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GFDL Past Events & Seminars - 2014

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Date Speaker Affiliation Title of Presentation
Jan. 9Michael PratherUC-IrvineCan global chemistry-climate models reproduce air quality extremes? Can we use them to predict what happens with climate change?
One would not choose global atmospheric chemistry models to study air quality on a metropolitan scale, yet we need them to assess change in air quality at those scales when global changes are occurring in composition and climate. This study seeks metrics that allow direct comparison of the surface air quality monitoring stations with the global chemistry-transport models (CTMs) on an equal footing; it began as an NSF REU summer project and morphed into Mr. Jordan Schnell's doctoral dissertation (due 2015). Comparison of point-based station measurements to modeled grid-cell values, while done for most CTMs, is fundamentally unjustified and described in the literature as incommensurability. It has been addressed in part through interpolation techniques such as Kriging or inverse-distance weighting, but is still regarded as an unresolved problem. We use these methods in a new approach that seeks to calculate a grid-cell averaged value based on the station data, thus allowing direct comparison with CTM simulations and providing clear statistical information on model errors under different conditions (e.g., summer vs. winter, 50th vs 95th percentile, clear vs. cloudy). With these new tools applied to all of the US and EU air quality monitoring sites, we are able to cleanly diagnose the UCI CTM errors in simulating the maximum daily 8-hour average surface O3. For this case study, the CTM was run at 1° x 1° horizontal resolution using ECMWF IFS forecast meteorology for the years 2005-2006. This hindcast was done to match the NASA Aura O3 measurements (OMI, MLS, TES, and HIRDLS) and to look for tropopause folds and strat-trop exchange. The CTM did very well in matching the enhanced tropospheric O3 near folds on an hour by hour basis (see Tang & Prather, 2010, 2012ab, ACP). No specific effort was made to look at surface O3 data, and our initial comparisons with US EPA O3 data identified clear model biases of order +30 ppb over polluted regions with a consistent relative error from 50th to 97th percentiles. These diagnostics provide some insight into the source of error as well as clear metrics of any model improvements. Over the 21st century, we anticipate that (i) local/regional emissions will change in response to air quality mitigation efforts; (ii) atmospheric composition will change globally; and (iii) meteorological conditions contributing to the worst pollution episodes will shift with climate. In addressing point (iii), we need to identify and characterize extreme air quality (pollution) episodes, which we designate AQX. Using the daily MDA8 O3 gridded values, we define AQX events for cell in a climatological manner (i.e., return time) as the 10 worst days in a year (97.3 percentile). A clustering algorithm is then used to construct multi-day, multi-cell AQX episodes. Processing observations and CTM in the same manner shows that the model has considerable skill (24% for US, 32% for EU) in hindcasting these episodes. About 75% of the individual cell events occur in coherent, multi-day, connected episodes covering areas greater than 1000 x 1000 square km, and model skill is greatest with these large episodes. All of these tests are specific to exact-day matches (concurrent sampling) and cannot be used for free-running chemistry-climate models. Even the traditional approach of using the probability distribution is interpreted incorrectly when used as climatology. The use of a fixed return time in a future climate has problems given changing baseline levels. We investigate the climate statistics of AQX episode size from a decade of observations and show that his may provide a clear measure of how climate change may alter the meteorology of extreme pollution episodes.
Jan. 15Stan BenjaminESRLNumerical Weather Prediction Development at Global and Regional Scales at ESRL
ESRL continues to participate toward improving NOAA operational numerical weather prediction guidance through developing innovations in dynamic model cores, model parameterizations, and data assimilation techniques. An overview of the ESRL-developed FIM global model, under consideration for incorporation as part of the NCEP global ensemble, will be presented. FIM uses an icosahedral horizontal grid and hybrid isentropic-sigma quasi-lagrangian vertical grid. Results showing equal or improved forecast skill over GFS for upper-air and tropical cyclone data will be presented. Applications of the inline-chemistry version of FIM with aerosols and of the coupled FIM-HYCOM atmospheric-ocean model will also be shown. ESRL has also been the primary development institution for the NCEP hourly updated Rapid Refresh (RAP) and upcoming 3-km High-Resolution Rapid Refresh (HRRR). New implementations of RAP and HRRR at NCEP will occur over the next few months, and design of the model and assimilation components will be described along with verification performance. Recent innovations in model physics for boundary-layer, land-surface, and convection have been developed within the context of RAP and HRRR but are also being widely used by the international WRF community of users. Innovations for assimilation of radar, cloud, and surface observations have also been successful for improving severe weather forecasting and subsequently applied to the NCEP-led GSI community data assimilation system. Applications of the HRRR and RAP models for energy, severe weather, and aviation applications will also be presented.
Jan. 16Michelle BellYaleHealth Impacts of Airborne Particles and Sources: Recent Evidence and Remaining Challenges
Many studies have demonstrated that ambient particulate matter is associated with an increase in risk of adverse health effects, such as mortality and hospital admissions. A growing number of studies suggest that some particles may be more harmful than others, although air quality regulations are based on particles' total mass within a given size distribution such as fine particles (PM2.5). Policies intended to improve public health through air quality would benefit from information on which types and sources of particles are most harmful. We investigated the health effects of particulate matter with an emphasis on whether effects differ by the chemical composition and source. Methods include source factorization and use of PM2.5 chemical component data. Health effects estimates were generated with city-specific Poisson models in a first stage. Effect estimates from multiple communities and the estimates' statistical uncertainty were combined in a second stage using Bayesian hierarchical modeling in a second stage. This presentation will provide findings from studies of particles and mortality, hospital admissions, and adverse birth effects using multi-city studies in the U.S. In addition, remaining challenges for this type of research will be discussed.
Jan. 23Paul O'GormanMIT Cambridge MAThe response of snowfall extremes to climate change: theory and simulations
The response of snowfall extremes to climate change: theory and simulations
Jan. 29Kikuro MiyakodaGFDL EmeritusOceanic Process of ENSO
Oceanic Process of ENSO
Jan. 30Lynne TalleyScripps Institution of Oceanography, LaJolla, CA Subantarctic Mode Water and Antarctic Intermediate Water processes in the southeast Pacific
Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are major water masses of the Southern Hemisphere thermocline, found north of the Subantarctic Front (SAF) of the Antarctic Circumpolar Current in all three oceans. The densest SAMW and lightest AAIW are formed in the southeastern Pacific in the region of deep mixed layers north of the SAF. This talk reviews multiple publications by co-investigators, postdocs and graduate students resulting from winter 2005 observations in the southeast Pacific, as well as circumpolar analysis of the Southern Ocean State Estimate (SOSE) for years 2005-2006. Winter 2005 observations revealed very thick surface mixed layers that become SAMW and AAIW. SAMW was identified as the mode water of greatest thickness, in the region of laterally higher salinity north of the SAF, while the AAIW salinity minimum was identified as the densest winter surface mixed layer just to the north of the SAF. Local air-sea fluxes were sufficient to create the very thick (> 400 m) mixed layers from the previous summer's stratified waters, but cross-SAF flux of fresh water was necessary for the observed eastward freshening of the mixed layers. Production rates are estimated using several independent methods; those from SOSE based on air-sea fluxes show the relative importance of freshwater and heat exchanges in producing SAMW from both thermocline inputs and northward cross-frontal exchange.
Feb. 19Felicity GrahamUniversity of TasmaniaDoes the cold tongue bias in coupled models generate a new flavor of ENSO?
Does the cold tongue bias in coupled models generate a new flavor of ENSO?
Feb. 20 Kentaro SuzukiJet Propulsion Lab, Pasadena, CAEvaluation of the warm cloud microphysical processes in global models using the CloudSat/A-Train multi-sensor satellite observations
Evaluation of the warm cloud microphysical processes in global models using the CloudSat/A-Train multi-sensor satellite observations
Feb. 26Huan GuoUCARA PDF-based turbulence and cloud scheme in AM3: AM3-CLUBB
A PDF-based turbulence and cloud scheme in AM3: AM3-CLUBB
Mar. 5Gabriel VecchiGFDLGFDL-FLOR: A new seasonal to decadal prediction model targeting regional water and extremes
GFDL-FLOR: A new seasonal to decadal prediction model targeting regional water and extremes
Mar. 12Tom PeacockMITDeep ocean topographic scattering of the low mode internal tide
Deep ocean topographic scattering of the low mode internal tide
Mar. 13Alex HallUCLA Developing a Physical Understanding of Regional Climate Change
I will present results from an ongoing study of climate change in the Los Angeles region. Using a combination of dynamical and statistical downscaling techniques, the region's climate is projected under "mitigation" and "business-as-usual" forcing scenarios. Two time slices are examined, mid-21st century and end-of century. All global climate models associated with the CMIP5 archive are downscaled. The project is an opportunity to address a number of general questions related to the science of climate change at the regional scale. These include: Can regional climate projections be trusted, and what information about regional scale change really is latent in the GCM projections? How can one downscale multiple models and multiple time slices efficiently so as to have conventional estimates of ensemble-mean outcomes and associated uncertainties? Given the generally higher levels of natural variability at the regional scale, how significant is climate change at the scales where climate change adaptation actually occurs?
Mar. 20Alex GuentherPNNLBiogenic volatile organic emissions from a changing planet
Biogenic volatile organic compounds (BVOC) emitted by terrestrial ecosystems into the atmosphere play an important role in determining atmospheric constituents including the oxidants and aerosols that control air quality and climate. Accurate quantitative estimates of BVOC emissions are needed to simulate these processes in earth system models in order to develop effective air quality and climate management strategies. BVOC emissions are highly sensitive to environmental factors that are variable in the short-term and have long-term trends including climate, landcover, and ecosystem stress. Considerable progress has been made in understanding how BVOC emissions will respond to these changes but there are also major gaps in our knowledge. BVOC emissions can influence, and are influenced by, climate and ecosystem stress resulting in the added complexity of potential feedback couplings that must be quantified. Our current understanding of the response of biogenic VOC to changes in the earth system will be described and potential feedback couplings will be discussed. Recently developed techniques for multi-scale (molecules to global) observations will be described and an approach for integrating these data to quantify BVOC emission factors using all available observations will be shown. These observations include BVOC fluxes from individual cells and leaves measured using enclosure systems, from whole canopies with instruments deployed on above-canopy tower platforms, from entire landscapes with airborne eddy covariance flux systems, and entire regions using satellite data. A strategy will be presented for improving BVOC emission estimates from our changing planet using the next version of the BVOC emission model, Model of Emissions of Gases and Aerosols from Nature (MEGAN), that will include better representation of biological and chemical diversity and more mechanistic emission algorithms.
Mar. 26John KrastingGFDLTrajectory sensitivity of the transient climate response to cumulative carbon emmissions
Trajectory sensitivity of the transient climate response to cumulative carbon emmissions
Mar. 27John FyfeCanadian Centre for Climate Modelling and Analysis (CCCma), Environment CanadaSouthern Wind, Ice and Carbon
I will present some of our recent results towards understanding the observed changes in Southern Hemisphere surface westerlies, Antarctic sea-ice extent and ocean carbon uptake. In the first part of my presentation I will argue against polar ozone depletion and Antarctic ice-sheet retreat as the main causes of the observed increase in Antarctic sea-ice extent, and will argue instead for internal variability. In the second part of my presentation I will show that systematic errors in simulated Southern Hemisphere surface westerlies may have caused Earth System Models to overestimate ocean carbon uptake in recent decades.
Apr. 2Elizabeth Drenkard Woods Hole Oceanographic InstitutionExploring Climate Change on Equatorial Pacific Islands
Exploring Climate Change on Equatorial Pacific Islands
Apr. 3Greg JohnsonPMEL, SeattleAntarctic Bottom Water Warming, Contraction, Freshening, and Contributions to Sea Level Rise
Antarctic Bottom Water Warming, Contraction, Freshening, and Contributions to Sea Level Rise
Apr. 9Yanni DingU of MarylandAtlantic-Arctic Ocean variability in CMIP5 historical simulations
Atlantic-Arctic Ocean variability in CMIP5 historical simulations
Apr. 10Katja FennelDalhousie University - HalifaxSimulating hypoxia in the northern Gulf of Mexico
A large hypoxic zone develops every summer in the northern Gulf of Mexico and is thought to negatively impact benthic fauna including species of commercial importance. In response to these concerns a Taskforce consisting of multiple U.S. state and federal agencies and tribes was formed and, in its 2008 Action Plan, called for a decrease of the hypoxic zone through nutrient management in the watershed. However, an interplay of several physical and biogeochemical factors controls the formation of the hypoxic zone, and interannual variations in its size are significant. A quantitative understanding of the relative importance of these different factors is thus important for developing effective nutrient reduction targets and for articulating reasonable expectations as to when nutrient reductions will produce measurable reductions in hypoxic conditions. Realistic numerical models help build mechanistic understanding of the processes underlying hypoxia formation and of interannual variability in hypoxic zone size. I will present such a model and, based on model results, will 1) assess the sensitivity of hypoxia predictions to model resolution, variations in sediment oxygen consumption and choice of physical horizontal boundary conditions, 2) present a framework for uncertainty analysis of model predictions resulting from errors in forcing terms and other model inputs, and 3) discuss the relative importance of nitrogen versus phosphorus limitation as well as wind forcing on hypoxia development and show the results of nutrient load reduction experiments.
Apr. 16Kathy Pegion ESRL/CIRESUntangling ENSO Precursors
Untangling ENSO Precursors
Apr. 17Chris BrethertonU of Washington, SeattleLow cloud feedback on climate change: A large-eddy simulation perspective
Low cloud feedback on climate change: A large-eddy simulation perspective
Apr. 24Axel TimmermannU of HawaiiExplaining the continuum of Dansgaard Oeschger variability
Explaining the continuum of Dansgaard Oeschger variability
May. 28Ryan AbernathyLamont-Doherty Earth Observatory, NYPhase speed of ocean eddy fluxes in the Pacific
Phase speed of ocean eddy fluxes in the Pacific
May. 29Claudia TebaldiNCARHow different is too different? How different is different enough?
As the climate modeling community approaches the time when a new experimental design for future climate simulations will be proposed to guide the next generation of coordinated coupled model experiments (CMIP Phase 6) we explore differences among scenarios based on the available experiments from CMIP5 (mostly through results from idealized 1% CO2 scenarios, but also checking for consistency of the results through historical and RCP4.5 simulations). Our two goals are achieved with the same methodological approach, despite being motivated by opposite concerns: on the one hand, we ask what level of radiative forcing -- or global average temperature change -- causes climate to change significantly enough to warrant exploring the underlying scenario through coupled model simulations. Significantly enough -- or different enough -- is defined here in terms of significantly different (both statistically and in magnitude) values of local temperature at a large-enough fraction of grid-points over the Earth's surface. On the other hand, the opposite concern may be answered by our results: what level of radiative forcing is small enough to prevent climate outcomes from being significantly (by the same definition as above) different from a baseline? The first type of answers can inform the choice of a set of scenarios to be run by CMIP6 participating ESMs, avoiding costly redundancies given the computational resources required to run these simulations. The second type can allow policy relevant explorations of alternative mitigation choices, in particular assessing the consequences of stabilization paths at slightly different target levels and of differently sized overshoots. It can also provide guidance when conducting impact analysis by matching pre-existing RCP-driven outcomes to new socio-economic scenarios. The latter may imply slightly different radiative forcing but our analysis could be used to decide if still comparable climate outcomes could be assumed.
Jun. 4Wenchang Yang Lamont Doherty Earth ObservatoryEast African precipitation and droughts: from decadal variability to annual cycle
East African precipitation and droughts: from decadal variability to annual cycle
Jun. 5Frederic VitartECMWF- Reading UKMJO and tropical cyclone prediction at ECMWF
MJO and tropical cyclone prediction at ECMWF
Jun. 11Stephen GriffiesGFDLA survey of Coordinated Ocean-ice Reference Experiments (COREs)
A survey of Coordinated Ocean-ice Reference Experiments (COREs)
Jun. 12T. N. KrishnamurtiFlorida State University, Tallahassee, FloridaA Monsoonal link to the rapid Arctic ice melt
This study provides a monsoonal link to the rapid Arctic ice melt. Each year the planetary scale African Asian monsoonal outflow near the tropopause carries a large anticyclonic gyre that has a longitudinal spread which occupies nearly half of the entire tropics. In recent years, the south Asian summer monsoon has experienced increased rainfall over the northwest of India and Pakistan and it has also contributed to more intense local anticyclonic outflows from this region. The western lobes of these intense upper high pressure areas carry outflows with large heat fluxes from the monsoon belt towards central Asia and eventually to the region of the rapid ice melt of the Canadian Arctic. In this study this spectacular pathway has been defined from air flow trajectories, heat content and heat flux anomalies. Most of these show slow increasing trends in the last 20 years. The monsoonal connection to the rapid Arctic ice melt is a new contribution of this study. This is shown from the passage of a vertical column of large positive values of the heat content anomaly that can be traced from the Asian monsoon belt to the Canadian Arctic. The heat flux along these episodic and intermittently active pathways is shown to be considerably larger than the atmospheric poleward flux across latitude circles and from the oceans.
Jun. 18 Sergey ZilitinkevichU of Helsinki, Finnish Meteorological InstituteNature, theory and parameterization of atmospheric convective planetary boundary layer
Nature, theory and parameterization of atmospheric convective planetary boundary layer
Jun. 25Michael Bueti University of Rhode Island Tropical Cyclone-Induced Thermocline Warming and its Regional and Global Impacts
Tropical Cyclone-Induced Thermocline Warming and its Regional and Global Impacts
Jul. 2Xianglei HuangU of MichiganThe spectral dimension of longwave feedbacks in the CMIP3 and CMIP5 experiments
TBA
Jul. 9Baylor Fox-KemperBrown University, Providence, Rhode IslandSurface wave effects on ocean fronts
Surface wave effects on ocean fronts
Jul. 16Poster Expo Organized by GFDLGFDLPoster Expo
Poster Exposition. For more information click here.
Jul. 23Hiroyuki MurakamiGFDL Projected future changes in tropical cyclones at regional scale
Projected future changes in tropical cyclones at regional scale
Jul. 30Tom DelworthGFDLA link between the hiatus in global warming and North American drought
A link between the hiatus in global warming and North American drought
Sep. 10Tra DinhAOSImpacts of thin cirrus clouds on humidity in the tropical tropopause layer and lower stratosphere
Impacts of thin cirrus clouds on humidity in the tropical tropopause layer and lower stratosphere
Sep. 17V. BalajiCICS PrincetonClimate computing: the state of play
Climate computing: the state of play
Sep. 18Alexander KhainHebrew UniversityHigh Resolution WRF Simulations of Hurricane Irene: Sensitivity to Aerosols and Choice of Microphysical schemes
High Resolution WRF Simulations of Hurricane Irene: Sensitivity to Aerosols and Choice of Microphysical schemes
Sep. 24Ian Grooms NYU Courant Institute of Mathematical SciencesStochastic parameterization of QG flows
Stochastic superparameterization (SP) is a multiscale framework for the development of stochastic parameterizations. This talk reviews the recent development and application of stochastic SP to mesoscale ocean eddy parameterization in coarse and eddy-permitting quasigeostrophic models. A main feature of the method is the inclusion of stochastic backscatter, which leads to increased model variability. The method is tested in a doubly-periodic `eddy-permitting' setting, where the model grid scale resolves the peak of the kinetic energy spectrum but not the deformation radius, and in a coarse-resolution channel model with complex topography. In the eddy permitting setting the stochastic backscatter parameterizes the inverse cascade of kinetic energy from unresolved scales, and is crucial for accurate representation of large-scale dynamics. In the channel model the primary impact of the method is increased temporal variability, though the variability remains too low overall. A path towards implementation in primitive equation models is sketched at the end.
Sep. 25Dave ThompsonCSUPeriodic Variability in the large-scale middle latitude atmsophere
Periodic Variability in the large-scale middle latitude atmosphere
Oct. 1Charlie StockGFDL Amplification of Ocean Productivity Trends in a Changing Climate
Amplification of Ocean Productivity Trends in a Changing Climate
Oct. 9Ted ShepherdUniversity of ReadingThe role of the stratosphere in tropospheric circulation
The role of the stratosphere in tropospheric circulation
Oct. 16Josefino ComisoNASA/GSFC Greenbelt, MDRapid Decline of the Arctic Sea Ice Cover and Environmental Impacts
Rapid Decline of the Arctic Sea Ice Cover and Environmental Impacts
Oct. 22David NeelinUCLAPrecipitation sensitivity and fast-process diagnostics
Precipitation sensitivity and fast-process diagnostics
Oct. 23Brian MapesU of MiamiPrecipitable water: the PV of the tropics?
Precipitable water: the PV of the tropics?
Oct. 29Lee MurrayColumbiaCharacterizing key processes controlling variability in the oxidative capacity of the atmosphere
Tropospheric abundances of the hydroxyl radical (OH) and other oxidants determine the "oxidative capacity"of the atmosphere, setting the lifetime before chemical removal of many air pollutants and reactive greenhouse gases such as methane. Understanding the mechanisms controlling oxidant variability in the past and present is important if we are to predict how their levels may change in a warming world. Here, I present work aimed at identifying, characterizing, and understanding the natural and anthropogenic processes that drive variability in the oxidative capacity. I first derive from steady-state assumptions a simple linear relationship relating global mean OH to convolved changes in tropospheric ozone photolysis frequencies, water vapor concentrations, and the ratio of reactive nitrogen to reduced carbon emissions, and show that this relationship holds across a range of emission and climate conditions in a single model (the global GEOS-Chem Chemistry-Transport Model). I then apply this relationship to explain the large spread in global mean OH and its temporal evolution simulated by the ensemble members of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), finding an additional strong sensitivity to inter-model variability in the NOx lifetime and the efficiency of the hydrocarbon oxidation mechanism. I next demonstrate that space-based satellite observations of stratospheric ozone (a proxy for tropospheric ozone photolysis frequencies), water vapor, and tropospheric columns of NO2 and CO can be combined into a proxy for temporal (monthly) variability in OH at the local scale over the past decade. Previous work has shown that lightning NOx emissions are a strong lever on tropospheric oxidizing capacity, and I conclude with preliminary estimates quantifying the chemistry-climate feedback from this key process on the oxidant chemistry across the 21st century, using the GISS ModelE2 chemistry-climate model.
Oct. 30Ka-Kit TungU of WashingtonVarying Planetary Heat Sink and the Global Warming Hiatus
Varying Planetary Heat Sink and the Global Warming Hiatus
Nov. 13Dorian AbbotU of ChicagoThe effect of clouds on Snowball Earth Deglaciation and the inner edge of the Habitable Zone
Clouds have a huge impact on the climate of a planet. On Earth, if the clouds forgot that part of their job is to reflect solar energy, we would suffer a runaway greenhouse and end up like Venus. If instead clouds forgot that part of their job is to absorb infrared radiation emitted by the surface and contribute to the greenhouse effect, we would enter global glaciation. Although they are very important for climate, clouds are very difficult to model and represent the largest source of uncertainty in climate modeling. This results both from insufficient resolution to resolve cloud-scale circulation and incomplete understanding of cloud microphysics. Cloud simulation is therefore the main reason our current models aren't better, and is a critical area to attack if we want to create generalized models that could be easily applied to different planets (the clouds might not be water clouds in this case). In this talk I will discuss how we can use the models we have to gain insight into cloud behavior in climates vastly different from modern Earth. The two examples I will focus on are the Snowball Earth episodes that occurred 600-800 million years ago and the runaway greenhouse of tidally locked super-Earths at the inner edge of the habitable zone of M-stars.
Nov. 19Monika BarcikowskaPrinceton UMulti-decadal sea surface temperature fluctuations and the global warming hiatus.
Multi-decadal sea surface temperature fluctuations and the global warming hiatus
Nov. 20Dan VimontU of WisconsinOptimal Growth of Central and East Pacific ENSO events
Optimal Growth of Central and East Pacific ENSO events
Dec. 3Baoqiang XiangGFDL/UCARMJO and tropical cyclone prediction in the GFDL coupled climate model
MJO and tropical cyclone prediction in the GFDL coupled climate model
Dec. 4Gabriel KatulDuke U.Multi-scale modeling of ecosystem structure and function
ABSTRACT: The dynamics of biological systems, from cells to communities and ecosystems, have been hypothesized to follow optimal trajectories shaped by selection pressure that force organisms to maximize their fitness and reproductive success. This concept has been particularly successful in explaining the form and function of terrestrial vegetation from an eco-hydrological and carbon-economy perspective, and across spatial and temporal scales. Any optimality model is based on three key ingredients: an objective function that describes the gain that needs to be maximized or loss to be minimized, a control variable that shifts the dynamics in the desired direction, and a set of constraints that account for environmental conditions and conservation laws bounding the system. All three ingredients are difficult to define and quantify - especially in complex biological and ecological systems. Despite these difficulties, optimality approaches may complement process-based approaches when mechanistic knowledge is scarce. At the leaf scale, it is often hypothesized that carbon gain is maximized, thus providing a quantifiable objective for a mathematical definition of optimality conditions. Eco-physiological trade-offs and fluctuating resource availability introduce natural bounds to this optimization process. In particular, carbon uptake from the atmosphere is inherently linked to water losses from the soil as water is taken up by roots and evaporated. Hence, fluctuating soil moisture constrains the amount of carbon that can be taken up and assimilated into new biomass. The problem of maximizing photosynthesis at a given water availability by modifying stomatal conductance, the plant-controlled variable to be optimized, has been traditionally formulated for short time intervals over which soil moisture changes can be neglected. This simplification led to a mathematically open solution, where the undefined Lagrange multiplier of the optimization (equivalent to the marginal water use efficiency) is heuristically determined via data fitting. Here, a set of models based on different assumptions that account for soil moisture dynamics over an individual dry-down are proposed so as to provide closed analytical expressions for the carbon gain maximization problem for varying soil moisture resources, atmospheric CO2 levels, and vapor pressure deficit. Bridging these theories with widely used empirical formulations of stomatal conductance is discussed. The newly proposed theories establish a blue print on how to link plant-water use strategies with optimality arguments at the leaf-scale. Upscaling these theories to tree and landscapes using soil- plant hydraulics are also presented.
Dec. 10Lakshmi KrishnamurthyUCAR/GFDLThe Seasonality of the Great Plains Low-Level Jet and ENSO Relationship
The Seasonality of the Great Plains Low-Level Jet and ENSO Relationship
Dec. 11Wenhong LiDuke UniversityEnhancement of precipitation extremes over the Southeastern US
Enhancement of precipitation extremes over the Southeastern US