Processes contributing to cloud dissipation and formation events on the North Slope of Alaska

Joseph Sedlar,Adele Igel,Hagen Telg

doi:10.5194/acp-21-4149-2021

Abstract

Abstract. Clear-sky periods across the high latitudes have profound impacts on the surface energy budget and lower atmospheric stratification; however an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is limited. A method to identify clear periods at Utqiaġvik (formerly Barrow), Alaska, during a 5-year period (2014–2018) is developed. A suite of remote sensing and in situ measurements from the high-latitude observatory are analyzed; we focus on comparing and contrasting atmospheric properties during low-level (below 2 km) cloud dissipation and formation events to understand the processes controlling clear-sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant during the periods bookending clear conditions, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation on the North Slope of Alaska. Further, meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear-sky period: namely, horizontal advection, which was dominant in winter and early spring, and quiescent air mass modification, which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud or fog layer developing near the surface. This low cloud formation is driven largely by air mass modification under relatively quiescent synoptic conditions. Near-surface aerosol particles concentrations changed by a factor of 2 around summer formation events. Thermodynamic adjustment and increased aerosol presence under quiescent atmospheric conditions are hypothesized as important mechanisms for fog formation.

Highlights

Over the Arctic, clouds are ubiquitous (e.g., Herman and Goody, 1976; Curry et al, 1996)
The annual distribution of monthly clear-sky frequency follows the annual trends of cloudiness in the high Arctic reported in the literature (Curry et al, 1996; Wang and Key, 2005; Shupe, 2011), where more clear periods are found during the seasons with relatively lower cloud fractions
These cloud formations dominate after clear periods from spring through early autumn, occurring for approximately 60 % to 90 % of all cloud formation events during these seasons

Summary

Introduction

Over the Arctic, clouds are ubiquitous (e.g., Herman and Goody, 1976; Curry et al, 1996). Detailed observations of the vertical structure of Arctic clouds from remote sensing “supersites” document the frequent presence of lower tropospheric clouds (e.g., Shupe et al, 2011). These clouds frequently contain both water and ice particles, known as mixed-phase clouds, which can persist for hours to days in a near-homogeneous state (Shupe, 2011). Clouds strongly modulate the incoming and outgoing radiative fluxes; over reflective sea ice, longwave radiation dominates the radiative energy budget at the surface (Walsh and Chapman, 1998; Shupe and Intrieri, 2004; Sedlar et al, 2011)

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