Observations of orographic Cloud Base Heights from satellite and in-situ measurements at the Monteverde Cloud Mist Forest Reserve, Costa Rica

Tropical montane cloud mist forests are among the most biologically rich and diverse ecosystems, providing habitats for many of the world's endangered species. These biodiversity hotspots occupy approximately 0.4% of the Earth's land surface but support 20% and 16% of the Earth's plants and vertebrates, respectively. Survival of these habitats depends strongly on regular and frequent immersion in orographic clouds. However, regional land use change and global climate change has lead to increasing cloud base heights at some of these forest locations.  At the Monteverde Cloud Mist Forest Reserve in Costa Rica, the bases of the clouds have shifted upslope, leading to anuran population crashes, an increase in the upper elevation of bird ranges on the Pacific slope, and longer dry season mist-free intervals.  Satellite remote sensing techniques have been developed to determine the orographic cloud base heights; these are tested for the dry season month of March 2003 over the Monteverde cloud forests.  The approach derives MODIS cloud top pressures and then converts them to cloud top heights using geopotential height profiles. The NCAR Land Use and Cloud Interaction Experiment (LUCIE), consisting of paired mobile radiosonde systems deployed in Costa Rica, provided the means for validating the retrievals. Results show that the four MODIS CO₂ slicing channels do not provide sufficiently accurate cloud top height values, although some of the differences are due to a mismatch in the observational periods.  An alternative approach using simulated geopotential height profiles from the CSU Regional Atmospheric Modeling System (RAMS) initialized with the LUCIE soundings provided superior results..  Cloud thicknesses are estimated using three different approaches of: 1) constant liquid water content (CLWC); 2) an empirical relationship; and 3) an adiabatic model.  The CLWC approach provided the most consistent results. The cloud base heights are computed from subtracting cloud thickness from  cloud top height.  Orographic cloud base heights derived from the combined MODIS/RAMS approach were then compared with values observed at the study sites.  Differences between the observed and remotely sensed values were on the order of 200m. The results suggest that it is possible to monitor global cloud mist forest cloud base heights using the combination of MODIS satellite imagery with model simulations. Further investigations using the Atmospheric Infrared Sounder (AIRS) is now being carried out to develop methods that are wholly dependent on satellite observations.