How Forest Cover and Climate Change Affect the Frequency and Distribution of Boundary Layer Cumulus Clouds in the Eastern United States—An Observational Analysis

Boundary layer cumulus clouds (BLcu) play a crucial role in modulating the exchange of momentum, heat, and moisture within and above the planetary boundary layer (PBL). In forested landscapes, the presence of BLcu is strongly associated with the structure, density and leaf state of underlying vegetation, due to its role in surface energy flux partitioning (i.e., the Bowen ratio). Freedman et al. (2000) and Freedman and Fitzjarrald (2001) found that BLcu are directly coupled with local and regional surface heat and moisture budgets. Their work, and research performed by Fitzjarrald et al. (2001) demonstrated the link between the seasonal frequency distributions of BLcu, leaf-out, and “spring intensity”— a measure of the Bowen ratio change during the spring transition throughout the forests of the eastern United States.

Over the past 350 years, the eastern half of the United States experienced extensive land cover changes. These began with land clearing in the 1600s, continued with widespread deforestation, wetland drainage, and intensive land use by 1920, Reforestation of eastern U.S. forests began with efforts to promote forest regrowth in support of environmental conservation movements that began in the 1880s, motivated by concerns about the negative consequences of land use change, specifically land and water resource degradation associated with deforestation and poor farming practices. The reforestation of the eastern United States, however, was also characterized by land cover transformation that included growing urbanization and other land use changes leading to increasing landscape fragmentation. Bythe late 1990s reforestation in the eastern United States had reached its peak, with slow but steady declines in forest cover during the last two decades. This natural experimentation in land use modification, occurring concurrently with climate change, raises a question regarding how the PBL and, more specifically, BLcu have responded. Fortunately, we now have an extensive data base of long-term surface-based cloud observations to interrogate—the Automated Surface Observing System (ASOS) network established by the Federal Aviation Administration and the National Oceanic and Atmospheric Administration National Weather Service over two decades ago.

Using twenty years of 5-minute ASOS observations, we present preliminary results of what the effects of real world afforestation/deforestation, combined with a changing climate, have had on the frequency and distribution of BLcu. Included in the analysis is the current climatology, trends, and inter-annual and seasonal variability of BLcu over the eastern United States.

Fitzjarrald, D. R., Acevedo, O. C., & Moore, K. E., 2001: Climatic consequences of leaf presence in the eastern united states. J. Climate, 14(4), 598-614. 

Freedman, J. M., D. R. Fitzjarrald, K. E. Moore, and R. K. Sakai, 2001: Boundary layer cumulus clouds and vegetation-atmosphere feedbacks. J. Climate, 14, 180-197.

Freedman, J. M. and D. R. Fitzjarrald, 2001: Post-frontal air mass modification. J. Hydrometeorology, 2, 419-437.