1. Introduction
Atmospheric radio wave ducting, which has a significant influence on electromagnetic wave propagation (Dockery 1988), is a common phenomenon in some areas (Abdul-Jauwad et al. 1991; Babin 1996); the probability of its occurrence is mainly related to the geographical position, season, and weather systems of the area. An atmospheric duct has significant influence on electronic equipment such as radars and communication equipment. It causes surveillance radar, navigation radar, and other radars over-the-horizon detection, increases radar clutter, and causes the appearance of such phenomena as over-the-horizon communication in VHF communication equipment. In particular, a surface duct has a greater impact on shipboard or ground-based electronic equipment. A surface duct is a grounded atmospheric duct (Babin and Rowland 1992; Babin 1996; Brooks et al. 1999); during the advection of a warm and dry air mass to the cold and wet sea surface, an atmospheric layer of inverse temperature and sharply declining humidity is formed, thus forming a surface duct.
The detection of atmospheric ducts is very important for research of the formation mechanism and characteristics of atmospheric ducts. Radiosondes are often used to detect atmospheric ducts (Patterson 1982; Babin 1996; Mentes and Kaymaz 2007). In recent years, the vertical resolution of radiosonde data can reach the meter level. Hayton and Craig (1997) used WMO radiosonde data to analyze the distribution of atmospheric ducts worldwide. However, radiosondes are unable to measure atmospheric ducts continuously. In addition, the sounding balloons drift downwind; thus, the values measured are not at the same position. The use of aircraft measurements is another effective method of atmospheric duct detection, including the use of helicopters to measure atmospheric temperature, humidity, and pressure to calculate the modified refractivity (Babin and Rowland 1992; Babin 1996), and the use of aircraft to take zigzag routes to measure atmospheric ducts (Brooks et al. 1999). Using aircraft to measure atmospheric ducts has similar shortcomings as radiosondes. A low-altitude rocket is a convenient atmospheric duct measurement device used on board. In addition, there is also a method for measuring the atmospheric duct by a tethered sounding balloon, but the measurement height by this method is limited to the boundary layer. All these methods have inevitable shortcomings: 1) unable to be observed continuously; 2) the equipment is complicated to use and inconvenient; 3) some measurement methods are lossy or less economical, such as the use of radiosondes and sounding rockets. Therefore, it is desirable to find a new remote sensing method to monitor atmospheric ducts, which can continuously observe atmospheric ducts without using sounding equipment.
Wind profiler radar (WPR) can measure the wind field, spectral width and volume refractivity (Tatarski 1961), atmospheric refractive index structural constant (VanZandt et al. 1978; Ruan et al. 2008), etc. The virtual temperature of the atmosphere could be measured if equipped with a radio acoustic sounding system (RASS) (Matuura et al. 1986; May et al. 1989; Tsuda et al. 1994). In this paper, WPR with RASS is used to measure the profile of atmospheric virtual temperature, spectral width, and the atmospheric refractive index structure constant; the Brunt–Väisälä frequency, the turbulence dissipation rate, and the atmospheric refractive index gradient profile are calculated, and then the profile of atmospheric modified refractivity is calculated to get the atmospheric duct value. To verify the height and intensity of the atmospheric duct, comparison experiments between WPR-RASS and radiosondes were carried out from June 2014 to June 2015 in Dalian, Liaoning Province, China. The difference of the atmospheric refractivity gradient and the atmospheric modified refractivity between the two methods is analyzed.
2. Method
a. Virtual temperature
The
b. Brunt–Väisälä frequency
Here,
c. Turbulence dissipation rate
Here
d. Refractive index structure constant
e. Refractive index gradient
Here
f. Atmospheric ducts
When
3. WPR-RASS detection atmospheric duct experiment
To verify the height and intensity of the atmospheric duct, comparison experiments between WPR-RASS and radiosondes were carried out from June 2014 to June 2015 in Dalian. During the period of observation, 16 experiments were carried out; 2 of them were under conditions of atmospheric ducts. The intensity of these two ducts was weak. The two times were 1915 LT 17 October 2014 and 1910 LT 1 June 2015.
The GTS1 Digital Radiosonde produced by Shanghai Changwang Meteotech Co., Ltd., was used in these experiments. The radiosonde launch site was 3 km away from the WPR-RASS; the WPR-RASS site was near the beach. The terrain elevation between the two sites was about 60 m. The observations of radiosondes and WPR-RASS were carried out at the same time.
Radiosondes took approximately 20 min to ascend to 5.7 km. To compare the WPR-RASS and radiosonde data, WPR-RASS data were extracted from the time of each launch until 20 min after. During this time, wind velocity is measured first, and the virtual temperature is measured about four minutes later. The WPR-RASS altitude above sea level was 70.8 m; the parameters of WPR-RASS are listed in Tables 1 and 2. In low-detection mode, the range of detection height for the WPR was from 150 to 3225 m, the range gate was 75 m, the number of fast Fourier transforms was 256, and the WPR had five beams: one vertical beam and four tilted beams with a zenith angle of 14.2°. The
The parameters of CFL-16 WPR.

The parameters of RASS.

The

(a) Comparison of
Citation: Journal of Atmospheric and Oceanic Technology 36, 4; 10.1175/JTECH-D-18-0009.1

Comparison of (a) the atmospheric refractivity gradient and (b) M from WPR-RASS (dashed line) and the radiosonde (solid line) at 1915 LT 17 Oct 2014.
Citation: Journal of Atmospheric and Oceanic Technology 36, 4; 10.1175/JTECH-D-18-0009.1
The profile of Brunt–Väisälä frequency squared
According to Eq. (12),
When the modified refractivity profile is calculated from WPR-RASS data, the first layer of modified refractivity is calculated approximately by Eqs. (15) and (16), the humidity of the first layer is calculated from WPR-RASS data by the method of Tsuda et al. (2001), and the air pressure of the first layer is calculated by ground pressure, which was measured at the WPR-RASS site and the height of first layer. The profile of modified refractivity is calculated from the refractivity gradient profile as shown by the dashed line in Fig. 2b, the top height of the duct is 595.8 m in the figure, and duct intensity
Another atmospheric duct was detected in summer. The

(a) Comparison of
Citation: Journal of Atmospheric and Oceanic Technology 36, 4; 10.1175/JTECH-D-18-0009.1

Comparison of (a) the atmospheric refractivity gradient and (b) M from WPR-RASS (dashed line) and the radiosonde (solid line) at 1910 LT 1 Jun 2015.
Citation: Journal of Atmospheric and Oceanic Technology 36, 4; 10.1175/JTECH-D-18-0009.1
The profile of
The profile of
4. Discussion and conclusions
WPR-RASS is used to detect atmospheric ducts. Two examples from different seasons show that the profile of modified refractivity and the atmospheric duct value by this method is consistent with the method by radiosondes. The validity of this method has been proved preliminarily, and the main conclusions are summarized below:
- The method of detecting atmospheric ducts using WPR-RASS is to measure the profile of
by the RASS and the profile of and by WPR; calculate , , , and atmospheric refractive index gradient ; and then calculate the profile of modified refractivity according to the atmospheric refractive index gradient. Finally, the atmospheric duct is determined according to the decision method of the atmospheric refractivity gradient, and the height and intensity of the atmospheric duct is calculated according to the profile of modified refractivity. - The accuracy of atmospheric duct detection is mainly determined by the accuracy of the refractive index gradient. The value of the refractive index gradient is determined by
, , and . Among them, the plays a major role, so the accuracy of is very important for atmospheric duct detection. - The method of detecting an atmospheric duct by WPR-RASS sometimes leads to missing ducts. This mainly occurs at times when the
deviation is larger or the measurement value is missing. There are three main conditions: 1) the wind speed is higher (>10 m s−1 for CFL-16); 2) humidity changes with height are rather significant; 3) close to the ground, there is influence of noise affecting the signal-to-noise ratio, which affects the measurement; it is particularly obvious at the of the bottom two layers. In addition, the CFL-16 WPR has a slightly larger spectral width than other WPRs. This leads to larger and smaller absolute values of the atmospheric refractive index gradient, thus leading to the duct being missed. In addition, because the measurement of sound speed and wind velocity is not completely simultaneous in the experiment, there are still vertical wind errors. - The vertical resolution of WPR-RASS data is 75 m in the low mode of CFL-16.The low vertical resolution of data has led to deviations of the measured atmospheric data from the reality, leading to errors in detecting atmospheric duct, or even missing the duct.
Continuous automatic detection of the atmospheric duct is of great significance for the study of the phenomenon and the mechanism of the atmospheric duct. It is also important to study the application of atmospheric ducts. To improve this method, the following research will be carried out: 1) studying how to improve the accuracy of
This research is supported by the National Natural Science Foundation of China under Grant 41276019. We deeply thank Dalian Meteorological Bureau for providing us with radiosonde data during the WPR-RASS experiments.
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