Cloud feedbacks in extratropical cyclones: insight from long-term satellite data and high-resolution global simulations
Authors: McCoy Daniel T. , Field Paul R. , Elsaesser Gregory S. , Bodas-Salcedo Alejandro , Kahn Brian H. , Zelinka Mark D. , Kodama Chihiro , Mauritsen Thorsten , Vanniere Benoit , Roberts Malcolm , Vidale Pier L. , Saint-Martin David , Voldoire Aurore , Haarsma Rein , Hill Adrian , Shipway Ben , Wilkinson Jonathan 
Published: January 2019

Abstract. A negative extratropical shortwave cloud feedback driven by changes in cloud
optical depth is a feature of global climate models (GCMs). A robust positive
trend in observed liquid water path (LWP) over the last two decades across
the warming Southern Ocean supports the negative shortwave cloud feedback
predicted by GCMs. This feature has been proposed to be due to transitions
from ice to liquid with warming. To gain insight into the shortwave cloud
feedback we examine extratropical cyclone variability and the response of
extratropical cyclones to transient warming in GCM simulations. Multi-Sensor
Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of
cyclone properties from the period 1992–2015 are contrasted with GCM
simulations, with horizontal resolutions ranging from 7 km to hundreds of
kilometers. We find that inter-cyclone variability in LWP in both
observations and models is strongly driven by the moisture flux along the
cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances the
LWP within cyclones. This relationship is replicated in GCMs, although its
strength varies substantially across models. It is found that more than
80 % of the enhancement in Southern Hemisphere (SH) extratropical cyclone LWP in GCMs in
response to a transient 4 K warming can be predicted based on the
relationship between the WCB moisture flux and cyclone LWP in the historical
climate and their change in moisture flux between the historical and warmed
climates. Further, it is found that that the robust trend in
cyclone LWP over the Southern Ocean in observations and GCMs is consistent with changes in the moisture flux. We propose two cloud feedbacks acting
within extratropical cyclones: a negative feedback driven by
Clausius–Clapeyron increasing water vapor path (WVP), which enhances the
amount of water vapor available to be fluxed into the cyclone, and a feedback
moderated by changes in the life cycle and vorticity of cyclones under
warming, which changes the rate at which existing moisture is imported into
the cyclone. Both terms contribute to increasing LWP within the cyclone.
While changes in moisture flux predict cyclone LWP trends in the current
climate and the majority of changes in LWP in transient warming simulations,
a portion of the LWP increase in response to climate change that is
unexplained by increasing moisture fluxes may be due to phase transitions.
The variability in LWP within cyclone composites is examined to understand
what cyclonic regimes the mixed-phase cloud feedback is relevant to. At a
fixed WCB moisture flux cyclone LWP increases with increasing sea surface temperature (SST) in the half
of the composite poleward of the low and decreases in the half equatorward of
the low in both GCMs and observations. Cloud-top phase partitioning observed
by the Atmospheric Infrared Sounder (AIRS) indicates that phase transitions
may be driving increases in LWP in the poleward half of cyclones.