Stability-dependent increases in liquid water with droplet number in the Arctic

Abstract

Abstract. The effects of aerosols on cloud microphysical properties are a large source of uncertainty when assessing anthropogenic climate change. The aerosol–cloud relationship is particularly unclear in high-latitude polar regions due to a limited number of observations. Cloud liquid water path (LWP) is an important control on cloud radiative properties, particularly in the Arctic, where clouds play a central role in the surface energy budget. Therefore, understanding how aerosols may alter cloud LWP is important, especially as aerosol sources such as industry and shipping move further north in a warming Arctic. Using satellite data, this work investigates the effects of aerosols on liquid Arctic clouds over open ocean by considering the relationship between cloud droplet number concentration (Nd) and LWP, an important component of the aerosol–LWP relationship. The LWP response to Nd varies significantly across the region, with increases in LWP with Nd observed at very high latitudes in multiple satellite datasets, with this positive signal observed most strongly during the summer months. This result is in contrast to the negative response typically seen in global satellite studies and previous work on Arctic clouds showing little LWP response to aerosols. The lower tropospheric stability (LTS) was found to be an important control on the spatial variations in LWP response, strongly influencing the sign and magnitude of the Nd–LWP relationship, with increases in LWP in high-stability environments. The influence of humidity varied depending on the stability, with little impact at low LTS but a strong influence at high LTS. The mean Nd state does not dominate the LWP response, despite the non-linearities in the relationship. As the Nd–LWP sensitivity changed from positive to negative when moving from high- to low-LTS environments, this work shows evidence of a temperature-dependent aerosol indirect effect. Additionally, the LWP–LTS relationship changes with Nd, generating an aerosol-dependent cloud feedback. As the LTS is projected to decrease and the boundary layer to become more polluted in a future Arctic, these results show that aerosol increases may produce lower cloud water paths. This shift to more unstable environments implies that LWP adjustments shift from enhancing the Twomey effect by 8 % to offsetting it by around 40 %, with this warming effect having potential consequences for sea ice extent.

Robust evidence for reversal of the trend in aerosol effective climate forcing

“…The trends of aerosols, clouds, and radiation in the observations are subject to not only changes in anthropogenic aerosol emissions, but also other influencing factors. These include changes in natural aerosol emissions, which remain poorly constrained and contribute a substantial fraction of total observed AOD, interannual variability, and responses to greenhouse-gas-induced global warming; aerosol-cloud interactions may also be altered in a changing climate Murray-Watson and Gryspeerdt, 2022).…”

Too Frequent and Too Light Arctic Snowfall With Incorrect Precipitation Phase Partitioning in the MIROC6 GCM

“…These three types of cloud regime are classified by the CloudSat column maximum radar reflectivity ( Z max ) as follows: Z max < −15 dBZ e for non‐precipitating clouds; −15 dBZ e < Z max < 0 dBZ e for drizzling clouds; and Z max > 0 dBZ e for precipitating clouds. SLWCs are rare in the Arctic (annual average of 1.7% of all Arctic single‐layer clouds), but the SLWCs in this region are important because of (a) the high magnitude of aerosol‐cloud interactions (Coopman et al., 2018) and (b) regime‐dependent LWP adjustment (Murray‐Watson & Gryspeerdt, 2022).…”

Observing short-timescale cloud development to constrain aerosol–cloud interactions

“…Geostationary satellites provide a natural pathway forward, although night-time retrievals of cloud properties are challenging. Future work should also account for the possibility that these relationships are not constant under warming (Zhang et al, 2022;Murray-Watson and Gryspeerdt, 2022).…”

“…Due to difficulties using the aerosol optical depth as a proxy for cloud condensation nuclei (CCN; Quaas et al, 2010;Stier, 2016), many recent observational studies have focussed on the N d -LWP relationship as a method for quantifying the aerosol impact on LWP. Although a positive relationship is found in some locations (Han et al, 2002;Murray-Watson and Gryspeerdt, 2022), these studies often identify a negative relationship that would indicate a LWP reduction with increasing aerosol (Michibata et al, 2016;Toll et al, 2019;Gryspeerdt et al, 2019). These studies may be negatively biased (overestimating the warming effect) due to correlated errors in the N d and LWP retrievals (Gryspeerdt et al, 2019).…”