The Impact of Supplemental Observations on the Intensity and Structure of Tropical Cyclones in the NCEP Hybrid 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 NCEP GFS model. In 2012, the NCEP 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 this change 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) and Hurricane Isaac (2012) in the GSI analysis and GFS model forecasts will be examined.

A detailed examination was conducted on the impact of the synoptic surveillance mission flown into Karen on 4 October using the NOAA Gulfstream-IV (G-IV) jet. Results from a data denial experiment show that the assimilation of the supplemental dropwindsondes produced an analysis with a slightly weaker, shallower, and more tilted vortex. The assimilation of the dropwindsonde data also contributed to 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 with the supplemental observations showed the tropical cyclone 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.

Results from subsequent runs of the GSI/GFS system through the remainder of Karen's life cycle will also be shown. In addition, preliminary results from data denial studies using the GSI/GFS system through six cycles of synoptic surveillance missions into Hurricane Isaac (2012) during the period of 23-26 August 2012 will also be presented.

Overall, results from the Karen case suggest that global data assimilation systems and models have reached a point where supplemental observations can influence model predictions of 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 future TC events.