An improved composite surface model for the calculation of the normalized radar backscattering cross section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JC00190

An improved composite surface model for the radar backscattering cross section of the ocean surface 1. Theory of the model and optimization/validation by scatterometer data

An airborne microwave scatterometer-radiometer system operated in X band and Ka band was applied to the observations of microwave backscattering signatures of the ocean. The normalized radar cross sections σ0 were measured as combined functions of microwave frequency (10.00 GHz and 34.43 GHz), polarization (HH and VV), incident angle (0°–70°), azimuth angle (0°–360°), and wind speed (3.2–17.2 m/s). The azimuth anisotropic signatures for Ka band are confirmed to be similar to those for X band, and the wind speed dependences are analyzed for each azimuth angle, polarization, and incident angle. For each parameter the behaviors of σ0 for microwave frequencies is shown as compared with the results obtained by other experiments and theories. The effective reflection coefficient, the mean square surface slope, and the two-dimensional wave number spectrum of the short surface waves are estimated from the microwave scattering signatures.

Measurement of microwave backscattering signatures of the ocean surface using X band and Ka band airborne scatterometers

A new method for retrieving rainfall profiles from a spaceborne radar is introduced. As a result of the frequencies necessary in spaceborne radar applications, attenuation by both rainfall and liquid cloud particles is nonnegligible and must be accurately accounted for before quantitative rainfall estimates can be made. The proposed method is based on the minimization of a cost function that allows one to account for attenuation at each level directly in the iteration process. In addition, the algorithm does not invoke the Rayleigh approximation and is, therefore, applicable at wavelengths characteristic of spaceborne radars. The method is flexible with regard to the parameters to be retrieved and is well-suited for the addition of measurements from other sensors, such as a passive microwave radiometer, to constrain the retrieval. Preliminary results, using simplified assumptions of drop size distribution and particle shape, illustrate the utility of the algorithm provided the attenuation is not ...

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An Estimation-Based Precipitation Retrieval Algorithm for Attenuating Radars

In a two-part study we investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation at several frequencies across the EHF and lower SHF portions of the microwave spectrum in order to explore the feasibility of using multichannel passive-microwave retrieval techniques for the estimation of precipitation from space-based platforms. A series of numerical sensitivity experiments have been conducted that were designed to quantify the impact of an evolving cumulus cloud in conjunction with a superimposed rain layer on the transfer to space of microwave radiation emitted and scattered from the cloud layers, rain layer and the underlying surface. The specification of cloud microphysics has been based on the results of a time-dependent two-dimensional numerical cumulus model developed by Hall (1980). An assortment of vertically homogeneous rain layers, described by the Marshall-Palmer rain drop distribution, has been inserted in the model a...

Radiative Transfer to Space through a Precipitating Cloud at Multiple Microwave Frequencies. Part I: Model Description

Spaceborne radars offer a unique three-dimensional view of the atmospheric components of the Earth's hydrological cycle. Existing and planned spaceborne radar missions provide cloud and precipitation information over the oceans and land difficult to access in remote areas. A careful look into their measurement capabilities indicates considerable gaps that hinder our ability to detect and probe key cloud and precipitation processes. The international community is currently debating how the next generation of spaceborne radars shall enhance current capabilities and address remaining gaps. Part of the discussion is focused on how to best take advantage of recent advancements in radar and space platform technologies while addressing outstanding limitations. First, the observing capabilities and measurement highlights of existing and planned spaceborne radar missions including TRMM, CloudSat, GPM, RainCube, and EarthCARE are reviewed. Then, the limitations of current spaceborne observing systems, with respect to observations of low-level clouds, midlatitude and high-latitude precipitation, and convective motions, are thoroughly analyzed. Finally, the review proposes potential solutions and future research avenues to be explored. Promising paths forward include collecting observations across a gamut of frequency bands tailored to specific scientific objectives, collecting observations using mixtures of pulse lengths to overcome trade-offs in sensitivity and resolution, and flying constellations of miniaturized radars to capture rapidly evolving weather phenomena. This work aims to increase the awareness about existing limitations and gaps in spaceborne radar measurements and to increase the level of engagement of the international community in the discussions for the next generation of spaceborne radar systems.

Spaceborne Cloud and Precipitation Radars: Status, Challenges, and Ways Forward.

An airborne microwave rain-scatterometer/radiometer system operated at 10 and 34-45 GHz was developed for the remote sensing of precipitation, especially rain, from an aeroplane. Airborne rain-scatterometers were developed as a first step for the development of the future spaceborne rain-scatterometer. Flight experiments of more than 30 hours were performed in co-operation with the ground-based C-band weather radar. The sensor system of the airborne microwave rain-scatterometer/radiometer, flight experiment in June 1981 cooperating with the ground-based weather radar, and results of data analyses are described.

Airborne microwave rain-scatterometer /radiometer

Data on the airborne microwave radiometer, which is one of the sensors of the airborne microwave rain-scatterometer/radiometer (AMRS) system, are analyzed to infer path-integrated rain rate measured from topside. The equation of radiative transfer is used to relate quantitatively the antenna temperature to the rain rate profile inferred by the scatterometer. The influence of the ocean surface temperature on the radiometer measurements of rain is evaluated by a model computation. The theoretical prediction agrees excellently with the measurements. The effect of nonuniform rain along the propagation path is also evaluated by using the experimental data. It is shown that the excess antenna temperature (difference between the antenna temperature under raining and no-rain conditions) in the 10 GHz band is proportional to the path-integrated rain rate, and a method for determining the reference temperature (antenna temperature under a no-rain condition) is suggested.

Takeyuki Ojima

Papers

Airborne microwave rain-scatterometer /radiometer

Quantitative Measurements of Path-Integrated Rain Rate by an Airborne Microwave Radiometer over the Ocean

Ocean Oil Slick Observation with an Imporoved Side-Looking Airborne Radar

Observations of Oil Pollution on the Ocean by an X-band SLAR System

Remote Sensing of Precipitations by the Airborne Microwave Rain-Scatterometer/Radiometer System