Wave and Wind Direction Effects on SFMR Brightness Temperatures

Many phenomena, such as hurricanes, can be studied using surface wind data. The stepped-frequency microwave radiometer (SFMR), which measures ocean surface wind speeds in hurricanes, is currently onboard the NOAA P-3 and Air Force C-130J hurricane hunter aircraft. It measures sea surface microwave brightness temperatures at six frequencies ranging from 4.7 to 7.2 GHz. Surface wind speed estimates are obtained from these brightness temperatures by using a retrieval algorithm that employs a geophysical model function relating surface emissivity and wind speed. Currently, SFMR measurements are only used at nadir during level flight. This is because of the complexity of the wave field and foam distribution when the SFMR views the surface off-nadir or during aircraft rolls. However, the effects of the wave field on the measurements can be investigated using measurements obtained during Hurricane Gustav (2008). SFMR data were collected during three full circles, one at a 30° roll angle and two at a 45° roll angle, in a precipitation-free region of the eyewall. Excess brightness temperatures were then calculated with respect to zero wind speed values and independent of wind direction. An asymmetry was found in the resulting excess brightness temperatures. It is hypothesized that this asymmetry is caused by the direction of wave propagation and the angle at which the wave field is viewed by the SFMR. Wind direction may also play a role in the asymmetry. To analyze the asymmetry further, measurements from other radiometers, such as WindSat, will be used.