Japanese

Assessment of precipitation attenuation
（by Mirror Image）(Results of Mr. Ji Li)

PR sends out microwaves from an antenna actively and detects the backscattered radiowaves by the precipitation. Rainfall rate can be estimated from their microwave measurements. By using an airborne or a spaceborne radar, such as TRMM PR, to detect the precipitation, there is a virtual rainfall system, that is, mirror image (MI) appeared below the ocean surface (Meneghini et al. 1983). It is because the incident power is scattered from the surface to the precipitation, intercepted by the precipitation, and again returned to the surface where it is scattered a final time back to the antenna. Comparing this mirror image with the direct precipitation image, a four-way integrated precipitation attenuation could be estimated (Meneghini & Atlas 1986, Meneghini & Nakamura 1988).

The 1C21 data and the 2A25 data from TRMM PR are utilized in this study. The dat a were observed in June 1998. The algorithms version 4 is used. One month data, totally 492 granules were checked and all the over ocean "rain certain" (see TRMM documents online at here) pixels were picked out. In order to calculate the MI reflectivity, we assume that the rain rate will not change from the surface to the height of 1000 meters. Because of th e contamination of surface, the PR can not detect the rain rate below the 500 meters height. We used the closed expression of MI return which derived by Meneghini and Atlas (1986).

Summary (This is an abstract of Li & Nakamura (2002).)

A mirror image is a virtual image of precipitation from “below” the ocean surface when an airborne or a spaceborne radar is used to view rainfall over the ocean. It is due to a reflection of energy from the sea surface to the precipitation and back to the radar via a second reflection at the sea surface. The mirror image characteristics were investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation radar data and the following was found. 1) The radar can detect the mirror image clearly over the ocean. 2) The mirror image echo corresponds well to the direct rain echo at nadir and near-nadir incidence angles. 3) In a weak rain region, the mirror echo intensity is nearly proportional to the direct echo power except near the radar noise level. 4) In the strong rain region, rain attenuation effects clearly appear. 5) The ratio of the mirror echo power to the direct echo power is affected by the rain attenuation, which varies with the brightband height and the range of the target rain from surface. Further, a simple simulation was performed in order to confirm the above characteristics. The signal fluctuation, noise contamination, rain attenuation, and surface cross section are taken into account in the simulation. The results of simulation confirmed the observation results.

Full text of Li & Nakamura (2002) is able to be downloaded from http://ams.allenpress.com/amsonline/?request=get-archive&issn=1520-0426&volume=019 .

References
Meneghini, R. and K. Nakamura, Proc. Intl. Symp. Tropical rainfall measurements., 235, 1988.
Meneghini, R. and D. Atlas, J. Atmos. Oceanic Technol., 3, 400-413, 1986.
Meneghini, R., J. Eckerman and D. Atlas, IEEE-Trans. Geosci. Remote Sensing., GE-21, 34-43, 1983.
Li, J. and K. Nakamura, J. Atmos. Oceanic Technol., 19, 145-158, 2002. "Characteristics of the Mirror Image of Precipitation Observed by the TRMM Precipitation Radar"