Vertical Temperature Distribution Research Articles

Controlling effects of cap rocks on the formation of deep geothermal resources

Cap rocks with high thermal insulation are important for deep geothermal systems at a depth of 3000‒6000 m. Based on the deep geothermal geological conditions in the Fujian-Guangdong-Hainan area of South China, this study established an ideal geological model of reservoir-cap rock assemblages and simulated the geothermal field distribution of cap rocks of different thicknesses and thermal conductivity. The simulation results show that the vertical geothermal temperature distribution in an uplifted area of a depression was present as inverted mirror reflections relative to the elevated area of the basement. Specifically, the isotherms above the elevated area are convex in shape, while those below the elevated area are concave. There is a temperature equilibrium line between the convex and concave isotherms. The heat flow moves from the depressed area to the uplifted area below the temperature equilibrium line and migrates in an opposite direction above the line. On this base, this study conducted the inversion of geothermal temperature fields in typical areas with thin, moderately thick, and thick cap rocks. The results indicate that, at the depth of 3000–6000 m, areas with thin cap rocks (igneous rock zone in the coastal area of Fujian) mainly host moderate- to low-temperature hydrothermal resources; areas with moderately thick cap rocks (Yuezhong Depression) have the geothermal temperature ranging between 100 °C and 200 °C and may develop moderate- to high-temperature hydrothermal resources and hot dry rocks (HDRs), with the former superimposing on the latter; and areas with thick cap rocks (onshore Beibuwan Basin) have a geothermal temperature of 120‒220 °C, and contains mainly moderate- to high-temperature hydrothermal resources and HDRs. Therefore, it is recommended that the evaluation, exploitation, and utilization of deep geothermal resources be carried out according to the burial depth of the temperature equilibrium line and the specific demand for geothermal resources.

Explicitly determined sea ice emissivity and emission temperature over the Arctic for surface‐sensitive microwave channels

AbstractData assimilation of satellite microwave measurements is one of the important keys to improving weather forecasting over the Arctic region. However, the use of surface‐sensitive microwave‐sounding channel measurements for data assimilation or retrieval has been limited, especially during winter, due to the poorly constrained sea ice emissivity. In this study, aiming at more use of those channel measurements in the data assimilation, we propose an explicit method for specifying the surface radiative boundary conditions (namely emissivity and emitting layer temperature of snow and ice). These were explicitly determined with a radiative transfer model for snow and ice and with snow/ice physical parameters (i.e. snow/ice depths and vertical distributions of temperature, density, salinity, and grain size) simulated from the thermodynamically driven snow/ice growth model. We conducted 1D‐Var experiments in order to examine whether this approach can help to use the surface‐sensitive microwave temperature channel measurements over the Arctic sea ice region for data assimilation. Results show that (1) the surface‐sensitive microwave channels can be used in the 1D‐Var retrieval, and (2) the specification of the radiative boundary condition at the surface using the snow/sea ice emission model can significantly improve the atmospheric temperature retrieval, especially in the lower troposphere (500 hPa to surface). The successful retrieval suggests that useful information can be extracted from surface‐sensitive microwave‐sounding channel radiances over sea ice surfaces through the explicit determination of snow/ice emissivity and emitting layer temperature.

Characterization of the vertical evolution of urban nocturnal boundary layer by UAV measurements: Insights into relations to cloud radiative effect

The complex structure of the nocturnal boundary layer (NBL) and its impact on air pollution remain poorly understood. In this study, we present in-situ nocturnal flight measurements onboard an unmanned aerial vehicle (UAV) during the wintertime of 2022 at an urban site in Hefei, China. Besides, co-located measurements of radiation intensity and total amount of cloud were conducted. The vertical distribution of temperature, particle number concentration, and relative humidity were obtained to study the structure of NBL and the key factors driving the evolution of the NBL. A multi-layer inversion boundary layer was observed during haze and fog episodes, which affects the vertical diffusion of particles near the surface and leads to a vertical gradient of particle number concentrations. The particle size distribution demonstrates a drastic vertical variation over different sections of the nocturnal boundary layer: homogeneously mixed in the SBL and the RL layer, sharply reduced in the IL. It is found that the temperature and particle number concentration differences between near-surface and at 500 m are highly related to variations of the radiation intensity and the amount of cloud. The decreased cloud cover enhances the surface cooling, creating a shallow NBL with multiple inversion layers, which reinforces the suppression of vertical diffusions and consequently promotes the accumulation of aerosols within the NBL. This reveals an important mechanism for the impact of NBL evolution modulated by cloud radiative effect on the formation of urban haze.

Development of non-isothermal wall jet integrated zonal model with enhanced accuracy using a multi-flow coefficient calibration method

A zonal model is an intermediate model between a nodal model and a computational fluid dynamics (CFD) model. This type of models has been widely used to predict indoor temperature distribution and airflow patterns by compromising prediction accuracy and computational efficiency. However, the zonal model was mainly used to predict the indoor environment of rooms with natural convection or isothermal air supply in previous literature, neglecting its application in non-isothermal air supply. Since driving flow under non-isothermal air supply is more common in air-conditioned spaces, this paper proposes a jet-integrated zonal model (JIZM) to predict the indoor environment of rooms with non-isothermal wall jet supply, extending the applicability of the zonal modelling technique. Meanwhile, a multi-flow coefficient calibration method was developed to improve the performance of the JIZM using a multi-objective genetic algorithm (MOGA). The applicability of the JIZM was evaluated under typical air supply scenarios. The results demonstrated that the JIZM with the proposed calibration method is able to using limited measurement data with calibrated model parameters to predict the air distributions with reasonable accuracy in a fast way, thus offering a better technique in predicting the vertical temperature distribution in air-conditioned rooms compared with a conventional zonal model.

Mapping Protoplanetary Disk Vertical Structure with CO Isotopologue Line Emission

High-spatial-resolution observations of CO isotopologue line emission in protoplanetary disks at mid-inclinations (≈30°–75°) allow us to characterize the gas structure in detail, including radial and vertical substructures, emission surface heights and their dependencies on source characteristics, and disk temperature profiles. By combining observations of a suite of CO isotopologues, we can map the two-dimensional (r, z) disk structure from the disk upper atmosphere, as traced by CO, to near the midplane, as probed by less abundant isotopologues. Here, we present high-angular-resolution (≲0.″1 to ≈0.″2; ≈15–30 au) observations of CO, 13CO, and C18O in either or both J = 2–1 and J = 3–2 lines in the transition disks around DM Tau, Sz 91, LkCa 15, and HD 34282. We derived line emission surfaces in CO for all disks and in 13CO for the DM Tau and LkCa 15 disks. With these observations, we do not resolve the vertical structure of C18O in any disk, which is instead consistent with C18O emission originating from the midplane. Both the J = 2–1 and J = 3–2 lines show similar heights. Using the derived emission surfaces, we computed radial and vertical gas temperature distributions for each disk, including empirical temperature models for the DM Tau and LkCa 15 disks. After combining our sample with literature sources, we find that 13CO line emitting heights are also tentatively linked with source characteristics, e.g., stellar host mass, gas temperature, disk size, and show steeper trends than seen in CO emission surfaces.

A Novel Near-Surface Wave-Coherent Instantaneous Profiling System for Atmospheric Measurements.

Large knowledge gaps concerning the effect of ocean surface waves on near-surface vertical distributions of temperature and humidity exist due to practical limitations and sensor fidelity challenges of direct measurements. Measurements of temperature and humidity are classically made using rocket- or radiosondes and fixed weather stations and can utilize a tethered profiling system. However, these measurement systems have limitations when obtaining wave-coherent measurements near the sea surface. Consequently, boundary layer similarity models are commonly employed to fill in near-surface measurement gaps despite the documented shortcomings of the models in this region. Thus, this manuscript presents a near-surface wave-coherent measurement platform that measures high-temporal-resolution vertical distributions of temperature and humidity down to ~0.3 m above the instantaneous sea surface. The design of the platform is described along with preliminary observations obtained during a pilot experiment. Ocean surface-wave phase-resolved vertical profiles are also demonstrated from the observations.

Investigating the temperature distribution of non-enclosed atriums with air infiltration in winter

Low-temperature air infiltration and vertical thermal stratification lead to an unsatisfied bottom-occupied environment of the atrium and increase the space heating load in winter. An integrated prediction model was proposed regarding the interaction between thermal stratification and air infiltration. It includes the extended Block model for predicting the vertical temperature distribution in a non-enclosed atrium, the air infiltration model for calculating the buoyancy-driven air infiltration rate, and the Velocity propagating model for describing the airflow pattern within a foyer. By applying this model, we quantitatively analysed the indoor thermal characteristics in different atrium/foyer spatial morphologies. Beyond the indoor net height, the atrium diameter is also a dominant factor affecting the flow rate of air infiltration introduced from the foyer. The lower the section aspect ratio (SAR) and the higher the bottom floor height of the atrium, the warmer the bottom-occupied environment. The foyer's buffer effect mainly promotes occupants' thermal comfort, as lengthening the foyer length increases the heating load of air infiltration into the atrium while helping reduce the horizontal temperature gradient (HTG) when occupants walk across. This study clarified the indoor environmental characteristics in such spaces in winter and explored the interrelated influencing factors. The methodology and conclusions provide insights into practical engineering.

Experimental study on vertical temperature distribution of the two-layer smoke flow in tunnel during construction

For a tunnel fire during construction, the formation of two-layer smoke flow underneath the ceiling accelerates the smoke descent threatening the personnel evacuation. As an important indicator of smoke diffusion, the gas temperature distribution inside tunnel is worthy of analysis in depth. However, the vertical temperature profiles within this kind of two-layer smoke flow inside tunnel have not been experimentally studied. To fill the gap, this paper conducts a series of fire experiments in a 1/20 reduced-scale tunnel model. Multi-variables including fire heat release rate, fire position, lifting-up height of fire source, inclined shaft slope, are considered. The visible flame shape, O2 volume fraction and gas temperature are recorded and measured. The burning of the propane gas persists steadily for 20 mins without the occurrence of self-extinction. The structural evolutions of the smoke layer are predicted by FDS simulation and traced by the laser sheet. The smoke descent process in the main tunnel is divided into three stages. The effects of fire source position, inclined shaft slope, fire heat release rate and lifting-up height on the gas temperature distributions inside tunnel are analyzed. The measured temperature rise and the perpendicular distance from the ceiling are normalized by ΔTmax and LT, respectively. At various longitudinal positions (1.82 ≤ │X-dm│/Hef ≤ 10.68), the normalized temperature distributions follow the same curve, independent of the values of Qm, dm and hm. The self-similarity of the vertical temperature distribution within the two-layer smoke flow are sufficiently proved. It is found that ΔT/ΔTmax with z/LT shows a two-regional variation. Based on over 1,740 temperature data from 36 experimental cases, an experimental equation is proposed to describe the gas temperature rise in the region between the fire source and the closed end. The proposed model shows good agreement with the measured data, which can help the researchers to better assess the smoke layer height inside tunnel.

Flow and heat transfer characteristics of natural convection in hydrothermal reactor with segmented heating

Hydrothermal reactor is very suitable to utilize low-grade heat especially concentrated solar energy. However, the heat flux generated by solar heating is usually non-uniform, which affects the natural convection inside the reactor. In this work, natural convection of near critical water in cylindrical hydrothermal reactor is investigated by numerical simulation. With constant total heat input into the cylinder, the sidewall is divided into two halves with different heat flux density. The effect of heating schemes and physical properties on flow structure, temperature distribution, fluid motion and wall heat transfer are analyzed. It is found that natural convection is dominated by a large vortex with high velocity when heat flux ratio is lower than 1, and is dominated by two independent weak vortices when heat flux ratio is higher than 1. In addition, the former case has more uniform vertical temperature distribution, stronger fluid flow and weaker wall heat transfer, while the latter case is in the opposite. When the temperature approaches the pseudocritical point, the flow structure remains qualitatively unchanged, but wall heat transfer is enhanced and fluid motion is generally weakened. The change of heat flux ratio significantly regulates fluid motion but slightly affects wall heat transfer. Besides, the regulating effect of heat flux ratio is weakened in near critical region.

Numerical Simulation on Smoke Temperature Distribution in a Large Indoor Pedestrian Street Fire

In order to study the characteristics of fire smoke spread and temperature distribution of a large indoor pedestrian street under different heat release rates and smoke exhaust modes, this paper focuses on the analysis of fire smoke spread, visibility, smoke temperature distribution and variation curves in an atrium. This paper uses a numerical simulation method to conduct research. PyroSim fire simulation software is used to calculate this project, which is based on a full-scale experimental design scheme. The numerical simulation results show that under the conditions of higher heat release rate, the smoke spread rate is greater than that under the conditions of lower heat release rate. Furthermore, the average temperature of smoke in the atrium is also greater, up to about 400 °C. The conditions of a higher heat release rate also show the characteristics of faster generation, faster spread and a larger volume of smoke. When the smoke exhaust system is turned on, the thickness of the smoke layer and the smoke temperature decrease. There then comes a situation where the stabile section of the fire ends in advance. The simulation results of vertical temperature distribution in an atrium can fit the modified McCaffrey plume model in any case. Under all cases, the smoke temperature reaches the maximum directly above the fire source. The horizontal dimensionless smoke temperature rises under the atrium roof, and decreases exponentially with the dimensionless distance from the fire source. The greater the heat release rate of fire source is, the smaller the attenuation coefficient is, with a more than 50% change. When the smoke exhaust system is turned on, the smoke flow accelerates and the smoke is cooled rapidly. Thus, the attenuation coefficient increases. Additionally, the effect of mechanical smoke exhaust is better than natural smoke exhaust, because mechanical smoke exhaust makes air flow and heat exchange more intense. The variation amplitudes of the attenuation coefficient under natural smoke exhaust and mechanical smoke exhaust are 13% and 22%, respectively.

Indoor thermal and humid stratification and statistical distribution in ice arenas

On-site measurements in four different ice arenas were carried out under multiple working conditions to study the indoor vertical temperature and humidity distributions. It's indicated that air distribution system has a great impact on the vertical profiles and brings about different inflection points. In order to achieve the same moisture level of around 3 g/kg in the occupied ice field, supply air humidity of jet ventilation should be reduced to 0.4 g/kg while that of ground displacement ventilation could be as high as 3.0 g/kg. The vertical profiles under different air distribution systems are simplified by calculating the air parameter discrepancy brought by dehumidification air supply in the present research. The discrepancy profile was characterized by Gaussian function with three shape coefficients. Based on the measured results, the variation of shape coefficients with different influencing factors was investigated. It is demonstrated that the extreme point coefficient can be estimated by the height of the air outlet, while the scaling range coefficient varied linearly with the relative air supply parameters. This study accumulated considerable measured data to comprehensively understand the current situation of temperature and humidity stratification in ice arenas. The proposed statistical variation distribution contributes to the precise environmental control and the efficient design of ice arenas.

A zonal model to predict the vertical temperature distribution of displacement ventilation system during human movement

The existence of stable and continuous temperature stratification in displacement ventilation systems could bring the advantages of energy savings and high ventilation efficiency. The disturbance effect of human body movement can weaken such kind of temperature stratification. Comprehending the variation of the temperature stratification during human body movement helps to formulate countermeasures in the design phase. Until now, empirical studies of the zonal model predicting the vertical temperature distribution with body movement in displacement ventilation are very scarce. This paper proposed a novel zonal model to predict indoor vertical temperature distribution with body movement disturbance based on the theory of piecewise linear distribution of temperatures. The vertical entrained air volume (me) caused by the movement is one of the important variables in the model. In this study, a series of simulations and experiments in the literature were applied to get the value of me, which distinguishes the difference between the traditional static models and present dynamic model. We found that the value of me is closely related to the dimensionless number Ri. me between two adjacent zones is almost 0 when Ri < 0.256. The relationship between Ri and me can be fitted by the equation (me = −0.197 × ln(Ri) - 0.269, R2 = 0.879) when Ri > 0.256. The predicted temperature stratification by present zonal model exhibits a great fit with experimental data. Our findings contribute to the prediction of the indoor vertical temperature distribution in the quasi-steady state with the disturbance from body movement, which reflects most of the real situations in DV.

Performance of Air Curtain located at the top of entrance opening - Difference among indoor/outdoor/double-sided installation

Large openings of buildings are often used open, which results in heat loss due to intrusion of outdoor airflow and leakage of indoor air through the opening. To reduce air-conditioning load and to improve thermal environment in such a building, installing an air curtain at the opening can be a beneficial technique. The airflow blowing out of the device suppresses the heat exchange by buoyancy-induced convection through the opening. The impact of temperature difference, blowing speed, and installation position of the air curtain on temperature distribution and invasion flow rate of outdoor air was investigated by full-scale experiment under heating operation. Thermocouples were installed to measure vertical temperature distribution. Outdoor air infiltration was measured using tracer-gas method. The amount of outdoor air infiltration was evaluated based on both temperature difference and CO2 concentration. Comparing the results, it was found that the indoor air curtain was able to suppress the airflow through the opening. The indoor-outdoor temperature difference and CO2 concentration were highest when the air velocity was 4 m/s, suggesting that there is an optimal air velocity at which the air curtain can effectively suppress the airflow through the opening.

The Earth Summit Mission-2022: Successful ozone soundings contribute to source identification in the north Mt. Qomolangma region

As part of “The Earth Summit Mission-2022” during the second Tibetan Plateau Scientific Expedition and Research (STEP) in April and May 2022, we conducted the ozone sounding experiment (an ozonesonde mated to a radiosonde) at Mt. Qomolangma Base Camp (MQBC; 86.85°E, 28.14°N; 5200 m), a location at an extremely high altitude. A total of ten sounding profiles were obtained between April 30 and May 06, 2022, of which seven profiles were above 35 km in altitude, with a maximum detection altitude up to 39.0 km. This study presents the temporal variation and vertical distributions of atmospheric temperature, humidity, and ozone during the MQBC campaign. The averaged ozone concentration was high (68.3 ppbv) at the surface and then increased smoothly until peaking (∼110 ppbv) in the middle troposphere (approximately 10 km), and afterward, the ozone concentration increased rapidly from the upper troposphere to a maximum of ∼10 ppmv at ∼30 km. The enhanced ozone concentration in the middle troposphere was associated with the blocking high pressure, and transport from the southern flank of the Himalayas occurred during the campaign period. The average total ozone column was 291.9±21.4 DU for the seven profiles exceeding 35 km in altitude. The ozonesonde measurements were also compared with the vertical ozone profiles retrieved from the space-borne ozone products from the Microwave Limb Sounder (MLS) onboard the Aura satellite and the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite.

Striking impacts of biomass burning on PM2.5 concentrations in Northeast China through the emission inventory improvement

Biomass burning exerts substantial influences on air quality and climate, which in turn to further aggravate air quality. The biomass burning emissions in particular of the agricultural burning may suffer large uncertainties which limits the understanding of their impact on air quality. Based on an improved emission inventory of the Visible Infrared Imaging Radiometer Suite (VIIRS) relative to commonly used Global Fire Emissions Database (GFED), we thoroughly evaluate the impact of biomass burning on air quality and climate during the episodes of November 2017 in Northeast China which is rich in agriculture burning. The results first indicate substantial underestimates in simulated PM2.5 concentrations without the inclusion of biomass burning emission inventory, based on a regional air quality model Weather Research and Forecasting model and Community Multiscale Air Quality model (WRF-CMAQ). The addition of biomass burning emissions from GFED then reduces the bias to a certain extent, which is further reduced by replacing the agricultural fires data in GFED with VIIRS. Numerical sensitivity experiments show that based on the improved emission inventory, the contribution of biomass burning emissions to PM2.5 concentrations in Northeast China reaches 32%, contrasting to 15% based on GFED, during the episode from November 1 to 7, 2017. Aerosol direct radiative effects from biomass burning are finally elucidated, which not only reduce downward surface shortwave radiation and planetary boundary layer height, but also affect the vertical distribution of air temperature, wind speed and relative humidity, favorable to the accumulation of PM2.5. During November 1–7, 2017, the mean daily PM2.5 enhancement due to aerosol radiative effects from VIIRS_G is 16 μg m−3, a few times higher than that of 2.8 μg m−3 from GFED. The study stresses the critical role of biomass burning, particularly of small fires easily missed in the traditional low-resolution satellite products, on air quality.

МИКРОВОЛНОВЫЙ МОНИТОРИНГ СОДЕРЖАНИЯ ОЗОНА В СРЕДНЕЙ АТМОСФЕРЕ В АПАТИТАХ И ПЕТЕРГОФЕ ЗИМОЙ 2021/22 Г

Results of synchronous microwave microwave monitoring of middle atmosphere ozone in Apatity (67° N, 33° E) and Petergof (60° N, 30° E) during the winter and spring of 2021/22 are presented. Identical mobile microwave spectrometers with an observation frequency of 110.8 GHz were used for ozone measurements. These instruments have identical techniques for observations and estimation of ozone profiles at the altitudes of 22-60 km. The main attention was given to changes in ozone (daily variations) in the mesosphere during the polar night and dynamic disturbances. For interpretation of the mesospheric ozone (60 km) behavior, MLS/Aura satellite data on the vertical distribution of temperature in the middle atmosphere over Apatity were used.

Analysis of the Correctness of Retrieving the Vertical Atmospheric Temperature Distribution from Lidar Signals of Molecular Scattering at the Main Lidar of the Siberian Lidar Station

Analysis of the Correctness of Retrieving the Vertical Atmospheric Temperature Distribution from Lidar Signals of Molecular Scattering at the Main Lidar of the Siberian Lidar Station

Impacts of Subgrid Temperature Distribution Along Elevation Bands in Snowpack Modeling: Insights From a Suite of Andean Catchments

AbstractThe implementation of elevation bands is a popular strategy to account for topographic heterogeneities in snowpack modeling. Here, we characterize the implications of subgrid temperature distribution along elevation bands through numerical experiments in nine mountainous basins of the Andes Cordillera, central Chile. Specifically, we analyze outputs from the Variable Infiltration Capacity model with six different setups: no elevation bands (i.e., flat grid cells; benchmark model) and elevation bands with vertical discretizations of 1,000, 750, 500, 200, and 100 m. The analyses are conducted in a wet period (April/1982–March/1987), dry period (April/2010–March/2015) and a climatological period (April/1982–March/2015). The results show that adding elevation bands yields little variations in simulated monthly or daily streamflow; however, there are important effects on the partitioning of precipitation between snowfall and rainfall, snowmelt, sublimation, and the spatial variability in 1 September snow water equivalent (SWE), suggesting a form of model‐structure equifinality. Vertical temperature distribution generally yields less basin‐averaged snowmelt and more (less) catchment‐scale sublimation across water‐limited (energy‐limited) basins. Further, the implications of subgrid temperature distribution vary with the analysis period: fluxes are more affected during the wet period, while variations in 1 September SWE are more noticeable during the dry period. In general, the effects of topographic temperature distribution are reduced with increasing vertical discretization and can differ among catchments. Finally, the grid cells that yield the largest sensitivities to vertical discretization have relatively more humid conditions, large intra‐annual variations in the water/energy budget, lower mean altitude, elevation ranges &gt;1,000 m, and steep slopes (&gt;15°).

Influence of sluice gate operation on salinity stratification and hypoxia development in a brackish estuary dam

The water quality in the artificially made Saemangeum Lake, located in an estuary, is poor; here, we explored the causes of hypoxia development. The field survey for spatial and vertical distribution of salinity and dissolved oxygen (DO) was conducted three times at 6 sites in the Mankyung Basin during the period when the salinity stratification was developed, and continuous observations for temporal and vertical distribution of water temperature, salinity, DO, and flow direction and velocity were performed at a site in the downstream of the lake for about 40 days. Hypoxia was detected at the bottom layer of the survey sites, which were directly affected by seawater inflow. At these sites, the salinity stratification, one of the causes of hypoxia, was clearly observed. The Richardson number and potential energy anomaly (PEA) calculated from the continuous observation measurement results indicated that the water column was very stable and strongly stratified during the investigation period. The vertical salinity structure due to the stabilization of the water column and the bottom temperature above 20 °C caused hypoxia of the bottom layer when insufficient seawater inflow or non-operation of sluice gate. Due to the horizontal salinity gradient by the seawater flowing in through the sluice gate operation, a baroclinic force acts from the sluice gate to the upstream direction at the bottom layer. In contrast, a barotropic force acts from the upstream inlet toward the sluice gate at the surface layer. The directional flow of each water layer changes in opposite directions to form a repeated reversing current which prevents the supply of DO through the inflow of fresh seawater. Therefore, to improve the water quality of Saemangeum Lake, it is necessary to control the short/ long-term reversing current that obstructs DO supplementation from seawater flowing through the sluice gate.

Transitional fluctuations of thermal boundary layer in horizontal convection

Fluctuations of thermal boundary layer (BL) in horizontal convection (HC) strongly affect the global heat transport through the HC sample and predominately determine the flow state. Here, we present a systematic experimental study of vertical profiles of mean temperature and temperature variance in the thermal BL of a HC flow that are heated and cooled from the bottom. Three rectangular HC samples filled with de-ionised water are used in the experiment. They have the same aspect ratios Γ≡L:W:H=10:1:1 but with different L (L, W and H are, respectively the length, width and height of the sample). The Rayleigh number Ra varies in the range 2×1010≲Ra≲1.3×1013, and the Prandtl number Pr is in the range 3.9≲Pr≲6.5. For all Ra in the studied range, we find that the cold thermal BL above the cooling plate remains laminar and the vertical profiles of measured mean temperature follow a universal scaling form predicted for laminar BL by Yan, Shishkina and He (J. Fluid Mech., vol. 915, 2021, R5). Above the heated plate, on the contrary, temperature fluctuations in the hot thermal BL undergo several transitional changes as Ra increases. For Ra≲6×1010, the thermal BL has a stable double-layer structure agreeing with the predicted scaling form for laminar BL and the temperature fluctuations are nearly zero. In the range 6×1010≲Ra≲1.1×1011, temperature fluctuations inside the BL have intermittent decayed oscillations, which results in deviations of the measured mean temperature profiles from the predicted scaling form. For 1.1×1011≲Ra≲1012, the temperature oscillations inside the BL become continuous and stable. Correspondingly, an intermediate layer emerges in the thermal BL, where the mean temperature profile has a sinusoidal variation on the distance z above the heated plate. For 1012≲Ra≲1.3×1013, the temperature fluctuations in the intermediate layer become irregular and intermittent, indicating a growing mixing intensity as Ra increases. When Ra exceeds 7×1012, the vertical distribution of mean temperature in the intermediate layer is uniform because of strong mixing. The temperature-variance vertical profiles follow a power-law function σT2(z)∼z−γ with the exponent γ=0.66, which agrees with the Priestley prediction for thermal convection over a heated horizontal surface. Our results thus reveal a series of transitional changes of temperature fluctuations, from laminar to chaotic states, in the hot thermal BL. They predominately account for the enhancement of global heat transport in the HC flow observed by Reiter and Shishkina (J. Fluid Mech., vol. 892, 2020, R1).