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Kenneth O. Wilson and Dayton G. Vincent

Abstract

A diagnosis of the scale interactions between areas of organized convective activity and their larger scale environment is presented for AVE/SESAME II, 19–20 April 1979. Two distinct areas of deep convection occurred during the period, one associated with frontal thunderstorms over the Great Plains and Southwest (referred to as CB1) and the other associated with a persistent region of airmass thunderstorms over the Texas–Louisiana Gulf Coast (referred to as CB2). It is found that a significant source of kinetic energy in both areas was the generation of energy due to the interaction between different scales of mass and motion. Furthermore, most of the scale-interaction generation occurred in the upper troposphere during the period of most active convection. In CB1, nearly all of the energy enhancement appears to be due to large-scale flow interesting with mesoscale (thunderstorm-produced) height gradients (referred to as GLD). In CB2, two processes appear to be equally important: GLD and the generation due to mesoscale flow interacting with large-scale height gradients (GDL). In CB1, a comparison is made with previously derived results from AVE/SESAME I which were obtained using the same methodology. Although the SESAME I case was characterized by much stronger dynamical forcing than the present case, the primary physical mechanism involving enhancement of kinetic energy due to scale interactions (i.e., large-scale flow down the mesoscale height gradient), was a common feature in both cases The results from SESAME I and II strongly suggest that a moderate increase in upper air sounding data can help quantify important physical processes in areas of active convection.

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Dennis Baldocchi, Eva Falge, Lianhong Gu, Richard Olson, David Hollinger, Steve Running, Peter Anthoni, Ch. Bernhofer, Kenneth Davis, Robert Evans, Jose Fuentes, Allen Goldstein, Gabriel Katul, Beverly Law, Xuhui Lee, Yadvinder Malhi, Tilden Meyers, William Munger, Walt Oechel, K. T. Paw U, Kim Pilegaard, H. P. Schmid, Riccardo Valentini, Shashi Verma, Timo Vesala, Kell Wilson, and Steve Wofsy

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S.

FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite.

Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO2 exchange of temperate broadleaved forests increases by about 5.7 g C m−2 day−1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

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