Modulation of Boundary-Layer Stability and the Surface Energy Budget by a Local Flow in Central Alaska

Abstract

The pre-ALPACA (Alaskan Layered Pollution And Chemical Analysis) 2019 winter campaign took place in Fairbanks, Alaska, in November–December 2019. One objective of the campaign was to study the life-cycle of surface-based temperature inversions and the associated surface energy budget changes. Several instruments, including a 4-component radiometer and sonic anemometer were deployed in the open, snow-covered University of Alaska Fairbanks (UAF) Campus Agricultural Field. A local flow from a connecting valley occurs at this site. This flow is characterized by locally elevated wind speeds (greater than 3 m s$$^{-1}$$ - 1 ) under clear-sky conditions and a north-westerly direction. It is notably different to the wind observed at the airport more than 3.5 km to the south-west. The surface energy budget at the UAF Field site exhibits two preferential modes. In the first mode, turbulent sensible heat and net longwave fluxes are close to 0 W m$$^{-2}$$ - 2 , linked to the presence of clouds and generally low winds. In the second, the net longwave flux is around − 50 W m$$^{-2}$$ - 2 and the turbulent sensible heat flux is around 15 W m$$^{-2}$$ - 2 , linked to clear skies and elevated wind speeds. The development of surface-based temperature inversions at the field is hindered compared to the airport because the local flow sustains vertical mixing. In this second mode the residual of the surface energy budget is large, possibly due to horizontal temperature advection.

Overview of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) Field Experiment

“…UAF-Farm generally has much shallower SBIs than CTC. This may be due to the influence of cold flows from a nearby valley , and the fact that the height of obstructions at the UAF-Farm is much lower than downtown. The UAF-Farm is a flat field for hundreds of meters, while downtown has an “urban canopy” consisting of buildings and trees, which are roughly 10–15 m tall.…”

“…Fairbanks, Alaska, is one of the most polluted cities in the U.S. during wintertime, regularly exceeding the Environmental Protection Agency (EPA) short term (24-h) outdoor standard of 35 μm m –3 for fine particulate matter (PM 2.5 , <2.5 μm in diameter) pollution. Past studies have highlighted the critical role of pollution trapping by temperature inversions in high pollution episodes. − These temperature inversions can exceed 0.5 °C/m temperature gradient at the surface, greatly hindering vertical mixing. − These same inversions trapped carbon monoxide (CO), causing Fairbanks to violate CO standard of 9 parts per million frequently up until the early 2000s, when better technology decreased their emissions from automotive sources. This source mitigation has been very successful, and now Fairbanks CO levels only reach about a third of the regulatory standard under the most severe inversion trapping conditions.…”

“… 32 − 36 These temperature inversions can exceed 0.5 °C/m temperature gradient at the surface, greatly hindering vertical mixing. 37 − 40 These same inversions trapped carbon monoxide (CO), causing Fairbanks to violate CO standard of 9 parts per million frequently up until the early 2000s, 41 when better technology decreased their emissions from automotive sources. This source mitigation has been very successful, and now Fairbanks CO levels only reach about a third of the regulatory standard under the most severe inversion trapping conditions.…”

Investigating processes influencing simulation of local Arctic wintertime anthropogenic pollution in Fairbanks, Alaska during ALPACA-2022

“…Thus, pollutant emissions from elevated sources, such as power plant chimney stacks, can be influenced by the presence of stably stratified layers (Pasquill and Smith, 1983;Briggs, 1984;Akingunola et al, 2018). Less stable conditions, with weak surface temperature inversions, can be induced by transient or cyclonic synoptic conditions or local sub-mesoscale flows under anti-cyclonic conditions (Maillard et al, 2022).…”