Abstract
Abstract. High-quality observations are powerful tools for the evaluation of climate models towards improvement and reduction of uncertainty. Particularly at low latitudes, the most uncertain aspect lies in the representation of moist convection and interaction with dynamics, where rising motion is tied to deep convection and sinking motion to dry regimes. Since humidity is closely coupled with temperature feedbacks in the tropical troposphere, a proper representation of this region is essential. Here we demonstrate the evaluation of atmospheric climate models with satellite-based observations from Global Positioning System (GPS) radio occultation (RO), which feature high vertical resolution and accuracy in the troposphere to lower stratosphere. We focus on the representation of the vertical atmospheric structure in tropical convection regimes, defined by high updraft velocity over warm surfaces, and investigate atmospheric temperature and humidity profiles. Results reveal that some models do not fully capture convection regions, particularly over land, and only partly represent strong vertical wind classes. Models show large biases in tropical mean temperature of more than 4 K in the tropopause region and the lower stratosphere. Reasonable agreement with observations is given in mean specific humidity in the lower to mid-troposphere. In moist convection regions, models tend to underestimate moisture by 10 to 40 % over oceans, whereas in dry downdraft regions they overestimate moisture by 100 %. Our findings provide evidence that RO observations are a unique source of information, with a range of further atmospheric variables to be exploited, for the evaluation and advancement of next-generation climate models.
Highlights
A key challenge in current representations of the climate system is the modelling of the atmospheric water cycle and its coupling with circulation
The Intergovernmental Panel on Climate Change (IPCC) finds differences in feedback processes to be the main reason for the spread of model uncertainty, stating in the latest world climate report: “Water vapour, lapse rate and cloud feedbacks each involve moist atmospheric processes closely linked to clouds and, in combination, produce most of the simulated climate feedback and most of its inter-model spread” (Boucher et al, 2013)
Most models agree that the combined net feedback from water vapour and lapse rate changes is positive (Boucher et al, 2013) with the net effect resulting in the amplification of a warming, which is largest in the tropical middle and upper troposphere
Summary
A key challenge in current representations of the climate system is the modelling of the atmospheric water cycle and its coupling with circulation. Most models agree that the combined net feedback from water vapour and lapse rate changes is positive (Boucher et al, 2013) with the net effect resulting in the amplification of a warming, which is largest in the tropical middle and upper troposphere. At low latitudes, the most uncertain aspect lies in the representation of moist convection and its interaction with large-scale dynamics, and improvements are necessary to reduce uncertainty in climate model projections (Stevens and Bony, 2013; Shepherd, 2014). This is one focus of the World Climate Research Programme (WCRP) and its grand challenge of clouds, circulation, and climate sensitivity. We focus on the vertical structure of moist and dry regimes in the tropics and inspect their representation in models and observations
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