An Objective Method for Clustering Observed Vertical Thermodynamic Profiles by Their Boundary Layer Structure

This study introduces a novel method for comparing vertical thermodynamic profiles, focusing on the atmo-spheric boundary layer, across a wide range of meteorological conditions. This method is developed using observed temperature and dewpoint temperature data from 31153 soundings taken at 0000 UTC and 32308 soundings taken at 1200 UTC between May 2019 and March 2020. Temperature and dewpoint temperature vertical profiles are first interpo-lated onto a height above ground level (AGL) coordinate, after which the temperature of the dry adiabat defined by the surface-based parcel's temperature is subtracted from each quantity at all altitudes. This allows for common sounding fea-tures, such as turbulent mixed layers and inversions, to be similarly depicted regardless of temperature and dewpoint tem-perature differences resulting from altitude, latitude, or seasonality. The soundings that result from applying this method to the observed sounding collection described above are then clustered to identify distinct boundary layer structures in the data. Specifically, separately at 0000 and 1200 UTC, a k-means clustering analysis is conducted in the phase space of the leading two empirical orthogonal functions of the sounding data. As compared to clustering based on the original vertical profiles, which results in clusters that are dominated by seasonal and latitudinal differences, clusters derived from trans-formed data are less latitudinally and seasonally stratified and better represent boundary layer features such as turbulent mixed layers and pseudoadiabatic profiles. The sounding-comparison method thus provides an objective means of catego-rizing vertical thermodynamic profiles with wide-ranging applications, as demonstrated by using the method to verify short-range Global Forecast System model forecasts.