南海西南季风爆发前后海-气通量交换系数研究

A STUDY ON THE AIR-SEA FLUXES EXCHANGE COEFFICIENTS OVER THE SOUTH CHINA SEA(SCS) DURING THE ONSET OF SOUTHWEST MONSOON

动量交换系数(CD)、感热交换系数(CH)和潜热(或水汽)交换系数(CE)是气候模式中参数化海-气通量必需的参数,不同地区、天气、海况的试验中计算结果各异。文章利用2002年4月24日至6月20日在西沙海区进行的第3次南海海-气通量观测试验资料,使用涡旋相关法和TOGA COARE2.5b版本通量计算方案,计算了西南季风爆发前后海-气界面动量、感热通量、潜热通量等的湍流交换系数,讨论了各通量交换系数的变化特征及其与气象要素变化的关系。结果表明:西南季风爆发前后,随着风向、风速、云量、降水、湿度及海面状态等变化,通量交换系数也发生变化:中性条件动量交换系数(CDn)在季风爆发前数值略小,季风爆发后数值增大;中性条件感热交换系数和潜热交换系数(CHn,CEn)对天气变化的反应不够敏感。动量交换系数主要受风速影响,但在不同风速区间相关关系有异。(CH)与海-气温差呈现正相关关系,和气温有明显的负相关关系。CE与风速的关系密切,但当风速>12 m/s,CE随风速的变化趋向一个稳定值。另外当海-气温差大约<2℃时,CE随着海-气温差增大相应增大,反映了通量交换系数不仅与风影响下的下垫面特性有关,而且还与稳定度参数有关。各通量交换系数与气象要素变化的关系可以拟合为多项式或者简单的线性关系式。

Flux exchange coefficients including drag coefficient （Co）, sensible and latent heat flux exchange coefficients （ CH and CE） are important parameters for fluxes computation when the bulk algorithms and routine observations are used. The computation results vary with the experiments over different regions, weather and sea states. The data of the third SCS air-sea fluxes observation experiment from 24 April to 20 June 2002 are used in this paper. Momentum, sensible and latent heat flux exchange coefficients before and after the onset of the SCS southwest monsoon are estimated using the eddy correlation and TOGA OCRES 2.5b algorithms. The features of these coefficients and their relationship with other meteorological parameters are also discussed. The results indicate that these flux exchange coefficients changed significantly with the variation of wind direction, wind speed, cloud, precipitation, humidity and sea state before and after the onset of SCS southwest monsoon. Under the neutral condition, the momentum exchange coefficient（ CDn）varied from a smaller value before to a large value after the onset ; and the sensible （CHn） and latent （CEn） heat exchange coefficients were not sensible to the synoptic change. The momentum exchange coefficient was mainly affected by wind speed, but the association between the both differed with wind speed size. The sensible heat exchange coefficient was positively proportional to the air-sea temperature difference and negatively to air temperature. The latent heat exchange coefficient had close correlation with wind speed, when the wind speed exceeded 12 m/s, CE gradually approached to a stable value. Beside, when the air-sea temperature difference was less than 2 ℃, CE increased with the temperature difference; and when it exceeded 2 ℃, the relationship between CE and the temperature difference turned weak. The above relationships between the coefficients and meteorological parameters could be expressed with simple linear relation or polynomial fitting.