A.R. Holt

Extracting Rainfall Rates from X-band CDR Radar Data by Using Differential Propagation Phase Shift

Abstract In this paper, the method of using differential propagation phase shift ΔΦ to measure rain rate R from X-band circular depolarization ratio (CDR) radar data is presented. The principle of this method is based on the fact that there is a relationship between ΔΦ and R. The relationship between ΔΦ and R is investigated under various parameters such as water temperature, raindrop shape, and drop-size distribution form. Differential backscattering phase shift ΔδHV is related to ΔΦ, and this makes it possible to extract ΔΦ from CDR radar data and thus to estimate rain rates. The “cleaning” procedures are utilized to minimize the effects of canting angle. Limitations due to noise and quantization in using ΔΦ to measure R are discussed. Rain-rate measurements obtained by raingage and by radar agree well.

An evaluation of the potential of polarimetric radiometry for numerical weather prediction using QuikSCAT

It has been proposed that wind vector information derived from passive microwave radiometry may provide an impact on numerical weather forecasts of similar magnitude to that achieved by scatterometers. Polarimetric radiometers have a lower sensitivity to wind direction than scatterometers at low wind speed but comparable sensitivity at high windspeed. In this paper, we describe an experiment which aimed to determine if an observing system only capable of providing wind direction information at wind speeds over 8 ms/sup -1/ can provide comparable impact to one providing wind vectors at wind speeds over 2 ms/sup -1/. The QuikSCAT dataset used in the experiments has a wide swath and is used operationally by several forecast centers. The results confirm that assimilation of wind vectors from QuikSCAT only for wind speeds above 8 ms/sup -1/ gives similar analysis increments and forecast impacts to assimilating wind vectors at all wind speeds above 2 ms/sup -1/. Measurements from the WindSat five frequency polarimetric radiometer are compared with calculations from Met Office global forecast fields, and this also confirms that WindSat measurement and radiative transfer model accuracy appears to be sufficiently good to provide useful information for numerical weather prediction.

Two new approaches to the calculation of rain induced attenuation and cross polarization

The authors review the recently presented Fredholm integral method for calculating the scattering of electromagnetic waves by dielectric spheroids and ellipsoids. The application to oblate spheroidal and ellipsoidal raindrops is given and the important properties of numerical stability and guaranteed convergence is discussed in relation to other scattering methods. Using a recent more general model of a rain filled medium in which a stochastic distribution of canting angles is assumed for the raindrops they provide results for terrestrial and satellite communication systems which they compare with experimental results, and with results obtained using the more usual deterministic model. Finally the conclusions of a study of multiple scattering in rain filled media are given and conditions presented which show that for frequencies up to 100 GHz as investigated, multiple scattering is negligible.AnalyseLes auteurs passent en revue la méthode de l’intégrale de Fredholm récemment présentée pour calculer la diffusion des ondes électromagnétiques par des sphéroïdes et des ellipsoïdes diélectriques. Ils appliquent cette méthode à des gouttes d’eau oblates ou ellipsoïdales et ils comparent avec d’autres méthodes de diffusion les propriétés importantes de stabilité numérique et de convergence garantie. En utilisant un modèle récent plus général d’un milieu pluvieux pour lequel la distribution stochastique des angles d’inclinaison des gouttes est admise, ils obtiennent des résultats pour les systèmes de communication terrestre ou par satellite qu’ils comparent avec les résultats expérimentaux et ceux obtenus en utilisant un modèle déterministe plus classique. Finalement, ils donnent les conclusions d’une étude de diffusion multiple dans un milieu pluvieux et présentent les conditions qui montrent que pour les fréquences allant jusqu’à 100 GHz, la diffusion multiple est négligeable.

A rainrate-attenuation-reflectivity relation for use in the spaceborne and airborne sensing of rain

A relationship linking rainfall rate with powers of radar reflectivity and specific attenuation is considered. It is shown that at frequencies used for the spaceborne monitoring of rain, such a relationship exists that is relatively insensitive to dropsize distribution (dsd) and temperature over a wide range of rainrates.