基于敦煌戈壁站观测对几套再分析产品夏季地表感热通量的评估

Evaluation of Summer Sensible Heat Flux in Multi-reanalysis Products Based on Measurements at Dunhuang Gobi Site

西北干旱区是欧亚大陆夏季感热输送的高值区,此区域感热输送对东亚季风气候系统的变异有重要影响。然而,再分析资料的感热通量在此区域存在很大的不确定性,影响该地区感热变化对东亚区域气候影响的认识。本文基于敦煌戈壁站2001~2014年夏季的观测数据,评估了NCEP/NCAR、NCEP/DOE、ERA-Interim和JRA-55这4套再分析产品的感热通量。敦煌戈壁站的感热通量是根据敦煌戈壁站常规观测数据和Y08方案计算得到,代表敦煌戈壁站的实际感热。结果显示,戈壁站夏季感热多年平均约为85.7 W m-2,但受局地降水的影响存在较大的波动;再分析资料的感热通量之间存在很大的不确定性,与观测相比,ERA-Interim的感热通量在大小和变化上好于其他再分析资料,在没有局地性降水的影响时比较接近观测。进一步分析了再分析感热与观测差异的原因。研究表明,再分析资料中的地表风速和地气温差与粗糙度设置和热力参数化方案相关联。各再分析资料均不同程度地低估了敦煌戈壁站的地气温差(观测值约6.5°C),这主要是由于再分析系统中对戈壁下垫面的粗糙度设置偏高以及热力参数化方案不太适用于戈壁下垫面造成的。相对而言,ERA-Interim的参数化方案在戈壁下垫面优于其他再分析产品的参数化方案,使得ERA-Interim的地气温差相对其他再分析资料更接近实际观测,感热通量较为合理。

The arid region of Northwest China is one of the regions with strong summer surface sensible heat flux transfer in Eurasia. Surface sensible heat flux over this arid region plays an important role in the variation of the East Asian monsoon system. However, large uncertainties in summer sensible heat flux over this region have been found in various reanalysis datasets, which hinders our exact understanding of its climatic influence on the East Asian monsoon system. Using the summer measurements from 2001 to 2014 at Dunhuang Gobi site, this paper evaluated summer sensible heat fluxes in four reanalysis datasets including NCEP/NCAR, NCEP/DOE, ERA-Interim, and JRA-55. Sensible heat flux observations at Dunhuang Gobi site were derived from routine meteorological observations combined with a reasonable surface flux parameterization scheme（Y08 scheme）. The results indicate that the average summer sensible heat flux at this site is about 85.7 W m-2, while its variation range is large due to the influence of local rainfall. There exist certain differences in the sensible heat flux among different reanalysis datasets. Compared with the observations, the sensible heat flux from the ERA-Interim is the best in both the magnitude and variation, and agrees well with observations when there is no local rainfall influence. The reasons for the uncertainty in sensible heat flux in the four reanalysis products and the differences between reanalysis and observations were investigated. Surface wind speed and differences between surface skin temperature and air temperature are associated with the specification of roughness and surface flux parameterization scheme. The difference between surface skin temperature and air temperature in the reanalysis products was much smaller than the observation（6.5 °C） mainly due to the unrealistically high value of roughness and inappropriate parameterization scheme for the Gobi surface in these reanalysis products. The parameterization scheme of ERA-Interim is relatively better than those of others in that the difference between surface skin temperature and air temperature is close to observations, which results in a relatively reasonable estimation of sensible heat flux.