Determining Atmospheric Boundary Layer Behavior over Mountainous Terrain Using Aircraft Vertical Profiles from NASA Student Airborne Research Program Data

The atmospheric boundary layer (ABL) height separates turbulently mixed air and pollutants emitted at the ground from the free troposphere above and is an important parameter in numerical weather prediction and air pollution dispersion models. Discerning the ABL height over mountainous terrain has historically been difficult because of, for example, complex interactions with upper level winds, venting of humidity and aerosols into the free troposphere, and large spatiotemporal variability. Mountain boundary layers can closely follow the terrain, be flat, or be shallower than surrounding valleys depending on the time of day, season, and synoptic conditions. To determine the extent to which the ABL heights follow terrain, meteorological and greenhouse gas (GHG) data collected by NASA aircraft during accents and descents over mountains across Central and Southern California during the 2012-2018 Student Airborne Research Programs (SARP) were analyzed. Meteorological parameters considered included water vapor, potential temperature, and turbulence. Carbon monoxide, methane, and carbon dioxide concentration ratios were used to consider when observed atmospheric layers were last in contact with the ground surface. United States Geological Survey elevation data were spatially joined with the aircraft data in a geographic information system when aircraft ground radar elevation data were unavailable, so that the topography underlying the aircraft vertical profiles could be determined. North American Regional Reanalysis data were also used to analyze the synoptic weather conditions during the NASA SARP research flights. Vertical profiles created by the aircraft ascending and descending over the southern Sierra Nevada, San Emigdio, and San Bernardino Mountains indicate that ABL heights closely follow the topography of these areas, being higher over ridges and lower over valleys. This new knowledge of ABL behavior can then be used to improve numerical weather and discern whether mountaintop GHG monitoring stations are sampling the free troposphere or within the ABL.