Satellite and surface observations on 9 July 2014 are used in the present study. In the following, details of the observations are introduced.
Four kinds of satellite observations (TRMM, CloudSat, Aqua and MTSAT-1R) are used. TRMM, CloudSat and Aqua are polar-orbiting satellites. They all passed over Naqu once on 9 July 2014. MTSAT-1R is geostationary satellite with temporal resolution of 30 minutes. Satellite retrieval products used in this study are listed below. (1) The 2A25 and 1B11 products are retrieved from measurements of the precipitation radar (PR) and microwave scanning radiometer (TMI) onboard TRMM. The PR operates at 13.8 GHz, providing information on a horizontal resolution of approximately 5 km, a total of 80 levels in the vertical with a vertical resolution of 250 m, and a detection range of 0–20 km. The 2A25 product provides three-dimensional distribution of radar reflectivity factor. The 1B11 product is calibrated microwave TBB retrieved from the TMI measurements. In this study, the 85-GHz polarization-corrected TBB extracted from the 1B11 product used. (2) The 2B-GEOPROF, 2B-CWC-RO, and 2B-CLDCLASS products are retrieved from observations of cloud profiling radar (CPR) onboard CloudSat. The CPR operates at 94 GHz, providing observations with a vertical resolution of 240 m at 125 vertical levels and a detection range of 0–30 km. The 2B-GEOPROF product contains vertical distributions of radar reflectivity factor along the orbital trajectory of CloudSat; the 2B-CWC-RO product includes liquid (ice) water content, liquid (ice) water number concentration, and effective radius of liquid (ice) water particles. In the 2B-CLDCLASS product, clouds are classified into eight types, i.e., cirrus (Ci), altostratus (As), altocumulus (Ac), stratus (St), stratocumulus (Sc), cumulus (Cu), nimbostratus (Ns) and deep convective clouds (Dc). (3) The MYD06_L2 product is retrieved from observations of MODIS onboard Aqua (Platnick et al., 2015). In this paper, cloud microphysical parameters such as TBB at 11-μm wavelength and COD and Re at 3.7-μm wavelength are used. The horizontal resolution of TBB is 5 km, and that of COD and Re is 1 km. (4) The MTSAT-1R product provides TBB observations on a 4-km horizontal resolution.
Surface observations used in this paper include the following: (1) hourly surface precipitation collected at 34 automatic weather stations in the central TP (30°–34°N, 88°–94°E) on 9 July 2014 and surface precipitation at 10-min intervals at Naqu weather station; (2) soundings (31.48°N, 92.06°E; 4508-m ASL) at 0800 and 2000 BT 9 July 2014 at Naqu weather station; (3) C-band Doppler weather radar (31.48°N, 92.06°E; 4526-m ASL) observations at Naqu; (4) observations of KA-band millimeter wave cloud radar (31.48°N, 92.01°E; 4507-m ASL), C-FMCW (31.48°N, 92.06°E; 4507-m ASL), and disdrometer (31.48°N, 92.06°E; 4507-m ASL) deployed at Naqu during the TIPEX-III. Table 1 lists the instruments, locations, and observation parameters and their resolutions. The geographical distribution of surface observing facilities is shown in Fig. 1.
Instrument Location Observation parameters and resolutions KA-band millimeter wave cloud radar 31.48°N,
4507 m ASL
Operating frequency: 33.44 GHz; observations: echo intensity, radial velocity, linear depolarization ratio, and power spectral density; vertical detection range: 0.12–15 km; vertical resolution: 30 m; and temporal resolution: 0.85 s C-band frequency-modulated
4507 m ASL
Operating frequency: 5530 ± 3 MHz; observations: echo intensity, radial
velocity, velocity spectral width, and echo power; vertical detection range:
0.02–15 km; vertical resolution: 30 m; and temporal resolution: 2–3 s
C-band Doppler weather radar 31.48°N,
4526 m ASL
Conventional C-band operational radar, providing echo intensity, radial
velocity and velocity spectral width; temporal resolution: 6 minutes
4507 m ASL
Wavelength: 650 nm; frequency: 50 kHz; transmit power: 3 m W; radius
ranges: 0.2–5 mm for liquid particles, 0.2–25 mm for solid particles;
types of particles in total: 1024 ; and temporal resolution: 1 minute
4508 m ASL
L-band, providing elements of temperature, pressure, humidity, winds, etc. at temporal resolution of 12 h (0800 and 2000 BT); extra observations at 1400 BT were conducted during aircraft observations in the TIPEX-III of 2014
Table 1. Instruments, locations, parameters, and resolutions of the observations
Figure 1. Geographical locations of surface observing facilities nearby Naqu over Tibetan Plateau. The symbols denote the following——●: automatic weather stations; ★: KA-band millimeter cloud radar; ■: C-band Doppler weather radar; ▲: C-band frequency-modulated continuous-wave radar; ○: sounding; □: disdrometer; and the grey shadings represent the topography.
Radar reflectivity data from the C-band Doppler weather radar deployed at Naqu are remapped to gridded data with horizontal resolution of 1 km and vertical resolution of 0.5 km. The data cover an area of 400 km × 400 km and 0.5–20 km in the vertical. The REORDER software package (Oye and Case, 1995) developed by the NCAR is used for radar data interpolation, which adopts a closest point weighting function. The effective radius used in the interpolation is 1 km for radar range, 1° for azimuth angle, and 2° for elevation angle. This method has been applied in previous research (Wang et al., 2014). In the present study, gridded observations of C-band Doppler weather radar are used to calculate composite radar reflectivity, whose horizontal distribution is used to demonstrate the echo evolution during the deep convective process.
The bright band is determined based on vertical distribution of C-FMCW radar echoes. For each observed echo profile, the maximum and minimum vertical gradients of radar reflectivity over 0.2–2 km above the C-FMCW radar are calculated, and their corresponding heights (H1 and H2, H1 < H2) are determined. The maximum curvature heights are then determined from H1 downward 300 m and H2 upward 300 m, respectively, and the maximum curvature heights are the bottom and top heights of the bright band. In order to ensure echoes inside the bright band maintain certain levels of intensity, it is required that the echo intensity in the bright band must be greater than –10 dBZ and the maximum value is not less than 5 dBZ.
Following the approach proposed by Rosenfeld and Lensky (1998) for analysis of vertical structure of cloud microphysics based on satellite data, cloud microphysical parameters (TBB, Re) under various cloud top heights are used to investigate Re changes with the cloud top height (represented by TBB). The result is displayed by the so-called T–Re relation, which simplifies microphysical processes in cloud into five main processes, i.e., diffusional droplet growth, coalescence droplet growth, rainout, mixed-phase precipitation, and glaciation. Cloud microphysical parameters (TBB, COD, and Re) retrieved from the Aqua MODIS data are used to obtain the T–Re relation, which is then applied to analysis of vertical structure of cloud microphysics. Similar to the method proposed by Yuan et al. (2010), deep convection pixels are defined as the pixels satisfying TBB < 260 K and COD > 30 in the MODIS observations.
The method to determine deep strong convection and deep weak convection is from Pan and Fu (2015) and Fu et al. (2016). Based on the TRMM observations, deep strong convection is determined if the storm height (the first layer at which the echo detected by PR exceeds 17 dBZ and echoes in three consecutive layers all exceed 17 dBZ) is above 7.5 km and reflectivity of the echo in at least one layer detected by PR exceeds 39 dBZ; deep weak convection is determined if the storm height is above 7.5 km while the reflectivities of whole layer echoes are all smaller than 39 dBZ.