Measurements of boundary layer depth in hurricanes

While images taken with optical sensors are dependent on sunlight for illumination, radar signals can be used to observe the earth`s surface at day and night and in all weather conditions. A synthetic aperture radar (SAR) not only records the intensity of the returned signal, but also the phase history of the backscattered radar signal and is processed to high spatial resolution (~30 meter) images. In the past decade several satellite borne synthetic aperture radars (SAR) have been put into orbit, for the observation of hurricanes RADARSAT 1 and ENVISAT ASAR Scan SAR images ( 400 x 400 km coverage ) are used. Their high resolution and large spatial coverage make them a well suited tool for measuring and observing synoptically geophysical parameters related to sea surface roughness in the scale of the radar wavelength like the surface wind field and the ocean wave spectrum.

We have been investigating the following features in SAR images of hurricanes:

- Eye size and eccentricity as a measure of hurricane development - Wavelength and Direction of Boundary layer rolls for information of mixed boundary layer depth - Sea State in terms of wavelength and -direction

On SAR images often so-called wind streaks are visible, ranging in wavelengths from 600 to 2,000 meters. This variation in sea surface roughness is explained by change in surface wind speed due to the formation of boundary layer rolls. The direction of these streaks is thus used to derive the wind direction and the wavelength is taken to be a measure of roll size and thus mixed layer depth. A set of RADARSAT 1 and ENVISAT ASAR images is used to derive statistics on roll size.

Two cases from the hurricane 2002 and 2003 season are investigated in more detail, SAR images of Isidore and Isabel at landfall. For hurricane Isidore comparisons to aircraft measurements of boundary layer roll size are available, for Isabel radar measurements of NEXRAD rain radar are used for validation.