DSpace collection: 期刊論文

Water Budget of Typhoon Nari (2001)

title: Water Budget of Typhoon Nari (2001) abstract: Although there have been many observational and modeling studies of tropical cyclones (TCs), the understanding of TCs' budgets of vapor and condensate and the changes of budgets after TCs' landfall is still quite limited. In this study, high-resolution (2-km horizontal grid size and 2-min data interval) model output from a cloud-resolving simulation of Typhoon Nari (2001) is used to examine the vapor and condensate budgets and the respective changes of the budgets after Nari's landfall on Taiwan. All budget terms are directly derived from the model except for a small residual term. For the vapor budget, while Nari is over the ocean, evaporation from the ocean surface is 11% of the inward horizontal vapor transport within 150 km of the storm center, and the net horizontal vapor convergence into the storm is 88% of the net condensation. The ocean source of water vapor in the inner core is a small portion (5.5%) of horizontal vapor import, consistent with previous studies. After landfall, Taiwan's steep terrain enhances Nari's secondary circulation significantly and produces stronger horizontal vapor import at low levels, resulting in a 22% increase in storm-total condensation. Precipitation efficiency, defined from either the large-scale or microphysics perspective, is increased 10%-20% over the outer-rainband region after landfall, in agreement with the enhanced surface rainfall over the complex terrain.

The impact of microphysical schemes on hurricane intensity and track

title: The impact of microphysical schemes on hurricane intensity and track abstract: During the past decade, both research and operational numerical weather prediction models [e.g. the Weather Research and Forecasting Model (WRF)] have started using more complex microphysical schemes originally developed for high-resolution cloud resolving models (CRMs) with 1-2 km or less horizontal resolutions. WRF is a next-generation meso-scale forecast model and assimilation system. It incorporates a modern software framework, advanced dynamics, numerics and data assimilation techniques, a multiple moveable nesting capability, and improved physical packages. WRF can be used for a wide range of applications, from idealized research to operational forecasting, with an emphasis on horizontal grid sizes in the range of 1-10 km. The current WRF includes several different microphysics options. At NASA Goddard, four different cloud microphysics options have been implemented into WRF. The performance of these schemes is compared to those of the other microphysics schemes available in WRF for an Atlantic hurricane case (Katrina). In addition, a brief review of previous modeling studies on the impact of microphysics schemes and processes on the intensity and track of hurricanes is presented and compared against the current Katrina study. In general, all of the studies show that microphysics schemes do not have a major impact on track forecasts but do have more of an effect on the simulated intensity. Also, nearly all of the previous studies found that simulated hurricanes had the strongest deepening or intensification when using only warm rain physics. This is because all of the simulated precipitating hydrometeors are large raindrops that quickly fall out near the eye-wall region, which would hydrostatically produce the lowest pressure. In addition, these studies suggested that intensities become unrealistically strong when evaporative cooling from cloud droplets and melting from ice particles are removed as this results in much weaker downdrafts in the simulated storms. However, there are many differences between the different modeling studies, which are identified and discussed.

The Impact of a Warm Ocean Eddy on Typhoon Morakot (2009): A Preliminary Study from Satellite Observations and Numerical Modelling

title: The Impact of a Warm Ocean Eddy on Typhoon Morakot (2009): A Preliminary Study from Satellite Observations and Numerical Modelling abstract: On 6 August 2009, typhoon Morakot encountered a giant warm ocean eddy approximately 700 km by 500 km in the southern eddy rich zone of the western North Pacific Ocean. Soon after passing over the warm ocean eddy, Morakot reached its peak intensity at category 2. Results based on multiple satellite observations and numerical modelling suggest very favourable ocean conditions provided by the warm ocean eddy during this earlier developmental stage of Morakot. It is found that in the presence of the observed warm ocean eddy, the upper ocean heat content increased significantly by similar to 100%, from similar to 60 to 120 KJ cm(-2). This very deep and warm subsurface temperature effectively reduced the negative feedback of typhoon-induced ocean cooling. As a result, the during-storm sea surface temperature remained high at similar to 29 - 30 degrees C. This very warm during-typhoon sea surface temperature (SST) provided an increase in air-sea enthalpy flux supply by similar to 200% (i.e., similar to 500W m(-2) under the warm eddy situation .vs. the similar to 170 W m(-2) under the without eddy situation). Also, since the during-typhoon SST remained high, the moisture supply was increased to enhance convective activities. Numerical experiments using the Weather Research and Forecasting (WRF) model suggest that the presence of the warm ocean eddy does not change the overall structure or characteristics of Morakot. Rather, it contributes to a similar to 10% increase in Morakot's precipitation. This research shows that in addition to the favourable atmospheric conditions such as the Intra-Seasonal Oscillation or the southwestern monsoon flow, there also exist favourable ocean conditions provided by the presence of a warm ocean eddy during the early developmental stage of Morakot. Further studies based on a full-physics typhoon-ocean coupled model are needed to quantify the role of the upper ocean features in affecting the evolution of Morakot, including its rainfall over Taiwan.

The Effect of Tropical Cyclones on Southwest Monsoon Rainfall in the Philippines

title: The Effect of Tropical Cyclones on Southwest Monsoon Rainfall in the Philippines abstract: Intense southwest monsoon (SWM) rainfall events causing massive landslides and flash floods along the western sections of the Philippines were studied. These rainfall events, are not directly coming from the tropical cyclones (TCs) for they are situated far north to northeast of Luzon Island. The heavy rainfall is hypothesized as caused by the interaction of strong westerlies with the mountain ranges along the west coast of Luzon that produces strong vertical motion and consequently generates heavy rainfall. Four of heavy SWM rainfall cases were examined to determine how the presence and position of tropical cyclones in the Philippine vicinity affect these SWM rainfall events; three cases with TC of varying positions within the Philippine area of responsibility (PAR) and the fourth case without IC. Using a spatial Fourier decomposition approach, the total streamfunction is decomposed into two flow regimes: monsoon basic flow (Waves 0-1) and tropical cyclone perturbation flow (Waves 2-23) over a domain of (20 degrees E-140 degrees W, 5 degrees S-35 degrees N). The purpose of this flow decomposition is to determine the latter's effect on or contribution to the monsoon activity. The analysis utilized the NCEP Final (FNL) data with 1 degrees long. x 1 degrees lat. resolution. Results show that the tropical cyclones over the Pacific Ocean located northeast of Luzon generate strong southwesterly winds over the west coast of Luzon. These in addition to the southwesterlies from the basic flow strengthened the southwest winds that interact with the high Cordillera Mountain ranges along the west coast of Luzon. When the tropical cyclone is located north or north-northwest of Luzon, it generates northwesterlies which converge with the southwesterlies from the basic flow. This results to enhancement of rising motion over western Luzon. The much stronger westerlies are then forced to rise above the mountains resulting to strong vertical motion that brings about heavy rainfall.