The impact of deforestation on orographic cloud formation in a complex tropical environment

Abstract

Ecological changes observed at cloud forests in Monteverde, Costa Rica, including disappearance of anuran populations and expansion of bird and bat ranges to higher elevations has been linked to an increasing trend in dry season mist free days. Prior studies suggest that the increasing trend in dry season mist free days may be influence by both large scale climate change processes and also regional scale changes in land use. Preliminary investigations exploring the impact of land use on cloud formation indicated that drying and warming of boundary layer air in response to deforestation leads to increased cloud base heights. In the present study, numerical model experiments utilizing realistic land use scenarios and atmospheric conditions are used to further explore the impact of land use on orographic cloud formation. The Regional Atmospheric Modeling System (RAMS) is used to simulate orographic cloud formation during the time period of 1-14 March, 2003 in the Monteverde region for pristine, current and future land use scenarios. These simulations are initiated from the same atmospheric conditions and are subject to similar lateral boundary conditions. Comparisons against observations show that RAMS realistically simulate the nature of orographic cloud formation and boundary layer thermodynamics. Numerical simulations show that deforestation in the lowlands and premontane areas results in an increase in average cloud base heights and a consequent decrease in the areal extent of montane forests immersed in clouds. In the current and future land use scenarios, an increase in Bowen ratio and warmer, drier air is found over lowlands and premontane areas. The simulated differences in cloud formation and air mass thermodynamics are positively correlated to the amount of deforestation in the lowland and premontane regions.