This study assesses the Moderate-resolution Imaging Spectroradiometer (MODIS) BRDF/albedo 8day 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.
•We evaluated the MCD43A Albedo during dormant and snow covered period.•Spatial representativeness analysis is necessary for the albedo evaluation.•MODIS albedo performs well during vegetation dormancy and snow cover.
The impact of increases in drought frequency on the Amazon forest’s composition, structure and functioning remain uncertain. We used a process- and individual-based ecosystem model (ED2) to quantify ...the forest’s vulnerability to increased drought recurrence.
We generated meteorologically realistic, drier-than-observed rainfall scenarios for two Amazon forest sites, Paracou (wetter) and Tapajós (drier), to evaluate the impacts of more frequent droughts on forest biomass, structure and composition.
The wet site was insensitive to the tested scenarios, whereas at the dry site biomass declined when average rainfall reduction exceeded 15%, due to high mortality of large-sized evergreen trees. Biomass losses persisted when year-long drought recurrence was shorter than 2–7 yr, depending upon soil texture and leaf phenology.
From the site-level scenario results, we developed regionally applicable metrics to quantify the Amazon forest’s climatological proximity to rainfall regimes likely to cause biomass loss > 20% in 50 yr according to ED2 predictions. Nearly 25% (1.8 million km2) of the Amazon forests could experience frequent droughts and biomass loss if mean annual rainfall or inter-annual variability changed by 2σ. At least 10% of the high-emission climate projections (CMIP5/RCP8.5 models) predict critically dry regimes over 25%of the Amazon forest area by 2100.
We examine the physical and biological responses of forest canopies to step changes in light caused by passing low cumulus clouds that intermittently block the direct solar beam. Using data obtained ...at a tropical rainforest and at a midlatitude deciduous forest, we estimate the course of sensible heat flux, net ecosystem exchange, evapotranspiration, and water-use efficiency in response to the rapid changes in the incident radiative flux. To describe these fluxes during the interval over which the effects of stomatal time delays can be most influential, eddy fluxes are estimated over minute or shorter intervals by invoking a conditional-sampling procedure based on forming a Reynolds-average ensemble. The most important differences between the two forests’ physical responses are in the thermal balances and heat-flux time response constants. During the initial period after the light transition the only mean variables that show appreciable changes are the blackbody and air temperatures, the other scalars being little affected. We find that a distinct transient thermal internal boundary layer appears ≈ 20 m thick above the temperate deciduous forest and ≈ 45 m thick above the tropical rainforest. At each forest, the effective thickness of the inferred thermal outer-canopy ‘big leaf’ is about 1 mm. Twenty minutes after the abrupt change in incident light, ensemble eddy-flux estimates approach those found using conventional time averaging, confirming the validity of the ensemble approach. Previously unrecognized transient maxima in net ecosystem exchange and evapotranspiration are evident 5–10 min following the shadow-to-light transition, longer than the average light interval between shadows observed on partly-cloudy days in each case. Short-term variations in sensible heat flux, net ecosystem exchange, and evapotranspiration approximate an exponential adjustment, implying that first-order time-dependent single-leaf models are adequate to describe whole-canopy processes in these cases, providing an experimental method for determining whole-canopy bulk stomatal time constants. During the sunlit interval (direct and diffuse radiative fluxes combined) net ecosystem exchange is enhanced, while under cloud shadow (only diffuse radiative flux) water-use efficiency increases. This light and shadow alternation provides a mechanism describing the observed enhanced net ecosystem exchange and water-use efficiency under certain types of partly-cloudy sky. We apply empirical flux-response curves to an idealized case of radiative flux varying in a regular on–off light and shadow pattern. For this case, an analytical solution for mean net ecosystem exchange flux as a function of diffuse-radiation fraction yields results that strongly resemble previous findings based on conventional time-averaged fluxes. Our analysis and modelling indicates that a well-known correlation between diffuse radiative flux and enhanced net ecosystem exchange and water-use efficiency on cloudy days is in many cases not causal, but rather to be a consequence of time averaging over light-and-shadow intervals. By linking processes associated with photosynthesis in fluctuating light at the leaf scale to the canopy scale, our efforts facilitate the scaling-up of leaf responses to the ecosystem scale.
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
∘
N
,
72.17
∘
W
) and one made at a similar tower in a tropical rain forest (Tapajós National Forest, Pará, Brazil:
2.86
∘
S
,
54.96
∘
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 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.
We examined the net-ecosystem-exchange (NEE)-based annual carbon-balance estimates obtained from eddy-covariance (EC) measurements at an unmanaged sedge-grass marsh ecosystem (Třeboň, Czech Republic, ...49°1′ N, 14°46′ E), seeking methods to improve the EC measurements in inhomogeneous environment. The data filtering procedure was developed using three thresholds: (a) a stationarity test; (b) a stability
u
∗
-threshold; and (c) a high relative humidity
RH
-threshold. This procedure was tested in 2014, a year without significant floods and drought events led to a stable water table, reducing the effect of soil respiration on the EC measurements. Estimates of annual carbon-balance were reduced from 182 to 234 ± 12 gC m
−2
year
−1
for the initial data to 39–44 ± 8 gC m
−2
year
−1
after the
RH
≤ 95% filtering and to 24–26 ± 7 gC m
−2
year
−1
after the further
u
∗
≥ 0.1 m s
−1
filtering. Applying the precipitation/fog threshold reduced this balance to 10–12 ± 7 gC m
−2
year
−1
, closer to carbon neutrality. Up to 9.5% of this identified shift occurred during apparent nocturnal downslope katabatic drainage flows or plume descent coming from the nearby town of Třeboň. High-
RH
conditions account for up to 27% of this shift. Moreover, both conditions together account for an additional 67% of the identified carbon-balance change. Removing these non-ecosystem-related processes brings EC measurements closer to values of an unmanaged-ecosystem productivity, providing a better NEE-based estimate for the net ecosystem production. The presented procedure is applicable to EC measurements conducted at different wetlands or terrestrial ecosystems with similar conditions.
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 Ricrat locations with “nonideal fetch.”
Observing subcanopy CO2 advection STAEBLER, Ralf M; FITZJARRALD, David R
Agricultural and forest meteorology,
04/2004, Letnik:
122, Številka:
3-4
Journal Article
Vegetation is a climate modifier: It is a primary modifier, such as the Amazon rain forest, or secondary modifier, such as the agricultural fields of Pannonian lowlands in Central Europe. At periods ...of winter crop spring renewal and the start of the orchard growing season, enhanced evapotranspiration shifts energy balance partitions from sensible toward latent heat flux. This surface flux alteration converges into the boundary layer, and it can be detected in the daily variations of air temperature and humidity as well as daily temperature range records. The time series of micrometeorological measurements and phenological observations in dominant plant canopies conducted by Forecasting and Reporting Service for Plant Protection of the Republic of Serbia (PIS) are explored to select indices that best record the signatures of plant growth stages in temperature and humidity daily variations. From the timing of extreme values and inflection points of relative humidity (R1 and R2) and normalized daily temperature range (DTR/Td), we identified the following stages: (a) start of flowering (orchard)/spring start of the growing season (crop), (b) full bloom (orchard)/development (crop), (c) maximum LAI reached/yield formation (orchard and crop), and (d) start of dormancy (orchard)/leaf drying (crop). The average day of year (DOY) for dominant plants corresponds to the timing obtained from climatological time series recorded on a representative climate station.
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.
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