Open AccessEffects of rain on Ku‐band backscatter from the ocean
Author(s) -
Contreras Robert F.,
Plant William J.,
Keller William C.,
Hayes Kenneth,
Nystuen Jeffrey
Publication year - 2003
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jc001255
Subject(s) - backscatter (email) , wind speed , environmental science , atmospheric sciences , meteorology , radar , rain rate , wind direction , azimuth , range (aeronautics) , remote sensing , geology , physics , optics , materials science , precipitation , telecommunications , composite material , computer science , wireless
During the Kwajalein Experiment (KWAJEX) in July and August 1999, measurements of the normalized radar cross section of the ocean, σ o , were made at Ku‐band with HH and VV polarizations from the R/V Ronald H. Brown . Data were collected at a variety of incidence angles during periods of rainfall as well as during clear conditions. During the experiment the rainfall rate ranged from 0 to 80 mm hr −1 . Coincident with the backscatter measurements, measurements of rain rate, wind speed, wind direction, and fluxes of heat and momentum were made. Since we were primarily interested in backscatter from the surface, we removed backscatter from the raindrops themselves for the σ o measurements reported here. As a secondary result we show that the backscatter from the rain drops is a good indicator of rain rate. Most of the data were collected with the ship stationary and the bow held into the wind. Thus the azimuth angle between the antenna look direction and the direction from which the wind came was predominantly between 0° and 90°. Over this range, rain was found to increase σ o at incidence angles of 30° to 75°, to have little effect near 20°, and to decrease σ o very slightly between 14° and 16°. At all incidence angles, no discernible dependence of σ o on wind speed was found during rainfall for wind speeds below 10 m s −1 ; within experimental error the level of σ o depended only on rain rate. For the lower incidence angles, this dependence was very small while at the higher incidence angles, σ o depended on rain rate to a power that varied between about 0.5 and 1.2, being somewhat higher at large incidence angles and higher for HH polarization. The implication of these results is that rain produces small‐scale surface displacements (wavelengths shorter than about 3 cm) that roughen the ocean surface much more than the wind for wind speeds below 10 m s −1 . The results also imply that when rain fills the entire scatterometer footprint on the surface and the wind speeds are low to moderate, scatterometry at Ku band is impossible.