Biophysical Regulation of Carbon Flux in Different Rainfall Regime in a Northern Tibetan Alpine Meadow

Inter-annual variability in total precipitation can lead to significant changes in carbon flux.In this study,we used the eddy covariance（EC） technique to measure the net CO_2 ecosystem exchange（NEE） of an alpine meadow in the northern Tibetan Plateau.In 2005 the meadow had precipitation of 489.9 mm and in 2006 precipitation of 241.1 mm,which,respectively,represent normal and dry years as compared to the mean annual precipitation of 476 mm.The EC measured NEE was 87.70 g C m^（-2） yr^（-1） in 2006 and-2.35 g C m^（-2） yr^（-1） in 2005.Therefore,the grassland was carbon neutral to the atmosphere in the normal year,while it was a carbon source in the dry year,indicating this ecosystem will become a CO_2 source if climate warming results in more drought conditions.The drought conditions in the dry year limited gross ecosystem CO_2 exchange（GEE）,leaf area index（LAI） and the duration of ecosystem carbon uptake.During the peak of growing season the maximum daily rate of NEE and Pmax and a were approximately 30%-50% of those of the normal year.GEE and NEE were strongly related to photosynthetically active radiation（PAR） on half-hourly scale,but this relationship was confounded by air temperature（Ta）,soil water content（SWC） and vapor pressure deficit（VPD）.The absolute values of NEE declined with higher Ta,higher VPD and lower SWC conditions.Beyond the appropriate range of PAR,high solar radiation exacerbated soil water conditions and thus reduced daytime NEE.Optimal T_a and VPD for maximum daytime NEE were 12.7℃ and 0.42 KPa respectively,and the absolute values of NEE increased with SWC.Variation in LAI explained around 77% of the change in GEE and NEE.Variations in R_e were mainly controlled by soil temperature（T_s）,whereas soil water content regulated the responses of R_e to T_s.

Comparison of Heavy Rainfall Events Originated from Different Directions of Beijing City

Strong rainfall events originated from the northeast(NE)and southwest(SW)directions of the plain area of Beijing City(BJP)over 8 recent warm seasons(May–September of 2009–2016)were analyzed by using hourly merged rainfall,satellite brightness temperature,and the fifth-generation ECMWF reanalysis(ERA5)data.Such heavy regional rainfall events(RREs)with different origins present quite different features in both the precipitation itself and its corresponding circulations.The heavy RREs originated from the SW occur more frequently in the flood season of North China(July and August),and the peak time of rainfall occurrences is in the early morning.They are linked with stronger large-scale circulation forcing,compared with the NE-originated events.Meanwhile,the ratio of heavy rainfall to the total rainfall in SW-originated events,the mean spatial coverage of rainfall,and associated convective index,are also larger,for the SW events.The heavy RREs from the NE occur more frequently in June and July(before the traditional flood season),with a more apparent afternoon peak.They exhibit stronger convective features,with higher maximum convective index values,but the large-scale forcing is weaker at the hour of onset.These features of the RREs from different directions of Beijing City and associated precursor circulation signals help better forecast RREs over the BJP.