Fresnel zone considerations for reflection and scatter from refractive index irregularities

Several different echoing mechanisms are proposed to explain VHF/UHF scatter from clear air; (1) anisotropic scatter; (2) Fresnel reflection, and (3) Fresnel scatter, in order to account for the spatial (angle and range) and temporal dependence of the echoes. The term diffuse reflection describes the echoing mechanism when both scatter and reflection coexist. A unifying formulation is presented incorporating a statistical approach that embraces all mechanisms the above mechanisms and gives conditions under which reflection or scatter dominates. A distinction between Fraunhofer and Fresnel scatter and a criterion is presented under which Fresnel scatter is important

Analysis of airborne Doppler lidar, Doppler radar and tall tower measurements of atmospheric flows in quiescent and stormy weather

The first experiment to combine airborne Doppler Lidar and ground-based dual Doppler Radar measurements of wind to detail the lower tropospheric flows in quiescent and stormy weather was conducted in central Oklahoma during four days in June-July 1981. Data from these unique remote sensing instruments, coupled with data from conventional in-situ facilities, i.e., 500-m meteorological tower, rawinsonde, and surface based sensors, were analyzed to enhance understanding of wind, waves and turbulence. The purposes of the study were to: (1) compare winds mapped by ground-based dual Doppler radars, airborne Doppler lidar, and anemometers on a tower; (2) compare measured atmospheric boundary layer flow with flows predicted by theoretical models; (3) investigate the kinematic structure of air mass boundaries that precede the development of severe storms; and (4) study the kinematic structure of thunderstorm phenomena (downdrafts, gust fronts, etc.) that produce wind shear and turbulence hazardous to aircraft operations. The report consists of three parts: Part 1, Intercomparison of Wind Data from Airborne Lidar, Ground-Based Radars and Instrumented 444 m Tower; Part 2, The Structure of the Convective Atmospheric Boundary Layer as Revealed by Lidar and Doppler Radars; and Part 3, Doppler Lidar Observations in Thunderstorm Environments

Dependence of reflectivity factor-rainfall rate relationship on polarization

The reflectivity factor (Z), rainfall rate (R) relationship for weather radars that probe precipitation at low elevation angles is sensitive to polarization. It is shown how to transform a relation that is valid with one polarization (vertical, horizontal or circular) to relations that are applicable to the other two polarizations. We present errors that occur if the transformations are not applied, and an example from literature in which two seemingly different Z, R relations are equivalent, tied by the polarization transformation

Error structure of multiparameter radar and surface measurements of rainfall. Part III: specific differential phase

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 NSSL Cimarron radar and the NCAR/CP-2 radar. As practical application of the analysis presented here, we have determined the range of applicability of the three rainfall rate estimators: R(ZH), R(ZH, ZDR) and R(KDP). Our simulations show that when the rainfall rate exceeds about 70 mm h-1, R(KDP) performs better than R(ZH, ZDR). This result is valid over a 1 km propagation path. At intermediate rainfall rates around 20≲R≲70 mm h−1, our simulations show that R(ZH, ZDR) gives the least error. However, there are other reasons which make R(KDP) useful; i.e., (i) its stability with respect to mixed phase precipitation, and (ii) the fact that it is a differential phase measurement and thus insensitive to system gain calibration. This last premise suggests an accurate method of system gain calibration based on the rain medium

Use of Ground Clutter to Monitor Polarimetric Radar Calibration

Abstract It is suggested that urban ground clutter can have a role in monitoring calibration of reflectivity factor ZH and differential reflectivity ZDR on polarimetric radars. The median and average values of these variables are considered. Analysis of data from 1 month of cold season in Germany (X-band radar) and 3.5 hot days in Oklahoma (S-band radar) is presented. In the presence of up to moderate rain or snow a reflectivity threshold suffices for separating significant clutter from precipitation observed with an X-band radar. The same threshold was suitable on observations with an S-band radar in Oklahoma because heavy precipitation was not present. The tests suggest the scheme is worthy considering for operational monitoring of ZH as its median values at both locations were within the quantization interval of 0.5 dB. Environmental factors that can influence reflectivities from clutter are examined. The effects on ZDR can be significant. These are quantified in the data and possible uses for calibration and monitoring radar status are indicated.</jats:p