The characteristics of system noise of TRMM/PR and their application to the rain detection algorithm

The characteristics of the system noise sampled by the precipitation radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite are investigated in this study. The long term trend of the system noise shows a periodical change relating to the temperature of the PR. This change is corresponding to the changes in the solar beta angle which is the angle between the satellite orbit plane and the direction to the sun. The amplitude of the fluctuation of the system noise by the solar illumination to the PR is about 0.15 dB. The fluctuation of the noise caused by surface emission over ocean is examined by averaging the system noise of no-rain condition for a long time (e.g. one month) in order to remove the effect of the temperature fluctuation relating to the change of solar illumination and fading effect. The resulting amplitude is about 0.05 dB and the spatial distribution almost corresponds to the pattern of the sea surface temperature (SST). These results indicate that the system noise of the PR is quite stable so that the fluctuation of the sampled system noise is mainly caused by the fading effect. The characteristics of the system noise in three rain categories, rain certain, rain possible and no rain, are examined by taking a histogram assuming that the PR\´s system noise distribution is caused by the fading effect The histogram of rain certain and no rain cases show a Gaussian shape with a few tenths of decibel difference caused by the emission from rain drops. On the other hands, rain possible case shows two-peak pattern with a large count peak existing at lower system noise. Since the rain detection algorithm compares the difference between the system noise and the received power with a threshold of about 1 decibel, this algorithm allows categorizing as "rain possible" the combination of the low system noise and high received power, both of which are caused by the fading effect, even under no-rain condition.