Abstract:Abstract. Lidar at 1064 nm and Ka-band millimetre-wave cloud radar (MMCR) are powerful tools for detecting the height distribution of cloud boundaries and can monitor the entire life cycle of cloud layers. In this study, lidar and MMCR
are employed to jointly detect cloud boundaries under different conditions.
By enhancing the echo signal of lidar at 1064 nm and combining its
signal-to-noise ratio (SNR), the cloud signal can be accurately extracted from
the aerosol signals and background noise. The interferenc… Show more
“…Except for the simplest single-point threshold determination in the early days, all cloud signal detection methods use the principle of cloud continuity to improve detection performance and reduce FAR and MDR. A common method is to use the continuity of Doppler velocity spectral width of cloud signals [17,18]. Only the spectral segment with N = 2N h + 1 continuous bins all over the threshold N s can be masked as the cloud signal.…”
“…To decrease both the FAR and MDR, most cloud detection methods utilize the spatiotemporal continuity of the cloud to discriminate the cloud from noise to achieve better FAR and MDR. Some of them perform the detection method at the Doppler power spectrum stage [17,18]. For example, Shupe [17] thought that spectral peaks of cloud signal should have a minimum width (in his use case, seven points) over the noise level.…”
A new method using three dimensions of cloud continuity, including range dimension, Doppler dimension, and time dimension, is proposed to discriminate cloud from noise and detect more weak cloud signals in vertically pointing millimeter-wave cloud radar observations by fully utilizing the spatiotemporal continuum of clouds. A modified noise level estimation method based on the Hildebrand and Sekhon algorithm is used for more accurate noise level estimation, which is critical for weak signals. The detection method consists of three steps. The first two steps are performed at the Doppler power spectrum stage, while the third step is performed at the base data stage. In the first step, a new adaptive spatial filter combined with the Kuwaraha filter and the Gaussian filter, using the ratio of mean to standard deviation as the adaptive parameter, is applied to initially mask the potential cloud signals to improve the detection performance at the boundary of cloud and noise. Simulations of boundary cases were performed to compare our adaptive filter and normal Gaussian filters. Box filters are used in steps two and three to remove the remaining noise. We applied our method to cloud radar observations with TJ-II cloud radar at the Nanjing University of Information Science & Technology. The results showed that our method can detect more weak cloud signals than the usual methods, which are performed only at the Doppler power spectrum stage or the base data stage.
“…Except for the simplest single-point threshold determination in the early days, all cloud signal detection methods use the principle of cloud continuity to improve detection performance and reduce FAR and MDR. A common method is to use the continuity of Doppler velocity spectral width of cloud signals [17,18]. Only the spectral segment with N = 2N h + 1 continuous bins all over the threshold N s can be masked as the cloud signal.…”
“…To decrease both the FAR and MDR, most cloud detection methods utilize the spatiotemporal continuity of the cloud to discriminate the cloud from noise to achieve better FAR and MDR. Some of them perform the detection method at the Doppler power spectrum stage [17,18]. For example, Shupe [17] thought that spectral peaks of cloud signal should have a minimum width (in his use case, seven points) over the noise level.…”
A new method using three dimensions of cloud continuity, including range dimension, Doppler dimension, and time dimension, is proposed to discriminate cloud from noise and detect more weak cloud signals in vertically pointing millimeter-wave cloud radar observations by fully utilizing the spatiotemporal continuum of clouds. A modified noise level estimation method based on the Hildebrand and Sekhon algorithm is used for more accurate noise level estimation, which is critical for weak signals. The detection method consists of three steps. The first two steps are performed at the Doppler power spectrum stage, while the third step is performed at the base data stage. In the first step, a new adaptive spatial filter combined with the Kuwaraha filter and the Gaussian filter, using the ratio of mean to standard deviation as the adaptive parameter, is applied to initially mask the potential cloud signals to improve the detection performance at the boundary of cloud and noise. Simulations of boundary cases were performed to compare our adaptive filter and normal Gaussian filters. Box filters are used in steps two and three to remove the remaining noise. We applied our method to cloud radar observations with TJ-II cloud radar at the Nanjing University of Information Science & Technology. The results showed that our method can detect more weak cloud signals than the usual methods, which are performed only at the Doppler power spectrum stage or the base data stage.
“…After getting the noise level, it is time to distinguish between noise and signal. A common method is to use the continuity of velocity spectral width of cloud signals (Shupe et al, 2004;Yuan et al, 2022). Only spectral segment with N = 2N h + 1 continuous bins all over the threshold N s can be masked as cloud signal.…”
Abstract. A set of Doppler power spectrum processing methods for TJ-II, a vertical sensing 94 GHz millimeter-wave cloud radar(MMCR), is proposed to distinguish clouds and enhance the data quality. The noise level is estimated by a modified segment method with 2-D segments. A two-step cloud signal mask method is proposed to distinguish clouds and noise. A Gaussian filter with adaptive standard variance is used to improve detection performance at the boundary of clouds and noise. Square signal blocks were constructed to test our method. Velocity dealiasing is carried out combined with a pre-dealiasing processing and the dual Pulse Repetition Frequency (PRF) technique to address a particular phenomenon called half-folded in MMCRs, which can not be fixed by post-processing at the base datum stage. Some observations of TJ-II are used to demonstrate our method. A comparison between our method as pre-processing and the method as post-processing proposed by Key Laboratory for Semi-Arid Climate Change of the Ministry of Education and College of Atmospheric Sciences, Lanzhou University, was carried out using one-day observation. It was found that our method shows some advantages in cloud base detection.
“…Due to the fluctuation of both the cloud signal and noise, simply using a threshold method will lead to an increased false alarm rate (FAR) and missed detection rate (MDR). One common method [15,16] is to exploit the continuity of the velocity spectrum of cloud signals, masking only N continuous bins over the threshold as cloud signals. In our application, however, the continuum points method did not fit because of its poor performance for weak cloud signals.…”
A 94 GHz pulse Doppler solid-state millimeter-wave cloud radar (MMCR), Tianjian-II (TJ-II), has been developed. It reduces the size and cost using a solid-state power amplifier (SSPA) and a single antenna. This paper describes the system design, including hardware and signal processing components. Pulse compression, segmented pulse, and dual pulse repetition frequency (PRF) technologies are employed to overcome the limitations imposed by the low power of the SSPA and the high frequency of 94 GHz. The TJ-II also features a dual-polarization, high-gain antenna for linear depolarization ratio detection and a time-division receive channel to improve channel consistency and save on costs. To achieve high flexibility and low interference in signal transmission and reception, the TJ-II uses software-defined radio technology, including direct digital synthesis, digital downconversion, and bandpass sampling. A series of Doppler power spectrum processing methods are proposed for detecting weak cloud signals and improving scene adaptability.
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