Micro Rain Radar

The Micro Rain Radar MRR-2 retrieves quantitative rain rates, drop size distributions, radar reflectivity, fall velocity of hydro meteors and other rain parameters simultaneously on vertical profiles up to several kilometers above the radar.

It operates with electromagnetic radiation at a frequency of 24 GHz with a modulation of 0.5 - 15 MHz depending on the height resolution (e.g. 300 m - 10 m). The radiation is transmitted vertically into the atmosphere where a small portion is scattered back to the antenna from rain drops or other formsof precipitation. Due to the falling velocity of the rain drops relative to the antenna a frequency deviation between the transmitted and the received signal (Doppler frequency) can be measured. This frequency is a measure for the falling velocity of the rain drops. Since drops with different diameters have different terminal falling velocities the backscattered signal consists of a distribution of different Doppler frequencies. The spectral analysis of the received signal yields a power spectrum which is spread over a range of frequencies lines corresponding to the Doppler frequencies of the signal.

Due to the high sensitivity and fine temporal resolution very small amounts of precipitation (below the threshold of conventional rain gauges)are detectable. The large scattering volume (compared to in situ sensors) allows to derive statistically stable drop size distributions within few seconds. The droplet number concentration in each drop-diameter bin is derived from the backscatter intensity in each corresponding frequency bin. In this procedure the relation between terminal falling velocity and drop size is exploited.

The backscatter cross section of rain drops increases with the fourth power of the radar frequency, if the target diameter is small compared to the wavelength (Rayleigh scattering). This is why a high frequency is useful in order to increase the sensitivity with respect to small drops. At very high frequencies the quantitatively interpretable height range becomes limited due to attenuation at moderate and higher rain rates. At 24 GHz, which is used here, attenuation effects may be noticeablebut should be weak enough to be correctable with sufficient accuracy.