Measuring Stress in Tropical Cyclones

Abstract:

Wind is air in motion and stress is the turbulent transport of momentum between the ocean and the atmosphere. There was no large-scale measurement of stress before the scatterometer and the stress we used was almost entirely derived from wind through a drag coefficient. 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. Even if we have good wind measurements, there is a large uncertainty in the drag coefficient in TC. 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. We 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, and 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. With abundant stress measurements by the scatterometers, the drag coefficient for TC are characterized and compared with conventional values. The variation of stress in TC will be used to understand the intensification of TC with respect to feedback by the ocean.