Thermodynamic and Dynamic Control of Upper Tropospheric Water Vapor

Abstract:

Upper tropospheric water vapor (UTWV) plays a key role in amplifying the global warming due to increasing CO₂ since small changes of water vapor in the upper troposphere can cause much larger impact on the water vapor greenhouse effect than those in the lower atmosphere. Realistic simulation of UTWV and its variability is thus essential to the accuracy of climate change predictions. Deep convection has a strong influence on the UTWV, making it one of the most poorly simulated quantities in climate models. We evaluate 23 AMIP-type model simulations of UTWV submitted for the IPCC Fifth Assessment report (AR5) by “A-Train” satellite observations and reanalysis data and perform detailed error diagnosis. We find that large-scale dynamics and convection play a dominant role in controlling UTWV errors in climatological mean, seasonal cycle, and interannual variability, while thermodynamic control associated with temperature biases are important for the water vapor simulations in the lower troposphere and near the tropopause.