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  • Author or Editor: I. Zawadzki x
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I. I. Zawadzki

Abstract

The decrease of the mean square and the variance of reflectivity due to radar beam smoothing and post-detection integration are studied in terms of the autocorrelation function of the field of reflectivity. An exponential form of this function is used to evaluate the results. Applications to the problem of radar beam resolution and design of radar displays are discussed.

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I. I. Zawadzki

Abstract

Space and time autocorrelation functions are defined for the precipitation process on a horizontal plane. An optical device was designed and used to measure these functions as well as the mean, the mean square, and the variance of the rainfall rate for a time sequence of precipitation patterns of a widespread convective storm. The input data were radar PPI records stored on film in which the transmittance was adjusted to be proportional to rainfall rate.

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I. I. Zawadzki

Abstract

Spacial smoothing by the radar beam as well as post-detection integration reduce the variability of the distribution of rainfall rate in space. It is shown that when radar data are compared with instantaneous point rainfall rate a random error and a bias are introduced by the smoothing. This could account for some of the difficulties in the hydrological use of radars. It is shown that when raingage data are smoothed in time there is an optimum smoothing time interval such that the random error and the bias are reduced to a negligible level. A method is suggested for the optimum comparison of radar and raingage data and the possibility of a determination of Z-R relationships from such comparisons is discussed.

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G. Drufuca and I. I. Zawadzki

Abstract

Ten years of raingage data are processed in order to evaluate duration, average and maximum rate, mean square, variance, autocorrelation function and total amount for each rain storm. A spatial interpretation of these quantities is also given. Further, various rainfall rate probabilities are evaluated.

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F. Trudeau and I. Zawadzki

Abstract

This study investigates whether part of the variability of rain rates not explained by the thermodynamic parameters could be explained by the vertical wind structure as revealed by standard aerological sounding observations. The correlation found between rain rates and vertical wind structure is at most marginally significant. Moreover, given the high degree of interdependence between the thermodynamic parameters and rain rates, the somewhat more significant correlation observed between the dynamic and thermodynamic variables may induce an automatic correlation between wind profiles and rain rates.

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R. V. Calheiros and I. Zawadzki

Abstract

In this work a method is presented to obtain R-Z ε relationships through comparison, in probability, of nonsimultaneous measurements of Z ε and R. Range dependent relationships obtained in this way are given for a radar situated at 20°21′30″S, 49°01′38″W. The method is tested by comparing actual river hydrographs from a number of basins with those simulated using data as input to a hydrological model.

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I. I. Zawadzki and C. U. Ro

Abstract

Daily 5, 10 and 30 min maxima of precipitation rate determined from a raingage network and radar were correlated with parcel convective energies, upper air humidity, height of parcel convection and maxima of surface conditions. After a selection of 54 well-documented cases the correlation coefficient between the maximum of rain rate over 5 min and the maximum convective energy was ρ = 0.79 for all cases and ρ = 0.89 for 15 air mass cases. Introducing the upper air humidity further improves the correlations.

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B. J. Turner, I. Zawadzki, and U. Germann

Abstract

Filtering of nonpredictable scales of precipitation can be used to improve forecast precision (rms). Previous papers have studied the scale dependence of predictability of patterns of instantaneous rainfall rate and of probabilistic forecasts. In this paper, motivated by the often localized, intermittent nature of rainfall, the wavelet transform is used to develop measures of predictability at each scale. These measures are then used to design optimal forecast filters. This method is applied to radar composites of rainfall reflectivity over much of the continental United States and is developed to be appropriate for operational forecasts of rainfall rates and raining areas. For the four precipitation events studied, the average correlation at 4-h lead time was increased from 0.50 for the original nowcasts to 0.62 with forecast filtering. This forecast filtering is incorporated into the McGill Algorithm for Precipitation Nowcasting by Lagrangian Extrapolation (MAPLE), which now includes variational echo tracking, a semi-Lagrangian advection scheme, scale-based filtering, and appropriate rescaling of the filtered nowcast fields.

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I. Zawadzki, W. Szyrmer, and S. Laroche

Abstract

Liquid water is produced in the updraft regions of subfreezing clouds when the generation of vapor excess over the water saturation value exceeds the vapor depletion through the depositional growth of the solid particles. A diagnostic technique for the presence of supercooled cloud in the presence of snow is presented here. The data required are single-Doppler observations of reflectivity and radial velocity as well as a nearby sounding of temperature. From these data, the 3D wind field is retrieved by a variational method. From the retrieved vertical motion, the supercooled water is derived from the steady-state balance relation between snow content and cloud liquid water. The method is tested with a kinematic model that includes the main microphysical processes expected to occur in stratiform subfreezing conditions. A comparison between aircraft in situ measurements of supercooled water content and the diagnosed as well as model-generated values shows good agreement.

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I. Zawadzki, J. Morneau, and R. Laprise

Abstract

Predictability, defined as the ability to forecast precipitation over an area by Lagrangian persistence, is studied for 11 radar precipitation patterns. After a time ranging between 40 and 112 min, depending on individual cases, all forecast skill is lost. Attempts at relating this range of predictability to larger-scale meteorological parameters lead to positive results when the convective available potential energy is considered alone or in combination with wind shear energy. It appears from this study that the limited range of scales properly sampled by a single radar severely hampers the possibility of establishing a solid empirical relationship between mesoscale predictability and synoptic-scale meteorological parameters.

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