367403 Importance of Physical Parameterization for Snowfall Forecasts: Implications from a Case Study of Heavy Snowfall over the Southern Coast of Japan

Tuesday, 14 January 2020
Hall B1 (Boston Convention and Exhibition Center)
Ginga Akimoto, JMA, Tokyo, Japan

The Tokyo metropolitan area has a lot of fine days in winter mainly due to the dry northwest monsoon blowing from mountainous regions. However, low-pressure systems generated in the East China Sea often develop over the southern coast of Japan and occasionally bring heavy snowfall for the Tokyo area.
Reliable snowfall prediction has long been desired because such snowfall events in the Tokyo area tend to have quite large impacts on transportation and logistics even when the amount of the snowfall is just a few centimeters. When a low-pressure system passes near the southern coast of Japan, in most cases, the temperature of the lower atmosphere is around the freezing point and a small difference in the temperature makes a huge difference in the total amount of snow. That kind of uncertainty makes accurate snowfall forecasts difficult.
To support domestic forecasters, the Japan Meteorological Agency (JMA) operates a regional high-resolution NWP system using a non-hydrostatic model named ASUCA (Ishida et al. 2009, 2010; Hara et al. 2012). The one of the most important task of the NWP system is to provide short-range forecasts, warnings, and advisories for such severe weather events.
This study focuses on a heavy snowfall event on January 22, 2018 in which the model prediction consistently underestimated the total amount of snow, to examine some hints for improvements of physical parameterization of the NWP model. This study clarified several issues on physical processes such as radiation, boundary layer, and snow melting. These processes determine the lower atmospheric temperature around the freezing point. In this case, an overestimation of downward shortwave radiative flux at the surface was found to be the primary source of the errors of temperature prediction and hence the amount of snow. This presentation will show the accurate behavior of physical parameterization such as boundary layer processes, radiative transfer process, and cloud microphysics is crucial as well as synoptic-scale representation of a low-pressure system for the winter precipitation forecasting in the Tokyo area.


  • Hara, T., K. Kawano, K. Aranami, Y. Kitamura, M. Sakamoto, H. Kusabiraki, C. Muroi, and J. Ishida, 2012: Development of Physics Library and its application to ASUCA. CAS/JSC WGNE Res. Activ. Atmos. Oceanic Modell., 42, 05.05–05.06.
  • Ishida, J., C. Muroi, K. Kawano, and Y. Kitamura, 2010: Development of a new nonhydrostatic model “ASUCA” at JMA. CAS/JSC WGNE Res. Activ. Atmos. Oceanic Modell., 40, 05.11–05.12.
  • Ishida, J., C. Muroi, and Y. Aikawa, 2009: Development of a new dynamical core for the nonhydrostatic model. CAS/JSC WGNE Res. Activ. Atmos. Oceanic Modell., 39, 05.09–05.10.
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