Y.M.M. Antar

Measurements of ice depolarization at 28.56 GHz using the COMSTAR Beacon simultaneously with a 16.5 GHz polarization diversity radar

Measurements on ice depolarization made at the National Research Council in Ottawa, using a 16.5-GHz polarization diversity radar concurrently with the 28.56-GHz COMSTAR beacon, are reported. The radar, which is normally operated in a circular polarization mode, provides data on polarization parameters such as the cancellation ratio and the degree of preferred orientation. Frequently, the differential propagation parameters can be determined. From these data, identification of different kinds of hydrometeors (i.e., rain, melting layer, and ice crystals) along the path is possible. The beacon receiver measures the differential attenuation and the differential phase shift between two linear orthogonal components of the received signal, and the copolar attenuation. Several occurrences of ice depolarization in the presence of rain were observed. From radar data, identification of the hydrometeors was made and the contribution of each type toward the differential propagation parameters was determined. Good agreement between radar and beacon receiver measurements was obtained. Also several events of ice depolarization in the absence of rain are reported. The results presented here provide new data on ice depolarization and demonstrate the usefulness of simultaneous polarization diversity radar and beacon measurements for obtaining a better understanding of the nature of ice depolarization. Because of the limited observation period, emphasis is given to the nature of the phenomenon rather than to the statistics of its occurrence.

Differential propagation constants on slant paths through snow and ice crystals as measured by 16.5 GHz polarization-diversity radar

Radar determinations of differential propagation constants at a wavelength of 1.82 cm on slant paths through heavy snow and ice crystals are reported. Also reported are measurements of differential propagation effects in the melting layer. The slant path results indicate an increase of differential phase shift with height, to a value at 2000 m which may exceed 1.3 deg/ km, being typically several times that at the surface. This effect is attributed to a change in the hydrometeors, as they precipitate, from ice crystals to snow aggregates. Differential propagation effects in the melting layer are small.AnalyseL’article présente les constantes de propagation différentielle mesurées par radar à la longueur d’onde de 1,82 cm, sur des trajets obliques dans de la neige épaisse et des cristaux de glace ainsi que les mesures des effets de la propagation différentielle dans la couche de fusion. Les résultats sur les trajets obliques font apparaître une augmentation du déphasage différentiel avec la hauteur, jusqu’à une valeur pouvant excéder 1,3 deg/km à 2000 m, soit plusieurs fois le déphasage en surface. On attribue cette augmentation à un changement d’état des hydrométéores: en tombant, ceux-ci qui étaient sous forme de cristaux de glace se transforment en agrégats de neige. Les effets de la propagation différentielle dans la couche de fusion sont faibles.