Fig. 1

The investigated area in the northern South China Sea from December 13rd to 22th, 2012. The black triangles are sounding observation sites"

Fig. 2

Schematic diagram of the basic parameters to describe the characteristics of atmospheric duct. The pink layer is the duct layer, where h0 is the bottom heigh of the duct layer, h1 is the height of trapping layer, and h2 is top height of the duct layer. d0 and ΔM respectively denotes the depth and strength of the duct. A point marks the place where the gradient of M turns into negative, B point marks the place where the gradient of M turns into positive, and C point marks the place where have the same value of M as B point"

Fig. 3

Vertical profile of the atmospheric modified refractive index M before (blue lines) and after polynomial fitting and moving average (red lines), the results of sounding No. 2 (a) and No. 17 (b) are shown as examples here"

Fig. 4

(a) The wind speed vector, sea level pressure, and relative humidity observed by the shipboard automatic weather station (AWS) from December 17th to 22nd; (b) The sea surface air temperature from AWS and the water temperature recorded by the SBE-21 sailing thermometer and salinometer installed on the bottom of the ship. The ordinate in Fig. 4a marks the absolute value of the wind vector, and the 0-value line is the tail of the wind vector. The magenta inverted triangle indicates the sounding release point and number"

Fig. 5

Comparison of the water temperature recorded by the SBE 21 from December 16th to 22nd and the OSTIA SST. n is the number of samples, which is the sum of the number of SBE 21 observation records closest to the OSTIA grid point in the study area on that day. Dashed line is the linear regression line with a slope of one"

Tab.1

Information for the northern South China Sea investigation observation from December 13rd to 22nd, 2012, including the GPS sounding observation time, sea surface temperature, and whether the atmospheric ducts were observed"

Tab. 2

Characteristics of the surface duct observed during the investigation, and only the atmospheric duct layer with ΔM > 1 M-unit was counted"

Tab. 3

Characteristics of the elevated ducts observed during the investigation, and only the waveguide layer with ΔM > 1 M-unit was counted"

Fig. 6

(a) Vertical profile of the atmospheric modified refractive index M (blue line) and its vertical gradient (black line) observed at 02:00 on December 15th; (b) Vertical profile of the virtual potential temperature gradient (units: K∙m-1); (c) Vertical profile of the virtual potential temperature (minus 290K, black line), temperature (minus 8 ℃, gray line), and specific humidity (units: ×10-3kg∙kg-1, blue line). The red and magenta painted parts on the profiles are the stable elevated duct layer identified by ΔM > 1 M-unit"

Fig. 7

(a) Vertical profile of the atmospheric modified refractive index M (blue line) and its vertical gradient (black line) observed at 01:00 on December 22nd; (b) Vertical profile of the virtual potential temperature gradient (units: K∙m-1); (c) Vertical profile of the virtual potential temperature (minus 290K, black line), temperature (minus 8 ℃, gray line), and specific humidity (units: ×10-3kg∙kg-1, blue line). The red painted parts on the profiles are the duct layer but ΔM > 1 M-unit"

Fig. 8

Daily average weather map of 925 hPa from December 13th to 21st. (a) December 13th; (b) December 15th; (c) December17th; (d) December 19th; (e) December 21st. The coloring is relative humidity, the blue line is the geopotential height, the green line is the temperature, and the gray line is coastline"

Fig. 9

The strength (ΔM) of the elevated duct layers shown in Tab. 3 (vertical axis) and the average value of the vertical gradient of the virtual potential temperature ($\partial {{\theta }_{\text{v}}}/\partial z$) in the elevated duct layers. The black square represents the elevated ducts with d0 > 50 m, and the black triangle represents the elevated ducts with d0 < 50 m. The right side of the graphic symbol is the sounding number. The gray dashed lines are the value range for 10 K·km–1 < $\partial {{\theta }_{\text{v}}}/\partial z$< 40 K·km–1"

Fig. 10

Daily average weather map of surface from December 13th to 21st. (a) December 13th; (b) December 15th; (c) December 17th; (d) December 19th; (e) December 21st. The coloring is relative humidity, the blue line is the temperature, and the gray line is coastline"

Fig. 11

Vertical profile of the atmospheric modified refractive index M (blue line) and its vertical gradient (black line, after three-points moving average) observed from 08:00 on December 15th to 02:00 on December 16th. The red painted parts on the profiles are the stable elevated duct layer identified by ΔM > 1 M-unit"

Figure 12

Vertical profile of the atmospheric modified refractive index M (blue line) and its vertical gradient (black line is no smoothing result and red line is the three-point moving average) observed from 07:00 to 20:00 on December 21st. There is no red painted part on the profiles due to the non-existence of the stable elevated duct layer identified by ΔM > 1 M-unit"