Exploration of daytime atmospheric boundary layer thermodynamics across fronts over land using in-situ airborne measurements

An accurate numerical weather prediction (NWP) model for the atmospheric boundary layer (ABL) has remained a demand of society, agriculture, energy sectors, policy makers, and urban planners. Even miniscule inaccuracies in initial and boundary conditions and associated parameterizations can cause errors in NWP forecasts. The combination of factors such as synoptic scale airmass exchange, convergence, lifting, subsidence, convection initiation, and cloud formation cause frontal systems to be particularly complex with respect to ABL kinematics and thermodynamics. Though previous studies provided extensive information on surface fronts, empirical evidence of the horizontal and vertical variability in front-relative daytime ABL thermodynamic features remained underexplored. We investigated the impacts of frontal passages on daytime ABL thermodynamics during summer and winter across three regions of the continental US using in-situ aircraft measurements of state variables. Results revealed that ABL moisture heterogeneity during frontal passages may occur due to terrain heterogeneity, front-induced precipitation and subsequent soil moisture impacts, and synoptic scale mixing near frontal boundaries. The vertical contrasts in moisture between the ABL and overlying free troposphere were larger in the warm sectors than the cold sectors for most cases due to cold air advection and enhanced turbulent mixing near the front. The vertical contrasts in moisture did not yield a clear pattern based on sector, meaning that the vertical moisture structures in our cases depended on the properties of the individual airmasses. Our findings on front-relative ABL thermodynamic features provide unprecedented information which could help improve parameterizations for more accurate NWP for frontal ABL regimes.