Evolution of the Nocturnal Inversion Layer at an Urban and Nonurban Location

Abstract

The evolutionary cycle of the nocturnal radiation inversion layer, from formation through the time of erosion under fair weather summer conditions was investigated by time-series analyses of observations of inversion base and top heights, and inversion strength at an urban and a nonurban site in St. Louis, Missouri. The time-dependent behavior of each inversion parameter is presented from statistical and least-squares regression analyses. Differences of inversion evolution between these sites are discussed. A surface-based inversion generally formed before sunset at the nonurban site, and the growth of the inversion top height has been described quite well by (2KTt)½, where KT is the thermal eddy coefficient. The average time of formation of an elevated inversion layer at the urban site was 2.5 h after sunset. The height of the nocturnal urban boundary layer decreased after formation under steady wind conditions. The urban mixing height was consistently higher during the morning than at the nonurban site, although the difference diminished with time since the nonurban mixing height growth rate was greater. The slower growth rate of the urban mixing height was attributed primarily to advection of relatively cold air and lower mixing heights from the upwind nonurban environment. An overall rise of the inversion top height after sunrise was believed to be due to urban-induced upward motions, caused by low-level convergence that produced increasing mixing height growth rates. The average time for the inversion layer to erode completely was 4 h after sunrise at both sites.