On the Seasonality of Cold Air Pooling Dynamics across Scales

The objective of this study was to understand how cold air pooling dynamics differ across scales at different times of a year. The dynamics of cold air pooling refers to the cycle of formation and dissipation of stable cold air which forms in valley depressions due to radiative cooling of the ground. The physical mechanisms responsible for describing the phenomenon across scales are known to be radiative cooling, along-valley and cross-valley flows, and stability of the boundary layer. The main question in this study was “how does the cold air pooling dynamics differ across scales in different times of a year?” To answer this question, the dynamics of cold air pooling was investigated at two different spatial scales during different seasons. The main study site was the HJA Forest in Oregon’s Western Cascades. The forest is a 6475-hectare experimental site with complex topography. The spatial scales were local (PRIMET and VANMET meteorological stations at the HJA forest) vs regional scales (The whole Andrews Forest). The Primet is located at 450 meters above sea level on a fluvial terrace while Vanmet is located at 1250 meters above sea level close to a ridge. July and December were selected as two months when the cold air pools show distinctive behaviors hourly and diurnally. The underlying hypothesis was that the cold air pools tend to persist in December whereas they follow a diurnal pattern of formation and dissipation in July. To test this hypothesis, the hourly mean temperature, wind speed, wind direction, incoming shortwave radiation, vapor pressure deficit, and relative humidity data for Primet and Vanmet within a 10-day temporal window from 14 to 26 of July and December in 2014 were acquired. The methods used in this study was based on an assumption that determining the stability of the boundary layer at local and regional scales is the first step in understanding the dynamics of the cold air pools in different seasons.

Regarding the dependability of the boundary layer stability to the incoming shortwave radiation and mean wind speed, two logical statements were introduced to determine the stability of the boundary layer at local scale (Primet and Vanmet) and regional scale (the HJA Forest). For the local scale, a value of 300 W m^-2 for the incoming shortwave radiation in July 2014 was used as a threshold for determining the stability of the boundary layer. For December 2014 the threshold values of 1 W m^-2 and 1 m/s were determined for the hourly mean incoming shortwave radiation wind speed from 8:00 am to 4:00 pm respectively. For the regional scale, the hourly environmental lapse rates were calculated based on the hourly deficit of the mean temperatures between Vanmet and Primet. The result show that there is a stark difference between July and December patterns of hourly mean incoming shortwave radiation and wind speed. The lower amount of hourly mean incoming shortwave radiation and wind speed plus high variability of wind speed and higher amount of relative humidity on December are convincing evidence for more persistent cold air pools that might have been formed during December 15, 20, and 25. In addition, the higher amount of relative humidity along with lower amounts of VPD on December 15, 20, and 25 in comparison to July 15, 20, and 25 are other convincing evidence for accepting the assumption that the cold air pool has formed during the mentioned dates. Furthermore, the comparison between July and December patterns of hourly mean incoming shortwave radiation and wind speed for Vanmet shows a stark difference in both the amount of the variables and their pattern. The higher values of wind speed plus lower values of hourly mean incoming shortwave radiation on December 15, 20, and 25 in comparison to July 15, 20, and 25 are supportive evidence for possible cold air pool formation. This possibility is even stronger on December 20 since the maximum hourly mean incoming shortwave radiation is 18 W m^-2 and the maximum mean wind speed reaches 3.5 m/s which is the highest among the days. In addition, the maximum relative humidity plus the lowest amount of VPD on December 20, are other convincing evidence for assuming that a cold air pool has been formed. The findings of this study show that at the regional scale on December the established cold air pools tend to persist whereas, on July the formed pools tend to follow a cycle of daily formation and dissipation.