Microwave emissions from snow

The radiation emitted from dry and wet snowpack in the microwave region (1 to 100 GHz) is discussed and related to ground observations. Results from theoretical model calculations match the brightness temperatures obtained by truck mounted, airborne and spaceborne microwave sensor systems. Snow wetness and internal layer structure complicate the snow parameter retrieval algorithm. Further understanding of electromagnetic interaction with snowpack may eventually provide a technique to probe the internal snow propertie

Severe storm identification with satellite microwave radiometry: An initial investigation with Nimbus-7 SMMR data

The severe weather characteristics of convective storms as observed by the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) are investigated. Low 37 GHz brightness temperatures (due to scattering of upwelling radiation by precipitation size ice) are related to the occurrence of severe weather (large hail, strong winds or wind damage, tornadoes and funnel clouds) within one hour of the satellite observation time. During 1979 and 1980 over the United States there were 263 storms which had very cold 37 GHz signatures. Of these storms 15% were severe. The SMMR detected hail, wind, and tornadic storms equally well. Critical Success Indices (CSI's) of 0.32, 0.48, and 0.38 are achieved for the thresholding of severe vs. nonsevere low brightness temperature events during 1979, 1980, and the two years combined, respectively. Such scores are comparable to skill scores for early radar detection methods. These results suggest that a future geostationary passive microwave imaging capability at 37 GHz, with sufficient spatial and temporal resolution, would allow the detection of severe convective storms. This capability would provide a useful complement to radar, especially in areas not covered by radar

Progress Towards the Remote Sensing of Aircraft Icing Hazards

NASA has teamed with the FAA, DoD, industry, and academia for research into the remote detection and measurement of atmospheric conditions leading to aircraft icing hazards. The ultimate goal of this effort is to provide pilots, controllers, and dispatchers sufficient information to allow aircraft to avoid or minimize their exposure to the hazards of in-flight icing. Since the hazard of in-flight icing is the outcome of aircraft flight through clouds containing supercooled liquid water and strongly influenced by the aircraft s speed and configuration and by the length of exposure, the hazard cannot be directly detected, but must be inferred based upon the measurement of conducive atmospheric conditions. Therefore, icing hazard detection is accomplished through the detection and measurement of liquid water in regions of measured sub-freezing air temperatures. The icing environment is currently remotely measured from the ground with a system fusing radar, lidar, and multifrequency microwave radiometer sensors. Based upon expected ice accretion severity for the measured environment, a resultant aircraft hazard is then calculated. Because of the power, size, weight, and view angle constraints of airborne platforms, the current ground-based solution is not applicable for flight. Two current airborne concepts are based upon the use of either multifrequency radiometers or multifrequency radar. Both ground-based and airborne solutions are required for the future since groundbased systems can provide hazard detection for all aircraft in airport terminal regions while airborne systems will be needed to provide equipped aircraft with flight path coverage between terminal regions

MERITXELL: the Multifrequency Experimental Radiometer with Interference Tracking for Experiments over Land and Littoral—instrument description, calibration and performance

MERITXELL is a ground-based multisensor instrument that includes a multiband dual-polarization radiometer, a GNSS reflectometer, and several optical sensors. Its main goals are twofold: to test data fusion techniques, and to develop Radio-Frequency Interference (RFI) detection, localization and mitigation techniques. The former is necessary to retrieve complementary data useful to develop geophysical models with improved accuracy, whereas the latter aims at solving one of the most important problems of microwave radiometry. This paper describes the hardware design, the instrument control architecture, the calibration of the radiometer, and several captures of RFI signals taken with MERITXELL in urban environment. The multiband radiometer has a dual linear polarization total-power radiometer topology, and it covers the L-, S-, C-, X-, K-, Ka-, and W-band. Its back-end stage is based on a spectrum analyzer structure which allows to perform real-time signal processing, while the rest of the sensors are controlled by a host computer where the off-line processing takes place. The calibration of the radiometer is performed using the hot-cold load procedure, together with the tipping curves technique in the case of the five upper frequency bands. Finally, some captures of RFI signals are shown for most of the radiometric bands under analysis, which evidence the problem of RFI in microwave radiometry, and the limitations they impose in external calibration.Peer ReviewedPostprint (published version

Microwave remote sensing from space for earth resource surveys

The concepts of radar remote sensing and microwave radiometry are discussed and their utility in earth resource sensing is examined. The direct relationship between the character of the remotely sensed data and the level of decision making for which the data are appropriate is considered. Applications of active and a passive microwave sensing covered include hydrology, land use, mapping, vegetation classification, environmental monitoring, coastal features and processes, geology, and ice and snow. Approved and proposed microwave sensors are described and the use of space shuttle as a development platform is evaluated

Global unsupervised assessment of multifrequency vegetation optical depth sensitivity to vegetation cover

Vegetation optical depth (VOD) has contributed to monitor vegetation dynamics and carbon stocks at different microwave frequencies. Nevertheless, there is a need to determine which are the appropriate frequencies to monitor different vegetation types. Also, as only a few VOD-related studies use multi-frequency approaches, it is needed to evaluate their applicability. Here, we analyze the sensitivity of VOD at three frequencies (L-, C- and X-bands) to different vegetation covers by applying a global-scale unsupervised classification of VOD. A combination of these frequencies (LCX-VOD) is also studied. Two land cover datasets are used as benchmarks and, conceptually, serve as proxies of vegetation density. Results confirm that L-VOD is appropriate for monitoring the densest canopies but, in contrast, there is a higher sensitivity of X-, C- and LCX-VOD to the vegetation cover in savannahs, shrublands and grasslands. In particular, the multi-frequency combination is the most suited to sense vegetation in savannahs. Also, our study shows a vegetation-frequency relationship which is consistent with theory: the same canopies (e.g., savannahs and some boreal forests) are classified as lighter ones at L-band due to its higher penetration (e.g., as shrublands), but labeled as denser ones at C- and X-bands due their saturation (e.g., boreal forests are labeled as tropical forests). This study complements quantitative approaches investigating the link between VOD and vegetation, extends them to different frequencies, and provides hints on which frequencies are suitable for vegetation monitoring depending on the land cover. Conclusions are informative for upcoming multi-frequency missions, such as the Copernicus Multi-frequency Image Radiometer (CIMR).This work has been funded by the Spanish Ministry of Science, Innovation and Universities and the European Regional Development Fund (ERDF, EU) through projects ESP2017-89463-C3-2-R, RTI2018-096765-A-100 and MDM-2016-0600, and by the project PID2020-114623RB-C32, funded by MCIN/ AEI /10.13039/501100011033. D. Chaparro has also received funding from “la Caixa” Foundation (ID 100010434), under agreement LCF/PR/MIT19/51840001 (MIT-Spain “La Caixa” Foundation Seed Fund), and from the XXXIII Ramón Areces Postdoctoral Fellowship.Peer ReviewedPostprint (published version

Summary of the Active Microwave Workshop, chapter 1

An overview is given of the utility, feasibility, and advantages of active microwave sensors for a broad range of applications, including aerospace. In many instances, the material provides an in-depth examination of the applicability and/or the technology of microwave remote sensing, and considerable documentation is presented in support of these techniques. An assessment of the relative strengths and weaknesses of active microwave sensor data indicates that satisfactory data are obtainable for several significant applications

Comparisons of precipitation measurements by the Advanced Microwave Precipitation Radiometer and multiparameter radar

Includes bibliographical references.Multiparameter microwave radar measurements are based on dual-polarization and dual-frequency techniques and are well suited for microphysical inferences of complex precipitating clouds, since they depend upon the size, shape, composition, and orientation of a collection of discrete random scatterers. Passive microwave radiometer observations represent path integrated scattering and absorption phenomena of the same scatterers. The response of the upwelling brightness temperatures TB to the precipitation structure depends on the vertical distribution of the various hydrometeors and gases, and the surface features. As a result, combinations of both active and passive techniques contain great potential to markedly improve the longstanding issue of precipitation measurement from space. The NASA airborne Advanced Microwave Precipitation Radiometer (AMPR) and the National Center for Atmospheric Research (NCAR) CP-2 multiparameter radar were jointly operated during the 1991 Convection and Precipitation/Electrification experiment (CaPE) in central Florida. The AMPR is a four channel, high resolution, across-track scanning total power radiometer system using the identical multifrequency feedhorn as the widely utilized Special Sensor Microwave/Imager (SSM/I) satellite system. Surface and precipitation features are separable based on the TB behavior as a function of the AMPR channels. The radar observations are presented in a remapped format suitable for comparison with the multifrequency AMPR imagery. Striking resemblances are noted between the AMPR imagery and the radar reflectivity at successive heights, while vertical profiles of the CP-2 products along the nadir trace suggest a storm structure consistent with the viewed AMPR TB. Directly over the storm cores, the difference between the 37 and 85 GHz TB was noted to approach (and in some cases fall below) zero. Microwave radiative transfer computations show that this is theoretically possible for hail regions suspended aloft in the core of strong convective storms.This work was supported by the NASA Earth Science and Applications Division under Grant NAG8-890. The National Center for Atmospheric Research is sponsored by the National Science Foundation