[Show abstract][Hide abstract] ABSTRACT: Ramp-like shapes in the turbulent scalar trace are the signature of coherent structures, and their characteristics (i.e., amplitude and duration) are resolved via a structure-function model for use in the surface renewal flux calculation. The potential for surface renewal to provide inexpensive sensible heat flux measurements has not been fully realized because this method has required calibration against eddy covariance or other more expensive flux measurement techniques. The calibration factor alpha is ideally 0.5, but a broad range of values have been reported in the surface renewal literature. Although it has been hypothesized that the sensor size, and hence sensor frequency response characteristics, influence alpha, no effort has been previously made to compensate the thermocouple signal in surface renewal measurements. We evaluate methods for compensating the frequency response of a thermocouple in the time domain and the frequency domain, and we present a novel method for compensation in the lag domain (i.e., compensating the structure function directly). We evaluated the compensation procedure as it affects the resolution of ramp characteristics at both the smallest and the second smallest scales of ramp-like turbulent shapes. The surface renewal sensible heat flux estimates from the compensated robust thermocouples (76 μm diameter wire) agree well with the estimates from the compensated fragile thermocouples (13 μm diameter). Using both the data collected for the present experiment and a meta-analysis of data in the surface renewal literature, we correct the surface renewal estimates for thermocouple frequency response characteristics to obtain alpha calibrations that converge to close to the predicted value of 0.5. We conclude that the frequency response characteristics of the thermocouple are the prevailing influence on the alpha calibrations reported in the literature.
Full-text · Article · Jun 2014 · Agricultural and Forest Meteorology
[Show abstract][Hide abstract] ABSTRACT: Approaches on modeling urban environment have often focused on specific components of cities complexity. In some simple models, cities have been simply described as a single-layer area showing aerodynamic and thermal variations from the surroundings. Multi-layer models have been used to describe vertical interactions within a multi-layered paradigm. These allow key features of turbulent exchange of physical quantities between levels, as well as vertically differentiated source and sink terms within the urban canopy. Another improvement is to describe anthropogenic fluxes of heat, later differentiating it between sensible and latent heat. However, usually heat fluxes from buildings have not also included traffic fluxes, and human-related fluxes have generally been ignored. Many of the building energy models used were usually run on a climatological basis, neglecting the actual daily and hourly local meteorological conditions. Furthermore, many models have ignored carbon dioxide fluxes , due both to the difficult modeling of anthropogenic emissions and to the lack of an adequate soil-canopy-atmosphere exchange model.
To overcome these limitations, the present work proposes an urbanized version of the UC Davis developed numerical SVAT model ACASA (Advanced Canopy-Atmosphere-Soil Algorithm). This model, initially created for vegetated canopies, has widely-tested and simulates CO2, moisture, radiation and sensible heat fluxes in natural environments. Its layered structure of soil and atmosphere permits a realistic third-order turbulent exchange of physical quantities. It can be run stand-alone (in-situ simulation on a footprint centered on a Eddy Covariance tower) or coupled interactively with the atmospheric model WRF (Weather Research and Forecasting) for regional scale simulation. The model ACASA has now been modified to include an improved, detailed urban scheme that accurately reproduces radiative and aerodynamic properties. A few basic anthropogenic fluxes (namely carbon dioxide from traffic and human respiration, and sensible heat from building heating) have been included, showing a discrete capability to reproduce observations. An additional, more complete set of urban parameterizations has been introduced, making the model able to simulate building and natural canopy in an integrated mode. No anthropogenic contributions are neglected, with the incorporation of fluxes of CO2, moisture and sensible heat from buildings, traffic and human population. Within the vertical ACASA structure, each of these source/sink contributions is assigned the proper vertical level, then the associated scalars and vector quantities are turbulently diffused between layers.
The present work simulates fluxes in a complex urban environment, starting from meteorological data and footprint morphology evaluation. These serve as inputs for the ACASA model that predicts fluxes of carbon dioxide, water vapour, and sensible heat. Predictions are then validated against experimental eddy-covariance data, focusing on impact of the newly introduced anthropogenic fluxes parameterization. Model performance improvements are objectively assessed through evaluation of widely accepted statistical parameters. Results are further compared with existing scientific literature.
[Show abstract][Hide abstract] ABSTRACT: Among the uncertain consequences of climate change on agriculture are changes in timing and quantity of precipitation together with predicted higher temperatures and changes in length of growing season. The understanding of how these uncertainties will affect water use in semiarid irrigated agricultural regions depends on accurate simulations of the terrestrial water cycle and, especially, evapotranspiration. The authors test the hypothesis that the vertical canopy structure, coupled with horizontal variation in this vertical structure, which is associated with ecosystem type, has a strong impact on landscape evapotranspiration. The practical result of this hypothesis, if true, is validation that coupling the Advanced Canopy–Atmosphere–Soil Algorithm (ACASA) and the Weather Research and Forecasting (WRF) models provides a method for increased accuracy of regional evapotranspiration estimates.
ACASA–WRF was used to simulate regional evapotranspiration from irrigated almond orchards over an entire growing season. The ACASA model handles all surface and vegetation interactions within WRF. ACASA is a multilayer soil–vegetation–atmosphere transfer model that calculates energy fluxes, including evapotranspiration, within the atmospheric surface layer.
The model output was evaluated against independent evapotranspiration estimates based on eddy covariance. Results indicate the model accurately predicts evapotranspiration at the tower site while producing consistent regional maps of evapotranspiration (900–1100 mm) over a large area (1600 km[superscript 2]) at high spatial resolution (Δx = 0.5 km).
Modeled results were within observational uncertainties for hourly, daily, and seasonal estimates. These results further show the robustness of ACASA’s ability to simulate surface exchange processes accurately in a complex numerical atmospheric forecast model such as WRF.
Full-text · Article · Mar 2014 · Journal of Hydrometeorology
[Show abstract][Hide abstract] ABSTRACT: Abstract: Changes in urban climate greatly affect cities' liveability and human health. The main factor altering urban climate is recognized to be the increase in GHGs concentration (mainly carbon dioxide) due to human activities in urban areas (such as traffic, domestic heating/cooling, etc.). To reduce the impact of urbanization on climate, it is crucial to monitor the urban environment and to develop planning strategies, which could help in reducing carbon emissions. A new project just started in Italy with the purpose of providing new tools,methodologies, and strategies to local and regional authorities of the Island of Sardinia for reducing carbon emissions of cities. The general aim of the project is to develop a methodology framework, including inventory tools, direct measurements and models, for identifying and planning future urban low carbon scenarios. The project aims to quantitatively estimate the urban metabolism components and for this reason an Eddy Covariance station will be used to monitor long-term energy, water, and carbon fluxes over a Sardinian city. An inventory approach will also allow for monitoring of the air quality and identification of the main emissions sources. An innovative point of the project is the development of a modeling system to study the impact of different urban planning strategies on carbon emission rates. At this stage, interactions with local stakeholders are crucial to understand the real needs of the city in terms of future development and to identify the scenarios to test. Four models are involved: 1) the land surface model ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) will be calibrated to simulate the urban metabolism components at local scale 2) a Cellular Automata model will simulate the urban land-use dynamics for the next 10-20 years 3) a transportation model will estimate the variation of the transportation network load 4) the coupled model WRF-ACASA will simulate the urban metabolism components for the entire municipality. In this way, the impact of changes in land use demand and transportation network load in the city will be tested at both local and regional scale. At the end of the project in 2014, results will provide local authorities with detailed information about the temporal and spatial dynamics of CO2 emissions and the main environmental factors affecting emission rates. Results will also help in assessing the sustainability and the applicability of the identified planning strategies in mitigating CO2 emissions. In addition, stakeholders will have information to implement local environmental policies to meet the strategic objectives identified in the framework of Europe 2020. A detailed description of project activities and methods will be reported here.
[Show abstract][Hide abstract] ABSTRACT: Micrometeorological methods and ecosystem-scale energy and mass flux density measurements have become increasingly important in soil, agricultural, and environmental sciences. For many scientists without formal training in atmospheric science, these techniques are relatively inaccessible. Eddy covariance, surface renewal, and other flux measurement methods require an understanding of boundary layer meteorology and extensive training in instrumentation and multiple data processing programs. In this paper, we present an open-source turnkey data logger program that performs flux data acquisition and post-processing, returning to the user a simple data table with the corrected energy balance fluxes and quality control parameters. The underlying theory of the flux measurements is briefly discussed and the program design and execution are described line by line. Data were collected over a wheat canopy and the program results were tested against other flux processing software, including EdiRe and R. The energy balance fluxes appear reasonable for the fair weather conditions found during data collection period. The daily cumulative evapotranspiration values from the flux tower and logger program show strong agreement with measurements from a precision weighing lysimeter. The logger program can be accessed at sites.google.com/site/tmshapland.
No preview · Article · Jan 2013 · Italian Journal of Agrometeorology
[Show abstract][Hide abstract] ABSTRACT: Structure functions are used to study the dissipation and inertial range scales of turbulent energy, to parametrize remote turbulence measurements, and to characterize ramp features in the turbulent field. Ramp features are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. The analysis of structure functions to identify ramp characteristics is used in surface renewal methods for estimating fluxes. It is unclear how commonly observed different scales of ramp-like shapes (i.e., smaller ramps and spikes embedded in larger ramps) influence structure function analysis. Here, we examine the impact of two ramp-like scales on structure function analysis using artificially generated data. The range of time lags in structure function analysis was extended to include time lags typically associated with isotropic turbulence to those larger than the ramp durations. The Van Atta procedure (Arch Mech 29:161–171, 1977) has been expanded here to resolve the characteristics of two-scale ramp models. This new method accurately, and in some cases, exactly determines the amplitude and duration of both ramp scales. Spectral analysis was applied to the structure functions for a broad range of time lags to provide qualitative support for the expanded Van Atta procedure results. The theory reported here forms the foundation for novel methods of analyzing turbulent coherent structures.
No preview · Article · Oct 2012 · Boundary-Layer Meteorology
[Show abstract][Hide abstract] ABSTRACT: Ramp features in the turbulent scalar field are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. Although finer scale ramp-like shapes embedded within larger scale ramp-like shapes can readily be perceived in turbulent scalar traces, their presence has largely been overlooked in the literature. We demonstrate the signature of more than one ramp scale in structure functions of the turbulent scalar field measured from above bare ground and two types of short plant canopies, using structure-function time lags ranging in scale from isotropic to larger than the characteristic coherent structures. Spectral analysis of structure functions was used to characterize different scales of turbulent structures. By expanding structure function analysis to include two ramp scales, we characterized the intermittency, duration, and surface renewal flux contribution of the smallest (i.e., Scale One) and the dominant (i.e., Scale Two) coherent structure scales. The frequencies of the coherent structure scales increase with mean wind shear, implying that both Scale One and Scale Two are shear-driven. The embedded Scale One turbulent structure scale is ineffectual in the surface-layer energy and mass transport process. The new method reported here for obtaining surface renewal-based scalar exchange works well over bare ground and short canopies under unstable conditions, effectively eliminating the α calibration for these conditions and forming the foundation for analysis over taller and more complex surfaces.
No preview · Article · Oct 2012 · Boundary-Layer Meteorology
[Show abstract][Hide abstract] ABSTRACT: Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present(1-3). Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity(3,4). The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming(5-8). Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30-298 Tg C yr(-1) during the 2000-2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions(7) and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177-623 Tg C yr(-1) in western North America could disappear by the end of the century.
[Show abstract][Hide abstract] ABSTRACT: Since urban population is growing fast and urban areas are recognized as
the major source of CO2 emissions, more attention has being dedicated to
the topic of urban sustainability and its connection with the climate.
Urban flows of energy, water and carbon have an important impact on
climate change and their quantification is pivotal in the city design
and management. Large effort has been devoted to quantitative estimates
of the urban metabolism components, and several advanced models have
been developed and used at different spatial and temporal scales for
this purpose. However, it is necessary to develop suitable tools and
indicators to effectively support urban planning and management with the
goal of achieving a more sustainable metabolism in the urban
environment. In this study, the multilayer model ACASA (Advanced
Canopy-Atmosphere-Soil Algorithm) was chosen to simulate the exchanges
of heat, water vapour and CO2 within and above urban canopy. After
several calibration and evaluation tests over natural and agricultural
ecosystems, the model was recently modified for application in urban and
peri-urban areas. New equations to account for the anthropogenic
contribution to heat exchange and carbon production, as well as key
parameterizations of leaf-facet scale interactions to separate both
biogenic and anthropogenic flux sources and sinks, were added to test
changes in land use or urban planning strategies. The analysis was based
on the evaluation of the ACASA model performance in estimating urban
metabolism components at local scale. Simulated sensible heat, latent
heat, and carbon fluxes were compared with in situ Eddy Covariance
measurements collected in the city centre of Florence (Italy).
Statistical analysis was performed to test the model accuracy and
reliability. Model sensitivity to soil types and increased population
density values was conducted to investigate the potential use of ACASA
for evaluating the impact of planning alternative scenarios. In this
contest, an in progress application of ACASA for estimating carbon
exchanges alternative scenarios is represented by its integration in a
software framework composed by: (i) a Cellular Automata model to
simulate the urban land-use dynamics; (ii) a transportation model, able
to estimate the variation of the transportation network load; (iii) the
ACASA model, and (iv) the mesoscale weather model WRF for the estimation
of the relevant urban metabolism components at regional scale. The CA
module is able to produce future land use maps, which represent a
spatial distribution of the aggregate land-use demand consistent with
the main rules governing the functioning of an urban system. Such future
land use maps, together with the street network including the current
traffic data, are used by the transportation module for estimating
future traffic data coherent with the assumed land uses trends. All
these information are then used by the coupled model WRF-ACASA for
estimating future maps of CO2 fluxes in the urban area under
consideration, allowing to estimate the impact of future planning
strategies in reducing C emissions. The in-progress application of this
system to the city of Florence is presented here.
[Show abstract][Hide abstract] ABSTRACT: Tundra ecosystems are especially sensitive to climate change, which is particularly rapid in high northern latitudes resulting in significant alterations in temperature and soil moisture. Numerous studies have demonstrated that soil drying increases the respiration loss from wet Arctic tundra. And, warming and drying of tundra soils are assumed to increase CO 2 emissions from the Arctic. However, in this water table manipulation experiment (i.e., flooding experiment), we show that flooding of wet tundra can also lead to increased CO 2 loss. Standing water increased heat conduction into the soil, leading to higher soil temperature, deeper thaw and, surprisingly, to higher CO 2 loss in the most anaerobic of the experimental areas. The study site is located in a drained lake basin, and the soils are characterized by wetter conditions than upland tundra. In experimentally flooded areas, high wind speeds (greater than ∼4 m s -1) increased CO 2 emission rates, sometimes overwhelming the photosynthetic uptake, even during daytime. This suggests that CO 2 efflux from C rich soils and surface waters can be limited by surface exchange processes. The comparison of the CO 2 and CH 4 emission in an anaerobic soil incubation experiment showed that in this ecosystem, CO 2 production is an order of magnitude higher than CH 4 production. Future increases in surface water ponding, linked to surface subsidence and thermokarst erosion, and concomitant increases in soil warming, can increase net C efflux from these arctic ecosystems.
Full-text · Article · Apr 2012 · Global Biogeochemical Cycles
[Show abstract][Hide abstract] ABSTRACT: A crucial point in urban sustainable development is to evaluate the
impact that future planning alternatives has on the main factors
affecting the citizens liveableness, as the development of the urban
heat island or the carbon emissions level. Recent advances in
bio-physical sciences have led to new methods and models to estimate
energy, water, and carbon fluxes. Also, several studies have addressed
urban metabolism issues, but few have integrated the development of
numerical tools and methodologies for the analysis of fluxes between a
city and its environment with its validation and application in terms of
future development alternatives. Over the past several years and most
recently within the European Project "BRIDGE", CMCC tested the ACASA
(Advanced-Canopy-Atmosphere-Soil Algorithm) land-surface model over
agricultural ecosystems (grapes), wild vegetation (forests and
Mediterranean maquis), and urban (Florence) or mixed urban/vegetated
land (Helsinki). Preliminary results show success in adapting the model
to mixed urban systems in each of the main fluxes of interest. The model
was improved to adapt it for urban environment, and key
parameterizations of leaf-facet scale interactions permit separate
accounting of both biogenic and anthropogenic flux sources and sinks,
and allow for easy scenario building for simulations designed to test
changes in land use or urban planning. In this way, sustainable planning
strategies are proposed based on quantitative assessments of energy,
water, and carbon fluxes. In this research, three planning alternatives
accounting for an increase in urbanization intensity were tested by
ACASA in Helsinki (Finland) for the year 2008. Helsinki is located at a
high latitude and is characterized by a rapid urbanization that requires
a substantial amount of energy for heating. The model behavior for the
baseline and alternatives scenarios (i.e., urban classes with low, mid,
and high residential intensity) during the entire year was investigated
and the model results were compared with in situ Eddy Covariance energy
and mass flux measurements. Model sensitivity to land use change and
increased population density values was tested individually first. Then,
the impact of the three urban classes was evaluated by analyzing energy
and mass fluxes produced by combining soil type classes, varying from
silty-clay-loam to sand and bedrock, to increased population density
values, respectively. Preliminary results are shown and statistical
analysis was performed in order to evaluate the model performance for
each scenario. From this first analysis, it appeared that ACASA model
was able to adequately reproduce the increase in urban heat island and
carbon emissions related to rapid urbanization. Also, the model could be
used to simulate urban fluxes at both local and regional scale (when
coupled to the mesoscale model WRF) and help local administration in
planning future sustainable development strategies.
[Show abstract][Hide abstract] ABSTRACT: Vegetation is an important element of the climate system due to its
influence on atmospheric processes through precipitation interception,
moisture exchange, surface roughness, energy conversion, and momentum
and gas exchanges (Monteith and Unsworth, 1990). Oversimplifying the
vegetation representation can lead to the lost of vital information that
can impact climate. This is especially true for regional scale studies
where the influence of vegetation increases with increasing spatial
resolution. This study examines the impact of vegetation canopy
structure and leaf area index on surface fluxes over California. High
spatial resolution satellite MODIS LAI data are used in the medium
complexity WRF-NOAH and high complexity WRF-ACASA coupled models to
simulate the interactions between the land surface and the atmosphere.
ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) is a multilayer land
surface model with interactive biophysical, meteorological and full
surface hydrological processes that allows micro-environmental variables
such as air and canopy temperature, wind speed and humidity to vary
vertically. Water vapor, energy and momentum fluxes between the
atmosphere and the land surface are calculated in ACASA through third
order closure turbulence equations and complex canopy physiology, such
as 10 leaf classes for better light and precipitation interception. It
allows counter-gradient transport that low-order turbulence closure
models are unable to simulate. Therefore, the coupled model WRF-ACASA
can take full advantage of high resolution MODIS LAI data to better
represent the land surface layer. The model simulations with and without
MODIS LAI data for both WRF-NOAH and WRF-ACASA are compared with the
surface observations to study the impact of a more realistic LAI
representation in the models on surface fluxes.
[Show abstract][Hide abstract] ABSTRACT: The measurement of ecosystem-scale energy and mass fluxes between the
planetary surface and the atmosphere is crucial for understanding
geophysical processes. Surface renewal is a flux measurement technique
based on analyzing the turbulent coherent structures that interact with
the surface. It is a less expensive technique because it does not
require fast-response velocity measurements, but only a fast-response
scalar measurement. It is therefore also a useful tool for the study of
the global cycling of trace gases. Currently, surface renewal requires
calibration against another flux measurement technique, such as eddy
covariance, to account for the linear bias of its measurements. We
present two advances in the surface renewal theory and methodology that
bring the technique closer to becoming a fully independent flux
measurement method. The first advance develops the theory of turbulent
coherent structure transport associated with the different scales of
coherent structures. A novel method was developed for identifying the
scalar change rate within structures at different scales. Our results
suggest that for canopies less than one meter in height, the second
smallest coherent structure scale dominates the energy and mass flux
process. Using the method for resolving the scalar exchange rate of the
second smallest coherent structure scale, calibration is unnecessary for
surface renewal measurements over short canopies. This study forms the
foundation for analysis over more complex surfaces. The second advance
is a sensor frequency response correction for measuring the sensible
heat flux via surface renewal. Inexpensive fine-wire thermocouples are
frequently used to record high frequency temperature data in the surface
renewal technique. The sensible heat flux is used in conjunction with
net radiation and ground heat flux measurements to determine the latent
heat flux as the energy balance residual. The robust thermocouples
commonly used in field experiments underestimate the sensible heat flux,
yielding results that are less than 50% of the sensible heat flux
measured with finer sensors. We present the methodology for correcting
the thermocouple signal to avoid underestimating the heat flux at both
the smallest and the second smallest coherent structure scale.
[Show abstract][Hide abstract] ABSTRACT: The number of urban metabolism studies has increased in recent years,
due to the important impact that energy, water and carbon exchange over
urban areas have on climate change. Urban modeling is therefore crucial
in the future design and management of cities. This study presents the
ACASA model coupled to the Weather Research and Forecasting (WRF-ARW)
mesoscale model to simulate urban fluxes at a horizontal resolution of
200 meters for urban areas of roughly 100 km^2. As part of the European
Project "BRIDGE", these regional simulations were used in combination
with remotely sensed data to provide constraints on the land surface
types and the exchange of carbon and energy fluxes from urban centers.
Surface-atmosphere exchanges of mass and energy were simulated using the
Advanced Canopy Atmosphere Soil Algorithm (ACASA). ACASA is a
multi-layer high-order closure model, recently modified to work over
natural, agricultural as well as urban environments. In particular,
improvements were made to account for the anthropogenic contribution to
heat and carbon production. For two cities four climate change and four
urban planning scenarios were simulated: The climate change scenarios
include a base scenario (Sc0: 2008 Commit in IPCC), a medium emission
scenario (Sc1: IPCC A2), a worst case emission scenario (Sce2: IPCC
A1F1) and finally a best case emission scenario (Sce3: IPCC B1). The
urban planning scenarios include different development scenarios such as
smart growth. The two cities are a high latitude city, Helsinki
(Finland) and an historic city, Florence (Italy). Helsinki is
characterized by recent, rapid urbanization that requires a substantial
amount of energy for heating, while Florence is representative of cities
in lower latitudes, with substantial cultural heritage and a
comparatively constant architectural footprint over time. In general,
simulated fluxes matched the point observations well and showed
consistent improvement in the energy partitioning over urban regions. We
present comparisons of observed (EC) tower flux observations from the
Florence (Ximeniano) site for 1-9 April, 2008 with results from two sets
of high-resolution simulations: the first using dynamically-downscaled
input/boundary conditions (Model-0) and the second using fully nested
WRF-ACASA (Model-1). In each simulation the model physics are the same;
only the WRF domain configuration differs. Preliminary results (Figure
1) indicate a degree of parity (and a slight statistical improvement),
in the performances of Model-1 vs. that of Model-0 with respect to
observed. Figure 1 (below) shows air temperature values from observed
and both model estimates. Additional results indicate that care must be
taken to configure the WRF domain, as performance appears to be
sensitive to model configuration.
[Show abstract][Hide abstract] ABSTRACT: The Advanced Canopy–Atmosphere–Soil Algorithm (ACASA) model is used to predict energy, water and carbon fluxes over a Mediterranean maquis site located in North-Western Sardinia (Italy) and the model performance is evaluated. Flux simulations are compared with Eddy Covariance field measurements collected from 2004 to 2007. The site experiences a drought season during the summer months in which the vegetation becomes water stressed. Results from the months of January, April, and July are analyzed
to demonstrate the model behavior in different environmental conditions. In general, simulated and observed fluxes matched when both the thermal and moisture regime are optimal. During the July water stress period the model underestimated latent heat and carbon fluxes due to a strong stress response linked to soil properties and plant physiological characteristics. The selection of values for key parameters, e.g. maximum ideal photosynthetic capacity (RUBISCO), wilting point, soil water content, and root and
leaf area ratio, is crucial to obtain close agreement between simulated and observed fluxes. The model was designed so that the most sensitive parameters are measurable quantities. Using the ACASA model to predict energy and mass fluxes between the vegetation and atmosphere appears promising in this context, and it could significantly improve our ability to estimate fluxes for use in future studies.
Full-text · Article · Feb 2011 · Agricultural and Forest Meteorology
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to evaluate the performance of the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model used to predict energy, water and carbon fluxes over a Mediterranean maquis ecosystem located in North-Western Sardinia (Italy). Vegetation at the experimental site is composed by sclerophillous shrub species experiencing a drought season during the summer months. ACASA consists of an advanced scaling model from the leaf and soil level to the canopy level. The model employs a process-based interactive set of modules that include radiative transfer within the ecosystem, ecophysiological response of the vegetation to soil and atmospheric conditions, column water, snow and ice hydrology, and sophisticated interlayer turbulent transfer physics. Parameters are added to account for soil moisture stress, which is simulated with a soil water transport model. These linked models automatically yield carbon dioxide exchange and transpiration by accounting for stomatal control of evapotranspiration. Turbulent exchange between the layers and the atmosphere is described by the ACASA higher-order closure model, which allows counter-gradient transport that simpler models are unable to describe. ACASA requires (1) plant and soil characteristics, (2) 30-minute meteorological data, and (3) initial soil water content conditions. Input data came from in situ measurements or were selected from the literature when observations were unavailable. Flux simulations were compared with Eddy Covariance field measurements collected from 2004 to 2007 (more details in the abstract Spano et al.). Results are showed for three months (January, April, and July) per each year. These months are selected since they can be assumed to be representative of the site environmental conditions during the year. January represents the typical winter time conditions of the experimental site, with little downwelling solar radiation, low air temperature values, and no water limitations. April is the most representative of the spring season showing an optimal combinations of environmental conditions. In the end, July represents the drought season (high vapor pressure deficit and water stress conditions). In general, simulated and observed fluxes matched when both the thermal and moisture regime are optimal. During the July water stress period, the model underestimated latent heat and carbon fluxes due to a strong stress response linked to soil properties and plant physiological characteristics. The selection of values for key parameters, e.g. maximum ideal photosynthetic capacity (Rubisco), wilting point, soil water content, and root and leaf area ratio, is crucial to obtain close agreement between simulated and observed fluxes. The model was designed so that the most sensitive parameters are measurable quantities. Using the ACASA model to predict energy and mass fluxes between the vegetation and atmosphere appears promising in this context, and it could significantly improve our ability to estimate fluxes for use in future studies.
[Show abstract][Hide abstract] ABSTRACT: This research focuses on the Wind Late Successional Reserve of Southern Washington where clear-cut logging over the past 100 years has created a fragmented landscape of coniferous forests that range in age from 0 to 500 years. In this study, we integrate several datasets to examine the environmental drivers of carbon exchange in this region across time and space. These sources include: (1) network of flux towers across a disturbance choronosequence, (2) MODIS Enhanced Vegetation Index, (3) aboveground net primary production (ANPP) from forest inventories, (4) and regional precipitation and air temperature measurements from the NOAA network of weather stations and PRISM reanalysis data. Net ecosystem exchange of carbon (NEE) has been measured at the Wind River Canopy Crane AmeriFlux site since 1998. The canopy crane is located in an old-growth forest composed of late seral Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). Two flux towers were erected in early seral stands to study the effects of silviculture on net ecosystem exchange. CO2 uptake at the old-growth stand is highest in the spring before bud break when air and soil temperatures and vapor pressure deficit are relatively low, and soil moisture and light levels are favorable for photosynthesis, while maximum CO2 uptake is observed two to three months later at the early seral stands and coincide with peak leaf area index. This CO2 pattern is driven by different water conserving strategies. A reduction in carbon exchange is observed at the old-growth forest when moisture becomes limiting and canopy conductance rates drop sharply after mid-morning in the summer. In contrast, inhibition in canopy conductance rates and CO2 exchange is not observed at the early seral stands until soil moisture levels become critically low at the very end of the summer. The regional MODIS data (200 km X 200 km area) from 2000-2008 show that annual variability in the Enhanced Vegetation Index (EVI) also can be linked to precipitation and temperature anomalies at the stand level and across the region. Regional EVI anomalies are strongly negatively correlated with the annual precipitation and air temperature anomalies once the MODIS pixels are carefully examined with regards to forest age. EVI data from the tower-centered pixel also correlate well with annual NEE at the AmeriFlux site and show promise for scaling sparse flux tower observations, even over old-growth forests. Lastly, permanent plots have been continuously measured in the old-growth stand since 1947 and provide long-term data on tree demographics, recruitment, growth and mortality, and show evidence of decadal variability in response to precipitation and air temperature anomalies, as well as to disturbance (e.g., a Douglas-fir beetle kill in the 1950's). We take advantage of the overlapping measurement period 1998-2004 and compare ANPP from the forest inventories to the flux tower estimates of NEE and MODIS EVI with focus on the regional environmental drivers.
[Show abstract][Hide abstract] ABSTRACT: The number of urban metabolism studies has increased in recent years, due to the important impact that energy, water and carbon exchange over urban areas have on climate change. Urban modeling is therefore crucial in the future design and management of cities. This study presents the ACASA model coupled to the Weather Research and Forecasting (WRF-ARW) mesoscale model to simulate urban fluxes at a horizontal resolution of 200 meters for urban areas of roughly 10 by 10 km. As part of the European Project ``BRIDGE'', these regional simulations were used in combination with remotely sensed data to provide constraints on the land surface types and the exchange of carbon and energy fluxes from urban centers.Surface-atmosphere exchanges of mass and energy were simulated using the Advanced Canopy Atmosphere Soil Algorithm (ACASA). ACASA is a multi-layer high-order closure model, recently modified to work over natural, agricultural as well as urban environments. In particular, improvements were made to account for the anthropogenic contribution to heat and carbon production. In order to more accurately simulate the mass and energy exchanges across larger urban regions, ACASA was coupled with a mesoscale weather model (WRF). Here we present ACASA-WRF simulations of mass and energy fluxes over over two different urban regions: a high latitude city, Helsinki (Finland) and an historic European city, Florence (Italy). Helsinki is characterized by recent, rapid urbanization that requires a substantial amount of energy for heating, while Florence is representative of cities in lower latitudes, with substantial cultural heritage, a huge tourist flow, and an architectural footprint that remains comparatively constant in time. The in-situ ACASA model was tested over the urban environment at local point scale with very promising results when validated against urban flux measurements. This study shows the application of this methodology at a regional scale with high spatial resolution for several urban centers in Europe. In general, simulated fluxes matched the point observations well and showed consistent improvement in the energy partitioning over urban regions.