Role of Background Climate in Estimation of Summertime Surface Energy Budgets Changes due to Historical Vegetation Changes in the Mid-Atlantic Region of the United States

Background climatic conditions determine how changes in surface energy budgets due to afforestation influence climate change in the mid-atlantic region (MAR) of the United States. In the 1900s, the MAR land-surface consisted of 38% Broadleaf Deciduous Forest (BDF), 32% crops, while the remaining areas were covered by evergreen trees and grasses. In 2000, in the MAR, BDF increased to 44%, cropland decreased to 17%, while the shares of evergreen and grass coverage were approximately constant. A land use conversion of such a large magnitude (~500,000 km2 ) has implications for surface radiative properties as well as the surface energy budgets. Large trees significantly differ in their sizes, root-systems, and leaf areas as compared to grasses which affect the rate of evaporation, transpiration, surface roughness length as well as the sub-surface hydrology of the region. Background climate conditions such as precipitation rates, cloud cover, shortwave radiation, soil moisture, water table depth, etc. play important roles in regulating the effect of large-scale vegetation changes. For example, higher precipitation rates reduce the moisture stress and decreases reflected shortwave radiation, and available energy is partitioned more into latent heat flux. Hence, to test the sensitivity of the surface fluxes to the background climate conditions, we perform numerical simulations using the Community Earth System Model (CESM2). We simulate the surface and atmospheric conditions for the climatic conditions of the 2000s as our control simulation. We then alter the land-surface conditions to the year 1900 keeping the sea-surface temperature and the carbon dioxide levels constant (referred to as our perturbed simulation). We simulate both the control and the perturbed cases using multiple initial conditions leading to different background climatic conditions. We report the summertime ( i.e. June-July-August (JJA)) sensitivity of the surface energy fluxes to the differing background climatic conditions.