Lifted Temperature Minimum during the atmospheric evening transition

Abstract

Abstract. Observations of Lifted Temperature Minimum (LTM) profiles in the nocturnal boundary layer were first reported by Ramdas and Atmanathan (1932). It was defined by the existence of a temperature minimum some centimeters above the ground. During the following decades, several research studies analyzed this phenomenon verifying its existence and postulating different hypothesis about its origin. The aim of this work is to study the existence and characteristics of LTM during the evening transition by using observations obtained during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. Data obtained from two masts instrumented with thermocouples and wind sensors at different heights close to the ground, and a mast with radiometers are used to study the role of mechanical turbulence and radiation in LTM development. The study shows that LTM profiles can be detected under calm conditions during the day–night transition, several hours earlier than reported in previous work. These conditions are fulfilled under weak synoptic forcing during local flow shifts associated with a mountain–plain circulation in relatively complex orography. Under these special conditions, turbulence becomes a crucial parameter in determining the ideal conditions for observing LTM profiles. Additionally, LTM observed profiles are also related to a change in the atmospheric radiative characteristics under calm conditions.