Alexander V. Ryzhkov

The Hydrometeor Classification Algorithm for the Polarimetric WSR-88D: Description and Application to an MCS

Abstract This paper contains a description of the most recent version of the hydrometeor classification algorithm for polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D). This version contains several modifications and refinements of the previous echo classification algorithm based on the principles of fuzzy logic. These modifications include the estimation of confidence factors that characterize the possible impacts of all error sources on radar measurements, the assignment of the matrix of weights that characterizes the classification power of each variable with respect to every class of radar echo, and the implementation of a class designation system based on the distance from the radar and the parameters of the melting layer that are determined as functions of azimuth with polarimetric radar measurements. These additions provide considerable flexibility and improve the discrimination between liquid and frozen hydrometeors. The new classification scheme utilizes all available polarimetric va...

Polarimetric method for ice water content determination

There have been a number of studies to estimate ice water content (IWC) of snow clouds using the radar reflectivity factor, Z. All of these studies show extreme variability in the IWC-Z relations, which appear to change from day to day and cloud to cloud. High diversity in the IWC-Z relations is primarily due to the fact that reflectivity factor is proportional to the product of IWC and average mass of ice hydrometeors; therefore at least one more independent measurement is needed to resolve this ambiguity. Since ice and snow hydrometeors are nonspherical the use of polarimetry is a natural way for estimation of bulk properties of snow clouds and precipitation. In this paper the authors develop a radar polarimetric model of a cloud of ice hydrometeors and obtain a polarimetric relation for computing TWC. They account for the diversity of crystal shape and the dependence of shape and density of scatterers on their size. Finally, they use polarimetric data from four Oklahoma snowstorms to check the consistency of the IWC estimates (obtained from the proposed polarimetric algorithm) with those derived using conventional IWC-Z relations.

Classification of Precipitation Types during Transitional Winter Weather Using the RUC Model and Polarimetric Radar Retrievals

AbstractA new hydrometeor classification algorithm that combines thermodynamic output from the Rapid Update Cycle (RUC) model with polarimetric radar observations is introduced. The algorithm improves upon existing classification techniques that rely solely on polarimetric radar observations by using thermodynamic information to help to diagnose microphysical processes (such as melting or refreezing) that might occur aloft. This added information is especially important for transitional weather events for which past studies have shown radar-only techniques to be deficient. The algorithm first uses vertical profiles of wet-bulb temperature derived from the RUC model output to provide a background precipitation classification type. According to a set of empirical rules, polarimetric radar data are then used to refine precipitation-type categories when the observations are found to be inconsistent with the background classification. Using data from the polarimetric KOUN Weather Surveillance Radar-1988 Dopple...

Areal rainfall estimates using differential phase

Radar polarimetric methods for rainfall measurements have received increasing attention in recent years. The one based on the estimate of specific differential phase K/sub DP/ uses the relation: R=aK/sub DP//sup b/ where R is rain rate. This method has several advantages compared to the conventional one which utilizes radar reflectivity factor Z. Differential phase is immune to radar miscalibration, microwave attenuation, partial beam blockage. It is less contaminated by hail and is less affected by drop size distribution variations. Because K/sub DP/ is a radial derivative of the total differential phase /spl Phi//sub DP/(K/sub DP/= 1/2 d/spl Phi//sub DP//dr), and the exponent b in the above equation is close to unity, the rainfall integrated over the radial interval (r/sub 1/,r/sub 2/) is approximately proportional to the difference between /spl Phi//sub DP/ values at the ends of the interval: /spl Phi//sub DP/(r/sub 2/)-/spl Phi//sub DP/(r/sub 1/). Similarly, areal rainfall is determined by the values of differential phase on the areal contour and, therefore, is not affected by distribution of differential phase inside the area of interest. This idea was first suggested by Raghavan and Chandrasekar (1994) as a useful technique with potential to obtain Area-Time Integral rain accumulations. In this paper the authors examine this technique using the data obtained with the 10-cm wavelength polarimetric radar and rain gauge data from the Agricultural Research Service (ARS) micronetwork in Oklahoma.

Effective shape of raindrops. Polarimetric radar perspective

In this study, polarimetric radar measurements are used for estimating mean shape of raindrops for several rain events. A linear dependence of the axis ratio on equivolume raindrop diameter is assumed, and the slope /spl beta/ of such dependence is obtained from the measurements of reflectivity factor Z, differential reflectivity Z/sub DR/, and specific differential phase K/sub DP/. Within-the-storm and between-the-storm variations of the parameter /spl beta/ are the primary focus of this investigation.

Full wave EM calculation of polarimetric variables for the atmospheric scatterers with nonspheroidal shape

A model of spheroid is commonly utilized to simulate polarimetric characteristics of natural scatterers in atmosphere such as hydrometeors or biota (birds, insects, bats, etc.) The estimates of polarimetric radar variables are usually obtained using either closed form Rayleigh solutions, if size of particle is much smaller than radar wavelength or T-matrix codes otherwise.

Development of a classification algorithm for operational polarimetric NEXRAD radar

Classification of meteorological and nonmeteorological radar echoes will be one of the key functions of the operational polarimetric NEXRAD radar. In this paper, basic principles of the classification for several cases are presented. The data were obtained from the polarimetric prototype of the NEXRAD radar.

Polarimetric signatures of ice crystals in clouds

Cloud ice crystals have important bearing on radiative transfer and associated temperature change of the atmosphere. Furthermore, some crystals grow to precipitation sized particles that replenish the Earths water supply. Until the advent of radar polarimetry, remote measurements of crystals in clouds consisted of attempts to obtain the ice water content (IWC) from the reflectivity factor Z. Because of their low dielectric constant and small size, ice crystals backscatter considerably weaker signal than liquid hydrometeors of the same size and concentration. Nonetheless, there is a substantial range of reflectivities that is common to both ice crystals and small drops. Therefore, the radar reflectivity by itself could not serve for discrimination between these hydrometeors. Environment temperature or knowledge of the melting layer height have been used to delineate regions of ice from rainy regions. Remarkable changes are being made in remote sensing of ice crystals thanks to the advent of radar polarimetry. The authors describe crystal signatures in the fields of polarimetric variables and suggest possibilities for quantitative estimation of IWC.