Abstract:Based on the new data such as mooring launch sounding, lidar and surface densely observed data of the Winter Olympic Games in Zhangjiakou Area, the NCEP 0.25°×0.25° reanalysis data, and the field observation of the cold-air-pool (CAP) process from 15 to 16 January 2020, this paper analyzes the temperature, humidity and three-dimensional wind field of the CAP, and constructs the conceptual model of the formation, development, maintenance and breakup process of the CAP. The results show that this CAP process occurred under the background of stable-static weather with weak high-pressure ridge control, middle-level warming, significantly reduced wind speed in the upper air, less cloud in the sky and breeze near the ground. The CAP began to build up gradually from sunset in the evening when atmosphere was neutral, the strong fall wind on both sides of the valley slopes carried the cold air, stacked up and converged at the bottom of the valley, producing updraft and replacing the warm air in the valley, lifting it up. Temperature inversion formed and rapidly developed upward. Around midnight about 4-5 hours after sunset, the CAP developed to the height of about 300 m, which is about 3/5 of the valley height, 300 m above which was the isothermal layer, i.e., warm zone. During the development of the CAP, obvious east-west wind shear layer appeared under the inversion layer. With the continuous rise of the inversion layer top, the wind shear layer also gradually rose. From midnight to sunrise, the CAP developed into maintenance period, with little change at the height of inversion layer top and temperature, while the temperature at the bottom of the CAP continued to decline slowly. In this period, the down-slope wind and down-valley wind failed to penetrate into the valley floor, and the cooling was mainly of long-wave radiation cooling. The CAP break up about four hours after sunrise. First, solar radiation heated the western slope of the valley, causing the air in the middle and upper levels of the valley to heat up rapidly, and then with the increase of solar altitude angle, up-slope wind and up-valley wind appeared on the eastern and western slopes of the valley, transporting the cold air from the valley bottom to the eastern and western slopes and out of the valley. The warm air in the middle and upper levels of the valley sank, the convective boundary layer decreased, and the inversion dissipated from top to bottom, which was obviously different from the inversion disappearing from bottom to top in the plain area.