A lifted temperature minimum (LTM) profile is characterized by an elevated temperature minimum close to the surface. Depending on the ground characteristics, LTM is typically located between 10 and 50 cm above the surface and observed at night. After sunset, if cloudless and calm conditions exist and ground and air emissivities have similar values, the air layer just above the ground can cool radiatively faster than the ground itself and a minimum temperature appears several centimetres above the surface. LTMs have been studied by means of observations , numerical simulations and laboratory experiments .

provided for the first time a detailed description of the unexpected temperature minimum neglecting advective effects, and suggested that the LTM might be related with radiation from the ground and the lower layer of the atmosphere. Several years later, , and confirmed the results obtained by , discarding instrumental errors by using more complex instruments. took measurements over different terrain types to verify that LTMs are not produced by advection and defined three different types of temperature profiles, distinguishing between profiles with the minimum temperature at the ground and LTM profiles caused by advection. Additionally, they made measurements at different latitudes to prove that the phenomenon was not restricted to the tropics. On the contrary, showed some skepticism about the existence of LTM. For instance, he wondered why LTMs are not overturned by convective instability. He was also concerned about the precision of the measurements close to the ground. Later on, suggested the existence of a haze layer near the ground to explain the appearance of the LTM. Nevertheless, this approach was discarded because this layer was never observed and the thermal diffusivity required for its explanation was not realistic .

More recent studies have shown that LTM observations are common over different natural, e.g. bare soil, snow and short grass and artificial surfaces such as concrete or thermofoam . studied in detail the importance of surface characteristics for the appearance of LTM. They demonstrated, by studying LTM formation over different surfaces (aluminum, thermofoam and concrete), that decreasing surface emissivity increases the intensity of an LTM and the near-ground temperature gradient. Lowering surface emissivity with respect the overlying atmosphere can act to change the temperature profile from a minimum temperature occurring at the ground to an elevated temperature minimum. Therefore, terrain with an emissivity close to that of the overlying air favours LTM formation. summarized the main mechanisms related to the occurrence of LTM. In his first summary, he introduced a brief description of a model, which was later described in detail in . They hypothesized that radiative cooling depends on ground emissivity and air emissivity gradient. When the air emissivity gradient is large, the temperature of the air close to the ground decreases faster than the temperature of the ground and an LTM can be observed. Even though the model presented a detailed solution for the air temperature evolution considering surface emissivity, ground cooling and turbulence, it did not include a detailed discussion of the energy budget near the ground, which was introduced afterwards by .

Apart from ground thermal characteristics, calm conditions with low mechanical turbulence are crucial to observe an LTM. For instance, LTM intensity is weaker for high roughness length surfaces because it increases both turbulence and emissivity . Moreover, field measurements and models show that advection was weak when an LTM was observed. The LTM has only been reported for a small number of cases where the friction velocities were above 0.1 ms-1, and in those cases LTM disappeared relatively quickly .

were the first to suggest a model which appears to be in good agreement with observations. They studied the importance of radiative, conductive and convective fluxes during LTM events. This model was accepted until and identified an error in the calculations of and introduced a new model based on the work by . This model includes the importance of suspended solid or liquid particles, which can enhance radiative cooling. , , and pointed out the importance of radiation in the formation of LTM. confirmed that near the surface, radiative cooling can be orders of magnitude greater than values elsewhere in the boundary layer. With very light winds, the importance of turbulence is nearly negligible compared with radiation. Therefore, temperature evolution is mainly governed by the radiative timescale . Moreover, 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 . The first process would drive the heat budget at 20–30 cm, but turbulence cooling would temporarily be dominant around 10–15 cm.

Finally, daytime LTM measurements have been reported when near-surface temperature inversions occur under specific conditions over the open Arabian Sea during the summer monsoon season . These atmospheric conditions, characterized by strong surface winds and high levels of sea salt particle concentration in the boundary layer, are far away from the conditions presented at night or here.

In summary, LTM occurrence varies depending on surface characteristics (emissivity and thermal inertia), prevailing wind conditions (turbulence) and atmospheric radiation. In contrast with previous studies, we analyse LTM occurrences during the evening transition period. It is during this period when the largest radiative cooling occurs . Our research objective is to study the relevance of wind characteristics driven by orography, turbulence, characterized by the Richardson number and radiation on the appearance of LTM during the evening transition.

The study of the appearance of LTM, besides increasing the knowledge of the physics of the surface layer, can also be relevant for agriculture. The lifted temperature minimum can modify the occurrence of frost, which has adverse effects on crops . Moreover, it can help to describe the presence of radiation fog because, as it will be shown, the presence of LTM is related with a variation of the radiation .

The paper is structured as follows. In Sect. 2 we explain the measurements used in this study, taken during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. In Sect. 3, the temperature profiles are analysed in detail and LTM characteristics are described. Section 4 investigates and presents the variables influencing LTM: wind characteristics and friction velocity, turbulence and radiation. Finally, Sect. 5 summarizes the results.

To investigate LTM during the evening transition, we analyse measurements acquired during the BLLAST field experiment . This campaign was performed from 14 June to 8 July 2011 in southern France, near to the Pyrenees. The campaign site extended over an area of approximately 100 km2 covered with heterogeneous vegetation: mainly grass, corn, moor and forest.

The most salient BLLAST objective was to obtain a detailed set of meteorological observations during the evening transition to better understand the physical processes that control it. For example, to improve the understanding of the effects of entrainment across the boundary layer top, surface heterogeneity, horizontal advection, clouds, radiation and gravity waves on the evening transition.

During intensive observational periods (IOPs), the atmosphere was heavily probed by in situ measurements from masts, towers, tethered balloons, radiosondes and manned and unmanned airplanes, as well as remote sensing instruments such as lidar and radar wind profilers.

For the present work, the near surface temperature evolution is analysed using the measurements taken at two masts (T1 and T2) separated by approximately 468 m. Figure  shows a plan view of the T2 area and a side view of the T1 and T2 instruments. T1 was located at 43.1275∘ N, 0.36583∘ E and T2 at 43.1238∘ N, 0.36416∘ E. T1 was a 10 m mast instrumented with four Campbell Scientific CSAT3 sonic anemometer thermometers and Campbell Scientific E-TYPE model FW05 (12.7 µm diameter) fine wire (FW) thermocouples at 2.23, 3.23, 5.2 and 8.2 m. Closer to the ground, there were four additional FW05 12.7 µm FWs at 0.091, 0.131, 0.191 and 0.569 m, which were only installed during the IOPs. Temperature data at T1 were recorded at 20 Hz. The influence of direct or indirect solar radiation has been taken into account in the measurements. Moreover, showed that as the size of the thermocouple goes down, the radiative influence is reduced. For a 25 µm sensor a 0.1 K of error was observed. Our sensor is half that size; hence, the error of the instrument should be lower than 0.1 K, which is smaller than the values of the LTM intensity.

T2 was a 2 m mast with eight FW3 (76.2 µm diameter) FWs located at 0.015, 0.045, 0.075, 0.14, 0.3, 0.515, 1.045 and 1.92 m recording temperature data at 10 Hz. Additionally, separated by approximately 2 m from T2, there was also a Campbell Scientific CSAT3 at 1.95 m, recording data at 20 Hz. To unify the measurements taken by the different instruments, all the recorded data were averaged over 5 min intervals . This information was complemented with an estimation of the skin temperature provided by a Campbell Scientific IR120 infrared remote temperature sensor pointing towards the surface. This infrared sensor measured temperature with a sampling frequency of 3 Hz before 21 June 2011 and of 1 Hz after this day.

Near T2, one Kipp & Zonen CNR1 net radiometer was installed. The CNR1 sensor is able to measure upwelling and downwelling components of both the shortwave solar (0.305–2.8 µm) and terrestrial radiation (5–50 µm) separately. The CNR1 was installed at 0.8 m above the ground.

The ground characteristics below both masts were conducive to observe LTM . The ground in both cases was covered by long grass, which has an emissivity of 0.986 . The vegetation cover has low thermal conductivities which vary from 0.05 to 0.46 Wm-1K-1 . However, the surface surrounding T1 was covered by long grass while the T2 surface had some cut grass over the terrain, which could cause some heterogeneity in the surface thermal properties.

pointed out that, over grass-covered surfaces, the minimum temperature during the night can be found just above the grass instead of right at the surface. This phenomenon, which is associated with the vegetative canopy, is sometimes confused with an LTM. observed an LTM at 0.02 m above the grass. In our case study, the grass height is short, around 0.03–0.07 m, and the observed LTM height occurred above 0.1 m from the ground, that is, always above the grass.

For the following analysis, we selected different favourable IOPs with good data availability from the T1 and T2 areas. 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. Almost all these IOPs were clear and calm days with a mountain–plain circulation characterized by weak northerly winds during the day switching to southerly at night. The synoptic situation did not show any notable perturbation.

During the BLLAST campaign, when LTM occurred it was observed at both masts. Figure  shows the evolution of potential temperature profiles where an LTM is observed on 24 June 2011 (top panels) and 1 July 2011 (bottom panels) recorded at T1 (left) and T2 (right). The LTM can be observed on both days at both masts.

As illustrated in Fig. , three sensors on each tower were used to detect and characterize LTM. First, the location of the minimum temperature was identified (θbase). Next, the sensor closest to the ground was defined as LTM▾. Finally, the sensor located just above the base sensor (LTM▴) was identified. An LTM is observed if θbase-θLTM▾<0andθLTM▴-θbase>0. During this period, LTM intensity is calculated following : LTMintensity=θbase-θLTM▾. The LTM duration was defined as the period when the LTM conditions outlined above were fulfilled. Table  presents a summary of the following LTM characteristics for the different IOPs: height, intensity absolute values and duration of the phenomenon.

An LTM was observed during the evening transition for all IOP days except on 27 June 2011. An LTM forms at similar heights on both towers. For example, at T1 a height of around 0.131 m was typical, while LTM heights were between 0.075 and 0.14 m (except on 25 June 2011) at T2. Unfortunately, limitations in the vertical resolution of the measurements prevent a more precise determination of the LTM heights. In spite of this consistency, there are clear differences between the detailed LTM characteristics on different IOPs and at the different towers. On 24 June 2011, an LTM was observed during 10 min at T1 and for 40 min at T2. Greater LTM intensity (0.7 K) was observed at T2 compared to T1 (0.35 K). On 25 June 2011, an LTM was detected at T2 at a slightly higher level, around 0.3 m with an intensity of 0.5 K. This height is in the range of LTM heights reported by . On 25 June 2011, FWs were installed at T1 after 19:30 UTC (universal time coordinated); therefore, LTM comparisons cannot be made.

A completely different situation was observed on 27 June 2011; with no clear LTM development. T2 measurements showed indications of an LTM formation which did not progress (not shown).

On 30 June 2011, T1 showed a slightly lower-intensity (0.3 K) LTM starting around 18:00 UTC and lasting less than 20 min. A slightly lower-intensity LTM was also observed at T2 with an intensity of 0.5 K. On 1 July 2011 a clearly marked (0.7 K) LTM was observed at T2 during 1 h. On the other hand, T1 showed a less pronounced LTM (0.35 K), which persisted only 20 min. Finally, on 2 July 2011 T2 showed an LTM intensity of around 0.5 K with a duration of more than 1 h. However, T1 showed an intensity of 0.35 K with a duration of 40 min.

Due to the variations in sensor heights at the two locations, the LTM intensity can vary from one tower to the other. Day to day variations at a single location, however, can be compared. Specifically, our definition of LTM intensity is based on the temperature measured closest to the ground which, in order to detect an LTM, needs to be warmer than the LTM. The elevation of the sensor closest to the ground differs for T1 and T2 (about 9 and 1.5 cm, respectively); thus, the two locations' intensities are not strictly comparable. As shown in Table 1, the LTM intensity at T2 is always roughly twice the value observed at T1, which is most likely due to the fact that the lowest thermocouple at T1 is still influenced by the cold air associated with the LTM and an additional increase in temperature towards the surface is not resolved.

The presence of a lifted temperature minimum during the evening transition is studied by means of observations taken during the 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 IOPs except on 27 June 2011.

With the instrumentation deployed during the campaign we were not able to verify all the previous hypotheses to explain the appearance of  LTM. For instance, the presence of aerosols at lower height were not monitored during the campaign.

Additionally, it would be difficult to analyse, by using observations, the budget between radiation warming and turbulence cooling during the evening transition. While small Kaijo–Denki sonics could be used at 15 and 30 cm to measure cooling via sensible heat flux divergence, radiation measurements would be much more difficult at those heights close to the surface, and not possible with commercial pyranometers.

Moreover, it is important to note that the research study focusses on the afternoon transition. To our knowledge, the heat budget (the competition between turbulent fluxes and radiation divergence) at the different levels close to the surface has not been studied during this period of the day. In fact, the current MATERHORN observational campaign was partially designed to study the evolution of the heat budget during the afternoon/evening transition.

By studying the wind conditions characterized by a mountain–plain flow, we conclude that the days with a more marked decrease of mean wind speed and wind speed fluctuations (24 June or 1 July 2011) have a more intense LTM. On the other hand, on the days without a reduction of wind speed, such as 27 June 2011, LTM cannot be observed during the evening transition.

Analysing Rig during the evening transition, we observe that the LTM is detected on days with a faster increase of Rig, i.e., a faster decrease of mechanical turbulence. However, due to the fact that ∂θv‾/∂z is changing sign during the evening transition, no threshold of Rig can be defined.

Finally, the longwave-radiative conditions are analysed. We study the differences in the decay rate of the upwelling longwave radiation at 0.8 m and the longwave radiation at LTM height. Longwave radiation at LTM height decays at two different rates in contrast to the upwelling longwave radiation decay at 0.8 m which is constant in time. This change in the radiative conditions can modify the temporal evolution of the potential temperature creating the LTM.

To conclude, during evening transition it is possible to observe the lifted temperature minimum over a terrain with moderate/large emissivity and thermal inertia. In this study case, really calm conditions were observed during evening transition due to the presence of the Pyrenees which produces an early evening calm period easily defined through a change in the wind velocity and turbulence. Moreover, a change in the radiative conditions was observed during an LTM period which confirms its radiative origin.