- The objective of this dissertation was to understand the physical mechanisms affecting inversion events in a complex forested mountain landscape. This work was motivated by the long-term studies of climate at the Andrews Forest, short-term studies of vertical temperature, light, wind, and moisture gradient in old-growth trees, and interest in the role of inversions in moderating the effects of climate change. Regional atmospheric conditions and local topographic conditions (i.e., topographic depressions) are known to influence the formation and dissipation of inversions. However, relatively little is known about the effect of the forest canopy on the dynamics of inversion events in mountainous landscapes. Although a few studies have examined this topic through simulation modeling, to the knowledge of this author, no studies have used high temporal resolution climate data to study inversions in forested mountain landscapes. Therefore, the novelty of this Ph.D. research is conducting empirical analysis using long-term meteorological data in a complex forested landscape in the Western Cascades, Oregon.
Chapter 1 examined the main published ideas on the cold air drainage and inversion events since 1949.
Chapter 2 examined the effect of different sun angles on radiation, air temperature, wind speed and direction in Discovery Tree, a 66-m tall tree located on the valley floor of a forested mountain basin, in the H.J. Andrews Experimental Forest, and related those patterns to processes of cold air drainage and pooling on clear, cloudless days at different times of year. The study developed a conceptual model of the energy budget of a hypothetical Douglas-fir tree in an old-growth forest stand as a function of sun angle under clear sky conditions. This conceptual model used a simple radiation budget to establish predictions about the vertical temperature profile in the canopy at various sun angles. Data from four two-week periods centered around the solstices and equinoxes of 2016 and 2017 were evaluated to test hypotheses and predictions about heat and energy exchange within the canopy and the surrounding landscape. Data on air temperature, wind speed and direction, radiation, relative humidity, and leaf wetness were analyzed from heights ranging from 1.5 to 56 m in the tree, along with data on radiation from a nearby meteorological station (Primet). In addition, the proportion of the nearby landscape that was shaded was calculated for sun angles at pre-dawn, local noon, and local dusk on the solstices and equinoxes. Predictions based on the simple radiation budget were generally consistent with observations at local noon on solstices and equinoxes. However, most observations did not match predictions during pre-dawn and dusk, because other elements of the energy budget, such as upward ground heat flux, canopy heat storage, and subsidence (advection) also play a role in controlling temperature, wind, and moisture in the forest canopy. As a result of landscape shading, the Discovery Tree is in high shade in the morning and illuminated in the afternoon. In general, inversions occurred more frequently during the day than at night. Drainage flow and down-valley flow may create turbulence and mixing during the night. In addition, intermittent gusts may create turbulence at canopy top and mix the boundary layer during the night. Moreover, upward soil heat flux and outgoing longwave radiation from the canopy during the night may contribute to a mixed nighttime mixed boundary layer. Overall, at low sun angles, other elements than net radiation, including shading of the surrounding landscape, influenced the energy budget of the tree, producing conditions opposite to those predicted by radiation alone.
Chapter 3 examined the influence of seasonal and daily topographic shading and weather variability on inversions in a forested mountain watershed, H.J. Andrews Forest, Oregon. The objective of this study was to investigate how solar illumination affects topography-induced shading, local winds, and the frequency of inversions (when the temperature increases with height) in a forested mountain landscape. The study site is the H.J. Andrews Experimental Forest in the western Cascades, Oregon. It was expected that the frequency of stable conditions, i.e. when the lapse rate is more positive than the environmental lapse rate, and the strength of inversions would be greatest when the landscape is less illuminated, and vice-versa. The study used digital elevation data as well as hourly data from a low elevation (Primet) and a high elevation (Vanmet) meteorological station in the Andrews Forest during months near the solstices and equinoxes (October, January, April, and July) of 2003, 2011, and 2014, which were years with the second highest, the lowest, and the highest average temperature over the period 1989-2015, when matched records are available from these two stations. In addition, the proportion of the Andrews Forest landscape that was shaded was calculated for sun angles at pre-dawn, local noon, and local dusk on the solstices and equinoxes. Landscape shading was greatest at 0900 hours on all four dates, and least at 1200 at the winter solstice and at 1500 h at the summer solstice and the spring and fall equinoxes. The highest frequency of stable conditions and the strongest inversions occurred in January of all three years. The frequency of inversions was related to solar radiation and the amount of the landscape in high shade. Inversions were more frequent during nighttime than daytime hours. In many cases, up-valley winds dominate in the daytime and down-valley winds dominate at night. When synoptic (upper-elevation, i.e., Vanmet) wind speeds were high, the wind speeds at valley floor (i.e., Primet) were low since Primet was decoupled from the synoptic wind conditions. Thus, the interaction of sun angle with landscape shading strongly influenced the strength and persistence of inversions in this forested mountain landscape.
Chapter 4 examined the effect of light, wind, and humidity, and sensor type on the measurement of air temperature in the Discovery Tree, Andrews Forest, Oregon. The temperature difference between HOBO and aspirated sensors was significantly related to the HOBO light intensity at 40m and 56 m height in the tree. These relationships indicate that at 30 m and above, an increase of 1000 lux is associated with an increase of 0.10 to 0.18 °C in the temperature difference between the HOBO sensor and the aspirated one. At these heights, and at wind speeds < 1 m/s, an increase of 1000 lux is associated with an increase of 0.17 °C in the temperature difference between the HOBO sensor and the aspirated sensor, whereas at wind speed ≥ 1 m/s, an increase of 1000 lux is associated with an increase of only 0.1°C (at 56 m) or 0.13°C (at 40 m) in the temperature difference between the HOBO sensor and the aspirated sensor. These relationships can be used to correct HOBO sensor measurements in locations lacking aspirated sensors.
Chapter 5 summarizes the main findings of the dissertation. Chapter 2 showed that at low sun angles, other elements than net radiation, including shading of the surrounding landscape, influenced the energy budget of the tree, producing conditions opposite to those predicted by radiation alone. Chapter 3 demonstrated that the interaction of sun angle with landscape shading strongly influences the strength and persistence of inversion events in the Andrews Forest. Chapter 4 showed that at 30 m and above in the Discovery Tree, an increase of 1000 lux is associated with an increase of 0.10 to 0.18 °C in the temperature difference between the HOBO sensor and the aspirated one, and the radiation-induced temperature error is reduced when the wind speed is high.