Study of Diurnal Variation of Cloud Macro Parameters in Three Important Weather Systems over the Tibetan Plateau Using Ka-Band Cloud Radar

The diurnal variation of cloud macro parameters over the Tibetan Plateau is affected by the combined effects of large-scale circulation, local solar radiation and surface processes, while it also remarkably affects the radiation budget and transmission and the distribution of sensible and latent heat. Due to the lack of continuous quantitative observation, the understanding of the diurnal variation characteristics of the cloud macro parameters in various weather systems is still quite insufficient. Ka-band cloud radar of atmospheric profiling synthetic observation system (APSOS) is the first radar to realize long-term cloud observation in the Tibetan Plateau. In this paper, using statistical and Fast Fourier Transform methods, the data of the APSOS Ka-band cloud radar in 2019 are used to study the time- and frequency-domain diurnal variation characteristics of cloud frequency, single-layer cloud top height, cloud bottom height, and cloud thickness under the influence of westerly trough, shear line, and vortex system. The major conclusions are as follows: (1) The daily mean cloud frequency is 56.9% for the westerly trough system, 50.8% for the shear line system, and 73% for the vortex system. (2) Although the origins of the westerly trough and shear line system are different, the diurnal variation trend and main harmonic period of the cloud macro parameters of the two systems are similar: the diurnal variation trend is sinusoidal; the minimum value appears before sunrise, and the maximum value appears before sunset. The main harmonics of cloud frequency are diurnal (24 h) and semidiurnal (12 h), and the diurnal harmonics have the largest amplitude among the main harmonics of cloud top height, bottom height, and thickness. (3) The diurnal variation characteristics of cloud macro parameters in a vortex system are different from that in the first two systems. The diurnal variation of cloud parameters in the vortex system is multipeak type. Although the harmonic amplitude of the diurnal period is the largest among the main harmonics of the cloud frequency, single-layer cloud top height, and cloud bottom height, the spectrum distribution is discrete, and the maximum harmonic period of cloud thickness amplitude is 4.8 h. (4) The statistical regression equations of the diurnal variation of the cloud frequency, single-layer cloud top height, cloud bottom height, and cloud thickness are provided.