Exploring Atmospheric Boundary Layer Depth Variability in Frontal Environments Over an Arid Region

Abstract

Details about atmospheric boundary layer (ABL) dynamics under advection over arid regions remain unexplored. Most numerical weather prediction (NWP) models strictly rely on ABL parametrization schemes under steady-state assumptions while observation-based research also considers horizontally homogeneous atmospheric conditions for estimating ABL depth (zi) growth rates. However, how different frontal systems modify ABL thermodynamic features, including zi, remains sparse in the literature. This work investigates how synoptic events impact daytime zi variability in different seasons and provides new insights on front-relative zi variability over two sites located in an arid region. Radiosonde-derived thermodynamic profiles obtained during more than 40 synoptic events in 2020 from two sites (Amarillo and Midland, Texas) have been used. Individual soundings are divided into a three-day period: prefrontal, frontal, and postfrontal. The ABL regimes and associated soundings are further classified into four categories: (1) Type-I with elevated mixed layer (EML) only, (2) Type-II with dryline only, (3) Type-III with both EML and dryline, and (4) Type-IV without dryline or EML. Results suggest that zi decreases substantially during frontal passages and is shallower in the cold sector than the warm sector. We also find that the zi variability in the postfrontal airmass is more uniform throughout the year compared to the zi in the prefrontal airmass indicating complexities associated with airmass advection. Regression analyses comparing frontal strength and observed Δzi (i.e., frontal contrasts in zi estimated via zi in warm sector minus zi in cold sector) do not yield any correlations, which suggests that advection from frontal passages has considerable influence in governing zi variability unlike in the conditions when zi is mainly dependent on surface forcings. By explaining how airmass exchange associated with frontal environments impacts overall ABL dynamics, new parametrizations for ABL modelling can be developed with an emphasis on zi advection.