The effect of land cover on growing-season convective precipitation

The Southeastern United States (SE) timberland ecosystems are among the most productive ecosystems in the U.S. and act as an important carbon sink within North America due to the mild climate and abundant growing-season (GS) precipitation. While the total timberland area did not significantly change over the past 50 years, its composition is undergoing significant changes. The proportion of planted pine forests (PP) to the total timberland area has increased from under 1.0% in the early 1950s to about 15.0% in 1999 and is projected to reach 32.0% by 2040. The impact of this land-cover change on growing-season precipitation remains a vexing research problem. Here, we use 9-year (1998-2006) measured water vapor and sensible heat flux record at a PP ecosystem, a 6-year record (2001-2006) from an adjacent hardwood (HW) and an old-field grassland (OF) ecosystem to investigate the possible soil moisture and atmospheric water vapor states that trigger convective precipitation during the GS. In particular, these half-hourly series were used to investigate how eco-physiological controls on sensible heat flux affect triggers of convective precipitation using a simple slab model for convective boundary layer growth. We showed that for a given relative humidity state in the atmosphere, the PP ecosystem is more likely to trigger convective precipitation than HW and OF in both wet and dry soil moisture states. An analytical model was formulated to diagnose the soil moisture thresholds within each ecosystem likely to induce convective rainfall at a given atmospheric state. The broader implications of this work is that increases in PP land cover in the SE may lead to increases in convective precipitation during the GS.