Physics Schemes in NCEP Operational Hafs v1 and the Impacts of TC-Specific Modifications

We summarize the physics schemes used in two configurations of the first version of the operational Hurricane Analysis and Forecast System (HAFSv1) at NOAA NCEP. The physics package in HAFSv1 is the same as that used in NCEP global forecast system (GFS) version 16 except for modifications to sea surface roughness lengths, boundary layer scheme, and the entrainment rate in the deep convection scheme, specifically for improving the simulation of tropical cyclones (TCs), as well as a different microphysics scheme used in one configuration of HAFSv1. The two configurations of HAFSv1 differ mainly in the adopted microphysics schemes and TC-specific modifications in addition to initialization. Experiments are made to highlight the impacts of those TC-specific modifications and different microphysics schemes on HAFSv1 performance. Retrospective simulations for TCs over the North Atlantic Basin in 2021 and 2022 show that the TC-specific modifications do improve intensity forecasts, particularly in mean intensity and rapid intensification, without degrading track forecasts. Challenges and plans for the development of physics schemes for future versions of operational HAFS are discussed.