Analysis of hurricane morphology, internal waves and boundary layer rolls observed from satellite SAR images

In 2008, the Canadian Space Agency sponsored the RADARSAT Hurricane Applications Project (RHAP), for researching new developments in the application of RADARSAT-1 synthetic aperture radar (SAR) data and innovative mapping approaches to better understand the dynamics of hurricane genesis, morphology and movement. Although hurricanes can be detected by many remote sensors, SAR can yield high-resolution (sub kilometer) and low-level storm information that cannot be seen below the clouds by other sensors. In addition to the wind field, and hurricane eye information, structures associated with atmospheric processes can also be detected by SAR. We have acquired 161 RADARSAT-1 SAR images in the Atlantic Ocean through RHAP between 2001 and 2007. Among these, 74 images show clear hurricane eye structure. In addition, we also acquired 10 images from the European Space Agency's ENVISAT SAR between 2004 and 2010 covering typhoons/hurricanes in both the Pacific and Atlantic Oceans.

In this study, we analyze these 84 (74 RADARSAT-1 and 10 ENVISAT) images with hurricane eye information along with ancillary hurricane intensity information from the archive to generate hurricane morphology statistics. Histograms of wave number asymmetry and intensity are presented. The statistics show that when the storm has higher intensity, the hurricane eye tends to become more symmetric, and the area of the hurricane eye, defined by the minimum wind area, tends to be smaller. Examples of fine-scale structures within the hurricane, i.e., eye-eyewall mesovortices, arch clouds, double eyewalls, oceanic internal waves, atmospheric gravity waves, and abnormally high wind within eyes, are presented and discussed. It is shown that a majority of data exhibit circular eye (wave number 1) and elliptical-shaped (wave number 2) asymmetries in the eye, suggesting that the nature of the inner-core asymmetry is dominated by these two wave numbers. We also find that a tropical cyclone with a smaller eye tends to develop into a stronger tropical cyclone because the reduction in eye size leads to the decrease of the area of high wind region, which lowers the kinematic dissipation to offset the generation of kinetic energy due to surface enthalpy flux. There is a ubiquitous presence of four types of rain band signatures in SAR tropical cyclone images. These mechanisms dominating the rain patterns in the SAR images are attenuation due to heavy rain, backscattering from rain drops in the air and ice particles, sea surface capillary waves induced by rain, damping of sea surface waves by rain-induced turbulence, and wind gusts. Different mechanisms will increase or decrease the normalized radar cross section (NRCS) measured by a SAR, depending on the observation geometry that varies over the image and the local wind/wave conditions. In addition, SAR can provide useful information for identifying tropical cyclone boundary layer rolls, because streak patterns in sea surface roughness can be explained by change in surface wind speed due to the formation of boundary layer rolls.

SAR images show a few unusual observations. One is that the storm pattern continues across the land–sea boundary. We conjecture that this is due to rain scattering and attenuation in the atmosphere. The other one is that higher NRCS values are observed within some storm eyes, which is usually believed to be a relatively calm area within the storm system. Possible explanations are rain, waves, and abnormally high wind. However, these phenomena cannot be addressed by SAR observation alone. With the increasing number of spaceborne SAR satellites in the next 2–3 years, we believe there will be more simultaneous observations of storm systems from different spaceborne, airborne, and in situ sensors to help researchers understand these phenomena.

Utilization of SAR imagery is a relatively new tool for tropical cyclone research and forecasting because of its limited coverage, lack of operational analysis tools, and high cost. All these impediments are in the process of being swept away. NOAA is implementing an operational SAR wind processing system. ESA will launch the Sentinel-1 in 2014 and announced its data policy will be free and open. Canadian Space Agency's Radarsat Constellation Mission (RCM) SAR missions in 2016. The RCM will be operational and data will be free and open as well. This study demonstrates the advantage of SAR sensors for the imaging of fine scale storm patterns on the sea surface beneath the storm clouds. The research results we presented in this study demonstrate the potential of using SAR as a hurricane forecast tool in the future.