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Monika Feldmann, Kerry Emanuel, Laiyin Zhu, and Ulrike Lohmann

Abstract

Tropical cyclones pose a significant flood risk to vast land regions in their path because of extreme precipitation. Thus it is imperative to quantitatively assess this risk. This study compares exceedance frequencies of tropical cyclone precipitation derived from two independent observational datasets with those estimated using a tropical cyclone rainfall algorithm applied to large sets of synthetic tropical cyclones. The modeled rainfall compares reasonably well to observed rainfall across much of the southern United States but does less well in the mid-Atlantic states. Possible causes of this disparity are discussed.

Open access
Monika Feldmann, Curtis N. James, Marco Boscacci, Daniel Leuenberger, Marco Gabella, Urs Germann, Daniel Wolfensberger, and Alexis Berne

Abstract

Region-based Recursive Doppler Dealiasing (R2D2) is a novel dealiasing algorithm to unfold Doppler velocity fields obtained by operational radar measurements. It specializes in resolving issues when the magnitude of the gate-to-gate velocity shear approaches or exceeds the Nyquist velocity. This occurs either in highly sheared situations, or when the Nyquist velocity is low. Highly sheared situations, such as convergence lines or mesocyclones, are of particular interest for nowcasting and warnings. R2D2 masks high-shear areas and adds a spatial buffer around them. The areas between the buffers are then identified as continuous regions that lie within the same Nyquist interval. Each region subsequently is assigned its most likely Nyquist interval by applying vertical and temporal continuity constraints, as well as supplemental wind information from an operational mesoscale model. The shear zones are then resolved using 2D continuity in azimuth and range. This 4D procedure is repeated until no further improvement can be achieved. Each iteration with fewer folds identifies fewer but larger continuous regions and less shear zones until an optimum is reached. Residual errors, often related to shear greater than the Nyquist velocity, are contained to small areas within the buffer zones. This approach maximizes operational performance in high-shear situations and restricts errors to minimal areas to mitigate error propagation.

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