TY - JOUR
T1 - A feasibility study on the high-resolution regional reanalysis over Japan assimilating only conventional observations as an alternative to the dynamical downscaling
AU - Fukui, Shin
AU - Iwasaki, Toshiki
AU - Saito, Kazuo
AU - Seko, Hiromu
AU - Kunii, Masaru
N1 - Funding Information:
We would like to express sincere thanks to the editor in charge and two anonymous reviewers for their critical reading and constructive comments on our manuscript. We were indebted to the Japan Meteorological Agency for giving us the permission to use the Nonhydrostatic Model developed by the staffs at the Meteorological Research Institute and the Numerical Prediction Division of the Japan Meteorological Agency. The numerical experiments were partly conducted with the use of the supercomputing resources at the Cyberscience Center, Tohoku University, and the Earth Simulator under the Strategic Project with Special Support. This study was funded by Advancement of meteorological and global environmental predictions utilizing observational “Big Data” of the social and scientific priority issues (Theme 4) to be tackled by using post K computer of the FLAGSHIP 2020 Project (Project ID: hp150287, hp160230, hp170246, hp180194) and Grant-in-Aid for Scientific Research (B) (Grant 16H04054) from Japan Society for the Promotion of Science. This work was done as part of a joint research project between Tohoku University and Meteorological Research Institute. This work was partly supported by Core Research Cluster of Disaster Science, Tohoku University.
Publisher Copyright:
© The Author(s) 2018.
Copyright:
Copyright 2019 Elsevier B.V., All rights reserved.
PY - 2018/12
Y1 - 2018/12
N2 - 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 by paying special attention to summer precipitation, through one-month experiment before conducting a long-term reanalysis. The system was designed to assimilate surface pressure and radiosonde upper-air observations using the Japan eteorological Agency’s nonhydrostatic model (NH) and the local ensemble transform Kalman filter (LETKF). It covered 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 overcame the problems with both types of dynamical downscaling approaches. It reproduced actual synoptic-scale systems and precipitation patterns better. It also realistically described spatial variability and precipitation intensity. The 5-km grid spacing regional reanalysis reproduced frequency of heavy precipi-tation and described anomalous local fields affected by topography, such as circulations and solar radiation, better than the coarser reanalyses. We optimized the NH-LETKF for long-term reanalysis by sensitivity experiments. The lateral boundary perturbations that were derived from an empirical orthogonal function analysis of JRA-55 brought stable analysis, saving computational costs. The ensemble size of at least 30 was needed, because further reduction significantly degraded the analysis. The deterministic run from non-perturbed analysis was adopted as a first guess in LETKF instead of the ensemble mean of perturbed runs, enabling reasonable simulation of spatial variability in the atmosphere and precipitation intensity.
AB - 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 by paying special attention to summer precipitation, through one-month experiment before conducting a long-term reanalysis. The system was designed to assimilate surface pressure and radiosonde upper-air observations using the Japan eteorological Agency’s nonhydrostatic model (NH) and the local ensemble transform Kalman filter (LETKF). It covered 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 overcame the problems with both types of dynamical downscaling approaches. It reproduced actual synoptic-scale systems and precipitation patterns better. It also realistically described spatial variability and precipitation intensity. The 5-km grid spacing regional reanalysis reproduced frequency of heavy precipi-tation and described anomalous local fields affected by topography, such as circulations and solar radiation, better than the coarser reanalyses. We optimized the NH-LETKF for long-term reanalysis by sensitivity experiments. The lateral boundary perturbations that were derived from an empirical orthogonal function analysis of JRA-55 brought stable analysis, saving computational costs. The ensemble size of at least 30 was needed, because further reduction significantly degraded the analysis. The deterministic run from non-perturbed analysis was adopted as a first guess in LETKF instead of the ensemble mean of perturbed runs, enabling reasonable simulation of spatial variability in the atmosphere and precipitation intensity.
KW - Data assimilation
KW - Reanalysis
KW - Regional climate
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U2 - 10.2151/jmsj.2018-056
DO - 10.2151/jmsj.2018-056
M3 - Article
AN - SCOPUS:85062539655
VL - 96
SP - 565
EP - 585
JO - Journal of the Meteorological Society of Japan
JF - Journal of the Meteorological Society of Japan
SN - 0026-1165
IS - 6
ER -