Environmental Simulation Center

An efficient implementation of the local ensemble transform Kalman filter (LETKF) with the error covariance localization by physical distances is introduced and assessed in this study. Instead of using local patches uniform in the model grid space to localize the error covariance, accurate physical distances are computed and used for the localization, so that the problem of analysis discontinuities in the Polar Regions is solved. Data assimilation cycle experiments with real observations are performed, which indicate less discontinuity in the Polar Regions. Moreover, the computational time is shorter and more robust for various localization scales. Thus, the implementation introduced in this study is a promising choice of future LETKF systems.

This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies. In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows. It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.