Integration of Feature Calibration and Alignment into ACAPS

In support of the National Center for Atmospheric Research (NCAR) development of the Air Force Weather Agency (AFWA) Coupled Analysis and Prediction System (ACAPS), a data assimilation preprocessor was developed to displace model fields using Feature Calibration and Alignment (FCA; Hoffman et al., 1995). A Fotran90 API was written to apply FCA displacements to WRF input as a preprocessor for 4DVar. A modified version of the vortex relocation scheme of Hsiao et al. (2010) was applied to maintain dynamical balance. Unlike Hsiao et al. (2010), sea level pressure was displaced rather than surface pressure, and relative humidity was used in the displacement in lieu of water vapor mixing ratio. A time-lagged test was run using the Hurricane Katrina default WRF case. The 12-hour control forecast was used as pseudo-observations to displace the 6-hour control forecast forward along the hurricane's track. After completing an additional 18-hour integration (for a total of 24 hours), the hurricane maintained its forward displaced position. The modifications of Hsiao's algorithm allowed for the application of displacement across the entire model domain rather than only over flat areas and significantly reduced terrain-induced artifacts.