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We examined climate trends in the northern North American Great Plains (NNAGP) from 1970 to 2015, a period that aligns with widespread land-use changes in this globally important agricultural region. Trends were calculated from the Climatic Research Unit (CRU) and other climate datasets using a linear regression model that accounts for temporal autocorrelation. The NNAGP warmed on an annual basis, with the largest change occurring in winter (DJF) at 0.4°C decade−1. January in particular warmed at nearly 0.9°C decade−1. The NNAGP cooled by −0.18°C decade−1 during May and June, nearly the opposite of global warming trends during the study period. The atmospheric vapor pressure deficit (VPD), which can limit crop growth, decreased in excess of −0.4 hPa decade−1 during climatological summer in the southeastern part of the study domain. Precipitation P increased in the eastern portion of the NNAGP during all seasons except fall and increased during May and June in excess of 8 mm decade−1. Climate trends in the NNAGP largely followed global trends except during the early warm season (May and June) during which 2-m air temperature Tair became cooler, VPD lower, and P greater across large parts of the study region. These changes are consistent with observed agricultural intensification during the study period, namely the reduction of summer fallow and expansion of agricultural land use. Global climate model simulations indicate that observed Tair trends cannot be explained by natural climate variability. However, further climate attribution experiments are necessary to understand if observed changes are caused by increased agricultural intensity or other factors.

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Feedbacks between atmospheric processes like precipitation and land surface fluxes including evapotranspiration are difficult to observe, but critical for understanding the role of the land surface in the Earth System. To quantify global surface-atmosphere feedbacks we use results of a process network (PN) applied to 251 eddy covariance sites from the LaThuile database to train a neural network across the global terrestrial surface. There is a strong land–atmosphere coupling between latent (LE) and sensible heat flux (H) and precipitation (P) during summer months in temperate regions, and between H and P during winter, whereas tropical rainforests show little coupling seasonality. Savanna, shrubland, and other semi-arid ecosystems exhibit strong responses in their coupling behavior based on water availability. Feedback couplings from surface fluxes to P peaks at aridity (P/potential evapotranspiration ETp) values near unity, whereas coupling with respect to clouds, inferred from reduced global radiation, increases as P/ETp approaches zero. Spatial patterns in feedback coupling strength are related to climatic zone and biome type. Information flow statistics highlight hotspots of (1) persistent land–atmosphere coupling in sub-Saharan Africa, (2) boreal summer coupling in the central and southwestern US, Brazil, and the Congo basin and (3) in the southern Andes, South Africa and Australia during austral summer. Our data-driven approach to quantifying land atmosphere coupling strength that leverages the global FLUXNET database and information flow statistics provides a basis for verification of feedback interactions in general circulation models and for predicting locations where land cover change will feedback to climate or weather. “Big data” methods reveal robust hotspots of land–atmosphere coupling. Sparse observations and inadequate analytical tools have hindered our understanding of land–atmosphere feedbacks, including the exchange of energy, water, and CO2. Emergent methods such as machine learning, however, offer new opportunities, as Tobias Gerken from Montana State University, USA, and colleagues, demonstrate. A “big data” approach is adopted to characterise the spatial and temporal variability of land–atmosphere coupling without a priori assumptions: information flows are computed from 251 FLUXNET sites which are subsequently used to train a neural network. Distinct regional differences in the magnitude of land–atmosphere feedbacks are found, related to climatic zone and biome type; coupling in semi-arid ecosystems, for example, are strongly related to seasonal water availability. Complementing model studies with such empirical approaches may assist in quantifying climate change impacts on ecosystem services.

Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO2) concentrations, but processes driving the IAV of net ecosystem CO2 exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long‐term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water‐limited ecosystems versus temperature‐ and radiation‐limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water‐limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature‐ and radiation‐limited ecosystems. The Lund‐Potsdam‐Jena (LPJ) model and the Multi‐scale Synthesis and Terrestrial Model Inter‐comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface.

As human encroachment increases in the Amazon rain forest, it is important to determine how anthropogenic emissions of reactive gases affect regional atmospheric chemistry. In the present study, we investigate the extent to which urban air plumes modify the levels of ozone (O3) and nitrogen oxides (NOX) in the downwind rain forest. The median mixing ratios of the background O3, NOX, and NOX oxidation products (NOZ) were 20 (15 parts per billion on a volume basis, ppbv), 0.6 ppbv (0.6 ppbv), and 1.0 ppbv (0.5 ppbv) during the dry (wet) season at the study site. Compared to the background environment, air plumes from the city of Manaus had enhanced median mixing ratios for O3 and NOZ by 30–50% and 40–90%, respectively. However, the enhancements of NOX in the air plumes were less than 20%, indicating that the majority of NOX was chemically converted to O3 and NOZ during transport. Results from a photochemical model showed that an injection of 8 ppbv of NOX into the rain forest can cause up to 260% and 150% increases in O3 and hydroxyl radical (OH) levels compared to the background conditions, indicating the likely extent that NOX can modify the air quality and oxidative capacity in the Amazon rain forest. Slopes of the O3-temperature linear relationships increased with NOX levels from 3.7 to 6.5 ppbv per degree Kelvin during the dry season and 1.7–5.5 ppbv per degree Kelvin during the wet season. Average rates of change of the slope with respect to NOZ were approximately 1.8 and 2.3 times higher than those with respect to NOX for the dry and wet seasons. One key conclusion of this study is that NOZ substantially contributed to the O3 formation in response to temperature under enhanced NOX conditions in the forested environment.

With 450,000 km2 Kobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000–6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf. (Less)

The vegetated canopy plays a key role in regulating the surface fluxes and, therefore, the global energy, water and carbon cycles. In particular, vulnerable ecosystems like the Amazonia basin can be very sensitive to changes in vegetation that exert subsequent shifts in the partition of the energy, water and carbon in and above the canopy. Despite this relevance, most 3D atmospheric models represent the vegetated canopy as a flat 2D layer with, at most, a rough imitation of its effect in the atmospheric boundary layer through a modified roughness length. Thus, the representations often describe quite crudely the surface fluxes. In this work, particular emphasis is placed in the biophysical processes that take place within the canopy and its impact above. Our approach is to represent the coupling of the flow between the canopy and the atmosphere including the following processes: radiative transfer, photosynthesis, soil evaporation and CO2 respiration, combined with the mostly explicit atmospheric turbulence within and above the canopy. To this end, we implemented in LES a detailed multi-layer canopy model that solves the leaf energy balance for sunlit and shaded leaves independently, regulating the exchange of heat, moisture and carbon between the leaves and the air around. This allows us to connect the mechanistically represented processes occurring at the leaf level and strongly regulated by the transfer of diffuse and direct radiation within the canopy to the turbulent mixing explicitly resolved at the meter scale.We test and validate this combined photosynthesis-turbulence-canopy model by simulating a representative clear day transitioning to shallow cumulus. We based our evaluation on observations by the GoAmazon2014/5 campaign in Brazil in 2014. More specifically, we systematically validate the in-canopy radiation profiles; sources, sinks and turbulent fluxes of moisture, heat and CO2, and main state variables within the canopy, and also study the effects of these in the air above. Preliminary results show an encouraging satisfactory match to the observed evolution of the profiles. As a first exploration and demonstration of the capabilities of the model, we test the effects of a coarser in-canopy resolution, a different radiation scheme and the use of a more simple 2D canopy representation.

The Atmospheric Carbon and Transport (ACT)-America NASA Earth Venture Suborbital Mission set out to improve regional atmospheric greenhouse gas (GHG) inversions by exploring the intersection of the strong GHG fluxes and vigorous atmospheric transport that occurs within the midlatitudes. Two research aircraft instrumented with remote and in situ sensors to measure GHG mole fractions, associated trace gases, and atmospheric state variables collected 1,140.7 flight hours of research data, distributed across 305 individual aircraft sorties, coordinated within 121 research flight days, and spanning five 6-week seasonal flight campaigns in the central and eastern United States. Flights sampled 31 synoptic sequences, including fair-weather and frontal conditions, at altitudes ranging from the atmospheric boundary layer to the upper free troposphere. The observations were complemented with global and regional GHG flux and transport model ensembles. We found that midlatitude weather systems contain large spatial gradients in GHG mole fractions, in patterns that were consistent as a function of season and altitude. We attribute these patterns to a combination of regional terrestrial fluxes and inflow from the continental boundaries. These observations, when segregated according to altitude and air mass, provide a variety of quantitative insights into the realism of regional CO2 and CH4 fluxes and atmospheric GHG transport realizations. The ACT-America dataset and ensemble modeling methods provide benchmarks for the development of atmospheric inversion systems. As global and regional atmospheric inversions incorporate ACT-America’s findings and methods, we anticipate these systems will produce increasingly accurate and precise subcontinental GHG flux estimates.

While large-scale floods directly impact human lives and infrastructures, they also profoundly impact agricultural productivity. New satellite observations of vegetation activity and atmospheric CO2 offer the opportunity to quantify the effects of such extreme events on cropland carbon sequestration, which are important for mitigation strategies. Widespread flooding during spring and early summer 2019 delayed crop planting across the U.S. Midwest. As a result, satellite observations of solar-induced chlorophyll fluorescence (SIF) from TROPOspheric Monitoring Instrument (TROPOMI) and Orbiting Carbon Observatory (OCO-2) reveal a shift of 16 days in the seasonal cycle of photosynthetic activity relative to 2018, along with a 15% lower peak photosynthesis. We estimate the 2019 anomaly to have led to a reduction of -0.21 PgC in gross primary production (GPP) in June and July, partially compensated in August and September (+0.14 PgC). The extension of the 2019 growing season into late September is likely to have benefited from increased water availability and late-season temperature. Ultimately, this change is predicted to reduce the crop yield over most of the midwest Corn/Soy belt by ~15%. Using an atmospheric transport model, we show that a decline of ~0.1 PgC in the net carbon uptake during June and July is consistent with observed CO2 enhancements from Atmospheric Carbon and Transport - America (ACT-America) aircraft and OCO-2. This study quantifies the impact of floods on cropland productivity and demonstrates the potential of combining SIF with atmospheric CO2 observations to monitor regional carbon flux anomalies.

While large‐scale floods directly impact human lives and infrastructures, they also profoundly impact agricultural productivity. New satellite observations of vegetation activity and atmospheric CO₂ offer the opportunity to quantify the effects of such extreme events on cropland carbon sequestration. Widespread flooding during spring and early summer 2019 induced conditions that delayed crop planting across the U.S. Midwest. As a result, satellite observations of solar‐induced chlorophyll fluorescence from TROPOspheric Monitoring Instrument and Orbiting Carbon Observatory reveal a 16‐day shift in the seasonal cycle of photosynthesis relative to 2018, along with a 15% lower peak value. We estimate a reduction of 0.21 PgC in cropland gross primary productivity in June and July, partially compensated in August and September (+0.14 PgC). The extension of the 2019 growing season into late September is likely to have benefited from increased water availability and late‐season temperature. Ultimately, this change is predicted to reduce the crop productivity in the Midwest Corn/Soy belt by ~15% compared to 2018. Using an atmospheric transport model, we show that a decline of ~0.1 PgC in the net carbon uptake during June and July is consistent with observed CO₂ enhancements of up to 10 ppm in the midday boundary layer from Atmospheric Carbon and Transport‐America aircraft and over 3 ppm in column‐averaged dry‐air mole fractions from Orbiting Carbon Observatory. This study quantifies the impact of floods on cropland productivity and demonstrates the potential of combining solar‐induced chlorophyll fluorescence with atmospheric CO₂ observations to monitor regional carbon flux anomalies.

American bison (Bison bison L.) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but their impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg year−1 of the estimated 9–15 Tg year−1 of pre-industrial enteric methane emissions, but few contemporary measurements have been made due to their mobile grazing habits and safety issues associated with direct measurements. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using eddy covariance. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = 0.0024 ± 0.042 μmol m−2 s−1) and were significantly greater than zero, 0.048 ± 0.082 μmol m−2 s−1 with a positively skewed distribution, when bison were present. We coupled an eddy covariance flux footprint analysis with bison location estimates from automated camera images to calculate a mean (median) methane flux of 38 μmol s−1 (22 μmol s−1) per animal, or 52 ± 14 g CH4 day−1 (31 g CH4 day−1), less than half of measured emission rates for range cattle. Emission estimates are subject to spatial uncertainty in bison location measurements and the flux footprint, but from our measurements there is no evidence that bison methane emissions exceed those from cattle. We caution however that our measurements were made during winter and that evening measurements of bison distributions were not possible using our approach. Annual measurements are ultimately necessary to determine the greenhouse gas burden of bison grazing systems. Eddy covariance is a promising technique for measuring ruminant methane emissions in conventional and alternate grazing systems and can be used to compare them going forward.

The explicit coupling at meter and second scales of vegetation's responses to the atmospheric‐boundary layer dynamics drives a dynamic heterogeneity that influences canopy‐top fluxes and cloud formation. Focusing on a representative day during the Amazonian dry season, we investigate the diurnal cycle of energy, moisture and carbon dioxide at the canopy top, and the transition from clear to cloudy conditions. To this end, we compare results from a large‐eddy simulation technique, a high‐resolution global weather model, and a complete observational data set collected during the GoAmazon14/15 campaign. The overall model‐observation comparisons of radiation and canopy‐top fluxes, turbulence, and cloud dynamics are very satisfactory, with all the modeled variables lying within the standard deviation of the monthly aggregated observations. Our analysis indicates that the timing of the change in the daylight carbon exchange, from a sink to a source, remains uncertain and is probably related to the stomata closure caused by the increase in vapor pressure deficit during the afternoon. We demonstrate quantitatively that heat and moisture transport from the subcloud layer into the cloud layer are misrepresented by the global model, yielding low values of specific humidity and thermal instability above the cloud base. Finally, the numerical simulations and observational data are adequate settings for benchmarking more comprehensive studies of plant responses, microphysics, and radiation.

The Amazon rain forest is a major global isoprene source, but little is known about its seasonal ambient concentration patterns. To investigate the environmental and phenological controls over isoprene seasonality, we measured isoprene mixing ratios, concurrent meteorological data, and leaf area indices from April 2014 to January 2015 above a rain forest in the central Amazon, Brazil. Daytime median isoprene mixing ratios varied throughout the year by a factor of two. The isoprene seasonal pattern was not solely driven by sunlight and temperature. Leaf age and quantity also contributed to the seasonal variations of isoprene concentrations, suggesting leaf phenology was a crucial variable needed to correctly estimate isoprene emissions. A zero-dimensional model incorporating the estimated emissions, atmospheric boundary layer dynamics, and air chemistry was used to assess the contributions of each process on the variability of isoprene. Surface deposition was an important sink mechanism and accounted for 78% of the nighttime loss of isoprene. Also, chemical reactions destroyed isoprene and during 6:00 to 18:00 h local time 56, 77, 69, and 69% of the emitted isoprene was chemically consumed in June, September, December, and January, respectively. Entrainment fluxes from the residual layer contributed 34% to the early-morning above-canopy isoprene mixing ratios. Sensitivity analysis showed that hydroxyl radical (HO) recycling and segregation of isoprene–HO played relatively lesser roles (up to 16%) in regulating ambient isoprene levels. Nitric oxide (NO) levels dominated isoprene chemical reaction pathways associated with consumption and production of HO under low-NO and high volatile organic compound (VOC) conditions. While surface deposition and oxidative processes altered isoprene levels, the relative importance of these factors varied seasonally with leaf phenology playing a more important role.

Tornadoes are among the most destructive natural events and occur most frequently in the United States. It is difficult to ascertain if the frequency of tornadoes in the U.S. is increasing because our ability to observe and report tornado occurrence has increased over time. Previous studies have demonstrated that tornado likelihood has shifted toward earlier dates across the south-central United States over the past seven decades, the region sometimes called “Tornado Alley”, if it can be assumed that seasonal observation effort has not shifted over time. It is unclear if such shifts in tornado seasonality have also occurred elsewhere, including the region of the southeastern United States where tornado likelihood has a bimodal annual distribution. We use circular methods to demonstrate that the date of observed peak tornado occurrence during the early tornado season has not changed in the past seven decades. However, the date of peak tornado occurrence during the later tornado season has shifted toward earlier dates by more than a week. The influence of tropical storms had no effect on changes in late-season tornado seasonality. The conclusions are robust with respect to whether tornado counts or tornado days are used as the response variable. Results demonstrate the ongoing need to encourage tornado preparedness in the southeastern U.S., where tornadoes tend to have a higher impact on humans, and to understand the mechanisms that underlie trends in tornado seasonality.

This work investigates the diurnal and seasonal behavior of the energy balance residual ( E ) that results from the observed difference between available energy and the turbulent fluxes of sensible heat ( H ) and latent heat ( LE ) at the FLUXNET BR-Ma2 site located in the Brazilian central Amazon rainforest. The behavior of E is analyzed by extending the eddy covariance averaging length from 30 min to 4 h and by applying an Information Flow Dynamical Process Network to diagnose processes and conditions affecting E across different seasons. Results show that the seasonal turbulent flux dynamics and the Bowen ratio are primarily driven by net radiation ( R n ), with substantial sub-seasonal variability. The Bowen ratio increased from 0.25 in April to 0.4 at the end of September. Extension of the averaging length from 0.5 (94.6% closure) to 4 h and thus inclusion of longer timescale eddies and mesoscale processes closes the energy balance and lead to an increase in the Bowen ratio, thus highlighting the importance of additional H to E. Information flow analysis reveals that the components of the energy balance explain between 25 and 40% of the total Shannon entropy with higher values during the wet season than the dry season. Dry season information flow from the buoyancy flux to E are 30–50% larger than that from H , indicating the potential importance of buoyancy fluxes to closing E . While the low closure highlights additional sources not captured in the flux data and random measurement errors contributing to E , the findings of the information flow and averaging length analysis are consistent with the impact of mesoscale circulations, which tend to transport more H than LE , on the lack of closure.

The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend’s “attached” eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k−1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k−1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

Land management impacts atmospheric boundary layer processes, and recent trends reducing the practice of summer fallow have led to increases in precipitation and decreases in temperature in the Canadian Prairie provinces during summer. It is unclear if such trends also impact the hydrometeorology of the adjacent U.S. northern Great Plains, parts of which have seen similar changes in land management. Here, MERRA-2 reanalysis data, eddy covariance observations, and a mixed-layer (ML) atmospheric modeling framework are combined to demonstrate that the likelihood of convectively preconditioned conditions has increased by approximately 10% since the mid-1980s and is now more sensitive to further decreases in the Bowen ratio (Bo) and maximum daily net radiation in northeastern Montana. Convective season Bo in the study area has decreased from approximately 2 to 1 from the 1980s until the present, largely due to simultaneous increases in latent heat flux and decreases in sensible heat flux, consistent with observed decreases of summer fallow and increases in cropping. Daily net radiation has not changed despite a significant decrease in May and June humidity lapse rates from the 1980s to present. Future research should determine the area of the U.S. Great Plains that has seen changes in the dynamics of the atmospheric boundary layer height and lifted condensation level and their crossings as a necessary condition for convective precipitation to occur and ascertain if ongoing changes in land management will lead to future changes in convective outcomes.

An analysis of the local atmospheric circulation in a northern Himalayan valley in the region of Mount Everest is presented. Data were collected using an automatic weather station over a one-year period in 2014. A ground-based wind profiler radar (WPR) and an in situ GPS radiosonde (RS) were also employed. This study focuses on the characteristics of afternoon strong wind events in the downstream of Rongbuk Valley. We found that: (1) The occurrence of the southwesterly wind during non-monsoon was in good consistency with high values of westerly wind at high levels over this region and confirmed to be driven by the strong westerly jet aloft. (2) The strong afternoon wind in monsoon season has a persistent southeasterly direction, which differs from the prevailing direction of the strong wind in non-monsoon. This flow was found to be independent of the wind aloft and was strongly seasonal, developing at Qomolangma Station (QOMS) when the subtropical jet stream had moved northward and was most stabl...

We present a theoretical model, based on a simple model of turbulent diffusion and first-order chemical kinetics, to determine air-parcel residence times and the out-of-canopy export of reactive gases emitted within forest canopies under neutral conditions. Theoretical predictions of the air-parcel residence time are compared to values derived from large-eddy simulation for a range of canopy architectures and turbulence levels under neutral stratification. Median air-parcel residence times range from a few sec in the upper canopy to approximately 30 min near the ground and the distribution of residence times is skewed towards longer times in the lower canopy. While the predicted probability density functions from the theoretical model and large-eddy simulation are in good agreement with each other, the theoretical model requires only information on canopy height and eddy diffusivities inside the canopy. The eddy-diffusivity model developed additionally requires the friction velocity at canopy top and a parametrized profile of the standard deviation of vertical velocity. The theoretical model of air-parcel residence times is extended to include first-order chemical reactions over a range of of Damkohler numbers (Da) characteristic of plant-emitted hydrocarbons. The resulting out-of-canopy export fractions range from near 1 for $$Da =10^{-3}$$ to less than 0.3 at $$Da = 10$$ . These results highlight the necessity for dense and tall forests to include the impacts of air-parcel residence times when calculating the out-of-canopy export fraction for reactive trace gases.

Evaporation (E) and transpiration (T) respond differently to ongoing changes in climate, atmospheric composition, and land use. Our ability to partition evapotranspiration (ET) into E and T is limited at the ecosystem scale, which renders the validation of satellite data and land surface models incomplete. Here, we review current progress in partitioning E and T, and provide a prospectus for how to improve theory and observations going forward. Recent advancements in analytical techniques provide additional opportunities for partitioning E and T at the ecosystem scale, but their assumptions have yet to be fully tested. Many approaches to partition E and T rely on the notion that plant canopy conductance and ecosystem water use efficiency (EWUE) exhibit optimal responses to atmospheric vapor pressure deficit (D). We use observations from 240 eddy covariance flux towers to demonstrate that optimal ecosystem response to D is a reasonable assumption, in agreement with recent studies, but the conditions under which this assumption holds require further analysis. Another critical assumption for many ET partitioning approaches is that ET can be approximated as T during ideal transpiring conditions, which has been challenged by observational studies. We demonstrate that T frequently exceeds 95 % of ET from some ecosystems, but other ecosystems do not appear to reach this value, which suggests that this assumption is ecosystem-dependent with implications for partitioning. It is important to further improve approaches for partitioning E and T, yet few multi-method comparisons have been undertaken to date. Advances in our understanding of carbon-water coupling at the stomatal, leaf, and canopy level open new perspectives on how to quantify T via its strong coupling with photosynthesis. Photosynthesis can be constrained at the ecosystem and global scales with emerging data sources including solar-induced fluorescence, carbonyl sulfide flux measurements, thermography, and more. Such comparisons would improve our mechanistic understanding of ecosystem water flux and provide the observations necessary to validate remote sensing algorithms and land surface models to understand the changing global water cycle.

We describe the salient features of a field study whose goals are to quantify the vertical distribution of plant-emitted hydrocarbons and their contribution to aerosol and cloud condensation nuclei production above a central Amazonian rain forest. Using observing systems deployed on a 50-m meteorological tower, complemented with tethered balloon deployments, the vertical distribution of hydrocarbons and aerosols was determined under different boundary layer thermodynamic states. The rain forest emits sufficient reactive hydrocarbons, such as isoprene and monoterpenes, to provide precursors of secondary organic aerosols and cloud condensation nuclei. Mesoscale convective systems transport ozone from the middle troposphere, enriching the atmospheric boundary layer as well as the forest canopy and surface layer. Through multiple chemical transformations, the ozone-enriched atmospheric surface layer can oxidize rain forest–emitted hydrocarbons. One conclusion derived from the field studies is that the rain forest produces the necessary chemical species and in sufficient amounts to undergo oxidation and generate aerosols that subsequently activate into cloud condensation nuclei.

Summer fallow – the practice of keeping a field out of production during the growing season – is a common practice in dryland wheat (Triticum aestivum L.) cropping systems to conserve soil water resources. Fallow also depletes soil carbon stocks and thereby soil quality. The area of summer fallow has decreased by tens of millions of hectares since the 1970s in the northern North American Great Plains as producers have recognized that avoiding fallow usually confers both economic and soil conservation benefits. Observed summertime cooling across parts of this region has coincided with fallow reduction, suggesting that the role of fallow in atmospheric processes needs to be ascertained. We measured carbon dioxide, latent heat, and sensible heat flux across a winter wheat – spring wheat – fallow sequence in Montana, USA to determine the effects of dryland crop management on ecosystem carbon resources and energy partitioning at the surface-atmosphere interface. Winter wheat and spring wheat fields were carbon sinks (Fc = −203 ± 52 g C CO2 m−2 and −107 ± 29 g C CO2 m−2), respectively, during the April to September study period, but the fallow field was a carbon source of 135 ± 73 g C CO2 m−2. Evapotranspiration in the wheat crops was over 100 mm greater than the 275 ± 39 mm observed in the fallow field during the study period. Modeled maximum daily atmospheric boundary layer height was on average 210 m higher and up to 900 m higher in fallow compared to the spring wheat field with more crossings of the modeled atmospheric boundary layer and lifted condensation level, suggesting that regional studies of the effects of fallow on near-surface temperature and moisture are necessary to understand the effects of fallow reduction on regional climate dynamics. Results demonstrate that fallow has a detrimental impact to soil carbon resources yet is less water intensive, with consequences for regional climate via its impacts on atmospheric boundary layer development and global climate via its carbon metabolism.

Multiresolution decomposition is applied to 10 months of nocturnal turbulence observations taken at eight levels within and above a forest canopy in Central Amazonia. The aim is to identify the contributions of different temporal scales of the flow above and within the canopy. Results show that turbulence intensity in the lower canopy is mostly affected by the static stability in the upper canopy. Horizontal velocity fluctuations peak at time scales longer than 100 s within the canopy, which correspond to the scale of non-turbulent submeso motions above the canopy. In the vertical velocity spectrum near the surface, the peak occurs at time scales around 100 s, which are larger than the time scales of the turbulent flow above the canopy. Heat-flux cospectra within the canopy peak at the same temporal scales as the vertical velocity fluctuations at that level, suggesting the existence of buoyancy driven turbulence. Case studies are presented as evidence that low-frequency fluctuations propagate towards the canopy interior more easily than does turbulence.

From April 2014 to January 2015, ozone (O3) dynamics were investigated as part of GoAmazon 2014/5 project in the central Amazon rainforest of Brazil. Just above the forest canopy, maximum hourly O3 mixing ratios averaged 20 ppbv (parts per billion on a volume basis) during the June–September dry months and 15 ppbv during the wet months. Ozone levels occasionally exceeded 75 ppbv in response to influences from biomass burning and regional air pollution. Individual convective storms transported O3-rich air parcels from the mid-troposphere to the surface and abruptly enhanced the regional atmospheric boundary layer by as much as 25 ppbv. In contrast to the individual storms, days with multiple convective systems produced successive, cumulative ground-level O3 increases. The magnitude of O3 enhancements depended on the vertical distribution of O3 within storm downdrafts and origin of downdrafts in the troposphere. Ozone mixing ratios remained enhanced for > 2 h following the passage of storms, which enhanced chemical processing of rainforest-emitted isoprene and monoterpenes. Reactions of isoprene and monoterpenes with O3 are modeled to generate maximum hydroxyl radical formation rates of 6 × 106 radicals cm−3s−1. Therefore, one key conclusion of the present study is that downdrafts of convective storms are estimated to transport enough O3 to the surface to initiate a series of reactions that reduce the lifetimes of rainforest-emitted hydrocarbons.

Flash droughts tend to be disproportionately destructive because they intensify rapidly and are difficult to prepare for. We demonstrate that the 2017 US Northern Great Plains (NGP) flash drought was preceded by a breakdown of land–atmosphere coupling. Severe drought conditions in the NGP were first identified by drought monitors in late May 2017 and rapidly progressed to exceptional drought in July. The likelihood of convective precipitation in May 2017 in northeastern Montana, however, resembled that of a typical August when rain is unlikely. Based on the lower tropospheric humidity index (HIlow), convective rain was suppressed by the atmosphere on nearly 50 % of days during March in NE Montana and central North Dakota, compared to 30 % during a normal year. Micrometeorological variables, including potential evapotranspiration (ETp), were neither anomalously high nor low before the onset of drought. Incorporating convective likelihood to drought forecasts would have noted that convective precipitation in the NGP was anomalously unlikely during the early growing season of 2017. It may therefore be useful to do so in regions that rely on convective precipitation.

The seasonal variability of strong afternoon winds in a northern Himalayan valley and their relationship with the synoptic circulation were examined using in situ meteorological data from March 2006 to February 2007 and numerical simulations. Meteorological observations were focused on the lower Rongbuk valley, on the north side of the Himalayas (4270 m MSL), where a wind profile radar was available. In the monsoon season (21 May–4 October), the strong afternoon wind was southeasterly, whereas it was southwesterly in the nonmonsoon season. Numerical simulations were performed using the Weather Research and Forecasting Model to investigate the mechanism causing these afternoon strong winds. The study found that during the nonmonsoon season the strong winds are produced by downward momentum transport from the westerly winds aloft, whereas those during the monsoon season are driven by the inflow into the Arun Valley east of Mount Everest. The air in the Arun Valley was found to be colder than that of the surroundings during the daytime, and there was a horizontal pressure gradient from the Arun Valley to Qomolangma Station (QOMS), China Academy of Sciences, at the 5200-m level. This explains the formation of the strong afternoon southeasterly wind over QOMS in the monsoon season. In the nonmonsoon season, the colder air from Arun Valley is confined below the ridge by westerly winds associated with the subtropical jet.

Tropical rainforests play a central role in the Earth system by regulating climate, maintaining biodiversity, and sequestering carbon. They are under threat by direct anthropogenic impacts like deforestation and the indirect anthropogenic impacts of climate change. A synthesis of the factors that determine the net ecosystem exchange of carbon dioxide (NEE) at the site scale across different forests in the tropical rainforest biome has not been undertaken to date. Here, we study NEE and its components, gross ecosystem productivity (GEP) and ecosystem respiration (RE), across thirteen natural and managed forests within the tropical rainforest biome with 63 total site-years of eddy covariance data. Our results reveal that the five ecosystems with the largest annual gross carbon uptake by photosynthesis (i.e. GEP > 3000 g C m(-2) y(-1)) have the lowest net carbon uptake - or even carbon losses versus other study ecosystems because RE is of a similar magnitude. Sites that provided sub canopy CO2 storage observations had higher average magnitudes of GEP and RE and lower average magnitudes of NEE, highlighting the importance of measurement methodology for understanding carbon dynamics in ecosystems with characteristically tall and dense vegetation. A path analysis revealed that vapor pressure deficit (VPD) played a greater role than soil moisture or air temperature in constraining GEP under light saturated conditions across most study sites, but to differing degrees from -0.31 to -0.87 mu mol CO2 m(-2) s(-1) hPa(-1). Climate projections from 13 general circulation models (CMIP5) under the representative concentration pathway that generates 8.5 W m(-2) of radiative forcing suggest that many current tropical rainforest sites on the lower end of the current temperature range are likely to reach a climate space similar to present-day warmer sites by the year 2050, warmer sites will reach a climate not currently experienced, and all forests are likely to experience higher VPD. Results demonstrate the need to quantify if and how mature tropical trees acclimate to heat and water stress, and to further develop flux-partitioning and gap-filling algorithms for defensible estimates of carbon exchange in tropical rainforests.

Kobresia pastures in the eastern Tibetan highlands occupy 450000 km2 and form the world’s largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresia pygmaea, which forms a lawn with a durable turf cover anchored by a felty root mat, and occurs from 3000 m to nearly 6000 m a.s.l. The existence and functioning of this unique ecosystem and its turf cover have not yet been explained against a backdrop of natural and anthropogenic factors, and thus its origin, drivers, vulnerability or resilience remain largely unknown. Here we present a review on ecosystem diversity, reproduction and ecology of the key species, pasture health, cycles of carbon (C), water and nutrients, and on the paleo-environment. The methods employed include molecular analysis, grazing exclusion, measurements with micro-lysimeters and gas exchange chambers, 13C and 15N labelling, eddy-covariance flux measurements, remote sensing and atmospheric modelling.The following combination of traits makes Kobresia pygmaea resilient and highly competitive: dwarf habit, predominantly below-ground allocation of photo assimilates, mixed reproduction strategy with both seed production and clonal growth, and high genetic diversity. Growth of Kobresia pastures is co-limited by low rainfall during the short growing season and livestock-mediated nutrient withdrawal. Overstocking has caused pasture degradation and soil deterioration, yet the extent remains debated. In addition, we newly describe natural autocyclic processes of turf erosion initiated through polygonal cracking of the turf cover, and accelerated by soil-dwelling endemic small mammals. The major consequences of the deterioration of the vegetation cover and its turf include: (1) the release of large amounts of C and nutrients and (2) earlier diurnal formation of clouds resulting in (3) decreased surface temperatures with (4) likely consequences for atmospheric circulation on large regional and, possibly global, scales.Paleo-environmental reconstruction, in conjunction with grazing experiments, suggests that the present grazing lawns of Kobresia pygmaea are synanthropic and may have existed since the onset of pastoralism. The traditional migratory rangeland management was sustainable over millennia and possibly still offers the best strategy to conserve, and possibly increase, the C stocks in the Kobresia turf, as well as its importance for climate regulation.

This chapter summarizes the significant findings of the research on coherent structures contributed by investigations conducted at the Waldstein-Weidenbrunnen site from several field campaigns. The description of the quasi-online wavelet detection algorithm and of the coherent flux computation method using a triple decomposition is followed by a presentation of their application to define and diagnose vertical and horizontal couplings in forest canopies. It is demonstrated that these exchange regimes provide physically and biologically meaningful proxies for the communication of air and integration of the spatially separated sinks and sources as a result of the stratified canopy architecture. We continue by presenting two innovative applications of the coherent forest exchange that include the computation of daytime respiration fluxes directly from above-canopy eddy-covariance measurements and the explanation of stationary gradients in the sub-canopy CO2 field causing systematic advection as a result of the spatial heterogeneity of the forest architecture. Advantages and limitations of both are discussed. The chapter concludes by formulating directions for future research and indicating new observational techniques that may have the potential to improve understanding and quantifying the forest coherent exchange.

This paper uses the cloud resolving Active Tracer High-resolution Atmospheric Model coupled to the interactive surface model Hybrid in order to investigate the diurnal development of a lake-breeze system at the Nam Co Lake on the Tibetan Plateau. Simulations with several background wind speeds are conducted, and the interaction of the lake breeze with topography and background wind in triggering moist and deep convection is studied. The model is able to adequately simulate the systems most important dynamical features such as turbulent surface fluxes and the development of a lake breeze for the different wind conditions. We identify two different mechanisms for convection triggering that are dependent on the direction of the background wind: triggering over topography, when the background wind and the lake breeze have the same flow direction, and triggering due to convergence between the lake-breeze front and the background wind. Our research also suggests that precipitation measurements at the centre of the basins on the Tibetan Plateau are not representative for the basin as a whole as precipitation is expected to occur mainly in the vicinity of the topography.

This paper introduces a surface model with two soil-layers for use in a high-resolution circulation model that has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and base temperature is assumed in each layer and extended to the surface. The model is tested at two sites on the Tibetan Plateau near Nam Co Lake during four days during the 2009 Monsoon season. In comparison to a two-layer model without explicit surface temperature estimate, there is a greatly reduced delay in diurnal flux cycles and the modelled surface temperature is much closer to observations. Comparison with a SVAT model and eddy covariance measurements shows an overall reasonable model performance based on RMSD and cross correlation comparisons between the modified and original model. A potential limitation of the model is the need for careful initialisation of the initial soil temperature profile, that requires field measurements. We show that the modified model is capable of reproducing fluxes of similar magnitudes and dynamics when compared to more complex methods chosen as a reference.

Erdős asked whether every sufficiently large set of points in general position in the plane contains six points that form a convex hexagon without any points from the set in its interior. Such a configuration is called an empty convex hexagon. In this paper, we answer the question in the affirmative. We show that every set that contains the vertex set of a convex 9-gon also contains an empty convex hexagon.

The Tibetan Plateau plays a significant role in atmospheric circulation and the Asian monsoon system. Turbulent surface fluxes and the evolution of boundary-layer clouds to deep and moist convection provide a feedback system that modifies the plateau's surface energy balance on scales that are currently unresolved in mesoscale models. This work analyses the land surface's role and specifically the influence of soil moisture on the triggering of convection at a cross section of the Nam Co Lake basin, 150 km north of Lhasa using a cloud-resolving atmospheric model with a fully coupled surface. The modelled turbulent fluxes and development of convection compare reasonably well with the observed weather. The simulations span Bowen ratios of 0.5 to 2.5. It is found that convective development is the strongest at intermediate soil moisture. Dry cases with soils close to the permanent wilting point are moisture limited in convective development, while convection in wet soil moisture cases is limited by cloud cover reducing incoming solar radiation and sensible heat fluxes, which has a strong impact on the surface energy balance. This study also shows that local development of convection is an important mechanism for the upward transport of water vapour, which originates from the lake basin that can then be transported to dryer regions of the plateau. Both processes demonstrate the importance of soil moisture and surface–atmosphere interactions on the energy and hydrological cycles of the Tibetan Plateau.