Tropical free-tropospheric humidity differences and their effect on the
clear-sky radiation budget in global strom-resolving models
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.