Tetsuya Takemi

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.

The effects of subgrid model mixing and numerical filtering in simulations of mesoscale cloud systems

Abstract Using the newly developed Weather Research and Forecasting (WRF) model, this study investigates the effects of subgrid mixing and numerical filtering in mesoscale cloud simulations by examining the sensitivities to the parameters in turbulence-closure schemes as well as the parameters in the numerical filters. Three-dimensional simulations of squall lines in both no-shear and strong-shear environments have been performed. Using the Smagorinsky or 1.5-order turbulent kinetic energy (TKE) subgrid model with standard values for the model constants and no explicit numerical filter, the solution in the no-shear environment is characterized by many poorly resolved grid-scale cells. In the past, such grid-scale noise was avoided by adding a numerical filter which, however, produces excessive damping of the physical small-scale eddies. Without using such a filter, it was found that by increasing the proportionality constant in the eddy viscosity coefficient in the subgrid turbulence models, the cells bec...

Structure and Evolution of a Severe Squall Line over the Arid Region in Northwest China

Abstract The structure and evolution of a long-lived squall line associated with a disastrous dust storm, called a black storm, that occurred over the arid region in northwest China are described. Data used in the present study were those from routine observations in China and those acquired from surface observations of the Heife River Field Experiment. The squall line evolved within an extremely dry environment in which surface water vapor mixing ratios were less than 2.5 g kg−1 and the level of free convection (LFC) was very high. The surface cold-air outflow that emanated from the squall line propagated rapidly, producing strong horizontal convergence at the leading edge of the cold pool. The deep, dry mixed layer that developed over the desert region, owing to strong solar surface heating, played a significant role in determining the squall line evolution. The maximum depth of the mixed layer was estimated as more than 4 km, which is often found over the desert regions of China. The growth of the deep...

Finescale Doppler Radar Observation of a Tornado and Low-Level Misocyclones within a Winter Storm in the Japan Sea Coastal Region

Abstract Life histories of low-level misocyclones, one of which corresponded to a tornado vortex within a winter storm in the Japan Sea coastal region on 1 December 2007, were observed from close range by X-band Doppler radar of the East Japan Railway Company. Continuous plan position indicator (PPI) observations at 30-s intervals at the low-elevation angle revealed at least four cyclonic misocyclones within the head of the comma-shaped echo of the vortical disturbance under winter monsoon conditions. The meso-β-scale vortical disturbance developed within the weak frontal zone at the leading edge of cold-air outbreaks. High-resolution observation of misocyclones revealed the detailed structures of these misocyclones and their temporal evolution. As the parent storm evolved, a low-level convergence line was observed at the edge of the easternmost misocyclone. This convergence line was considered to be important for the initiation and development of the misocyclones and the tornado through vortex stretching...

LES Analysis of the Aerodynamic Surface Properties for Turbulent Flows over Building Arrays with Various Geometries

AbstractThis paper describes aerodynamic roughness properties for turbulent flows over various building arrays that represent realistic urban surface geometries. First, building morphological characteristics such as roughness density λf and building height variability Vh, defined respectively as the ratio of total frontal area of roughness elements to the total surface area and the ratio of standard deviation in building height to the average building height of the study site, were investigated. Next, large-eddy simulations (LESs) of turbulent flows over building arrays were performed with various surface geometries characterized by a wide range of values for both λf and Vh, based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z0 and drag coefficient were evaluated for the central Tokyo area from the values of z0 and Vh using the LES results. The values of z0 and as a function of both λf and Vh were comparable to those found in earlier studies. Th...

Numerical Experiments on the Mechanisms for the Development and Maintenance of Long-Lived Squall Lines in Dry Environments

Abstract The mechanisms responsible for the development and maintenance of long-lived squall lines in dry environments are investigated through two-dimensional numerical experiments by using a nonhydrostatic cloud model. The squall line environments are characterized by a low convective available potential energy (CAPE), low moisture content, and a high level of free convection (LFC), which are based on observations of a squall line over an arid region in China. Although these environments seem to be unfavorable for the development of convective systems, a long-lived squall line is simulated in the environment of a well-mixed moisture profile within a deep, mixed boundary layer. During the mature stage of this simulated squall line, the air parcels originating in the upper part of the mixed layer ahead of a surface cold-air pool are lifted to the upper troposphere. On the other hand, the air parcels originating in the lower part of the mixed layer are forced to go rearward, never reaching the upper levels...

A TRANSITION MECHANISM FOR THE SPONTANEOUS AXISYMMETRIC INTENSIFICATION OF TROPICAL CYCLONES

AbstractA mechanism for the transition of tropical cyclones (TCs) to the spontaneous rapid intensification (RI) phase is proposed based on numerical results of a three-dimensional full-physics model. The intensification phase of the simulated TC is divided into three subphases according to the rate of intensification: 1) a slowly intensifying phase, 2) an RI phase, and 3) an adjustment phase toward the quasi-steady state. The evolution of a TC vortex is diagnosed by the energy budget analysis and the degree of axisymmetric structure of the TC vortex, and the simulated TC is determined to be axisymmetrized 12 h before the onset of RI. It is found that equivalent potential temperature θe in the lowest layer suddenly increases inside the radius of maximum azimuthally averaged horizontal wind rma after the TC becomes nearly axisymmetric. Forward trajectory analyses revealed that the enhanced convective instability in the TC core region where the eyewall subsequently forms results from the increased inertial s...

Climate change effects on the worst-case storm surge: a case study of Typhoon Haiyan

Effects of climate change on the worst case scenario of a storm surge induced by a super typhoon in the present climate are investigated through the case study of Typhoon Haiyan. We present the results of our investigation on super-typhoon Haiyan by using a super high resolution (1 km grid) regional model that explicitly handles cloud microphysical processes. As the parent model, we adopted the operational weekly ensemble experiments (60 km grid) of the Japan Meteorological Agency, and compared experiments using sea surface temperatures and atmospheric environmental parameters from before the beginning of anthropogenic climate change (150 years ago) with those using observed values throughout the typhoon. We were able not only to represent the typhoons intensity but also to evaluate the influences of climate change on worst case storm surges in the Gulf of Leyte due to a typhoon with high robustness. In 15 of 16 ensemble experiments, the intensity of the simulated worst case storm in the actual conditions was stronger than that in a hypothetical natural condition without historical anthropogenic forcing during the past 150 years. The intensity of the typhoon is translated to a disaster metric by simulating the storm surge height by using a shallow-water long-wave model. The result indicates that the worst case scenario of a storm surge in the Gulf of Leyte may be worse by 20%, though changes in frequency of such events are not accounted for here.

Environmental stability control of the intensity of squall lines under low‐level shear conditions

[1] The environment for the development and evolution of linearly organized convective systems, i.e., squall lines is diverse for their existence in various climate regions. Understanding the behavior of squall lines under various environmental conditions is required for diagnosing and forecasting the development and intensity of the convective systems. The present study investigates the effects of environmental static stability on the squall-line intensity by conducting a systematic series of idealized cloud-resolving simulations of squall lines that develop in line-perpendicular, low-level westerly shear. Changing the temperature lapse rate with convective available potential energy (CAPE) being unchanged, we showed that the environmental stability in a convectively unstable layer well describes the intensity of the simulated squall lines. A less stable stability is favorable for generating stronger convective systems. The amount of CAPE does not account for the difference in the squall-line intensity in different temperature environments. An environment with a less static stability leads to the development of stronger cold pool, which will strongly controls the scale and strength of convective updrafts, the intensity of tropospheric overturning, and thus the organization and intensity of squall lines. The CAPE value can only be a good measure for diagnosing the development and intensity of the convective systems so long as the environmental static stability is identical. The static stability is a controlling parameter in determining the intensity of squall lines.

Over 5,000 Years of Ensemble Future Climate Simulations by 60-km Global and 20-km Regional Atmospheric Models

AbstractAn unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the a...