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Harvard Forest Symposium Abstract 2005

  • Title: A regional climatology of cloud and aerosol for forest-atmosphere exchange
  • Primary Author: Qilong Min (SUNY at Albany)
  • Abstract:

    A regional climatology of cloud and aerosol for forest-atmosphere exchange: observation and modeling





    Qilong Min+ and Xiangqing Wu*


    +State University of New York, Albany, New York


    *Iowa State University, Ames, Iowa





    The impact of aerosols and clouds on CO2 uptake and water use efficiency at Harvard Forest has been studied by using collocated turbulent flux and radiation measurements [Min, 2005]. Optical properties of aerosols and clouds have significant impacts on photosynthesis not only through changes of the total amount of PAR, but also through changes of its spectral distribution (or light quality) and its partitioning between direct and diffuse components. It is crucial to monitor forest-atmosphere exchanges in regional scale to understand climate impacts on ecosystem, particularly for monitoring ET and carbon uptake under all-weather conditions and accurately estimating the spring onset and growing season duration. We have used a combination of visible, infrared and microwave measurements to retrieve MEDI from SSM/I and to estimate forest-atmosphere exchange [Min and Lin, 2005a,b]. This study demonstrates the potential of microwave remote sensing on monitoring ecosystem exchange processes, particularly under cloudy conditions where cloud-vegetation interaction takes place.





    Furthermore, ecosystems affect both cloud formation through the process of evapotranspiration, and cloud properties, by influencing aerosol and water vapor concentrations. We applied our newly developed retrieval algorithms to the hourly averaged shortwave measurements at Harvard Forest from 1997 to 2000. We exam seasonal variations of clouds, shown in the Figure 1. The monthly occurrence of thin clouds shows a distinct peak in July. On the other hand, occurrence of thick clouds appears no seasonal trends, but closely links to the precipitation. The occurrence of thin clouds starts to increase as the leave emergence indicated by shortwave albedo as well as PAR albedo. Changes of thin cloud occurrence are strongly correlated with the water vapor flux measured at the EMS tower. With the commencement of transpiration, the additional moisture is bumped into the boundary layer, resulting lower lifting condensation level and formation of boundary layer clouds. With reduction of water vapor flux in the late growing season, the occurrence of thin clouds is also reduced. Those statistics indicate that most of thin clouds are formed as a result of forest-atmospheric exchange. The thick clouds, on the other hand, are due to the large-scale circulations.





    Observation evidence illustrates the need to understand ecosystem impacts on both cloud formation and cloud properties. More importantly, how large scale circulation coupled with ecosystem influences cloud formation. We utilize cloud resolving model (CRM) to study the surface-atmosphere interaction on cloud formation. Large-scale forcing data sets (temperature and moisture advections) are produced by the variational analysis of observations over the ARM Southern Great Plains (SGP). Therefore, we set up our 2D CRM domain (600 km long and 40 km deep) at the ARM SGP site. The model has a 3-km horizontal resolution and a stretched grid in the vertical with a time step of 15s. Periodic lateral boundary conditions are used to facilitate a mathematically consistent CRM framework. Figure 2 shows sensible heat flux and latent heat flux observed at the ARM SGP site for year 2000, which are used as. surface boundary conditions for CRM simulations. Observed precipitation during year 2000 is also shown in Figure 2. Seasonal changes of surface fluxes is similar to that at Harvard Forest site.





    In order to isolate impacts of surface and large scale circulations on cloud formation, we set up the CRM in two simulations: one is control run with large scale forcing and surface sensible and latent heat fluxes; and the other is without the forcing. Monthly statistics of cloud vertical distributions with large scale forcing are shown in Figure3, while cloud statistics without forcing are shown in Figure 4. Without the forcing, shown in Figure 4, cloud occurrences increase with increase of latent heat flux (LHF), which consistent with observed cloud statistics at Harvard Forest (See Figure 1). With sufficient supply of moisture from surface on July, atmospheric convection is strengthen and clouds are formed at a large range of altitudes. Contrasting to the run without the forcing, the large scale forcing reduces high cloud formation on July, resulting in relatively low occurrences of high clouds. The forcing advects moisture and enhances cloud formation on months with low LHF, shown in Figure 3. As shown in Figure 5, without the forcing, simulated precipitation is strongly correlated with LHF with much smaller magnitudes. For the control run, precipitation is much stronger and has no apparently seasonal pattern as observed at both Harvard Forest and SGP sites.





    Reference


    Min, Q.-L, Impacts of aerosols and clouds on forest-atmosphere carbon exchange, J. Geophys. Res, in press, 2005.


    Min, Q.-L., and B. Lin, Remote sensing of forest-atmosphere exchanges, to be submitted, 2005.


    Min, Q.-L., and B. Lin, Remotely detecting spring onset and growing season duration, submitted, 2005





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