Cloud feedbacks in extratropical cyclones: insight from long-term satellite data and high-resolution global simulations

Abstract

<p><strong>Abstract.</strong> Extratropical cyclones provide a unique set of challenges and opportunities in understanding variability in cloudiness over the extratropics (poleward of 30°). We can gain insight into the shortwave cloud feedback from examining cyclone variability. Here we contrast global climate models (GCMs) with horizontal resolutions from 7 km up to hundreds of kilometers with Multi-Sensor Advanced Climatology Liquid Water Path (MAC-LWP) microwave observations of cyclone properties from the period 1992–2015. We find that inter-cyclone variability in both observations and models is strongly driven by moisture flux along the cyclone's warm conveyor belt (WCB). Stronger WCB moisture flux enhances liquid water path (LWP) within cyclones. This relationship is replicated in GCMs, although its strength varies substantially across models. In the southern hemisphere (SH) oceans 28–42 % of the observed interannual variability in cyclone LWP may be explained by WCB moisture flux variability. This relationship is used to 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. We show that changes in moisture flux drive can explain the observed trend in Southern Ocean cyclone LWP over the last two decades. Transient warming simulations show that the majority of the change in cyclone LWP can be explained by changes in WCB moisture flux, as opposed to changes in cloud phase. The variability 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 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.</p>

The Role of Thermodynamic Phase Shifts in Cloud Optical Depth Variations With Temperature

“…Although investigation of the physical processes responsible for the contribution of ice and liquid clouds to d ln 𝜏 dT (second and third terms on the RHS of equation ( 5), respectively) is beyond the scope of this study, these terms are nevertheless shown to put the contribution of thermodynamic phase shifts to d ln 𝜏 dT (first term on the RHS of equation ( 5)) in context. Processes related purely to liquid clouds include but are not limited to changes in adiabatic water content, moisture fluxes into the warm conveyor belt (McCoy et al, 2019), precipitation, cloud-top entrainment, and other processes related to boundary-layer decoupling for low clouds. Some processes related to ice clouds include precipitation, riming, ice nucleation, and ice splintering.…”

Southern Ocean Cloud Properties Derived From CAPRICORN and MARCUS Data

“…Liquid clouds within this environment respond to large‐scale meteorological forcing and moisture transports (Field & Wood, 2007; Govekar et al., 2011; Kelleher & Grise, 2019; Klein et al., 2017; D. T. McCoy et al. 2019; Naud et al., 2018; Wall et al., 2018), as well as the local aerosol environment. The large‐scale environment provides the thermodynamic conditions for producing clouds, and the aerosol properties control the detailed processes that determine when and how low‐level clouds precipitate (Savic‐Jovcic & Stevens, 2008).…”

Warm Conveyor Belts and Their Role for Cloud Radiative Forcing in the Extratropical Storm Tracks

“…This concerns the microphysical properties of mixed-phase clouds in the ascent regions as well as cirrus clouds in the WCB outflow. Whether changes in NetCRF in a warmer climate are produced by changes in the underlying circulation and availability of moisture or if they are caused by changes in the clouds' microphysical properties remains unclear (Ceppi and Hartmann 2015;McCoy et al 2018). The method of decomposing the climatological signal into a signal produced by an atmosphere without WCBs (C NOWCB ), the contribution from WCBs (WCB effect; C WCB 2 C NOWCB ), and the frequency of WCBs (f WCB ) [see Eqs.…”

“…Govekar et al (2011) evaluated the distribution of CRF based on satellite data in cyclone composites and showed that the longwave cloud forcing is highest over the warm front where also most of the high cirrus clouds occur. A study by McCoy et al (2018), which is also based on satellite data, showed that stronger moisture inflows enhance the liquid water path in cyclones and thus their shortwave cloud forcing.…”