Rain Attenuation Studies in the Microwave Band over a Southern Latitude

Brightness temperatures at 22.234, 23.834 and 30 GHz and corresponding rain rates were measured using a ground based radiometer and a fast response optical rain gauge co-located with the radiometer during the year 2009 at Cauchuria, Paulista (22.57 0 S, 89 0 W), Brazil. A statistical analysis reveals that the number of occurrences goes beyond 200 times for rain rate up to 15 mm/hr. It was observed also that the number of events suddenly falls to 75 for rain rates 15-25 mm/hr. Hence it suggests that at the place of experiment the most abundant rain rate was up to 25 mm/hr. The brightness temperature as measured by the radiometer sharply increases up to rain rate 10-12 mm/hr. But above this rain rate, radiometer brightness temperature slowly increases and has a tendency of saturation at all frequencies. A slight departure is observed in variation pattern of attenuation at 22.234 GHz during 21:35:00 to 23:43:25 hrs (local time), when rain rate is very low. Below 5 mm/hr, the measured attenuation is found to be higher than the calculated attenuation. This might be due to the fact that the heated earth surface evaporates water vapor and subsequently is filling up the antenna beam by a more amount of vapor when rain drop falls on the surface. This becomes prominent only at 22.234 GHz since the frequency of the radiometer lies just at the water vapor resonance line, although weak. This kind of anomaly is not recognized at the pressure independent frequency 23.834 GHz. So it appears that rain attenuation measurements at 22.234 GHz are contaminated by the unwanted presence of water vapor. This effect is minimized at the window frequency region (around 30 GHz) where attenuation due to water vapor is very less. It was also noticed that rain attenuation increases monotonically with rain rate with an exception at 23.834 GHz. During our study it was also observed at 23.834 GHz the attenuation always provides a minimum irrespective of any rain rate within the water vapor band. Hence, 23.834 may be considered as a good choice of radiometric measurement in the water vapor band. It is interesting to point out that effective rain height measured at 23.834 matches well with the physical rain height which was found to be 3.62 km. (