Based on the CMA tropical cyclone(TC) best track data as well as the reanalysis datasets from the NCEP/NCAR and NOAA, the variation characteristics of TC number from 1949 to 2013 over the western North Pacific(including the South China Sea) are examined. Notably, the time series of TC number exhibits a significant abrupt change from more to less around 1995. Comparative analysis indicates that the environmental factors necessary to TC formation also change significantly around the mid-1990 s. After 1995, accompanying with anomalous warm sea surface temperature(SST) in western equatorial Pacific, a La Nia-like pattern in tropical Pacific appears obviously. However,compared with the period before 1995, the vertical upward movement decreases, vertical shear of tropospheric zonal wind increases, and sea level pressure(SLP) rises, all of which are unfavorable to TC formation and work together to make TC number reduce markedly after 1995. Furthermore, when the typical interannual more and less TCs years are selected in the two separate stages before and after 1995, the relative importance of oceanic and atmospheric environments in interannual TC generation is also investigated respectively. The results imply that the SST over the tropical Pacific exerts relatively important influence on TC formation before 1995 whereas the atmospheric circulation plays a more prominent role in the generation of TC after 1995.
This study uses ECMWF fifth-generation reanalysis, ERA5, which extends to the mesopause, to construct the Initial Conditions(IC) for WACCM(Whole Atmosphere Community Climate Model) simulations. Because the biases between ERA5 and Sounding of the Atmosphere using Broadband Emission Radiometry(SABER) temperature data are within ±5 K below the lower mesosphere,ERA5 reanalysis is used to construct IC in the lower atmosphere. Four experiments are performed to simulate a Stratospheric Sudden Warming(SSW) event from 5 to 15 February 2016. The simulation using the WACCM default climatic IC cannot represent the sharp meteorological variation during SSW. In contrast, the 0～4 d forecast results driven by ERA5-constructed IC is consistent with ERA5 reanalysis below the middle mesosphere. Comparing with WACCM climatology ICs scheme, the ICs constructing method based on ERA5 reanalysis can obtain 67%, 40%, 22%, 4% and 6% reduction of temperature forecast RMSE at 10 hPa, 1 hPa, 0.1 hPa, 0.01 hPa and 0.001 hPa respectively. However,such improvement is not shown in the lower thermosphere.
WANG YeguiJING WenqiCUI YuanyuanCAI QifaLAN WeirenFANG HanxianWENG LibinNIU Jun
基于WACCM+DART(Whole Atmosphere Community Climate Model,Data Assimilation Research Test-Bed)临近空间资料同化预报系统,以2016年2月的一次平流层爆发性增温(SSW)事件为例,开展了临近空间SABER(Sounding of the Atmosphere using Broadband Emission Radiometry)和MLS(Microwave Limb Sounder)温度观测资料集合滤波同化试验.结果表明:同化SABER和MLS温度观测资料可显著降低WACCM模式在中间层和平流层中上部(0.001~10 hPa)大气温度场的预报误差,改善CR试验在SSW发生时中间层变冷现象偏强、纬向风场首次发生反转的层次偏低以及增温恢复阶段0.1~10 hPa的东风层提前消退、纬向风速偏大、平流层顶位置偏高等现象.基于ERA5(The Fifth Generation of ECMWF Reanalyses)再分析资料的检验表明:同化SABER和MLS温度资料明显有利于减小北半球高纬度地区(60°-90°N)平流层中上层和下中间层(0.1~14 hPa)纬向风场以及平流层和中间层中下层(0.01~100 hPa)温度场的分析误差;同化低层大气观测也有利于减小0.1~14hPa纬向风场和0.01~100 hPa温度场的分析误差,但是不如同化SABER和MLS温度资料对临近空间纬向风场和温度场分析误差的改善效果显著.
