Turbulent Mechanisms for the Deep Convective Boundary Layer in the Taklimakan Desert

The deep convective boundary layer (CBL) in the Taklimakan Desert plays an important role in the climate system in East Asia. Based on the observation experiment and large-eddy simulation, turbulent mechanisms for its formation were revealed in this study. This explained why the daily maximum CBL depth was independent of surface heating. In the late-morning, there was a weak temperature inversion and a near-neutral residual layer (RL) above the CBL.   With the development of the CBL, stronger convection could penetrate the RL and even overshoot the top of the RL. The distinctive boundary-layer process entrained free-tropospheric air to warm the RL and then promoted the entrainment of the warmed air in the RL into the CBL. This extra energy supply effectively contributed to the growth of the CBL. With further positive feedback between the CBL and RL depths, a deeper CBL would form in consecutive fair-weather conditions.

Methods
The data were collected in July 2016 at the Tazhong meteorological station (39° 00'N, 83° 40'E), situated in the hinterland of the TD. The station is surrounded by relatively homogeneous shifting sand and dunes. The radiosonde data were collected from the GPS radiosonde system developed by the Beijing Institute of Radio Measurement. The variations of air density and specific heat capacity at constant pressure with height were empirically calculated using radiosondes. The turbulence data were obtained by the eddy covariance system (IGRASON, Campbell Scientific, Inc., USA) mounted on a mast, at a height of 3 m above the sand surface.

塔克拉玛干沙漠（Taklimakan Desert）中的深对流边界层（deep convective boundary layer, CBL）在东亚气候系统中发挥着重要作用。本研究基于观测试验与大涡模拟（large-eddy simulation），揭示了其形成的湍流机制，阐明了为何每日最大CBL深度与地表加热无关。上午晚些时段，CBL上方存在弱温度逆温层与近中性残留层（residual layer, RL）。随着CBL的发展，更强的对流可穿透残留层甚至越过其顶部。这一独特的边界层过程会夹带自由对流层空气以加热残留层，进而促进被加热的残留层空气被夹带进入CBL。这种额外的能量供给有效推动了CBL的增长。随着CBL与残留层深度之间形成进一步的正反馈，在连续晴朗天气条件下可形成更深的CBL。

方法
数据采集于2016年7月，地点位于塔克拉玛干沙漠腹地的塔中气象站（39°00'N，83°40'E）。该站周边为相对均一的流动沙丘与沙地。探空数据取自北京无线电测量研究所研发的GPS探空系统。研究人员通过经验公式，利用探空资料计算了空气密度与定压比热容随高度的变化。湍流数据由安装在沙面上方3米高桅杆上的涡动协方差系统（eddy covariance system, IGRASON，Campbell Scientific, Inc., 美国）获取。