Polar-Air Outbreak and Air-Mass Transformation over the East Coast of Asia as Simulated by an AGCM

Abstract

The present report studies features of the polar-air outbreak and the associated air-mass transformation over the east coast of Asia simulated in an AGCM (T106L52: a primitive equation spectral model, which has 52 σ-levels and triangular spectral truncation at wave-number 106) that used climatological SST in comparison with the features described in several observational studies.The large-scale circulations are properly reproduced for January Y07 (the 7th year after the spin up integration). A typical case of polar-air outbreak simulated in January is studied in detail. During the polar-air outbreak, cyclonic northwesterly polar-air streams over the Sea of Japan and the northwestern Pacific, and anticyclonic northeasterly polar-air streams over the East China Sea and the South China Sea are reasonably simulated. During the polar-air outbreak, the sensible and latent heat fluxes simulated over the Sea of Japan reach to ∼150 and ∼250 W m−2, while those over the East China Sea reach to ∼75 and ∼250 W m−2, respectively. These simulated fluxes agree with the fluxes evaluated in observational studies. The multi-layer structure of the transformed air-mass, including the unstable surface layer, the subcloud layer, and the cloud layer capped by the stable layer, simulated over the coastal areas of Asia is consistent with the observations for the cases of typical polar-air outbreak.However, the precipitation over the coastal areas of Japan simulated during the polar-air outbreak is significantly small as compared with the observation. This is due to the insufficient horizontal resolution of the T106L52 to simulate mesoscale circulation systems which induce snowfalls. In addition, the duration of the polar-air outbreak simulated in the AGCM is relatively short as compared with that in the real atmosphere, since the anticyclone over the continent tends to extend eastward in the model.