Year‐Round In Situ Measurements of Arctic Low‐Level Clouds: Microphysical Properties and Their Relationships with Aerosols
Abstract Two years of continuous in situ measurements of Arctic low-level clouds have been made at the Mount Zeppelin Observatory (78°56'N, 11°53'E), in Ny-Alesund, Spitsbergen. The monthly median value of the cloud particle number concentration (Nc) showed a clear seasonal variation: Its maximum appeared in May-July (65 ± 8 cm' ), and it remained low between October and March (8 ± 7 cm' ). At temperatures warmer than 0 °C, a clear correlation was found between the hourly Nc values and the number concentrations of aerosols with dry diameters larger than 70 nm (N70), which are proxies for cloud condensation nuclei (CCN). When clouds were detected at temperatures colder than 0 °C, some of the data followed the summertime Nc to N70 relationship, while other data showed systematically lower Nc values. The lidar-derived depolarization ratios suggested that the former (CC N-controlled) and latter (CCN-uncontrolled) data generally corresponded to clouds consisting of supercooled water droplets and those containing ice particles, respectively. The CCN-controlled data persistently appeared throughout the year at Zeppelin. The aerosol-cloud interaction index (ACI = dlnNc/(3dlnN70)) for the CCN-controlIed data showed high sensitivities to aerosols both in the summer (clean air) and winter-spring (Arctic haze) seasons (0.22 ± 0.03 and 0.25 + 0.02, respectively). The air parcel model calculations generally reproduced these values. The threshold diameters of aerosol activation (Dact)> which account for the Nc of the CCN-con trolled data, were as low as 30-50 nm when N70 was less than 30 cm' , suggesting that new particle formation can affect Arctic cloud microphysics.
The annual average Arctic temperature has increased at almost twice the rate as that of the rest of the world over the past few decades (IPCC, 2013). The main driver of this warming is an increase in the global concentration of carbon dioxide; however, various other climate forcers and feedback processes are amplifying the magnitude of warming in the Arctic (e.g., Serreze & Barry, 2011). In the Arctic, cloud radiative forcing at the surface is positive throughout the year, except during a short time period in summer (Curry & Ebert, 1992), and it is considered to play a significant role in the recent warming in the Arctic (e.g., Graversen & Wang, 2009). In fact, possible changes in the cloud amounts in the Arctic associated with changes in the sea ice have been reported (e.g.. Palm et al., 2010).
Aerosols, which can act as cloud condensation nuclei (CCN) and ice-nucleating particles (INP), can affect Arctic clouds (i.e., indirect effects). In addition to the shortwave cloud albedo effect that is exerted all over the globe (Twomey, 1977), the cloud particle size dependence of longwave emissivity can result in a positive radiative forcing at the Arctic surface, partly because the optical thickness of Arctic clouds is generally thin (Garrett & Zhao, 2006; Lubin & Vogelmann, 2006). In the Arctic, very low CCN concentrations (