A Regional Modeling Study of the Influence of Urban Land Cover Change on the Lower Atmosphere in Baltimore-Washington DC

The land-use and land cover (LULC) history of the Baltimore-Washington region has been intensively studied through a variety of environmental research collaborations and regional partnerships. One such partnership, the Baltimore-Washington Regional Collaboratory, involved multiple Federal and local agencies cooperating on a 200-year urban growth study in the Chesapeake region. Information from this study on pre-1900 and current LULC conditions for the Baltimore-Washington DC area was integrated with data from other sources to construct different lower boundary conditions for a series of simulations using the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). We use the RAMS simulations to diagnose the extent and nature of the effect of urban anomalies in surface heat, moisture, and momentum on mid-summer local and regional climate. The experimental design adopted for this study takes advantage of the grid-nesting capability of RAMS. A fine grid with 1km horizontal resolution was nested into a coarser 5 km grid, which extends from southern Pennsylvania, Maryland, to parts of Virginia, and West Virginia. Two sets of month-long simulations for July 2000 were conducted with RAMS running in parallel on a 26-processor cluster of computers at the Cooperative Institute for Research in the Atmosphere (CIRA), CSU. In the first set of simulations, we initially used satellite-derived current land cover data as the lower boundary condition in a 31-day RAMS run. We then replaced this data with the pre-1900 land cover data for the same region and ran a similar RAMS simulation. Identical observed meteorology was retained for the lateral boundary conditions in both cases. The model results for the initial run were validated with July 2000 surface climate data and flux measurements of sensible and latent heat from sites located within the fine grid model domain. Additional simulations were conducted to compare RAMS model performance using prescribed land surface conditions with results from a physically based scheme for urban energy budget coupled to RAMS. The urban surface scheme used here is the Town Energy Budget (TEB) model recently developed at the Center for Meteorological Research (CNRM), France. The TEB model allows for a refinement of model computed radiative budgets, heat and momentum based on a generalization of the classic canyon approach. Results from both sets of simulations, and the implications, for surface climate, of the driving human-induced land cover transformations are discussed.