Is the Arctic Boundary Layer De-stabilizing? Insights from Recent Shipborne Field Campaigns and 20 Years of ARM NSA Data

Over the last several decades, the region of the globe inside the Arctic Circle has warmed faster than any other region of the globe. Sea ice extent and concentration have decreased because of this temperature change, and by extension, has resulted in changes to the Arctic environment that haven’t been fully captured by present-day climate models. In this study, we hypothesize that the increase in surface temperature, sensible and latent heat fluxes from decreasing sea ice extent/concentration and increase in latent energy transported by baroclinic eddies (showed recently in Dzambo et al., 2023, GRL) into the Arctic have caused the Arctic boundary layer (ABL) to destabilize over the last 80 years. To investigate this hypothesis, European Center for Medium Range Weather Forecasting (ECMWF) ERA-5 reanalysis data are used to investigate a number of parameters related to the ABL including estimated inversion strength (EIS), lower tropospheric stability (LTS) and overlying vertical velocity (subsidence rate) to assess if the thermodynamic structure of the ABL is changing. In addition to these parameters from the ERA-5, cloud macrophysical data (e.g., cloud fraction, cloud top height and maximum reflectivity factor) are assessed from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) and Surface Heat Budget of the Arctic Ocean (SHEBA) field campaigns in 2019-20 and 1997-98 respectively, as well as 20 years of data from the Atmospheric Radiation Measurement (ARM) program’s Utqiaġvik site in northern Alaska (NSA). Results from the ERA-5 data show that both EIS and LTS have decreased at a rate of ~0.5 to 0.8 K per decade, with some dependence on location within the Arctic Circle. With this said, the ABL is perpetually stable with 97% of ERA-5 EIS data positive and with a mean EIS of 15-17 K around the MOSAiC, SHEBA and ARM NSA domains. Recent results examining MOSAiC radiosonde data confirm this persistently extreme stability, also showing over 97% of soundings had a stable inversion above the ABL. Subsidence is persistent across all three domains, but no significant trend in vertical velocity is found at this time. While both EIS and LTS have been decreasing on average each decade, EIS and LTS are still extremely stable. Low cloud fraction across the Arctic Circle is perpetually high with a median cloud fraction of 92%, however, median low cloud fraction from the recent MOSAiC (SHEBA) campaign was, on average, 79% (68%) throughout the campaign. Differences between these two campaigns can be explained, among other processes, with the higher frequency of unstable ABL’s (i.e., EIS < 0 K) during SHEBA. Combined SHEBA, MOSAiC and ARM NSA site cloud fraction data suggest that this subtle low cloud fraction might be the result of cloud type increasingly becoming more isolated cumulus, rather than typical stratocumulus that explain the perpetually high cloud fraction. CWPs at the ARM NSA site have consistently measured between 40-60 g/m3 over the last two decades, however, clouds at the ARM NSA site (irrespective of cloud type) have seen a statistically significant increase in the number of “outlier” cloud water path (CWP) estimates, with the ARM NSA site having experienced nearly 10 additional days (on average) over the last decade of CWPs exceeding 150 g/m3 compared to the period from 2000-2009. Nearly all of this increase in observed CWP occurs during the northern hemisphere summer and fall seasons, which coincides with the climatological minimum in LTS and EIS as well as aforementioned decreases in Arctic LTS and EIS. A similar trend is found for precipitable water vapor over the ARM NSA site. Overall, these set of results show that changes to the ABL have resulted in observable changes to cloud morphology and CWP consistent with a destabilizing ABL, however, current ABL stability (as measured by LTS and EIS) will need to destabilize at the current rate for another 10-15 decades before becoming “unstable”.