Using a Scatterometer-based, Low-wavenumber Analysis Tool to Evaluate Surface Wind Asymmetries in Tropical Cyclones

Because the Stepped Frequency Microwave Radiometer (SFMR) is known for obtaining reliable wind speeds in hurricanes, a Fourier-decomposed, low-wavenumber analysis tool was developed to assess the surface wind asymmetry in such conditions. These SFMR data were mainly collected over the North Atlantic and Eastern Pacific tropical cyclone (TC) basins and provide a starting point for evaluation of the wind asymmetry against other observational data (i.e. flight level winds). These analyses have also been useful for developing an alternative method of model intensity evaluation. However, the SFMR low wavenumber dataset is limited by its size so that statistical examinations stratified by storm intensity or intensity change (i.e. rapid intensification [RI]) are likely not sufficient. Despite the advantages of aircraft data obtained in TCs, satellite-based instruments are more capable to collect TC related data worldwide and are therefore advantageous for global studies. Scatterometer data, which provide global coverage of surface winds, are available through NASA's Jet Propulsion Laboratory archive between the years of 2000 and 2011 and could potentially be utilized to expand upon the SFMR low wavenumber dataset.

In the present work, evaluation of this global expansion of the low wavenumber wind asymmetry using scatterometer data is of primary importance and is expected to provide a more robust statistical result. Firstly, a scatterometer-relevant low wavenumber analysis tool is developed, similar to that of the SFMR. Please note that the scatterometer wind speeds used are corrected using a Neural Network (NN) algorithm, which provides reliable wind speeds up to ~60 m/s. Comparison of scatterometer analyses against SFMR analyses were performed to validate the scatterometer analysis method, and despite some drawbacks in terms of radial resolution, the scatterometer analyses compare well to those of the SFMR. Available scatterometer data were processed using the validated low wavenumber analysis method, and a large database (~3800 cases worldwide) was produced. Composite analyses are then produced in storm- and shear-direction-relative frameworks for the Saffir-Simpson based wind speed categories for each basin and for the entire database. Additional composite analyses are provided for rapidly intensifying TCs, specifically looking at periods of RI initiation and continuation. Initial evaluation of the surface wind asymmetry indicates that from tropical storm to major hurricane strength, the maximum low wavenumber wind speed rotates from down-motion to right-of-motion and from downshear to downshear-left locations. Additionally, wind shear has less influence on the location of the maximum as intensity increases. Further examination of the relationship between storm motion and vertical wind shear reveals that some of the impacts are basin-dependent, may be influenced by the magnitude of the separation of the motion direction and shear direction, and that these wind speed asymmetries are relatable to precipitation asymmetries. Future work includes determining a connection to satellite-based precipitation analyses and further evaluation of model intensity prediction.