Showing papers by "Guifu Zhang published in 2004"

Drop Size Distribution Retrieval with Polarimetric Radar: Model and Application

Abstract: Polarimetric radar measurements are used to retrieve properties of raindrop distributions. The procedure assumes that drops are represented by a gamma distribution and retrieves the governing parameters from an empirical relation between the distribution shape and slope parameters and measurements of radar reflectivity and differential reflectivity. Retrieved physical characteristics of the drop size distribution (DSD) were generally well matched with disdrometer observations. The method is applied to select storms to demonstrate utility. Broad DSDs were determined for the core (high reflectivity) regions of thunderstorms. Largest drop median volume diameters were at the leading edge of the storm core and were displaced slightly downwind from updrafts. Rainy downdrafts exhibited what are believed to be equilibrium DSDs in which breakup and accretion are roughly in balance. DSDs for stratiform precipitation were dominated by relatively large drops. Median volume diameters at the ground were closely related to the intensity of an overlying bright band. The radar measurements suggest that, although DSDs in stratiform rain were also broad and nearly constant in the rain layer, they were not at equilibrium but were merely steady. DSD invariance is attributed to small total drop numbers, which result in few collisions.

Polarimetric Radar Estimators Based on a Constrained Gamma Drop Size Distribution Model

Abstract: Raindrop size distribution (DSD) retrieval from remote radar measurements or from in situ disdrometer measurements is an important area of research. If the shape (μ) and slope (Λ) of a three-parameter gamma distribution n(D) = N0Dμ exp(−ΛD) are related to one another, as recent disdrometer measurements suggest, the gamma DSD model is simplified to a two-parameter DSD, that is, a constrained gamma DSD. An empirical relation between the μ and Λ was derived using moments estimated from video-disdrometer measurements. Here, the effects of DSD truncation on a μ and Λ relation were analyzed. It was shown that characteristic size and variance of size of a constrained gamma DSD depend only on the shape parameter μ. Assuming that a constrained gamma DSD is valid, S-band polarimetric radar–based estimators for rain rate, median volume diameter, specific propagation phase, attenuation, and differential attenuation were derived. The radar-based estimators were used to obtain the spatial distribution of DSD p...

Comparison of Polarimetric Radar Drop Size Distribution Retrieval Algorithms

Abstract: Recently, two physically based algorithms, the “beta” (β) method and the “constrained-gamma” method, have been proposed for retrieving the governing parameters of the gamma drop size distribution (DSD) from polarimetric radar measurements. The β method treats the drop axis ratio as a variable and computes drop shape and DSD parameters from radar reflectivity (Z), differential reflectivity (ZDR), and specific differential phase (KDP). The constrained-gamma method assumes that the axis ratio relation is fixed and computes DSD parameters from reflectivity, differential reflectivity, and an empirical relation between the DSD slope and shape parameters. In this paper, the two approaches are evaluated by comparing retrieved rain DSD parameters with disdrometer observations and examining derived fields for consistency. Error effects on the β method retrievals are analyzed. The β approach is found to be sensitive to errors in KDP and to be inconsistent with observations. Large retrieved β values are foun...

Performance of correlation estimators for spaced‐antenna wind measurement in the presence of noise

Abstract: [1] The theoretical accuracy of baseline winds, estimated using spaced antennas (SA) and a full correlation analysis (FCA) method to process signals in the presence of noise, is derived assuming horizontally isotropic refractive index perturbations with a horizontal scale small compared to the transmitting antenna diameter D. This performance of the FCA method is compared with the theoretical performance of another correlation-based approach (i.e., the cross-correlation ratio method, CCR). The theoretical results of the error analysis are supported with numerical simulations and experimental data. It is shown that the theoretical analysis is valid and the results can be applied to improve wind estimates obtained from SA signals contaminated with additive white noise. The theory shows that the effect noise has on SA wind estimates depends on system configuration and lag spacing, and cannot be correctly accounted by a reduced correlation coefficient due to noise as hypothesized by May [1988].

Comparison of spaced‐antenna baseline wind estimators: Theoretical and simulated results

Abstract: [1] Formulas for the theoretical precision of cross-beam winds measured with spaced-antenna profilers are developed and compared with results obtained from simulations for conditions of high signal-to-noise-ratios. These formulas relate the precision of wind measurement to radar and atmospheric parameters. Formulas for Briggs' Full Correlation Analysis, the Intersection method, and the Slope-at-Zero-Lag method are each presented for two implementations-estimating parameters for assumed Gaussian shaped correlation functions, and a direct finite difference method where this assumption is not necessary. For each wind measurement method and implementation, these formulas are used to evaluate, as an example, the theoretical performance of MAPR, NCAR's 915 MHz, Multiple Antenna Profiling Radar. The theory is also compared with the standard error obtained from simulations presented by Kawano et al. [2002] for the MU radar in Shigaraki, Japan. Comparisons show that the Intersection and Briggs' FCA methods are identical and provide the best performance.

Radar/radiometer combination to retrieve cloud characteristics for icing detection

Abstract: Remote measurements of cloud liquid water content (LWC) and characteristic droplet size (e.g., radar estimated size: RES) are required for quantifying potential aircraft icing hazard (Politovich, 1989; Politovich and Bernstein, 2002; Vivekanandan et al., 2001). During the 2004 Winter Icing and Storm Project (WISP04), research radars and radiometers were deployed at NCAR’s Marshall experimental site near Boulder, Colorado to evaluate remote sensing techniques for characterizing cloud icing conditions. The dataset included radar and radiometer measurements. The best case for single wavelength radar and twochannel radiometer-based retrievals was on March 1011, when a shallow, fairly uniform stratus cloud in the temperature range of ~ -5 to -15C was observed. High liquid water content and little ice was found in the cloud. This cloud began with some patches of relatively high reflectivity (~10-20 dBZ) and snow showers at the ground. It then evolved to low (<-10 dBZ) reflectivity with lots of liquid, as evidenced by numerous pilot reports of icing in the Denver area. Strong ground clutter at S-band limits the usefulness of the data for a dual-wavelength application. Nevertheless, Ka-band radar reflectivity and radiometer measurements were available for retrieving cloud characteristics. In this paper, we present retrievals for cloud LWC and RES from single frequency radar reflectivity and dual-channel radiometer measurements. Retrieval methods are reviewed in Section 2. The retrieval results and comparisons are presented in Section 3. A summary and discussion are given in Section 4.