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Tropical forests are an important part of global water and energy cycles, but the mechanisms that drive seasonality of their land‐atmosphere exchanges have proven challenging to capture in models. Here, we (1) report the seasonality of fluxes of latent heat (LE), sensible heat (H), and outgoing short and longwave radiation at four diverse tropical forest sites across Amazonia ‐‐ along the equator from the Caxiuanã and Tapajós National Forests in the eastern Amazon to a forest near Manaus, and from the equatorial zone to the southern forest in Reserva Jaru; (2) investigate how vegetation and climate influence these fluxes; and (3) evaluate land surface model (LSM) performance by comparing simulations to observations. We found that previously identified failure of models to capture observed dry‐season increases in evapotranspiration was associated with model over‐estimations of (1) magnitude and seasonality of Bowen ratios (relative to aseasonal observations in which sensible was only 20‐30% of the latent heat flux) indicating model exaggerated water limitation, (2) canopy emissivity and reflectance (albedo was only 10 to 15% of incoming solar radiation, compared to 0.15‐0.22% simulated), and (3) vegetation temperatures (due to underestimation of dry‐season evapotranspiration and associated cooling). These partially compensating model‐observation discrepancies (e.g. higher temperatures expected from excess Bowen ratios were partially ameliorated by brighter leaves and more interception/evaporation) significantly biased seasonal model estimates of net radiation (Rn), the key driver of water and energy fluxes (LE ~ 0.6Rn and H ~ 0.15Rn). Though these biases varied among sites and models. A better representation of energy‐related parameters associated with dynamic phenology (e.g. leaf optical properties, canopy interception, and skin temperature) could improve simulations and benchmarking of current vegetation‐atmosphere exchange and reduce uncertainty of regional and global biogeochemical models.

Aqueous chemical processing within cloud and fog water is thought to be a key process in the production and transformation of secondary organic aerosol mass, found abundantly and ubiquitously throughout the troposphere. Yet, significant uncertainty remains regarding the organic chemical reactions taking place within clouds and the conditions under which those reactions occur, owing to the wide variety of organic compounds and their evolution under highly variable conditions when cycled through clouds. Continuous observations from a fixed remote site like Whiteface Mountain (WFM) in New York State and other mountaintop sites have been used to unravel complex multiphase interactions in the past, particularly the conversion of gas-phase emissions of SO2 to sulfuric acid within cloud droplets in the presence of sunlight. These scientific insights led to successful control strategies that reduced aerosol sulfate and cloud water acidity substantially over the following decades. This paper provides an overview of observations obtained during a pilot study that took place at WFM in August 2017 aimed at obtaining a better understanding of Chemical Processing of Organic Compounds within Clouds (CPOC). During the CPOC pilot study, aerosol cloud activation efficiency, particle size distribution, and chemical composition measurements were obtained below-cloud for comparison to routine observations at WFM, including cloud water composition and reactive trace gases. Additional instruments deployed for the CPOC pilot study included a Doppler lidar, sun photometer, and radiosondes to assist in evaluating the meteorological context for the below-cloud and summit observations.

Fifty years ago the Line Islands Experiment was launched to obtain in situ surface, soundings, and aircraft observations in a convectively active region of the tropical oceans close to the nadir point of the new geosynchronous meteorological satellite. Two related tropical field experiments, in 1968 and 1969, followed in the vicinity of Barbados in the western Atlantic. Component parts of these three field experiments are recalled in this presentation that resonated over the subsequent half century.

In Amazon forests, the relative contributions of climate, phenology, and disturbance to net ecosystem exchange of carbon (NEE) are not well understood. To partition influences across various timescales, we use a statistical model to represent eddy-covariance-derived NEE in an evergreen eastern Amazon forest as a constant response to changing meteorology and phenology throughout a decade. Our best fit model represented hourly NEE variations as changes due to sunlight, while seasonal variations arose from phenology influencing photosynthesis and from rainfall influencing ecosystem respiration, where phenology was asynchronous with dry-season onset. We compared annual model residuals with biometric forest surveys to estimate impacts of drought disturbance. We found that our simple model represented hourly and monthly variations in NEE well (R2=0.81 and 0.59, respectively). Modeled phenology explained 1 % of hourly and 26 % of monthly variations in observed NEE, whereas the remaining modeled variability was due to changes in meteorology. We did not find evidence to support the common assumption that the forest phenology was seasonally light- or water-triggered. Our model simulated annual NEE well, with the exception of 2002, the first year of our data record, which contained 1.2 MgC ha−1 of residual net emissions, because photosynthesis was anomalously low. Because a severe drought occurred in 1998, we hypothesized that this drought caused a persistent, multi-year depression of photosynthesis. Our results suggest drought can have lasting impacts on photosynthesis, possibly via partial damage to still-living trees.

Cloud shadows lead to alternating light and dark periods at the surface, with the most abrupt changes occurring in the presence of low-level forced cumulus clouds. We examine multiyear irradiance time series observed at a research tower in a midlatitude mixed deciduous forest (Harvard Forest, Massachusetts, USA: $$42.53{^{\circ }}\hbox {N}$$ , $$72.17{^{\circ }}\hbox {W}$$ ) and one made at a similar tower in a tropical rain forest (Tapajos National Forest, Para, Brazil: $$2.86{^{\circ }}\hbox {S}$$ , $$54.96{^{\circ }}\hbox {W}$$ ). We link the durations of these periods statistically to conventional meteorological reports of sky type and cloud height at the two forests and present a method to synthesize the surface irradiance time series from sky-type information. Four classes of events describing distinct sequential irradiance changes at the transition from cloud shadow and direct sunlight are identified: sharp-to-sharp, slow-to-slow, sharp-to-slow, and slow-to-sharp. Lognormal and the Weibull statistical distributions distinguish among cloudy-sky types. Observers’ qualitative reports of ‘scattered’ and ‘broken’ clouds are quantitatively distinguished by a threshold value of the ratio of mean clear to cloudy period durations. Generated synthetic time series based on these statistics adequately simulate the temporal “radiative forcing” linked to sky type. Our results offer a quantitative way to connect the conventional meteorological sky type to the time series of irradiance experienced at the surface.

We examine cases of a regional elevated mixed layer (EML) observed during the Hudson Valley Ambient Meteorology Study (HVAMS) conducted in New York State, USA in 2003. Previously observed EMLs referred to topographic domains on scales of 10 $$^{5}$$ –10 $$^{6}$$ km $$^{2}$$ . Here, we present observational evidence of the mechanisms responsible for the development and maintenance of regional EMLs overlying a valley-based convective boundary layer (CBL) on much smaller spatial scales (

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.

O objetivo deste trabalho é verificar os impactos ocasionados pelos três últimos eventos de seca na saúde da população de catorze municípios na Amazônia, agrupados em quatro grupos com características pluviométricas homogêneas. Utilizaram-se dados diários e mensais de precipitação e temperatura do ar disponibilizado pelo Banco de Dados Meteorológicos para Ensino e Pesquisa do Instituto Nacional de Meteorologia e de internações por doenças respiratórias do Sistema Único de Saúde, DATASUS, do período de 2000 a 2015. Levando-se em consideração as especificidades dos dados em estudo utilizou-se a modelagem via Equações de Estimação Generalizada para captar associações significativas e o risco relativo de aumento da ocorrência das internações da população exposta a ausência de chuva. Os resultados mostraram que as estações que muito sofreram com a seca de 2005 (região A) localizada a sudoeste da Amazônia também foram pouco afetadas negativamente pelo evento de 2015. Embora o evento de seca de 2010 tenha sido mais extenso espacialmente do que 2005, o efeito da diminuição das chuvas a partir dos dados observados de um evento pra outro só se mostrou perceptível nas estações da região B, mais ao centro da Amazônia. Já a seca de 2015 teve maior impacto nas estações das regiões C e D. As populações consideradas mais vulneráveis por meio dos riscos captados pelo modelo foram Itaituba (região C), Monte Alegre e Porto de Moz (região D). Conclui-se que ao comparar as regiões percebe-se que os três eventos de seca afetam de forma diferente os quatros grupos, o que provavelmente está associado ao efeito diferencial dos mecanismos causadores das secas nas diferentes áreas da Amazônia.

Systematic biases in eddy covariance measurements of net ecosystem-atmosphere carbon dioxide exchange (NEE) are ubiquitous in forests when turbulence is low at night. We propose an alternative to the conventional bias correction, the friction velocity (u*) filter, by hypothesizing that these biases have two separate, concurrent causes: (1) a subcanopy CO2 storage pool that eludes typical storage measurements, creating a turbulence-dependent bias, and (2) advective divergence loss of CO2, creating a turbulence-independent bias. We correct for (1) using a simple parametric model of missing storage (MS). Prior experiments have inferred (2) directly from atmospheric measurements (DRAINO). For sites at which DRAINO experiments have not been performed or are infeasible, we estimate (2) empirically using a PAR-extrapolated advective respiration loss (PEARL) approach. We compare u* filter estimates of advection and NEE to MS-PEARL estimates at one temperate forest and two tropical forest sites. We find that for tropical forests, u* filters can produce a range of extreme NEE estimates, from long-term forest carbon emission to sequestration, that diverge from independent assessments and are not physically sustainable. Our MS model eliminates the dependence of nighttime NEE on u*, consistent with findings from DRAINO studies that nighttime advective losses of CO2 are often not dependent on the strength of turbulence. Our PEARL estimates of mean advective loss agree with available DRAINO measurements. The MS-PEARL correction to long-term NEE produces better agreement with forest inventories at all three sites. Moreover, the correction retains all nighttime eddy covariance data and is therefore more widely applicable than the u* filter approach, which rejects substantial nighttime data—up to 93% at one of the tropical sites. The full MS-PEARL NEE correction is therefore an equally defensible and more practical alternative to the u* filter, but leads to different conclusions about the resulting carbon balance. Our results therefore highlight the need to investigate which approach’s underlying hypotheses are more physically realistic.

The authors examine how terrain texture and topography influence nocturnal mixing rates. Local topographic curvature and site sheltering exhibit systematic influences on nocturnal heat and momentum fluxes and the near-surface potential temperature distribution. This influence is particularly evident in hilly terrain with patchy forested areas, typical of eastern North America and many other regions. Exposure to local obstacles, quantified using Fujita’s “transmission factor,” has its maximum influence on mixing during strong winds (>5 m s−1), whereas the effects of local terrain curvature dominate under weaker winds. Such complementary dominance conditions currently limit direct comparison of the two effects. Even with a limited network of 10 stations, it is clear that preferred regions for mixing can be identified in advance given knowledge of land cover and topography. When designing a network of surface stations to be deployed in heterogeneous terrain, one should consider site curvature, slope, and exposure in addition to spatial coverage.

Average heat and momentum fluxes observed by a network of surface stations during the Hudson Valley Ambient Meteorology Study (HVAMS) were found as functions of a spatially representative bulk Richardson number Ribr. Preferential sites were identified for the occurrence of strong turbulence under mesoscale stability conditions common to all stations. Locally sensed turbulence intermittency depends on the mesoscale flow stability. Nearly continuous turbulence with few long-lived intermittent events occurs when Ribr < Ricr, the critical gradient Richardson number. Less-continuous mixing associated with a larger number of events occurs when Ricr < Ribr < 5, with the weakest turbulence and fewer events observed for Ribr ≫ Ricr. It was found that the need to allow for extra mixing above the conventional critical bulk Richardson number in numerical weather prediction models is primarily a consequence of spatial averaging in a heterogeneous landscape and is secondarily the result of turbulence above Ricr at locations with “nonideal fetch.”

CHUVA, meaning “rain” in Portuguese, is the acronym for the Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (GPM). The CHUVA project has conducted five field campaigns; the sixth and last campaign will be held in Manaus in 2014. The primary scientific objective of CHUVA is to contribute to the understanding of cloud processes, which represent one of the least understood components of the weather and climate system. The five CHUVA campaigns were designed to investigate specific tropical weather regimes. The first two experiments, in Alcantara and Fortaleza in northeastern Brazil, focused on warm clouds. The third campaign, which was conducted in Belém, was dedicated to tropical squall lines that often form along the sea-breeze front. The fourth campaign was in the Vale do Paraiba of southeastern Brazil, which is a region with intense lightning activity. In addition to contributing to the understanding of cloud process evolution from storms to thunderstorms, this fourth campaign also provided a high-fidelity total lightning proxy dataset for the NOAA Geostationary Operational Environmental Satellite (GOES)-R program. The fifth campaign was carried out in Santa Maria, in southern Brazil, a region of intense hailstorms associated with frequent mesoscale convective complexes. This campaign employed a multimodel high-resolution ensemble experiment. The data collected from contrasting precipitation regimes in tropical continental regions allow the various cloud processes in diverse environments to be compared. Some examples of these previous experiments are presented to illustrate the variability of convection across the tropics.

We studied the interplay of atmospheric and canopy structure factors driving the canopy light environment (Photosynthetic Photon Flux Density, PPFD) in primary moist tropical forest in the Tapajos National Forest, Brazil. We quantified the temporal and spatial length scales that characterize intact rain forest inhomogeneities, asking: Are seasonal changes in the canopy radiation balance evident at these scales? We sought to describe the components of intensity, duration and spatial variation of within-canopy PPFD in light of these inhomogeneities. Do fluctuating atmospheric conditions, especially the presence of clouds and precipitation, affect the radiative inputs at both the canopy top and the forest floor? We examined the characteristic scales of heterogeneity in the vertical and the horizontal using a two-part approach. For radiation we combined long-term continuous high-frequency measurements of down- and up-welling short-wave and visible wavebands above the canopy with similarly frequent observations from a dense sensor network at forest floor. Vertical variations in canopy structure, obtained with a ground-based LIDAR, similarly combined intensive observations at the sensor network with occasional large-scale transects along the forest floor. Close similarities in both radiation and canopy structure at both scales support the representativeness of local observations of the wider area. A composite broadband measure similar to the Normalized Difference Vegetation Index (denoted cbNDVI) was constructed from above-canopy observations to compare with reports of remotely sensed canopy reflectivity at this site. We estimated the canopy Leaf Area Index (LAI) by combining observations of the variation in understory transmittance by solar elevation with a commonly-used algorithm. We obtained the conventional whole-canopy extinction coefficient, by applying the Beer-Lambert law. Over the course of a year this forest receives 11,795 mol m−2, only 62% of potential clear-day PPFD – atmospheric transmissivity is reduced by clouds, precipitation, smoke and haze. Very little PAR (≈ 2%) is reflected from the canopy and only 5.7% penetrates to 1 m above the forest floor - overall 92% of PAR is absorbed by the canopy. All radiation balance components closely tracked the dynamics of incoming light, showing little seasonal variation. Understory light observations across the 7.5–28.5 m spanning the understory array sensors showed essentially constant correlation between sensor pairs over time. There was a high degree of local persistence in the understory spatial pattern that varied slowly and directionally with the changing geometry of the sun and canopy structure. Over larger distances (to 1000 m), the patterns of spatial autocorrelation of understory PPFD and outer canopy structure were remarkably similar in shape, both declining rapidly to a more-or-less constant level around 15–20 m, a scale consistent with the dimensions of outer canopy crowns. The vertical pattern of transmission and absorption was estimated by combining understory transmittance with the distribution of canopy surface area obtained from the ground-based LIDAR system. It showed the maximum absorption relatively low in the canopy (8–19 m above ground). Although rather tall (canopy height is at 41.5 m), the extremely elaborate outer surface of the forest suggests the layers highest above ground are of little consequence to the PAR absorption budget. The cbNDVI measure exhibited seasonal variation consistent with other reports (somewhat higher in the wet season) but in contrast to a recent argument that the forest ‘greens up’ during the early dry season. However, except for the variations caused by the angle and intensity of incoming light modulated by atmospheric effects, there was little seasonality in the forest light environment at km67, including: canopy reflectances in several wavebands, all PPFD radiation budget components, the estimated LAI and mid-day PAR extinction coefficient and the length scales of understory PPFD. Canopy transmission was somewhat greater (6.4%) under diffuse skies compared to the least diffuse conditions (6.0%), and slightly more PPFD was absorbed under diffuse (92.7%) versus the least diffuse conditions (92.3%), but was not a significant contribution to the budget. The large diurnal variations in the extinction coefficient seriously affects its utility as a descriptor of canopy radiative properties. We propose an alternative approach for transmitted light: the canopy behaves as: 1.) a constant fraction filter under diffuse conditions, combined with 2.) a variable filter depending on solar elevation for sunfleck conditions. These regimes may be described with simple parameters each having mechanistic relations to canopy structure. In summary, we demonstrate that obtaining the radiation signal at forest floor at high data rate for long periods exploits seasonal sun angle changes to probe canopy structure. Combined with occasional long transect information, temporal and vertical sampling spatially allow improved definition of characteristic scales to describe both the understory light environment and canopy structure.

We examined the diurnal cycle of the rainfall in a coastal tropical mountainous region in central Veracruz State, Mexico (18&deg, &ndash, 21&deg, N, 95.5&deg, 98.5&deg, W), featuring a striking topographic gradient running from sea level at the Gulf of Mexico coast to 5000 m above sea level (m.a.s.l.) in less than 100 km horizontal distance. During the summer, this unique location leads to regular the interaction between the easterly moisture inflow and the mountainous barrier. Over the complex terrain, forced ascent leads the occurrence of maximum rainfall during the afternoon (16&ndash, 19 local time, LT &asymp, 1&frac12, hours ahead of solar time in summer), first along the slope and later over the coast. Along the coastal plain, the precipitation continues until the early morning consistent with there being convergence between land breezes and the trade winds. Observations obtained during a measurement campaign from 28 June to 3 July 2015, indicate that during the early evening downslope winds move against easterly flow, likely due to katabatic outflows previously observed over the region. These features are confirmed using spatial (0.88&deg, ) and temporal (30 min) resolution CMORPH rainfall estimates, since we observed evening episodes initiating along the slope during the afternoon (14&ndash, 17 LT) moving later towards the coast.

This study assesses the Moderate-resolution Imaging Spectroradiometer (MODIS) BRDF/albedo 8 day standard product and products from the daily Direct Broadcast BRDF/albedo algorithm, and shows that these products agree well with ground-based albedo measurements during the more difficult periods of vegetation dormancy and snow cover. Cropland, grassland, deciduous and coniferous forests are considered. Using an integrated validation strategy, analyses of the representativeness of the surface heterogeneity under both dormant and snow-covered situations are performed to decide whether direct comparisons between ground measurements and 500-m satellite observations can be made or whether finer spatial resolution airborne or spaceborne data are required to scale the results at each location. Landsat Enhanced Thematic Mapper Plus (ETM +) data are used to generate finer scale representations of albedo at each location to fully link ground data with satellite data. In general, results indicate the root mean square errors (RMSEs) are less than 0.030 over spatially representative sites of agriculture/grassland during the dormant periods and less than 0.050 during the snow-covered periods for MCD43A albedo products. For forest, the RMSEs are less than 0.020 during the dormant period and 0.025 during the snow-covered periods. However, a daily retrieval strategy is necessary to capture ephemeral snow events or rapidly changing situations such as the spring snow melt.

On the moderately complex terrain covered by dense tropical Amazon Rainforest (Reserva Biologica do Cuieiras—ZF2—02°36′17.1′′ S,60°12′24.4′′ W), subcanopy horizontal and vertical gradients of the air temperature, CO2concentration and wind field were measured for the dry and wet periods in 2006. We tested the hypothesis that horizontal drainage flow over this study area is significant and can affect the interpretation of the high carbon uptake rates reported by previous works at this site. A similar experimental design as the one byTóta et al.(2008) was used with a network of wind, air temperature, and CO2sensors above and below the forest canopy. A persistent and systematic subcanopy nighttime upslope (positive buoyancy) and daytime downslope (negative buoyancy) flow pattern on a moderately inclined slope (12%) was observed. The microcirculations observed above the canopy (38 m) over the sloping area during nighttime presents a downward motion indicating vertical convergence and correspondent horizontal divergence toward the valley area. During the daytime an inverse pattern was observed. The micro-circulations above the canopy were driven mainly by buoyancy balancing the pressure gradient forces. In the subcanopy space the microcirculations were also driven by the same physical mechanisms but probably with the stress forcing contribution. The results also indicated that the horizontal and vertical scalar gradients (e.g., CO2) were modulated by these micro-circulations above and below the canopy, suggesting that estimates of advection using previous experimental approaches are not appropriate due to the tridimensional nature of the vertical and horizontal transport locally. This work also indicates that carbon budget from tower-based measurement is not enough to close the system, and one needs to include horizontal and vertical advection transport of CO2into those estimates.

Recent studies suggest that vegetation can drive large-scale atmospheric circulations and substantially influence the hydrologic cycle. We present observational evidence to quantify the extent of coupling between vegetation and the overlying atmosphere. Within the context of vegetation–atmospheric interactions, we reanalyze existing climatological data from springtime leaf emergence, emissivity, dew point temperatures, and historical records of precipitation and forest coverage. We construct new rainfall transects based on a robust global climatology. Using isotopic analysis of precipitation, we find that rain in Amazonia comes primarily from large-scale weather systems coupling interior regions to the ocean and is not directly driven by local evaporation. We find that changes in vegetative cover and state influence the temperature and moisture content of the surface and atmospheric boundary layer but are not reflected in observable precipitation changes. This analysis reaffirms the view that changes in precipitation over continental reaches are a product of complex processes only partly influenced but not controlled by local water sources or vegetation.

Summary We introduce a new method to estimate rainfall interception and demonstrate its use for data obtained in an old-growth rain forest in the eastern Amazon basin. The approach is to use eddy covariance evaporation observations to estimate the ‘excess’ evaporation that occurs following individual events. Ensemble averaged water vapor fluxes were calculated from original high frequency data both for rain event and for base state dry days. Interception was inferred from the difference between observed evaporation for selected times during and following rainfall events from baseline evaporation estimates. This method allows the interception evaporation to be directly measured rather than determined from the residual of incident precipitation and throughfall. In conventional studies, large differences in throughfall can occur on a site due to varying forest canopy density, structure and the appearance of canopy gaps. This problem is mitigated when using the current approach, which provides an average interception value over the flux footprint area. Identification of light rainfall events not detected by an on-site tipping bucket rain gauge was aided by the use of a ceilometer. The mean interception for all events in the study (daytime and nocturnal) was 11.6%, comparable to some recent conventional studies in this region. We found an approximately 15% increase of evaporative fraction on the rain days as compared to dry baseline days, with the energy being supplied by a corresponding decrease in the canopy heat storage. Since net radiation is used to scale the evaporation in this method, this method may be applicable to data from other tower sites in varying surface and climatic types. We did not find that bulk stomatal resistance vanishes just after rainfall. The effective bulk stomatal resistance can be used as the observational equivalent of the wet fraction of canopy parameter used in interception models.

The use of friction velocity u* as the turbulence scale for correcting eddy-covariance carbon dioxide fluxes in low-mixing conditions is questioned. This is done because u* is, itself, a flux and, therefore, its value is highly dependent on the temporal scale used for the analysis. The multiresolution decomposition is applied to data from three different ecosystems in Brazil, to show that u* is well behaved and related to the turbulent mixing only up to the scale that separates the turbulent mixing from the low-frequency exchange. For larger temporal scales, mesoscale fluxes may induce large variability in the friction velocity, so that time series with low turbulent mixing may show an elevated value for u*, and vice-versa. We propose, as an alternative, the use of σ w , the standard-deviation of the vertical velocity fluctuations. It is shown that σ w has no variability within the mesoscale range and that, therefore, it is a much better scale to quantify the turbulent exchange than u*. The relationship between the two velocity scales is shown to depend on the scale and to be universal for the scales of the turbulent exchange. It is shown that curves of the turbulent carbon dioxide fluxes as a function of the turbulence scale are smoothed when using the friction velocity. Using σ w instead of u* in data filtering procedures has two main consequences: easier determination of the threshold for filtering and larger respiration rates of the series classified as turbulent. The improvement is larger for sites where very stable conditions are common.

Long-term eddy-covariance measurements from the LBA-ECO Project were used to study the behavior of turbulence spectra and cospectra above the Amazon rainforest as well as over a nearby deforested area in the central part of Amazon region. Results are separated according to the wind speed and stability conditions. Under stable low wind speed conditions the Kolmogorov inertial subrange law is not observed. For all other situations simple empirical relations are obtained and length scales associated with the more energetic eddies are calculated. Above the forest canopy, the cospectra are more peaked than those observed over the deforested area, confirming previous observations made over a midlatitude forest.

Large-scale agriculture is increasing in anthropogenically modified areas in the Amazon Basin. Crops such as soybean, maize, oil palm, and others are being introduced to supply the world demand for food and energy. However, the current challenge is to enhance the sustainability of these areas by increasing efficiency of production chains and to improve environmental services. The Amazon Basin has experienced a paradigm shift away from the traditional slash-and-burn agricultural practices, which offers decision makers the opportunity to make innovative interventions to enhance the productivity in previously degraded areas by using trees to ecological advantage. This study describes a successful experiment integrating the production of soybean and parica (Glycine max L. and Schizolobium amazonicum) based on previous research that indicated potential topoclimatic zones for planting parica in the Brazilian state of Para. This paper shows that a no-tillage system reduces the effects of drought compared to conventional tillage still used by many farmers in the region. The integrated system was implemented during the 2014/2015 season in 234.6 ha in the high-potential zone in the municipality of Ulianopolis, Para. Both soybean and parica were planted simultaneously. Parica was planted in 5 m x 2 m inter-tree spacing totaling 228x10 3 trees per hectare and soybean, in 4 m x 100 m spacing, distributed in nine rows with a 0.45 m inter-row distance, occupying 80% of the area. The harvested soybean production was 3.4 t ha -1 , higher than other soybean monocultures in eastern Para. Parica benefited from soybean fertilization in the first year: It exhibited rapid development in height (3.26 m) and average diameter (3.85 cm). Trees and crop rotation over the following years is six years for forest species and one year for each crop. Our results confirm there are alternatives to the current production systems able to diminish negative impacts resulting from monoculture. In addition, the system provided environmental services such as reduced soil erosion and increased carbon stock by soil cover with no-tillage soybean cultivation. The soybean cover contributes to increased parica thermal regulation and lower forestry costs. We concluded that innovative interventions are important to show local farmers that it is possible to adapt an agroforest system to large-scale production, thus changing the Amazon.

O objetivo principal do trabalho é identificar possíveis tendências de elevação nas séries de temperatura média máxima de municípios localizados na Amazônia Central, especificamente no Oeste do Pará. Para tanto, foram utilizados dados da temperatura do ar de Belterra, Monte Alegre, Óbidos e Porto de Moz, disponibilizados pelo Banco de Dados Meteorológicos para Ensino e Pesquisa (BDMEP), do Instituto Nacional de Meteorologia (INMET) no período de 30 anos (1980 a 2013). A análise inicial consistiu em agrupar e ordenar os dados, cronologicamente, de forma sazonal e anual. A técnica de Análise de Variância (ANOVA) foi adotada a fim de verificar se há entre os municípios diferença significativa nas médias da temperatura. Para detectar as possíveis tendências, utilizou-se o teste não paramétrico de Mann-Kendall e sua magnitude por meio do estimador de declive de Sen. Os resultados constataram tendência de elevação nas séries de temperatura média máxima estatisticamente significativa na escala sazonal, com exceção da amostra da primavera nas séries de Monte Alegre e Óbidos, e no outono na série de Porto de Moz. Destacando que os maiores valores da tendência foram observados no inverno em todas as localidades em estudo. Pela ANOVA constatou-se por meio do p-valor para um nível de significância de 1% que as diferenças entre as médias anuais e sazonais são significativas entre as localidades. Espera-se assim contribuir no auxílio do desenvolvimento de várias atividades na região, como também dar subsídios aos gestores para planejamento de políticas ambientais.

This study aimed to describe the vertical profile of CO2 concentration during least rainy period and rainy periods of 2006 , into an area of tropical forest in the Central Amazon, Brazil . It were used data of CO2, collected at various heights by a system of measures including an infrared gas analyzer (Licor 7000), in a tower with 40 meters of height. From the data collected it was observed that the concentration of CO2 decreases as one moves from the ground to the top of the canopy. This suggests that the largest source of CO2 is from the soil, due to processes such as respiration and decomposition of plant biomass. When is compared the least rainy and the most rainy periods, it is noted significant differences in the amounts of CO2 concentration in these periods. The highest concentrations were observed during least rainy period, with monthly mean values above 440 ppm near the surface.

A better understanding of forest subcanopy flows is needed to evaluate their role in the horizontal movement of scalars, particularly in complex terrain. This paper describes detailed measurements of the canopy structure and its variability in both the horizontal and vertical directions at a deciduous forest in complex terrain (the Harvard Forest, Petersham, Massachusetts). The effects of the trunks and subcanopy shrubs on the flow field at each of six subcanopy array locations are quantified. The dynamics of the subcanopy flow are examined with pragmatic methods that can be implemented on a small scale with limited resources to estimate the stress divergence, buoyancy, and pressure gradient forces that drive the flow. The subcanopy flow at the Harvard Forest was driven by mechanisms other than vertical stress divergence 75% of the time. Nocturnal flows were driven predominantly by the negative buoyancy of a relatively cool layer near the forest floor. The direction of the resulting drainage flows followed the azimuth of the longest forest-floor slope. Similar results were found at a much flatter site at Borden, Ontario, Canada. There was no clear evidence of flow reversals in the subcanopy in the lee of ridges or hills at the Harvard Forest even in high wind conditions, contrary to some model predictions.

Hydrologic records provide some of the most widespread, long-term data available that can be expected to contain signals of climate variation over time. Yet such data have not been used to identify the widespread effects of spring onset in the forested eastern United States, where streamflow is strongly influenced by vegetation. Three independent runoff characteristics are affected by the enhanced evapotranspiration (ET) that occurs with annual leaf emergence: (i) P − R, the difference between precipitation and runoff, returns to dormant season values and increases due to ET; (ii) the streamflow recession time constant (a measure of the time required after rainfall for streams to return to their baseflow levels) shortens; and (iii) the diurnal streamflow amplitude increases. The smallest watersheds in our dataset (area

Ecosystem carbon budgets depend on there being good representative surface flux observations for all land use types during the entire diurnal cycle. In calm conditions that often occur at night, especially in areas of small roughness (such as pastures), ecosystem respiration rate is poorly measured using the eddy covariance (EC) technique. Nocturnal vertical profiles of temperature, humidity and winds were observed using tethered balloon soundings in a pasture in the eastern Amazon during two campaigns in 2001. The site is characterized by very weak winds at night, so that there is insufficient turbulence for the EC technique to determine fluxes accurately. To compensate, the time evolution of the profiles is used to determine surface fluxes at early morning and these are compared with those observed by EC at a nearby micrometeorological tower. The nocturnal boundary layer thickness h is determined as the height to which the surface fluxes must converge so that energy budget closure is achieved. The estimated values range from 30 m, around 22:00 hours LST, to more than 100 m just before dawn. These are in good agreement with the observed thickness of a frequently observed fog layer during the middle of the night. During the early portion of the night, when the accumulation layer is shallow, there is appreciable decrease of dCO2/dt with height. On calm nights, CO2 accumulation rate is larger near the surface than at higher levels. On windier nights, this accumulation rate is vertically uniform. Hence, extrapolation of tower profiles for estimating fluxes must be done carefully. Although uncertainties remain large, an alternate approach to the EC method is described for measuring nighttime surface CO2 fluxes under stable atmospheric conditions.

Underestimation of nocturnal CO 2 respiration under calm conditions remains an unsolved problem at many forest flux stations. In this paper, the hypothesis is tested that horizontal mean transport, not previously measured, may account for the missing CO 2 . A systematic methodology was developed that comprises characterizing the subcanopy motions, determining the appropriate size of the subcanopy network required to make the measurements, developing a method of integrating the measurements in the vertical, and determining the required averaging time. Measurements were performed at the Harvard Forest (Petersham, MA), over 4 years. The appropriate size of the network of wind and CO 2 sensors was shown to be on the order of 100 m, ensuring that sensors were generally observing coherent processes on this scale or larger and thus displayed some correlation. Horizontal transport of CO 2 at Harvard Forest was found to be restricted to the bottom ∼10 m of the forest. The fraction of the negative buoyancy force in the sum of dynamic driving forces described nights with missing flux problems (“deficit nights”) significantly better than the commonly used friction velocity criterion. Including the measured horizontal transport terms did not on average fully account for the observed difference in NEE of 1.2±0.3 μmol m −2 s −1 between deficit and non-deficit nights, but decreased the difference to 0.7±0.5 μmol m −2 s −1 . Horizontal transport did account for the difference, to within measurement error, during summer months, but not during spring or fall.

The interaction between atmospheric and construction conditions and the exothermic, temperaturedependent hydration reactions of the concrete’s binding components may produce adverse conditions in curing concrete, thereby reducing the quality of that concrete. Accurate model forecasts of concrete temperatures and moisture would help engineers determine an optimal time to pour, an optimal mix design, and/or optimal curing practices. Existing models of curing concrete bridge decks and road surface prediction models lack realistic boundary conditions. The concrete models contain unnecessarily detailed hydration heat generation mechanisms for a simplified field forecast model. In this paper, a new energy balance model (SLABS), which can be easily adapted to predict road surface conditions, is described and applied to predict the temperatures and moisture of curing concrete bridge decks made with New York State Department of Transportation’s Class HP concrete. Highest concrete temperatures occurred at high air temperatures, humidities and initial concrete temperatures and at low cloud cover fractions and wind speeds. Peak concrete temperatures can exceed 60 °C. To minimise concrete temperatures and temperature gradient magnitudes, concrete should be placed during the late afternoon or early evening. As a field forecast model for which the meteorological inputs are taken from NGM MOS forecasts, the outputs of SLABS include the peak concrete temperature (to within 2 °C of the observed in one application), peak temperature gradient, evaporation rate at the time of placement and several warning messages indicating adverse field conditions.

Intermittent breakdowns that accompany wind gusts at the surface are responsible for a large fraction of the turbulent exchange between the surface and the upper boundary layer in the core of clear nights. Vertical and horizontal structure of the breakdowns are investigated using data from a network of 26 stations in an area of 30 km × 30 km. Surface heterogeneity in the area includes complex terrain with different types of land cover. We treat the fine-scale landscape structure near sensors (‘sheltering’) as a separate component of heterogeneity. These features have important consequences on the spatial distribution of mean variables and surface fluxes. We found that breakdowns connect the surface layer to a higher level (level HC). Weak wind gusts below a threshold (approximately 1.5 m s-1) mix the air down to the colder ground, cooling the surface layer. On the other hand, wind gusts above this threshold promote mixing with upper levels, warming the surface layer. The spatial maximum of surface temperature over the network can be used as an estimate of the temperature at HC, allowing vertical gradients and stability to be approximated. Minimum temperature is a function of topography and sheltering. Appreciable surface fluxes at night occur primarily at high, open locations, and can be large enough there to influencearea-averaged values. Surface-fluxparameterizations currently used in mesoscale models were tested first by estimating fluxes at each station and aggregating, and then by formingarea-averages before estimating fluxes. Results show that these formulations underestimate the average surface fluxes over a region for most of the nights.

Eddy covariance flux observations at a deciduous temperate forest site (83 days) and at a boreal forest site (21 days) are analyzed for midday periods (1100–1400 LT). Approximate stationarity of the time series is demonstrated, and the ensemble-averaged roughness sublayer cospectra are presented. Spectral and cospectral forms in the roughness sublayer are more peaked than those found in an inertial sublayer. They exhibit similar forms dependent on (z − d)/(h − d), where d is the displacement height and h is the canopy height. The inertial-layer spectral forms are recovered when observations are made where this scaled height is approximately 4. For a sample summer at the midlatitude deciduous forest, large eddies with periods from 4 to 30 min contribute about 17% to surface eddy fluxes of heat, water vapor, and carbon dioxide (CO2). Much larger contributions can occur in light-wind conditions. This effect, likely caused by the passage of convective boundary layer eddies, is not observed when using...

For several days after concrete is poured, atmospheric conditions influence the hydration reactions of concrete's binder components and so may influence its long-term durability. Accurate concrete temperature and moisture forecasts would help engineers determine an optimal pour time. Some existing curing concrete models include complicated chemistry and/or microstructure development parameterizations or do not allow for mix design changes. A bimolecular heat generation expression that is simple but sufficiently detailed to account for mix design changes was improved for Class HP concrete. Analysis of published calorimetry data and those determined in this study indicated that a second-order formulation adequately describes the heat generation. Class HP binder has an activation energy of ∼35 kJ mol−1. After 72 h, Class HP pastes evolved 250–280 kJ kg−1. A method to account for the effect of retarders on Stage II length and Stage III hydration rates was developed. A curing concrete bridge model with the bimolecular expression predicted concrete temperatures to within 2 °C of observed temperatures and reasonable 72-h hydration fractions (∼0.6).

Net uptake of carbon dioxide (CO 2 ) measured by eddy covariance in a 60- to 80-year-old forest averaged 2.0 ± 0.4 megagrams of carbon per hectare per year during 1993 to 2000, with interannual variations exceeding 50%. Biometry indicated storage of 1.6 ± 0.4 megagrams of carbon per hectare per year over 8 years, 60% in live biomass and the balance in coarse woody debris and soils, confirming eddy-covariance results. Weather and seasonal climate (e.g., variations in growing-season length or cloudiness) regulated seasonal and interannual fluctuations of carbon uptake. Legacies of prior disturbance and management, especially stand age and composition, controlled carbon uptake on the decadal time scale, implying that eastern forests could be managed for sequestration of carbon.

Atmospheric conditions for several days after concrete is poured influence the exothermic, temperaturedependent, hydration reactions of concrete’s cementitious (binding) components. Because excessively high concrete temperatures or lack of water eventually can lead to cracking, the initial days are critical to determining the concrete’s long-term durability. Accurate model forecasts of concrete temperatures and moisture would help engineers to determine an optimal time to pour. Such forecasts require adequate environmental predictions. Existing models of curing concrete bridge decks employed by engineers lack realistic boundary conditions and so cannot handle many atmospheric conditions. Atmospheric energy exchange parameterizations typically are intended for use over areas much larger than bridges and so may not be useful as boundary conditions in curingconcrete models. To determine proper boundary conditions for the curing-concrete model discussed here, energy balances of four curing concrete bridge decks were estimated from observations made in the atmosphere as well as inside the concrete. Common meteorological techniques to estimate energy balance terms were used to bound the estimates. Most (70%‐85%) of the concrete heat transfer occurred at the bridge’s top. Sensible, latent, net radiative, and runoff water (sprayed on the top surface) heat fluxes, respectively, contributed 6%‐24%, 15%‐ 58%, 10%‐34%, and 0%‐73% of the top surface heat transfer. Bottom heat transfer was less than 30% of the top surface transfer. Laboratory calorimetry and the energy balance results agree to within 20% that the hydration reactions evolved about 190 kJ kg21 by 24 h after mixing. This agreement validates the exchange coefficients proposed for the heat and moisture balances of these small areas both during periods when the concrete generated heat and later when the concrete was more passive in its environment.

On clear nights with appreciable radiative cooling, rates of change of mean quantities observed in the first 1 or 2 h after sunset are many times larger than they are subsequently until sunrise. These variations include large temperature drops, specific humidity increases, and abrupt wind speed decay. The early evening transition (EET) is dominated by vertical surface flux convergence as the turbulent mixing layer becomes confined to a shallow stable layer near the surface. Effects of surface heterogeneities are enhanced by the subsequent small eddy size, so that spatial variability of mean variables peaks during the EET. Hilltops do not experience such large variations at EET, as turbulence persists longer at those locations. Wind speed decays faster at obstructed sites, which show earlier transitions. The Richardson number increases exponentially at the EET, and the rate of exponential increase is proportional to the rate of wind decay. Qualitative aspects of the EET at a single point are resolved by a two-layer model, similar to those currently used in parameterizations of the surface exchange in mesoscale simulations. However, in order to simulate the different behaviors observed in an area as big as one grid cell in a mesoscale model, very different values of the geostrophic wind need to be imposed. A large eddy model is used to verify that the specific humidity jump and maximum cooling rate during the EET are primarily the consequence of enhanced vertical flux divergence, though in later stages of the EET, advective effects become more important.

The northeastern United States is subject to relatively frequent passages of frontal systems during the growing season. After a frontal passage, the newly arrived air mass is gradually modified by the underlying, mostly vegetated landscape. For the 1995‐98 growing seasons, 25 frontal sequences with at least 4 days between frontal passages were identified; 16 had sufficient data continuity for rigorous analysis. A composite of sequences featuring the daily appearance of boundary layer cumulus clouds (BLcu) indicates a diminished role for entrainment and other external forcings because of the daily occurrence of a rapid growth phase in the mixedlayer (ML) diurnal evolution subsequent to day 1. Between frontal passages, net heat and moisture flux convergence in the ML is near zero, but during the warming and moistening phase, the surface flux terms, through a net radiation‐BLcu feedback, are the principal controls on the tendencies of the ML temperature u and specific humidity q. The combination of the u and q tendencies leads to a nearly constant lifting condensation level, relative humidity, and BLcu cloud fraction during the latter part of the sequences. The presence of BLcu enhances water use efficiency and net afternoon carbon uptake throughout the sequence, with day 4 featuring optimal conditions. A multiday box model was used to perform sensitivity studies on subsidence, the lapse rate g uy above the ML, cloud mass flux, and the regional surface Bowen ratio breg. The effects of subsidence and g uy on ML processes are most conspicuous on day 1; during subsequent days, the rapid growth phase dominates the ML growth equation and reduces the impact of these external terms. Increasing breg to 3.5 reduces BLcu fraction to less than 20% and produces little net moistening of the ML, whereas reducing breg by 30% increases sequence BLcu coverage by 30%‐80%. In sum, the presence of a net radiation‐BLcu feedback allows for the establishment of an equilibrium in the ML heat and moisture tendencies and ensures the appearance of BLcu on each day of the sequence, thus sustaining favorable conditions for forest‐atmosphere exchange (i.e., carbon uptake).

At the time of leaf emergence in deciduous forests, markedly enhanced evapotranspiration leads to a rapid drop in the Bowen ratio. A small fraction of this surface flux alteration converges into the boundary layer, and this can be detected in the mean temperature and humidity daily increments at the surface. A simple technique is presented for identifying this response in surface climate data and extracting time series for the date of spring onset and for the “spring intensity,” a measure of surface energy budget partition change in spring. A tendency Bowen ratio B′ is found from changes in the daily increment of temperature and humidity in multidecadal averages. The spring date determined using this criterion for stations along the U.S. east coast corresponds to the date at which the normalized difference vegetation index (NDVI) reaches 80% of its seasonal maximum. Northward movement of the vernal front is similar to that obtained using Hopkins’ empirical rule; it is linearly related to leaf eme...

An analysis of boundary layer cumulus clouds and their impact on land surface–atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (∼1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (∼1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995–98 but reve...

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer-term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as QE, latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2–4 mm s−1) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single-variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with QE for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced QE relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen-covered surface in the overall water vapor exchange; subcanopy QE was 26% of the total.

Fluxes of carbon dioxide, water vapor, sensible heat, and momentum obtained over the boreal forest from the Twin Otter aircraft and six tower-based systems are compared. These measurements were collected as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) during three intensive field campaigns between May 25 and September 17, 1994. The representativeness of the tower-based measurements collected during BOREAS is discussed. Even though the net radiation from aircraft- and tower-based systems agreed well, in general, the aircraft tended to observe larger latent heat and smaller sensible heat fluxes than the towers. The CO2 fluxes from the aircraft were substantially less than from the tower, while the differences were relatively small for the momentum fluxes. The relationships between aircraft and tower-based flux measurements obtained by making repeated runs past various towers are used to scale up tower-based fluxes to a 16 × 16 km2 area near Prince Albert, Saskatchewan. It is demonstrated that except for a couple of cases primarily due to rapidly changing radiation conditions, this combination of measurements provides regional flux estimates of momentum, CO2, and sensible and latent heat similar to those obtained by flying a grid pattern over the area.

Tower fluxes measured at the Boreal Ecosystem-Atmosphere Study southern study area old aspen (SSA-OA), southern study area old jack pine (SSA-OJP), and northern study area old jack pine (NSA-OJP) sites during the three 1994 intensive field campaigns (IFCs) (May 24 to June 16 (IFC-1), July 19 to August 10 (IFC-2), and August 30 to September 19 (IFC-3)) were compared to fluxes simulated by a land surface model for the same period. Comparisons were limited to the average diurnal cycle for these periods to mitigate large day-to-day variability in the observations and problems with missing data. For consistency with the global implementation of the model, vegetation and soil parameters were not set to site-specific values but rather were the generic needleleaf evergreen and broadleaf deciduous vegetation and the generic sandy and loamy soil used in the global model. Despite the use of generic vegetation and soil, the model reasonably simulated the diurnal cycle of sensible heat, latent heat, net radiation, and CO2 fluxes for the SSA-OJP and SSA-OA sites. The main errors were that the model did not reproduce the midday reduction in latent heat seen at the SSA-OJP site during IFC-1 and IFC-2 and had less photosynthetic CO2 uptake than observed at the SSA-OA site. Differences in vegetation structure and physiology between the two sites were important to accurately simulate the fluxes. The needleleaf evergreen vegetation resulted in higher net radiation and a higher Bowen ratio than the broadleaf deciduous vegetation. Soil differences were less important. The NSA-OJP site was not so well simulated: midday latent heat flux was overestimated, and photosynthetic CO2 uptake was underestimated during each IFC. The only difference in the simulated southern and northern jack pine sites was in their atmospheric forcings; vegetation structure and soil types were the same. These results suggest the model is able to reproduce variability between vegetation types but not within vegetation types.

O contraste termico entre o rio Tapajos e FLONA Tapajos aliado as ocorrencias de ventos alisios fracos faz com que ocorra a brisa no sentido rio-floresta, alterando significativamente as caracteristicas termodinâmicas da CLA local. Os resultados mostram que existe um horario padrao de termino da brisa de rio com mais de duas horas, de duracao ate as 18 horas, com inicio variavel. As sondagens com balao cativo nao apresentam padroes caracteristicos, ocorrendo ou nao brisa do rio e, em outras, duas camadas de ar acopladas com caracteristicas bem definidas com informacoes do rio e floresta.