Proposing a Representative Scale for Aggregating Surface Energy Fluxes for the Qinghai-Tibet Plateau Based on Fine-Scale Modeling and Topographical Analysis

Lidar‐derived forest structural diversity (FSD) metrics—including measures of forest canopy height, vegetation arrangement, canopy cover (CC), structural complexity and leaf area and density—are increasingly used to describe forest structural characteristics and can be used to infer many ecosystem functions. Despite broad adoption, the importance of spatial resolution (grain and extent) over which these structural metrics are calculated remains largely unconsidered. Often researchers will quantify FSD at the spatial grain size of the process of interest without considering the scale dependency or statistical behaviour of the FSD metric employed. We investigated the appropriate scale of inference for eight lidar‐derived spatial metrics—CC, canopy relief ratio, foliar height diversity, leaf area index, mean and median canopy height, mean outer canopy height, and rugosity (R T )‐‐representing five FSD categories—canopy arrangement, CC, canopy height, leaf area and density, and canopy complexity. Optimal scale was determined using the representative elementary area (REA) concept whereby the REA is the smallest grain size representative of the extent. Structural metrics were calculated at increasing canopy spatial grain (from 5 to 1000 m) from aerial lidar data collected at nine different forested ecosystems including sub‐boreal, broadleaf temperate, needleleaf temperate, dry tropical, woodland and savanna systems, all sites are part of the National Ecological Observatory Network within the conterminous United States. To identify the REA of each FSD metric, we used changepoint analysis via segmented or piecewise regression which identifies significant changepoints for both the magnitude and variance of each metric. We find that using a spatial grain size between 25 and 75 m sufficiently captures the REA of CC, canopy arrangement, canopy leaf area and canopy complexity metrics across multiple forest types and a grain size of 30–150 m captures the REA of canopy height metrics. However, differences were evident among forest types with higher REA necessary to characterize CC in evergreen needleleaf forests, and canopy height in deciduous broadleaved forests. These findings indicate the appropriate range of spatial grain sizes from which inferences can be drawn from this set of FSD metrics, informing the use of lidar‐derived structural metrics for research and management applications.

As the “Third Pole of the World,” the Tibetan Plateau (TP) is an important thermal forcing to the South Asian summer monsoon (ASM) and even the global atmospheric circulation. In this paper, surface heat fluxes from the ERA-Interim reanalysis data during March–October of 1979–2016 in the TP and its surrounding areas are examined and analyzed. The results are as follows. (1) From March to May (before the ASM onset), the main body of the TP is dominated by sensible heat flux, which increases rapidly with high (low) values in the west (east), while the change of latent heat flux is small but it increases with time. (2) From June to August (after the ASM onset), sensible heat flux over the TP decreases, while latent heat flux increases rapidly with high (low) values in the east (west). (3) From September to October (after the ASM withdrawal), sensible and latent heat fluxes are comparable to each other in strength, again with high (low) sensible heat flux in the west (east). (4) During 1979–2016, surface sensible heat flux in the whole TP shows a slightly downward trend, while latent heat flux shows an increasing trend. Specifically, in the western TP, sensible (latent) heat flux shows a weak decreasing (an increasing) trend; while in the eastern TP, sensible (latent) heat flux decreases (increases obviously). These variations are consistent with the observed wanning and moistening in the TP region. The above results are useful for further analysis of the change of atmospheric heat sources and surface heat fluxes over the TP based on the data from the Third Tibetan Plateau Atmospheric Science Experiment (TIPEX-III).

Encompassing some of the major hotspots of biodiversity on Earth, large mountain systems have long held the attention of evolutionary biologists. The region of the Qinghai‐Tibet Plateau (QTP) is considered a biogeographic source for multiple colonization events into adjacent areas including the northern Palearctic. The faunal exchange between the QTP and adjacent regions could thus represent a one‐way street (“out of” the QTP). However, immigration into the QTP region has so far received only little attention, despite its potential to shape faunal and floral communities of the QTP. In this study, we investigated centers of origin and dispersal routes between the QTP, its forested margins and adjacent regions for five clades of alpine and montane birds of the passerine superfamily Passeroidea. We performed an ancestral area reconstruction using BioGeoBEARS and inferred a time‐calibrated backbone phylogeny for 279 taxa of Passeroidea. The oldest endemic species of the QTP was dated to the early Miocene (ca. 20 Ma). Several additional QTP endemics evolved in the mid to late Miocene (12–7 Ma). The inferred centers of origin and diversification for some of our target clades matched the “out of Tibet hypothesis’ or the “out of Himalayas hypothesis” for others they matched the “into Tibet hypothesis.” Three radiations included multiple independent Pleistocene colonization events to regions as distant as the Western Palearctic and the Nearctic. We conclude that faunal exchange between the QTP and adjacent regions was bidirectional through time, and the QTP region has thus harbored both centers of diversification and centers of immigration.

Jia, Q.; Zhou, L.; Yu, W.; Wang, X.; Wen, R., and Xie, Y., 2020. Surface energy flux changes and budget in a typical coastal reed wetland (Liaohe Delta, China). Journal of Coastal Research, 36(5), 1005–1012. Coconut Creek (Florida), ISSN 0749-0208.Coastal wetlands are located between the continents and oceans, and their surface energy flux changes and budget play an important role in regulating regional climate. Liaohe Delta is a typical coastal reed wetland in China. The latent heat, sensible heat, and net radiation flux of reed wetlands in Liaohe Delta were studied for four consecutive years (2012–2015) on the basis of the eddy correlation system. The average annual accumulation of latent heat flux was 1525.39 MJ m–2, with a large seasonal variation. The latent heat flux was positive, and the peak value did not exceed 320 W m–2 in the daytime. The latent heat flux increased with the increase of temperature each day. The annual average sensible heat flux was 550.96 MJ m–2, with the minimum sensible heat in June and July, and the daily average change peak did not exceed 160 W m–2. The annual net radiation accumulation was 2868.32 MJ m–2, and the net radiation in winter decreased. The maximum value of net radiation in summer reached 622 W m–2. Latent heat flux accounted for 41%–63% of solar radiation, followed by storage flux, and sensible heat flux had the lowest value. This typical reed wetland has a strong water vapor regulation ability. These results contribute to research on land surface processes in wetlands.

Interannual and seasonal variability in evapotranspiration and energy partitioning over the alpine riparian shrub Myricaria squamosa Desv. on Qinghai–Tibet Plateau

Studying the daily evolution of turbulent fluxes modulated by snowfall over the Tibetan Plateau (TP) is of great importance to understand the features of the change in the TP heat source/sink and its contribution to Asian atmospheric circulation and weather processes. However, the lack of data over the TP restricts the detailed studies. Based on observations from four sites of the Third TP Atmospheric Scientific Experiment, the process of surface energy balance impacted by snow is investigated. The results show that the surface albedo largely increases on the first day of snow and then slowly decreases. Correspondingly, the sensible heat (H) flux sharply decreases after snow and then gradually recovers to the original level during the following approximately 10 days. The latent heat (LE) flux becomes more active and stronger after snowfall and persists for a longer period than H, since the soil moisture may still contribute to a high LE after snowmelt. As the synergistic result of H and LE modulated by snow, the surface turbulent heating (i.e., the sum H and LE) of the TP decreases at the early period of snow events and then even enhances to a higher level after the snowmelt than before snow. Comparison analyses reveal that the impact of snow on the H and LE over the TP is much stronger than over similar latitude low‐altitude regions in North America and Europe, which may be partly attributed to the larger and more drastic change of the surface net solar radiation associated with snow processes in the TP. The ERA5 and CFS reanalysis data sets fail to reproduce the modulation of snow on the heat fluxes, which suggests that the physical schemes of the models should be further improved based on the observational analyses over TP. This study may help further understand the detailed physical processes of modulation of snow events on Asian weather processes during winter and is also conducive to the improvement of surface parameterization schemes of models.

The Tibetan Plateau (TP) has undergone extreme changes in climatic and land surface conditions that are due to a warming climate and land-cover changes. We examined the change in vegetation dynamics from 1982 to 2015 and explored the associations of vegetation with atmospheric variables over the alpine grasslands in the western TP during May as an early growing season. The linear regression analysis of area-averaged normalized difference vegetation index (NDVI) over the western TP in May demonstrated a 7.5% decrease of NDVI during the period from 1982 to 2015, an increase of NDVI by 11.3% from 1982 to 1998, and a decrease of NDVI by 14.5% from 1999 to 2015. The significantly changed NDVI in the western TP could result in the substantial changes in surface energy balances as shown in the surface climatic variables of albedo, net solar radiation, sensible heat flux, latent heat fluxes, and 2-m temperature. The land and atmosphere associations were not confined to the surface but also extended into the upper-level atmosphere up to the 300-hPa level as indicated by the significant positive associations between NDVI and temperatures in both air temperature and equivalent temperature, resulting in more than a 1-K increase with NDVI. Therefore, we concluded that the increasing or decreasing vegetation cover in the western TP during May can respectively increase or decrease the temperatures near the surface and upper atmosphere through a positive physical linkage among the vegetation cover, surface energy fluxes, and temperatures. The positive energy processes of vegetation with temperature could further amplify the variations of temperature and thus water availability. Significance Statement The Tibetan Plateau (TP) is an important landmass that plays a significant role in both regional and global climates. This study aims to examine the vegetation change in the TP during May as an early growing season to examine the changes in the near-surface and upper-level climatic conditions associated with vegetation change and to identify the plausible physical processes of the vegetation effects on atmosphere. The satellite-derived vegetation index showed a 7.5% decrease from 1982 to 2015 in the western TP during May. This study identified the positive associations of vegetation activity with temperature and proposed a positive energy process for land–atmosphere interactions over the alpine grasslands in the western region of TP during the transition period from winter to spring.

This study aimed to evaluate the applicability of the land‐surface model CLM 4.5 in the southeastern Tibetan Plateau by utilizing field observation data from Kabu and Pailong sites. Furthermore, the study investigated the impact of night‐time precipitation changes on radiation, surface energy fluxes and soil hydrothermal characteristics in this region. Results revealed that CLM 4.5 accurately simulated upward shortwave/longwave radiation, surface energy fluxes and soil temperature in this region. However, the simulation of soil moisture exhibited a slight fluctuation of the simulated value during precipitation, resulting in poorer performance during heavy precipitation and a failure to simulate the rapid change of latent heat flux and soil moisture. Sensitivity experiments demonstrated that night‐time precipitation significantly influenced upward shortwave/longwave radiation, energy budge and soil hydrothermal processes, with each element changing considerably with changes in precipitation. Surface energy fluxes and hydrothermal characteristics were found to be more sensitive to increased precipitation than decreased precipitation. Additionally, soil moisture was found to play a crucial role in the impact of precipitation changes on surface energy fluxes and hydrothermal characteristics. Furthermore, the study also found that changes in precipitation had a more pronounced effect on relatively dry areas and seasons than already extremely wet areas and seasons. Finally, the variation in soil moisture was found to be restricted by the original soil moisture. In conclusion, this study highlighted the significant influence of night‐time precipitation on surface hydrothermal characteristics and surface energy budget in the southeastern Tibetan Plateau.

This paper addresses interannual and seasonal variability in the thermal regime and surface energy fluxes in central Great Slave Lake during three contiguous open-water periods, two of which overlap the Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) water year. The specific objectives are to compare the air temperature regime in the midlake to coastal zones, detail patterns of air and water temperatures and atmospheric stability in the central lake, assess the role of the radiation balance in driving the sensible and latent heat fluxes on a daily and seasonal basis, quantify magnitudes and rates of the sensible and latent heat fluxes and evaporation, and present a comprehensive picture of the seasonal and interannual thermal and energy regimes, their variability, and their most important controls. Atmospheric and lake thermal regimes are closely linked. Temperature differences between midlake and the northern shore follow a seasonal linear change from 6°C colder midlake in June, to 6°C warmer in November–December. These differences are a response to the surface energy budget of the lake. The surface radiation balance, and sensible and latent heat fluxes are not related on a day-to-day basis. Rather, from final lake ice melt in mid-June through to mid- to late August, the surface waters strongly absorb solar radiation. A stable atmosphere dominates this period, the latent heat flux is small and directed upward, and the sensible heat flux is small and directed downward into the lake. During this period, the net solar radiation is largely used in heating the lake. From mid- to late August to freeze up in December to early January, the absorbed solar radiation is small, the atmosphere over the lake becomes increasingly unstable, and the sensible and latent heat fluxes are directed into the atmosphere and grow in magnitude into the winter season. Comparing the period of stable atmospheric conditions with the period of unstable conditions, net radiation is 6 times larger during the period of stable atmosphere and the combined latent and sensible heat fluxes are 9 times larger during the unstable period. From 85% to 90% of total evaporation occurs after mid-August, and evaporation rates increase continuously as the season progresses. This rate of increase varies from year to year. The time of final ice melt exerts the largest single control on the seasonal thermal and energy regimes of this large northern lake.

Threat of air pollution in the cleanest plateau

The relationship between sensible and latent heat flux and diurnal variation in soil surface temperature and moisture under four freeze/thaw soil conditions was investigated using observed soil temperature and moisture and simulated sensible and latent heat flux. The diurnal range of latent heat flux had a similar temporal change pattern as that of unfrozen soil water at depths of 0–3 cm during the freezing stage. Also, there was a better relationship with the diurnal range of unfrozen soil water at depths of 3–6 cm during the thawing stage. Diurnal variation in latent heat flux was significant and depended mostly on solar radiation during the completely thawed stage. However, while diurnal variation in solar radiation during the completely frozen stage was significant, for latent heat flux it was quite weak due to low unfrozen soil water content. Thus, diurnal variation in latent heat flux depended mostly on unfrozen soil water content during this stage. During the freezing and thawing stages, diurnal variation in latent heat flux was also significant and depended mostly on diurnal variation in unfrozen soil water content. However, the impacts of air temperature change from solar radiation on latent heat flux could not be ignored.

ABSTRACTLittle is known about the combined effects of vegetation and topography on hillslope water table dynamics. In forested headwater catchments, complex terrain and vegetation intersect to impose large spatial and temporal variability in the vertical and lateral redistribution of water from hillslopes to streams. Here, we demonstrate, using empirical data from the Northern Rocky Mountains, that vegetation interacts with landscape topography to influence hillslope–riparian–stream hydrologic connectivity. We compared a measured relationship between hillslope contributing area and hydrologic connectivity during the growing season to LiDAR‐derived vegetation characteristics and found that two behavioural regimes emerged. Among some hillslopes, hydrologic connectivity decreased as vegetation density increased, demonstrating that growing season hydrologic connectivity is subject to the balance between evapotranspiration and lateral redistribution of soil water. Among other hillslopes, hydrologic connectivity increased as vegetation density increased. For the latter set of hillslopes, hydrologic connectivity cannot be explained by topography and vegetation alone. When we compared joint distributions of vegetation density and modelled solar irradiance between the two regimes as another indicator of evapotranspiration, we found that conditions were generally more favourable for higher transpiration on hillslopes where hydrologic connectivity decreased as vegetation density increased than on hillslopes where the opposite behaviour was observed. Our results demonstrate not only the importance of vegetation heterogeneity for hillslope–riparian–stream connectivity but also the importance of other spatially distributed variables such as energy availability when considering the influence of topography on hydrological processes. Copyright © 2013 John Wiley & Sons, Ltd.

Significant temporal and spatial variability in soil moisture (SM) is observed during the warm season in China and its surrounding regions. Because of the existence of two different evapotranspiration regimes, i.e., soil moisture-limited and energy-limited, averaging the land–atmosphere (L–A) coupling strength for all soil wetness scenarios may result in the loss of coupling signals. This study examines the daytime-only L–A interactions under different soil moisture conditions, by using two-legged metrics in the warm season from May to September 1981–2020, partitioning the interactions between SM and latent heat flux (SM–LH, the land leg) from the interactions between latent heat flux and the lifting condensation level (LH–LCL, the atmospheric leg). The statistical results reveal large regional differences in warm-season daytime L–A feedback in China and its surrounding areas. As the soil becomes wetter, the positive SM–LH coupling strength increases in arid regions (e.g., northwest China, Hetao, and the Great Indian Desert) and the positive feedback shifts to the negative one in semi-arid/semi-humid regions (northeast and northern China). The negative LH–LCL coupling is most pronounced in wet soil months in arid regions, while the opposite is true for the Tibetan Plateau. In terms of intraseasonal variation, the large variability of SM in north China, the Tibetan Plateau, and India due to the influence of the summer monsoon leads to the sign change in the land segment coupling index, comparing pre- and post-monsoon periods. To further examine the impact of SM anomalies on L–A coupling and to explore evapotranspiration regimes in the North China Plain, four sets of sensitivity experiments with different soil moisture levels over a period of 10 years were conducted. Under relatively dry soil conditions, evapotranspiration is dominated by the soil moisture-limited regime with positive L–A coupling, regardless of external moisture inflow. The critical soil moisture value separating a soil moisture-limited and an energy-limited regime lies between 0.24 m3/m3 and 0.29 m3/m3. Stronger positive feedback under negative soil moisture anomalies may increase the risk of drought in the North China Plain.

Simulation of sensible and latent heat fluxes on the Tibetan Plateau from 1981 to 2018

34 Study of land surface heat fluxes and water cycle over the Tibetan plateau