmospheric Applications
Pages 24-53

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From page 24...
... In order to collect the necessary data, active remote sensing of the atmosphere is conducted from both ground- and satellite-based radar systems. Our climate is changing faster than ever, breaking the normal cycles established since before the beginning of human civilization.1 Recent extreme weather events in the United States have received significant attention on both traditional and social media sites (Figures 2.1 and 2.2)
From page 25...
... Some of the fundamental atmospheric variables that can be retrieved and monitored by active sensors include vapor and liquid water content, wind vectors, cloud cover, rainfall rate, precipitation type, and ice cloud content. To develop and enhance climate/weather models, it is necessary to have a global atmospheric database of geophysical parameters, which can then be used to evaluate risk management options and develop preparedness plans, thereby reducing the risks of disasters.
From page 26...
... can enhance our mitigation/adaptation response and help make informed decisions to reduce vulnerability to extreme events. The database also supports the develop ment of improved weather prediction models.
From page 27...
... LAYERS OF THE ATMOSPHERE AND SCIENTIFIC APPLICATIONS From a scientific perspective, the atmosphere is segmented into layers (troposphere, stratosphere, mesosphere, and thermosphere) based on temperature characteristics as a function of height, with temperature defined as the average random kinetic energy of the molecules in the particular atmospheric stratum.
From page 28...
... SOURCE: NOAA, National Weather Service, "Standardized Temperature Profile" [image] , page last modified March 5, 2013, http://www.
From page 29...
... and advanced Doppler weather radars, which can characterize precipitation over hundreds of kilometers. These data are often fed into numerical weather prediction (NWP)
From page 30...
... 5  W.K. Hocking, Measurement of turbulent eddy dissipation rates in the middle atmosphere by radar techniques: A review, Radio Science 20(6)
From page 31...
... As discussed in later sections, weather radars are designed so that their transmitted electromagnetic waves are scattered primarily by hydrometeors (e.g., raindrops, ice crystals, hailstones)
From page 32...
... The choice of optimum frequencies is different for precipitation radar than for cloud radar. This is because the radar backscattering cross section of a spherical or quasi-spherical water droplet is governed by Rayleigh scattering if r/λ < 0.1 (where r is the radius of the droplet and λ is the electromagnetic wavelength)
From page 33...
... SOURCE: W.K. Hocking, "Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review," Radio Science, November 1985.
From page 34...
... Fresnel Reflection (or Partial Reflection) In the upper atmosphere, large-scale vertically stratified discontinuities in the refractive index can cause what is called Fresnel reflection or partial reflection.
From page 35...
... Since severe storms are primarily confined to the troposphere, the altitude coverage of weather radars is also limited to this atmospheric layer. Using a pencil beam, weather radars are scanned for a full 360 degrees of azimuth and then sequentially positioned to several higher elevation angles in order to fill the volume of interest.13 This type of volume scanning is used by the NEXRAD network and has proven successful in providing the data forecasters need to protect lives and 12  T.D.
From page 36...
... National Weather Service.
From page 37...
... Peak power levels can be as high as 1 MW for large systems with duty cycles of approximately 0.1 percent. As mentioned earlier, the beam shape is always a pencil beam requiring extensive scanning in both azimuth and elevation in order to provide complete volumetric coverage.
From page 38...
... As with weather radars, cloud radars use a pencil beam and sometimes have dual-polarization capabilities. Wind Profiling Radar Measuring winds in the upper atmosphere is fundamental to our understand ing of global circulation and for numerical modeling.
From page 39...
... Given the weak echoes from clear-air turbulence, peak power levels are as high as 1 MW with approximately 0.2 percent duty cycles. As is the case with most atmospheric radars, wind profilers use a pencil beam, but in this case a typical beamwidth is 5‑10 degrees.
From page 40...
... By receiving the reflected signal on several spatially separated antennas, cross-correlation techniques can be used to find the delay of the diffraction pattern on the ground, thereby providing a method to estimate the horizontal velocity. This so-called Spaced Antenna Drift method has been used for decades with HF/MF radars20 as well as with wind profiling radars.
From page 41...
... Meteor Radar As with HF/MF radars, meteor radars were developed in order to obtain wind measurements in the upper mesosphere and lower thermosphere. Although the physical mechanism by which meteor radars operate is the same as HF/MF radars (Fresnel reflection)
From page 42...
... , ~915 MHz or ~1.3 GHz (boundary layer radars) Bandwidth <1 MHz Antenna Large phased array, pencil beam, typical 3-5 degree beamwidth, ± 10 degree zenith angles Peak power 500 kW-1 MW Duty cycle <0.2% HF/MF Radar Scattering/reflection Fresnel Altitude coverage 70-100 km, depends on ionospheric state Frequency ~1-60 MHz Bandwidth <1 MHz Antenna Various dipole designs, 20 degree beamwidth Peak power 10-200 kW Duty cycle <1% Meteor Radar Scattering/reflection Fresnel Altitude coverage 80-100 km, altitudes of meteor ablation Frequency ~20-60 MHz Bandwidth <1 MHz Antenna Crossed-Yagi, ~100 degree beamwidth for all-sky coverage, interferometer Peak power 8-40 kW Duty cycle <1%
From page 43...
... was established with the goal of predicting severe weather using advanced radar technology in conjunction with numerical weather prediction models.23 An important aspect of the CASA radar concept, called distributed collaborative adaptive sensing (DCAS) , is to use a network of low-cost X-band radars, which provides the important advantage of coverage of the lower troposphere, the source of severe weather that directly impacts life and property.
From page 44...
... Their data are also essential for feeding climate and weather prediction models used to understand interactions within the entire biosphere -- croplands, forests, lakes, oceans, and the like, including urban and coastal areas. Radar remote sensing instruments operating from satellites are particularly effective in the observation of precipitation, clouds, and near-surface winds over the ocean, providing invaluable data of environmental parameters that are vital for a wide variety of scientific, commercial, and military applications and that enhance our ability to protect human life and property.
From page 45...
... The GPM dual-frequency precipitation radar (DPR) uses two frequencies, 13.6 GHz (KuPR)
From page 46...
... It measures ocean winds to support weather prediction, monitor hurricanes, and perform basic research. QuikScat's measurements were so essential to the prediction community that when QuikSCAT
From page 47...
... In addition to supporting basic scientific research, the data are operationally used in numerical weather forecasting and are especially vital during tropical cyclones for monitoring and predicting land-falling hurricanes and predicting storm intensity.24 These sensors provide data used to retrieve the motion of ice and rain within storms and to support climate studies and hurricane evolution studies and other scientific applications. SPECTRUM USAGE REQUIREMENTS FOR SATELLITE-BASED SENSING Atmospheric transmission windows impose restrictions on what frequencies can be used for remote sensing atmospheric parameters.25 In addition, some frequencies are desirable (or undesirable)
From page 48...
... 2007 Measure near-surface wind speed and direction over the global oceans. Monitor severe weather events such as hurricanes and typhoons (ESA)
From page 49...
... Active atmospheric sensors are designed to meet their performance requirements so long as RFI does not exceed the interference criteria specified in Recommendation ITU-R RS.1166, where I/N refers to the ration of interference to noise power. According to the said recommendation, Performance criteria for active sensors is defined in terms of the precision of mea surement of physical parameters and the availability of measurements free from harmful interference.
From page 50...
... for Active Space borne Sensors," Recommendation RS.1166-4, 2009, http://www.itu.int/rec/R-REC-RS.1166-4-200902-I. Table 2.5 summarizes the maximum allowable interference levels for current precipitation radars based on Recommendation ITU-R RS.1166 for each operating frequency.
From page 51...
... storm-related deaths, and one-third of this savings, $1 billion, to a reduction in property-related damage."26 Although NASA's missions are mostly scientific, not operational, its active sensors provide important meteorological and space weather data in near-real time that is also used to improve forecasting, monitoring, and mitigation planning.27 Table 2.7 lists some of the yearly savings that accrue to industries like agriculture and railways and to society in general given in the NOAA report. Active remote sensing contributes to these annual savings by providing data to weather services and continually improving the understanding of Earth's processes.
From page 52...
... Weiher, and A Khotanzad, The economic value of temperature forecasts in electricity generation, Bulletin of the American Meteorological Society 86:1765-1771, 2005.
From page 53...
... can only be observed at particular radio frequency bands. Similar physically based constraints exist for other atmospheric remote sensing techniques, such as turbulence, rain, and clouds.


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