著者
AONO Kenji IWASAKI Toshiki SASAI Takahiro
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2020-017, (Released:2019-12-24)

This study examined the roles of wind-evaporation feedback in the tropical cyclone (TC) intensification, with special attention devoted to the feedback in weak wind areas (domains where the 10-m wind speed is smaller than 5, 10 and 15 m s−1). This was done by setting lower limits of the 10-m wind speed in the calculation of water vapor exchange between the atmosphere and the underlying ocean in a nonhydrostatic cloud-resolving model. As a result, the surface evaporation is enhanced in outer regions of a TC where the actual wind speed is smaller than the prescribed lower limit(s). Results show that increasing the lower limit reduces the radial water vapor contrast in the lower troposphere (below 100 m) and suppresses the TC size and intensity at the mature stage by 30-33 % and 5-14 %, respectively, compared to the control run with all standard model settings. The increased evaporation enhances the outer convective activity and reduces the radial pressure gradient in the lower troposphere. As a result, the inflow and thus the inward advection of angular momentum were reduced and the enhanced convection in the outer region suppressed eyewall updraft, and thus reduced the secondary circulation and finally the TC intensity. Moreover, the outer region convection suppresses the rainband activity (within a radius of 300 km from the TC center). The contribution of the wind-evaporation feedback to the enhancement of the radial contrast of water vapor in the lower troposphere is a fundamentally important element for TC intensification, suggesting that the TC development process can be revealed more accurately by elucidating the role of the weak wind area.
著者
SUZUKI Kento IWASAKI Toshiki YAMAZAKI Takeshi
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2021-002, (Released:2020-10-21)

Local fronts formed near the coast of the Kanto Plain mainly in a cold season, so-called “coastal fronts”, tend to be forecast on the inland side of their actual positions by the operational mesoscale Numerical Weather Prediction (NWP with a horizontal grid spacing of 5 km) model at Japan Meteorological Agency (JMA). In this study, we confirm a systematic NWP error through statistical validations of coastal fronts that occurred with southerly onshore winds during 2015-2018. Using a nonhydrostatic numerical model (JMA-NHM), we explore the relevant physical mechanisms through sensitivity experiments involving different horizontal resolution, envelope orography, and physics parameterization schemes for three cases with typical errors. The operational NWP model is shown to have a systematic error, with local fronts being consistently shifted to the inland side of their actual positions when the forecast period exceeds 5 hours, regardless of precipitation. The sensitivity experiments suggest that the systematic error associated with coastal fronts may be primarily caused by an underestimation of the mountain barrier surrounding the Kanto Plain in the model. The northwestward distance error of coastal fronts, averaged over the three illustrative cases, can be reduced by 27 % and 37 % by increasing the horizontal resolution from 5 km to 2 km and 1 km, respectively, and can be almost entirely eliminated by using the envelope orography. Moreover, the evaporative cooling of precipitation shifts coastal fronts to the seaward. Most coastal fronts are thought to take the form of cold air trapped on the southeastern slope of the mountains surrounding the Kanto Plain, where the elevation angle of the frontal surface is roughly controlled dynamically. The local front shifts to the seaward when the ridgelines of the mountains become higher, and shifts to the seaward through the reduction of the elevation angle when the trapped air becomes colder.
著者
FUKUI Shin IWASAKI Toshiki SAITO Kazuo SEKO Hiromu KUNII Masaru
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-056, (Released:2018-09-14)
被引用文献数
1

The feasibility of regional reanalysis assimilating only conventional observations was investigated as an alternative to dynamical downscaling to estimate the past three-dimensional high-resolution atmospheric fields with long-term homogeneity over about 60 years. The two types of widely applied dynamical downscaling approaches have problems. One with a serial long-term time-integration often fails to reproduce synoptic-scale systems and precipitation patterns. The other with frequent reinitializations underestimates precipitation due to insufficient spin-up. To address these problems maintaining long-term homogeneity, we proposed the regional reanalysis assimilating only the conventional observations. We examined it paying special attention to summer precipitation, through one-month experiment before conducting a long-term reanalysis. The system is designed to assimilate surface pressure and radiosonde upper-air observations, using the Japan Meteorological Agency's nonhydrostatic model (NHM) and the local ensemble transform Kalman filter (LETKF). It covers Japan and its surrounding area with a 5-km grid spacing and East Asia with a 25-km grid spacing, applying one-way double nesting in the Japanese 55-year reanalysis (JRA-55). The regional reanalysis overcomes the problems with both types of dynamical downscaling approaches. It reproduces actual synoptic-scale systems and precipitation patterns better. It also realistically describes spatial variability and precipitation intensity. The 5-km grid spacing regional reanalysis reproduces frequency of heavy precipitation and describes anomalous local fields affected by topography such as circulations and solar radiation better than the coarser reanalyses. We optimized the NHM-LETKF for long-term reanalysis by sensitivity experiments. The lateral boundary perturbations derived from an empirical orthogonal function analysis of JRA-55 brings stable analysis, saving computational costs. The ensemble size of at least 30 is needed because further reduction significantly degrades the analysis. The deterministic run from non-perturbed analysis is adopted as first guess in LETKF, instead of the ensemble mean of perturbed runs, enabling reasonable simulation of spatial variability in the atmosphere and precipitation intensity.
著者
YAMAGUCHI Junpei KANNO Yuki CHEN Guixing IWASAKI Toshiki
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2019-015, (Released:2018-11-24)
被引用文献数
8

An extreme cold surge event caused record-breaking low temperatures in the East Asia during 20–25 January 2016. The planetary- and synoptic-scale feature of the event is investigated quantitatively using the isentropic cold air mass analysis with the threshold potential temperature of 280 K. Because cold air mass is adiabatically conservative quantity, it is suitable for tracing and examining the extreme cold surges. We further introduce a metric named mean wind of cold air mass, which divides the factor of cold air mass evolution into convergence and advection parts. The new metric allowed us to trace the evolution of the cold air mass with dynamic consistency for a period of more than a week. A thick cold air mass built up over southern Sakha by a convergent cold air mass flow during 16–18 January. It migrated westward and reached Lake Baikal. On 20 January, an intense Siberian High developed with an eastward-moving mid-upper-level ridge, producing a strong surface pressure gradient over coastal regions of the Asian continent. This ridge and a cutoff low to the adjacent east formed a northerly flow in the mid-upper troposphere. The resultant southward flow through the troposphere blew the cold air mass over 480 hPa in thickness to the subtropical region of East Asia, causing strong cold surges there on 24 and 25 January. The abnormality of the event is further quantified using extreme value theory. The cold air mass gradually became rare along the path of the cold air mass from Lake Baikal to eastern China, which experienced as thick a cold air mass as once in 200 years. The cold air mass itself shows little change in thickness. Therefore, the migration of a cold air mass over 540 hPa in thickness from northern Siberia is the major cause of this cold surge extreme.