DEsert-oasis COnvergence line and Deep convection Experiment (DECODE)

Prof. Zhiyong Meng
Peking University
  1 Nov, 1pm

Convection initiation (CI) has always been a big challenge in weather forecasting worldwide. Boundary layer convergence line is one of the important lifting mechanisms of CI. One major cause of boundary layer convergence line is heterogeneous heating caused by land surface contrast. Previous studies on boundary layer convergence lines and their associated CI over heterogeneous land surface have mainly focused on land-sea, mountain-valley and land-lake transition zones. Studies on the mechanism of convection initiation and evolution due to underlying surface vegetation contrast have been quite limited. Along the border of Kubuqi Desert and the Hetao Irrigation District of Inner Mongolia, China, which is the largest one-head irrigation area in east Asia, boundary layer convergence lines often occur and deep convections often initiate over the desert side near oasis-desert transition zone. Our statistical analysis showed that about 60 convergence lines form along the desert-oasis border from June to August each year, and 44% of which initiate deep moist convections. These conventions sometimes propagate and intensify downstream, causing severe convective disasters. In order to understand the convection initiation, organization, and evolution mechanisms due to vegetation variability across oasis-desert transition zone in Hetao region, DEsert-oasis COnvergence line and Deep convection Experiment (DECODE) was launched from July 5-August 9, 2022. About 100 scientists and students participated in the experiment.

Vertical profiles of Planetary Boundary layer as well as convection and its ambient environment features were observed at two oasis sites and four desert sites using laser wind lidar, microwave-radiometer, soil temperature and moisture sensor, eddy flux observation system, portable radiosonde, two C-band radars with one dual-polarized, airplane, and satellite FY4B-GHI rapid scan with horizontal resolution of 250 m and time interval of 1 min. More than 800 radiosondes were launched during the observation period in the experiment regions with time interval from 30 min to 3 hours, capturing vertical profiles of wind, moisture and temperature in the PBL before and after the formation of the boundary layer convergence line and associated convection.

During the experiment, 23 cases of convergence lines with various morphology occurred, and 11 of which initiated convection. In addition to the convergence lines, there also occurred 20 gust fronts, 9 times of horizontal convective rolls, and 1 supercell splitting and 1 tornado. Our preliminary results include: 1) the fine line of the convergence line on radar was made of both dust and hydrometers; 2) a multi-perspective image of convergence lines were obtained by combining the observations from surface and airplane; 3) Differences in surface air temperature, wind and soil moisture between oasis and desert sites were obtained; 4) Real-time 1-km WRF forecasting was run during field experiment, demonstrating a quite promising performance in capturing the evolution of both the convergence lines and associated convection.