Universidade Federal de Santa Maria

Ci. e Nat., Santa Maria v.42, ed. esp.: meteorologia, e15, 2020

DOI:10.5902/2179460X55316

ISSN 2179-460X

Received: 22/09/20   Accepted: 22/09/20  Published: 30/09/20

 

 

Variabilidade Climática Clima e Oceano

 

Physical processes analysis of contrails formation over the South Brazil Region

 

Análise dos processos físicos na formação de contrails na região Sul do Brasil

 

Vanessa Lins I

Renato Ramos da Silva II

 

I Universidade Federal de Santa Catarina, Florianópolis, Brazil. E-mail: van.lins@outlook.com.

II Universidade Federal de Santa Catarina, Florianópolis, Brazil. E-mail: renato.ramos@ufsc.br.

 

 

ABSTRACT

Contrails are clouds in the shape of condensation trails formed from hot air and particles exiting the engines of airplanes. These clouds form from the isobaric mixing of hot and humid air masses emitted by airplanes with cold ambient air. Their formation can alter the sky through cirrus clouds and the radiation balance. In this study, photographic images, satellite data and atmospheric reanalysis and radiosonde data were used to assess the occurrence of these events in the South Brazil region. The results showed that there were observed several cases of contrails in the region, mainly when the upper layer where the aircraft passed was colder and moister. Initially, several cases were selected from the observations and the Terra, Aqua and Suomi satellite images. Also, radiosonding data from Curitiba, Florianópolis and Porto Alegre were applied to the thermodynamic Appleman diagram to study the physical processes involved. The results showed that temperatures below -50 °C from cold advection and moister air at the cruising level of airplanes contribute to formation of more persistent contrails and cirrus clouds. Therefore, monitoring the environmental conditions may improve the prediction of contrails formation and better understanding the impacts on the radiation balance and climate.

Keywords: Contrails climatology; Cirrus clouds; Appleman Diagram.

 

 

RESUMO

Contrails são nuvens em forma de trilhas de condensação formadas a partir de ar quente e partículas que saem dos motores dos aviões. Essas nuvens se formam a partir da mistura isobárica de massas de ar quente e úmida emitidas por aviões com ar ambiente frio. Sua formação pode alterar o céu através das nuvens cirrus e do balanço de radiação. Neste estudo, imagens fotográficas, dados de satélite e reanálise atmosférica e dados de radiossondas foram utilizados para avaliar a ocorrência desses eventos na região Sul do Brasil. Os resultados mostraram que foram observados vários casos de contrails na região, principalmente quando a camada superior por onde a aeronave trafegou apresentava-se mais fria e úmida. Inicialmente, vários casos foram selecionados a partir das observações e das imagens dos satélites Terra, Aqua e Suomi. Além disso, dados de radiossondagem de Curitiba, Florianópolis e Porto Alegre foram aplicados ao diagrama termodinâmico de Appleman para estudar os processos físicos envolvidos. Os resultados mostraram que temperaturas abaixo de -50 oC  e ar mais úmido no nível de cruzeiro dos aviões contribuem para a formação de contrails e contrails-cirrus mais persistentes. Portanto, o monitoramento das condições ambientais pode melhorar a previsão da formação destes contrails e também permitem entender melhor os impactos no balanço de radiação e no clima.

Palavras-chave: Climatologia de contrails; Nuvens cirrus; Diagrama de Appleman.

 

 

1 Introduction

Through observations on clear days, we can witness the tracks of some planes that mark the atmosphere. These trails, known as contrails or condensation trails, can have a short life span, or may persist for several hours. The main condition of life span of these clouds depends on the thermodynamics conditions of the atmosphere. If a contrail forms on a dry environment, they are generally not persistent. However, with sufficiently moist and cold environment air they can last for long hours and in many cases form broader cirrus contrails (SCHUMANN, 2002, 2005; LEWELLEN, 2014).

These condensation trails are formed from aircraft engines that emit hot moist gases and form a plume after incomplete combustion (SCHUMANN and HEYMSFIELD, 2017). This hot plume is usually composed of carbon dioxide (CO2), water vapor (H2O), nitrogen oxide (NOx), sulfur oxide (SOx), hydrocarbon (HC), and soot particles. Sulfur and hydrocarbons are the main ice particle precursors (KÄRCHER and YU, 2009). In general, ice begins to form at the jet's edge where the temperature is low and the vapor exhaust from aircraft engine is available for condensation. The mixture between the aircraft exhaust and the environmental air can reach supersaturation and therefore producing condensation and formation of ice crystals. Observations from airplanes of the microphysical properties show that contrails can alter the ice crystals formation upon the ambient conditions of saturation and the local ice concentration number (KÜBBELER et al., 2011). Table 01 summarizes a state-of-the-art of some recent cloud microphysics observations under contrail formation. Their results show that contrails can reach a broad variety of ice particle concentration, size and crystals habits shape. In general, the growth of the crystal ice is dependent of the ambient air moisture, temperature and number of particles concentration. Also, some observations show that in the absence of contrails, soot particles can modify the cirrus clouds forming ice cores away from the original plane flight location (KÄRCHER and YU, 2009).

 

Table 1 – Contrail observed microphysical studies, ice particle number concentration (N), crystals mean diameter (D) and major observing findings

Study

Study N, D and key findings

KNOLLENBERG (1972)

Contrail evolved into natural cirrus, D > 0.5 mm

GAYET et al. (1996)

N up to 0.175 ,, D > 50 µm

BAUMGARDNER and GANDRUD 1998

N 200 , D (bimodal) 0.7 and 2 µm

GOODMAN et al. (1998)

N ~ 5-10 cm-3, D ~ 4-5 µm, crystals type plates

HEYMSFIELD et al. (1998)

N 10-100 cm-3, D 1-10 µm, contrail visible for > 6h

LAWSON et al. (1998)

N ~1 cm-3, D 1-20 µm, crystals type columns and rosettes

POELLOT et al. (1999)

N > 10 cm-3, D ~ 10 µm

SCHRÖDER et al. (2000)

N > 100 cm-3, D 1-10 µm, spherical ice particles

SCHUMANN (2002)

Compilation of ice water content

GAO et al. (2004)

Ice crystals with HNO3

ATLAS et al. (2006)

Contrails tracked with satellite and lidar

 

The isobaric mixing between the ambient air and the airplane hot vapor and particles result in an environmental mixed air that promotes ice supersaturation and the contrail formation. The contrail becomes visible due to formation of ice crystals caused by supersaturation to ice. It occurs due to the exponential decrease of ice saturation vapor pressure as local temperature decreases, the Clausius-Clapeyron equation. The thermodynamic conditions of formation can be estimated by the Schmidt-Appleman criteria that are obtained by the conservation laws (SCHUMAN et al., 1996). The Appleman diagram can be used as a tool for estimating the ambient conditions for the contrail formation (APPLEMAN, 1953). Studies for North America and Europe based on satellite data shows that the ambient temperatures has to be below -40 oC to promote contrails formation (IWABUCHI, 2012). However, still there are no studies on contrails formation for the South Brazil region.

Gas emissions from the aircraft engines cause changes in the chemical composition of the upper troposphere and lower stratosphere. These changes occur at altitudes between 9 and 12 km in an eventual airplane flight track (PAOLI and SHARIFF, 2016). In general, these altitudes are adopted by airplanes with long flights. In addition to local changes, recent studies show that contrails can cause a disturbance in the global radiation balance (SCHUMANN et al., 2013). Sulfur oxide (SOx) and black carbon (BC) can significantly alter the behavior of radiation in the atmosphere. SOx reflects short-wave solar radiation and the BC absorbs short-wave solar radiation and long-wave terrestrial radiation (GELTEMANN et al., 2013). A major change in optical depth was also observed by the contrails for the northern hemisphere region, which were detected using the MODerate-resolution Imaging spectrometer (MODIS) (BEDKA et al., 2013). Understanding the formation of these clouds is therefore important to improve estimates of environmental conditions evolution and climate impacts.

Temperature and humidity profiles may differ from region to region. Analysis for the North Atlantic shows that the weather conditions have important impact on the contrail formation (IRVINE et al., 2012). In general, particles are important for the cloud microphysical processes of contrail evolution (HEYMSFIELD et al., 2009). For instance, the soot (particles resulting from incomplete fuel combustion) from the aircraft engines can act as a cloud condensation nucleai (CCN). However, a recent global summary of observations show that the environmental thermodynamics is the primary controlling factor for contrail formation (IPCC 1999). If the plume emitted by the airplane's engine mixes with a drier ambient air, the contrail should have a shorter life span. On the other hand, if mixing occurs in a more humid and cold environment leading to ice supersaturation, the contrails may persist for a longer time. If the ambient air is supersaturated in relation to the ice, persistent contrails can evolve into cirrus clouds (KÄRCHER et al., 2015).

Contrails can impact the atmosphere by altering the radiation balance. Recent studies have shown that persistent contrails that form cirrus have an important impact on the local climate (BURKHARDT and KÄRCHER, 2011; SCHUMAN, 2005). Analysis of temperature changes during the three day without airplanes in the sky after the September 11th attack in New York showed that contrails have an impact on the regional temperature range (TRAVIS et al., 2002). Modeling studies also show that the major impact of contrails on the radiation balance occurs due to formation of cirrus clouds (BURKHARDT and KÄRCHER, 2011; SCHUMANN, 2005). Radiative transfer models show that contrails can alter the atmospheric optical depth cooling the surface during the daytime and warming the atmosphere during the night time (MEERKÖTTER et al., 1999). The estimates of the impacts of cirrus for the north hemisphere show an important effect of the atmospheric moisture (MINNIS et al., 2004). Observations show that the estimated impact on the radiation from the induced contrail cirrus clouds may be in the order of 10-80 mW m-2 (STORDAL et al., 2005; LEE et al., 2009).

Radiative forcing (RF) is generally used to estimate the climate impact of contrails. This forcing may be on the longwave and shortwave radiation fluxes.  Recent studies show that the effects on longwave radiation dominates over the shortwave (STUBER and FORSTER, 2007; RAP et al., 2010; BURKHARDT and KÄRCHER, 2011). However, observations suggest an overall positive net RF effect (HAYWOOD et al., 2009). Table 2 summarizes the most recent estimates of the contrails impact on the radiative forcing and projected scenarios. The results show that there are still overall uncertainties on the impacts mainly due to uncertainties on contrail coverage, optical depth and cloud microphysics. Scenario estimates for 2050 indicates a projected increase of about 4-4.7% (LEE et al., 2009), but some more accurate modeling results suggest an increase in the order of 3% (BOCK and BURKHARDT, 2019).

 

Table 2 – Contrails mean Radiative Forcing RF (mW m-2) estimated impacts and period of traffic study

 

RF (mW m-2)

Period

IPCC (1999)

48.5

1992

STORDAL et al., (2005)

30 (10-80)

2000

SAUCEN et al., 2005

47.8

2000

LEE et al., (2009)

55 (23–87)

2000-2005

 

78 (38–139) *

2000-2005

BOUCHER et al., (2013)

10 (5-30)

2011

 

50 (20-150) *

2011

BOCK and BURKHARDT (2019)

160-180 **

2050

* combined contrail and contrail-cirrus RF, ** 2050 compared to 2006

In this study, several cases of contrails were identified from local sky observations and satellite images for the South Brazil region. The identified cases were then studied with the application of the Appleman diagram using local measured thermodynamic profiles from the closer airport radiosondes. Also, atmospheric reanalysis and satellite data were used to estimate temperature and moisture at the level of airplane flights. Thus, we could identify the major dynamic and thermodynamic conditions for persistent contrails and their cirrus cloud evolution in the region of study.

 

 

2 Material and Methods

Initially, the contrails identification was estimated from sky photographic images on some locations. The study region includes the South Brazil where several flights occur between the major airports. The most predominant flight routes and the height where the exhaust gases from the engine aircrafts fly were obtained from the Fligtht Aware website (https://pt.flightaware.com/). Figure 01 shows the region of study and examples of local major flights.

In addition to photographic images, daily satellite data was analyzed to check their possible registration and formation of persistent cirrus clouds. The satellites data used were the visible channel from the Terra, Aqua and Suomi NPP and were provided by National Aeronautics and Space Administration (NASA) at https://worldview.earthdata.nasa.gov. This portal also provides information of the satellite orbits. The data show that in their polar orbits, these satellites cross over the South Brazil in different times of the day. The Terra satellite usually record local images in the morning time; and the Aqua and Suomi satellites record local images around the afternoon. The period of analysis included the years between 2016 and 2019. The satellite observations provided information on the frequency of these clouds, their persistence in the atmosphere, and coverage area.

To examine the atmospheric thermodynamic conditions the next step was to explore the vertical atmospheric profile of temperature, winds and relative humidity of the local atmosphere. The atmospheric profiles were obtained from radiosondings carried out at the South Brazil region including data from Curitiba, Florianópolis and Porto Alegre International Airports. The Skew-T thermodynamic diagrams of these profiles were obtained from the University of Wyoming portal at http://weather.uwyo.edu/upperair/sounding.html. The closer airport radiosonde Skew-T profile of the observed contrail region was used to estimate the local dynamic and thermodynamic conditions.

The temperature profiles were then applied to the Appleman diagram, which has been used as a contrails estimator (APPLEMAN, 1953). In the construction of the Appleman diagram, it is initially considered that a mixture occurs between the gas expelled by the aircraft and the ambient air. The final mixture that may or may not form the contrail depends mainly on the pressure and relative humidity of the air and the amount of gas mixed with the environment. Based on observations, it is possible to obtain an estimate of an increase in the vapor mixing ratio of the order of 0.0336 grams per kilogram for each degree of temperature increase. Therefore, depending on the initial conditions of pressure, temperature and atmospheric humidity and the amount of ambient air mixed after the aircraft passes, we will have a final saturated or unsaturated atmosphere. Tables with the conditions for supersaturation of the environment after the passage of an aircraft can then be established. These data allow determining the critical temperature of contrails formation in function of variable conditions of pressure and ambient humidity. With these data, several curves can be established to determine the possibility of supersaturation of the atmosphere and therefore the formation of contrails. These curves were used to construct the Appleman diagram. A script example in python developed for this study is available at: https://github.com/renatordasilva/atmos.git.

The temperature radiosonde data from the airports were used in this diagram at the levels between 100 and 500 hPa that include the layers where most of the airplanes fly across the South Brazil region. Using this diagram, it was possible to estimate three likely states: (i) always contrails, (ii) may be contrails or (iii) no contrails. Also, the diagram permitted to evaluate the possible environmental conditions for persistent contrails.

The NCEP/NCAR Reanalysis II (KANAMITSU et al., 2002) fields at 250 hPa and surroundings were used to estimate the atmospheric conditions during the observed contrail cases. This level corresponds to approximately 11-km height at this region where most of the airplanes fly. The temperature maps and local time series of the surrounding layers were analyzed in order to evaluate the spatial and temporal impacts of temperature conditions on the contrails formation and persistence. Since the area of study covers the entire South Brazil region and focus on the high troposphere, this dataset can be considered suitable for synoptic scale analysis. This data is available from the NCEP-DOE portal at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis2.html.

The Atmospheric Infrared Sounder (AIRS) sensor on board of Aqua polar satellite product was use to estimate the atmospheric moisture. Maps of daily vapor mixing ratio were analyzed for each case study at the level of 250 hPa. This data is available at 1x1 degree of spatial resolution and can be obtained at the giovanni portal from NASA (https://giovanni.gsfc.nasa.gov).

 

Figure 1 – Example of flights between airports of Campinas-Porto Alegre (green), Cordoba-Rio de Janeiro (light blue), Buenos Aires-São Paulo (red)

 

 

3 Results and Discussion

Results obtained from a total of five well documented case studies are presented below. The analyses are based on sky photographic (when available), visible satellite images, thermodynamic profile diagrams and reanalysis and satellite maps of temperature and moisture for the South Brazil region.

 

3.1 Case study 01 – 15 June 2016

Figure 02 shows results for the observed contrail occurred on 15 June 2016 as observed at around 13:10 UTC by the Terra (Fig. 2a) and around 17:30 UTC by the Aqua (Fig. 2b) satellites. These images show several contrail lines over the continent and over the sea. The figures show also that several of these contrails evolved to broader cirrus contrails. In some locations these cirrus contrails reached about 13 km of width. Their locations over the sea are due to strong southwesterly winds at the 250 hPa layer (Fig. 2c). The Appleman diagram shows that at this layer, there were conditions for contrails formation as the temperatures above 250 hPa were colder than -50 ºC and shows conditions for persistent contrails (Fig. 2d).

Figure 03 shows the air temperature field at 250 hPa and its surroundings layers local time evolution. The temperature over the region of the observed contrails was in the order of -47 ºC to -50 ºC (Fig. 3a). The local time evolution shows that at the high levels the temperature dropped after 14th of June and remained colder than -48 ºC for most of the days after this time (Fig.3b). Since there was cold advection from the southwest winds at these layers, this condition was an important dynamic feature for the contrails transport and the cooling was an important thermodynamic condition for the contrails formation and persistence. Further satellite analysis shows that some contrails were observed also on June 15th (not shown).

 

Figure 2 – Figure 2(a) Terra satellite image, (b) Aqua satellite image, (c) Skew-T profile from Florianópolis and (d) Appleman diagram for 15 June 2016. Tr00, Tr30, and Tr100 are thresholds for Relative Humidity of 00%, 30% and 100%, respectively

 

Figure 04 shows the distribution of water vapor mixing ratio at 250 hPa. The field shows higher moisture over the local of contrails formation with amounts above 0.07 g/kg surrounded by drier atmospheric areas. It shows that environmental moisture also was an important parameter for the local contrails formation at this region.

 

Figure 3 – (a) Air temperature (ºC) at 250 hPa on 15 June 2016, (b) time series at 28oS and 49oW, from NCEP Reanalysis data

 

Figure 4 – Vapor mixing ratio (g/kg) at 250 hPa on 15 June 2016 from AIRS Aqua satellite sensor

 

 

3.2 Case study 02 – 29 June 2017

The sky photography recorded on 29 June 2017 at the city of Florianópolis shows the presence of two persistent contrails, with physical characteristics of cirrus (Fig. 5a).The Terra satellite image  shows several contrails and the presence of natural cirrus stratus that precedes the passage of a cold front over the state of Santa Catarina (Fig. 05b). Several of the contrail cirrus were located further east of the continent and over the Atlantic Ocean due to strong southwesterly winds at this layer (Fig. 5c). The Skew-T diagram shows an overall dry troposphere, but a high relative humidity around 11km height (i.e. 250 hPa) reaching moisture in the order of 75% (Fig. 05c). The Applemann diagram shows temperature conditions for the formation of persistent contrails above the 225 hPa level, where temperatures are colder than -50 ºC (Fig. 5d).

 

Figure 5 - (a) sky photography of a contrail in the city of Florianópolis (source RRSilva), (b) Terra satellite image, (c) Skew-T profile and (d) Appleman diagram for 29 June of 2017. Tr00, Tr30, and Tr100 are thresholds for Relative Humidity of 00%, 30% and 100%, respectively

 

Figure 06 shows that colder temperatures occurred near the coast (Fig 06a) where most of the contrails were observed (Fig. 5b). These cold temperatures were observed during most of the days, having a further drop on layers below 200 hPa (Fig. 06b), highlighting the importance of the cold advection in those layers. Further small contrails were observed at this region by satellite images (not shown) from 30th of June until 02nd of July.

 

Figure 6 – (a) Air temperature (ºC) at 250 hPa on 29 June 2017, (b) time series at 28oS and 49oW, from NCEP Reanalysis data

 

Figure 07 shows the presence of higher moisture over the state of Santa Catarina and over the sea. At this region, the mixing ratio was higher than 0.12 g/kg and therefore was an important parameter for the contrails formation and persistence.

 

Figure 7 – Vapor mixing ratio (g/kg) at 250 hPa on 29 June 2017 from AIRS Aqua satellite sensor

 

3.3 Case study 03 - 08 September 2017

The sky photography recorded on September 8, 2017 shows a contrail that are more spread out showing a structure that may dissipate and perhaps not develop into a broad cirrus cloud (Fig. 08a). The Suomi satellite image shows persistent contrails near the city of Florianópolis and over the sea (Fig. 8b). It is important to note that the same contrail cloud can take different forms along its length. This is because, from one location to another several factors such as changes in temperature, humidity, or even cloud cover at the time of the plane's passage may affect the contrail formation. The skew-T diagram for this day shows a dry atmosphere on most of its layers (Fig. 08c). Thus, this environment may be able to form contrails but not have conditions to form broad cirrus clouds. However, the persistent observed contrails were formed from the mixing of the airplane plume with the colder atmosphere.  Again, the Appleman diagram shows that temperatures reached -50 ºC at the layers above 250 hpa (Fig. 8d). These temperatures were low enough for the formation of observed persistent contrails.

 

Figure 8 – (a) Sky photography of a contrail recorded at Florianópolis (source V. Lins), (b) Suomi satellite image, (c) Skew-T profile and (d) Appleman diagram for 08 September 2017. Tr00, Tr30, and Tr100 are thresholds for Relative Humidity of 00%, 30% and 100%, respectively

 

Figure 09 shows that the temperatures over the region of contrail observations were in the order of -47 ºC (Fig. 09a). This colder temperature was observed on the days close to the day of contrail observation (Fig. 09b). The temperature dropped on most of the layers after 5th of September and shows again that cold advection from southwest was responsible for the ambient temperature change on the observed location of contrails.

 

Figure 9 – (a) Air temperature (ºC) at 250 hPa (a) on 08 September 2017; (b) time series at 28oS and 49oW, from NCEP Reanalysis data

 

Figure 10 shows again that moisture was important for the contrail formation. The mixing ratio higher than 0.11 g/kg was spread over the center domain including the state of Santa Catarina where most of the contrails were observed.

 

Figure 10 – Vapor mixing ratio (g/kg) at 250 hPa on 08 September 2017 from AIRS Aqua satellite sensor

 

3.4 Case study 04 – 12 July 2019

The Terra satellite image for 12 July 2019 shows contrails located over the state of Paraná, close to the city of Curitiba (Fig. 11a). Further in the afternoon, the Aqua satellite image shows formation of cirrus contrail that extends from the state of Santa Catarina and gained physical characteristics quite different from the initial ones (Fig. 11b). At the time when contrails were formed near the city of Curitiba, the skew-T diagram shows a profile with a very dry atmosphere, but shows a high relative humidity near 12 km height reaching moisture of about 53% (Fig. 11c). The Appleman diagram shows again a quite low temperature reaching -50 ºC above the layer of 250 hPa (Fig. 11d). As these contrails are transported to different locations from the westerly winds, they can change their initial characteristics according to the environmental conditions. At this dispersion phase, parameters that can modify their structure include wind shear and atmospheric turbulence (PAOLI et al., 2014), and radiation heating exchange (UNTERSTRASSER and GIERENS 2010).

 

Figure 11 – (a) Terra satellite image, (b) Aqua satellite image, (c) Skew-T profile from Curitiba and (d) Appleman diagram for 12 July of 2019. Tr00, Tr30, and Tr100 are thresholds for Relative Humidity of 00%, 30% and 100%, respectively

 

Figure 12 shows that over the observed region of contrails formation, the temperature was in the order of -47 ºC (Fig. 12a). Furthermore, the southwestern cold advection was again responsible for the local drop in temperature. During most of the days, the local temperature was colder and the southwestern winds were transporting the cold layers towards the east (Fig. 12b). Temperatures dropped after10th of July at high layers and this advection reached the layers below on 12 of July. A few contrails were observed also on 10th and 11th of July from the Aqua satellite images (now shown).

 

Figure 12 – (a) Air temperature (ºC) at 250 hPa for 12 July 2019, (b) time series at 27oS and 50oW, from NCEP Reanalysis data

 

Figure 13 shows again that higher moisture was spread over the local of contrails observations. Vapor mixing ratio was higher than 0.06 g/kg over the central-north of the domain, including the city of Curitiba. It shows that moisture advection was also important for the contrails formation at this region.

 

Figure 13 – Vapor mixing ratio (g/kg) at 250 hPa on 12 July 2019 from AIRS Aqua satellite sensor

 

3.5 Case study 05 – 15 August 2019

Figures 14a and 14b show the contrails recorded of August 15th, 2019 by the Terra and Suomi satellites, respectively. Both images show the presence of natural cirrus stratus along with contrails over the southern region of Brazil. The presence of these clouds is an indication of ice crystals which helps in the formation of a cirrus contrail. In the Figure 14b, it is possible to observe that some contrails developed very horizontally, gaining the appearance of a cirrus. In some places the original contrail is so spread out that it is difficult to distinguish it from the natural cirrus. The Skew-T shows again westerly winds and a moist layer reaching a relative humidity of 60% around 11 km height (Fig. 14c). The Appleman diagram shows again that in this layer, it was observed cooler temperatures than -50 ºC above 250 hPa (Fig 14d). Thus, the ambient conditions were favorable for the formation of persistent contrails and cirrus clouds.

 

Figure 14 –Satellite image (a) Terra, (b) Suomi, (c) Skew-T profile from Porto Alegre and (d) Appleman diagram for 15 August of 2019. Tr00, Tr30, and Tr100 are thresholds for Relative Humidity of 00%, 30% and 100%, respectively

 

Figure 15 shows a very cold region at the east of the continent over the south of the domain were most of the contrails were observed reaching temperature colder than -50 ºC (Fig. 15a). The temperature dropped over most of the upper layers reaching the coldest temperature between 14th and 15th of August (Fig. 15b). Again, the winds at this level show a southwestern cold advection with temperatures that remained cold for a few days (Fig. 15b).

 

Figure 15 – (a) Air temperature (ºC) at 250 hPa for 15 August 2019, (b) time series at 31oS and 52oW, from NCEP Reanalysis data

 

Figure 16 shows a high moisture layer over the east coast in the south. The vapor mixing ratio reached 0.06 g/kg over the regions of the south where most of the contrails were observed including the region of the city of Porto Alegre. Therefore, the higher moisture of these regions along with colder temperatures was also an important parameter for the contrails formation and persistence.

 

Figure 16 – Vapor mixing ratio (g/kg) at 250 hPa on 15 August 2019 from AIRS Aqua satellite sensor

 

 

4 Conclusion

Sky photography, satellite images and data, thermodynamics radiosonde profiles and reanalysis data were used to identify the occurrence and the physical processes of the contrails formation over the South Brazil region. This novel study for this region showed several observed persistent cases on different locations.

The satellite images along with the thermodynamic diagrams showed that strong southwestern winds transported most of the contrails towards eastern or the domain. Furthermore these profiles along with the application of the Appleman diagram showed that the formation of persistent contrails and cirrus clouds occurred over a colder environment reaching temperatures in the order of -47 oC to -50 oC. This temperature was colder than observed in other regions, such as the -40 oC observed over Europe and North America (IWABUCHI, 2012).

Furthermore, the formation of broader cirrus occurred on moister ambient that reached relative humidity in the order of 60%, which seems to be a moisture required for more persistent contrails and the evolution to cirrus clouds. Maps of water vapor mixing ratio showed that it was higher over the regions of observed contrails. Therefore, these two parameters, cold temperatures and moisture were important for the formation of contrails in this region as they set conditions for ice supersaturation.

The composition of atmospheric profiles of winds and temperature shows that the advection was important for setting the local ambient conditions for temperature and moisture that allowed the contrails formation. All the cases showed a westerly wind component between 200 and 250 hPa and therefore it is an important weather dynamic condition for the possible persistent contrail and cirrus formation at this region.

In the recent years important improvements were achieved on the numerical weather predictions. Temperature and moisture are now well predicted at the high tropospheric levels due to improvements on the models data assimilation processes. Thus, a future step could develop a contrail forecast system based on the current numerical weather prediction models.

The results show not only the presence of persistent contrails in this region but also provided important dynamic and thermodynamic conditions for their formation. The evolution of these contrails on broader cirrus can alter the radiation balance and therefore induce to regional climate changes. Therefore, the results of this study provided not only parameters for the contrails formation but also insights for a possible future environmental change. For instance, the scenarios from climate models show a possible warming of the troposphere and cooling of the stratosphere caused by the increase of greenhouse gases (LASTOVICKA et al., 2006; SANTER et al., 2013). Thus, future monitoring of the contrail formation will be of great importance for climate change and variability in this region.

 

 

Acknowledgments

Thanks to reviewers, to NASA Worldview for the use of imagery satellite, to NCEP for the reanalysis data and to UFSC Laboratory of Climate and Meteorology for the local support.

 

 

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