The Impact of Supplemental Observations on the Analysis and Forecast of the Intensity and Structure of Tropical Storm Karen in the NCEP Hybrid Gridpoint Statistical Interpolation (GSI) Scheme

Previous work examining the impact of synoptic surveillance missions flown into tropical cyclones (TCs) has focused solely on the improvements to track forecasts in the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) model. In 2012 the NCEP Gridpoint Statistical Interpolation (GSI) data assimilation scheme was upgraded to utilize a hybrid ensemble-variational scheme with characteristics of both a 3D-Var approach and an Ensemble Kalman Filter. Given these changes and increases in the effective horizontal grid resolution of the GFS model, the impact of these supplemental observations on the analyses and forecasts of the structure and intensity of Tropical Storm Karen (2013) in the GSI analysis and GFS model forecasts will be examined.

Despite a moderate to high shear environment, intensity guidance and the official National Hurricane Center (NHC) forecast early in Karen's life cycle showed the cyclone intensifying to near hurricane strength as it approached the northern Gulf Coast. Given the possible hurricane threat to the U.S. coastline, a synoptic surveillance mission was flown into Karen on 4 October using the National Oceanic and Atmospheric Administration Gulfstream-IV (G-IV) jet. Control and data denial runs of the GSI/GFS system were conducted where the G-IV dropsondes were included (excluded) from the control (data denial) run. Results indicate that the assimilation of the supplemental dropsondes produced an analysis with a slightly weaker, shallower, and more tilted vortex. In addition, the control run showed stronger vertical wind shear and drier air in the upper troposphere near and west of Karen. During the subsequent GFS model forecast, the control run showed the Karen vortex gradually weakening, while the data denial run showed Karen weakening initially but then restrengthening as it interacted with a mid-latitude trough over the northern Gulf of Mexico.

These results suggest that global data assimilation systems and global models have reached a point where supplemental observations can influence TC intensity and structure in addition to track. These results will help guide future studies and could lead to changes in how supplemental observations, including the G-IV, are deployed in tropical cyclone events.