GHz Water Vapor Absorption Line Research Articles

Waveguide-Based Split-Ring Resonators for Narrow-Band Filters Near 380 GHz

This work addresses the design of sub-terahertz narrow-band resonators for high performance and low-cost manufacturability. The intended application for these resonators is to realize narrow-band filters for passive millimeter-wave sounding of upper atmospheric humidity using the 380 GHz water vapor absorption line. Various narrow-band resonator designs and manufacturing processes were considered for this application. A design based on a waveguide split-ring resonator topology was selected to be developed and manufactured using laser machining. Experimental results are presented and compared with results from simulations for ten narrow-band resonators fabricated with a design center frequency in the WR-2.2 (325–500 GHz) waveguide band.

Impact of the Detection Channels Added by Fengyun Satellite MWHS-II at 183 GHz on Global Numerical Weather Prediction

Fine spectral detection can basically solve the problem of low vertical resolution at the 183 GHz water-vapor absorption line, and it is expected to become one of the main methods for next-generation geostationary and polar-orbiting satellites. Here, using data from Microwave Humidity Sounder II (MWHS-II) onboard the Chinese Fengyun 3D (FY-3D) satellite in the Global/Regional Assimilation and Prediction System (GRAPES) Four-Dimensional Variational (4D-Var) system of the China Meteorological Administration (CMA), we explore the assimilation application of the water-vapor absorption line at 183.31 ± 1 GHz, 183.31 ± 3 GHz and 183.31 ± 7 GHz, as well as 183.31 ± 1.8 GHz and 183.31 ± 4.5 GHz, two added channels, to assess the impact of adding the 183.31 ± 1.8 GHz and 183.31 ± 4.5 GHz sampling channels on data assimilation and numerical weather prediction. Our findings reveal a significant increase in the specific-humidity increment, which in the middle–upper troposphere is numerically much larger than in the lower troposphere. Specifically, the assimilation of 183.31 ± 1.8 GHz observations, positioned near the center of the water-vapor absorption line, results in a pronounced adjustment compared with the 183.31 ± 4.5 GHz observations. And under the strong constraint of the numerical model, the Root Mean Square Error (RMSE) of the wind field diminishes more significantly (by an average of 2–4%) after assimilating the water-vapor observations at greater heights.

Spaceborne differential absorption radar water vapor retrieval capabilities in tropical and subtropical boundary layer cloud regimes

Abstract. Differential absorption radar (DAR) near the 183 GHz water vapor absorption line is an emerging measurement technique for humidity profiling inside of clouds and precipitation with high vertical resolution, as well as for measuring integrated water vapor (IWV) in clear-air regions. For radar transmit frequencies on the water line flank away from the highly attenuating line center, the DAR system becomes most sensitive to water vapor in the planetary boundary layer (PBL), which is a region of the atmosphere that is poorly resolved in the vertical by existing spaceborne humidity and temperature profiling instruments. In this work, we present a high-fidelity, end-to-end simulation framework for notional spaceborne DAR instruments that feature realistically achievable radar performance metrics and apply this simulator to assess DAR's PBL humidity observation capabilities. Both the assumed instrument parameters and radar retrieval algorithm leverage recent technology and algorithm development for an existing airborne DAR instrument. To showcase the capabilities of DAR for humidity observations in a variety of relevant PBL settings, we implement the instrument simulator in the context of large eddy simulations (LESs) of five different cloud regimes throughout the trade-wind subtropical-to-tropical cloud transition. Three distinct DAR humidity observations are investigated: IWV between the top of the atmosphere and the first detected cloud bin or Earth's surface; in-cloud water vapor profiles with 200 meter vertical resolution; and IWV between the last detected cloud bin and the Earth's surface, which can provide a precise measurement of the sub-cloud humidity. We provide a thorough assessment of the systematic and random errors for all three measurement products for each LES case and analyze the humidity precision scaling with along-track measurement integration. While retrieval performance depends greatly on the specific cloud regime, we find generally that for a radar with cross-track scanning capability, in-cloud profiles with 200 m vertical resolution and 10 %–20 % uncertainty can be retrieved for horizontal integration distances of 100–200 km. Furthermore, column IWV can be retrieved with 10 % uncertainty for 10–20 km of horizontal integration. Finally, we provide some example science applications of the simulated DAR observations, including estimating near-surface relative humidity using the cloud-to-surface column IWV and inferring in-cloud temperature profiles from the DAR water vapor profiles by assuming a fully saturated environment.

Instrument Design and Performance of the High-Frequency Airborne Microwave and Millimeter-Wave Radiometer

The high-frequency airborne microwave and millimeter-wave radiometer (HAMMR) is a cross-track scanning airborne radiometer instrument with 25 channels from 18.7 to 183.3 GHz. HAMMR includes: low-frequency microwave channels at 18.7, 23.8, and 34.0 GHz at two linear-orthogonal polarizations; high-frequency millimeter-wave channels at 90, 130 and 168 GHz; and millimeter-wave sounding channels consisting of eight channels near the 118.75 GHz oxygen absorption line for temperature profiling and eight additional channels near the 183.31 GHz water vapor absorption line for water vapor profiling. HAMMR was deployed on a twin otter aircraft for a west coast flight campaign (WCFC) from November 4–17, 2014. During the WCFC, HAMMR collected radiometric observations for more than 53.5 h under diverse atmospheric conditions, including clear sky, scattered and dense clouds, as well as over a variety of surface types, including coastal ocean areas, inland water and land. These measurements provide a comprehensive dataset to validate the instrument.

Remote Sensing of Cloud Ice Water Path from SAPHIR Microwave Sounder Onboard Megha- Tropiques

This study derives the ice water path of the atmospheric column from the microwave sounder SAPHIR onboard Megha-Tropiques. SAPHIR (Sondeur Atmospherique du Profil d’Humidite Intertropicale par Radiometrie) is a cross-track, multichannel microwave humidity sounder with six channels ranging from 183.3 ± 0.2 to 183.3 ± 11 GHz near the 183.31 GHz water vapor absorption line. It measures the earth emitted radiation at these six frequencies. In this paper, Concurrent and collocated observations of Channel 183.31 ± 6.6 GHz, and 183.3 ± 11 GHz from SAPHIR and IWP (Ice water Path) from CloudSat have been used in the development the algorithm. A total of five sets of neural network model, each for 10° of incidence angle of SAPHIR have been developed. The model shows a correlation of 0.83 and RMSE of 195 g/m2 with an independent test dataset. The validation of the algorithm has been done by comparing the retrieval with various satellite derived IWP products such as CloudSat, GMI (Global precipitation measuring mission Microwave Imager) and MSPPS (Microwave Surface and Precipitation Products System). The instantaneous comparisons of IWP over a cyclonic storm ROANU demonstrate a good agreement between NN (Neural Network) derived IWP and CloudSat. A probability distribution of IWP indicates consistency between SAPHIR and CloudSat. A comparison of zonal mean between all the IWP products shows that SAPHIR performs better than GMI, and MSPPS.

The feasibility of water vapor sounding of the cloudy boundary layer using a differential absorption radar technique

Abstract. The feasibility of differential absorption radar (DAR) for the spaceborne remote profiling of water vapor within the cloudy boundary layer is assessed by applying a radar instrument simulator to large eddy simulations (LES). Frequencies near the 183 GHz water vapor absorption line attenuate too strongly to penetrate the large vapor concentrations that are ubiquitous in the boundary layer. However it is shown that lower frequencies between 140 and 170 GHz in the water vapor absorption continuum and on the wings of the absorption line, which are attenuated less efficiently than those near the line center, still have sufficient spectral variation of gaseous attenuation to perform sounding. The high resolution LES allow for assessment of the potential uncertainty in the method due to natural variability in thermodynamic and dynamic variables on scales smaller than the instrument field of view. The (160, 170) GHz frequency pair is suggested to best maximize signal for vapor profiling while minimizing noise due to undesired spectral variation in the target extinction properties. Precision in the derived water vapor is quantified as a function of the range resolution and the instrument precision. Assuming an observational spatial scale of 500 m vertical and 750 m full width at half maximum (FWHM) horizontal, measurement precision better that 1 g m−3 is achievable for stratocumulus scenes and 3 g m−3 for cumulus scenes given precision in radar reflectivity of 0.16 dBZ. Expected precision in the column water vapor (CWV) is achievable between 0.5 and 2 kg m−2 on these same spatial scales. Sampling efficiency is quantified as a function of radar sensitivity. Mean biases in CWV due to natural variability in the target extinction properties do not exceed 0.25 kg m−2. Potential biases due to uncertainty in the temperature and pressure profile are negligible relative to those resulting from natural variability. Assuming a −35 dBZ minimum detectable signal, 40 %(21.9 %) of stratocumulus(cumulus) atmospheric boundary layer range bins would be sampled. Simulated surface reflectivities are always greater than −5 dBZ, which implies the DAR technique could provide near spatially continuous observation of the CWV in subtropical boundary layers at a spatial resolution better than 1 km.

Optimization of Background Information and Layer Thickness for Improved Accuracy of Water-Vapor Profile Retrieval from Ground-Based Microwave Radiometer Measurements at K-Band

Ground-based microwave radiometers operating at frequencies near the 22.235 GHz (K-band) water vapor absorption line have been used extensively for remote sensing of water vapor in the troposphere, both the integrated amount and its profile. This paper explores the potential to use ground-based, zenith-pointing K-band radiometer measurements along with optimized background data sets consisting of radiosonde profiles to detect dynamic changes and gradients in water vapor profiles. To explore this capability, the HUMidity EXperiment 2011 (HUMEX11) was conducted at the U.S. Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Site near Lamont, OK, USA. This enables the choice of appropriate retrieval parameters to monitor temporal changes in atmospheric water vapor profiles. The results of this study illustrate that in a retrieval algorithm both the choice of the size of the background data set measured near the radiometer measurement time and the choice of atmospheric layer thickness affect the ability to remotely sense dynamic changes in water vapor. In general, it is found that background data sets of larger size provide better accuracy in a statistical sense but inhibit the ability to detect gradients.

Processing X/K band radio occultation data in the presence of turbulence

Canonical transform (CT) and full‐spectrum inversion (FSI) methods can be used in processing radio occultation data to retrieve transmission profiles, which are necessary in addition to bending angle profiles for the retrieval of atmospheric humidity independent of temperature. Low Earth orbiter–low Earth orbiter cross‐link occultations with X/K band frequencies can provide transmission data along the wing of the 22 GHz water vapor absorption line. The computation of transmission assumes spherical symmetry of the atmospheric real and imaginary refractivity. This condition is broken in the presence of turbulence, which can result in significant errors in transmission. We suggest the computation of the differential transmission from the differential CT/FSI amplitude to correct for the effect of turbulence. The efficiency of the method is tested by quasi‐realistic numerical end‐to‐end simulations. We explicitly modeled turbulence as random refractivity fluctuation field based on power form of the fluctuation spectrum. The simulations demonstrate that the error in the retrieved transmission can be significant in a single frequency channel, while the differential transmission can be retrieved with high accuracy. The new method does not impose any significant restriction on the frequency difference between the channels and provides very good differential transmission sensitivity if spacings of a few gigahertz are chosen (e.g., 10 and 17 GHz or 17 and 20 GHz).

Space-Borne High-Power Microwave Beam at 22.2 GHz (Water-Vapor Absorption Line) and its Potential for Atmospheric Modifications

AbstractThe National Aeronautics and Space Administration has been studying the feasibility of orbiting a Space Solar Power Laboratory at synchronous heights. The laboratory would he equipped with a 65 km2 solar-cell army, a 2.3 GHz, 10 GW transmitter, and a narrow-beam antenna. The steerable antenna beam can be oriented toward any region of the earth’s surface within tine of sight with power density as high as 100 to 1000 W m-2 at the earth’s surface. By retuning the transmitter to 22.2 GHz, a substantial amount of energy can be transferred to the atmosphere via absorption by atmospheric water vapor, water droplets from clouds, and rain droplets, as well as to the lossy terrain. These mechanisms have the potential of causing substantial atmospheric weather modifications—for example, the dissipation of fog layers or the amelioration of severe atmospheric pollution.