Impact of Assimilating Airborne Doppler Radar Winds on the Inner-Core Structure and Intensity of Hurricane Ike (2008)

of a Thesis at the University of Miami. Thesis supervised by Dr. Shuyi S. Chen. No. pages in text. (111) Accurate prediction of Tropical Cyclones (TC) is vital for the adequate protection of life and property in areas that are prone to their destructive forces. Over the last two decades, significant improvements have been made in forecasting hurricane track but intensity remains a challenge for most operational Numerical Weather Prediction (NWP) models. Previous studies suggest that better prediction of TC intensity requires an adequate initial description of the TC inner-core region since this area plays a crucial role in hurricane intensity changes. Therefore, there must be adequate observations of the inner-core area of the TC and effective data assimilation (DA) methods of ingesting these data into the NWP models. However, these requirements are seldom met due to the relatively low resolution of operational global NWP models and the lack of routine observations assimilated in the TC inner-core. In this study the impact of assimilating Airborne Doppler Radar (ADR) winds on the initial structure and subsequent intensity forecast of Hurricane Ike (2008) is investigated. Most global and some regional operational models failed to predict Ike’s intensity accurately with errors as high as 40 kts at landfall. The hypothesis is that their GORDON, RONALD Impact of Assimilating Airborne Doppler Radar Winds on the Inner-Core Structure and Intensity of Hurricane Ike (2008) (M.S. Meteorology and Physical Oceanography) (August 2011) inability to initialize the inner-core structure of the storm may have resulted in such forecast errors. By initializing the inner-core structure with observed winds and performing subsequent forecasts, this hypothesis is tested in this study. Additionally, the difference of using a flow dependent but computationally expensive DA method such as the 4-Dimensional Variational (4DVar) versus the relatively inexpensive 3DVar method is investigated. The results showed that assimilating the inner-core winds with both DA methods helped in defining the inner wind maximum that a control with no ADR DA could not initialize. Subsequent short-range 12-h forecasts showed that 4DVar was able to maintain this structure within the DA phase while it dissipated before the end of such forecasts using 3DVar. This was shown to be because of the dynamic nature of 4DVar’s analysis increments which helped in producing a lower sea-level pressure and stronger near-core pressure gradient. However, during a 60-h extended forecast the inner-wind maximum dissipated in both the 3DVar and 4DVar forecast about 12 -h before it did in the observed storm. The presence of this feature in the early stages of the DA storms, however, prevented rapid early intensification as was seen in the control. In the later stages (>24-h forecasts), the main factor determining intensification of the storm was no longer this inner-core feature but instead the outer environment shear. All the models had similar outer environment taken from GFDL and were able to capture this shear reasonably well.