Mapping the atmospheric water vapor by integrating microwave radiometer and GPS measurements

This paper deals with a procedure to generate maps of the integrated precipitable water vapor (IPWV) over the Mediterranean area by using estimates from a global positioning system (GPS) network over land and from the Special Sensor Microwave/Imager (SSM/I) over sea. In particular, we investigate the application of the kriging geostatistical technique to obtain regularly spaced IPWV values. The horizontal spatial structure of water vapor retrieved by SSM/I is explored by computing variograms that provide a measure of dissimilarity between pairs of IPWV values for the region of interest. Because the water vapor density decreases with height, the GPS station elevation is accounted for in the interpolation procedure. In this respect, the potential of the kriging with external drift relative to the ordinary kriging is evaluated by applying a test based on the cross-validation approach. Case studies are presented and qualitatively compared to the corresponding Meteosat infrared images. A quantitative comparison with an independent source of information, such as IPWV computed from radiosonde observations and from European Centre for Medium-Range Weather Forecasts analysis, is also performed.     

Performance characteristics of a 300-GHz radiometer and some atmospheric attenuation measurements

A 300-GHz Dicke-type superheterodyne radiometer receiver was used for measurements of atmospheric attenuation of electromagnetic waves over an open path at frequencies near 300 GHz. The average measured values of horizontal attenuation at 304 GHz and 316 GHz, presumably due to atmospheric water vapor absorption, were, respectively, 3.35 dB/km and 5.55 dB/km per g/m³of water vapor density. Absorption variations at 304 GHz with respect to water vapor density were shown in the measured results. The variation of the effective zenith sky temperature with respect to atmospheric water vapor density was also determined. The minimum detectable temperature difference(Delta T)_{min}, was obtained by measuring the rms value of noise in the receiver output. The best value achieved was3.16degK. Based on this result, the receiver noise figure and the mixer conversion loss were determined indirectly. The results were 31.4 dB and 22.9 dB, respectively. A blackbody radiation source served to calibrate the radiometer.     

An airborne radiometer for stratospheric water vapor measurements at 183 GHz

The Airborne Millimeter- and Submillimeter Observing System (AMSOS) is a total-power radiometer for observations of the 183.3-GHz water vapor rotational line, operated onboard a Learjet aircraft of the Swiss Air Force. The radiometer is also used to observe the 175.45-GHz ozone line in the other sideband. The neatly designed quasi optics provide a regular and narrow output beam with a half-power beam-width angle of 1.2/spl deg/ and efficient sideband switching. A /spl lambda//4-quasi-optical isolator is used for baseline reduction securing attenuation of internal reflections by more than 30 dB. A low noise temperature of the ambient-temperature-operating system (1900 K) and excellent target pointing (better than 0.1/spl deg/) provide a good duty cycle and reliable calibration. A reliable control over the radiometer's operational parameters, like system stability and system temperatures, and higher automatization were required to come up with high demands of an onboard operation. The measured spectra look typical for the region and time where they were observed.     

The effect of the half-width of the 22-GHz water vapor line on retrievals of temperature and water vapor profiles with a 12-channel microwave radiometer

We show that observed biases in retrievals of temperature and water vapor profiles from a 12-channel microwave radiometer arise from systematic differences between the observed and model-calculated brightness temperatures at five measurement frequencies between 22 and 30 GHz. Replacing the value for the air-broadened half-width of the 22-GHz water vapor line used in the Rosenkranz absorption model with the 5% smaller half-width from the HITRAN compilation largely eliminated the systematic differences in brightness temperatures. An a priori statistical retrieval based on the revised model demonstrated significant improvements in the accuracy and vertical resolution of the retrieved temperature and water vapor profiles. Additional improvements were demonstrated by combining the MWRP retrievals with those from the GOES-8 sounder and by incorporating brightness temperature measurements at off-zenith angles in the retrievals.     

MIR measurements of atmospheric water vapor profiles

Three subjects related to atmospheric water vapor profiling using the 183.3 GHz absorption line are discussed. First, data acquired by an airborne millimeter-wave imaging radiometer (MIR) over ocean surface in the western Pacific are used to estimate three-dimensional (3D) distribution of atmospheric water vapor. The instrument's radiometric measurements with mixed vertical and horizontal polarizations require modifications to the retrieval algorithm used in the past. It is demonstrated that, after the modifications, the new algorithm can provide adequate retrieval of water vapor profiles, even though the measured data are of mixed polarizations. Next, the retrieved profiles, in terms of water vapor mixing ratio ρ (g/kg), are compared with those measured in near concurrence by dropsondes from a research aircraft in the western Pacific and by a ground-based Raman lidar at Wallops Island, Virginia. The ratio of the standard deviation to the mean ρ is found to be 0.12 at 0.25 km altitude and gradually degraded to 0.67 at the highest altitude of the retrieval of 10.25 km. Finally, the effect of the “initial guess” relative humidity profile on the final retrieved product is analyzed with respect to the condition for the convergent retrieval. It is found that the effect is minimal if the initial profile is not unrealistically different from the true one. If the initial profile is very different from the true one, the final retrieved product could be subject to a substantial error. Tightening of the convergent condition in the retrieval helped reduce magnitude of the error, but not remove it totally. It is concluded that an initial profile based on climatology is likely to provide most reliable retrieval results     

The measurement of atmospheric water vapor: radiometer comparisonand spatial variations

Two water vapor radiometer (WVR) experiments were conducted to evaluate whether such instruments are both suitable and necessary to correct for propagation effects that are induced by precipitable water vapor (PWV) on signals from the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). WVRs are suitable for these corrections if they provide wet path delays to better than 0.5 cm. They are needed if spatial variations of PWV result in complicated, direction-dependent propagation effects that are too complex to be parameterized in the GPS or VLBI geodetic solution. In the first experiment, the suitability of radiometers were addressed by comparing six WVRs at Stapleton International Airport in Denver, Colorado, for two weeks. The second experiment addressed the question whether radiometers are needed for the detection of inhomogeneities in the wet delay. Three JPL D-series radiometers were operated at three sites in Colorado approximately 50 km apart. The WVRs simultaneously sampled PWV at different azimuths and elevations in search of spatial variations of PWV     

A new simplified water vapor measurement using a multifrequencymicrowave radiometer with two-different-beamwidth antennas

A simplified method for the measurement of the atmospheric water vapor is presented. A multifrequency microwave radiometer having two different beamwidth antennas is used to measure solar extinction in terms of the difference temperature of the antennas and the relative intensity at each pair of different frequency points. The atmospheric emission component and the effect of solar fluctuation are removed automatically. Cloud absorption can be estimated by simultaneous measurement at two different frequency pairs and compensated for by the water vapor absorption. Using the specific nature of the composite weighting function defined in the opacity integral, the radiometric data can be related to the integrated water vapor and the water vapor profile     

Comparison of MM5 integrated water vapor with microwave radiometer, GPS, and radiosonde measurements

A large dataset of concurrent integrated precipitable water vapor (IPWV) estimates from ground-based microwave radiometers (MWRs), global positioning system (GPS) ground-receivers, and radiosonde observations (RAOBs) has been collected in five different sites in Central Italy. Both MWRs and GPS have shown a capability of accurate and continuous water vapor monitoring. These data are used to study the seasonal and spatial variability of IPWV. A comparison of these data with the IPWV field produced operationally by the nonhydrostatic Mesoscale Model (MM5), running at the University of L'Aquila/Center of Excellence (CETEMPS) is performed in order to find either model shortcomings and to corroborate the IPWV behavior highlighted by the measurements. Both measurements and model outputs span over a period of about one year allowing for a systematic statistical analysis for all the examined stations. The statistical analysis shows a good agreement between GPS and MWR data, whereas discrepancies are found between RAOBs and the other techniques. The IPWV shows the largest diurnal variability, approximately 3%, during the summer season. An overall good agreement is found between the forecasted and observed IPWV. The related statistical parameters show a very low bias (0.001 cm) with a good correlation coefficient (0.939). On the other hands, the seasonal analyses highlight a few discrepancies, mostly due to the MM5 difficulties in correctly forecasting the diurnal cycle.     

The observed relationship between the south pole 225-GHz atmospheric opacity and the water vapor column density

We compare our previously reported measurements of South Pole 225 GHz atmospheric opacity,τ, to the column of precipitable water vapor (PWV) which was derived from concurrent upper air soundings. From this comparison we found thatτ=(2.8±0.1)×10^?2+(6.9±0.2)×10^?2×PWV withτ in units of nepers/airmass andPWV in units of mm of precipitable H₂O. We compared our results to predictions from Grossman's AT atmospheric transparency model which is widely used in the radio astronomy community. The coefficient of the second term of the above relation, 0.069, was consistent with the predictions from the model; however, the first term, 0.028, which represents the dry air opacity, was about five to ten times larger than expected. Most of this discrepancy between the observed and the predicted dry air opacity can be accounted for by including contributions from continuum emission from N₂ and O₂ as is done in Liebe's MPM atmospheric model.     

A new correlation radiometer

A correlation radiometer is described in which the input signal is divided amongpamplifiers. The outputs of thepamplifiers are cross-correlated in all possible combinations and the cross-products added to give the system output. It is shown that aspgoes to infinity the sensitivity of the radiometer approaches that of an ideal total-power system. A practical system of six amplifiers offers an improvement of 2.2 dB over a conventional two-amplifier correlation radiometer, or an improvement of 5.2 dB over a Dicke system.     

L-band radiometer measurements of soil water under growing clover grass

A field experiment with an L-band radiometer at 1.4 GHz was performed from May-July 2004 at an experimental site near Zurich, Switzerland. Before the experiment started, clover grass was seeded. Thermal infrared, in situ temperature, and time-domain reflectometer (TDR) measurements were taken simultaneously with hourly radiometer measurements. This setup allowed for investigation of the microwave optical depths and mode opacities (parallel and perpendicular to the soil surface) of the clover grass canopy. Optical depths and opacities were determined by in situ analysis and remotely sensed measurements using a nonscattering radiative transfer model. Due to the canopy structure, optical depth and opacity depend on the polarization and radiometer direction, respectively. A linear relation between vegetation water-mass equivalent and polarization-averaged optical depth was observed. Furthermore, measured and modeled radiative transfer properties of the canopy were compared. The model is based on an effective-medium approach considering the vegetation components as ellipsoidal inclusions. The effect of the canopy structure on the opacities was simulated by assuming an anisotropic orientation of the vegetation components. The observed effect of modified canopy structure due to a hail event was successfully reproduced by the model. It is demonstrated that anisotropic vegetation models should be used to represent the emission properties of vegetation. The sensitivity of radiometer measurements to soil water content was investigated in terms of the fractional contribution of radiation emitted from the soil to total radiation. The fraction of soil-emitted radiation was reduced to approximately 0.3 at the most developed vegetation state. The results presented contribute toward a better understanding of the interaction between L-band radiation and vegetation canopies. Such knowledge is important for evaluating data generated from future satellite measurements.     

The atmospheric water vapor continuum below 300 GHz

Absolute attenuation rates due to water vapor and moist nitrogen have been measured in the laboratory at 138 GHz, 282 and 300 K, pressures up to 1.5 atm, and relative humidities from 80 to 100 percent. The computer-controlled measuring system is comprised of a millimeter wave resonance spectrometer (0.15 km effective path length) and a humidity simulator. Several shortcomings of earlier measurement attempts have been rectified. The data are interpreted as a water vapor continuum spectrum consisting of two terms, namely strong self-broadening (H₂O?H₂O) plus foreign-gas-broadening (H₂O?N₂) contributions. Implications of the new results for modeling atmospheric EHF window transparencies and for revising established H₂O line broadening theory are discussed.     

A wide-range 22-GHz LC-based CMOS voltage-controlled oscillator

This work presents a novel voltage-controlled oscillator (VCO) design and simulations that combine a varactor bank with a transformer in the LC tank to achieve a high-frequency range. While the varactor bank is responsible for changing the capacitance in the LC tank, the transformer acts as a means to change the value of the inductance, hence allowing tune-ability in the two main components of the VCO. A control mechanism utilises a mixed-mode circuit consisting of comparators and a state machine. It allows efficient tuning of the VCO by controlling the capacitance and transformer in the LC tank. The VCO has a 10.75–22.43 GHz frequency range and the VCO gain, K_VCO, is kept at a low value ranging from 98.6 to 175.7 MHz/V. The simulated phase noise is ?111 dBc/Hz at 1 MHz offset from the 10.75 GHz oscillation frequency. The circuit is designed and simulated in 28 nm CMOS technology and uses a 1 V supply drawing a typical power of 14.74 mW.     

A 22-GHz push-push CMOS oscillator using micromachined inductors

This letter presents a low phase noise 0.35-/spl mu/m CMOS push-push oscillator utilizing micromachined inductors. This oscillator results in an improvement in phase noise compared with the previously published Si-based voltage-controlled oscillators (VCOs) around 20GHz. With the high-Q inductors introduced by the micromachined structure, the oscillator achieves an oscillating frequency of 22.2GHz while exhibiting an output power of -7.5dBm with a phase noise of -110.1dBc/Hz at 1-MHz offset. This work also demonstrates the highest operating frequency among previously published Si-based VCOs using micromachined structures.     

A low-power up-conversion CMOS mixer for 22-29-GHz ultra-wideband applications

A double-balanced, low-power, and low-voltage dual-gate up-conversion mixer working at K-band is designed and fabricated in the UMC 130-nm logic CMOS process. The mixer achieves a 3-dB conversion-gain bandwidth of 1.8 GHz at the input IF port and a 3-dB conversion-gain bandwidth of 10 GHz at the output RF port. The mixer achieves an output referred 1-dB compression point as high as -5.8 dBm and an output referred third-order intercept point as high as 5.8 dBm, while consuming 8.0 mW from a 1.2-V supply. This study demonstrates that the implementation of low-power mixers operating in the 22-29-GHz band for ultra-wideband automotive radar applications is possible in low-cost and low-voltage logic CMOS technology.     

Retrievals of column water vapor using millimeter-wave radiometric measurements

The airborne Millimeter-wave Imaging Radiometer (MIR) measurements conducted over the Midwest region of the continental United States during January/February 1997 and over the Alaska-Arctic region during May 1998 are used to estimate column water vapor W<0.8 g/cm/sup 2/ under a clear sky. On board the same aircraft are two other instruments, the Cloud Lidar System (CLS) and MODerate-resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS), which provide cloud cover information and independent measurements of W, respectively. The MIR-estimated W values are compared and found to be in very good agreement with those measured by rawinsondes at near concurrence. A close correlation is found between the MIR-estimated W and that estimated from the MAS near-IR reflectance ratios. Water surface emissivities at several MIR frequencies are obtained in the process of the W retrieval from several flights over the Midwest lakes. These estimated emissivities compared favorably with values calculated for a calm water surface, which are based on a di-electric permittivity model and MAS-measured surface temperatures. The results from all comparisons strongly demonstrate the soundness of the technique for estimating W.     

Retrieval of precipitable water vapor by the millimeter-waveimaging radiometer in the arctic region during FIRE-ACE

Millimeter-wave radiometric measurements obtained from the NASA ER-2 aircraft over the arctic region on May 20, 1998, were used to estimate precipitable water (PW) in the range⩽0.60 g/cm². The approach is a modified version of the recent work by J. Miao (1998), which utilized the radiometric measurements at 150, 183.3±3, and 183.3±7 GHz of the SSM/T-2 sensor to retrieve PW over the antarctic region. However, Miao has implicitly assumed a surface emissivity that is frequency independent over the 150-183 GHz range. This assumption turns out not to be a good one based on the airborne measurements described below and the errors introduced in the PW estimation were substantial in many cases. It is shown below that four-frequency radiometric measurements in the frequency range of 150-220 GHz provided a robust retrieval of PW, while allowing for a surface emissivity that varied linearly with frequency. The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations. The differences between the retrieved and calculated values are not more than ±0.02 g/cm², which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band nature of the 183.3 GHz measurements in the radiative transfer calculations for development of the retrieval algorithm. The PW values estimated from the algorithm developed from single side band, 183.3 GHz radiative transfer calculations could be in error by as much as ±0.10 g/cm² . Finally, the effect of surface temperature variations is shown to introduce only a small error in the estimation of PW     

A 22--24-GHz Cryogenically Cooled GaAs FET Amplifier

This paper describes the design and performance of a cryogenically cooled low-noise FET amplifier operating in the 22-24-GHz range. The amplifier employs five cascaded single-ended gain stages and an integral bandpass filter. Noise temperatures in the 200 K range with an associated gain of 28 dB are typical for the nine cooled units built to date.     

On the anomalies in submillimeter absorption spectrum of atmospheric water vapor

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm. The measurements have not found any spectral features mentioned repeatedly in the literature, besides the usually observed excess of the measured absorption above the calculated one for water vapor monomers.