Study On Tropopause Characteristics Over The Tibetan Plateau Using COSMIC Radio Occultation Data

The importance of the Tibet Plateau climatic characteristics study is self-evident,but the complex topographic conditions restrict the observation of conventional data.Global Positioning System Radio Occultation(GPS RO)data provides a possibility for innovating the plateau numerical prediction level and improving the ability of long-term climate detection,with its advantages of high accuracy,high vertical resolution,all-weather and global distribution.Firstly COSMIC(Constellation Observing System for Meteorology,Ionosphere,and Climate)RO data are collocated in space and time with ECMWF(European Centre for Medium-Range Weather Forecasts)analyses during 7 years period from 2007 to 2013 over Tibet Plateau and its surrounding areas.Then the radiosonde data is used to estimate the applicability of COSMIC RO data in determining the Tibet Plateau tropopause.And the seasonal variation of the tropopause over the Tibet Plateau is analyzed based on the WMO tropospheric height discrimination method.Finally the COSMIC RO data is used to discuss the water vapor distribution over the Tibet plateau in different seasons,and the differences between ECMWF analysis and COSMIC RO data in describing water vapor characteristics are studied.The results show that atmospheric refractivity,temperature and relative humidity derived from COSMIC GPS ROs are compared with those of the ECMWF analysis.It is found the COSMIC GPS RO refractivity observations are systematically greater than the refractivity calculated from ECMWF analyses in summer and autumn.The fractional refractivity bias over Tibet Plateau is larger than that over southwest monsoon area and plain with the value of 0.7%.In winter and spring the refractivity bias over Tibet Plateau is positive,while that over southwest monsoon area and plain is negative.Temperature and water vapor of GPS RO are derived from refractivity.The positive bias of refractivity is highly correlated with positive water vapor bias and negative temperature bias.The bias of relative humidity and temperature are 7% and 0.5k,respectively over Tibet Plateau.It is noted that relative humidity bias can reach 11% at the top of troposphere over southwest monsoon area.The lower tropospheric negative refractivity biases are related to the multi-path effect,and the positive biases in the middle and lower troposphere are the result of the influence of cloud.The occurrence of relative humidity bias near the tropopause is due to the inaccuracy of the ECMWF mode results.The tropospheric heights determined by COSMIC RO data and radiosonde data have a high linear correlation.The correlation coefficient is as high as 0.976,the mean deviation is 0.448 km,and the root mean square error is 0.672 km,proving that COSMIC RO data is suitable for the study of tropopause over the Tibetan Plateau.Overall the tropopause height shows the zonal distribution characteristics of low in the South and high in the north,which is the highest in summer and the lowest in winter.When the Tibet Plateau becomes a powerful heat source in summer,there is highest tropopause over the Plateau with the value of 17 km.Due to the coarse vertical resolution of ECMWF analysis data,the errors mainly occur near the tropopause when analyzing water vapor characteristics in cooperation with COSMIC occultation data.In spring,winter and autumn,moisture mainly concentrates over low latitudes.In the summer,under the joint action of deep convection caused by heat source,horizontal transport of monsoon and vertical elevation of South Asia high,high water vapor value areas appear over the QinghaiTibet Plateau,and water vapor exchange between the stratosphere and troposphere is active.