| dc.description.abstract |
The Variational Doppler Radar Analysis System (VDRAS) developed by National Center for Atmospheric Research (NCAR) is able to assimilate Doppler radar observations by using 4DVar method, and to provide high temporal and spatial meteorological states in convective scale. Due to lack of sea surface observation, it has been challenging to generate realistic analysis by using VDRAS in Taiwan and vicinity. As we know, satellite can measure in wide range on the ocean. For example, ASCAT (Advanced SCATterometer) data can be retrieved as product of sea surface 10 meters wind data, which avoids contamination caused by hydrometeors in the atmosphere. Therefore, this study is aimed to improve the accuracy of analysis by additionally assimilating satellite data by using VDRAS.
An improved surface data assimilation scheme has also been developed in updated version of VDRAS that was implemented with a terrain-resolving scheme (IBM_VDRAS). Note that horizontal wind data measured by mesonet and retrieved by the ASCAT are both processed and assimilated. A real case of squall line observed at 13UTC of June 23, 2013 near offshore of southwestern Taiwan is selected in current study.
Test on different surface assimilation schemes is conducted first, which indicates that, compare to previous scheme, modified surface data assimilation scheme can generate smoother and more reasonable analysis. With the updated surface data assimilation scheme, a series of experiments is then conducted to investigate the impacts of assimilation of different kinds of observational data. It reveals that the low-level wind convergence associated with the squall line is successfully analyzed with additional surface data assimilation. The enhanced prevailing southwesterly flow and seaward flow are clearly shown, which even modifies analyzed thermodynamic, dynamical and microphysical structures of the squall line. In addition, evaluations also demonstrate that, with merging of surface data, significant correction of underestimated southwesterly flow on the ocean and better minimization of rainwater mixing ratio are present. | en_US |