Frequency Modulation Continuous Wave Profiling Radar for Precipitation and Observations in Different Regions of China During 2013-2016

Vertical structure is important in understanding how the distribution of latent heating affects the atmospheric circulation and how to better parameterize precipitating cloud systems in numerical models. The profiling radars are ideal instruments for examining the vertical structure of precipitating cloud systems. The C band frequency modulation continuous wave profiling radar(C-FMCW) has been developed by State Key Laboratory of Severe Weather/Chinese Academy of Meterological Sciences (LaSW/CAMS) in 2013.

1.C-FMCW profiling radar system

Understanding higher temporal and spatial changes in the precipitating clouds drive the development of radar technology. Compared with pulse Doppler radar, frequency modulation continuous wave (FMCW) radar can provide detail characteristics and rapid evolutions of clouds.

Pulse Doppler radar using a narrow pulse transmitting method to improve the detecting resolution, but the bandwidth reduced the sensitivity of receiver. The technology of wide pulse, narrow pulse receiving pulse compression lift the system noise level, the result is affects the detection capability. So the pulse Doppler radar is difficult to obtain higher range resolution without loss of detection ability conditions.

The distance measure of FMCW radar is modulated the transmitted continuous wave signal and demodulated for return signal. The advantage of FMCW radar system is high precision, no blind zone and lower peak power.

LaSW’s C-FMCW radar is mainly used for accurate detection of high vertical resolution in the precipitation cloud. The technology include the bistatic dual antenna system, weighted processing method of the FFT spectrum transform and sidelobe suppression. Table 1 give the main parameters specification. The spatial resolution is 15m-30m and the resolution temporal is 2s-3s ,C-FMCW radar can obtain the rapid evolution process of cloud and precipitation from 15m to 24km height range.

Table 1．Specification of CFMCW vertically pointing radar

Parameter	Specification
Frequency	5530MHz±3MHz/±3.5MHz
Pulse Repetition Period	600、700μs
Band width	6MHz±3MHz/±3.5MHz
Typical sampling time	2s,3s
Range resolution	15m、30m
Height range	15m-15km;15m-12km;30m-24km ;30m-24km -
Antenna	Bistatic; paraboloid
Antenna radius	1.6m
Antenna gain	≥35dB
Beam width	≤2.6°
Transmitting power	≥150W
Number of FFT	First/Second： 512/512
Number of spectral average	1~10

The minimum detectable reflectivity is -50dBZ near surface and -25dBZ at 12km height. For the precipitation cloud there is no precipitation signal attenuation.

2. Observations introduce

The C-FMCW radar has observed during the southern China Monsoon Rainfall Experiment (SEMREX) and operated well at the 4507m altitude in the Third Tibetan Plateau Atmospheric Scientific Experiment III(TIPEX III). And also observed during Meiyu front rainy season of Yangze river in 2013 and 2015.The statistical characteristics of reflectivity & velocity of these three different regions from the continue observation data of the season are given. During the pre-summer rainy season in south China the melting layer near the 5km(AGL) in the stratiform precipitation. The convective cells are appeared dispersibility triggered by low-level disturbance after the precipitation process moved out. The main characteristic of precipitation cloud is deep convection during the cold vortex monsoon season in Tibetan plateau. The height of the melting layer is about 1.0-1.5km(AGL). The strong updraft can exceed 20m/s at the mid-upper level in the cold vortex convection. The stratiform is most stable during Meiyu precipitation season in Yangze river. The convection trigged by the surface unstable and deep convection are infrequent. The unstabitily appeared near the middle level and enhanced the rainfall mostly. The spectrum distribution of reflectivity is width and the velocity distribution is concentrated over the bright band height in stratiform, which indicated that different masses of solid particles exit over the melting layer, and the difference of shape and density of particles show the difference of reflectivity, but the change of their fall speed is smaller. The strong updraft air motions are common in the convective cloud.

Fig.1 Photo of C-FMCW radar observed at Naqu Tibet(31°29´,92°04´4507m)

Fig.2 Time-height distribution of convective cloud from 1730 to 1930BJT 04 Jul. 2014

Fig.3 Time-height distribution of stratiform from 0700 to 1500BJT 28 Jun. 2015

Fig.4 Time-height distribution of stratiform&convection from 13 to 14 Jun. 2016

Fig.5 CFAD with the continue observation during the month of C-FMCW

3. Data Preliminary analysis of TIPEX III

During the TIPEX III from Jul to Aug 2014, C-FMCW observed at Naqu Tibet(31°29´,92°04´4507m) . Removal of equipment fault, power outages and other disturbing influences, nearly 52 days normal vertical profiles are observed. Fig.1 is the photo of C-FMCW observed at Naqu,Tibet. Some results of pre-analysis using the total number of 1442634 profiles from C-FMCW ground vertical observation:

1) The statistic results confirm the multi-peak of Cloud top. The CTop with double-peaked at 7.8km and 11.9km altitude respectively，and the larger peak at lower levels and the smaller peak at higher altitudes. Peak at lower levels are mostly appear throughout the day, in addition to slightly lower at noon ,and the peak at higher levels have larger frequency during 1600-2200.

2) It was found the deep convective event are occurred from afternoon to nightfall, the radiation heating have a strengthening contribution to the convective development, and the rain rate is large during this period.

3) From the features of 9 convective cases, the peak updraft values are between the 10 m/s-22m/s. The strong updrafts at upper levels are together with the strong downdrafts, their intensity appears positive correlation. The height of the peak air motion are often above 10km altitude, the highest is can arrive 18km altitude. The deep convective clouds are coursed by the strong uplift of atmosphere.

4) The maximum and the larger reflectivity are mainly at the lower level mostly below the height of 0°C temperature. The strengthening of the melt strength of echo contribution It shows that the dynamic contribution is at the height over melting level, mostly at higher level even  stronger.