MIC261~1.PPT

Canada Centre for Remote Sensing, Natural Resources Canada Other Active and Passive Microwave Sensors Natural Resources Ressources naturelles CanadaCanada Introduction Microwave sensors are either active (space-borne and ground based RADARs, radiometers, altimeters and scatterometers) or passive (ra...

Canada Centre for Remote Sensing, Natural Resources Canada Other Active and Passive Microwave Sensors Natural Resources Ressources naturelles CanadaCanada Introduction Microwave sensors are either active (space-borne and ground based RADARs, radiometers, altimeters and scatterometers) or passive (radiometers); While active microwave sensors generate microwave energy, transmit and receive backscattered or emitted energy; passive microwave sensors depend on microwave energy generated by the sun; Several active microwave sensors provide data for meteorology, hydrology, oceanography, and climate monitoring e.g. QuikSCAT Seawinds scatterometer, TRMM Precipitation Radar (PR), Jason, Envisat, and Navy GEOSAT altimeters, RADARSAT and ERS Synthetic Aperture Radars (SAR). Other missions will include the Metop Advanced Scatterometers (ASCAT), NPOESS RADAR altimeters, Jason2 and Navy GFO altimeters, and RADARSAT-2 SAR. Canada Centre for Remote Sensing, Natural Resources Canada Canada Centre for Remote Sensing, Natural Resources Canada Ground Based RADAR Systems Natural Resources Ressources naturelles CanadaCanada Ground based Weather RADAR Precipitation intensity is measured by a ground-based radar that bounces radar waves off of precipitation. The Local Radar base reflectivity product is a display of echo intensity (reflectivity) measured in dBZ (decibels). "Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. Often the reduction of speckle is desired to improve classification and/or for enhancement. The maximum range of the base reflectivity product is 143 miles (230 km) from the radar location. Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR NEXRAD (Next Generation Radar) can measure both precipitation and wind. The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves are scattered in all directions. A small portion of that scattered energy is directed back toward the radar. This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse. The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR Doppler effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the frequency change of the reflected pulse of energy and then convert that change to a velocity of the object, either toward or from the radar. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to "see" the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice. Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR The National Weather Service's 148 WSR-88D Doppler radars can detect most precipitation within approximately 90 mi of the radar, and intense rain or snow within approximately 155 mi. However, light rain, light snow, or drizzle from shallow cloud weather systems are not necessarily detected. Included in the NEXRAD data are the following products, all updated every 6 minutes if the radar is in Precipitation Mode or every 10 minutes if the radar is in Clear Air Mode (continue scrolling for further definitions) The products are described in subsequent slides Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR Base Reflectivity: This is a display of echo intensity (reflectivity) measured in dBZ; Composite Reflectivity: This product is used to reveal the highest reflectivity in all echoes. When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms. Storm Relative Mean Radial Velocity: This is the same as the Base Radial Velocity, but with the mean motion of the storm subtracted out. Vertically Integrated Liquid Water (VIL): VIL is the amount of liquid water that the radar detects in a vertical column of the atmosphere for an area of precipitation. Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR Base Radial Velocity: This is the velocity of the precipitation either toward or away from the radar (in a radial direction). No information about the strength of the precipitation is given. Precipitation moving toward the radar has negative velocity (blues and greens). Precipitation moving away from the radar has positive velocity (yellows and oranges). Precipitation moving perpendicular to the radar beam (in a circle around the radar) will have a radial velocity of zero, and will be colored grey. Echo Tops: The Echo Tops image shows the maximum height of precipitation echoes. The radar will not report echo tops below 5,000 feet or above 70,000 feet, and will only report those tops that are at a reflectivity of 18.5 dBZ or higher. In addition, the radar will not be able to see the tops of some storms very close to the radar. Echo top information is useful for identifying areas of strong thunderstorm updrafts. Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR Storm Total Precipitation: The Storm Total Precipitation image is of estimated accumulated rainfall, continuously updated, since the last onehour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event. 1 Hour Running Total Precipitation: The 1 Hour Running Total Precipitation image is an estimate of one-hour precipitation accumulation on a 1.1x1.1 nm grid. This product is useful for assessing rainfall intensities for flash flood warnings, urban flood statements and special weather statements. Canada Centre for Remote Sensing, Natural Resources Canada Ground based Weather RADAR Velocity Azimuth Display (VAD) Wind Profile: The VAD Wind Profile image presents snapshots of the horizontal winds blowing at different altitudes above the radar. These wind profiles will be spaced 6 to 10 minutes apart in time, with the most recent snapshot at the far right. Other example of Weather Radar is the Terminal Doppler Weather Radar (TDWR): is an advanced technology weather radar deployed near 45 of the larger airports in the U.S. They are higher resolution, and can "see" details in much finer detail close to the radar. Canada Centre for Remote Sensing, Natural Resources Canada https://www.youtube.com/watch? v=NNxSPJkKfak Canada Centre for Remote Sensing, Natural Resources Canada Space-borne Non-imaging Active Microwave Sensors Natural Resources Ressources naturelles CanadaCanada Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatteromete rs Natural Resources Ressources naturelles CanadaCanada Microwave Scatterometers A scatterometer transmits radar pulses and receives backscattered energy, the intensity of which depends on the roughness and dielectric properties of a particular target. Scatterometers were originally designed to measure oceanic surface winds, where the amount of backscatter depends on two factors – the size of the surface ripples on the ocean, and their orientation with respect to the propagation direction of the pulse of radiation transmitted by the scatterometer. The first is dependent on wind stress, and hence wind speed at the surface, while the second is related to wind direction. As a result, measurements by scatterometers may be used to derive both wind speed and direction. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers The main aim of these instruments is to achieve high accuracy measurements of wind vectors (speed and direction), so resolution is of secondary importance. (They generally produce wind maps with a resolution of order 25–50 km). Because scatterometers operate at microwave wavelengths, the measurements are available irrespective of weather conditions. Spaceborne scatterometers have provided continuous synoptic microwave coverage of the Earth for nearly two decades, starting with the ERS series in 1991, NSCAT on ADEOS, SeaWinds on QuikSCAT, and more recently ASCAT on Metop. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers The ERS and NSCAT instruments employed a fanbeam (multi-incidence) wind retrieval technique, whereas QuikSCAT employs a conically scanning (fixed incidence) technique. Increases in swath width capability now mean that a single instrument can provide around 90% coverage of global oceans on a daily basis. Information from scatterometers provides a unique source of data on sea surface wind speed and direction. This has important applications in weather and wave forecasting, the investigation of climate models and elaboration of marine wind climate. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers The assimilation of scatterometer data into atmospheric forecasting models greatly improves the description of cyclonic features which are so important in predicting future weather patterns. A large number of new, unforeseen, terrestrial and sea ice applications has emerged beyond the original ocean winds mission of scatterometers. These include: the measurement of sea ice extent and concentration; soil moisture; snow accumulation; and regional monitoring of ice shelves, rainforests and deserts. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers The daily global coverage of scatterometers in the polar regions and their ability to discriminate sea ice, ice sheets and icebergs, despite poor solar illumination and frequent cloud cover, make them excellent instruments for large-scale systematic observations of polar ice. Depending on the wavelength of the radiation a distinction can be made between C-band and Kuband scatterometers. The two radar types are compared with each other in Table 1. The wavelength of the radar is chosen according to the sampling scale. Thus, Ku-band radars with a wavelength of about 2 cm are sensitive to rain. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Table 2: Comparison of C-band and Ku-band scatterometers. C-band Ku-band Frequency 5.255 GHz 13.4 GHz Wavelength 5 cm 2 cm Scatterometer ASCAT-B RapidSCAT Polarization VV-pol Dual-polarization Sampling 12.5-25 km 25 km- 50 km Geometry Static Rotating antenna Swath Double (550km each) Single Limitations (C-band): less detection in the higher wind range (> 60 kt), sea ice, coastal coverage - Land contamination; Limitations (Ku-Band): sensitive to rain, coastal coverage Land contamination Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Figure 8: Overview of finished, current and proposed satellite missions with scatterometeres onboard. The only exception is 'RapidSCAT', which is a scatterometer on the international space ship (ISS). (Source: http://ceos.org/ourwork/virtualCanada Centre for Remote Sensing, Natural Resources Canada constellations/osvw/). Microwave Scatterometers The scattering mechanism that occurs when electromagnetic radiation and water waves interact with similar wave lengths is called Bragg scattering. The Bragg scattering explains the effects of the reflection of electromagnetic waves on periodic structures whose distances are in the order of the wavelength. Only this Bragg scattering makes it possible to derive wind speed and direction as it changes the total amount of energy that returns to the satellite. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Bragg scattering is dependent on the incidence angle. This means that there is an optimal range of the incidence angle for scatterometer measurements. Typically angles between 30° and 60° are used as they provide the largest sensitivity to changes in wind speed. The design of scatterometers takes these facts into account which means that scatterometers only see the ocean surface from the side, never directly from above. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers By measuring the backscattered signal by the ripples and waves, scatterometers derive wind direction and speed from the roughness of the sea. The backscattering is dependent on the incidence and azimuth angles: the dependence on the incidence angle helps to derive wind speed, while the dependence on the azimuth angle allows deriving wind direction. Two kinds of instruments are used for this measuring principle:  fan beam scatterometers (ASCAT)  rotating pencil beam scatterometers (RapidSCAT) Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Measuring geometry of a fan beam antenna onboard of ASCAT. The working method of RapidScat as an example of rotating pencil beam scatterometers https://climate.nasa.gov/climate_resources/227/qu ikscat-a-pioneer-of-satellite-scatterometry-video/ Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Limitations In general, the quality of scatterometer winds is quite good. But there are some situations where the data can be compromised: (a) sea ice and land contamination (b) large spatial wind variability (for example in the vicinity of fronts and low pressure centers, e.g. downbursts) (c) rain - especially in Ku-band systems (d) higher wind speeds (>60 knots) - especially in Cband systems Canada Centre for Remote Sensing, Natural Resources Canada Microwave Scatterometers Example for sea ice and land contamination from the QuikScat scatterometer Canada Centre for Remote Sensing, Natural Resources Canada Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeters Natural Resources Ressources naturelles CanadaCanada Microwave Altimeter An altimeter is also referred to as an altitude meter; it is a device to compute an object's height above a stable point. The estimation of altitude is known as altimetry. Bathymetry, which is the computation of depth beneath the sea surface, is associated with altimetry. The height is usually measured between the altimeter platform, i.e., satellite or aircraft, and the Earth's surface. In other words, altimeters are nadir-looking pulseradars; they transmit short microwave pulses and measure the round trip time delay to targets to determine their distance from the air- or spaceborne sensor. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter https://www.youtube.com/watch?v=OXf4 Mf4TQeI Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter An altimeter is also referred to as an altitude meter; it is a device to compute an object's height above a stable point. The estimation of altitude is known as altimetry. Bathymetry, which is the computation of depth beneath the sea surface, is associated with altimetry. The height is usually measured between the altimeter platform, i.e., satellite or aircraft, and the Earth's surface. In other words, altimeters are nadir-looking pulseradars; they transmit short microwave pulses and measure the round trip time delay to targets to determine their distance from the air- or spaceborne sensor. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter Radar altimeters are active sensors which use the ranging capability of radar to measure the surface topography profile along the satellite track. They provide precise measurements of a satellite’s height above the ocean and, if appropriately designed, over land/ice surfaces by measuring the time interval between the transmission and reception of very short electromagnetic pulses. To date, most spaceborne radar altimeters have been nonimaging, wide-beam (pulse-limited) systems operating from low Earth orbits. Such altimeters are useful for relatively smooth surfaces such as oceans and low relief land surfaces, but are less effective over high relief continental terrain as a result of their large radar footprint (of the order of 25 km). Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter Radar altimeters have been flown on a number of satellites. Seasat was the first ocean-oriented mission carrying an altimeter package (including a precise orbit determination system) for the measurement of ocean circulation. A satellite altimetry revolution happened with the launch in 1992 of the US-French Topex/Poseidon mission. Carrying two high-precision altimeters, a multichannel microwave radiometer, and several precise orbit determination devices on a dedicated, high-altitude (1,336 km), low inclination (66°), non-Sun-synchronous orbit, it enabled the largescale ocean circulation to be accurately measured. The European ERS-1 (from 1991) and ERS-2 (complete July 2011) also provided long time-series of complementary altimetric observations from a Sun-synchronous polar orbit. These observations were continued with Jason-1 (launched in 2001), Envisat (launched in 2002), and Jason-2 (launched June 2008). Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter A variety of parameters may be inferred using the information from radar altimeter measurements. These include: time-varying sea surface height (ocean topography), the lateral extent of sea ice and the altitude of large icebergs above sea level, as well as the topography of land and ice sheets, and even that of the sea floor. Topographical maps of the structure of the Arctic sea floor have not only revealed new mineral deposits, but they also provide new insights into how a large part of the ocean basin was formed about 100 million years ago. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Altimeter Observations by current and future radar altimeters of trends in the ice masses of the Earth are of principal importance in testing the predicted thinning of Arctic sea ice due to global warming. Satellite altimetry also provides information which is used in mapping sea surface wind speeds and significant wave heights. Precision ocean altimetry applications for sea level monitoring and ocean circulation studies require more accurate, independent measurements of the geoid – derived from the instruments described in the ‘gravity field’ category. Canada Centre for Remote Sensing, Natural Resources Canada Canada Centre for Remote Sensing, Natural Resources Canada Microwave Radiometers Natural Resources Ressources naturelles CanadaCanada Microwave Radiometer Microwave radiometers measure the thermal radiation of the atmosphere, which provides information about the water-vapor and liquid-water content. Also temperature profiles of the lower troposphere can be inferred. Typically, microwave radiometers are not sensitive to the thermal radiation of ice. Thus, only information about the water-vapor and liquid-water content are retrieved, which can be used for the investigation of mixed-phase clouds. Microwave radiometers measure the thermal radiation of the atmosphere. When appropriate detection frequencies are used, the emission of microwave radiation of atmospheric trance gases, of liquid water, and of ice crystals can be measured. Canada Centre for Remote Sensing, Natural Resources Canada Microwave Radiometer Microwave radiometers measure the thermal radiation of the atmosphere, which provides information about the water-vapor and liquid-water content. Also temperature profiles of the lower troposphere can be inferred. Typically, microwave radiometers are not sensitive to the thermal radiation of ice. Thus, only information about the water-vapor and liquid-water content are retrieved, which can be used for the investigation of mixed-phase clouds. Microwave radiometers measure the thermal radiation of the atmosphere. When appropriate detection frequencies are used, the emission of microwave radiation of atmospheric trance gases, of liquid water, and of ice crystals can be measured. https://www.esa.int/ESA_Multimedia/Videos/2017/11/Sentinel-3_microwave_ra diometer Canada Centre for Remote Sensing, Natural Resources Canada