Backscattering enhancement from random hydrometeors should increase
as wavelengths of radars reach millimeter regions. For 95 GHz radars,
the reflectivity of backscattering is expected to increase by 2 dB,
due to multiple scattering including backscattering enhancement, for
water droplets of diameter of 1 mm with a density of 5 x 103 m-3. Previous theoretical studies of backscattering enhancement considered infinitely extending plane waves. In this paper, we expand the theory to spherical waves with a Gaussian antenna pattern, including depolarizing effects. While the differences from the plane wave results are not great when the optical thickness is small, as the latter increases the differences become significant, and essentially depend on the ratio of radar footprint radius to the mean free path of hydrometeors. In this regime, for a radar footprint that is smaller than the mean free path, the backscattering-enhancement reflectivity corresponding to spherical waves is significantly less pronounced than in the case of the plane wave theory. Hence this reduction factor must be taken into account when analyzing radar reflectivity factors for use in remote sensing applications.
Concept and expected performance of cloud profiling radar (CPR) for EarthCARE are described based on preliminary design study conducted to date. High sensitivity and Doppler capability are two significant new features in this CPR. Particularly, Doppler capability is the first attempt to spaceborne atmospheric radar, which requires great efforts in technical development and feasibility validation. We have developed a new numerical simulation method to assess Doppler velocity accuracy applicable to this application, and results are compared with conventional approximation method. Validity and limitation of the approximation method are indicated from comparison with numerical method. It is shown that requirements to radar sensitivity and Doppler measurements will be satisfied. However, because these requirements to CPR are very tough, further detailed study on both design optimization and assessment technique development are necessary. Under radar operation with very high pulse repetition frequency (PRF) required in this CPR, surface clutter interference caused through antenna sidelobes is an important issue. Analysis on this issue and preliminary requirements to the antenna sidelobes are also discussed.