*************************************************************************** IERS Message No. 12 August 17, 2001 *************************************************************************** IERS Bulletin A changes to AAM assimilation Predicted values of atmospheric angular momentum (AAM) were introduced into the IERS Bulletin A combination more than a year ago in an effort to improve our near-term UT1-UTC predictions. However, late last year we observed that the AAM-based values sometimes degraded the UT1 predictions as much as they helped. We suspended the use of AAM data at the beginning of 2001 to re-evaluate our entire AAM process. Starting with the 14 August 2001 issue of IERS Bulletin A (Rapid Service/Prediction of Earth orientation) -- Vol. XIV, No. 65 -- the AAM data have been restored to Bulletin A. The AAM results continue to come from the National Centers for Environmental Prediction (NCEP) at <ftp://ftp.ncep.noaa.gov/pub/cpc/long/aam/nmc/>. After the additional studies, our approach for assimilating AAM data has been changed. Differences include use of both pressure and wind AAM terms, use of the previously unavailable AAM analysis data at the 24-hour interval for the latest AAM analysis file, smoothing of the AAM-based LOD before integrating to UT1R, and removal of a best-fit sinusoid from the AAM UT1R time series. The analysis data value at the 24th hour was previously unavailable and allows for a better merging of the analysis and forecast periods into a single time series. Then, the resulting 15-day AAM-based LODR series (10 days of the most recent analysis + 5 days of forecast) are smoothed to remove variability with frequencies greater than the Bulletin's 2-day Nyquist frequency. The AAM-derived LODR is then integrated and the differences between a UT1 combination of geodetic-only data and the AAM-based UT1R values for the 10-day AAM analysis period are computed. A best-fit linear trend and sinusoid are fit to this difference time series. These fits are then weighted and removed from the entire AAM UT1R time series (analysis and forecast). The resulting rectified AAM-derived UT1R time series is then included in the Bulletin A UT1 combination together with the geodetic data. This procedure reduces the effects of systematic trends (both periodic and linear) that do not appear to be present in actual UT1 variability. Our studies indicate that this processing of this AAM-derived UT1R time series may reduce the UT1 prediction error by ~50% at 5 days into the future. The table below shows the UT1 RMS errors for our daily Bulletin A solutions (without AAM-based UT1R) and for the new AAM-based UT1R series for a 160-day period earlier this year; in both cases our "final" series is used retrospectively as truth. Assimilation ------- RMS UT1 Error versus Prediction Interval ------- Test Method 0 Day 1 Day 2 Days 3 Days 4 Days 5 Days wrt finals.data (us) (us) (us) (us) (us) (us) ------------------------------------------------------------------------- BullA (no AAM) 136.8 248.3 405.8 600.5 827.7 1084.3 AAM UT1R 97.2 150.4 220.4 294.4 381.4 483.1 ------------------------------------------------------------------------- [for the 160 days from 06 February through 15 July 2001] This work has been done by Tom Johnson (tj@CasA.usno.navy.mil) and Brian Luzum (bjl@maia.usno.navy.mil). Sincerely yours, Jim Ray IERS Bulletin A Web site: http://maia.usno.navy.mil/ *************************************************************************** IERS Messages are edited and ditributed by the IERS Central Bureau. To subscribe or unsubscribe, please write to <central_bureau@iers.org>. Archives: http://www.iers.org/iers/publications/messages/ ***************************************************************************