Using water stable isotopes to better evaluate the 
		atmospheric and land surface components of climate 
		models

			Abstract

Despite continuous improvements in climate models, uncertainties persist
regarding the amplitude of the projected global warming and the associated
hydrological changes. Better evaluating the representation of physical
processes such as clouds, water vapor transport, atmospheric convection
and land surface-atmosphere interactions is necessary to assess the
credibility in climate change projections. To what extent could water
isotopic measurements, combined with meteorological measurements, provide
observable, process-oriented diagnostics to evaluate the representation of
these processes in models?

First, I will show the added value of water isotopic measurements to
understand the reasons for a widespread bias in climate models: the moist
bias in the mid and upper tropical troposphere. Using an isotopic
atmospheric general circulation model (IAGCM) and various isotopic
measurements, we find that the subtropical isotopic seasonality
constitutes an observable diagnostic of the reason for the moist bias in
models. Applying this diagnostic to 6 other IAGCMs, we find that the most
frequent reason for the moist bias is excessively diffusive vertical
advection or insufficient vertical resolution. We also show that models
exhibiting this moist bias are likely to overestimate the future upper
troposphere drying.

Then, I will present the added value of water isotopic measurements for
evaluating two other model components that are key in hydrological change
projections: atmospheric deep convection and land surface feedbacks on
precipitation.