Search Results

You are looking at 1 - 10 of 29 items for

  • Author or Editor: A. Chandrasekar x
  • Refine by Access: All Content x
Clear All Modify Search
Steven A. Rutledge
and
V. Chandrasekar

Abstract

Great strides have been made over the past decades in educating radar meteorologists. These advances appear to be loosely associated with the arrival of new hardware in the field, for example, Doppler radars followed by polarimetric radars. Many radar meteorologists received a substantial portion of their early training through participation in field programs utilizing this new hardware. In this study, a brief look at the evolution of radar education will first be offered, followed by an assessment of the current state of this field. Finally, a view of the future will be offered. Future educational thrusts in radar meteorology will take full advantage of Internet technology, allowing radar systems to be brought into remote classrooms in a “virtual” sense. This study is purposely limited to meteorological radar and is focused on graduate-level education.

Full access
A. Mudukutore
,
V. Chandrasekar
, and
E. A. Mueller

Abstract

The measurement of the differential propagation phase and copolar correlation coefficient are affected by the differential phase pattern of the antenna system when operating in an alternate horizontal and vertical transmitting scheme. Direct phase pattern measurements of a large dish such as that of the CSU CHILL is difficult because of the need to obtain phase reference. A simple technique is devised to measure the differential phase pattern of the CSU CHILL antenna system. The measurements are subsequently used in the evaluation of the antenna imposed limit on the copolar correlation coefficient.

Full access
Vinodkumar
,
A. Chandrasekar
,
K. Alapaty
, and
Dev Niyogi

Abstract

This study investigates the impact of the Flux-Adjusting Surface Data Assimilation System (FASDAS) and the four-dimensional data assimilation (FDDA) using analysis nudging on the simulation of a monsoon depression that formed over India during the 1999 Bay of Bengal Monsoon Experiment (BOBMEX) field campaign. FASDAS allows for the indirect assimilation/adjustment of soil moisture and soil temperature together with continuous direct surface data assimilation of surface temperature and surface humidity. Two additional numerical experiments [control (CTRL) and FDDA] were conducted to assess the relative improvements to the simulation by FASDAS. To improve the initial analysis for the FDDA and the surface data assimilation (SDA) runs, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) simulation utilized the humidity and temperature profiles from the NOAA Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS), surface winds from the Quick Scatterometer (QuikSCAT), and the conventional meteorological upper-air (radiosonde/rawinsonde, pilot balloon) and surface data. The results from the three simulations are compared with each other as well as with NCEP–NCAR reanalysis, the Tropical Rainfall Measuring Mission (TRMM), and the special buoy, ship, and radiosonde observations available during BOBMEX. As compared with the CTRL, the FASDAS and the FDDA runs resulted in (i) a relatively better-developed cyclonic circulation and (ii) a larger spatial area as well as increased rainfall amounts over the coastal regions after landfall. The FASDAS run showed a consistently improved model simulation performance in terms of reduced rms errors of surface humidity and surface temperature as compared with the CTRL and the FDDA runs.

Full access
Brenda Dolan
,
Steven A. Rutledge
,
S. Lim
,
V. Chandrasekar
, and
M. Thurai

Abstract

A new 10-category, polarimetric-based hydrometeor identification algorithm (HID) for C band is developed from theoretical scattering simulations including wet snow, hail, and big drops/melting hail. The HID is applied to data from seven wet seasons in Darwin, Australia, using the polarimetric C-band (C-POL) radar, to investigate microphysical differences between monsoon and break periods. Scattering simulations reveal significant Mie effects with large hail (diameter > 1.5 cm), with reduced reflectivity and enhanced differential reflectivity Z dr and specific differential phase K dp relative to those associated with S band. Wet snow is found to be associated with greatly depreciated correlation coefficient ρ hv and moderate values of Z dr. It is noted that large oblate liquid drops can produce the same electromagnetic signatures at C band as melting hail falling quasi stably, resulting in some ambiguity in the HID retrievals. Application of the new HID to seven seasons of C-POL data reveals that hail and big drops/melting hail occur much more frequently during break periods than during monsoon periods. Break periods have a high frequency of vertically aligned ice above 12 km, suggesting the presence of strong electric fields. Reflectivity and mean drop diameter D 0 statistics demonstrate that convective areas in both monsoon and break periods may have robust coalescence or melting precipitation ice processes, leading to enhanced reflectivity and broader distributions of D 0. Conversely, for stratiform regions in both regimes, mean reflectivity decreases below the melting level, indicative of evaporative processes. Break periods also have larger ice water path fractions, indicating substantial mixed-phase precipitation generation as compared with monsoonal periods. In monsoon periods, a larger percentage of precipitation is produced through warm-rain processes.

Full access
S. Lim
,
V. Chandrasekar
,
P. Lee
, and
A. P. Jayasumana

Abstract

Monitoring of precipitation using higher-frequency radar systems such as X band is becoming popular. At X-band frequency, weather radar signals are attenuated along their paths due to precipitation. A network-based reflectivity retrieval technique has been developed for the Collaborative Adaptive Sensing of the Atmosphere (CASA) system, which is a radar network that can observe a weather event simultaneously by multiple radars. This paper describes the design and implementation of an architectural framework for real-time processing of the network-based attenuation correction system. The benchmarks presented here show that the system accomplishes networked attenuation correction within a few seconds, making it well suited for the CASA system. This paper presents the performance of the network-based attenuation correction system in terms of the metrics of attenuation correction as well as computational performance using CASA Integrated Project 1 (IP1) data during 2007–09 field experiments. The results show that the network-based attenuation correction algorithm works robustly in real time while retrieving attenuation-corrected reflectivity.

Full access
V. Chandrasekar
,
William A. Cooper
, and
V. N. Bringi

Abstract

Axis ratios were determined for about 3500 raindrop images recorded in summertime rainshowers by an instrumented aircraft. These ratios were used to determine the mean axis ratios and oscillation amplitudes of raindrops. A filtering algorithm using Fourier descriptors was applied to the raindrop images to reduce the quantization noise and the systematic errors, and simulations were used to estimate the standard errors of the measurement procedure. Drops with diameters <4 mm were observed to be slightly more spherical than would be expected for drops in equilibrium. Oscillation amplitudes were found to be typically ±10% in axis ratio for light to moderate rainfall rates, and such oscillations can account for the departures from equilibrium values. The effects of these axis ratios and oscillations on the differential radar reflectivity of rain are calculated and discussed.

Full access
Gianfranco Scarchilli
,
Eugenio Goroucci
,
V. Chandrasekar
, and
Thomas A. Seliga

Abstract

The accuracy of radar measurements and their derived parameters, such as rainfall rate, are compromised by errors caused by propagation effects at C-band frequencies. The radar measurements of reflectivity factor Z and differential reflectivity Z DR are affected by the absolute and differential attenuation through the rain medium. Another useful radar-derived parameter, differential propagation phase shift ΦDP, is contaminated by the differential phase on backscatter δ, which attains significant values in rainfall at C-band frequencies. In this paper we present a technique to correct these propagation and backscatter effects by application of an algorithm that corrects first Z and Z DR, using relationships between the specific and differential attenuations versus phase shift, which is followed by estimation of the differential backscatter phase shift parameter δ from the corrected Z DR. Simulation results are presented to demonstrate the effectiveness of this correction procedure for two cases: (a) uniform rainfall along the path, and (b) rainfall varying with range. We also present estimates of accuracy in the measurement of radar-derived rainfall rates made after applying this correction procedure.

Full access
V. Chandrasekar
,
Yoong-Goog Cho
,
D. Brunkow
, and
A. Jayasumana

Abstract

The Virtual CHILL (VCHILL) system makes it possible to transfer the educational and research experience of the Colorado State University dual polarization radar to remote locations over the Internet. The VCHILL operation includes remote control of radar and display of radar images, as well as the ability to locally process high-bandwidth radar data transferred over data networks. The low-bandwidth VCHILL operation allows the distant users to access the archived and real-time data estimated at the radar site and simultaneously display them on their local systems. A parallel receiver was developed exclusively for the high-bandwidth VCHILL. End-system architectures were designed to accommodate the demands of the high-bandwidth VCHILL operations in real time. A graphic user interface was also developed with the objective of easy installation and usage at various end-user institutions. The VCHILL not only expands the education experience provided by the radar system, but also stimulates the development of innovative research applications for atmospheric remote sensing. The VCHILL is being used by several universities for research and education.

Full access
L. Liu
,
V. N. Bringi
,
V. Chandrasekar
,
E. A. Mueller
, and
A. Mudukutore

Abstract

Recent research has suggested that the copolar correlation coefficient termed ρ hν(0) can be used to identity large hail and improve polarization estimates of rainfall. The typical measured values of ρ hν(0) at S band vary approximately between 0.8 and 1.0. For applications to hail identification, the required accuracy should be within ±0.01, while for rainfall improvement a higher accuracy is necessary, for example, within ±0.001. The statistics of the estimator of ρ hν(0) using the Gaussian spectrum approximation from both an analytical approach and using simulations are discussed. The standard deviation and bias in ρ^ hν(0) are computed as a function of number of samples. Doppler spectral width, and mean value of ρ hν(0). The effect of finite signal-to-noise ratio and phase noise are also studied using simulations. Several other estimators of ρ hν(0) are evaluated, Time series data collected with the Colorado State University–University of Chicago and Illinois State Water Survey (CSU–CHILL) radar are analyzed and compared with the simulations. Antenna pattern effects as they affect the accuracy of ρ^ hν(0) are also discussed.

Full access
Elizabeth J. Thompson
,
Steven A. Rutledge
,
Brenda Dolan
,
Merhala Thurai
, and
V. Chandrasekar

Abstract

Dual-polarization radar rainfall estimation relationships have been extensively tested in continental and subtropical coastal rain regimes, with little testing over tropical oceans where the majority of rain on Earth occurs. A 1.5-yr Indo-Pacific warm pool disdrometer dataset was used to quantify the impacts of tropical oceanic drop-size distribution (DSD) variability on dual-polarization radar variables and their resulting utility for rainfall estimation. Variables that were analyzed include differential reflectivity Z dr; specific differential phase K dp; reflectivity Z h ; and specific attenuation A h . When compared with continental or coastal convection, tropical oceanic Z dr and K dp values were more often of low magnitude (<0.5 dB, <0.3° km−1) and Z dr was lower for a given K dp or Z h , consistent with observations of tropical oceanic DSDs being dominated by numerous, small, less-oblate drops. New X-, C-, and S-band R estimators were derived: R(K dp), R(A h ), R(K dp, ζ dr), R(z, ζ dr), and R(A h , ζ dr), which use linear versions of Z dr and Z h , namely ζ dr and z. Except for R(K dp), convective/stratiform partitioning was unnecessary for these estimators. All dual-polarization estimators outperformed updated R(z) estimators derived from the same dataset. The best-performing estimator was R(K dp, ζ dr), followed by R(A h , ζ dr) and R(z, ζ dr). The R error was further reduced in an updated blended algorithm choosing between R(z), R(z, ζ dr), R(K dp), and R(K dp, ζ dr) depending on Z dr > 0.25 dB and K dp > 0.3° km−1 thresholds. Because of these thresholds and the lack of hail, R(K dp) was never used. At all wavelengths, R(z) was still needed 43% of the time during light rain (R < 5 mm h−1, Z dr < 0.25 dB), composing 7% of the total rain volume. As wavelength decreased, R(K dp, ζ dr) was used more often, R(z, ζ dr) was used less often, and the blended algorithm became increasingly more accurate than R(z).

Full access