Abstract
Abstract. Observations of lifted temperature minimum (LTM) profiles in the nocturnal boundary layer were first reported in 1932. It was defined by the existence of a temperature minimum some centimetres above the ground. During the following decades, several research studies analysed this phenomenon verifying its existence and postulating different hypotheses 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 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 when the 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. Additionally, LTM observed profiles are also related to a change in the atmospheric radiative characteristics under calm conditions.
Highlights
A lifted temperature minimum (LTM) profile is characterized by an elevated temperature minimum close to the surface
The presence of a lifted temperature minimum during the evening transition is studied by means of observations taken during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign
The campaign site presented ground characteristics suitable for observing LTM with large ground emissivity and thermal inertia. During this period of the day, LTMs were observed at different heights and with different intensity and duration during all intensive observational periods (IOPs) except on 27 June 2011
Summary
A lifted temperature minimum (LTM) profile is characterized by an elevated temperature minimum close to the surface. Mukund et al (2014) showed that a heterogenous distribution of aerosol concentration can cause hyper-cooling close to the surface, which modifies the atmospheric radiative cooling Another hypothesis explaining the appearance of LTM (or the temperature maximum at upper levels, around 20–30 cm) during the night in stable conditions is based on the competition between the radiative warming of the lower layers (up to 50–70 cm) of the atmosphere, over a rapidly cooling surface, and the turbulence cooling (Savijarvi, 2006, 2014; Edwards, 2009a, b). The analysis is based on the observations taken on 24, 25, 27, and 30 June and 1 and 2 July 2011 During these IOPs, we have measurements from both towers, the infrared surface temperature sensor and the radiometer.
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