Impacts of Aeolus horizontal Line‐Of‐Sight (HLOS) wind assimilation on the Korean integrated model (KIM) forecast system

• Published in: Atmospheric science letters 2023-03, Vol.24 (3), p.n/a
• Main Authors: Lee, Sihye, Song, Hyo‐Jong, Kwon, In‐Hyuk, Kang, Jeon‐Ho
• Format: Article
• Language: English
• Subjects: Aeolus; Aircraft; Antarctic zone; Assimilation; Atmospheric boundary layer; Atmospheric lasers; Atmospheric sciences; Data assimilation; Doppler sonar; Errors; Experiments; HLOS wind; Hybrid systems; Impact analysis; KIM; Lasers; Line of sight; Localization; Mathematical models; Meridional wind; Northern Hemisphere; Quality control; Southern Hemisphere; Stratosphere; Weather forecasting; Wind; Wind effects; Wind observation
• ISSN: 1530-261X; 1530-261X
• DOI: 10.1002/asl.1138

The Korean Integrated Model (KIM) forecast system, based on a hybrid four‐dimensional ensemble‐variational method, was extended to assimilate Horizontal Line‐Of‐Sight (HLOS) wind observations from the Atmospheric Laser Doppler Instrument (ALADIN) on board the Atmospheric Dynamic Mission Aeolus satellite. In a global cycling experiment, assimilation of Aeolus HLOS wind observations led to reductions in the average root‐mean‐square error of 0.8 and 0.5% for the zonal and meridional wind analyses when compared against European Center for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) analyses. Even though the observed variable is wind, there was also an overall beneficial impact on analyses of the mass variables. In the Southern Hemisphere (SH), the reduced analysis errors led to forecast skill improvements out to 72 h. In contrast, in the Northern Hemisphere (NH) there was relatively little reduction of analysis errors, but wind forecasts were nevertheless improved, and these positive impacts lasted longer – out to 120 h rather than 72 h. Experiments suggest that the relatively poor long‐range performance in the SH high latitudes was due to problems with the mass increments derived from Aeolus wind increments via the ensemble‐based part of the hybrid background error covariance (B), which eventually led to adverse effects on the wind variables as forecasts progressed in the SH. This study shows that it is necessary to estimate the ensemble B in the Antarctic region with its high elevation more accurately in order to effectively use the Aeolus observation information. In the KIM forecasting system, using hybrid‐4DEnVar in its operational configuration, it was confirmed that the assimilation of HLOS winds has a beneficial average effect of 0.5 ~ 0.8% on wind analyses, when using IFS analyses as “truth.” In the NH, wind forecasts were improved out to 120 h, but in the SH initial benefits were gradually neutralized, leading to a negative impact on wind forecast skill by 120 h. Experiments suggest that the cause of the performance degradation in the SH was due to issues with the mass increments induced from the wind increments, via the mass‐wind relationships provided by the ensemble part of the hybrid background error covariance.