Characterization of surface wind and stress in tropical cyclone with scatterometer

Monday, 15 December 2014: 9:15 AM
W. Timothy Liu, Wenqing Tang and Xiaosu Xie, Jet Propulsion Laboratory, Pasadena, CA, United States
Wind is air in motion and stress is the momentum exchange between ocean and atmosphere. While the strong wind of a tropical cyclone (TC) causes destruction at landfall, it is the surface stress that drags down the TC. The relations that were established to retrieve moderate wind speeds from the normalized radar cross-section, or backscatter power, measured by Ku-band and C-band scatterometers do not apply well to TC-scale winds. It has been difficult to establish new relations at strong winds because credible strong winds coincident with scatterometer measurements are not sufficient. We will give credence to our hypothesis that there is no distinct physics of radar backscatter from ocean surface for weather phenomenon like the TC. The relation between backscatter and surface roughness or stress does not change under TC, and the same retrieval algorithm can be extended to the TC. The need for changes in wind retrieval algorithm is explained through the change of the drag coefficient that relates wind to stress in TC. We aspire to separate the sensor parameters that affect backscatter, such as, incident angle, azimuth angle, polarization and backscatter frequencies, from the secondary factors related to the physics of the air-sea interface and turbulent transport, such as air stability (shear and buoyance), air density, sea states, and sea sprays, so that we can establish a simple approximation of surface stress from the backscatter averaged over the relevant spatial and temporal scales. We established a relation between backscatter and surface stress over a moderate range of wind speed, where wind measurements coincident with satellite observations are abundant, and the drag coefficient is well established to convert wind measurements to stress. This relation is applied to retrieve stress from the scatterometer measurement in the high wind range of TC. The characteristic of the drag coefficient in TC-scale winds will be discussed. The difference between wind and stress in a TC will be demonstrated.