High-Resolution Numerical Simulation of the Extreme Rainfall Associated with Typhoon Morakot. Part I: Comparing the Impact of Microphysics and PBL Parameterizations with Observations

Typhoon Morakot hit Taiwan the night of 7 August 2009 as a Category 1 storm and caused up to 3000 mm of rain, leading to the worst flooding there in 50 years as well as devastating mudslides. The Weather Research and Forecasting model (WRF) is used at high resolution to simulate this extreme weather event. The model results indicate that WRF is able to capture the amount and location of the observed surface rainfall and that the typhoon-induced circulation, orographic lifting and a moisture-abundant southwest flow are the main mechanisms that together produced the tremendous rainfall in this case. Furthermore, the model results suggest that the agreement with the observed rainfall is due to the simulated storm track and intensity being in relatively good agreement with the observed. Additional simulations were made to examine the sensitivity of this case to model physics (microphysics and planetary boundary layer or PBL). Both warm rain only as well as improved microphysics yield similar significant rain amounts at the same locations as the control case. The improved microphysics lead to a better storm intensity early on but later exceed the observed intensities by about 10 hPa. The stronger storm arises from less evaporative cooling from cloud and rain and consequently weaker simulated downdrafts. Warm rain results closely match the control (i.e., the track, intensity, and maximum rainfall locations/amounts), implying ice processes (i.e., additional heat release due to ice processes) have only a secondary effect on surface rainfall. Results are less sensitive to using different PBL schemes than different microphysics

Preface to the Special Issue on Severe Weather Research in Taiwan

The special section of “Severe Weather Research in Taiwan” is printed in this volume. There are total of 5 papers in this special section. The call-for-paper for this special section started on 2002 right after the “International Conference on Mesoscale Meteorology and Typhoon in East Asia” held at Taiwan in September 2001 and organized by Taiwan Weather Research Program Office and the Chinese Geoscience Union. It was a lengthy process. Totally there were 9 papers submitted and only 5 of them got accepted through the review processes. The papers published consist of a wide spectrum, from summertime hailstorm in the Taipei basin (Chen and Chou) to Mei-Yu rainfall estimation by using satellite (Hu and Chen) and to typhoon studies. There are 3 papers related to typhoon research, i.e., the simulation of discontinuous track of Typhoon Dot (1990) while passing by the Taiwan Island by using MM5 (Jian et al.), formation of Typhoon Robyn (1993) by numerical simulation (Cheung and Elsberry), and concentric eyewall formation study by using simple axis-symmetric model (Peng et al.)

Time-Lagged Ensemble Quantitative Precipitation Forecasts for Three Landfalling Typhoons in the Philippines Using the CReSS Model, Part I: Description and Verification against Rain-Gauge Observations

In this study, the 2.5 km Cloud-Resolving Storm Simulator was applied to forecast the rainfall by three landfalling typhoons in the Philippines at high resolution: Mangkhut (2018), Koppu (2015), and Melor (2015), using a time-lagged strategy for ensemble. The three typhoons penetrated northern Luzon, central Luzon, and the middle of the Philippine Archipelago, respectively, and the present study verified the track and quantitative precipitation forecasts (QPFs) using categorical statistics against observations at 56 rain-gauge sites at seven thresholds up to 500 mm. The predictability of rainfall is the highest for Koppu, followed by Melor, and the lowest for Mangkhut, which had the highest peak rainfall amount. Targeted at the most-rainy 24 h of each case, the threat score (TS) within the short range (≤72 h) could reach 1.0 for Koppu at 350 mm in many runs (peak observation = 502 mm), and 1.0 for Mangkhut and 0.25 for Melor (peak observation = 407 mm) both at 200 mm in the best member, when the track errors were small enough. For rainfall from entire events (48 or 72 h), TS hitting 1.0 could also be achieved regularly at 500 mm for Koppu (peak observation = 695 mm), and 0.33 at 350 mm for Melor (407 mm) and 0.46 at 200 mm for Mangkhut (786 mm) in the best case. At lead times beyond the short range, one third of these earlier runs also produced good QPFs for both Koppu and Melor, but such runs were fewer for Mangkhut and the quality of QPFs was also not as high due to larger northward track biases. Overall, the QPF results are very encouraging, and comparable to the skill level for typhoon rainfall in Taiwan (with similar peak rainfall amounts). Thus, at high resolution, there is a fair chance to make decent QPFs even at lead times of 3–7 days before typhoon landfall in the Philippines, with useful information on rainfall scenarios for early preparation

The Formation of Concentric Eyewalls with Heat Sink in a Simple Tropical Cyclone Model

A linearized, two-layer axisymmetric model analogous to Schubert el al. (1980) is used to simulate the formation of concentric eyewalls in an ideal strong tropical cyclone. By imposing a heat sink near the center of a cyclone the induced perturbation wind, through thermodynamic adjustment to the heat sink, forms a double-peak structure when the disturbance is added to the basic state tangential wind. The heat sink represents, in a crude way, evaporative cooling of precipitation falling from cloud during late stage convective activity or a cooling through environmental advection. Detailed profiling of the induced double-peak wind structure is dependent on the radial profile of the imposed heat sink. After the double-peak tangential wind structure is formed, if a heat source corresponding to a new convective activity is generated inside the outer maximum tangential wind, the outer eyewall contracts and strengthens while the inner eyewall weakens. This result suggests that thermodynamic adjustments to changes in the heating of a tropical-cyclone-core region may contribute to the formation of the double-eyewall phenomenon

Doppler Velocity Signatures of Idealized Elliptical Vortices

Doppler radar observations have revealed a class of atmospheric vortices (tropical cyclones, tornadoes, dust devils) that possess elliptical radar reflectivity signatures. One famous example is Typhoon Herb (1996) that maintained its elliptical reflectivity structure over a 40-hour period. Theoretical work and dual-Doppler analyses of observed tropical cyclones have suggested two physical mechanisms that can explain the formation of two types of elliptical vortices observed in nature, namely, the combination of a circular vortex with either a wavenumber two vortex Rossby wave or a deformation field. The characteristics of these two types of elliptical vortices and their corresponding Doppler velocity signatures have not been previously examined