Improved Land Use and Leaf Area Index Enhances WRF-3DVAR Satellite Radiance Assimilation： A Case Study Focusing on Rainfall Simulation in the Shule River Basin during July 2013

The application of satellite radiance assimilation can improve the simulation of precipitation by numerical weather prediction models. However, substantial quantities of satellite data, especially those derived from low-level（surface-sensitive）channels, are rejected for use because of the difficulty in realistically modeling land surface emissivity and energy budgets.Here, we used an improved land use and leaf area index（LAI） dataset in the WRF-3 DVAR assimilation system to explore the benefit of using improved quality of land surface information to improve rainfall simulation for the Shule River Basin in the northeastern Tibetan Plateau as a case study. The results for July 2013 show that, for low-level channels（e.g., channel 3）,the underestimation of brightness temperature in the original simulation was largely removed by more realistic land surface information. In addition, more satellite data could be utilized in the assimilation because the realistic land use and LAI data allowed more satellite radiance data to pass the deviation test and get used by the assimilation, which resulted in improved initial driving fields and better simulation in terms of temperature, relative humidity, vertical convection, and cumulative precipitation.

增温对青藏高原高寒草甸3种植物氮磷化学计量与重吸收的影响

全球气候变化将对生态系统造成重要影响,尤其是那些对气候变化极为敏感的高寒草甸生态系统。然而,全球变暖对高寒草甸植物氮磷化学计量特征和重吸收作用的影响仍不清楚。为了研究青藏高原高寒草甸植物的氮磷化学计量特征和重吸收作用,本研究采用开顶箱(OTCs)进行了3年的人工增温实验。选择3种优势物种和4种不同高度的OTC(0.4、0.6、0.8、1 m),设置4种增温处理(全年增温、冬季增温、夏秋冬季增温和春夏秋季增温)。结果表明,在不同增温方法下,土壤温度随OTC高度的增加而显著升高。随着温度的上升,高山嵩草(Kobresia pygmaea)的氮限制增加,矮生嵩草(K.humilis)的磷限制增加,而钉柱委陵菜(Potentilla saundersiana)的氮磷限制对温度的增加不敏感。在温度升高的情况下,氮磷重吸收效率比(NRE:PRE)与氮磷比(N:P)都表现出相同的变化趋势。3种物种的化学计量特征对温度升高的响应存在差异,显示出氮、磷限制对温度变化的反应是多方面的,而且草地生态系统可能具备一定程度的自我调节能力。上述结果表明,全球气候变暖情景下,仅依赖单一优势物种的化学计量指标来代表整个生态系统的氮、磷限制并不准确,使用N:P比值来反映营养限制可能会产生误导。

Estimation of aboveground biomass using in situ hyperspectral measurements in five major grassland ecosystems on the Tibetan Plateau

Aims There are numerous grassland ecosystem types on the Tibetan Plateau.These include the alpine meadow and steppe and degraded alpine meadow and steppe.This study aimed at developing a method to estimate aboveground biomass(AGB)for these grasslands from hyperspectral data and to explore the feasibility of applying air/satellite-borne remote sensing techniques to AGB estimation at larger scales.Methods We carried out a field survey to collect hyperspectral reflectance and AGB for five major grassland ecosystems on the Tibetan Plateau and calculated seven narrow-band vegetation indices and the vegetation index based on universal pattern decomposition(VIUPD)from the spectra to estimate AGB.First,we investigated correlations between AGB and each of these vegetation indices to identify the best estimator of AGB for each ecosystem type.Next,we estimated AGB for the five pooled ecosystem types by developing models containing dummy variables.At last,we compared the predictions of simple regression models and the models containing dummy variables to seek an ecosystem type-independent model to improve prediction of AGB for these various grassland ecosystems from hyperspectral measurements.Important findings When we considered each ecosystem type separately,all eight vegetation indices provided good estimates of AGB,with the best predictor of AGB varying among different ecosystems.When AGB of all the five ecosystems was estimated together using a simple linear model,VIUPD showed the lowest prediction error among the eight vegetation indices.The regression models containing dummy variables predicted AGB with higher accuracy than the simple models,which could be attributed to the dummy variables accounting for the effects of ecosystem type on the relationship between AGB and vegetation index(VI).These results suggest that VIUPD is the best predictor of AGB among simple regression models.Moreover,both VIUPD and the soil-adjusted VI could provide accurate estimates of AGB with dummy variables integrated in regression models.Therefore,ground-based hyperspectral measurements are useful for estimating AGB,which indicates the potential of applying satellite/airborne remote sensing techniques to AGB estimation of these grasslands on the Tibetan Plateau.

Spatial variations in responses of vegetation autumn phenology to climate change on the tibetan Plateau

Aims Information about changes in the start and end of the vegetation growing season(SOS and EOS)is crucial for assessing ecosystem responses to climate change because of the high sensitivity of both to climate and their extensive influence on ecological processes in temperate and cold regions.climatic warming substantially advanced SOS on the tibetan Plateau from 1982 to 2011.However,it is unclear why EOS showed little delay despite increasing tem-perature over this period.Methods We used multiple methods to determine EOS from the satellite-observed normalized-difference vegetation index and investigated the relationships between EOS and its potential drivers on the tibetan Plateau over 1982-2011.Important findings We found a slight but non-significant delay in regionally averaged EOS of 0.7 day decade−1(P=0.18)and a widespread but weak delaying trend across the Plateau over this period.the inter-annual variations in regionally averaged EOS were driven mainly by pre-season temperature(partial R=0.62,P<0.01),and precipitation and insolation showed weak impact on EOS(P>0.10).Pre-season warming delayed EOS mainly in the eastern half and north-western area of the plateau.In the south-west,EOS was significantly and positively related to SOS,suggesting potentially indirect effects of winter weather conditions on the following autumn’s phenology through regulation of spring phenology.EOS was more strongly related with pre-season temperature in colder and wetter areas,reflecting vegetation adaptation to local climate.Interestingly,pre-season temperature had weaker delaying effects on EOS for vegeta-tion with a shorter growing season,for which SOS had a stronger control on inter-annual variations in EOS than for vegetation with a longer growing season.this indicates that shorter-season tibetan Plateau vegetation may have lower plasticity in adjusting the length of its growing season,whenever it begins,and that climate change is more likely to shift the growing season than extend it for that vegetation.

Diurnal and seasonal variations in light-use efficiency in an alpine meadow ecosystem:causes and implications for remote sensing

Aims Light-use efficiency(LUE)is an important tool for scaling up local CO_(2)flux(F_(CO_(2)))tower observations to regional and global carbon dynamics.Using a data set including F_(CO_(2))and environmental factors obtained from an alpine meadow on the Tibetan Plateau,we examined both diurnal and seasonal changes in LUE and the environmental factors controlling these changes.Our objectives were to(i)characterize the diurnal and daily variability of LUE in an alpine meadow,(ii)clarify the causes of this variability,and(iii)explore the possibility of applying the LUE approach to this alpine meadow by examining the relationship between daily LUE and hourly LUE at satellite visiting times.Methods First,we obtained the LUE—the ratio of the gross primary production(GPP)to the absorbed photosynthetically active radiation(APAR)—from the flux tower and meteorological observations.We then characterized the patterns of diurnal and seasonal changes in LUE,explored the environmental controls on LUE using univariate regression analyses and evaluated the effects of diffuse radiation on LUE by assigning weights through a linear programming method to beam photosynthetically active radiation(PAR)and diffuse PAR,which were separated from meteorological observations using an existing method.Finally,we examined the relationships between noontime hourly LUE and daily LUE and those between adjusted noontime hourly and daily LUE because satellites visit the site only once or twice a day,near noon.Important Findings The results showed that(i)the LUE of the alpine meadow generally followed the diurnal and seasonal patterns of solar radiation but fluctuated with changes in cloud cover.(ii)The fraction of diffuse light played a dominant role in LUE variation.Daily minimum temperature and vapor pressure deficit also affected LUE variation.(iii)The adjusted APAR,defined as the weighted linear sum of diffuse APAR and beam APAR,was linearly correlated with GPP on different temporal scales.(iv)Midday adjusted LUE was closely related to daily adjusted LUE,regardless of the cloud cover.The results indicated the importance of considering radiation direction when developing LUE-based GPP-estimating models.

喜马拉雅山区高山冰缘植物并不能从变暖中受益

普遍认为变暖有利于高山植被生态系统,并已经导致冰缘植物向高海拔的扩张.珠峰山谷金字塔气象站(海拔5050 m)1994~2018的气象记录显示,高海拔山区呈显著的暖干化趋势.然而,我们依然不清楚暖干化是否对冰缘植被更新和分布造成显著影响?本研究在喜马拉雅山中段3个山谷建立了8块(30 m×120 m)滇藏方枝柏(Juniperus indica)和高山柏(Juniperus squamata)灌木线样地(海拔介于4344~5074 m之间).样地调查揭示,自20世纪90年代之前所有样地种群更新呈增加的趋势,然而90年代以来更新持续下降.种群更新与春季、夏季温度之间呈现显著的负相关关系,暗示了暖干化加剧了水分胁迫,并限制了种群的更新.另外,近60年来所有灌木线位置稳定.近30年遥感数据也证实,研究区冰缘植被无变绿趋势,甚至出现2%-6%的植被生长减弱现象.因此,进一步研究应该探讨冰缘植被生长减弱对喜马拉雅山区水循环和生物多样性的影响.

Spatial sampling inconsistency leads to differences in phenological sensitivity to warming between natural and experiment sites

Plant spring phenology is receiving increasing attention owing to the recognition of its high sensitivity to ongoing climatic warming [1]. Changes in plant spring phenology can substantially influence a wide range of ecosystem structure and functions, which can not only affect human-beings but also in turn affect climate [2]. Warming experiments have been widely conducted to help understand, and thus predict plant phenological response to climate. Most of these experiment-based studies have focused on reporting the signs and magn让udes of phenological responses, and a few have included temperature sensitivity (phenological shifts per unit temperature change). However, applying the outputs of these experiments to predict future phenological response to climate change remains challenging.

Estimation of aboveground biomass using in situ hyperspectral measurements in five major grassland ecosystems on the Tibetan Plateau

In the the original published figure,the‘Achnatherum grassland’chart showed six lines when there should have been three.Please see below the corrected figure.