Estimation of Liquid Water Contents by the Dual-frequency Cloud Radar

1. Introduction

The dual-frequency cloud radar, a part of Mobile Multi-parameter Radar System we have recently developed, uses a single Cassegrain antenna with 2 m diameter both for the Ka-band and the W-band subsystems. Therefore, the Ka-band radar and the W-band radars can observe the same direction. In order to utilize this characteristic, we plan to measure liquid water contents (LWC) of clouds by the dual-wavelength method.

2. The dual-wavelength method

Attenuation of a millimeter-wavelength radio wave by a cloud is relatively high, and therefore compensation of the attenuation is one of the major problems of millimeter-wavelength meteorological radars. Since attenuation by clouds is proportional to the LWC, and the attenuation coefficient depends on a radio frequency, the LWC can be estimated from the ratio of echo powers observed with two wavelengths.

3. Results of simulations

Table 1 is the specification of the radar model used in the computer simulations, which is based on the actual radar system. The meteorological model consists of 2 horizontal layers; layer 1 is a clear-air layer and layer 2 is a cloud layer.

Figure 1 is a result of computer simulation. LWC is 0.5 g/m³ and the height of the cloud bottom is 1km in this simulation case. Figure (d) is a distribution of estimated LWC, and figure (e) is a standard deviation of the estimation. From this figure, LWC is accurately estimated up to 2.2 km altitude and 1km horizontal range.

4. Conclusion

In this paper, we examined performance of LWC measurements with the dual-frequency cloud radar we have developed by computer simulations. The actual performance of the radar system will be verified by field data in the near future.

 

Table 1

	Ka-band	W-band
Frequency	35 GHz	95 GHz
Peak power	100 kW	1.5 kW
Antenna diameter	2 m (common antenna)
Antenna gain	52 dB	58 dB
Antenna scan	0.5 rpm
Beam width	0.4 deg.	0.2 deg.
Pulse width	0.5 micro sec.	1 micro sec.
Noise Figure	5.0 dB	8.0 dB

 

 

Figure 1