Tuesday, 30 January 2024
Hall E (The Baltimore Convention Center)
Atmospheric reanalyses have become invaluable tools with many applications in synoptic climatology. Since the mid-1990s, several global-scale reanalyses have been developed that assimilate various kinds of atmospheric observations and whose outputs extend into the pre-satellite era—notably, NCEP/NCAR Reanalysis 1 (since 1948), ECMWF ERA-40 reanalysis (1957–2002), the Japanese 55-year reanalysis, or JRA-55 (since 1958), and, most recently, ECMWF Reanalysis v5, or ERA5 (since 1940). Several reanalyses provide a considerably longer time series by assimilating only a limited number of surface variables—including NOAA-CIRES-DOE Twentieth Century Reanalysis version 3, or 20CRv3 (1836–2015) and ECMWF twentieth century reanalysis, or ERA-20C (1900–2010).
With new observational datasets comes the need for their inter-comparison. Here, we address this need by comparing the six above-mentioned reanalyses, focusing on the accordance of daily types of atmospheric circulation, identified in daily mean sea level pressure fields separately for boreal (DJF) and austral (JJA) winters. The circulation types are independently identified in 30° × 20° (longitude × latitude) regions by a k-means algorithm, each region centered on one of the total of 9,360 grid points that cover the Earth from 80°S to 80°N at the step of 2.5° × 2.5°.
Preliminary results for the boreal winter show marked differences between the reanalyses over the Southern Hemisphere during the pre-satellite era, which tend to gradually decrease over time. There are, however, several regions in which notable differences persist into the 21st century—namely, central South America, sub-equatorial Africa, northern half of the Indian subcontinent, western half of China, and Antarctica. In our presentation, we will assess the sensitivity of these findings to the choice of season, circulation variable, and classification method. We will also address the relevance of the lack of reanalyses’ accordance by assessing the synoptic skill of classifications (i.e., their ability to explain variations in different surface weather elements)
With new observational datasets comes the need for their inter-comparison. Here, we address this need by comparing the six above-mentioned reanalyses, focusing on the accordance of daily types of atmospheric circulation, identified in daily mean sea level pressure fields separately for boreal (DJF) and austral (JJA) winters. The circulation types are independently identified in 30° × 20° (longitude × latitude) regions by a k-means algorithm, each region centered on one of the total of 9,360 grid points that cover the Earth from 80°S to 80°N at the step of 2.5° × 2.5°.
Preliminary results for the boreal winter show marked differences between the reanalyses over the Southern Hemisphere during the pre-satellite era, which tend to gradually decrease over time. There are, however, several regions in which notable differences persist into the 21st century—namely, central South America, sub-equatorial Africa, northern half of the Indian subcontinent, western half of China, and Antarctica. In our presentation, we will assess the sensitivity of these findings to the choice of season, circulation variable, and classification method. We will also address the relevance of the lack of reanalyses’ accordance by assessing the synoptic skill of classifications (i.e., their ability to explain variations in different surface weather elements)
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