Doppler Lidar Wind Profiling in Fairbanks (Interior of Alaska) During the 2022 ALPACA Field Campaign

A seven-week field campaign took place in Fairbanks, Interior of Alaska, during January and February 2022, in the framework of the Alaskan Layered Pollution And Chemical Analysis (ALPACA) international project. A compact Doppler Lidar was deployed (Leosphere WindCube v2) to profile winds in the atmospheric boundary layer from 40 to 290 m above ground level (a.g.l.). The first objective of this project was to study the influence of strong atmospheric stratification and temperature inversion layering on air pollution. For this purpose, the Lidar was first installed in the urban canopy downtown Fairbanks city (~3.5 weeks). The second objective was to study the influence of shallow cold flows (SCF) on surface-based temperature inversions and the energy budget. Therefore, the Lidar was then re-deployed in a suburban site located nearby the foothills bordering the city, at the outlet of a small valley. Measurements on this site also included surface turbulence by eddy-covariance, radiative fluxes and temperature profiles, both in-situ during tethered balloon launches and remote sensing using a Microwave Radiometer.

The conditions for operating a near-infrared Doppler Lidar were very stringent, as Arctic environments are generally pristine, except for urban areas in wintertime. However, the wind dataset availability up to 100 m a.g.l. was ~47% at the urban site and ~30% at the suburban site. The relationship between Lidar performance and aerosol size distribution is under evaluation at both sites, using observations from Scanning Mobility Particle Sizers. At the suburban site, the SCF descending from the nearby valley was regularly visible, with wind speeds reaching 5-6 m.s^-1 at 40 m a.g.l. Cases of continuous flow lasting for more than 24 hours were observed, during which the SCF depth remained stable and very shallow (40 to 60 m a.gl.). Pulsated episodes also occurred, that could be as short as 2 hours, and were characterized by a more variable SCF depth (up to 100 m a.g.l.). The local and regional conditions triggering the onset, variability and characteristics of the SCF are under investigation. Another unexpected observation was made: at both sites, snow precipitations were always found to be associated with ascending winds (up to 1.2 m.s^-1). 

To improve our understanding of the dynamical processes highlighted by the Lidar wind observations, the Polar-WRF (Weather Research and Forecasting) model will be run at a very high resolution (~1 km). The focus will be to investigate, and potentially improve, the ability of the Polar-WRF surface layer and land-surface model (via the roughness length for momentum and the stability functions) to reproduce the surface energy budget, the very and weakly stable regimes and the transition between the two at the SCF onset in a continental, high-latitude context.