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Recent research has demonstrated the value of polarimetric measurements for the correction of rain-path attenuation at X-band radar frequency and the estimation of rain parameters including drop-size distributions (DSD). The issue this paper is concerned with is to what degree uncertainties in attenuation correction can affect the estimation of DSD. Since attenuation-correction uncertainty enhances with rain path, our hypothesis is that DSD retrieval uncertainty at X-band may deteriorate with range. In this paper, we evaluate the relative accuracy of X-band DSD retrieval against DSD estimates from S-band radar observations and in situ disdrometer spectra. We present comparisons of various techniques for estimating DSD model parameters from attenuation-corrected X-band dual-polarization radar data. Coincident X-band polarimetric-radar (XPOL) and S-band polarimetric-radar dual-polarized radar measurements from the International [H.sub.2]O Project experiment as well as coincident XPOL (MP-X) measurements over disdrometer during a typhoon storm case in Japan are used to assess the accuracy of the different DSD retrieval algorithms applied to X-band radar measurements. Index Terms--Attenuation correction, drop-size distribution (DSD) retrievals, polarimetric radar, S-band radar, X-band radar.

The accuracy of attenuation correction for X-band reflectivity ([Z.sub.H]) and differential reflectivity ([Z.sub.DR]) measurements in rainfall is analyzed using coincident X-band and S-band polarimetric radar observations collected during the International [H.sub.2]O Project in the period of May-June 2002 at northwestern Oklahoma. Two distinct attenuation correction techniques that use the differential propagation phase shift ([[PHI].sub.DP]) information, which is not a power-dependent measurement, as a means to provide independent estimates of path-integrated attenuation are assessed. The study is facilitated by nonattenuated X-band [Z.sub.H] and [Z.sub.DR] profiles simulated based on raindrop size distribution parameters retrieved from matched multiparameter ([Z.sub.H], [Z.sub.DR], and [Z.sub.DP]) S-band observations. The major outcome of this assessment is that [[PHI].sub.DP]-based attenuation correction for both techniques can reach almost unbiased measurements (within 5% mean relative error) and low random error (15-20% relative standard deviation). The study shows moderate differences in the error statistics of the evaluated techniques. The sensitivity of attenuation correction uncertainty with respect to the assumed variability in raindrops' oblateness-size relation and the noise in [[PHI].sub.DP] measurement is also shown. Index Terms--Attenuation, polarimetric radar, rainfall, S-band, X-band.

The early histories of radar echo and polarization differential reflectivity ([Z.sub.DR]) from growing cumulus clouds observed in Florida with a 10-cm-wavelength radar are reported in detail. Raindrops 1 to several millimeters in diameter are present at about cloud-base level in most cases as soon as any identifiable precipitation echo is seen within cloud (distinct from Bragg scattering and echo from cloud droplets). This is in most cases by the time of the first 10-dBZ radar echo aloft. The very early occurrence of large drops is consistent with origination directly from coalescence on ultragiant aerosol. However, they appear to exist so early and so low in the clouds as to be unexpected if the cumulus were single, vigorous thermals. The explanation may lie in the presence of a more gradual, very early cloud stage that is generally not observed in any detail. Simultaneous [Z.sub.e] and [Z.sub.DR] measurements in the early stages of developing, warm cumulus will provide a powerful test of understanding the onset of drop growth by coalescence. (Manuscript received 15 November 2000, in final form 30 August 2001)

NCAR/EOL is investigating potential configurations for the next generation airborne phased array radar (APAR) that is capable of retrieving dynamic and microphysical characteristics of clouds and precipitation. The APAR will operate at C-band. The APAR will use the electronic scanning (e-scan) feature to acquire the optimal number of independent samples for recording research quality measurements. Since the airborne radar has only a limited time for collecting measurements over a specified region (moving aircraft platform ~ 100 m/s), beam multiplexing will significantly enhance its ability to collect high-resolution, research quality measurements. Beam multiplexing reduces errors in radar measurements while providing rapid updates of scan volumes. Beamwidth depends on the size of the antenna aperture. Beamwidth and Directivity of elliptical, circular and rectangular antenna apertures are compared and radar sensitivity is evaluated for various polarimetric configurations and transmit/receive elements. In the case of polarimetric measurements, alternate transmit with alternate receive (single channel receiver) and simultaneous reception (dual channel receiver) is compared. From an overall architecture perspective, element level digitization of transmit/receive (T/R) module versus digital sub-array is considered with regard to flexibility in adaptive beamforming, polarimetric performance, calibration, and data quality. Methodologies for calibration of the radar and removing bias in polarimetric measurements are outlined. The above-mentioned engineering options are evaluated for realizing an optimal APAR system suitable for measuring the high temporal and spatial resolutions of Doppler and polarimetric measurements of precipitation and clouds. [ABSTRACT FROM AUTHOR]

Quantitative precipitation forecasting is currently limited by the paucity of observations on sufficiently fine temporal and spatial scales. Three-dimensional water vapor fields can be retrieved with improved spatial coverage from measurements obtained using a network of scanning microwave radiometers. To investigate this potential, an observation system simulation experiment was performed in which synthetic examples of retrievals using a network of radiometers were compared with results from the Weather Research and Forecasting model at a grid scale of 500 m. These comparisons show that the 3-D water vapor field can be retrieved with an accuracy of better than 15%-20%. A ground-based demonstration network of three compact microwave radiometers was deployed at the Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma. Results using these network measurements demonstrated the first retrieval of the 3-D water vapor field in the troposphere at fine spatial and temporal resolutions. [ABSTRACT FROM AUTHOR]

The early histories of radar echo and polarization differential reflectivity (Z[sub DR] ) from growing cumulus clouds observed in Florida with a 10-cm-wavelength radar are reported in detail. Raindrops 1 to several millimeters in diameter are present at about cloud-base level in most cases as soon as any identifiable precipitation echo is seen within cloud (distinct from Bragg scattering and echo from cloud droplets). This is in most cases by the time of the first 10-dBZ radar echo aloft. The very early occurrence of large drops is consistent with origination directly from coalescence on ultragiant aerosol. However, they appear to exist so early and so low in the clouds as to be unexpected if the cumulus were single, vigorous thermals. The explanation may lie in the presence of a more gradual, very early cloud stage that is generally not observed in any detail. Simultaneous Z[sub e] and Z[sub DR] measurements in the early stages of developing, warm cumulus will provide a powerful test of understanding the onset of drop growth by coalescence. [ABSTRACT FROM AUTHOR]