利用COSMIC掩星资料研究青藏高原地区大气边界层高度

Estimating planetary boundary layer height over the Tibetan Plateau using COSMIC radio occultation data

以往关于青藏高原边界层的研究都是基于个别站点的常规观测,对青藏高原边界层的整体性认识受限。GPS掩星资料具有测量精度高和垂直分辨率高的特性,其廓线中含有大量有价值的边界层信息。利用2007—2013年COSMIC掩星资料,通过计算大气折射率最小梯度来确定边界层高度,并用无线电探空资料对结果进行了检验。在此基础上,对青藏高原地区边界层高度的特征及其形成机制展开了研究,比较了COSMIC掩星确定的边界层高度和ERA-Int的差别,讨论了最小梯度法用于边界层研究的不确定性。结果表明:青藏高原上COSMIC掩星和无线电探空数据检测的边界层高度相关系数为0.786,平均值偏差为0.049 km,均方根误差为0.363 km,COSMIC掩星数据检测的边界层高度和无线电探空的结果非常接近。青藏高原上边界层高度呈现西高东低的分布特征,高原中西部边界层高度主要为1.8—2.3 km,而高原东部边界层为1.4—1.8 km,最大值在高原西南部。青藏高原地区边界层有明显的季节差异,冬季高原上大部分地区边界层高度超过2.0km;春季大部分地区高度降低,但在受印度季风影响的高原南部有明显的抬升,最大值可超过3.0km;夏季高原上边界层高度开始升高,大部分地区超过1.8 km;秋季又开始回落。青藏高原以北塔克拉玛干沙漠和高原以南印度季风活动区是两个高值区,北部的沙漠地区边界层高度在夏季最高,南部印度季风活动区在季风爆发前(4月)达到全年最大值。青藏高原中西部地区有水平风辐合以及广泛的上升运动,为边界层的发展提供了动力条件,而东部的下沉运动对边界层的发展有抑制作用。青藏高原边界层各个季节的空间分布与地表感热通量分布一致。COSMIC掩星资料确定的边界层高度和ERA-Int相比,空间分布基本一致但ERA-Int边界层高度明显偏低。当有系统性强逆温存在的时候,或者云中液态水或冰水含量较大时,用最小梯度法检测的边界层高度不确定性增加。

Many previous studies on the planetary boundary layer（PBL）over the Tibetan Plateau（TP）were based on sparse conventional observations,which led to limitations for overall understanding of the PBL over the TP.Global Positioning System Radio Occultation（GPS RO）measurements contain useful information about the PBL due to its high accuracy and high vertical resolution.COSMIC（Constellation Observing System for Meteorology,Ionosphere and Climate）RO measurements from2007 to 2013 were used to derive the PBL height over the TP.The top of the PBL is defined as the height at which the vertical gradient of the refractivity is the minimum.The PBL height derived from COSMIC RO is highly correlated with collocated radiosonde data with a correlation coefficient of 0.786,a mean PBL height difference of approximately 0.049 km and a root-mean-square difference near 0.363 km.The PBL height over the TP decreases from west to east with the amplitude of annual mean PBL height ranging with in1.8 to 2.3 km over the central-western TP and 1.4 to 1.8 km over the eastern TP.The maximum PBL height is found over southern TP.In addition,the PBL height over the TP has a distinct seasonal variation.The PBL height is more than 2.0 km during winter over most TP regions.Following the approach of spring,the PBL height starts to shrink over most TP regions except southern TP,where the PBL height increases to 3.0 km due to the influence of the Indian monsoon.The PBL over the TP strengthens during summer with the PBL height more than1.8 km.The PBL height reduces again during the autumn.There are two maxima over Taklimakan Desert and the monsoon region,which is located at the north and south of the TP,respectively.The maxima of PBL height occur at summer over the desert and April before the monsoon onset over the monsoon region.The convergence of horizontal winds and extensive ascending motions over the central and western TP provide the impetus for the development of the PBL,and the large-scale descending motions restrain the development of PBL over the eastern TP.The seasonal distributions of PBL height are consistent with that of sensible heat flux over the TP.The PBL height derived from COSMI RO have similar spatial and temporal patterns with that from ERA-Interim（European Centre for Medium-Range Weather Forecasts Reanalysis Interim）,but the PBL height derived from ERA-Interim is underestimated almost everywhere over the TP.The GPS RO can capture the structure of PBL over TP very well.However,the presence of strong inversion layer,or cloud with large liquid or ice water content,may cause uncertainties in the derived PBL height.