The Amazon biome contains more than half of the remaining tropical forests of the planet and has a strong impact on aspects of meteorology such as the planetary boundary layer (PBL). In this context, the objective of this study was to conduct observational evaluations of the daily cycle of the height of the PBL during its stable (night) and convective (day) phases from data that were measured and/or estimated using instruments such as a radiosonde, sodar, ceilometer, wind profiler, lidar and microwave radiometer installed in the central Amazon during 2014 (considered a typical year) and 2015 during which an intense El Niño–Southern Oscillation (ENSO) event predominated during the GoAmazon experiment. The results from the four intense observation periods (IOPs) show that during the day and night periods, independent of dry or rainy seasons, the ceilometer is the instrument that best describes the depth of the PBL when compared with in situ radiosonde measurements. Additionally, during the dry season in 2015, the ENSO substantially influenced the growth phase of the PBL, with a 15 % increase in the rate compared to the same period in 2014.
The Amazon basin covers about a third of the South American continent and
extends for approximately
In this region there is a substantial quantity of convective activity that occurs during the entire year, but there are significant seasonal differences due to annual variation in atmospheric circulation and thermodynamic structure (Marengo and Espinoza, 2016), and these wet (or rainy) and dry seasons are well-defined. In this context, during the years 2014 and 2015, in the central Amazon region, the Green Ocean Amazon (GoAmazon) project was conducted with the objective of observing the influence of the complex interaction between the pollution plume generated in the city of Manaus-Amazonas and clouds and vegetation (Martin et al., 2016). This project approached research questions from a multidisciplinary perspective, and one of the studied topics was the physics involved in convective processes in the Amazon with emphasis on differences between wet and dry seasons.
The PBL is a turbulent layer of the atmosphere near the surface that results from the interaction between the surface and the atmosphere. The knowledge of the properties of the PBL has important scientific and practical applications because through this understanding of the PBL operational models of weather and climate forecasting can be refined, pollutant dispersion processes can be adequately described, the eolic potential of a region can be objectively determined, patterns of ventilation in urban areas can be estimated and improvements in agricultural techniques can be made (Englberger and Dörnbrack, 2017). Furthermore, a more realistic representation of processes that occur in the PBL can benefit numeric models of weather forecasting with better parameterization of convection, clouds and rain (Holtslag et al., 2013). The PBL characteristics, related to surface processes, provide important information regarding the priming of the atmosphere for convective initiation (Tawfik and Dirmeyer, 2014).
Holtslag et al. (2013) state that the PBL, since it is the lowest level of
the atmosphere, is in continuous interaction with the Earth's surface, with
significant turbulent transfer of heat, mass and
Neves and Fisch (2015) emphasize that an important characteristic of the PBL is the determination of its height because it is then possible to estimate the volume into which the source of pollution will be dispersed, and this is an important parameter for modeling of atmospheric dispersion. During its daily cycle the PBL undergoes atmospheric processes generated by thermal and mechanical convection during the day and displays stable conditions at night. The height of the PBL during the atmospheric instability phase (principally during the day) is called the convective boundary layer (CBL), and that during the stable period (principally at night) is named the nocturnal boundary layer (NBL).
In this context, observational evaluation of the daily cycle of the PBL, with emphasis on the CBL and NBL, represents an important field of study since within the PBL there occur processes that have a large impact on society and the terrestrial environment. Therefore, the objective of this study was to contribute to a more thorough understanding of the daily cycle of the height of the PBL through integration and comparison of data that were measured and/or estimated using instruments such as a radiosonde, sodar, ceilometer, wind profiler, lidar and microwave radiometer installed by the GoAmazon experiment. Furthermore, this study attempted to verify if the observational methods were representative of the daily variation in the cycle of the CBL and NBL in the central Amazon during 2014 (stated as a normal year) and 2015 and to elucidate the influence of an intense El Niño–Southern Oscillation (ENSO) event that occurred in 2015/2016.
In order to conduct this observational study, data from the GoAmazon project
2014/5 were used. The article by Martin et al. (2016) describes the details
of the experiment wherein these data were collected, its principal
objectives and some results. These data were collected using the structure
that was installed at a research station called T3 (03
Location of the atmospheric measurement experiments in Manacapuru, Amazonas, Brazil.
At the experimental site of the T3 station, instruments were installed to
obtain measurements of the hydrological cycle, PBL energy flux and other
micrometeorological variables, and the data from these measurements are
available on the web site of ARM – Climate Research Facility
(
In order to measure the height of the PBL, instruments that probe the lower troposphere including a wind profiler (WP), ceilometer, sodar, microwave radiometer profiler (MWP) and lidar were used, and these data were compared to data taken in situ obtained using radiosondes (RSs), and this method is described below.
In this experiment, RS measurements were obtained using a system that
included a DigiCORA (MW12) (Vaisala Inc., Finland) with radiosonde model
RS92SVG. The RS was coupled to a meteorological balloon that had an average
ascension rate of 5 m s
From these data, the potential temperature (
In order to construct a wind profiler (WP) at the study site, a radio acoustic sound system (RASS) model RWP915 from Vaisala Inc. (Finland) was used for direct and continuous measurements of the PBL. The WPs are Doppler instruments used to detect the vertical wind profile, and they function at a frequency of 50 MHz to 16 GHz. The WP–RASS installed at the study site operates at 915 MHz to measure the wind profile. The RASS transmitter aids in the measurement of the profiles of the vertical temperature. The WP–RASS operates through transmission of electromagnetic waves in the atmosphere and measures the intensity and frequency of the backscatter of the waves, assuming that atmospheric dispersion elements are moving with the average wind profile.
Since this is an instrument that operates at a high frequency and using smaller intervals of space between layers, it is frequently used for tropospheric observations, especially for the PBL. The method used in this study is described by Wang et al. (2016), wherein the height of the PBL was estimated using the vertical profile of electromagnetic refraction of WP, where the maximum of this index occurs in the upper part of the PBL.
In the study area a mini-sodar (sound detection and ranging) (model Sodar
MFAS and RASS A032002, Scintec, Rottenburg, Germany) was installed. This
monostatic equipment consists of an emission-receiving antenna with an area
of 1.96 m
Through remote sensor measurement by the sodar the height of the PBL was calculated for its night phase (nocturnal boundary layer, NBL) through the determination of the maximum wind height (jet). This method was suggested by Neves and Fisch (2011) and showed good results for the Amazon due to its operational limit (400 m) and taking into account that the NBL in the region has an average depth of 100 to 300 m.
Lidar was also used to estimate the height of the PBL using a lidar model
Stream Line
These instruments employ a laser transmitter operating at a wavelength of
1.5
Lidar uses a technique of heterodyne detection (method of extraction of coded information as a phase modulation and/or the frequency of a wavelength) in which the return signal is mixed with a reference laser beam (a local oscillator) of a known frequency. A computer within the instrument then processes the signal determining the Doppler frequency change using the spectrum from the signal. The energy content of the Doppler spectrums can also be used to determine attenuated backscattering.
Lidar operates in the near-infrared wavelength and is sensitive to
retro-diffusion of aerosols at the micrometer scale; therefore it is capable
of measuring wind speeds under clear-sky conditions with very high
precision (normally 10 cm s
The PBL was also monitored using a ceilometer model CL31 from Vaisala Inc. (Finland). The Vaisala ceilometers are a type of lidar remote sensing instrument that operate through a maximum vertical range of 7700 m and register the intensity of optical backscattering at the near-infrared wavelength between 900 and 1100 nm through the emission of a vertical pulse that is autonomously executed. These measurements are used to produce derived products that are recorded: the height of the cloud base, the retrieval of the particle backscatter coefficient and PBL height (Wiegner et al., 2014; Shukla et al., 2014; Morris, 2016; Geiß et al., 2017; Carneiro et al., 2020). Although the ceilometer measures the reflection of the aerosol layer (thus the mixing layer height), it was assumed to be the diurnal PBL height since the entrainment zone is very shallow. Thus, throughout this paper, this information (backscatter aerosols) was assumed to be PBL height. The ceilometer is a high-temporal-resolution instrument with a measurement interval of 2 s and a sampling rate of 16 s, and it is a powerful tool for measuring the height of the PBL during its daily cycle (day and night phases) to a high level of detail. The ceilometer signal results from light backscattered by particles at the atmosphere; the intensity of backscattering depends on the concentration of particles in the air (Morris, 2016). Ceilometers use pulsed diode laser lidar (light detection and ranging) technology to determine the attenuated backscatter, and the particle backscatter coefficients are obtained from these data. Subsequently the heights of the cloud base and the PBL are calculated (Wiegner et al., 2014; Kotthaus et al., 2016; Morris, 2016; Geiß et al., 2017).
The standard procedure for the PBL heights determination from Vaisala ceilometers is the software package BL-View developed by the manufacturer (see more details in Morris, 2016; Geiß et al., 2017; Geisinger et al., 2017).
Data were also used from a microwave radiometer profiler model MP3000A from Radiometrics Corp., Boulder, CO, USA. This instrument provides vertical profiles of temperature, humidity and liquid water content at a sampling rate of 60 s and average values at intervals of approximately 5 min. The profiles are deduced from measurements of radiance values of absolute microwaves (expressed as “brightness temperature”) obtained at 12 different frequencies at intervals of 22–30 and 51–59 GHz. This type of data are useful as input into numerical models of weather forecasting which need high-resolution profiles in continuous time.
To obtain the potential temperature throughout the daily cycle, it was necessary to interpolate the pressure profiles of the RS using to the method of polynomial interpolation. Together with the MWR air temperature profiles, the daily cycle of the potential temperature profile was calculated. Thus, the height of the PBL was estimated using the profile method.
This table presents a synthesis of the instruments used in this study, the observation periods, and temporal and spatial resolutions.
The remote sensor instruments capture multiple layers from the heights of the PBL in the transition interval of day to night (between 17:00 and 18:00 LT) shown in Fig. S1 (presented in the Supplement). However, as one of the goals of this paper is to have a complete picture of the PBL cycle, the NBL heights in this interval were neglected in Figs. 4 and 6, in order to show only the decay of the CBL convection.
Determination of monthly rainfall for both study years was carried out using data
taken with a disdrometer model Parsivel
The measurements of radiation and soil heat flux were taken every 30 min using the Surface Energy Balance System (SEBS), which consists of measurements of solar and terrestrial radiation collected using radiometers, and the radiation balance by a net radiometer model CNR4/CNF4, (Kipp & Zonen, Delft, the Netherlands) installed 2 m above the ground. There was a coupling of sensors measuring soil heat flux by flux plates (HFT-3, Hukseflux Thermal Sensors, Delft, the Netherlands) buried at 0.02 m depth in this system.
In the results obtained, the average and standard deviation values were
computed for different time intervals along the PBL daily cycle (Tables 2
and 3). The computed Pearson's correlation coefficient (
Analysis of the meteorological variables revealed that accumulated precipitation was different between years (Fig. 2). The year 2014 was similar to the normal climatology (for the city of Manaus – data extracted from INMET, 2018) for the region (2300 mm), with a total of 2451 mm. This high rate of rainfall can be understood as a response of the dynamic fluctuation of the nearly permanent center of convection, associated with a high rate of local evapotranspiration, which contributed to recycling of water vapor and rainfall (Nobre et al., 2009; Rocha et al., 2017). In contrast, the year 2015 registered a significant reduction (approximately 30 %) of the total rainfall in relation to the previous year, with a total accumulation of 1764 mm, well below the normal climatological average. This reduction is associated with the occurrence of the El Niño–Southern Oscillation (ENSO) event of that year (ECMWF, 2017; Macedo and Fisch, 2018; Newman et al., 2018).
Distribution of accumulated monthly precipitation (mm) for the years 2014 and 2015 and the normal climatological pattern.
During 2014, monthly accumulated precipitation was always above 50 mm per month, and during representative months of IOP1 (February and March 2014) the total accumulated precipitation was 720 mm. However, during IOP2 (September and October 2014) the total accumulated precipitation was 185 mm, yielding a reduction of nearly 75 % of accumulated precipitation during IOP1. According to Ferreira et al. (2005), this difference between the rainy and dry seasons occurs because rainfall distribution in the Amazon is very irregular, with high spatial and temporal variability. Marengo et al. (2017) provide a more detailed explanation of this characteristic of central Amazonia and include the large-scale forcing role in the description of rainfall during the rainy and dry seasons.
While 2015 had a large reduction in rainfall, during the months of the period of intense observation of the rainy season (IOP3) the accumulated precipitation was 398 mm, representing a reduction of approximately 50 % of the total accumulated precipitation during the rainy season of 2014 (IOP1). During IOP4, the total accumulated precipitation was well below the normal climatological value, as well as in comparison to the same period in 2014 (IOP2), with a total registered precipitation of 68 mm, a reduction of approximately 65 % compared to IOP2. This occurred due to the EN event being more intense during these months (ECMWF, 2017; Newman et al., 2018). The ENSO event (2015/2016) was considered one of the most intense in recent years, with an intensity similar to that which occurred during 1982/83 and 1997/98 (ECMWF, 2017).
Daily cycles of 30 min averages of the components of the balance of energy are presented in Fig. 3 for IOP1 (Fig. 3a) and IOP2 (Fig. 3b) for 2014. The shaded area for Fig. 3 which represents the standard deviations values was computed for each 30 min time interval. It is also shown in Figs. 4 to 6.
In these figures, the radiation balance (
The latent heat flux (
Average daily cycle of the radiation balance (
In the Amazon, especially during the rainy season, only a small fraction of
Nevertheless, only a small percentage of
Figure 4 shows the hourly average of the heights of the PBL for the IOP1 (Fig. 4a) and IOP2 (Fig. 4b), and Table 2 shows standard deviations values in different time intervals along the PBL daily cycle. The sunrise and sunset times were marked by the vertical lines of 06:00 and 18:00 LT, respectively, since the study area is close to the Equator line and there are not changes at these times. The RS (in situ measurements) was considered the truth depth of the boundary layer, while the others presented were estimated by remote sensing.
Daily cycle of the height of the PBL during the IOP1
During the phase in which the NBL is formed (between 00:00 and 06:00 LT), IOP1 showed small vertical oscillations of its depth due to the occurrence of sporadic rainfall (Fig. 4a). The results obtained from the ceilometer between 00:00 and 03:00 LT showed that the depth of the PBL varied between 180 and 280 m, and after 04:00 LT there was an increase in the maximum height to 350 m, which was reduced in the following hours to 275 m by 06:00 LT (sunrise).
The measurements made with the WP also showed some oscillations in the height of the NBL, with a reduction in height between 00:00 and 02:00 LT from 280 to 250 m and then an increase to a maximum of 350 m at 04:00 LT, remaining constant until 06:00 LT. The sodar results during this interval showed lower variation in NBL depth during this same interval. The variation observed from the measurements by the different sensors is related to intermittent mechanical turbulence which could be the result of the presence of clouds and rain on some days and not on others, thus provoking an increase in wind variability during the night, which has the effect of deepening the NBL. However, in this same interval during IOP2 (Fig. 4b) the NBL was very stable with an average height of 250 m for all sensors (ceilometer, WP, sodar, MWR and lidar), thus corroborating the explanation of the influence of rainfall on the determination of variability of depth of the NBL.
The results found for the height of the NBL were similar to those reported by Neves and Fisch (2011), in a study using sodar in the southwestern Amazon, where the authors observed NBL heights varying from 150 to 329 m. However, Acevedo et al. (2004), also studying in a pasture site in the Amazon (in Santarém-PA), observed lower NBL heights than those from the current study (between 50 and 150 m), and this difference occurs because of different geographic conditions (influence of river breeze, fog formation, etc.). As an example of these influences, in the Santarém region the authors observed several cases of formation of fog during the night, which was not observed at the pasture site in Rondônia (see Neves and Fisch, 2015) or at T3 the site.
Standard deviation calculated for PBL height measurements of instruments at different intervals of the daily cycle.
* “X” represents where absence measurements occurred.
The phase of the erosion of the NBL according to Stull (1988) begins after
sunrise (at 06:00 LT in the Amazon), and the complete erosion of the NBL occurs
when the whole layer is mixed. The potential vertical gradient is almost
null and as a consequence there is a high growth rate (above 100 m h
In contrast, in the IOP2 (Fig. 4b), as a function of greater stability of
the NBL and the positive values of
The transition from nighttime to daytime is very complex. Although the
In IOP1, after complete erosion of the NBL the development phase of the
convective boundary layer (CBL) begins, and due to the slow erosion of the
NBL the growth of the CBL begins at 11:00 LT with a typical height of 850 m and
an average increase of 102 m h
During IOP2, with the NBL being rapidly degraded, the CBL that subsequently
formed had a more rapid development, with an average growth rate of 175.2 m h
The energy fluxes for IOP3 (Fig. 5a) and IOP4 (Fig. 5b) show that
during the 2015 rainy season,
The
Average of the daily cycle of net radiation (
The daily cycle of CBL during IOP3 (Fig. 6a), as well as in IOP1, showed vertical oscillations of the NBL's height (between 00:00 and 06:00 LT) of 200 m (00:30 LT) to 375 m (03:00 LT). The WP and the sodar yielded lower depths than the ceilometer and the MWR. However, during IOP4 (Fig. 6b) the NBL was more stable, with an average height of 250 m, similar to what was observed during IOP2. The result from 00:00 to 06:00 LT confirms that in the Amazon region the NBL is more stable during the dry season compared to the rainy season, when it has larger variation in its depth. Table 3 shows standard deviation values in the different time intervals.
Daily cycle of the height of the PBL during the IOP3
The IOP3 demonstrated a similar erosion pattern for the NBL to that observed
during IOP1, with the NBL still established between 06:00 and 08:00 LT. From this
time onward there was an increase in depth of the CBL with an average growth
rate of 19.6 m h
The development during IOP3 occurs in an analogous manner to that observed
during IOP1, with weak vertical development of the maximum depth. At 10:00 LT
the CBL begins to develop and has a height of 830 m and a growth rate of
100.3 m h
Standard deviation calculated for PBL height measurements of instruments at different intervals of the daily cycle.
* “X” represents where absence measurements occurred.
During the four IOPs the results show that during daytime and nighttime intervals, independent of weather conditions, the ceilometer is a promising sensor with good accuracy for direct and continuous measurement of the height of the PBL (which on average ranged from 250 m – NBL to 1900 m – CBL) when compared to in situ RS. The RS, in spite of it being a proven high-precision method, in this experiment was launched only on synoptic times plus an extra at 15:00 UTC. Hence, it did not capture a high-temporal-resolution (like the remote sensors) daily cycle evolution of the height of the PBL, due to the long time interval between launches (each 6 h). While the MWR, WP and the lidar were satisfactory for estimates of the convective phase (CBL) of the PBL, during the nocturnal phase (NBL) these sensors overestimated heights. Additionally, the sodar under- and overestimated the NBL during these periods.
The intense EN event of 2015/2016 influenced the development phase of the CBL during the dry season of IOP4, and it had a growth rate of about 15 % higher than the results from IOP2 and a sensible heat flux (responsible for heating the air) that was higher than the standard values for the central Amazon. As a consequence, more intense convective movements occurred and contributed to a stronger vertical development of the layer.
The NBL erosion showed differences between seasons, presenting an erosion time of 2 h in the dry IOPs (2 and 4), and 3 h in the wet IOPs (1 and 3). A more detailed analysis of NBL erosion is being elaborated in Carneiro and Fisch (2020).
The data sets used in this publication are available at
the ARM Climate Research Facility database for the GoAmazon 2014/5 experiment
(
The supplement related to this article is available online at:
RGC and GF designed the numerical experiments, and RGC performed the simulations as a part of his PhD. RGC performed data analysis, assisted by GF. RGC and GF prepared the manuscript.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) (ACP/AMT/GI/GMD inter-journal SI)”. It is not associated with a conference.
Institutional support was provided by the National Institute of Space Research (INPE), the National Institute of Amazonian Research (INPA) and Amazonas State University (UEA). Rayonil G. Carneiro acknowledges the Brazilian National Council for Scientific and Technological Development (CNPq) graduate fellowship (140726/2017-9). Rayonil G Carneiro and Gilberto Fisch thank the GoAmazon project group for providing the data available for this study.
This research has been supported by the National Council for Scientific and Technological Development (grant no. 140726/2017-9).
This paper was edited by Maria Assuncao Silva Dias and reviewed by two anonymous referees.