Arctic Cloud, Radiation and their Interactions with Sea Ice

Abstract

Over the past few decades, the Arctic has experienced dramatic changes, the most recognized being the amplified warming and substantial decline in both sea ice coverage and thickness at impressive rates. The cloud-radiation feedback is believed to be one of the critical reasons for Arctic sea ice long-term trends and variability. Therefore, this study aims to investigate the role of cloud-radiation feedback in modulating Arctic sea ice changes from seasonal to interannual scales utilizing an integrative analysis. The reanalysis product is a convenient and necessary tool in the Arctic climate study. However, their uncertainties should be quantified first before utilizing these reanalyses in Arctic climate studies. Therefore, we first estimate the uncertainties of Arctic cloud and radiative properties in five global reanalysis products using NASA satellite retrievals and conclude that all five reanalysis products estimate radiative fluxes better than cloud properties, and MERRA-2 and JRA-55 have relatively better performance on the representation of Arctic cloud and radiation properties. This study paves the way for us to properly use these products in the following Arctic climate studies. Then we investigate the impacts of cloud and radiation on Arctic sea ice variations at various time scales. Specifically, we show how cloud and radiation properties modulate the timing of melt onset in spring based on global reanalysis. In early melting events, the higher-than-average cloud fractions and cloud water paths result in increased downwelling longwave radiative flux at the surface, which triggers the initial melt of sea ice. In contrast, the late melt onset is usually linked to lower-than-average precipitable water vapor and downward longwave flux at the surface. The increased downward shortwave radiation during the period from middle to late June plays a more important role in accelerating the melting, aided further by the stronger than normal cloud warming effects. On seasonal scale, the relationships between long-term trend of cloud and radiation in spring and September sea ice retreat have been studied. Based on NASA satellite retrievals, it is hypothesized that increasing springtime cloud fraction and downward longwave flux at the surface tend to enhance sea ice melt via strong cloud warming effect, while surface shortwave fluxes play a more important role in determining September sea ice extent during late spring and early summer. Therefore, we conduct several model simulations using Community Earth System Model (CESM) to further prove this hypothesis and demonstrate their causal relationships. To further explore the impacts of Arctic clouds on Arctic sea ice variations, we examine two important factors controlling Arctic clouds: large-scale circulation and cloud microphysics. First, we explore how large-scale atmospheric circulation variability regulates changes in low clouds, as well as the important role of low clouds on modulating sea ice melt in summer. Using both observations and model simulations, we conclude that the summertime low clouds play a crucial role in driving sea ice melt by amplifying the warming effect induced by a stronger anticyclonic circulation. Moreover, we find that a less efficient Wegener–Bergeron–Findeisen (WBF) process in the model leads to a better simulation of Arctic cloud and radiative properties in CESM compared to satellite retrievals. As a response, the sea ice tends to melt over the North Atlantic Ocean. These results improve our understanding of cloud process and its influence on surface energy budget as well sea ice melt over the Arctic. This will inform the Earth System Models with a more realistic cloud-radiation process that governs the seasonal sea ice evolution.