Kazuhisa Tsuboki

Local amplification of storm surge by Super Typhoon Haiyan in Leyte Gulf

Typhoon Haiyan, which struck the Philippines in November 2013, was an extremely intense tropical cyclone that had a catastrophic impact. The minimum central pressure of Typhoon Haiyan was 895 hPa, making it the strongest typhoon to make landfall on a major island in the western North Pacific Ocean. The characteristics of Typhoon Haiyan and its related storm surge are estimated by numerical experiments using numerical weather prediction models and a storm surge model. Based on the analysis of best hindcast results, the storm surge level was 5–6 m and local amplification of water surface elevation due to seiche was found to be significant inside Leyte Gulf. The numerical experiments show the coherent structure of the storm surge profile due to the specific bathymetry of Leyte Gulf and the Philippines Trench as a major contributor to the disaster in Tacloban. The numerical results also indicated the sensitivity of storm surge forecast.

Large-Scale Parallel Computing of Cloud Resolving Storm Simulator

A sever thunderstorm is composed of strong convective clouds. In order to perform a simulation of this type of storms, a very fine-grid system is necessary to resolve individual convective clouds within a large domain. Since convective clouds are highly complicated systems of the cloud dynamics and microphysics, it is required to formulate detailed cloud physical processes as well as the fluid dynamics. A huge memory and large-scale parallel computing are necessary for the computation. For this type of computations, we have developed a cloud resolving numerical model which was named the Cloud Resolving Storm Simulator (CReSS). In this paper, we will describe the basic formulations and characteristics of CReSS in detail. We also show some results of numerical experiments of storms obtained by a large-scale parallel computation using CReSS.

Determination of a Z-R Relationship for Snowfall Using a Radar and High Sensitivity Snow Gauges

Abstract A best-fit power-law relationship (Z = 427 R1.09) between 1-minute integrated averages of snowfall rate (R) and radar reflectivity factor (Z) was determined on the basis of observations made by using high sensitivity snow gauges (accuracy 0.03 mm h−1) and a radar (wavelength 3.2 cm, beamwidth 1.1°) of three 1987 Sapporo snowstorms. The relationship Z = 554R0.88, using 30-minute integrated averages of Z and R, produced the best radar estimate of total snowfall. The ratio of the estimated to the observed amount of snowfall decreased with increasing density of new fallen snow ρ, the ratio roughly equaling 1, when ρ ≈ 0.05 g cm−3.

A numerical study on the effects of Taiwan topography on a convective line during the Mei-Yu season

Abstract During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m2 s−2. Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast–southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s−1. In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and co...

Effects of Asymmetric Latent Heating on Typhoon Movement Crossing Taiwan: The Case of Morakot (2009) with Extreme Rainfall

AbstractTyphoon Morakot struck Taiwan during 6–9 August 2009, and it produced the highest rainfall (approaching 3000 mm) and caused the worst damage in the past 50 yr. Typhoon–monsoon flow interactions with mesoscale convection, the water vapor supply by the monsoon flow, and the slow moving speed of the storm are the main reasons for the record-breaking precipitation. Analysis of the typhoon track reveals that the steering flow, although indeed slow, still exceeded the typhoon moving speed by approximately 5 km h−1 (1 km h−1 = 0.28 m s−1) during the postlandfall period on 8 August, when the rainfall was the heaviest. The Cloud-Resolving Storm Simulator (CReSS) is used to study the dynamics of the slow storm motion toward the north-northwest upon leaving Taiwan. The control simulations with 3-km grid size compare favorably with the observations, including the track, slow speed, asymmetric precipitation pattern, mesoscale convection, and rainfall distribution over Taiwan. Sensitivity tests with reduced moi...

Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

AbstractCyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation.The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band ...

Precipitation features observed by Doppler radar at Tuktoyaktuk, Northwest Territories, Canada, during the Beaufort and Arctic Storms Experiment

In the fall of 1994, the Beaufort and Arctic Storms Experiment (BASE) was held to collect information on the structure and evolution of mesoscale weather systems over the southern Beaufort Sea and the Mackenzie River delta of the western Canadian Arctic. As part of the experiment, X-band Doppler radar observations were carried out at Tuktoyaktuk, a village on the shore of the Beaufort Sea. In this paper, the precipitation features, structure, and moisture transport associated with two distinctly different weather systems that were observed during BASE are described with a variety of datasets. Climatologies of storm activity in the area suggest these two types of different weather systems, the so-called Pacific origin and storm track disturbances, are the most frequently observed in this region during the fall months. The characteristic feature of a Pacific origin weather system is a pronounced layering of the air masses. In the upper layer, the air mass is of Pacific origin and is associated with a deep low in the Gulf of Alaska. As a result it is moist and is capable of producing precipitation. In contrast, the lower layer is initially of continental origin and is associated with a secondary lee cyclogenesis event in the Mackenzie River basin. As the secondary disturbance moves to the east, there is a shift in the wind direction that advects air from the Beaufort Sea into the lower layer. This results in a moistening of the lower layer that allows precipitation from the upper layer that had previously evaporated in the lower layer to be enhanced and reach the surface. The detailed structure of this type of storm is strongly affected by the topography of the region and the presence of open water in the southern Beaufort Sea. The storm track weather system is markedly different and is associated with the passage of a mesoscale low over the southern Beaufort Sea. In this sort of system, there is a well-defined frontal structure of a type previously identified in the midlatitudes. Two different precipitation regimes are identified that are associated with the passage of the warm and cold front. In this sort of system, the sources of moisture are the Bering Sea and the open water in the southern Beaufort Sea.

Future increase of supertyphoon intensity associated with climate change

Increases of tropical cyclone intensity with global warming have been demonstrated by historical data studies and theory. This raises great concern regarding future changes in typhoon intensity. The present study addressed the problem to what extent supertyphoons will become intense in the global warming climate of the late 21st century. Very high resolution downscale experiments using a cloud-resolving model without convective parameterizations were performed for the 30 most intense typhoons obtained from the 20 km mesh global simulation of a warmer climate. Twelve supertyphoons occurred in the downscale experiments, and the most intense supertyphoon attained a central pressure of 857 hPa and a wind speed of 88 m s−1. The maximum intensity of the supertyphoon was little affected by uncertainties that arise from experimental settings. This study indicates that the most intense future supertyphoon could attain wind speeds of 85–90 m s−1 and minimum central pressures of 860 hPa.

High-Resolution Simulations of High-Impact Weather Systems Using the Cloud-Resolving Model on the Earth Simulator

High-impact weather systems occasionally cause huge disasters to human society owing to heavy rainfall and/or violent wind. They consist of cumulonimbus clouds and usually have a multiscale structure. High-resolution simulations within a large domain are necessary for quantitatively accurate prediction of the weather systems and prevention/reduction of disasters. For the simulations, we have been developing a cloud-resolving model named the Cloud Resolving Storm Simulator (CReSS). The model is designed for a parallel computer and was optimized for the Earth Simulator in the present study. The purpose of the present research is highresolution simulations of high-impact weather systems in a large calculation domain with resolving individual cumulonimbus clouds using the CReSS model on the Earth Simulator. Characteristic high-impact weather systems in East Asia are the Baiu front, typhoons, and winter snowstorms. The present chapter describes simulations of these significant weather systems. We have chosen for the case study of the Baiu front the Niigata—Fukushima heavy rainfall event on July 13, 2004. Typhoons for simulations are T0418, which caused a huge disaster due to strong wind, and T0423, which caused severe flood over the western Japan in 2004. Snowstorms were studied by an idealized numerical experiment as well as by a simulation of cold air outbreak over the Sea of Japan. These experiments clarified both the overall structures of weather systems and individual clouds. The high-resolution simulations resolving individual clouds permit a more quantitative prediction of precipitation. They contribute to accurate prediction of wind and precipitation and to reduction of disasters caused by high-impact weather systems.

A simulation of a lake effect snowstorm with a cloud resolving numerical model

[1] Lake-effect snowstorms (LES), linearly organized bands of convective clouds, are a major source of snowfall and severe weather in the North American Great Lakes region. LES develop as cold and dry air flows over the warm lake surfaces triggering convection that is often organized into quasi-linear structures known as band clouds. The small horizontal width of these bands, often less than 5 km, combined with their regional-scale evolution that is impacted by the distribution of open water, lake-ice and land makes the forecasting of LES particularly challenging. Here, we describe the simulation of an observed LES event using a cloud resolving numerical model in a domain that includes much of the Great Lakes region. The model was able to successfully capture many of the characteristics associated with the event. This simulation suggests that it soon may be possible to forecast the development of this class of convective weather systems.