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Brice Boudevillain and Hervé Andrieu

Abstract

Vertically integrated liquid (VIL) water content is a parameter obtained from a radar performing voluminal scanning. This parameter has proven useful in the detection of severe storms and may be a worthwhile indicator for very short-term rainfall forecasting methods. Unfortunately, no information is available on the accuracy of VIL radar measurements. The present paper addresses this issue by means of simulation. Reference VILs are defined from vertical profiles of drop size distributions (DSD). These profiles make it possible to simulate the corresponding vertical profiles of reflectivity as well as the radar measurements used to deduce the VIL, as estimated classically (i.e., application of a classical relationship between equivalent radar reflectivity factor Ze and liquid water content M adapted to raindrops). A comparison of the reference VIL to the corresponding estimate then allows estimating radar measurement error. The VIL measurement error is first studied from two hypothetical, yet realistic, vertical profiles of DSD: one typical of stratiform rain and the other typical of a convective situation. A sensitivity analysis with respect to both meteorological conditions and radar operating conditions is also performed on these two profiles. For the convective case, use of a classical ZeM relationship adapted to liquid water results in a significant underestimation of the reference VIL value. The same effect applies to the stratiform profile, even though brightband phenomena can compensate for this underestimation and lend the impression of smaller measurement error. A simple alternative method is proposed in order to reduce measurement errors. Both conventional and alternative VIL measurement methods are tested on the two theoretical profiles as well as on a series of actual vertical profiles of reflectivity. Better measurements are obtained with the alternative method, provided the altitude of the 0°C isotherm and the density of ice particles can be determined with reasonable precision. This alternative method for estimating VIL from radar data could serve to improve VIL measurement accuracy and would be worth applying to a longer series of observed data.

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Guy Delrieu, Jean Dominique Creutin, and Hervé Andrieu

Abstract

The aim of the present study is to characterize mountain returns measured with a ground-based weather radar operating in a mountainous region. A computation code based on the use of a digitized terrain model is developed for calculating the areas illuminated by the radar beam. Partial and total screening effects am accounted for in the calculation. The angular and range weighting functions of the radar measurement am modeled using Gaussian approximations to give the so-called weighted illuminated areas for various sizes of the radar resolution volume. Radar measurements are compared to the computed illuminated areas in order to determine the average backscattering coefficient of partly grass-covered, partly forested mountains: 87% of the measured time-averaged mountain return variance is explained by the computed values when the 15-dB resolution volume is considered. Additional geometrical information, provided by the calculated angles of incidence, is accounted for to yield a linear σ(dB) 0(α) model relevant for the so-called near-grazing region since most of the angles of incidence are in the 70°–900° range. Here 92% of the measurement variance is explained when the σ(dB) 0(α) model is used.

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Mark N. French, Hervé Andrieu, and Witold F. Krajewski

Abstract

Radar reflectivity is used to estimate meteorological quantities such as rainfall rate, liquid water content, and the related quantity, vertically integrated liquid (VIL) water content. The estimation of any of these quantities depends on several assumptions related to the characteristics of the physical processes controlling the occurrence and character of water in the atmosphere. Additionally, there are many sources of error associated with radar observations, such as those due to brightband, hail, and drop size distribution approximations. This work addresses one error of interest, the radar reflectivity observation error; other error sources are assumed to be corrected or negligible. The result is a relationship between the uncertainty in VIL water content and radar reflectivity measurement error. An example application illustrates the estimation of VIL uncertainty from typical radar reflectivity observations and indicates that the coefficient of variation in VIL is much larger than the coefficient of variation in radar reflectivity.

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Olivier Caumont, Véronique Ducrocq, Guy Delrieu, Marielle Gosset, Jean-Pierre Pinty, Jacques Parent du Châtelet, Hervé Andrieu, Yvon Lemaître, and Georges Scialom

Abstract

A full radar simulator for high-resolution (1–5 km) nonhydrostatic models has been developed within the research nonhydrostatic mesoscale atmospheric (Meso-NH) model. This simulator is made up of building blocks, each of which describes a particular physical process (scattering, beam bending, etc.). For each of these blocks, several formulations have been implemented. For instance, the radar simulator offers the possibility to choose among Rayleigh, Rayleigh–Gans, Mie, or T-matrix scattering methods, and beam bending can be derived from an effective earth radius or can depend on the vertical gradient of the refractive index of air. Moreover, the radar simulator is fully consistent with the microphysical parameterizations used by the atmospheric numerical model.

Sensitivity experiments were carried out using different configurations for the simulator. They permitted the specification of an observation operator for assimilation of radar reflectivities by high-resolution nonhydrostatic numerical weather prediction systems, as well as for their validation. A study of the flash flood of 8–9 September 2002 in southeastern France, which was well documented with volumetric data from an S-band radar, serves to illustrate the capabilities of the radar simulator as a validation tool for a mesoscale model.

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