Low-level mixed-phase clouds in a complex Arctic environment

Rosa Gierens,Matthew D Shupe,Marion Maturilli,Stefan Kneifel,Kerstin Ebell,Ulrich Löhnert

doi:10.5194/acp-20-3459-2020

Abstract

Abstract. Low-level mixed-phase clouds (MPCs) are common in the Arctic. Both local and large-scale phenomena influence the properties and lifetime of MPCs. Arctic fjords are characterized by complex terrain and large variations in surface properties. Yet, not many studies have investigated the impact of local boundary layer dynamics and their relative importance on MPCs in the fjord environment. In this work, we used a combination of ground-based remote sensing instruments, surface meteorological observations, radiosoundings, and reanalysis data to study persistent low-level MPCs at Ny-Ålesund, Svalbard, for a 2.5-year period. Methods to identify the cloud regime, surface coupling, and regional and local wind patterns were developed. We found that persistent low-level MPCs were most common with westerly winds, and the westerly clouds had a higher mean liquid (42 g m−2) and ice water path (16 g m−2) compared to those with easterly winds. The increased height and rarity of persistent MPCs with easterly free-tropospheric winds suggest the island and its orography have an influence on the studied clouds. Seasonal variation in the liquid water path was found to be minimal, although the occurrence of persistent MPCs, their height, and their ice water path all showed notable seasonal dependency. Most of the studied MPCs were decoupled from the surface (63 %–82 % of the time). The coupled clouds had 41 % higher liquid water path than the fully decoupled ones. Local winds in the fjord were related to the frequency of surface coupling, and we propose that katabatic winds from the glaciers in the vicinity of the station may cause clouds to decouple. We concluded that while the regional to large-scale wind direction was important for the persistent MPC occurrence and properties, the local-scale phenomena (local wind patterns in the fjord and surface coupling) also had an influence. Moreover, this suggests that local boundary layer processes should be described in models in order to present low-level MPC properties accurately.

Highlights

The Arctic is warming more rapidly than any other area on Earth due to climate change (Serreze et al, 2009; Solomon et al, 2007; Wendish et al, 2017)
The frequency of occurrence was found to depend on free-tropospheric wind direction, and most P-mixedphase clouds (MPCs) were associated with westerly winds
Less frequent persistent low-level mixed-phase clouds (P-MPCs) and with higher cloud-base height were found with easterly winds compared to westerly winds, and these clouds had lower Liquid water path (LWP) and Ice water path (IWP)

Summary

Introduction

The Arctic is warming more rapidly than any other area on Earth due to climate change (Serreze et al, 2009; Solomon et al, 2007; Wendish et al, 2017). It is well established that clouds strongly impact the surface energy budget in the Arctic (Dong et al, 2010; Shupe and Intrieri, 2004), but feedback processes that include clouds are not well characterized (Choi et al, 2014; Kay and Gettelman, 2009; Serreze and Barry, 2011). Low-level mixed-phase clouds are important for the warming of near-surface air (Shupe and Intrieri, 2004; Intrieri et al, 2002; Zuidema et al, 2005). Improvements in the process-level understanding are still required to improve the description of low-level mixed-phase clouds in climate models (McCoy et al, 2016; Kay et al, 2016; Klein et al, 2009).

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Conclusion