12.3
Modeling the Local Climatology of Mixing Layer Heights over two Midlatitude Hardwood Forests
Hong-Bing Su, Indiana University, Bloomington, IN; and C. S. B. Grimmond
The local climatology of the atmospheric boundary layer, particularly development of the mixing layer height (zi), is very important in the interpretation (e.g., quantification of the source area and/or footprint) of ground tower-based long-term measurements of surface-atmosphere exchanges of momentum, heat, carbon dioxide and water vapor at sites throughout the AmeriFlux and FLUXNET network. However, the mixing layer height is largely not measured at most sites.
At the Morgan-Monroe State Forest (MMSF) AmeriFlux site (Indiana, USA), profiles of potential temperature and specific humidity were collected with airsondes for 26 anticyclonic days over a 12-month period (2000-2001). It was found that the mixing layer heights derived from these profiles follow trends in cumulative and daytime total sensible heat flux measured from the eddy-covariance system at the 46-m level of the MMSF tower. When observed profiles of potential temperature and specific humidity were used as initial conditions and surface forcing (e.g., sensible heat flux) measured from the tower as input, a simple slab model is found to estimate zi and its growth during the daytime quite well in comparison with the observations. Model performance remains relatively good when profiles from regional National Weather Service (NWS) locations are used to initialize the model.
Based upon the above results, in this study, the local climatology of the daytime development of the mixing layer is studied using this modeling approach. Nearby NWS sounding profiles are combined with tower-based measurements as initial conditions, and the continuous eddy-covariance measurements of surface fluxes from the AmeriFlux tower were used to drive the growth and development of the mixing layer. Factors influencing the sensible heat and its daytime cumulative total, the essential forcing for the boundary layer development are discussed in terms of seasonal changes in daytime length, incoming solar radiation (cloudiness), forest morphology, particularly leaf emergence and senescence thus available energy (net radiation) and energy partitioning, etc.
Session 12, Coupled Surface-BL studies
Thursday, 18 July 2002, 10:30 AM-12:30 PM
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