HR: 09:15h
AN: A21B-04    [Abstracts]
TI: Using Observations to Assess the Increasing Trend in Storm Track Activity Found in NCEP/NCAR and ERA40 Reanalyses Data
AU: * Chang, E K
EM: kmchang@notes.cc.sunysb.edu
AF: ITPA/MSRC, Stony Brook University, SUNY, Stony Brook, NY 11794-5000 United States
AB: Based on analyzing reanalysis data, many recent works have shown that Northern Hemisphere baroclinic wave and cyclone activity in winter have increased significantly over the second half of the twentieth century. Studies have found that the average number of deep cyclones have increased both in the Pacific and Atlantic, and eddy energy and heat transports have also increased. However, since the storm tracks peak over the oceans, very little data are assimilated from 1948 to the early 1970s, except at the surface. Since the mid-1970s, much more aircraft and satellite data are available to constrain the reanalyses. Hence there is a possibility that such changes in data coverage could introduce biases in the trends found in reanalysis data. In this study, storm track activity computed from reanalysis data are compared to similar quantities computed directly from observations to assess whether there are biases in the reanalysis trends. For eddy activity at the upper troposphere, variances computed based on radiosonde and aircraft observations are compared to those computed based on NCEP/NCAR and ERA40 reanalyses. The results suggest a possible high bias in the trend computed from the NCEP/NCAR reanalysis, with the weaker trend seen in the ERA40 more consistent with the trend determined from observations.The high bias in the NCEP/NCAR reanalysis trend appears to be worst over the Pacific. The storm track trend near the surface has also been examined. Near the surface, the trends computed from the NCEP/NCAR and ERA40 reanalyses are comparable. However, both trends appear to be higher than trends computed based on COADS ship observations. Again, the biases appear largest over the Pacific. Taken together, the results suggest a significant upward trend in storm track activity over the Atlantic during the second half of the twentieth century, but the trend over the Pacific is less robust. Results of this study suggest that a "data consistent" reanalysis may be needed to better quantify the upward trend in Northern Hemisphere storm track activity.
DE: 3309 Climatology (1620)
DE: 3319 General circulation
DE: 3337 Numerical modeling and data assimilation
SC: Atmospheric Sciences [A]
MN: 2005 Joint Assembly