A Feasibility Study on the High-Resolution Regional Reanalysis over Japan Assimilating Only Conventional Observations as an Alternative to the Dynamical Downscaling

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  • FUKUI Shin
    Graduate School of Science, Tohoku University, Sendai, Japan
  • IWASAKI Toshiki
    Graduate School of Science, Tohoku University, Sendai, Japan
  • SAITO Kazuo
    Atmosphere and Ocean Research Institute, the University of Tokyo, Kashiwa, Japan Meteorological Research Institute, Tsukuba, Japan
  • SEKO Hiromu
    Meteorological Research Institute, Tsukuba, Japan Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  • KUNII Masaru
    Japan Meteorological Agency, Tokyo, Japan

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<p> 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.</p><p> The system was 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 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).</p><p> 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 precipitation and described anomalous local fields affected by topography, such as circulations and solar radiation, better than the coarser reanalyses. </p><p> We optimized the NHM-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.</p>

収録刊行物

  • 気象集誌. 第2輯

    気象集誌. 第2輯 96 (6), 565-585, 2018

    公益社団法人 日本気象学会

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