Height-resolved Structure Function Analysis of Water Vapor Variability in the Tropics

Abstract

The correct modelling of the albedo feedback of small-scale shallow cumulus clouds in the tropics is crucial for predicting the response of the climate system on the global warming. However, the horizontal resolution of current numerical models does not allow for explicit representation of these clouds, and different parametrization strategies yield in a large diversity of results. The understanding of how large-scale processes, which can be resolved by the models, are coupled to small-scale processes, which should be parametrized, is a key factor for improvement of existing models. The investigation of this coupling can be done by quantifying the scale dependence of spatial variability of different atmospheric parameters, obtained by observations. In the present work, the height-resolved analysis of the tropical water vapor variability was performed on the data obtained with the WALES (Water Vapor Lidar Experiment in Space) instrument on board the DLR HALO (High Altitude and Long-range Research Aircraft ) aircraft during the NARVAL ( Next-Generation Aircraft Remote Sensing for Validation ) campaigns, which took place in December 2013 and August 2016 over the Atlantic Ocean east of Barbados. The vertical dependence of scaling properties of the troposphere in the trade wind region was investigated by means of the structure function method, aiming to relate these results to existing studies of the water vapor scaling. Since two NARVAL campaigns represent the water vapor field during different seasons, the seasonal dependence of atmospheric scaling could be studied. Moreover, a good vertical resolution of the data and favourable meteorological conditions during several flights allowed for performing a detailed analysis of vertical profiles of scaling exponents and intermittency in the lower troposphere. Additionally, the diversity of meteorological conditions during the summer campaign NARVAL-2 made it possible to investigate the influence of the Saharan dust and the vicinity of deep convection on the water vapor scaling. Prior to analysing the final results, the influence of noise, data gaps, shape of the trajectory, and aircraft speed variations was studied.