Thermal Internal Boundary Layer Research Articles

CALOTRITON: a convective boundary layer height estimation algorithm from ultra-high-frequency (UHF) wind profiler data

Abstract. Long time series of observations of atmospheric dynamics and composition are collected at the French Pyrenean Platform for Observation of the Atmosphere (P2OA). Planetary boundary layer depth is a key variable of the climate system, but it remains difficult to estimate and analyse statistically. In order to obtain reliable estimates of the convective boundary layer height (Zi) and to allow long-term series analyses, a new restitution algorithm, named CALOTRITON, has been developed. It is based on the observations of an ultra-high-frequency (UHF) radar wind profiler (RWP) from P2OA with the help of other instruments for evaluation. Estimates of Zi are based on the principle that the top of the convective boundary layer is associated with both a marked inversion and a decrease in turbulence. Those two criteria are respectively manifested by larger RWP reflectivity and smaller vertical-velocity Doppler spectral width. With this in mind, we introduce a new UHF-deduced dimensionless parameter which weighs the air refractive index structure coefficient with the inverse of vertical velocity standard deviation to the power of x. We then search for the most appropriate local maxima of this parameter for Zi estimates with defined criteria and constraints such as temporal continuity. Given that Zi should correspond to fair-weather cloud base height, we use ceilometer data to optimize our choice of the power x and find that x=3 provides the best comparisons. The estimates of Zi by CALOTRITON are evaluated using different Zi estimates deduced from radiosounding according to different definitions. The comparison shows excellent results with a regression coefficient of up to 0.96 and a root-mean-square error of 71 m, which is close to the vertical resolution of the UHF RWP of 75 m, when conditions are optimal. In more complex situations, that is when the atmospheric vertical structure is itself particularly ambiguous, secondary retrievals allow us to identify potential thermal internal boundary layers or residual layers and help to qualify the Zi estimations. Frequent estimate errors are observed nevertheless; for example, when Zi is below the UHF RWP first reliable gate or when the boundary layer begins its transition to a stable nocturnal boundary layer.

Influence of coastal planetary boundary layer on PM2.5 with unmanned aerial vehicle observation

Field measurement with an Unmanned Aerial Vehicle (UAV) platform was conducted at coastal area of Bohai in winter and summer. 135 flights were obtained and characteristics of boundary layer structure and its influence on PM2.5 were studied. The averaged profiles showed that meteorological parameters and PM2.5 changed significantly below 300 m. Weak suspension inversion existed between 30–300 m in the morning, ground inversion existed below 70 m at night in winter, temperature inversion was only observed below 200 m at night in summer. PM2.5 peaked at 300 m, 100 m and 200 m during morning, daytime and night period, respectively. During the middle and later stages of winter pollution episode, southwest and southeast wind, northwest and northeast wind formed obvious convergence at coastal area in turn, resulted in maximum PM2.5 concentration of 160 μg m−3 and 218 μg m−3 at night and morning, respectively. Isothermal layer was observed below 200 m, the maximum wind speed and high PM2.5 concentration all appeared at 200 m at night. Special stratification between 50–100 m was observed at 11:00, where wind speed decreased and high PM2.5 concentration maintained. During the summer pollution episode, the sea breeze formed at 13:00 and ended at 22:00, sea breeze extend westward 50 km to the central urban area at 16:00. Thermal internal boundary layer formed below 100 m at 15:00 and extended upward to 150 m at 17:00, weak temperature inversion layer was observed between 50 m ∼ 100 m, the maximum wind speed was 8 m s−1, the wind direction changed from southeast to south, and maximum PM2.5 concentrations of 160 μg m−3 were observed at 100 m.

Thermodynamic structure of the atmospheric boundary layer over a coastal station in India for contrasting sky conditions during different seasons

The thermodynamic structure of the atmospheric boundary layer for contrasting sky conditions over Chennai, a coastal station in the Indian subcontinent, is investigated through conserved variable analysis of equivalent potential temperature and specific humidity. Simultaneous radiosonde and micropulse lidar measurements undertaken in 2018 constitute the primary database in this investigation. One of the most prominent features of this analysis is a consistent occurrence of a double mixed layer structure during the clear-sky and cloudy conditions throughout the year. The occurrence frequency of the double mixing lines is higher during the pre-monsoon season compared with the winter and northeast (NE) monsoon seasons. The advection mainly dominates the formation of double mixing lines during the winter and pre-monsoon seasons. In contrast, convection and advection dominate during the southwest (SW) and NE monsoon seasons. The frequent double mixing lines over Chennai occur mainly from the restratification of the convective boundary layer (CBL) due to the sea-breeze onset and the cloud layer. Occasionally, a triple mixing line structure is also observed during the fair-weather boundary layer (FWBL) of the pre-monsoon and SW monsoon seasons. Among the 355 total observations collected during 2018, the first, second, and third mixing lines occurred 100%, 70%, and 14%, respectively. The thermal internal boundary layer (TIBL), CBL, and FWBL occur ∼50%, ∼97%, and ∼ 30%, respectively. The first mixing line is represented by both the TIBL and CBL, and CBL and FWBL represent the second mixing line, whereas the third mixing line is represented solely by the FWBL. The first and second mixing line shows strong seasonal variations with lower altitudes during the pre-monsoon season and higher altitudes in the SW monsoon season, almost in the same phase as the CBL variation but in the opposite phase of the TIBL variations. The CBL height attains a minimum during the winter season and maximum during the SW monsoon season, while TIBL becomes minimum during the pre-monsoon and SW monsoon seasons and maximum during winter and NE monsoon seasons.

A Study of Urban Haze and Its Association with Cold Surge and Sea Breeze for Greater Bangkok.

This study deals with haze characteristics under the influence of the cold surge and sea breeze for Greater Bangkok (GBK) in 2017-2022, including haze intensity and duration, meteorological classification for haze, and the potential effects of secondary aerosols and biomass burning. A total of 38 haze episodes and 159 haze days were identified. The episode duration varies from a single day to up to 14 days, suggesting different pathways of its formation and evolution. Short-duration episodes of 1-2 days are the most frequent with 18 episodes, and the frequency of haze episodes decreases as the haze duration increases. The increase in complexity in the formation of relatively longer episodes is suggested by a relatively higher coefficient of variation for PM2.5. Four meteorology-based types of haze episodes were classified. Type I is caused by the arrival of the cold surge in GBK, which leads to the development of stagnant conditions favorable for haze formation. Type II is induced by sea breeze, which leads to the accumulation of air pollutants due to its local recirculation and development of the thermal internal boundary layer. Type III consists of the haze episodes caused by the synergetic effect of the cold surge and sea breeze while Type IV consists of short haze episodes that are not affected by either the cold surge or sea breeze. Type II is the most frequent (15 episodes), while Type III is the most persistent and most polluted haze type. The spread of haze or region of relatively higher aerosol optical depth outside GBK in Type III is potentially due to advection and dispersion, while that in Type IV is due to short 1-day episodes potentially affected by biomass burning. Due to cold surge, the coolest and driest weather condition is found under Type I, while Type II has the most humid condition and highest recirculation factor due to the highest average sea breeze duration and penetration. The precursor ratio method suggests the potential effect of secondary aerosols on 34% of the total haze episodes. Additionally, biomass burning is found to potentially affect half of the total episodes as suggested by the examination of back trajectories and fire hotspots. Based on these results, some policy implications and future work are also suggested.

Abrupt High PM Concentration in an Urban Calm Cavity Generated by Internal Gravity Waves and a Shallow Coastal Atmospheric Boundary Layer with the Influence of the Yellow Dust from China

Using GRIMM-1107 aerosol sampler, GOES-9 DCD satellite images, HYSPLIT model of backward trajectory and 3D-meteorological WRF-3.6 model, high particulate matter concentrations were investigated at Gangneung city in the Korean east coast which consists of Mt. Taeglyung in its west and the East Sea in its east on 00:00LST March 26~00:00 LST 4 April 2004. During a Yellow Dust period, the maximum PM10 (PM2.5 and PM1) concentration at the city was about 3.3 (1.1 and 1.01) times higher than one in the non-dust period. After the transported dust from the Gobi Desert and Nei-Mongo by strong northwesterly wind passed over Mt. Taegulyang and moved down toward the city. Then the dust was trapped inside a calm cavity generated by the confront of internal gravity waves (IGW) over the city and the eastward movement of the trapped dust is prohibited by the easterly onshore wind from the East Sea, and the trapped dust further combined with particulate and gaseous emitted from the road vehicles and heating boilers of the city at 09:00LST, March 30 (beginning time of office hours), causing high PM concentrations. On mid-day, as the combined dust due to daytime sufficient thermal convection rises up to the top of the thermal internal boundary layer (TIBL) of a 300 m depth from the coast to the top of the mountain, the ground-based PM concentrations in the city are much lower at 15:00LST due to the higher thickness of the TIBL than at 09:00LST. At night, particulates emitted from many road vehicles after the end of office hours and residential heating boilers could combine with both dust transported from the Nei-Mongo and falling dust uplifted from the ground surface of the city during the day, and they were trapped inside a calm cavity by the IGV under much shallower stable nocturnal surface inversion layer than the TIBL, causing more dust to be accumulated near the surface and showing the maximum PM concentrations at 20:00 LST.

Interaction between heterogeneous thermal stratification and wakes of wind turbine arrays

AbstractThe thermal heterogeneity between the land and sea might affect the wind patterns within wind farms (WF) located near seashores. This condition was modeled with a large‐eddy simulation of a numerical weather prediction model (Weather Research and Forecasting) that included the wind turbine actuator disk model (ADM). The assumed condition was that the downstream surface temperature was relatively higher (unstably stratified condition) than the neutrally stratified upstream wind. Under this condition, a thermal internal boundary layer (TIBL) was developed from an area where a step‐changed surface temperature was implemented. The combined effect of the wake deficit due to the WF and velocity recovery as a result of enhanced mixing under unstable stratification showed significant modulation of the wind speed at the hub height when local atmospheric stability affected the wind turbine (WT). We show that TIBL height depends on the variables to be evaluated as the threshold. A precise prediction of the TIBL height is beneficial for better estimation of power generation. A prediction model was proposed as an extension of the internal boundary layer (IBL) model for neutral stratification, and the results tracked TIBL development reasonably well. The effects of WFs on surface properties (e.g., friction velocity, heat flux, and Obukhov length) and the tendency of IBL growth were minor. A single WT wake was also assessed under several TIBL developmental stages (i.e., location) and thermal stratification conditions. The standard deviation of the wake deficit increased vertically during the development stage of the TIBL. In contrast, the coefficients in the horizontal and vertical directions were comparable when the WT was deep inside the TIBL.

Quantitative attribution of wintertime haze in coastal east China to local emission and regional intrusion under a stagnant internal boundary layer

A thermal internal boundary layer (TIBL) can cause the fumigation effect and reduce the dispersion capacity of air pollutants. The paper's goal is to quantify the interactions of pollutants between cities subjected to enhanced fumigation effects. To that end, an observational campaign was launched at the Shandong Peninsula of China in the winter of 2020. The campaign focused on monitoring vertical profiles of aerosols, wind field, temperature, and humidity for the boundary-layer dynamic-thermodynamic structure. The Nested Air Quality Prediction Modeling System (NAQPMS) coupled with online source-tagging model was also employed to delve into the source-receptor relationships of the cities involved in the fumigation effect. The results indicated that the presence of a TIBL structure triggered the prolonged and widespread pollution episodes. This resulted in a temperature inversion (250–1000 m in altitude) near the convergence line on surface and rendered the atmospheric boundary layer more stably stratified, i.e., a situation that was unfavorable for the diffusion of pollutants against a persistently sinking flow (with a mean velocity of roughly 0.2 m s−1) for a prolonged time. Meanwhile, TIBLs prevented the fumigation areas from interacting with adjacent inland areas (Shandong Peninsula's local contribution accounting for >90% of the PM2.5) and facilitated inter-city transport of pollutants inside this peninsula (the transport contribution between cities reached up to 55–75%). Such distinct behaviors as observed over this coastal area render air pollution control more of an elusive issue, as is referenced to several inland cities.

Investigation of Coastal Atmospheric Boundary Layer and Particle by Unmanned Aerial Vehicle under Different Land-sea Temperature

During summer daytime, the surface PM2.5 concentration tends to rise briefly in the west coast of Bohai Sea, which might be related to the thermal internal boundary layer (TIBL) formed by the land-sea thermal interaction. In this study, we investigated the relationship between land-sea air temperature and pollutant concentration in summer over a coastal region of northern China using an unmanned aerial vehicle (UAV)-measurement platform to obtain vertical meteorological data and pollutant concentration. In midday, when only considered onshore winds, PM2.5 concentration was significantly higher than that in not considered onshore winds, meanwhile the high positive correlation between land-sea temperature difference and particle concentration was identified. It can be seen from the UAV observed profiles that the TIBLs were formed at the bottom of the atmospheric layer in the daytime with height values in the range of 44–97 m, resulting from the impact of onshore winds caused by land-sea thermal difference. This land-sea thermal difference influences the atmospheric boundary layer (ABL) and TIBL structures, also the PM2.5 concentration diffusion. We found that larger land-sea temperature difference could induce lower coastal ABL height and larger potential temperature vertical gradients below TIBL, even delaying the stable layer establishment. When TIBL and ABL heights increased, as a result, the height of the maximum PM2.5 concentration also increased. In addition, TIBL could lead to an increase in surface PM2.5 concentration lasting for 2 hours during the daytime. Our results establish the relationship between land-sea temperature difference and particle diffusion, and have an important role in coastal air quality forecast.

Large eddy simulation of boundary-layer turbulence over the heterogeneous surface in the source region of the Yellow River

Abstract. Lake breezes are proved by downdrafts and the divergence flows of zonal wind in the source region of the Yellow River (SRYR) in the daytime based on ERA-Interim reanalysis data. In order to depict the effect of the circulations induced by surface anomaly heating (patches) on the boundary-layer turbulence, the UK Met Office Large Eddy Model was used to produce a set of 1D strip-like surface heat flux distributions based on observations, which were obtained by a field campaign in the Ngoring Lake basin in the summer of 2012. The simulations show that for the cases without background wind, patch-induced circulations (SCs) promote the growth of convective boundary layer (CBL), enhance the turbulent kinetic energy (TKE), and then modify the spatial distribution of TKE. Based on phase-averaged analysis, which separates the attribution from the SCs and the background turbulence, the SCs contribute no more than 10 % to the vertical turbulent intensity, but their contributions to the heat flux can be up to 80 %. The thermal internal boundary layer (TIBL) reduces the wind speed and forms the stable stratification, which produces the obvious change of turbulent momentum flux and heat flux over the heterogeneous surfaces. The increased downdrafts, which mainly occur over the lake patches, carry more warm, dry air down from the free atmosphere. The background wind inhibits the SCs and the development of the CBL; it also weakens the patch-induced turbulent intensity, heat flux, and convective intensity.

Vertical Profiles of Turbulence Parameters in the Thermal Internal Boundary Layer

The correct simulation of pollutant dispersion in coastal regions demands understanding the turbulence structure of the thermal internal boundary layer (TIBL), which typically occurs in daytime when maritime air is advected over the continent. Such a structure is investigated using 10 levels of turbulence observations made at a 140-m micrometeorological mast installed at 3500 m from the shoreline in south-eastern Brazil, with TIBL dimensionless vertical profiles of the turbulence parameters commonly used in Lagrangian and Eulerian dispersion models determined. To accomplish that, the TIBL height $$z_i$$ is estimated using a vertical-flux-convergence approach, developed here. The values experimentally obtained for $$z_i$$ agree with those predicted by a widely-used model for TIBL growth. In general, the normalized turbulent profiles evaluated for the TIBL differ from those previously obtained in the convective boundary layer (CBL). The horizontal eddy diffusivities evaluated for the TIBL are larger than those typically observed in the CBL, while the vertical ones are similar for both boundary layers. Finally, it is shown that CBL similarity relationships can be used to describe turbulent parameters as long as the proper empirical constants are used.

Seasonal characteristics of sea breeze and thermal internal boundary layer over Indian east coast region

Characteristics of the sea breeze and thermal internal boundary layer (TIBL) during different seasons are studied using mini micropulse lidar (MPL) at Kattankulathur (12.83° N, 80.04° E, 45 m) and radiosonde and surface meteorological observations at Meenambakkam (13.0° N, 80.06° E, 16 m) during the period January 2016–June 2018 over Chennai, in the Indian east coast region. Study of the sea breeze is important because it is one of the main factors responsible for most of the convective rainfall during summer monsoon season over Chennai. The sea breeze onset occurs from morning ~ 07:00 IST to late afternoon during winter and pre-monsoon seasons whereas it occurs from mid-morning ~ 09:00 IST to late evening ~ 19:00 IST during the summer monsoon and post-monsoon seasons. The cessation of the sea breeze lies between the late evening ~ 18:00 IST and early morning ~ 04:00 IST on the next day. Sea breeze prevails for the longest (shortest) duration during pre-monsoon (summer monsoon) season. The temperature, moisture, refractivity, and aerosol profiles show double mixed layer characteristics indicating TIBL as the first layer and convective boundary layer (CBL) or residual layer (RL) as the second layer. TIBL occurs throughout the year with maximum occurrence during pre-monsoon season. It varies from 0.2 to 0.8 km (0.4–1.0 km) with peak at 0.4 km (0.6 km) over Meenambakkam (Kattankulathur). TIBL height is found to be at altitude ~ 0.54 ± 0.16 km and 0.62 ± 0.13 km at Meenambakkam and Kattankulathur, respectively. TIBL shows a weak seasonal variation with maximum height during winter and minimum height during summer in contrast to the seasonal variation of the CBL.

Investigating microclimate effects in an oasis-desert interaction zone

To investigate oasis-desert microclimate effects, we performed a series of numerical simulations in an idealized oasis-desert system based on an improved computational fluid dynamics (CFD) model for simulating atmospheric boundary layer flows, air temperature and humidity. Numerical simulations were designed based on the hydrometeorological observations obtained during the HiWATER-MUSOEXE (Heihe Watershed Allied Telemetry Experimental Research, Multi-Scale Observation Experiment on Evapotranspiration over heterogeneous land surfaces) campaign. The results are summarized as follows: (1) Oasis-desert interactions are significantly affected by background wind conditions. We observed the oasis-desert local circulation under calm background wind conditions and the oasis thermal internal boundary layer under low wind speed conditions induced by hydrothermal contracts. These interactions will disappear when the background wind speed is sufficiently high, and there is only an oasis dynamic internal boundary layer caused by the aerodynamic roughness length contrast. (2) Oasis-desert interactions lead to a series of microclimate effects, including the oasis cold-wet island effect, air humidity inversion effect within the surrounding desert and oasis wind shield effect, which are important for the stability and sustainability of the oases-desert ecosystem. (3) The hydrothermal conditions due to the difference between the oasis and desert, the vegetation fraction and distribution patterns impact the oasis-desert microclimate effects. The intensity of oasis-desert interactions increases with the land surface temperature (LST) difference in the oasis-desert. The oasis-desert interactions are gradually strengthened with the increase of the vegetation fraction within the oasis. Integrated ecological and economic benefits of the oasis, the oasis vegetation pattern, which includes the croplands and shelterbelts staggered within the oasis and the shelterbelts surrounding the outside, is beneficial to limiting the loss of water vapor and preventing sandstorms from the oasis. The findings of the current study improve the fundamental understanding of the microclimate and provide implications for maintaining the sustainability of oasis-desert ecosystems.

Assessment of Stability-Based Characterizations of North Carolina’s Offshore Wind Resource Using a Nested Boundary Layer Method

SODAR wind profiler measurements from a land-based site within 1000 m of the coastal ocean are compared with predicted wind profiles based on upwind conditions and a stability-based model. The thermally dynamic influence of the Gulf Stream near the site of measurement, Cape Hatteras, has been found to complicate wind speed estimation at height through its impacts on atmospheric stability and the resulting changes to wind shear. The COARE 3.0 algorithm was used to calculate a Monin-Obukhov Similarity Theory (MOST) scaling parameter, quantifying the degree of stability of the marine environment as well as the friction velocity and roughness length. Knowledge of upwind conditions aided in estimating the height of nested roughness and thermal internal boundary layers (IBLs) to identify a range of the measured wind profile corresponding to winds over the water. This work suggests that MOST through COARE 3.0 provides a good estimate of wind-at-height behaviour, with some exceptions. The layers identified in the IBL approximations indicate that (1) effects of stability can improve wind speed estimation when upwind conditions are well defined, but (2) unexpectedly high levels of wind shear make this method unreliable within unstable layers above stable layers. These results impact the estimate of the available wind resource, as well as other properties applicable to wind energy engineering, including shear and turbulence characteristics.

Impact of Sea Breeze Dynamics on Atmospheric Pollutants and Their Toxicity in Industrial and Urban Coastal Environments

Sea breeze (SB) phenomena may strongly influence air quality and lead to important effects on human health. In order to study the impact of SB dynamics on the properties and toxicity of aerosols, an atmospheric mobile unit was deployed during a field campaign performed in an urbanized and industrialized coastal area in Northern France. This unit combines aerosol samplers, two scanning lidars (Doppler and elastic) and an air-liquid interface (ALI, Vitrocell®) in vitro cell exposure device. Our study highlights that after the passage of an SB front, the top of the atmospheric boundary layer collapses as the thermal internal boundary layer (TIBL) develops, which leads to high aerosol extinction coefficient values (>0.4 km−1) and an increase of PM2.5 and NOx concentrations in the SB current. The number-size distribution of particles indicates a high proportion of fine particles (with diameter below 500 nm), while the volume-size distribution shows a major mode of coarse particles centered on 2–3 µm. Individual particle analyses performed by cryo-transmission scanning electron microscopy (cryo-TSEM)-EDX highlights that submicronic particles contained a high fraction of secondary compounds, which may result from nucleation and/or condensation of condensable species (vapors or gaseous species after photo-oxidation). Secondary aerosol (SA) formation can be enhanced in some areas, by the interaction between the SB flow and the upper continental air mass, particularly due to the effect of both turbulence and temperature/humidity gradients between these two contrasting air masses. Potential areas of SA formation are located near the ground, during the SB front passage and in the vicinity of the SB current top. During the sea breeze event, an increase in the oxidative stress and inflammation processes in exposed lung cells, compared to the unexposed cells, can also be seen. In some instances, short singularity periods are observed during SB, corresponding to a double flow structure. It consists of two adjacent SB currents that induce an important increase of the TIBL top, improving the pollutants dispersion. This is associated with a substantial decrease of aerosol mass concentrations.

Simulation of Sea Breeze Events in Gulf of Jakarta under Different Synoptic Condition: An Application of WRF Model

The behaviour of the sea breeze developing along the Gulf of Jakarta during days with different synoptic condition is examined using observational data and simulations from the Weather Research and Forecasting (WRF). An objective and systematic selection method is used to detect sea breeze days from observational data. The simulation used three nested grids at 27, 9 and 3 km resolution covering Gulf of Jakarta and its joining region for sea breeze event at November 22th, 2016 which formed under offshore large-scale ambient wind, January 20th, 2017 which formed under westerly large-scale ambient wind, and August 10th, 2017 which formed under easterly large-scale ambient wind. Results show that the development of sea breeze circulation under offshore ambient wind produced lower 10-m wind speed simulation, while sea breeze conditions under westerly large-scale ambient wind are associated with highest surface wind speeds reaching 9.5 ms-1 in Gulf of Jakarta. Sea breeze simulation also indicates the formation of calm zone which is 22 - 56 km away from the Jakarta Gulf coastline. Model also shows the formation of thermal internal boundary layer that are about 300-700 m near the coast gradually merged with the mixing layer inland. Performance of WRF on the simulated circulation is studied by conducting value of correlation, RMSE, and MBE. Overall, model simulation agrees fairly well with the observations taken at a near-surface meteorological station.

Observing the Whole-Canopy Short-Term Dynamic Response to Natural Step Changes in Incident Light: Characteristics of Tropical and Temperate Forests

We examine the physical and biological responses of forest canopies to step changes in light caused by passing low cumulus clouds that intermittently block the direct solar beam. Using data obtained at a tropical rainforest and at a midlatitude deciduous forest, we estimate the course of sensible heat flux, net ecosystem exchange, evapotranspiration, and water-use efficiency in response to the rapid changes in the incident radiative flux. To describe these fluxes during the interval over which the effects of stomatal time delays can be most influential, eddy fluxes are estimated over minute or shorter intervals by invoking a conditional-sampling procedure based on forming a Reynolds-average ensemble. The most important differences between the two forests’ physical responses are in the thermal balances and heat-flux time response constants. During the initial period after the light transition the only mean variables that show appreciable changes are the blackbody and air temperatures, the other scalars being little affected. We find that a distinct transient thermal internal boundary layer appears ≈ 20 m thick above the temperate deciduous forest and ≈ 45 m thick above the tropical rainforest. At each forest, the effective thickness of the inferred thermal outer-canopy ‘big leaf’ is about 1 mm. Twenty minutes after the abrupt change in incident light, ensemble eddy-flux estimates approach those found using conventional time averaging, confirming the validity of the ensemble approach. Previously unrecognized transient maxima in net ecosystem exchange and evapotranspiration are evident 5–10 min following the shadow-to-light transition, longer than the average light interval between shadows observed on partly-cloudy days in each case. Short-term variations in sensible heat flux, net ecosystem exchange, and evapotranspiration approximate an exponential adjustment, implying that first-order time-dependent single-leaf models are adequate to describe whole-canopy processes in these cases, providing an experimental method for determining whole-canopy bulk stomatal time constants. During the sunlit interval (direct and diffuse radiative fluxes combined) net ecosystem exchange is enhanced, while under cloud shadow (only diffuse radiative flux) water-use efficiency increases. This light and shadow alternation provides a mechanism describing the observed enhanced net ecosystem exchange and water-use efficiency under certain types of partly-cloudy sky. We apply empirical flux-response curves to an idealized case of radiative flux varying in a regular on–off light and shadow pattern. For this case, an analytical solution for mean net ecosystem exchange flux as a function of diffuse-radiation fraction yields results that strongly resemble previous findings based on conventional time-averaged fluxes. Our analysis and modelling indicates that a well-known correlation between diffuse radiative flux and enhanced net ecosystem exchange and water-use efficiency on cloudy days is in many cases not causal, but rather to be a consequence of time averaging over light-and-shadow intervals. By linking processes associated with photosynthesis in fluctuating light at the leaf scale to the canopy scale, our efforts facilitate the scaling-up of leaf responses to the ecosystem scale.

Effect of sea breeze propagation on the urban boundary layer of the metropolitan region of Sao Paulo, Brazil

Thermally driven circulations over urban areas, such as sea, lake, and land breezes and mountain-valley and urban heat island (UHI) circulations, strongly determine local weather and air quality conditions over highly populated areas. This study investigates the influence of sea breeze (SB) propagation on the development of the urban boundary layer (UBL) in the Metropolitan Region of Sao Paulo (MRSP), Brazil, an urban area with complex topography. This investigation is undertaken observationally by analyzing surface and upper air observations conducted in 2013 during the MCITY BRAZIL Project field campaigns and is conducted numerically by performing a set of Weather Research and Forecasting (WRF) simulations. The observational analysis highlights two SB events in the MRSP, one in February (austral summer) and the other in August (winter). In both seasons, the passage of the SB front disrupts the convective growth of the UBL and establishes a thermal internal boundary layer, thereby reducing the UBL height. The cooling also momentarily disrupts the UHI circulation. The results indicate that topography enhances SB propagation because it helps the marine air to go up the MRSP coastal escarpment and to reach the plateau while forcing adiabatic cooling of this air, intensifying the thermal gradient between the southern part of the plateau and the urban area. The UHI further increases the thermal gradient (from 0.5 K to 2 K in summer and 1 to 2.5 K in winter), accelerating the SB propagation. Synoptic conditions determine whether the SB front reaches the urban area and also may displace the convergence center of the UHI circulation.

Analysis of four years of ceilometer-derived aerosol backscatter profiles in a coastal site of the western Mediterranean

We present the analysis of four years of measurements by a CL51 ceilometer in Burjassot (39.51 N, 0.42 W), a research station in the western Mediterranean coast. The 1-min resolution profiles of the CL51 are corrected, calibrated, grouped and cloud-screened to create a 1-h resolution database of aerosol backscatter profiles at 910 nm (βa) spanning from July 2013 to August 2017. A total of 21,247 βa profiles are obtained, covering 58% of the considered period. The analysis of the βa profiles as a function of the main aerosol in the atmosphere reveals that dust arrives at Burjassot mainly in the form of elevated layers, with a highest impact between 1 and 4 km. The βa profiles obtained throughout the year show a strong seasonal trend: in summer we obtain, on average, the highest βa coefficients distributed over a stratified profile; on the contrary, in winter the mean βa coefficients have the lowest value and decrease exponentially with height. Our observations indicate that these seasonal changes are related to seasonal changes in the distribution of dust and to the wind direction at surface level. Regarding this last point we observe that conditions favoring aerosol re-circulation and thermal internal boundary layer lead to the highest βa values and to the stratification of the aerosol profile. On the contrary, when these mechanisms are impeded we find lower βa values and no aerosol layering. Finally, we observed that the βa follows a daily cycle that varies depending on the season the wind regime.

Atmospheric Boundary Layer Characterization Using Multiyear Ground-Based Microwave Radiometric Observations Over a Tropical Coastal Station

The continuous ground-based microwave radiometer profiler (MRP) observations of lower atmospheric temperature and humidity profiles are used to investigate the diurnal evolution of atmospheric boundary layer height (BLH) over a tropical coastal station. The BLH estimated from the MRP observations is compared with concurrent and collocated measurements of mixing layer height using a Micropulse Lidar and the BLH derived from radiosonde ascends. The monthly mean diurnal variation of the BLH derived from the multiyear (2010–2013) MRP observations exhibits strong diurnal variation with the highest around the local afternoon (~12:00–15:00 IST) and the lowest during the nighttime (~100–200 m). The daytime convective BLH is maximum during the premonsoon season (March–May) with the peak value (~1300 m) occurring in April and minimum in the month of July (~600 m). This paper presents the potential of MRP observations to investigate the continuous diurnal evolution of the BLH over a tropical coastal region manifested by a thermal internal boundary layer (TIBL) at much better time resolution, which is essential for understanding the rapid growth of the boundary layer and the TIBL during the forenoon period.

Monitoring the Micrometeorology of a Coastal Site next to a Thermal Power Plant from the Surface to 140 m

AbstractA 140-m micrometeorological tower has been operating since August 2016 at 4 km from the coastline and 250 m from a thermal power plant that releases heat from its 20-m stacks in southeastern Brazil. The measurements include 11 levels of turbulence observations and 10 levels of slow-response temperature and humidity. The observed atmospheric structure is largely dependent on the wind direction with respect to the power plant. When winds blow from the plant to the tower, the air layer between 20 and 60 m of the atmosphere may be warmed by as much as 2°C. In these circumstances there are events when the emissions pass directly by the tower. They allow the analysis of turbulence structures of thermal plumes generated from the plant’s heat release in comparison with those generated by the surface heating. In the more common case of winds blowing from the tower to the plant, the observations allow a detailed description of the local atmospheric boundary layer. During the day, vertical profiles of turbulent quantities and their spectral distributions show a cycle controlled by interactions between the land and oceanic surfaces, such as a thermal internal boundary layer. At night, there is a systematic tendency of progressive stabilization throughout the period, suitable for the analysis of the boundary layer transition from weakly to very stable conditions. The data also grant the inference of detailed vertical profiles of turbulent diffusion coefficients directly from observations.