The MRR (Micro Rain Radar) is a vertically looking Doppler radar that measures the velocity spectrum of the radar reflectivity factor of precipitating particles at several altitude levels above it. Initially designed to measure raindrops, the size spectrum of the drops can be deduced from the velocity spectrum by means of theoretical velocity-diameter relations adapted to rain. From this size spectrum, other variables such as water content or rainfall intensity can be deduced.

It is an "FM-CW" radar, i.e. it transmits continuously (at low power) and therefore has to modulate its transmission frequency in order to estimate the distance to the targets from which it detects the backscattered signal in return (unlike pulsed radars whose transmission frequency is fixed and for which the time interval between the pulses allows telemetry).

It is a light-configuration radar with millimetre-waves (1.24 mm, i.e. 24.2 GHz, K-band), which explains the relatively small antenna size (60 cm in diameter) and low cost for a weather radar. However, these frequency bands are sensitive to attenuation by precipitation, which makes this instrument less relevant for the observation of thunderstorms where rain intensities are higher. Moreover, the high vertical air velocities in these convective systems are added to the falling velocities of the precipitating particles, which makes the interpretation of the measured velocities more difficult. This is why this instrument is better suited to the observation of finer rainfall in systems where convection is less strong (stratiform systems).

In recent years, the MRR has also been used to measure snowfall. Algorithms have been proposed to improve the sensitivity of the instrument (snow backscatters less than rain) and to widen the possible range of measured speeds.

In the iFROG project, the MRR will be used to identify the boundary altitudes of the snow melting layer in precipitating systems, with the upper boundary corresponding to the altitude of the 0°C isotherm and the lower boundary corresponding to the altitude where all the snowflakes are melted. This layer is easily identifiable with the MRR because the falling velocities increase as the particles melt and increase in density and because the backscatter signal from melting snow is significantly higher than that from snow and from rain (bright band phenomenon). Melting layer altitudes estimated by MRR from the ground will be used in the iFROG project to validate those estimated by GPM (Global Precipitation Mission) products, whose main satellite carries on board two Ku- and Ka-band (13.6 and 35.5 GHz) vertical profiler radars.

Updated on 10 June 2022