Frontal Modification of Atmospheric Boundary Layer Dynamics over Land in Midlatitudes

Mid-latitude cyclones and associated passages of cold and warm fronts over the land surface lead to high impact weather events including deep moist convection and extreme precipitation. The atmospheric boundary layer (i.e. the lowest 1-2 km of the atmosphere) plays an important role in modulating the kinematics and thermodynamics of these systems. In general, the ABL plays an important role in a number of atmospheric processes like convection initiation, turbulence mixing of tracers and pollutants, momentum exchange, aerosol-cloud-microphysics, and land-atmosphere feedback mechanisms. We hypothesize that during quasi-periodic passages of cyclonic and anti-cyclonic flows, the ABL encounters vigorous changes (both vertically and horizontally) due to three key competitive forcings: land-surface forcing via changes in soil moisture regimes due to precipitation, subsidence over the high-pressure-dominated cold sector, and cloud coverage and frontal lifting. We aim to understand the frontal modification of ABL dynamics, in particular changes in ABL depths during pre-to-post frontal conditions so that a comparison between ABL depths in warm (pre-frontal) versus cold (post-frontal) sectors can be revealed.

Whereas the ABL depth variability on diurnal, intra-diurnal, synoptic and seasonal time scales is well documented, changes in ABL processes, before, during and after frontal passages or ABL depth variability as a function of weather patterns largely remain unexplored. To understand the frontal modification of ABL features, we explored 5-years (2014-2018) of rawinsonde observations over 17 sites located in the central and eastern US. While selecting sites for this study from the IGRA (Integrated Global Radiosonde Archive) database, we confirmed that the sites were not affected by inflow of urban boundary layer airmass, coastal flow advecting marine boundary layers, and terrain-modulated elevated mixed-layers. Thus, all the sites chosen were far from the sources of different air mass (i.e., urban boundary layers, elevated mixed-layers over mountains, and marine boundary layers). We will present selected results obtained from analyses investigating ABL depth changes during warm and cold front passages in four seasons determined using more than 17,000 rawinsonde profiles and 14,000 surface map analyses. We will also compare our findings of ABL depth variability observed during frontal passage cases versus typical fair-weather anticyclonic conditions. The knowledge gained from this work will advance our understanding of moist boundary-layer processes and single out potential sources that trigger drastic changes in ABL dynamics during, before, and after frontal passages.