A feasibility study on the high-resolution regional reanalysis over Japan assimilating only conventional observations as an alternative to the dynamical downscaling

Shin Fukui*, Toshiki Iwasaki, Kazuo Saito, Hiromu Seko, Masaru Kunii

*この研究の対応する著者

    研究成果: Article査読

    2 被引用数 (Scopus)

    抄録

    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.

    本文言語English
    ページ(範囲)565-585
    ページ数21
    ジャーナルJournal of the Meteorological Society of Japan
    96
    6
    DOI
    出版ステータスPublished - 2018 12 1

    ASJC Scopus subject areas

    • 大気科学

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