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1. Introduction
Elevated mixed layers (EMLs) are layers of constant vertical distribution of potential temperature θ, that is, layers having dry-adiabatic temperature lapse rates, not coupled to the ground (Carlson and Ludlam 1968; Lanicci and Warner 1991a, hereafter LW91). These layers occur mainly to the east (downstream) of great mountain ranges, a result of horizontal advection of surface-heated air over higher terrain and/or ageostrophic circulations in the lee of the mountains. The Rockies in North America (NA; e.g., Carlson and Ludlam 1968; Banacos and Ekster 2010, hereafter BE10) and the Tibetan Plateau in East Asia (e.g., Das et al. 2014) are known to favor the formation of EMLs that can have a great impact on the atmospheric conditions over the lower terrain downstream due to their association with hazardous weather events (Cordeira et al. 2017). Observations and case studies by Rasmussen and Houze (2016) show EMLs occur east of the Andes in South America (SA). The purpose of this paper is to document SA EMLs and to compare and contrast their formation processes with NA EMLs.
A complete climatology of NA EMLs was done by LW91 and Lanicci and Warner (1991b,c) in three sequential papers. They used observed NA soundings from 1983 to 1986 and applied several criteria to automatically search for EMLs. The classic model of EML formation depicts an approaching trough in the mid- and upper troposphere and associated southwesterly flow over the high terrain in northern Mexico/southwestern United States. This southwesterly flow advects a dry, mixed air mass from over higher terrain eastward over somewhat less warm but much more moist planetary boundary layer (PBL) air at lower elevations over the Great Plains. This setup can create an environment of high potential instability over the Great Plains and is often observed from spring to summer, when solar radiation is strong enough to form deep, surface-based mixed PBLs over the Rockies.
The mechanisms for EML formation and maintenance in NA were investigated by BE10. The authors presented an equation for the local tendency of temperature lapse rate (γ; γ ≡ −dT/dz, where T is temperature and z is height). The local γ tendency is given by
[Inline formula omitted: See PDF]
where Q is the...