Ecosystem-biogeochemistry-climate

Whereas in the past, meteorology and climatology were separate fields, be it only because of disparate time (and length scales as well), it appears today that the two fields are strongly coupled, not only as the climate gives the boundaries for investigating the weather, but also because localized events can influence the larger climatological scales. The specific items on which ESSL scientists focused in this year are related to the role of aerosols in climate and weather, to the coupling of eco-systems, biochemistry and climate, to climate change, climate variability and extreme weather such as hurricanes, to interactions of the water cycle with climate and weather, to the impacts of climate and weather on society and ecosystems and finally to megacities and the effects of urbanization; the latter priority is a highlight for NCAR and concerns the international multi-agency field campaign that took place in Mexico city and combined interactive modeling as well. The laboratory highlights are related to the role of aerosols, to the regional carbon cycle, to a numerical simulation of turbulence, to landfall of hurricanes, to the global and regional water cycles and to polar climate.

Regional carbon cycle [Highlight] - TIIMES
Numerical simulation of turbulence [Highlight] - MMM
Fine mesh land model - TIIMES
Global biogeochemical cycles - TIIMES
Bioemissions and photochemical processing - ACD
Emission inventories and application - ACD
iLeaps Contributions - ACD
Ecosystem - biogeochemistry - climate interactions: Carbon Cycle - CGD
Structure and evolution of clear and cloudy atmospheric boundary layers - MMM
Land- atmosphere coupling - MMM
Atmosphere/ocean interactions - MMM

Regional carbon cycle

The top panel shows the spatial distribution of averaged CO2 at 1 m above the ground over about a 1 km x 1km area and the bottom panel shows the CO2 along the distance across Como Creek (140 m). The plot demonstrates that nighttime CO2 accumulation was transported toward low lying grounds, even toward the 2-m-wide creek along the main slope; however, the spatial distribution of CO2 was also affected by low respiration from the water and local mixing generated by the thermal contrast between the water and its surrounding land, leading to low CO2 right above the water (not included in the top panel). High resolution figure

NCAR was one of the hosts of the seventh International Carbon Dioxide Conference. Measurements and instrument development projects have provided key NCAR contributions to the multi-agency North American Carbon Program (NACP) and CarboEurope. These projects include:

• Rocky RACCOON (The Regional Atmospheric Continuous CO₂ Network in the Rocky Mountains) expands the CO₂ monitoring network to the western U.S. to include four sites installed in fall of 2005 and spring of 2006:  Niwot Ridge T-Van (NWR), near Ward, Colorado; Storm Peak Laboratory (SPL) near Steamboat Springs, Colorado; Fraser Experimental Forest (FEF), near Fraser Colorado; and Hidden Peak (HDP), near Snowbird, Utah.
• AIRCOA (The Autonomous Inexpensive Robust CO₂ Analyzer) measures CO₂ concentrations at 3 levels on a tower, producing individual measurements every 2.5 minutes precise to 0.1 ppm CO₂ and closely tied to the World Meteorological Organization (WMO) CO₂ scale. A key component in the robustness of these analyzers is near real-time data processing with extensive automated diagnostic tests to verify normal operation and make new results available from a web interface every day. The success of the AIRCOA units has generated considerable outside interest, and the design has been shared with trained university (Pennsylvania State University, Oregon State University, Stanford University) collaborators installing similar systems around the U.S. NCAR researchers are also collaborating with member institutions of the CarboEurope project to deploy AIRCOA sensors at 5 sites in Europe. With support from the Earth Observing Laboratory (EOL) and the Director’s office, we have initiated a project in collaboration with a University of Colorado graduate student (Sherri Heck) to install AIRCOA units for CO₂ measurements on the Navajo Reservation and in Africa, to provide carbon cycle insights, and to provide a basis for local education and capacity building.
• Community Airborne Oxygen Instrument has been developed. It is based on a vacuum-ultraviolet absorption technique. Because of the unique relationships between industrial, terrestrial, and oceanic exchange of carbon and oxygen, this instrument promises valuable insights into these processes.
• MEDUSA (Multiple Enclosure Device for Unfractionated Sampling of Air) has completed upgrades to and certification of a flask sampler in anticipation of participation in the Brazilian-Amazonia Regional Carbon Airborne (BARCA) study scheduled for this fall, as well as in future studies on the C130 and GV aircraft.
• O₂ / CO₂ Calibration Facility is now complete and available for community use. The Research Aviation Facility (RAF) of the EOL continues to support these efforts and RAF chemistry measurements by providing reference gases tied to internationally-recognized calibration scales. We have participated in two international round-robin cylinder comparison exercises in the past year, WMO (CO₂) and GOLLUM (O₂). Preliminary results show that our calibration system is performing very well in comparison to other participating laboratories. These instruments have also supported the work of two international visitors to NCAR.
• Analysis of light-aircraft vertical CO₂ profiles measured at 12 global sites by 6 international laboratories. The results suggest a significant revision to the consensus view of the global carbon cycle by revealing systematic biases in atmospheric models that predict large northern terrestrial CO₂ uptake and large tropical CO₂ releases.

The difficulty in understanding the carbon dioxide (CO₂) budget over complex terrain, and particularly its contribution to the global carbon balance, has led to two complementary field studies: the Carbon in the Mountain Experiment (CME) and the Airborne Carbon in the Mountain Experiment (ACME04). CME was an intensive ground-based field campaign over the foothills of the Colorado Front Range from 10 June to 5 October 2004. ACME was an airborne campaign conducted on a large-scale area of the Front Range from mid-May to the beginning of August, 2004. This unique combination allowed us to scale up the CO₂ budget from several hundreds of meters to regional scales.

Nighttime-respired CO₂ accumulated in North Park, Colorado, a bowl-shaped area surrounded by mountains. The disappearance of the accumulated high CO₂ concentration in the morning due to the reverse of the slope flow and convective mixing was slower than expected. The CO₂ concentration decreased exponentially with height even around 10 a.m. On the local scale, we found that at the Niwot Ridge CME site, the CO₂ concentration increased with decreasing altitude. High CO₂ was transported toward Como Creek, which runs along the main slope and is only about 2 m wide (Figure 1 - top). However the spatial variation of CO₂ was also affected by the low respiration of CO2 from the cold water in Como Creek and local mixing generated by the thermal contrast between the creek and the surrounding land. As a result, the CO₂ concentration was lower just over the water surface than it was over its banks (Figure 1 - bottom).

For the future, NCAR is collaborating with investigators at University of Colorado, Colorado State University, and Harvard University to plan and conduct the ACME07 campaign on the University of Wyoming King Air. This campaign will be the second Airborne Carbon in the Mountains Campaign, focusing on Colorado and Wyoming, and will include flights from early spring through fall. The payload will include the RAF community oxygen instrument to be completed this fall. Two additional Rocky RACCOON sites will be added in the coming year in further support of NACP and will collaborate with Pennsylvania State University, Colorado State University, Ohio State University, and other institutions to formalize a national CO₂ observing network and pursue improvements in inter-site comparability and data management. The MEDUSA sampler will participate in the BARCA campaign, collecting flasks using during 4 weeks of flights on a Brazilian Lear Jet over the Amazon, and analyzing the measurements made on these flasks at Scripps, NOAA/GMD, and CU INSTAAR. The Community Airborne Oxygen Instrument will be completed and tested as we pursue collaborations for its use on NSF aircraft. Five more AIRCOA samplers will be completed and deployed in the Netherlands, Germany, France (2), and Spain in collaboration with the CarboEurope project. We will continue our work on science and capacity building through the NCAR-CU Navajo and African CO₂ measurement project. Continuing data analysis from both CME and ACME will focus on problems such as how topography affects CO₂ transport differently during day and night, how this deviates from CO₂ transport over flat terrain, and how regional and global terrestrial carbon budgets are affected.

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Numerical simulation of turbulence

Contours of potential temperature (left) and vertical velocity (right) fluctuations on a horizontal cross section at z=100 m from the outer-domain LES. Results show both flows generated from the outer and inner domains blend smoothly across the nest boundaries and behave the same statistically. The expansion of LESs for real-world PBLs will advance our understanding of the role of PBLs in climate, weather and air quality, and will improve the representation of PBL processes in large-scale forecast models. High resolution figure

The representation of the planetary boundary layer (PBL) and its associated clouds (e.g., subtropical marine stratocumulus and shallow cumulus) remains one of the biggest challenges for climate modeling and weather forecasting. The problem has been tackled in the past using the Large-Eddy Simulation (LES) approach. However, so far, LES applications have been limited to idealized PBLs over horizontally uniform surfaces or periodic striplike heterogenous surfaces where periodic boundary conditions can be applied in the horizontal directions. For simulations of real-world PBLs, which are mostly over complex terrain, periodic lateral boundary conditions are no longer adequate. One way to solve the problem is to nest LES domain(s) of interest inside mesoscale domain(s), where mesoscale and turbulence motions can both be resolved and interact. With increasing computer power, this multi-scale nesting approach for turbulence is becoming feasible. However, it is necessary to first examine the capability of horizontal nesting for LES in simplified situations.

Using the dynamic framework of the WRF-ARW model, ESSL/MMM scientists examined the model's horizontal nesting capability by designing an LES-within-LES experiment, where a finer-grid LES is horizontally nested inside a coarser-grid LES. For this first experiment, horizontally homogeneous PBLs were considered and thus the outer-domain LES can still use periodic boundary conditions in the horizontal directions, just like traditional LESs. However, the inner-domain LES no longer uses periodic boundary conditions; rather, it uses the specified flow field along the nest boundaries based on the outer-domain flow. Both LESs are driven by the same forcing under the same large-scale condition and thus are expected to behave the same statistically. Having the two domains interacting with each other is a strict test for the LES's sensitivity to grid nesting and to subgrid-scale modeling because any difference in their net statistical behavior would be amplified near the nest boundaries. With proper modifications to the WRF model, this LES-within-LES experiment shows that two-way grid nesting can be applied to LES types of turbulence simulations. The two simulated turbulent flows (see figure) blend in smoothly across the nesting boundaries, and produce similar statistics, which also compare well with previous LESs. Future plans for ESSL/MMM scientists include expanding this nest-LES study for more complex and realistic PBLs, such as those with clouds, over realistic surface conditions, and interacting with mesoscale events and deep convection.

Support for this work includes NSF and the Office of Naval Research.

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Fine mesh land model

The top panels show the spatial distribution of averaged CO2 at 1 m above the ground over about a 1 km x 1km area and the bottom panel shows the CO2 along the distance across Como Creek (140 m). The plot demonstrates that nighttime CO2 accumulation was transported toward low lying grounds, even toward the 2-m-wide creek along the main slope; however, the spatial distribution of CO2 was also affected by low respiration from the water and local mixing generated by the thermal contrast between the water and its surrounding land, leading to low CO2 right above the water (not included in the top panels). High resolution figure

Several parallel activities at NCAR have demonstrated the need for land models to improve their spatial representation of key land-atmosphere exchange processes for simulating and predicting land surface processes at length scales far finer than those of the parent atmospheric models. The goal of this project is to develop and implement a framework for modeling land surface processes using NCAR community land models at scales several multiples finer than those of parent atmospheric models. The approach that has been taken is to enhance and implement the fine-mesh model of Hahmann and Dickinson (2001) within the Community Atmosphere Model - Community Land Model (CAM - CLM) climate model.

Completed Work: Work during FY06 has proceeded in the following three areas:

• Development of a sub-grid land model architecture within CAM-CLM to address the question, What is the effect of fine-resolution, sub-grid land use/land cover specification on the simulated climate? In order to implement the fine-mesh adaptation of Common Land Model (CLM) in the coupled CAM-CLM model, new array structure and mapping procedure has been developed. Results from new simulations show a very modest effect on the global atmospheric circulation and thermodynamic structure, although regionally-specific responses are more pronounced.
• Implementation of a precipitation disaggregation scheme into the fine-mesh model which stochastically determines the sub-grid area occurrence and intensity of precipitation based on the coarse grid model precipitation. Work under this topic addresses the science question, How does the sub-grid character and disaggregation of precipitation affect land-atmosphere exchanges in a multi-scale climate modeling framework?  This code is currently being implemented, and current climate simulations are forthcoming. 
• Improved representation of sub-grid scale topography onto the fine mesh implementation of CLM seeks to address the question, What is the influence of a fine-resolution, sub-grid representation of topography on the simulated climate? Three key improvements were required for a spatially-explicit sub-grid representation of topography including implementation of fine-mesh topographic datasets, a dynamic lapse rate interpolation/extrapolation, and partitioning of rain versus snow. Preliminary results are highlighted in Figure 1.

Plans for FY07 and Beyond - The fine-mesh model as developed under this project now provides a general framework for improvement of the land model and sets the stage for improvement in biogeochemical modeling. Continued development and testing of the fine-mesh model in order to refine the upscaling and downscaling methodologies used to pass information between the land surface model and the parent atmospheric model. Alternative methodologies to disaggregate and aggregate highly heterogeneous fluxes such as precipitation, heat, moisture and momentum (i.e. mountain wave drag) need to be evaluated. NCAR has anticipated petascale computing capabilities for application to numerical representations of the Earth System as a computational problem. The fine-mesh approach to climate modeling provides an ideal application in the peta-scale computing that can be applied in a coupled model mode or in an uncoupled mode, with very high resolution representation of the land surface. A demonstration project is underway for FY07 to perform long-term uncoupled CLM simulations at 0.1 deg × 0.1 deg grid spacing to study carbon, water and nitrogen exchange at landscape-relevant scales.

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Global biogeochemical cycles

As is clearly articulated in the Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC), to be released in 2007, there is increasingly strong motivation to examine terrestrial and oceanic carbon fluxes on regional to continental scales, to understand the coupling of the carbon cycle to the climate system and to other biogeochemical cycles, to understand the processes responsible for present uptake of anthropogenic carbon, to predict future trends in these fluxes under various climate change scenarios, and to assess potential strategies for increasing carbon uptake and storage into the future. The challenges of scaling up from local measurements and scaling down from global constraints are being addressed in TIIMES through the development and application of advanced observational and modeling tools.

There are systematic biases in atmospheric models that predict large northern terrestrial CO₂ uptake and large tropical CO₂ releases based on analysis of light-aircraft vertical CO₂ profiles measured at 12 global sites by 6 international laboratories - Northern Hemisphere sites include Poker Flat, Alaska, USA (PFA); Harvard Forest, Massachusetts, USA (HFM); Briggsdale, Colorado, USA (CAR); Park Falls, Wisconsin, USA (LEF); Estevan Point, British Columbia, Canada (ESP); Molokai Island, Hawaii, USA (HAA); Orleans, France (ORL); Zotino, Russia (ZOT); Sendai/Fukuoka, Japan (SEN); and Surgut, Russia (SUR).  Southern Hemisphere sites include Rarotonga, Cook Islands (RTA) and Bass Straight/Cape Grim, Australia (AIA) [Upper Panel]. These new findings suggest that previous estimates of the Northern Hemisphere “missing terrestrial carbon sink” were overestimated.

Carbon-nitrogen (C-N) coupling reduces by about a factor of four the global terrestrial carbon uptake response to increasing atmospheric CO₂ concentration, compared to the carbon-only response. Global integrated responses of Net Ecosystem Exchange (NEE) to variation in temperature and precipitation are damped by C-N coupling [Lower Panel]. The carbon-only model predicts that the NEE responses to variation in temperature and precipitation increase in magnitude under increasing atmospheric CO₂ concentration, while the C-N model predicts that these responses decrease in magnitude. Fertilization responses in the C-N model to increasing CO₂ and increasing mineral nitrogen deposition are in qualitative agreement with results from field experiments.

The changes in the North and South Atlantic sea surface temperatures (SSTs) resulting from North African atmospheric dust transport may account for up to 50% of the Sahel  precipitation reduction, but this effect could be a duet of factors other than desert dust aerosols (e.g., biomass burning aerosols and other low-frequency processes). Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant due to the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, our analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation (Yoshioka, Mahowald et al. submitted to Journal of Climate).

FY07 plans are to continue moving towards more fully coupled representations of biogeochemical cycles in the Community Climate System Model (CCSM). Key foci for the next year include: a) examination of the role of biomass burning, b) working towards coupling the chemistry and biogeochemistry through emission and deposition models, c) a continuing commitment to the carbon model inter-comparison, d) continued development of the coupled carbon nitrogen model, e) the CCSM Community Land Model (CLM) C-N model, and f) combining observations and models to understand both trends and inter-annual variability in the biogeochemical exchanges.

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Bioemissions and photochemical processing

Figure 1. Left panel: (MVK+MAC)/isoprene ratio plotted versus GPS altitude. The black line corresponds to the median and the dotted gray line to the mean for all data. The dashed blue lines represent lower and upper limits of modeled ratios. Right panel: Lower (180 pptv NO x and 30% cloud cover) and upper (320 pptv NO x and 100% cloud cover) limits of modeled OH density plotted as a function of GPS altitude. Red circle is the calculated OH density according to the PBL budget approach. Horizontal blue lines indicate the depth of the cloud layer. High resolution figure

ACD scientists conducted a number of laboratory and field studies to further characterize biogenic emissions of volatile organic compounds (VOCs). The laboratory experiments included studies of methanol, acetone, and isoprene. Methanol and acetone are among the dominant contributors to total biogenic VOC emissions and are important for atmospheric chemistry in some regions. ACD scientists characterized variations in biogenic methanol and acetone emission rates and used the results as the basis of a new numerical model of methanol and acetone emissions. The model is able to simulate most of the observed variations. Previous ACD field observations have demonstrated that high isoprene emissions can be induced by short term exposure to elevated ozone concentrations. ACD scientists and visitors Paul Palmer and Jed Sparks (Cornell University) investigated this phenomenon under controlled laboratory conditions. The studies demonstrated that the response of isoprene emission to elevated ozone is negligible in most cases.

Field studies were conducted by ACD scientists in Mexico, Japan, the U.S. Mojave Desert, and Brazil. As part of the MIRAGE field campaign in Mexico, ACD scientists measured ozone, VOC, particles and direct/diffuse radiation at the MIRAGE T1 supersite. A tethered balloon system was used to measure vertical variations in these constituents from the surface to about 1 km above ground level. An increase in solar radiation, and a decrease in the diffuse component, was observed with increasing altitude when the site was impacted by the Mexico City plume. In addition, ACD scientists deployed a PTRMS system on the NCAR C130 to measure VOC at a very high sampling rate. They are using measurements of VOC concentration fluctuations to characterize fluxes of these compounds.   ACD scientists are also collaborating with Jerome Fast (DOE) to use WRF-CHEM to examine the impact of the Mexico City plume on regional biogenic emissions. In addition, they are developing a fire emissions inventory (including urban fires) that will be available to all MIRAGE investigators to estimate the impact of fires on background air quality during the MIRAGE study.  

Observations of OH and other trace gases at rural and remote sites suggest that OH losses are considerably higher than what can be accounted for by the measured OH sinks. ACD scientists collaborated with Prof. Yoshizumi Kajii's research group in Tokyo Metropolitan University to investigate OH lifetime and sinks in August 2006 at the Tomakomai Experimental Forest in Hokkaido, Japan. Emissions and ambient concentrations of a wide range of biogenic compounds were measured to characterize compounds, including those not measured in previous studies, that contribute to OH loss. Scientists in both groups are currently analyzing the data.

Isoprene, monoterpenes and sesquiterpenes are all thought to be important contributors to biogenic secondary aerosol formation. ACD scientists used a new sampling/analytical technique to measure sesquiterpenes (SQTs) and even larger and heavier compounds (e.g. oxygenated sesquiterpenes and diterpenes). They collaborated with Mark Potosnak and Maria Papiez (U. Nevada Desert Research Institute) to quantify emissions of these compounds from plants growing in the Mojave Desert near Las Vegas, NV in July 2006. Additional measurements were made in Colorado at the NCAR Foothills Lab. greenhouse and at Niwot Ridge. From those experiments, they discovered that compounds heavier than sesquiterpenes are comparable in emission rate to monoterpenes and sesquiteprenes. Efforts to identify these unknown compounds are currently underway but it is likely that they are highly reactive with high aerosols yields (similar to sesquiterpenes) and so have a major impact on tropospheric chemistry.

Tropical landscapes are thought to be responsible for about 80% of global biogenic VOC emissions and yet are among the least understood.   Global chemistry and transport models often perform poorly when using the biogenic VOC emission rates recommended by current emission models. This could be due to uncertainties in emissions but could also be a result of inaccurate characterization of boundary layer meteorology and/or chemistry.   Aircraft measurements of biogenic VOC and their oxidation products were used to investigate the ability of photochemical models to predict atmospheric concentrations of OH and other oxidants.   The results illustrated in Figure 1 show that a photochemical model underestimates the OH by about a factor of 5 in comparison to a planetary boundary layer (PBL) approach.   

In order to have photochemical reactions accurately represented in models, the reactions and rate constants must have been studied extensively in the laboratory. ACD scientists from the chemical kinetics group have been heavily involved in writing a reference book Mechanisms of Atmospheric Oxidation of the Alkanes. Co-authors in the project are Jack Calvert (Emeritus ACD), Dick Derwent (RGD Consulting, Bracknell, UK) and Tim Wallington (Ford Motor Company). The project is partially funded by the Coordinating Research Council. The book includes summaries and recommendations for the kinetics of OH, Cl and NO 3 with alkanes and their first generation reaction products; photolysis data for the reaction products; evaluated data for the peroxy and oxy radicals involved in the oxidation; in depth discussions of the reaction mechanisms of the common alkanes; and a discussion of the treatment of alkane chemistry in atmospheric models on different scales. The book builds upon the experimental data acquired in the ACD labs over the years particularly with regard to peroxy and alkoxy radical chemistry. Because of the relatively long lifetimes of the alkanes, special attention is paid to the chemistry at the lower temperatures found in the free troposphere. The book, which should be completed in the Fall of 2006, will be a valuable resource for scientists working on the chemistry of organics in both polluted and clean areas. This has a bearing on both regional and global pollution.

FY2007 work will continue the investigation of factors affecting biogenic emissions in the laboratory and in the field. Ongoing laboratory studies of kinetics will also continue. This work was funded by NSF/NCAR and EPA.

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Emission inventories and application

	MEGAN - Global distribution of landscape-average isoprene emission factors (mg isoprene m−2 h−1). Spatial variability at the base resolution (~1 km) is shown by regional images of the southeastern U.S. and southeastern Australia. High resolution figure

ACD scientists have developed the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is a modeling system for estimating the net emission of gases and aerosols from terrestrial ecosystems into the atmosphere. It is driven by landcover, weather, and atmospheric chemical composition. MEGAN is a global model with a base resolution of ~ 1 km. It can run as a stand-alone model for generating emission inventories but is also being incorporated as an on-line component of chemistry/transport and earth system models. ACD scientists completed the development of the isoprene component of the MEGAN model and a manuscript describing this model was published and made available to the scientific community on the NCAR data portal. Several efforts are underway to incorporate MEGAN into chemistry and transport model systems including NCAR's WRF-CHEM and CCSM-CLM and the USEPA's regulatory models. Improved landcover for the U.S. was incorporated into the model and a method was developed to characterize urban vegetation.

ACD scientists also developed the North American wildfire emission model.   The model is designed to predict daily emissions from fires for all of North America at a 1km resolution. A manuscript describing the approach was published and has been made available to other researchers and regulatory air quality modelers.

FY2007 work will involve incorporation of MEGAN into the models listed above. This work is funded by NSF/NCAR and EPA.

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iLeaps Contributions

	The BEACCHON Project would be a study of the impact of biogenic aerosols on clouds and precipitation, especially focused on the links between carbon, nitrogen and water cycles. Its goal is to characterize and understand interactions between biogeochemical and water cycles across scales and their response to climate and land-use change. High resolution figure

ESSL scientists hosted the first conference of the Integrated Land Ecosystem-Atmosphere Processes Study (iLEAPS), a new ten-year land-atmosphere project of the International Geosphere-Biosphere Programme (IGBP). The goal of iLEAPS is to understand how interacting physical, chemical, and biological processes transport and transform energy and matter through the land-atmosphere interface. The project is designed to study interactions and feedbacks on scales from molecules to the entire globe, and from minutes to centuries, both past and future. The project brings together multi- and cross-disciplinary scientists to collaborate, distribute ideas and results rapidly, and increase scientific relations with developing countries. The iLEAPS International Project Office is based at the University of Helsinki in Finland.

Currently iLEAPS promotes 10 international research projects studying essential phenomena related to global climate change. These include the Amazon rainforests, West African and Asian monsoons, Southern Hemisphere fires, and Arctic and boreal vegetation zones.

In FY2007 ESSL scientists are currently developing a new land-atmosphere research initiative, BEACCHON, that will be proposed for sponsorship by iLEAPS. This planning work is funded by NSF/NCAR.

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Ecosystem - biogeochemistry - climate interactions: Carbon cycle

Figure 1. Relationship between growing season length and observed cumulative annual net ecosystem exchange (NEE), 1999 - 2004. “Growing season length” is defined as the interval between the spring onset of net negative NEE and the fall transition back to positive NEE: the interval between zero crossings. Longer growing seasons are associated with less annual CO2 uptake, probably because of the effects of snowmelt timing on water availability. The best-fit line, shown here, is given by the equation y = 1.37x - 305 (r2 = 0.45). High resolution figure

Change in climate, land use and disturbance regimes are driving increasingly evident changes to ecosystems. The impacts of these three related forces are complex and non-linear and current state of the art coupled carbon-climate models show divergent results. Reducing this uncertainty requires improved parameterizations of key ecosystem processes and better estimates of pertinent rate constants. The regulation of many ecosystem physiological processes is, to first order, understood. However, model comparisons show that different simulations, broadly including similar understanding, diverge widely when examined in climate change scenarios. The relationships between the modeled hydrological cycle and carbon dynamics, the effects of CO2 on photosynthesis and the dynamics of soil carbon are particular uncertainties. The subtle and nonlinear interactions amongst physiological and substrate level control of both photosynthesis and respiration are becoming clearer, and reveal that even small errors in simulated environmental regulation of fluxes can cause the net flux to be in error or even have the wrong sign.

While global modelers are working to develop new and more realistic approaches, estimation of accurate parameters has become a larger and larger problem. Developing and parameterizing models to capture these interactions and forecast them accurately has forced the development of a new family of modeling techniques, known in the literature as “data assimilation” and “model data fusion”, drawing on techniques pioneered in control theory, statistics and weather forecasting. Most of the extant papers in this genre have focused on understanding climate controls over ecosystem carbon fluxes, and their application to this problem is relatively paralleling the increase in process level understanding. While climate controls over terrestrial carbon fluxes are critical, changes to land use and disturbance regimes are at least as important and may interact with climate in complex ways. NCAR and University scientists have pioneered developments in ecosystem data assimilation in studying land use effects on biogeochemistry and have begun to actively address the role of disturbance. We are now integrating the relatively continuous, albeit nonlinear, effects of climate with the often abrupt or even discontinuous effects of land use and disturbance in an assimilation or model-data fusion approach. The multi-scaled, multi-process (biology, biophysics, wildfire and other dynamics, all responding to similar environmental drivers) nature of the earth system interactions poses a unique opportunity to interface novel and state-of-the art numerical and computational approaches within the terrestrial ecosystem domain. The achievement of these objectives will result in the rapid improvements in the realism and predictability of terrestrial biosphere simulation system. While the underlying process-level science and numerical techniques associated with climate and ecosystem simulations are relatively mature, our capacity to efficiently apply these data assimilation and error analysis techniques to complex models and to applications where significant non-linear behaviors occur is currently not well developed. Students of the land surface and terrestrial carbon problem have known for decades that this family of problems requires coupling “fast” and “slow” processes from turbulence to soil carbon dynamics, seconds to millennia, and more recently the importance of abrupt events has become evident (fires, severe storms). The mathematical legacy from control theory and meteorological data assimilation does not emphasize such multi-scaled processes. We are developing an assimilation system designed for the multi-scaled problem. Preliminary results computing fluxes from aircraft (over 100s of km) show interesting comparisons to local (1 km) flux observations, and suggest that process information from local observations can be combined with spatially integrated observations from aircraft or concentration observing networks.

The results suggest that seasonal cycle patterns can be observed using integrative regional patterns and interpreted using intensive local observations, combined within a consistent modeling framework. Preliminary results from this project show that carbon uptake can be estimated at multiple spatial scales even in mountainous landscapes, that soil moisture is perhaps the critical driver of variability in carbon fluxes, and that the principal sign of potential climate change in the Rockies, warmer springs and longer growing seasons, reduces carbon uptake by intensifying the mid-summer drought. These results are being used in the evaluation of process models and coupled models and should lead to an improved generation of simulations of the control of terrestrial carbon exchange by temperature, precipitation and soil moisture. In another line of work, global carbon assimilation models are being used with real and simulated data to assess future measurement strategies. An NCAR-developed 4-D variational assimilation model has been used to evaluate the information gain from proposed satellite missions. The NCAR CDAS (Carbon Data Assimilation) model suggests that the planned OCO (Orbiting Carbon Observatory) will significantly improve the retrieval of global CO2 fluxes, taking into account OCO’s sampling and precision characteristics. The model is also being used to evaluate potential strategies for routine data analysis after OCO flies. In summary, the NCAR carbon cycle analysis activities complement the development of the coupled carbon climate model through the development and testing of specific parameterizations, the development of benchmarks for the fully coupled model and the design of next-generation observations.

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Structure and evolution of clear and cloudy atmospheric boundary layers

Two techniques have been used to estimate the entrainment rate of the cloud-capped boundary layer in DYCOMS-II from aircraft measurements. The first uses the jump in eddy flux and mean concentration of a scalar across the boundary-layer top; the second uses a direct measurement of divergence from the wind field measured during the circular flight legs in the trade-wind boundary layer and the rate of change with time of the boundary-layer top. Estimates of the divergence from the 7 DYCOMS-II flights show that, with the exception of flight 4, the values are in reasonable agreement with the expected divergence, advancing our understanding of the evolution of the boundary layer. High resolution figure

Studies of marine stratocumulus and trade-wind boundary layers is an important element of the ESSL/MMM program. In July 2001, the NCAR C-130 aircraft conducted a research study in the marine stratocumulus regime off the southern California coast to investigate their structure and evolution. Globally, these persistent clouds significantly cool the Earth because they radiate at a temperature similar to the surface, and yet reflect a much larger fraction of solar radiation than clear ocean regions. A key variable needed to understand their evolution is the rate at which this cloud-capped boundary layer entrains overlying air, since this controls their liquid-water content and extent of cloud cover. The Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) Experiment utilized new technology and measurement strategies to obtain entrainment rates with unprecedented accuracy.

This technology was then applied to the trade-wind regime over the eastern Caribbean Sea during the Rain in Cumulus over the Ocean (RICO) Experiment from November 2004 through January 2005, in collaboration with ground-based and ship-based active remote sensing of clouds by radar. A major goal of RICO is to evaluate how shallow trade-wind cumulus clouds precipitate, how the precipitation affects the structure of individual cumuli, and the role of precipitation in determining the aggregate behavior of fields of clouds. A wide array of instrumentation was deployed, centered around an approximately 100-km radius area scanned by the NCAR S-Band Polarization Radar (SPol). In addition to SPol, RICO featured insitu measurements by three aircraft, ground sampling at Antigua, surface measurements from Spanish Point Barbuda, and a wide array of surface layer and active remote sensing from the NOAA ship RV Seward Johnson. Analysis of data from RICO is still in its early stages, but studies are underway to evaluate trade-wind cumulus cloud precipitation and its affect on the structure of individual cumuli and the agregate behavior of cloud fields.

The DYCOMS data set has provided the basis for an improved understanding of marine stratocumulus. Result highlights include: 1) The stratocumulus-topped boundary layer entrains less vigorously than previously imagined, perhaps by as much as 50%, and cloud-top-entrainment instability is not associated with local instability and enhanced mixing; 2) Large-Eddy Simulations tend to over-predict entrainment unless great care is taken to limit cloud-top mixing, both by reducing the vertical grid spacings and by ensuring that subgrid scale models are not active at cloud top; and 3) drizzle is prevalent and associated with morphologically distinct cloud forms, which we call POCs, for pockets of open cells. Support for this work includes NSF and NSF/ROCEW.

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Land-atmosphere coupling

	A photograph of one of the four arrays of sonic anemometers deployed in the Central Valley of California during the original HATS campaign, 2000. This array of sonic anemometers was used to measure subfilter scales of motion in the surface layer above short and sparse vegetation. High resolution figure

Over the last forty years, atmospheric research has shown that the land-atmosphere coupling is of critical importance for weather and climate prediction. Trees are a dominant feature on planet Earth. Forests cover some 30% of the world's land area, accommodate two-thirds of life on Earth, and are responsible for 90% of the biomass on solid ground. Due to its diverse nature, a tall canopy's influence on turbulent exchange is extremely complex, e.g., because of their spatial distribution, seasonal variability, flexibility, porosity, etc. Within the layer of the atmosphere directly influenced by the canopy, turbulence exhibits dramatically varying properties depending on the detailed structure of the roughness elements and cannot be described by traditional similarity relationships. Where vegetation covers the surface, it becomes the important momentum sink and a key player in distributing sources and sinks of moisture, heat and other chemical species. Parameterization of turbulent transport in and above tall canopies remains somewhat elusive but is essential for accurate weather and climate prediction. Large-eddy simulation (LES) is an important tool for studying the coupling between microscale and mesoscale motions. LES can also incorporate the influence of vegetation on momentum, energy, and scalars. Because observing three-dimensional and time-dependent fields of all quantities of interest is difficult, LES has become a direct link between currently observable quantities and larger-scale models which are forced to parameterize all of the turbulence.

A photograph of one of the walnut orchards being considered for the CHATS campaign. Scientists will deploy an array of sonic anemometers (similar to that seen in Figure 1) within a canopy of walnuts to examine the influence of distributed canopy sources/sinks of momentum and scalars on subfilter scales of motion. High resolution figure

LES needs to be validated and improved to deal with complex flows, especially for surface layers where dependence on the subfilter-scale (SFS) model increases. To address this issue, NCAR, in collaboration with several university groups, recently carried out two pioneering observational studies to improve subfilter-scale parameterizations over flat terrain with short sparse vegetation (Horizontal Array Turbulence Study, HATS, see Fig. 1) and over the ocean (Ocean HATS; OHATS). These studies provided an observational basis for testing and improving closure approximations used in LES and they have substantially increased our confidence in parameterizations developed using LES as their basis.

With NSF funding, NCAR scientists will conduct a third HATS-type experiment in the spring of 2007 called CHATS (Canopy HATS, http://www.eol.ucar.edu/rtf/projects/CHATS/isff/). In CHATS, researchers will investigate SFS transport of momentum and scalars within a walnut canopy in a central California orchard (see Fig. 2). At this point, the character of within-canopy SFS motions is not known, nor the role that the eddies shed in the lee of the plant elements play, nor how these wake-scale motions affect scalar and momentum transport. Therefore, the objectives of CHATS are to: 1) measure SFS variables in a complex environment linking the biosphere, geosphere, and the atmosphere, and 2) study the fundamental interaction between vegetation and atmospheric turbulence. CHATS will allow for validation of currently utilized SFS models and improvement of parameterizations representing this critical regime.

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Atmosphere/ocean interactions

{figure a} Comparison of modeled and observed SFS scalar flux in the atmospheric surface layer over a stationary rough surface (from HATS). Horizontal and vertical SFS scalar flux are indicated by black and red dots respectively, the solid black line is the 1-to-1 correlation between the observations and model predictions. Research such as this has provided new impetus to improve parameterizations in large eddy simulation (LES) codes. High resolution figure

Air-sea interaction occurs over a wide spectrum of scales ranging from millimeters (spray droplets and air bubbles) to hundreds of kilometers (synoptic-scale storms) and even larger (global climate). A goal of marine-surface-layer research is to identify and quantify coupling mechanisms that connect the atmospheric and oceanic boundary layers (the ABL and OBL) and surface waves. Some of the specific problems of interest in the ABL include the effects of wave age, swell, surface roughness, and wind-wave misalignment. In the OBL beneath waves, NCAR scientists are examining mixing in the presence of wave-averaged forces due to the mean Lagrangian current (Stokes drift), which induce Langmuir circulations, and a stochastic representation of breaking waves. Wave influences on the OBL are of particular importance under high-wind conditions.

Large-eddy simulation (LES) plays an important role in this air-sea interaction research and has provided insight into the couplings between imposed waves and turbulence. However, the fidelity of subfilter-scale (SFS) parameterizations used in LES for flows over complex geometry, e.g., a moving surface gravity wave field, is untested. Recent field campaigns such as the Horizontal Array Turbulence Study (HATS), conducted over land (http://www.eol.ucar.edu/rtf/projects/sgs2000/) have provided new impetus to improve parameterizations in LES codes. A natural progression in these observational investigations is to study increasingly complex flows. Results from the field campaign, the Ocean Horizontal Array Turbulence Study (OHATS), specifically directed at the measurement of SFS variables in the marine surface layer in the presence of surface waves (see http://www.eol.ucar.edu/rtf/projects/OHATS04), are described here. These observations can be used to improve the SFS parameterization in LES codes, and more broadly, to examine the impacts of water waves on surface-layer turbulence under a variety of atmospheric stability conditions and wave states. OHATS is a joint collaboration between NCAR/ESSL, Woods Hole Oceanographic Institute, and Pennsylvania State University scientists.

{figure b} Comparison of modeled and observed SFS scalar flux in the marine surface layer over moving waves (from OHATS). Horizontal scalar flux is indicated by open blue circles; vertical scalar flux [f3] is indicated by open green and orange circles; cases with significant wave height greater than 1 m are shown in orange. The results from HATS are the black and red dots. Observations such as these can be used to improve the subfilter-scale (SFS) parameterization in LES codes, and more broadly, to examine the impacts of water waves on surface-layer turbulence. High resolution figure

From the nearly three-month-long OHATS database, about 275 hours were identified for detailed analysis, which is approximately “12 days of data.” This subset of the OHATS database covers a wide range of atmospheric stability conditions and wave states. A comparison with flow over a rough land surface shows both similarities and differences. For example, the variation of the normal components of the SFS momentum flux are similar over land and water, while the larger scale (resolved-resolved) component of the vertical momentum flux is clearly enhanced by the presence of surface waves. One of the most surprising results concerns the impact of surface waves on SFS scalar flux as shown in figures (a) and (b). Over land, a simple rate equation model, developed by Wyngaard (2006), accurately predicts the variation of all components of the SFS scalar flux. However, the same model applied over ocean waves noticeably over predicts the SFS vertical scalar flux especially in cases with significant wave heights greater than 1 m and surface convection. Scientists’ first suspicion is that the wave field is inducing motions that are not accounted for in the modeling of the pressure destruction term and, in particular, the phase relationship between the wave induced pressure and scalar fields, which is important. In order to explore this speculation the wave correlated motions in the velocity, pressure, and scalar fields need to be identified. In the upcoming year, direct numerical simulation results for stratified flow over waves will be further analyzed to gain insight into the present results. In addition to continued analysis of OHATS data, NCAR scientists also plan to examine the impact of surface waves on OBL mixing, especially entrainment, under high-wind conditions. This work is supported by NSF and Office of Naval Research.

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