V. Chandrasekar

Raindrop Size Distribution in Different Climatic Regimes from Disdrometer and Dual-Polarized Radar Analysis

Abstract The application of polarimetric radar data to the retrieval of raindrop size distribution parameters and rain rate in samples of convective and stratiform rain types is presented. Data from the Colorado State University (CSU), CHILL, NCAR S-band polarimetric (S-Pol), and NASA Kwajalein radars are analyzed for the statistics and functional relation of these parameters with rain rate. Surface drop size distribution measurements using two different disdrometers (2D video and RD-69) from a number of climatic regimes are analyzed and compared with the radar retrievals in a statistical and functional approach. The composite statistics based on disdrometer and radar retrievals suggest that, on average, the two parameters (generalized intercept and median volume diameter) for stratiform rain distributions lie on a straight line with negative slope, which appears to be consistent with variations in the microphysics of stratiform precipitation (melting of larger, dry snow particles versus smaller, rimed ic...

Short wavelength technology and the potential for distributed networks of small radar systems

The NSF Engineering Research Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) is advancing a new approach to radar network design based on dense networks of short-range radars. The centers concept is to deploy small radars atop communication towers, rooftops, and other elements of the infrastructure as a means to comprehensively map winds, rainfall, and other atmospheric and airborne objects throughout the atmosphere with resolution, low-altitude coverage, Doppler wind vector measurement, and other capabilities that are substantially beyond the current state-of-the-art. The technology has the potential to supplement - or perhaps replace - the large long-range civil infrastructure radars in use today.

An Examination of Propagation Effects in Rainfall on Radar Measurements at Microwave Frequencies

Propagation effects in rainfall are examined at three microwave frequencies corresponding to S (3.0 GHz), C (5.5 GHz), and X ( 10.0 GHz) bands. Attenuation at horizontal polarization, as well a s differential attenuation and differential propagation phase between horizontal (HI and vertical ( V ) polarizations are considered. It is shown that at the three frequencies both attenuation and differential attenuation are nearly linearly related to differential propagation phase ( cbDP). This is shown through simulation using ( a ) gamma raindrop size distributions (RSD) with three parameters (No, DO. m ) that are varied over a very wide range representing a variety of rainfall types, and ( b ) measured raindrop size distributions at a single location using a disdrometer. Measurements of X-band specific attenuation and S-band specific differential phase in convective rainshafts using the National Center for Atmospheric Research CP-2 radar are presented in order to experimentally demonstrate the linear relationship between attenuation and differential propagation phase. Correction procedures for reflectivity and differential reflectivity (&) are developed assuming that differential propagation phase is measured using a radar that alternately transmits H and V polarized waves with copolar reception through the same receiver and processor system. The correction procedures are not dependent on the actual rainrate profile between the radar and the range location of interest. The accuracy of the procedure depends on, (a) RSD fluctuations, (b) variability in the estimate of differential propagation phase due to measurement fluctuations, and ( c ) nonzero values of the backscatter differential phase ( & ) between H and V polarizations. Simulations are used to gauge the accuracy of correction procedures at S- and C-bands assuming 6 is negligible. The correction accuracy for attenuation at S-band is estimated to be -0.05 dB while at C-band it is estimated to be within I dB if (IDP G 60 deg). Simulations further indicate that C-band differential attenuations effects can be corrected to within -35% of the mean value.

Correcting C-band radar reflectivity and differential reflectivity data for rain attenuation: a self-consistent method with constraints

Quantitative use of C-band radar measurements of reflectivity (Z/sub h/) and differential reflectivity (Z/sub dr/) demands the use of accurate attenuation-correction procedures, especially in convective rain events. With the availability of differential phase measurements (/spl Phi//sub dp/) with a dual-polarized radar, it is now possible to improve and stabilize attenuation-correction schemes over earlier schemes which did not use /spl Phi//sub dp/. The recent introduction of constraint-based correction schemes using /spl Phi//sub dp/ constitute an important advance. In this paper, a self-consistent, constraint-based algorithm is proposed and evaluated which extends the previous approaches in several important respects. Radar data collected by the C-POL radar during the South China Sea Monsoon Experiment (SCSMEX) are used to illustrate the correction scheme. The corrected radar data are then compared against disdrometer-based scattering simulations, the disdrometer data being acquired during SCSMEX. A new algorithm is used to retrieve the median volume diameter from the corrected Z/sub h/, corrected Z/sub dr/, and K/sub dp/ radar measurements which is relatively immune to the precise drop axis ratio versus drop diameter relation. Histograms of the radar-retrieved D/sub 0/ compared against D/sub 0/ from disdrometer data are in remarkable good agreement lending further validity to the proposed attenuation-correction scheme, as well as to confidence in the use of C-band radar for the remote measurement of rain microphysics.

THE SEVERE THUNDERSTORM ELECTRIFICATION AND PRECIPITATION STUDY

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

Error Structure of Multiparameter Radar and Surface Measurements of Rainfall. Part III: Specific Differential Phase

Abstract Parts I and II of this three part paper dealt with the error structure of differential reflectivity and X-band specific attenuation in rainfall as estimated by radar and surface disdrometers. In this Part III paper we focus on the error structure of the specific differential phase (KDP, °km−1) measurement in rainfall. This allows us to analyze three estimators of rainfall rate, the first based on the reflectivity factor ZH, the second based on combining reflectivity and ZDR, [R(ZH, ZDR)], and the third based on KDP alone, R(KDP). Simulations are used to model random errors in ZH, ZDR and KDP. Physical variations in the raindrop size distribution (RSD) are modeled by varying the gamma parameters (N0, D0, m) over a range typically found in natural rainfall. Thus, our simulations incorporate physical fluctuations onto which random measurement errors have been superimposed. Radar-derived estimates of R(ZH, ZDR) and R(KDP) have been intercompared using data obtained in convective rainfall with the NSS...

Simulation of radar reflectivity and surface measurements of rainfall

Abstract Raindrop size distributions (RSDs) are often estimated using surface raindrop sampling devices (e.g., disdrometers) or optical array (2D-PMS) probes. A number of authors have used these measured distributions to compute certain higher-order RSD moments that correspond to radar reflectivity, attenuation, optical extinction, etc. Scatter plots of these RSD moments versus disdrometer-measured rainrates are then used to deduce physical relationships between radar reflectivity, attenuation, etc., which are measured by independent instruments (e.g., radar), and rainrate. In this paper we simulate RSDs of the gamma form as well as radar reflectivity (via time series simulation) to study the correlation structure of radar estimates versus rainrate as opposed to RSD moment estimates versus rainrate. Simulations offer a powerful method of studying the statistics of radar and surface RSD measurements since the “natural” RSD fluctuations can be introduced separately. In our simulations we vary the parameter ...

Hydrometeor classification system using dual-polarization radar measurements: model improvements and in situ verification

A hydrometeor classification system based on a fuzzy logic technique using dual-polarization radar measurements of precipitation is presented. In this study, five dual-polarization radar measurements (namely horizontal reflectivity, differential reflectivity, specific differential phase, correlation coefficient, and linear depolarization ratio) and altitude relating to environmental melting layer are used as input variables of the system. The hydrometeor classification system chooses one of nine different hydrometeor categories as output. The system presented in this paper is a further development of an existing hydrometeor classification system model developed at Colorado State University (CSU). The hydrometeor classification system is evaluated by comparing inferred results from the CSU CHILL Facility dual-polarization radar measurements with the in situ sample data collected by the T-28 aircraft during the Severe Thunderstorm Electrification and Precipitation Study.

Correction of Radar Reflectivity and Differential Reflectivity for Rain Attenuation at X Band. Part II: Evaluation and Application

Abstract In this paper, the attenuation-correction methodology presented in Part I is applied to radar measurements observed by the multiparameter radar at the X-band wavelength (MP-X) of the National Research Institute for Earth Science and Disaster Prevention (NIED), and is evaluated by comparison with scattering simulations using ground-based disdrometer data. Further, effects of attenuation on the estimation of rainfall amounts and drop size distribution parameters are also investigated. The joint variability of the corrected reflectivity and differential reflectivity show good agreement with scattering simulations. In addition, specific attenuation and differential attenuation, which are derived in the correction procedure, show good agreement with scattering simulations. In addition, a composite rainfall-rate algorithm is proposed and evaluated by comparison with eight gauges. The radar-rainfall estimates from the uncorrected (or observed) ZH produce severe underestimation, even at short ranges from...

Correction of Radar Reflectivity and Differential Reflectivity for Rain Attenuation at X Band. Part I: Theoretical and Empirical Basis

Abstract In this two-part paper, a correction for rain attenuation of radar reflectivity (ZH) and differential reflectivity (ZDR) at the X-band wavelength is presented. The correction algorithm that is used is based on the self-consistent method with constraints proposed by Bringi et al., which was originally developed and evaluated for C-band polarimetric radar data. The self-consistent method is modified for the X-band frequency and is applied to radar measurements made with the multiparameter radar at the X-band wavelength (MP-X) operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan. In this paper, characteristic properties of relations among polarimetric variables, such as AH–KDP, ADP–AH, AH–ZH, and ZDR–ZH, that are required in the correction methodology are presented for the frequency of the MP-X radar (9.375 GHz), based on scattering simulations using drop spectra measured by disdrometers at the surface. The scattering simulations were performed under ...