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Harvard Forest Symposium Abstract 2011
- Title: Meteorological field observations at Harvard Forest
- Primary Author: David Fitzjarrald (SUNY at Albany)
- Additional Authors: Ricardo Sakai (SUNY at Albany)
- Abstract:
Task 1: Observing subcanopy CO2 advection; documenting how flow over Prospect Hill affects measurements in an around Harvard Forest flux towers.
Introduction. Subcanopy advection may explain why ecosystem respiration appears to be underestimated on calm nights. Understanding drainage flows and intermittent mixing at night is only now getting attention in the carbon flux community.
Site and instrumentation Harvard Forest (Petersham, MA; 42°30'30'' N; 72°12'28'' W) is a transition mixed deciduous and coniferous forest. Dominant species are red oak (Quercus rubra), red maple (Acer rubrum), black birch (Betula lenta), white pine (Pinus strobus), and eastern hemlock (Tsuga canadensis).. The current study site is focuses at the Little Prospect Hill (LPH) site, where a subcanopy array of 4 2D sonic anemometers where installed around the flux tower at 1.7 m. All sonics point to West. At the tower flux 3 3D sonics were installed at 21, 7, and 1.7 m. The tower flux is measuring almost continuously throughout the year; the subcanopy array measurement campaigns are during the warmer seasons The subcanopy array shows a strong advection within the canopy that is decoupled with the air aloft at nighttime. Only significant horizontal CO2 gradients were observed during spring and summer at the forest floor. These observations show that the canopy storage is less important than either the flux through canopy top or the advection on the sides of the domain. (Note that we performed no ‘u* filtering’ for the CO2 flux). At night, horizontal advection is about the same magnitude as the observed vertical flux, occasionally exceeding the above-canopy average CO2 flux.
Conclusions. We found good agreement among both sodars and tower flux measurements. Nowadays, the task is to test the ‘flow over hill’ hypothesis in which Bernoulli effects associated with flow over hills strongly affect subcanopy motions, possibly altering horizontal CO2 advection. We aim to categorize above-canopy flows by direction and intensity, compare flow upwind and downwind
of the major topographic features, and determine the extent to which subcanopy flows are altered. The final step is to document if any effects are sufficient to modify the role of horizontal advection on the subcanopy CO2 budget.
This work has been fully supported by DOE, Project ID 0013717. Final report is being prepared as are two publications.
Task 2: Observing how clouds modulate incident radiation and affect carbon uptake at Harvard Forest.
Falconer [1948] looked the time series of daylight in the northern sky, qualitatively associating the patterns with the cloud types reported by meteorological observers. In recent years, more studies of the statistics of instantaneous and time-averaged diffuse fraction in cloudy conditions are found in the solar energy rather than the ecological literature. We discuss new observations and new analyses of existing data to associate cloud presence. Further we outline a technique of using high frequency radiative flux and turbulent sensors to estimate how carbon uptake varies with the temporal pattern of incident radiation. We will outline our plan to estimate cloud fraction in situ with the deployment of a ceilometer at HF and to extend our analysis geographically using the newly available 30-s ASOS ceilometer data; NCDC data set DSI-6404, and 1-minute ASOS temperature and humidity records for the currently commissioned stations in North America. These data will complement existing estimates of aerosol optical depth at Harvard Forest. We will begin these observations during summer 2011.
This then will generate æartificial sky
patterns, following the methodology described by [Beyer et al., 1994]. Suehrcke and McCormick [1988; 1989] presented early versions of the probability density functions of incident light (direct and diffuse
components) on cloudy days. Woyte et al. [2007] updated their work, introducing wavelet analysis of fluctuations in the instantaneous clearness index, an approach we propose to follow. Multiple frequency rotating shadowband radiometer (MFRSR) observations
will be used to assess two primary impacts of clouds: decreased overall radiance and increased light use efficiency with emphasis on spectral response.
This work is supported by DOE TCP, in collaboration with researchers from Harvard University.
Task 3. Sodar Observations in support of the Harvard Forest proposal for wind energy
Sodar measurements are planned for the Hemlock walkup tower site. ASRC/SUNY will collect data, and provide a detailed analysis of historical data. Current data will be collected for a period of at least 3 months and be provided to the engineers that working on including the SODAR data into other aspects of the MassCEC wind turbine feasibility study.
- Research Category: Forest-Atmosphere Exchange
