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Below are short descriptions of my research interests, along with corresponding past and present papers, as well as pictures of ongoing projects.

Design and Calibration for Polarimetric Phased Array Weather Radar

Radar performance depends on the system configuration and specification. This is specificially true for agile beam Polarimetric Phased Array Radar (PPAR) for weather surveillance. The mostly used array configuration for PAR is a planar array antenna. The planar configuration, however, has significant deficiencies for polarimetric measurements, as well as other limitations such as increases in beamwidth, decreases of sensitivity, and changes in the polarization basis when the beam scans off its broadside. The Cylindrical Polarimetric Phased Array Radar (CPPAR) is proposed to avoid these deficiencies. The CPPAR principle and potential performance are demonstrated through theoretical analysis and simulation. It is shown that the CPPAR has the advantage of a scan-invariant polarization basis, and thus avoids the inherent limitations of the planar PPAR (i.e., PPPAR).

Polarimetric Phased Array Radar for Weather Measurement: A Planar or Cylindrical Configuration?
Phased Array Radar Polarimetry for Weather Sensing: A Theoretical Formulation for Bias Corrections

Remote sensing for understanding and quantifying weather and earth environment

Remote sensing is to obtain information without contact measurements. It is fast becoming a primary tool to monitor Earth environments, including weather. Weather radar measures the wave scattering of hydrometeors in order to quantify them in terms of their sizes, densities, shapes and motions. Ground-based radar and satellite observations have recently been used in our studies.

An assessment of droplet size and liquid water content using dual-frequency radar measurements for aircraft icing detection

Wave propagation and scattering in geophysical media

Radar transmits and receives electromagnetic wave scattered from the resolution volume of a targeted medium. Wave scattering and propagation in geophysical media subjected to turbulent mixing and filled with hydrometers and other objects are studied. The relations between radar observables and physical parameters will be established. Remote sensing techniques and retrieval methods for physical parameters are our interests.

Angular correlation function and scattering coefficient of electromagnetic wave scattered by a buried object under a two-dimensional rough surface
Angular correlation function of wave scattering by a random rough surface and discrete scatterers and its application in the detection of a buried object

Radar polarimetry for quantitative precipitation estimation and forecast

Improving Microphysics Parameterizations and Quantitative Precipitation Forecast through Optimal Use of Video Disdrometer, Profiler and Polarimetric Radar Observations

Understanding and characterizing precipitation microphysics are important for accurate quantitative precipitation estimation (QPE) and for improving quantitative precipitation forecasts (QPF). The objective of this proposal is to improve precipitation estimation and numerical forecasts of quantitative precipitation with the optimal use of observations from advanced instruments, including 2D video disdrometers, profiler and dual-polarization radars.

A 2D video disdrometer (2DVD) provides detailed information about the size, shape and density of precipitating particles. A vertically pointing wind profiler measures precipitation (or clear air) characteristics at various heights. Polarization radar measurements have shown great potential in classifying hydrometeor types and in retrieving hydrometeor drop size distributions (DSD) with a large spatial coverage. A combination of polarization radar, profiler and disdrometer measurements makes it possible to develop and verify observation-based microphysics parameterizations with case-dependent and time-evolving DSDs.

The project includes deployment of 2DVDs at sites within the coverage of a prototype WSR-88D dual-polarization radar (KOUN in Norman, Oklahoma) and the Oklahoma City WSR-88D (KTLX) radar. This will enable the project team to (1) better understand cloud/precipitation microphysics with further study of drop size distribution and their evolution through in-situ measurements and radar observations, (2) improve microphysics parameterization, (3) develop forward observation operators for polarization radar variables and improved QPE algorithms employing polarimetric radar data (PRD), and (4) assimilate PRD in NWP models for initialization/forecasts and optimal retrieval of microphysical parameters with the use of improved microphysics. Error structure of radar measurements and retrievals will be quantified. Observation-based and model-based microphysical retrievals will be cross-validated and verified with in-situ measurements.

The increasing needs for improved microphysics parameterization as research and operational numerical weather prediction (NWP) models start to resolve convection and precipitation explicitly and the planned upgrade of the national network of operational WSR-88D radars to dual-polarization capability in the next five years make the proposed research both urgent and timely. Further, the fact that the QPF improvement has been slow and difficult over the years provides another impetus for the research.

A method for estimating rain rate and drop size Distribution from polarimetric radar measurements

Phased array radar interferometry to measure wind, shear and turbulence

Weather radars, like WSR-88Ds, measure only Doppler velocity (radial wind) of hydrometeors. The National Weather Radar Testbed (NWRT) Phased Array Radar (PAR) allows for Spaced Antenna Interferometry for crossbeam wind measurements. The theory of measuring crossbeam wind, shear, and turbulence within the radar’s resolution volume V6 is being developed. Weather radar interferometry is formulated for such measurements using phased-array weather-radar. The formulation for a Spaced Antenna Interferometer (SAI) includes shear of the mean wind, allows turbulence to be anisotropic, and allows receiving beams to have elliptical cross sections. Auto- and cross-correlation functions are derived based on wave scattering by randomly distributed particles. Antenna separation, mean wind, shear, and turbulence all contribute to signal de-correlation. Crossbeam wind cannot be separated from shear and thus crossbeam wind measurements are biased by shear. It is shown that SAI measures an apparent crossbeam wind (i.e., the angular shear of the radial wind component). Whereas the apparent crossbeam wind and turbulence within V6 cannot be separated using monostatic Doppler techniques, angular shear and turbulence can be separated using the SAI.

Spaced antenna interferometry to measure crossbeam wind, shear, and turbulence: Theory and formulation
Bistatic Interferometry to Measure Clear Air Wind

Cloud/precipitation microphysics and model parameterization

Precipitation microphysics is studied by using disdrometer and polarimetric radar observations. Disdrometer measurements provide information regarding the size, shape, orientation and falling speed of hydrometeors. Hence, drop-size distributions are also obtained.The results of our research on this topic are represented by on the page ""Disdrometer Data." More data is available by request.

Diagnosing the intercept parameter for exponential raindrop size distribution based on video disdrometer observations: Model development
Improving parameterization of rain microphysics with disdrometer and radar observations

Ou's disdrometer was deployed at Kessler Farm to collect drop-size distribution data
for studying precipitation microphysics and verifying polarimetry radar measurements.