The effect of rain on microwave backscatter from the ocean: measurements and modeling

Abstract

Microwave backscatter is used to probe the atmosphere, ocean, and land surface. The SeaWinds scatterometer on QuikSCAT and the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) are both microwave instruments that observe the state of the atmosphere. The Kwajalein Experiment (KWAJEX) was carried out in the summer of 1999 for the purpose of validating the TRMM PR. During KWAJEX measurements of the effect of rain on Ku-band microwave backscatter were made. At moderate and high incidence angles rain significantly increases surface backscatter. At low incidence angles few of the measurements proved to be statistically significant and a relationship between rain and surface backscatter could not be established. To verify our understanding of the effect of rain on sea-surface backscatter and to extend it to lower incidence angles, higher wind speeds, and other microwave frequencies, microwave backscatter from a rain-disturbed ocean surface is modeled. The modeled surface accounts for damping of surface waves by rain-induced subsurface turbulence and enhancement of gravity-capillary waves by rain-generated ring waves. The spectral representation of the ocean surface is used as input to a multiple scale (multiscale) scattering model, which separates surface waves into three distinct scales (short, intermediate, and long) and evaluates cross sections for each scale. At low incidence angles, Ku-band backscatter remains relatively unchanged because the enhancement of roughness by ring waves is compensated for by the damping of short gravity waves. At moderate and high incidence angles, rain increases the cross section and diminishes the wind relative directionality of the backscatter. To match the observations it is necessary to consider backscatter from stationary splash products which is substantial at high incidence angles. Although backscatter is dependent upon the wind at high wind speeds, relationships derived in rain-free conditions are invalid. Other commonly used microwave frequencies are significantly altered by rain. L-band backscatter decreases at low wind speeds, at moderate and high incidence angles. C- and Ka-band backscatter both increase and decrease, depending on the incidence angle and rain rate. It is also evident from the simulations that the contribution from stationary splash products will be significant at these frequencies.