Temporal Variations in the Scaling Properties of Rain Echoes during the Development of a Cold Low in Saskatchewan

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  • 1 Climate Processes and Earth Observation Division, Atmospheric Environment Service, National Hydrology Research Centre, Saskatoon, Saskatchewan, Canada
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Abstract

A better understanding of the scaling properties of rain could assist in parameterizing rain events in hydrological models and support the development of improved “weather generators.” A technique developed for the analysis of the scaling characteristics of snow patches during melt has been adapted to study the development of rain echoes observed by radar. The technique is applied to echo ensembles detected by a weather radar located at Elbow, Saskatchewan, for a 48-h period from 1800 LST 30 June to 1800 LST 2 July 1991.

The scaling properties and other statistical measures of ensembles of radar echoes from 2-km MSL CAPPI (constant-altitude plan position indicator) maps wore analyzed for a 240 km × 240 km square domain centered on Elbow. The 0.2 mm h−1 rain rate was used to define the echoes. During the 48-h period, the sizes, numbers, and shapes of echoes within the domain changed considerably as a result of a cold low that intensified just outside the study area and then moved eastward. Analyses of some scaling properties of these ensembles were carried out along with tests of their sensitivity to reflectivity threshold.

The echo ensembles were characterized by the perimeter/area and Korcak parameters. The perimeter/area parameter provides a measure of the “roughness” of the edges of the echoes. Values of the perimeter/area parameter from this study agreed well with the results obtained by other investigators who used different data sources and techniques. The Korcak parameter, which represents the “clumpiness” of the pattern, was more variable. Changes in this parameter may serve as a precursor to significant changes in the overall rain pattern. Both the statistical properties and the scaling properties of echo ensembles were found to vary depending on the effects of the dynamic and thermodynamic atmospheric controls on rain formation.

Abstract

A better understanding of the scaling properties of rain could assist in parameterizing rain events in hydrological models and support the development of improved “weather generators.” A technique developed for the analysis of the scaling characteristics of snow patches during melt has been adapted to study the development of rain echoes observed by radar. The technique is applied to echo ensembles detected by a weather radar located at Elbow, Saskatchewan, for a 48-h period from 1800 LST 30 June to 1800 LST 2 July 1991.

The scaling properties and other statistical measures of ensembles of radar echoes from 2-km MSL CAPPI (constant-altitude plan position indicator) maps wore analyzed for a 240 km × 240 km square domain centered on Elbow. The 0.2 mm h−1 rain rate was used to define the echoes. During the 48-h period, the sizes, numbers, and shapes of echoes within the domain changed considerably as a result of a cold low that intensified just outside the study area and then moved eastward. Analyses of some scaling properties of these ensembles were carried out along with tests of their sensitivity to reflectivity threshold.

The echo ensembles were characterized by the perimeter/area and Korcak parameters. The perimeter/area parameter provides a measure of the “roughness” of the edges of the echoes. Values of the perimeter/area parameter from this study agreed well with the results obtained by other investigators who used different data sources and techniques. The Korcak parameter, which represents the “clumpiness” of the pattern, was more variable. Changes in this parameter may serve as a precursor to significant changes in the overall rain pattern. Both the statistical properties and the scaling properties of echo ensembles were found to vary depending on the effects of the dynamic and thermodynamic atmospheric controls on rain formation.

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