A Case Study of the Mechanisms Modulating the Evolution of Valley Fog
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We present a valley fog case study in which radiation fog is modulated by topographic effects using data obtained from a field campaign conducted in Heber Valley, Utah from January 7–February 1, 2015, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program. We use data collected on January 9, 2015 to gain insight into relationships between typical shallow radiation fog, turbulence, and gravity waves associated with the surrounding topography. A ≈ 10–30 m fog layer formed by radiative cooling was observed from 0720 to 0900 MST under cold air temperatures (≈−9 °C), near-saturated (relative humidity with respect to water ≈95 %), and calm wind (mostly <0.5 m s−1) conditions. Drainage flows were observed occasionally prior to fog formation, which modulated heat exchanges between air masses through the action of internal gravity waves and cold-air pool sloshing. The fog appeared to be triggered by cold-air advection from the south (≈200°) at 0700 MST. Quasi-periodic oscillations were observed before and during the fog event with a time period of about 15 min. These oscillations were detected in surface pressure, temperature, sensible heat flux, incoming longwave radiation, and turbulent kinetic energy measurements. We hypothesize that the quasi-periodic oscillations were caused by atmospheric gravity waves with a time period of about 10–20 min based on wavelet analysis. During the fog event, internal gravity waves led to about 1 °C fluctuations in air temperatures. After 0835 MST when net radiation became positive, fog started to dissipate due to the surface heating and heat absorption by the fog particles. Overall, this case study provides a concrete example of how fog evolution is modulated by very weak thermal circulations in mountainous terrain and illustrates the need for high density vertical and horizontal measurements to ensure that the highly spatially varying physics in complex terrain are sufficient for hypothesis testing.
KeywordsIce fog internal gravity wave mountain complex terrain radiation fog turbulence–wave interaction
This research was funded by the Office of Naval Research Award #N00014-11-1-0709, Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. We are grateful the John Pace and Dragan Zajic from the U.S. Army Dugway Proving grounds for their gracious help and instrument contributions to the project. The authors want to thank Stephan de Wekker for providing data from the automatic weather station. We would also like to thank Alexei Perelet, Derek Jensen, and Matt Jeglum for their help in the field. We are also extremely grateful to Grant Kohler and the Kohler family for the use of their farm during the experiment as well as all of the additional help that they regularly provided during the experiment. The authors are extremely grateful for all of the help during the field campaign, and the scientific insight provided by the MATERHORN team.
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