Using Aircraft Measurements to Investigate Boundary Layer Thermodynamic Variability Across Frontal Systems Over Land

For decades, an accurate numerical weather prediction (NWP) model which applies for all spatial scales (micro-, meso-, and synoptic-scale) has remained a demand of society, local governments, energy sectors, policy makers, and urban planners. It has been shown that even miniscule errors in initial and boundary conditions and associated parameterizations can cause significant forecast errors. For instance, ongoing research suggests that errors in simulating water vapor mixing ratio as small as 1-2 g/kg in the atmospheric boundary layer (ABL) can be the decisive factor in determining whether convection initiation occurs. Frontal systems are a particularly complicated environment due to complexities in kinematics and thermodynamics in the warm sectors, cold sectors, and frontal boundaries. However, our knowledge regarding thermodynamics across frontal systems within the ABL in different seasons and regions has been underexplored. A comprehensive understanding of the vertical and horizontal variability of thermodynamic state variables during synoptic-scale weather events will help improve parameterization schemes and therefore reduce forecast errors in the complex frontal environment. Within this work, we used airborne measurements of state variables and cloud physics lidar measurements obtained during NASA’s Atmospheric Carbon and Transport - America (ACT-America) campaign to better characterize the impact of frontal passages on ABL thermodynamics. During the campaign, in-situ and remote sensing measurements were collected within and above the ABL across the cold sectors, warm sectors, and frontal boundaries in all four seasons across three regions of the United States: the Midwest, the Mid-Atlantic, and the Southeast. We used the level-leg flight measurements and the vertical profiles of these variables to investigate horizontal and vertical variability in water vapor mixing ratio, potential temperature, and boundary layer depths across fronts and their adjacent warm and cold sectors to analyze how frontal passages impact the ABL across different seasons and regions. We further analyzed the contrast in mixing ratio and potential temperature between the ABL and free troposphere (FT), as this has ramifications for stability and mixing between the two layers. This will provide a basis to build new parameterizations for more accurate numerical weather prediction for ABL regimes in the presence of fronts. Six cases have been analyzed for three regions in the summer, with two cases from each region. Results from these cases confirm that water vapor mixing ratio and potential temperature generally drop from the warm sector to the cold sector, but the location of the drop can vary, and heterogeneity within each sector may arise due to clouds and precipitation, elevation, and distance from the front. These results also show that the ABL depths across fronts are case-dependent because of impacts from location, elevation, the presence of clouds, precipitation, and stage of frontogenesis. For instance, elevated mixed layers (EMLs) can advect from more mountainous areas and deepen the boundary layers of areas lower in elevation. In terms of ABL-to-FT contrasts, for the six summer cases there is a maximum in magnitude of ABL-to-FT potential temperature difference near the frontal boundary in the cold sectors. The magnitudes of the contrasts are generally higher in the cold sectors, which confirms that the cold sectors exhibit greater stability. To continue this work, we will compare these measurements and this analysis to the High-Resolution Rapid Refresh (HRRR) and the North American Regional Reanalysis (NARR) model outputs to verify frontal ABL structure and evaluate any bias or discrepancies we find within the models using these six summer cases and six winter cases.