Effects of rain on Ku‐band backscatter from the ocean

[1] During the Kwajalein Experiment (KWAJEX) in July and August 1999, measurements of the normalized radar cross section of the ocean, σo, 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 σo 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 σo at incidence angles of 30° to 75°, to have little effect near 20°, and to decrease σo very slightly between 14° and 16°. At all incidence angles, no discernible dependence of σo on wind speed was found during rainfall for wind speeds below 10 m s−1; within experimental error the level of σo depended only on rain rate. For the lower incidence angles, this dependence was very small while at the higher incidence angles, σo 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.

[1]  Murray J. Smith,et al.  Doppler radar backscatter from ring waves , 1998 .

[2]  Zhizhang Yang,et al.  An Experimental Study of Rain Effects on Fine Structures of Wind Waves , 1997 .

[3]  Bernard Le Mehaute,et al.  Gravity–capillary rings generated by water drops , 1988, Journal of Fluid Mechanics.

[4]  Christophe Craeye,et al.  Rain generated ring-waves: Measurements and modelling for remote sensing , 1997 .

[5]  William J. Plant,et al.  Measurements of the Marine Boundary Layer from an Airship , 1998 .

[6]  C. Craeye,et al.  Scatterometric signatures of multivariate drop impacts on fresh and salt water surfaces , 1999 .

[7]  M. Gade,et al.  Investigation of multifrequency/multipolarization radar signatures of rain cells over the ocean using SIR‐C/X‐SAR data , 1998 .

[8]  Lewis B. Wetzel,et al.  On the theory of electromagnetic scattering from a raindrop splash , 1990 .

[9]  William J. Plant,et al.  The dependence of microwave backscatter from the sea on illuminated area: Correlation times and lengths , 1994 .

[10]  J. Nystuen.  A note on the attenuation of surface gravity waves by rainfall , 1990 .

[11]  F. J. Wentz,et al.  A model function for ocean radar cross sections at 14.6 GHz , 1984 .

[12]  E. W. S. ASHTON,et al.  Effect of Rain in Calming the Sea , 1949, Nature.

[13]  J. P. Hansen.  Rain backscatter tests dispel old theories , 1986 .

[14]  R. Gunn,et al.  THE TERMINAL VELOCITY OF FALL FOR WATER DROPLETS IN STAGNANT AIR , 1949 .

[15]  A. Dyer.  A review of flux-profile relationships , 1974 .

[16]  David Atlas,et al.  Footprints of storms on the sea: A view from spaceborne synthetic aperture radar , 1994 .

[17]  E. F. Bradley,et al.  Flux-Profile Relationships in the Atmospheric Surface Layer , 1971 .

[18]  Richard K. Moore,et al.  Preliminary study of rain effects on radar scattering from water surfaces , 1979 .

[19]  The splash of a drop and allied phenomena , 1896 .

[20]  J. Marshall,et al.  THE DISTRIBUTION OF RAINDROPS WITH SIZE , 1948 .

[21]  Martin Gade,et al.  Radar backscattering measurements of artificial rain impinging on a water surface at different wind speeds , 1999, IEEE 1999 International Geoscience and Remote Sensing Symposium. IGARSS'99 (Cat. No.99CH36293).

[22]  Lutz Hasse,et al.  A Ship Rain Gauge for Use in High Wind Speeds , 1998 .

[23]  A. R. Jameson,et al.  Improved Coupled Z-R and k-R Relations and the Resulting Ambiguities in the Determination of the Vertical Distribution of Rain from the Radar Backscatter and the Integrated Attenuation , 1995 .