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Tropical free-tropospheric humidity differences and their effect on the clear-sky radiation budget in global strom-resolving models
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  • Theresa Lang,
  • Ann Kristin Naumann,
  • Bjorn Stevens,
  • Stefan Alexander Buehler
Theresa Lang
Universität Hamburg, Universität Hamburg

Corresponding Author:[email protected]

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Ann Kristin Naumann
Max Planck Institute for Meteorology, Max Planck Institute for Meteorology
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Bjorn Stevens
Max Planck Institute for Meteorology, Max Planck Institute for Meteorology
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Stefan Alexander Buehler
Universität Hamburg, Universität Hamburg
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Abstract

Reducing the model spread in free-tropospheric relative humidity (RH) and its response to warming is a crucial step towards reducing the uncertainty in clear-sky climate sensitivity, a step that is hoped to be taken with recently developed global storm-resolving models (GSRMs). In this study we quantify the inter-model differences in tropical present-day RH across GSRMs, making use of DYAMOND, a first 40-day intercomparison. We find that the inter-model spread in tropical mean free-tropospheric RH is reduced compared to conventional atmospheric models, except from the the tropopause region and the transition to the boundary layer. We estimate the reduction to approximately 50-70% in the upper troposphere and 25-50% in the mid troposphere. However, the remaining RH differences still result in a spread of 1.2 Wm-2 in tropical mean clear-sky outgoing longwave radiation (OLR). This spread is mainly caused by RH differences in the lower and mid free troposphere, whereas RH differences in the upper troposphere have a minor impact. By examining model differences in moisture space we identify two regimes with a particularly large contribution to the spread in tropical mean clear-sky OLR: rather moist regimes at the transition from deep convective to subsidence regimes and very dry subsidence regimes. Particularly for these regimes a better understanding of the processes controlling the RH biases is needed.