中国西北干旱区戈壁下垫面夏季的热力输送

Summer heat transfer over a Gobi underlying surface in the arid region of Northwest China

摘要:

以敦煌戈壁站2004年6月和2008年8月的常规观测和超声观测为例,分析了西北干旱区戈壁下垫面夏季热力输送的一般过程及特征。首先评价了湍流通量的观测质量以及仪器观测的地表能量通量闭合问题,结果表明敦煌戈壁站的观测在白天总体较好。夏季地表能量通量的平均日变化显示,潜热通量整天都很小,可以忽略,白天到达地表的短波辐射以及地表向上的长波辐射非常强,地表净辐射主要转化为感热输送(敦煌戈壁站在中午时平均分别达380 W·m^-2以上和250 W·m^-2以上);夜间土壤释放热量以平衡地表的辐射冷却,感热通量略低于0。白天时地表大气经常触发自由对流活动,影响动量通量的观测质量,并有效输送地表热力至上层大气中,有助于形成超厚大气边界层。分析了戈壁下垫面的动量粗糙度特征和热力粗糙度特征(敦煌戈壁站动量粗糙度约为0.6 mm),热力粗糙度基本小于动量粗糙度一个量级,这符合目前对干旱区戈壁下垫面热力输送特征的初步认识。

Abstract:

Using the routine meteorological measurements and supersonic measurements at Dunhuang Gobi site in June 2004 and August 2008 as an example,This paper analyses the general process of summer heat transfer and its characteristics over a Gobi underlying surface in the arid region of Northwest China.Observation qualities of turbulent fluxes and measured surface energy closure are first evaluated,indicating that the system in the site measures well during the daytime overall.The typical diurnal variations of surface energy fluxes in summer show that the latent heat flux is generally little and can be ignored.In the daytime,both downward solar radiation and upward longwave radiation near surface are strong,and surface net radiation is mainly transformed into sensible heat flux(e.g.,mean surface net radiation and sensible heat flux at noon at Dunhuang Gobi site can reach more than 380 W·m^-2 and 250 W·m^-2,respectively).Soil releases energy to balance surface radiation cooling in the nighttime,and sensible heat flux is a little less than 0.The frequently occurred free convection activity near-ground is found in the daytime,which influences the observation quality of momentum flux.It can effectively transfer near-ground heat into upper air and contributes to the super depth of the atmospheric boundary layer.The characteristics of momentum roughness length and thermodynamic roughness length over the Gobi surface are further researched.The momentum roughness length at Dunhuang Gobi site is determined as 0.6 mm.The thermodynamic roughness length is an order of magnitude lower than the momentum roughness length,which agrees with the primary understanding of thermodynamic transport characteristics over the arid region.