The Impact of Assimilating Radar-estimated Rain Rates on Simulation of Precipitation in the 17--18 July 1996 Chicago Floods |
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Authors: | Xingbao WANG M K YAU B NAGARAJAN and Luc FILLION |
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Institution: | Deptartment of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada,Deptartment of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada,Deptartment of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada,Recherche en Prevision Numerique, Environment Canada, Dorval, Montreal, Canada |
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Abstract: | Rainfall prediction remains one of the most challenging problems in weather
forecasting. In order to improve high-resolution quantitative precipitation
forecasts (QPF), a new procedure for assimilating rainfall rate derived from
radar composite reflectivity has been proposed and tested in a numerical
simulation of the Chicago floods of 17--18 July 1996. The methodology is
based on the one-dimensional variation scheme (1DVAR) assimilation approach
introduced by Fillion and Errico but applied here using the Kain-Fritsch
convective parameterization scheme (KF CPS). The novel feature of this work
is the continuous assimilation of radar estimated rain rate over a three
hour period, rather than a single assimilation at the initial (analysis)
time. Most of the characteristics of this precipitation event, including the
propagation, regeneration of mesoscale convective systems, the frontal
boundary across the Midwest and the evolution of the low-level jet are
better captured in the simulation as the radar-estimated precipitation rate
is assimilated. The results indicate that precipitation assimilation during
the early stage can improve the simulated mesoscale feature of the
convection system and shorten the spin-up time significantly. Comparison of
precipitation forecasts between the experiments with and without the 1DVAR
indicates that the 1DVAR scheme has a positive impact on the QPF up to 36
hours in terms of the bias and bias equalized threat scores. |
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Keywords: | quantitative precipitation forecasts 1DVAR data assimilation |
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