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Observation of clouds and precipitations with radars in millimeter waves is one of the most fruitful method to investigate those interior structures because the sensitivities for small particles and droplets are much better than those in longer wavelengths. We have developed the cloud profiling FMCA (Frequency Modulated Continuous Wave) 95 GHz Doppler Radar, named “FALCON-I” (Fig.1; FALCON=FMCW Radar for Cloud Observations), at Chiba University. FALCON-I consists of two 1-m diameter antennas and has a spatial resolution of 0.18 degree (which corresponds to 15m at the height of 5km) and ranging resolution of typically 50m. We make regular observations at zenith with the temporal interval of 10 seconds and make scanning observations with +/- 5-degree in one direction from the zenith with the interval of 15 seconds when we want to observe spatial extent of clouds.
Fig.2 shows clouds and precipitations observed with FALCON-I on 15 August 2017 at Chiba University. Time-height intensity map from 00:00-02:00 UT (09:00-11:00 JST; Fig.2a) and Doppler profile map at 00:50 UT (Fig.2b) are presented. The intensity map shows that precipitations started around 00:35 UT at the height of 5 km. The Doppler profile map shows the cloud at 5-7km in height has Doppler velocities about 0 to -2 m/s, which negative velocity means downward motion. At the bottom of the cloud i.e. 5km in height, rain droplets with the Doppler velocities of up to -6 m/s are abruptly produced within a few hundred meters in height. Terminal velocity of 6 m/s corresponds to about 2 mm diameter droplets. Rain droplets were produced intermittently in this case and one of the gropes of droplets is marked with a solid line which just started falling below the cloud bottom as shown in Fig.2b. We can recognize several groups of droplets whose inclines in Fig.2b are getting steeper with falling down. We can derive droplet number distribution N(D) from the Doppler spectra by assuming the observed velocities are terminal velocities. Successive Doppler profile maps and movies from 00:30 to 01:00 UT show dynamic phenomena in the beginning phase of precipitations such as production and acceleration of rain droplets at the bottom of the cloud, evaporation of droplets, changes of number distributions during the falling way, and so on. These results of analysis show that observations of millimeter-wave Doppler radar FALCON-I are powerful methods to investigate micro processes in clouds and precipitations.
Fig.1. Cloud Profiling Doppler Radar FALCON-I consists two 1-m antennas and observes clouds and precipitations at 95GHz with high spatial resolution of 0.18 degree.
Fig.2. Clouds and precipitations observed with FALCON-I on 15 August 2017 at Chiba University. Time-height intensity map from 00:00-02:00 UT (a) and Doppler profile map at 00:50 UT (b) are presented. The intensity map shows that precipitations occurred around 00:35 UT at the height of 5 km. The Doppler profile map shows the cloud at 5-7km in height has Doppler velocities about 0 to -2 m/s, which negative velocity means downward motion. At the bottom of the cloud i.e. 5km in height, rain droplets with the Doppler velocities of up to -6 m/s are abruptly produced within a few hundred meters. Terminal velocity of 6 m/s corresponds to about 2 mm diameter droplets. Rain droplets were produced intermittently in this case and falling as shown in (b).
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