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Fog and stratus formation on the coast of Brazil
Atmospheric Research 87 (2008) 268 – 278 www.elsevier.com/locate/atmos
Fog and stratus formation on the coast of Brazil Natalia Fedorova a,⁎, Vladimir Levit a , Dmitry Fedorov b b
a Institute of Atmospheric Science, Federal University of Alagoas, Brazil Electrical and Computer Engineering Department, University of California, Santa Barbara, USA
Abstract The physical and synoptic processes of fog and stratus cloudiness formation at the northern and southern coasts of Brazil were investigated. The frequencies of both phenomena were higher at the southern coast. The synoptic situation patterns for fog/stratus formation were identified using the following products of NCEP reanalysis: streamlines, pressure, omega and relative humidity at different levels. Fog/stratus formation is associated, in general, with a wave disturbance in the trade winds (WDTW) field at the north coast. Moreover, stratus clouds were observed on the cold front periphery, Easterly wave and under the Upper Tropospheric Cyclonic Vortex. The principal synoptic processes of fog formation at the southern coast are a High dislocation along the east coast of South America and a warm core barotropic Low occurrence north of Argentina. Fog formation is initiated between these two synoptic systems. There were 1 or 2 h of fog duration at the north coast and on average 12 h at the south coast. A variety of meteorological elements and phenomena were studied before and during the fog/stratus days, and results of an atmosphere instability analysis are presented. A classification of vertical profiles (from NCEP reanalysis data) of temperature and dew-point for different fog/stratus physical processes was also developed. At the north coast no stable layers in the vertical profiles were observed and the humid layer was very narrow in all cases. An intense inversion or stable layer with a high humidity up to 950–670 hPa is responsible for fog development at the south coast. © 2007 Elsevier B.V. All rights reserved. Keywords: Fog/stratus formation; Synoptic analysis; Atmosphere instability
1. Introduction There is a paucity of research in fog/stratus development in South America, specifically Brazil. Scarce surface radiosonde data for coastal regions of southern and northern Brazil make fog/stratus studies more complicated. Two specific regions at the southern (Pelotas city, state of Rio Grande do Sul) and northern (Maceio city,
⁎ Corresponding author. E-mail addresses: [email protected] (N. Fedorova), [email protected] (V. Levit), [email protected] (D. Fedorov). 0169-8095/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2007.11.008
state of Alagoas) coasts of Brazil were chosen for the present investigation (Fig. 1). Both cities are located on the eastern coast of Brazil and are under the influence of maritime winds. On the other hand, a dryland region is present near the northern coast, but swamps and very humid regions are found near the southern coast. Synoptic processes responsible for weather are very different for these cities (Fig. 1). A constant intrusion of baroclinic systems (High and Low with frontal zones) is the typical synoptic process for the southern coast year round. Warm core barotropic lows situated north of central South America also influence the weather during a warm season. Typical tropical synoptic systems such as trade winds, easterly waves and the intertropical
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Fig. 1. Observation stations location and study area map: The full map represents the calculation area for the NCEP reanalysis products; white colored area (north of 20°S) has a predominant tropical synoptic system; grey color area (south of 20°S) has a predominant extratropical synoptic system.
convergence zone (ITCZ), form weather patterns affecting the northern coast. According to a climatological analysis of the northern coast of Brazil, fog was detected in Recife and Salvador (nearest points of radio-sound data to Maceio city, Fig. 1) on 13 and 37 days per year, respectively (Ratisbona, 1976). The same analysis found more frequent fog on the southern coast in Porto Alegre (point of the nearest sounding to Pelotas city, Fig. 1), 57 days per year. A climatological analysis of fog formation in the south-eastern state of San Paulo demonstrated that a significant growth in fog occurrence was registered during the past two decades (Araújo et al., 2001). The most probable reason is the instability in the ocean surface temperature. Therefore, a future increase in fog frequency is probable for such coastal regions. Analyses were also carried out for the southern Brazil coast and two fog forecast methods were developed: (a) radiation fog forecast for Porto Alegre airport (Lima 1982) and (b) fog forecast for Pelotas city (Oliveira, 1998; Oliveira and Fedorova, 1998). According to this forecast method, the radiation fog formed at 19:00 local time under the following weather conditions: extra-tropical High, east and southeast wind direction, wind velocity less than 1.4 m/s, air temperature between 16 and 22 °C and a dewpoint depression less than 1 °C. As a result, simple and multiple linear regression equations of fog probability
for Pelotas were obtained. The developed fog forecast algorithm utilized several parameters: a) vertical profiles of temperature and dew-point temperature (from radiosonde data for Porto Alegre and from NCEP reanalysis data for Pelotas), b) synoptic situation as well as conventional meteorological data such as wind velocity, relative humidity and temperature as predictors. The radiation fog formation in Porto Alegre was investigated by sounding (Piva and Fedorova, 1999). As a result, two types of vertical profiles of temperature and humidity were determined. They are distinguished by: 1) temperature inversion, 2) a moist layer at low levels and a dry layer in the rest of the atmosphere and 3) wind velocity at the earth surface. A unique study of stratus cloud formation in southern Brazil (Pelotas) was developed (Fedorova et al., 2002), in which patterns of tropospheric vertical structures for stratus days were elaborated. A few investigations of fog/stratus formation were made for the northern coast. One of them (Silveira, 2003) was developed for the Maceio airport and shows the tropospheric vertical structure and synoptic scale systems for the studied days. But this study is not sufficient for the daily operation of the airport. It should be noted here that Maceio is a very special region for international tourism, attracted by beautiful beaches and the ocean. After the construction of a new airport, the growth of air
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traffic made it necessary to carry out more detailed studies of fog/stratus development in order to elaborate higher precision weather forecasts. The forecast of lowlevel stratus clouds and of their associated precipitation is very important for the normal operation of the Maceio airport. The main goal for the present research is to make a comparison of the physical and synoptic processes of fog/stratus formation for the northern and southern coasts of Brazil.
manually confirm the NCEP profiles (Fig. 1). Types of temperature and humidity vertical profiles were determined for fog and stratus cloud days using the NCEP reanalysis data. Tropospheric thermal stability, localization of humid/dry layers, occurrence, intensity and inversion and/or localization of isothermal layers were investigated for all the fog and stratus days. Convective Available Potential Energy (CAPE), Lifting Condensation Level (LCL) and Level of Free Convection (LFC) were calculated (Djuric, 1994).
2. Data source and methodology
2.3. Synoptic systems associated with fog/stratus formation
The fog and stratus formation were compared for two coastal regions of Brazil: the northern coast (Maceío city, latitude 9.7°S, longitude 35,8°W) and the southern coast (Pelotas city, 32.7°S, longitude 52.5°W) (Fig. 1). A methodology was elaborated to understand the main differences of fog and stratus cloud formation in the extremely different regions of Brazil for short-term weather forecasts. Therefore, meteorological data, applicable in daily operational forecasts, were used. A description of the data and the method used for comparison are presented in the following.
The stabilization of synoptic patterns of any meteorological phenomenon is the first necessary step for synoptic weather forecast elaboration for any given region. Scarce meteorological research of fog/stratus formation in Brazil and the complete absence of any numerical methods for forecast of these phenomena near the coast make the elaboration of synoptic patterns very important for short-term weather forecasts. Byers (1959) developed a new classification by unifying previous research of synoptic patterns. Nevertheless, it is now necessary to elaborate more detailed synoptic patterns for specific regions. Synoptic scale information (e.g. maps and sections of different meteorological parameters, satellite images) was used to elaborate these patterns. It is important to note that this information was applied to the synoptic scale analysis. The synoptic situation during a fog/stratus day and two previous days were analyzed using different products from NCEP reanalysis: streamlines (at the levels of 1000, 925, 500 and 200 hPa), surface pressure, omega, temperature and relative humidity at the 925, 850, and 1000 hPa levels. The software “Grads” (Doty, 2001) was used to elaborate all maps in the region between 10°E–90°W and 20°N–60°S (Fig. 1). The figures below show some fragments of this observed area according to the synoptic process. The synoptic system analysis is presented herein. The wave disturbance in the field of trade winds (WDTW) at the northwest periphery of subtropical High in the southern Atlantic Ocean was identified by streamline maps at low levels according to the description by Molion and Bernardo (2000). The WDTW are associated with cyclonic curvature of trade winds at the levels of 1000 and 925 hPa. Easterly waves were determined by cyclonic air currents at the low and middle levels (Vasquez, 2000). The existence of WDTW and an Easterly wave were confirmed by the development of a synoptic scale cloud system on infrared channel satellite images. The images were obtained by CPTEC/INPE (http://www.cptec.inpe. br/). It is noteworthy that all the satellite images were used
2.1. Conventional meteorological data and fog/stratus identification The hourly surface data (temperature, relative humidity, wind speed, cloud types and meteorological phenomenon) were used for the fog/stratus days, as well as for the previous and following days. These data were obtained in 1995 by the Pelotas university's meteorological station and in 1996 by the meteorological station at Zumbi dos Palmares Maceio airport. Visibility data were gathered only by the airport meteorological station. In addition, the frequency of stratus clouds was studied over a period of three years (1998–2001) for the southern coast. The unavailability of meteorological data for the southern and northern coasts of Brazil for the same period is the reason why different periods are presented in this study. Conventional data were used for identification of fog/stratus days, phenomenon duration and frequency analysis. 2.2. Vertical profiles and structure analysis The vertical profiles were elaborated from NCEP reanalysis data for Maceio and Pelotas. Moreover, the radiosonde data for the Porto Alegre airport (southern coast, 32.7°S, near Pelotas) and the Recife Airport (northern coast, 8.0°S, near Maceio) were used only to
N. Fedorova et al. / Atmospheric Research 87 (2008) 268–278 Table 1 Fog and stratus (St) cloud yearly frequency (number of fog and St days) for the northern and southern coasts Month
1 2 3 4 5 6 7 8 9 10 11 12 ∑
Northern coast
Southern coast
Fog
St
Fog
St
0 0 0 0 0 1 1 0 0 0 0 0 2
2 0 2 8 4 4 3 7 1 0 2 0 33
2 2 14 12 15 8 10 12 6 9 3 0 93
18 10 10 17 19 18 18 18 17 21 11 5 182
only for synoptic scale system identification and were never used for fog identification. The absence of archived high resolution images didn't permit the identification of fog/stratus regions. Upper Tropospheric Cyclonic Vortices (UTCV) were identified by streamlines at high levels (Kousky and Gan, 1981). The intensity of anticyclones has been identified by the pressure value in the anticyclone centre (P), using Satyamurty and Lima (1994) criterions: intense (P N 1030 hPa), moderate (1021 b P b 1030 hPa) and weak (1012 b P b 1020 hPa). The Low type was determined according to the classical description of a barotropic Low (Vasquez, 2000) and by regional research (Seluchi et al., 2003). Furthermore, the Low or High parts were determined when fog was observed near Pelotas. The traditional geographical determination of positions of Low/High parts are the following: center, north, northeast, east, southeast, south, southwest, west and northwest. For example, fog may be observed in the High center or in the west periphery of the High. The frontal zone positions were identified by the traditional synoptic method: troughs on the pressure maps, streamline convergence and a high temperature gradient (Petterssen, 1956; Zverev, 1968; Djuric, 1994). The satellite images on the infrared channel were obtained by CPTEC/INPE and were used to confirm the frontal position. 3. Results 3.1. Frequency of fog and stratus clouds Fog was observed in southern Brazil throughout the whole experimental period except for December
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(Table 1). During the cold period (March through August), fog was registered more frequently (8–14 days per month) with the greatest occurrence in May (15 days). In northern Brazil, fog was registered only two days per year, in June and July. Fog formation for the southern coast was observed between 23 h and 04 h local time, though only during 1 h at 05 h local time on the northern coast. Stratus clouds were observed throughout the year, with the highest frequency being during the cold period (May–October) for the southern coast of Brazil. The greatest stratus cloud occurrence was detected in the beginning and the end of that period (19 and 21 days per month, respectively, Table 1). The cold period for the northern coast of Brazil (April–August) is associated with the rainy season and the greatest stratus cloud occurrence was observed exactly during that period. The stratus cloud frequency (3–8 days per month) for the northern coast was lower than for the southern coast. The highest stratus cloud frequency reached 8 and 7 days per month in the beginning and in the end of the cold period for the northern coast. Stratus clouds were observed for 10 days in February and March, for 11 days in November and for 5 days in December for the southern coast. During all other months, the frequency was higher, about 17 to 21 days per month. A comparison of stratus cloud frequency in the period of five years (Tables 1 and 2) for the southern coast has shown a great variation in the number of stratus cloud days. The highest frequency was registered in 1995 (182 days per year), fewer days were registered in 1998 (75 days per year) and the lowest frequency was in 1999, 2000 and 2001 (24–32 days per year). Stratus clouds
Table 2 Frequency of stratus (St) clouds (number of St days) between 1998 and 2001 for the southern coast Month
1998
1999
2000
2001
∑
1 2 3 4 5 6 7 8 9 10 11 12 ∑
7 10 9 12 8 2 7 7 4 5 2 2 75
4 0 0 2 2 4 4 4 5 3 2 1 31
4 1 0 2 2 2 2 0 3 8 0 0 24
0 0 0 0 7 10 6 2 5 2 0 0 32
15 11 9 16 19 18 19 13 17 18 4 3 162
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were least observed in November and December during the entire period of study. 3.2. Vertical structure of troposphere for fog and stratus clouds days 3.2.1. South coast 3.2.1.1. Classification of stratus cloud days. Three types of vertical profiles for temperature and humidity were determined for stratus cloud days at the southern
coast (Fig. 2). This classification for types and subtypes was elaborated through average values according to the distributions of humidity levels and air instability at low levels using NCEP reanalysis data for stratus cloud days presented in Tables 1 and 2. Type I includes cases of high humidity and conditional stability. Type II groups cases of stable air (isothermal levels) at low levels. All cases for types I and II are characterized by the absence of positive CAPE or by low positive CAPE at low levels. All cases of positive CAPE (at an average 311 J/kg) over the entire troposphere, or
Fig. 2. Vertical profiles of Type I (subtypes Ia, Ib and Ic) (A), II and III (B) for temperature (solid line) and dew-point (Td) (dotted line) for stratus cloud days on the southern coast of Brazil. The scale distance of the T-axis corresponds to 3 °C. Dark regions correspond to a positive Convective Available Potential Energy and circled numbers are (T − Td) values.
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only at middle/high levels, were grouped into type III. It is important to note that type III cases were only observed when morning altostratus clouds and/or cirrus clouds were replaced later by stratus clouds. These situations were always associated with frontal zones. Vertical profiles of Type I cases were divided into subtypes Ia, Ib and Ic: Ia) cases of positive CAPE (at an average 40 J/kg, up to 800 hPa) and high humidity at low levels (temperature and dew-point difference (T − Td) less or equal to 3 °C at an average of up to 870 hPa); Ib) cases of the absence of positive CAPE and deeper levels of high humidity (T − Td ≤ 3 °C) at low levels (at an average of up to 830 hPa); Ic) cases of positive CAPE and dryer air (3 °C ≤ T − Td ≤ 6 °C) at low levels. Type II vertical profiles were divided in subtypes IIa and IIb: IIa) cases of high humidity (T − Td ≈ 1 °C) only near the surface; isothermal level up to 925 hPa; IIb) cases of temperature inversion between surface and 1000 hPa, and humid air (T − Td b 5 °C) at low levels up to, on average, 640 hPa. 3.2.1.2. Classification of fog days. Two types of vertical profiles of temperature and humidity from NCEP reanalysis data for fog days (presented in Table 1) on the southern coast were identified. It was also seen that these types depend on the existence and the height of the thermal inversion levels (Fig. 3). Type I includes cases with no inversion and Type II includes cases with inversion. A sub-classification based on the height of high humidity levels includes subtypes for heights of: Ia) 950 hPa, on average, and Ib) 670 hPa, on average. Subtype IIa includes cases of more intensive inversion (the temperature difference between surface and the highest inversion level (ΔT) of, on average, 8 °C). These cases were characterized by low inversion altitude (960 hPa, on average) and high humidity (T − Td near 2 °C). Cases of weak inversion (ΔT of, on average, 5 °C) have different height and humidity. Subtype IIb includes cases of higher humidity (T ≈ Td) and low inversion altitude (up to 1000 hPa,). Subtype IIc groups cases of inversion height up to 900 hPa, which were associated with dryer air (T − Td ≈ 3 °C). There are two principal differences in vertical profiles for fog and stratus cloud days. The first difference is higher humidity at low levels (T ≈ Td) for fog days without temperature inversion than for stratus cloud days. The
Fig. 3. Vertical profiles of Types I (subtypes Ia and Ib) (A) and II (subtypes IIa, IIb and IIc) (B) for temperature (solid line) and dew-point (Td) (dotted line) for fog days on the southern coast of Brazil. The scale distance of T-axis corresponds to 3 °C. Circled numbers are (T −Td) values. Squared numbers are temperature differences between surface and highest inversion levels.
second difference is a more intense temperature inversion (up to 8 °C) for fog days than for stratus cloud days. 3.2.2. North coast On the northern coast the stratus clouds were observed only together with altostratus and cumulus clouds. Vertical profiles of temperature and humidity (elaborated from the NCEP reanalysis data for fog days presented in Table 1) were similar for all stratus cloud days. The humidity layer (1 °C ≤ T − Td ≤ 3 °C) was observed at low levels of up to 925 hPa (Fig. 4) and the low positive CAPE (at an average 370 J/kg) was detected at levels of up to 950 hPa.
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Inter-Tropical Convergence Zone (ITCZ). For example, some maps of synoptic systems associated with a more intensive fog day are presented in Fig. 6. The WDTWs were detected by cyclonic curvature of streamlines only at low levels (Fig. 6A). They were associated with weak ascendant movement observed in the WDTW region (Fig. 6B). A high relative humidity (85–100%) was typical for all WDTWs areas and provoked fog formation processes.
Fig. 4. Vertical profiles of temperature (solid line) and dew-point (Td) (dotted line) for stratus cloud days on the northern coast of Brazil. The scale distance of T-axis corresponds to 3 °C. Dark regions correspond to a positive Convective Available Potential Energy and circled numbers are (T − Td) values.
Vertical profiles for fog and stratus cloud days were similar. Positive CAPE layers (at an average of 220 J/kg) began at 970 hPa for cases of fog days. The humidity layer reached 850 hPa for cases of intensive fog and remained close to the surface for weak fog days.
3.3.1.2. Stratus days. Stratus cloud formation (Table 1) was more frequently associated with a weak WDTW at low levels and the synoptic processes were very similar to the ones of fog formation, described above in 3.3.1.1. Some other synoptic systems were present only during three days. Firstly, the stratus clouds were observed in the cold front periphery during the event on 23/04/1996 (Fig. 7A). According to climatological data, cold front peripheries pass over the southern part of the state of Bahia every month, predominantly in rainy (winter) seasons, starting in March and ending in September (Kousky, 1979). The front periphery was more rarely observed in the state of Alagoas. A unique observation in this state was obtained in 2003 with only one front per year (Gemiacki, 2005). The infrared satellite images
3.3. Synoptic situation for fog/stratus days 3.3.1. Northern coast of Brazil 3.3.1.1. Fog days. Fog was registered only during two days on the northern coast of Brazil in June and July (Table 1). The variations of conventional meteorological parameters for the day of more intensive fog are presented in Fig. 5. Thick lines display the time period of the fog presence (with relative humidity 100%). These data show that the air temperature rises during a period of fog (Fig. 5A). A minimum visibility of 200 m was observed during the absence of rain. The wind velocity was about 1–2 ms− 1 during the night of the first fog day and it was calm during the second fog day. The wind direction of the first fog day slowly changed from East to North–North-West before the fog and changed rapidly to the initial position after the phenomenon (Fig. 5B). It was proposed that the additional humidity from Mundau River (located North–North-West of Maceio) helps in the fog formation. In both cases fog was formed on WDTWs, localized on the northwest periphery of the South Atlantic subtropical High and on the southern periphery of the
Fig. 5. Conventional meteorological parameters variations for the intensive fog at the Maceio Airport meteorological station from 1200 15 June 1996 to 2400 16 June 1996 (local time): A) relative humidity (RH, %, solid line) and air temperature (T, °C, dashed line); B) relative humidity and wind direction (D; 0°−360°, dashed line). Thick parts of the lines indicate fog presence.
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Fig. 6. Maps of different meteorological parameters near the surface level (1000 hPa) for a fog formation day in wave disturbance in the trade winds (WDTW) on the northern coast of Brazil 0600 UTC 16 June 1996: A) stream lines and wind velocity (m s− 1, numbers are the maximum wind velocity in the nucleus; a white dotted line shows the WDTW trough axis); B) omega (m s− 1, numbers are the maximum omega in the nucleus). The arrow shows the location of Maceio.
were used to identify the synoptic scale process and have shown the presence of a frontal cloud band above the Atlantic Ocean and frontal periphery above Alagoas. Some convection was registered by the same image above the ocean and in the western region of Alagoas. The stratus clouds were observed together with altostratus and cumulus clouds at the Maceio Airport (eastern coastal region), as already mentioned in Section 3.2.2. In the second case, on 21 January 1996 (Fig. 7B), stratus clouds were localized under Upper Tropospheric Cyclonic Vortices (UTCV). The UTCV is a very typical synoptic process during the summer season for northeastern Brazil (NEB) and it is localized at the high levels (therefore, stream lines at 200 hPa provide the best information for UTCV identification). The UTCV is one of the most important precipitation production systems on NEB (Kousky and Gan, 1981). The UTCV central region is usually associated with a descendent movement in the middle and high levels and, therefore, promotes stratus cloud formation. In the case presented in Fig. 7B, the UTCV centre was registered near the eastern coast in the city of Natal (5°S), near Alagoas, and the descendent movements were identified at the high and middle levels. The third case presented the most intensive stratus cloud event and was associated with a Easterly wave on 28 April 1996. The stratus cloud's duration was 6 h and the cloud's base height was 120 m during the entire period. The minimal visibility registered was 1200 m
and a visibility lower than 2000 m was registered during 5 h. Satellite infrared images were used, as usual, for Easterly wave identification and show the synoptic scale cloud system with intensive convection to the north, east and southern directions of Maceio (around Maceio airport). Cumulus, Altostratus and Stratocumulus clouds were also registered at the airport meteorological station during the entire day. The association of stratus clouds on the northern coast with other clouds was mentioned before in Section 3.2.2. Generally, low levels troughs of easterly waves are associated with convective activity (Vasquez, 2000). In the case presented in this investigation, convective clouds from the satellite image were associated with a trough at the northern periphery of the Subtropical South Atlantic High. This trough was more intensive at the 925 hPa level. The weak ascendant movement at low levels (Fig. 8) and high humidity on the easterly wave area were similar to the WDTW area for the fog days. 3.3.2. South coast of Brazil The association of fog formation with synoptic scale processes, such as High and frontal zones, was described in various synoptic research papers, for example Petterssen (1956), Byers (1959), Guseva and Razimor (1986). Some specific synoptic situations associated with fog formation on the southern coast of Brazil are presented below.
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Fig. 7. Typical synoptic situations of stratus clouds formation for the northern coast of Brazil for winter (A) and summer (B) seasons: A) satellite infrared image 00 UTC 23 April 1996 of the cold front periphery cloudiness (dotted line shows the frontal zone position) and B) Upper Tropospheric Cyclonic Vortex (UTCV), identified by stream lines at the 200 hPa, 0600 UTC 21 January 1996 (black dot shows the UTCV centre). The arrow shows the location of Maceio in both images.
On the southern coast, fog was usually observed in two of the following synoptic conditions: 1) High movement from South-West to East/North-East of South America and at the same time barotropic Low (trough) presence in northern Argentina and 2) baroclinic Low with a frontal zone passage through the South-East of South America. High and Low were observed together in the first synoptic process. The typical positions of High and Low for fog days are presented in Fig. 9. According to a classical and universal methodology for High/Low identification, the surface pressure map was used to identify positions of High and Low. All observed trajectories of Highs were typical for cold air intrusion to the south of Brazil (Satyamurty and Lima, 1994). Although, not all Highs were associated with fog formation, predominately moderated ones (in 82% of cases) with intensified, or constant, pressure in the centre. A cyclone is a warm core barotropic Low (thermal Low) and is more typical for that region in the warm season (Seluchi et al., 2003). Fog was formed between two of these
synoptic systems and more frequently was associated with a circulation Low (41% of all fog events) than with a High (34%, Table 3).
Fig. 8. Map of omega (m s− 1, numbers are the maximum omega in the nucleus) at the level 925 hPa for a more intensive stratus cloud event 1800 UTC 28 April 1996. The black dot shows the location of Maceio.
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Table 4 Frequency of fog days (number of fog days and percentage) on the southern coast associated with different Low/High parts (center—C, north—N, northeast—NE, east—E, southeast—SE, south—S, west— W, northwest—NW) and frontal types (cold, warm)
Fig. 9. Typical High (H) and Low (L) positions for fog days on the southern coast; example of fog event 1800 UTC 30 May 1995. The black dot shows the location of Pelotas.
Detailed analyses of these Highs showed that, more frequently, fog formation was detected in the western part of a High (72%) than in any other part of a High (Table 4). Fog was less often formed in the northwestern part of a High (18%) and near a High's center (10%). No other High's part was associated with fog formation. Research of these barotropic Low positions show that fog formation occurs more frequently in the east/southeast Low periphery (34 and 32%, respectively, Table 4). More rarely, fog was formed in the south, northeast and northern Low peripheries (16, 10 and 8%, respectively). Fog formation was never registered in any other part of the Low. The second principal synoptic process indicated above is a frontal zone influence. Fog was associated with the frontal zone passing in 25% of all fog events (Table 3) and predominately with a cold front (96% of all frontal events, Table 4). It should be noted that warm fronts are rare synoptic systems in this region (Fedorova
Table 3 Frequency of fog days (number of fog days and percentage) on the southern coast, associated with different synoptic systems (Low, High and Front) Synoptic system
Days
%
Low High Front ∑
38 32 23 93
41 34 25 100
Synoptic system part
Days
%
Low N NE E SE S ∑
3 4 13 12 6 38
8 10 34 32 16 100
High NW W C ∑
6 23 3 32
18 72 10 100
Front Cold Warm ∑
22 1 23
96 4 100
and Carvalho, 2000). The frequency of fog formation before and after a cold front's passage was approximately 48 and 52%, respectively. 4. Conclusions A comparison of vertical structure and synoptic situations shows different processes for stratus clouds and fog formation on the southern and northern coasts of Brazil. Fog events are more frequent on southern coasts than on the northern coasts (93 days and 2 days of fog per year, respectively). Stratus clouds were observed during 61 and 33 days per year for these regions, respectively. The greatest stratus cloud occurrences were at the beginning and end of the cold period and were similar for both coasts. Vertical profiles for fog/stratus days are different for the southern and northern coasts. The temperature inversion and isothermal layers are associated with fog/stratus on the southern coast and were not registered on the northern coast. The absence of inversion layers and weak ascendant motion are typical for fog/stratus formation days on the northern coast. The union of stratus clouds with altostratus and cirrus clouds was observed in the south during the frontal zone passages and with altostratus and cumulus clouds in the north for all observed days. The base of CAPE positive level for these cases was higher in the south (750 hPa) than in the north (950 hPa).
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A high relative humidity at a superficial level of up to 850 hPa, formed by the influence of weak ascendant movement in Trade Winds, is the main process of fog/stratus formation on the northern coast. On the other hand, the accumulation of humidity under the stable layers (under isothermal/inversion layers) provoked fog/stratus formation on the southern coast. On the northern coast, fog and stratus clouds were associated with WDTW at low levels, which were localized at the southern periphery of the ITCZ and on the northwestern periphery of the South Atlantic subtropical High. Also, stratus clouds were observed on the cold front periphery, the easterly wave and under the Upper Tropospheric Cyclonic Vortex. The principal synoptic processes of fog formation on the southern coast are: a High dislocation from the South to the East/North-East of South America along the eastern coast and a warm core barotropic Low occurrence in northern Argentina. Fog formation initiated between two of these synoptic systems, predominately on the east and south-eastern Low periphery and at the western part of the High. It was associated with Low circulation more frequently than with High. More rarely, fog was associated with a cold frontal passage. In these cases it was formed before and after the front with the same probability. Acknowledgments Partial financial support was provided by FAPEAL (Research Support Foundation of the Alagoas State), CNPq (National Council of Scientific and Technological Development) and CAPES (Coordination of the Level Superior Personal Improvement). References Araújo, G.P., Freitas, E.D., Gonçalves, F.L.T., 2001. Climatological analysis and numerical modeling to the fog events at Sao Paulo metropolitan area. 2nd International Conference on Fog and Fog Collection. SBMET, San Paulo, pp. 417–420. Byers, H.R., 1959. Fog. General Meteorology. McGraw Hill Book Company, New York, pp. 480–510. Djuric, D., 1994. Weather Analysis. Prentice Hall, New Jersey. Doty, B., 2001. The Grid analysis and display system. Version 1.8SL11, http://grads.igds.org/grads/.
Fedorova, N., Carvalho, M.H., 2000. Processos sinóticos em anos de La Niña e de El Niño. Parte II: Zonas frontais. Revista Brasileira de Meteorologia 15 (2), 57–72. Fedorova, N., Carvalho, M.H., Nunes, F.C., 2002. The first results about stratus clouds formation on South of Brazil. The XII Brazilian Congress of the Meteorology. SBMET, San Paulo, CD. Gemiacki, L., 2005. Frontal zone on North-east of Brazil. MSc. Thesis, Federal University of Alagoas, Maceio, Brazil. Guseva, N.N., Razimor, M.Y., 1986. Fog and visibility forecasting. Short-term Weather Forecasting. Hydrometeoisdat, Leningrad (in Russian). Kousky, V.E., 1979. Frontal influences on northeast Brazil. Monthly Weather Review 107, 1140–1153. Kousky, V.E., Gan, M.A., 1981. Upper tropospheric cyclonic vortices in the tropical South Atlantic. Tellus 33, 538–551. Lima, J.S., 1982. Previsão da ocorrência de nevoeiro em Porto Alegre: método objetivo. The Second Brazilian Congress of Meteorology. SBMET, San Paulo, pp. 275–292. Molion, L.C.B., Bernardo, S.O., 2000. Dinâmica das chuvas no Nordeste Brasileiro. The XI Brazilian Congress of the Meteorology. SBMET, San Paulo, CD. Oliveira, V.M., 1998. Condições para a formação de nevoeiro em Pelotas. MSc. Thesis, Federal University of Pelotas, Pelotas, Brazil. Oliveira, V.M., Fedorova, N., 1998. Condition for fog formation in the South coast of Brazil. First International Conference on Fog and Fog Collection. International Development Research Centre (IDRC), Ottawa, Canada, pp. 233–236. Petterssen, S., 1956. Motion and motion systems. weather and weather systems. In: Weather Analysis and Forecasting, vol.1, vol. 2. McGraw-Hill, New York, Toronto, London. 266p. Piva, E.D., Fedorova, N., 1999. Um Estudo sobre a formação de nevoeiro de radiação em Porto Alegre. Revista Brasileira de Meteorologia 14 (2), 47–62. Ratisbona, L.R., 1976. The climate of Brazil. Chapter 5. In: Schwerdtfeger, W. (Ed.), Climates of Central and South America. Elsevier Scientific Publishing Company, Oxford, pp. 219–269. Satyamurty, P., Lima, L.C.E., 1994. Movimento e intensificação de anticiclones extratropicais na região sul americana. The VIII Brazilian Congress of the Meteorology, vol. 2. SBMET, San Paulo, pp. 75–77. Seluchi, M.E., Saulo, A.C., Nicolini, M., Satyamurty, P., 2003. The northwestern Agentinean Low: a study of two typical events. Monthly Weather Review 131, 2361–2378. Silveira, P.S., 2003. Analysis of the fog cases and Stratus clouds in Airport of Maceio. MSc. Thesis, Federal University of Alagoas, Maceio, Brazil. Vasquez, T., 2000. Weather Forecasting Handbook. Weather Graphics Technologies, Garland, Texas. Zverev, A.S., 1968. Synoptic meteorology and numerical weather forecasting. Hydrometeoisdat, Leningrad. (in Russian).
Fog and stratus formation on the coast of Brazil Natalia Fedorova a,⁎, Vladimir Levit a , Dmitry Fedorov b b
a Institute of Atmospheric Science, Federal University of Alagoas, Brazil Electrical and Computer Engineering Department, University of California, Santa Barbara, USA
Abstract The physical and synoptic processes of fog and stratus cloudiness formation at the northern and southern coasts of Brazil were investigated. The frequencies of both phenomena were higher at the southern coast. The synoptic situation patterns for fog/stratus formation were identified using the following products of NCEP reanalysis: streamlines, pressure, omega and relative humidity at different levels. Fog/stratus formation is associated, in general, with a wave disturbance in the trade winds (WDTW) field at the north coast. Moreover, stratus clouds were observed on the cold front periphery, Easterly wave and under the Upper Tropospheric Cyclonic Vortex. The principal synoptic processes of fog formation at the southern coast are a High dislocation along the east coast of South America and a warm core barotropic Low occurrence north of Argentina. Fog formation is initiated between these two synoptic systems. There were 1 or 2 h of fog duration at the north coast and on average 12 h at the south coast. A variety of meteorological elements and phenomena were studied before and during the fog/stratus days, and results of an atmosphere instability analysis are presented. A classification of vertical profiles (from NCEP reanalysis data) of temperature and dew-point for different fog/stratus physical processes was also developed. At the north coast no stable layers in the vertical profiles were observed and the humid layer was very narrow in all cases. An intense inversion or stable layer with a high humidity up to 950–670 hPa is responsible for fog development at the south coast. © 2007 Elsevier B.V. All rights reserved. Keywords: Fog/stratus formation; Synoptic analysis; Atmosphere instability
1. Introduction There is a paucity of research in fog/stratus development in South America, specifically Brazil. Scarce surface radiosonde data for coastal regions of southern and northern Brazil make fog/stratus studies more complicated. Two specific regions at the southern (Pelotas city, state of Rio Grande do Sul) and northern (Maceio city,
⁎ Corresponding author. E-mail addresses: [email protected] (N. Fedorova), [email protected] (V. Levit), [email protected] (D. Fedorov). 0169-8095/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2007.11.008
state of Alagoas) coasts of Brazil were chosen for the present investigation (Fig. 1). Both cities are located on the eastern coast of Brazil and are under the influence of maritime winds. On the other hand, a dryland region is present near the northern coast, but swamps and very humid regions are found near the southern coast. Synoptic processes responsible for weather are very different for these cities (Fig. 1). A constant intrusion of baroclinic systems (High and Low with frontal zones) is the typical synoptic process for the southern coast year round. Warm core barotropic lows situated north of central South America also influence the weather during a warm season. Typical tropical synoptic systems such as trade winds, easterly waves and the intertropical
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Fig. 1. Observation stations location and study area map: The full map represents the calculation area for the NCEP reanalysis products; white colored area (north of 20°S) has a predominant tropical synoptic system; grey color area (south of 20°S) has a predominant extratropical synoptic system.
convergence zone (ITCZ), form weather patterns affecting the northern coast. According to a climatological analysis of the northern coast of Brazil, fog was detected in Recife and Salvador (nearest points of radio-sound data to Maceio city, Fig. 1) on 13 and 37 days per year, respectively (Ratisbona, 1976). The same analysis found more frequent fog on the southern coast in Porto Alegre (point of the nearest sounding to Pelotas city, Fig. 1), 57 days per year. A climatological analysis of fog formation in the south-eastern state of San Paulo demonstrated that a significant growth in fog occurrence was registered during the past two decades (Araújo et al., 2001). The most probable reason is the instability in the ocean surface temperature. Therefore, a future increase in fog frequency is probable for such coastal regions. Analyses were also carried out for the southern Brazil coast and two fog forecast methods were developed: (a) radiation fog forecast for Porto Alegre airport (Lima 1982) and (b) fog forecast for Pelotas city (Oliveira, 1998; Oliveira and Fedorova, 1998). According to this forecast method, the radiation fog formed at 19:00 local time under the following weather conditions: extra-tropical High, east and southeast wind direction, wind velocity less than 1.4 m/s, air temperature between 16 and 22 °C and a dewpoint depression less than 1 °C. As a result, simple and multiple linear regression equations of fog probability
for Pelotas were obtained. The developed fog forecast algorithm utilized several parameters: a) vertical profiles of temperature and dew-point temperature (from radiosonde data for Porto Alegre and from NCEP reanalysis data for Pelotas), b) synoptic situation as well as conventional meteorological data such as wind velocity, relative humidity and temperature as predictors. The radiation fog formation in Porto Alegre was investigated by sounding (Piva and Fedorova, 1999). As a result, two types of vertical profiles of temperature and humidity were determined. They are distinguished by: 1) temperature inversion, 2) a moist layer at low levels and a dry layer in the rest of the atmosphere and 3) wind velocity at the earth surface. A unique study of stratus cloud formation in southern Brazil (Pelotas) was developed (Fedorova et al., 2002), in which patterns of tropospheric vertical structures for stratus days were elaborated. A few investigations of fog/stratus formation were made for the northern coast. One of them (Silveira, 2003) was developed for the Maceio airport and shows the tropospheric vertical structure and synoptic scale systems for the studied days. But this study is not sufficient for the daily operation of the airport. It should be noted here that Maceio is a very special region for international tourism, attracted by beautiful beaches and the ocean. After the construction of a new airport, the growth of air
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traffic made it necessary to carry out more detailed studies of fog/stratus development in order to elaborate higher precision weather forecasts. The forecast of lowlevel stratus clouds and of their associated precipitation is very important for the normal operation of the Maceio airport. The main goal for the present research is to make a comparison of the physical and synoptic processes of fog/stratus formation for the northern and southern coasts of Brazil.
manually confirm the NCEP profiles (Fig. 1). Types of temperature and humidity vertical profiles were determined for fog and stratus cloud days using the NCEP reanalysis data. Tropospheric thermal stability, localization of humid/dry layers, occurrence, intensity and inversion and/or localization of isothermal layers were investigated for all the fog and stratus days. Convective Available Potential Energy (CAPE), Lifting Condensation Level (LCL) and Level of Free Convection (LFC) were calculated (Djuric, 1994).
2. Data source and methodology
2.3. Synoptic systems associated with fog/stratus formation
The fog and stratus formation were compared for two coastal regions of Brazil: the northern coast (Maceío city, latitude 9.7°S, longitude 35,8°W) and the southern coast (Pelotas city, 32.7°S, longitude 52.5°W) (Fig. 1). A methodology was elaborated to understand the main differences of fog and stratus cloud formation in the extremely different regions of Brazil for short-term weather forecasts. Therefore, meteorological data, applicable in daily operational forecasts, were used. A description of the data and the method used for comparison are presented in the following.
The stabilization of synoptic patterns of any meteorological phenomenon is the first necessary step for synoptic weather forecast elaboration for any given region. Scarce meteorological research of fog/stratus formation in Brazil and the complete absence of any numerical methods for forecast of these phenomena near the coast make the elaboration of synoptic patterns very important for short-term weather forecasts. Byers (1959) developed a new classification by unifying previous research of synoptic patterns. Nevertheless, it is now necessary to elaborate more detailed synoptic patterns for specific regions. Synoptic scale information (e.g. maps and sections of different meteorological parameters, satellite images) was used to elaborate these patterns. It is important to note that this information was applied to the synoptic scale analysis. The synoptic situation during a fog/stratus day and two previous days were analyzed using different products from NCEP reanalysis: streamlines (at the levels of 1000, 925, 500 and 200 hPa), surface pressure, omega, temperature and relative humidity at the 925, 850, and 1000 hPa levels. The software “Grads” (Doty, 2001) was used to elaborate all maps in the region between 10°E–90°W and 20°N–60°S (Fig. 1). The figures below show some fragments of this observed area according to the synoptic process. The synoptic system analysis is presented herein. The wave disturbance in the field of trade winds (WDTW) at the northwest periphery of subtropical High in the southern Atlantic Ocean was identified by streamline maps at low levels according to the description by Molion and Bernardo (2000). The WDTW are associated with cyclonic curvature of trade winds at the levels of 1000 and 925 hPa. Easterly waves were determined by cyclonic air currents at the low and middle levels (Vasquez, 2000). The existence of WDTW and an Easterly wave were confirmed by the development of a synoptic scale cloud system on infrared channel satellite images. The images were obtained by CPTEC/INPE (http://www.cptec.inpe. br/). It is noteworthy that all the satellite images were used
2.1. Conventional meteorological data and fog/stratus identification The hourly surface data (temperature, relative humidity, wind speed, cloud types and meteorological phenomenon) were used for the fog/stratus days, as well as for the previous and following days. These data were obtained in 1995 by the Pelotas university's meteorological station and in 1996 by the meteorological station at Zumbi dos Palmares Maceio airport. Visibility data were gathered only by the airport meteorological station. In addition, the frequency of stratus clouds was studied over a period of three years (1998–2001) for the southern coast. The unavailability of meteorological data for the southern and northern coasts of Brazil for the same period is the reason why different periods are presented in this study. Conventional data were used for identification of fog/stratus days, phenomenon duration and frequency analysis. 2.2. Vertical profiles and structure analysis The vertical profiles were elaborated from NCEP reanalysis data for Maceio and Pelotas. Moreover, the radiosonde data for the Porto Alegre airport (southern coast, 32.7°S, near Pelotas) and the Recife Airport (northern coast, 8.0°S, near Maceio) were used only to
N. Fedorova et al. / Atmospheric Research 87 (2008) 268–278 Table 1 Fog and stratus (St) cloud yearly frequency (number of fog and St days) for the northern and southern coasts Month
1 2 3 4 5 6 7 8 9 10 11 12 ∑
Northern coast
Southern coast
Fog
St
Fog
St
0 0 0 0 0 1 1 0 0 0 0 0 2
2 0 2 8 4 4 3 7 1 0 2 0 33
2 2 14 12 15 8 10 12 6 9 3 0 93
18 10 10 17 19 18 18 18 17 21 11 5 182
only for synoptic scale system identification and were never used for fog identification. The absence of archived high resolution images didn't permit the identification of fog/stratus regions. Upper Tropospheric Cyclonic Vortices (UTCV) were identified by streamlines at high levels (Kousky and Gan, 1981). The intensity of anticyclones has been identified by the pressure value in the anticyclone centre (P), using Satyamurty and Lima (1994) criterions: intense (P N 1030 hPa), moderate (1021 b P b 1030 hPa) and weak (1012 b P b 1020 hPa). The Low type was determined according to the classical description of a barotropic Low (Vasquez, 2000) and by regional research (Seluchi et al., 2003). Furthermore, the Low or High parts were determined when fog was observed near Pelotas. The traditional geographical determination of positions of Low/High parts are the following: center, north, northeast, east, southeast, south, southwest, west and northwest. For example, fog may be observed in the High center or in the west periphery of the High. The frontal zone positions were identified by the traditional synoptic method: troughs on the pressure maps, streamline convergence and a high temperature gradient (Petterssen, 1956; Zverev, 1968; Djuric, 1994). The satellite images on the infrared channel were obtained by CPTEC/INPE and were used to confirm the frontal position. 3. Results 3.1. Frequency of fog and stratus clouds Fog was observed in southern Brazil throughout the whole experimental period except for December
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(Table 1). During the cold period (March through August), fog was registered more frequently (8–14 days per month) with the greatest occurrence in May (15 days). In northern Brazil, fog was registered only two days per year, in June and July. Fog formation for the southern coast was observed between 23 h and 04 h local time, though only during 1 h at 05 h local time on the northern coast. Stratus clouds were observed throughout the year, with the highest frequency being during the cold period (May–October) for the southern coast of Brazil. The greatest stratus cloud occurrence was detected in the beginning and the end of that period (19 and 21 days per month, respectively, Table 1). The cold period for the northern coast of Brazil (April–August) is associated with the rainy season and the greatest stratus cloud occurrence was observed exactly during that period. The stratus cloud frequency (3–8 days per month) for the northern coast was lower than for the southern coast. The highest stratus cloud frequency reached 8 and 7 days per month in the beginning and in the end of the cold period for the northern coast. Stratus clouds were observed for 10 days in February and March, for 11 days in November and for 5 days in December for the southern coast. During all other months, the frequency was higher, about 17 to 21 days per month. A comparison of stratus cloud frequency in the period of five years (Tables 1 and 2) for the southern coast has shown a great variation in the number of stratus cloud days. The highest frequency was registered in 1995 (182 days per year), fewer days were registered in 1998 (75 days per year) and the lowest frequency was in 1999, 2000 and 2001 (24–32 days per year). Stratus clouds
Table 2 Frequency of stratus (St) clouds (number of St days) between 1998 and 2001 for the southern coast Month
1998
1999
2000
2001
∑
1 2 3 4 5 6 7 8 9 10 11 12 ∑
7 10 9 12 8 2 7 7 4 5 2 2 75
4 0 0 2 2 4 4 4 5 3 2 1 31
4 1 0 2 2 2 2 0 3 8 0 0 24
0 0 0 0 7 10 6 2 5 2 0 0 32
15 11 9 16 19 18 19 13 17 18 4 3 162
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were least observed in November and December during the entire period of study. 3.2. Vertical structure of troposphere for fog and stratus clouds days 3.2.1. South coast 3.2.1.1. Classification of stratus cloud days. Three types of vertical profiles for temperature and humidity were determined for stratus cloud days at the southern
coast (Fig. 2). This classification for types and subtypes was elaborated through average values according to the distributions of humidity levels and air instability at low levels using NCEP reanalysis data for stratus cloud days presented in Tables 1 and 2. Type I includes cases of high humidity and conditional stability. Type II groups cases of stable air (isothermal levels) at low levels. All cases for types I and II are characterized by the absence of positive CAPE or by low positive CAPE at low levels. All cases of positive CAPE (at an average 311 J/kg) over the entire troposphere, or
Fig. 2. Vertical profiles of Type I (subtypes Ia, Ib and Ic) (A), II and III (B) for temperature (solid line) and dew-point (Td) (dotted line) for stratus cloud days on the southern coast of Brazil. The scale distance of the T-axis corresponds to 3 °C. Dark regions correspond to a positive Convective Available Potential Energy and circled numbers are (T − Td) values.
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only at middle/high levels, were grouped into type III. It is important to note that type III cases were only observed when morning altostratus clouds and/or cirrus clouds were replaced later by stratus clouds. These situations were always associated with frontal zones. Vertical profiles of Type I cases were divided into subtypes Ia, Ib and Ic: Ia) cases of positive CAPE (at an average 40 J/kg, up to 800 hPa) and high humidity at low levels (temperature and dew-point difference (T − Td) less or equal to 3 °C at an average of up to 870 hPa); Ib) cases of the absence of positive CAPE and deeper levels of high humidity (T − Td ≤ 3 °C) at low levels (at an average of up to 830 hPa); Ic) cases of positive CAPE and dryer air (3 °C ≤ T − Td ≤ 6 °C) at low levels. Type II vertical profiles were divided in subtypes IIa and IIb: IIa) cases of high humidity (T − Td ≈ 1 °C) only near the surface; isothermal level up to 925 hPa; IIb) cases of temperature inversion between surface and 1000 hPa, and humid air (T − Td b 5 °C) at low levels up to, on average, 640 hPa. 3.2.1.2. Classification of fog days. Two types of vertical profiles of temperature and humidity from NCEP reanalysis data for fog days (presented in Table 1) on the southern coast were identified. It was also seen that these types depend on the existence and the height of the thermal inversion levels (Fig. 3). Type I includes cases with no inversion and Type II includes cases with inversion. A sub-classification based on the height of high humidity levels includes subtypes for heights of: Ia) 950 hPa, on average, and Ib) 670 hPa, on average. Subtype IIa includes cases of more intensive inversion (the temperature difference between surface and the highest inversion level (ΔT) of, on average, 8 °C). These cases were characterized by low inversion altitude (960 hPa, on average) and high humidity (T − Td near 2 °C). Cases of weak inversion (ΔT of, on average, 5 °C) have different height and humidity. Subtype IIb includes cases of higher humidity (T ≈ Td) and low inversion altitude (up to 1000 hPa,). Subtype IIc groups cases of inversion height up to 900 hPa, which were associated with dryer air (T − Td ≈ 3 °C). There are two principal differences in vertical profiles for fog and stratus cloud days. The first difference is higher humidity at low levels (T ≈ Td) for fog days without temperature inversion than for stratus cloud days. The
Fig. 3. Vertical profiles of Types I (subtypes Ia and Ib) (A) and II (subtypes IIa, IIb and IIc) (B) for temperature (solid line) and dew-point (Td) (dotted line) for fog days on the southern coast of Brazil. The scale distance of T-axis corresponds to 3 °C. Circled numbers are (T −Td) values. Squared numbers are temperature differences between surface and highest inversion levels.
second difference is a more intense temperature inversion (up to 8 °C) for fog days than for stratus cloud days. 3.2.2. North coast On the northern coast the stratus clouds were observed only together with altostratus and cumulus clouds. Vertical profiles of temperature and humidity (elaborated from the NCEP reanalysis data for fog days presented in Table 1) were similar for all stratus cloud days. The humidity layer (1 °C ≤ T − Td ≤ 3 °C) was observed at low levels of up to 925 hPa (Fig. 4) and the low positive CAPE (at an average 370 J/kg) was detected at levels of up to 950 hPa.
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Inter-Tropical Convergence Zone (ITCZ). For example, some maps of synoptic systems associated with a more intensive fog day are presented in Fig. 6. The WDTWs were detected by cyclonic curvature of streamlines only at low levels (Fig. 6A). They were associated with weak ascendant movement observed in the WDTW region (Fig. 6B). A high relative humidity (85–100%) was typical for all WDTWs areas and provoked fog formation processes.
Fig. 4. Vertical profiles of temperature (solid line) and dew-point (Td) (dotted line) for stratus cloud days on the northern coast of Brazil. The scale distance of T-axis corresponds to 3 °C. Dark regions correspond to a positive Convective Available Potential Energy and circled numbers are (T − Td) values.
Vertical profiles for fog and stratus cloud days were similar. Positive CAPE layers (at an average of 220 J/kg) began at 970 hPa for cases of fog days. The humidity layer reached 850 hPa for cases of intensive fog and remained close to the surface for weak fog days.
3.3.1.2. Stratus days. Stratus cloud formation (Table 1) was more frequently associated with a weak WDTW at low levels and the synoptic processes were very similar to the ones of fog formation, described above in 3.3.1.1. Some other synoptic systems were present only during three days. Firstly, the stratus clouds were observed in the cold front periphery during the event on 23/04/1996 (Fig. 7A). According to climatological data, cold front peripheries pass over the southern part of the state of Bahia every month, predominantly in rainy (winter) seasons, starting in March and ending in September (Kousky, 1979). The front periphery was more rarely observed in the state of Alagoas. A unique observation in this state was obtained in 2003 with only one front per year (Gemiacki, 2005). The infrared satellite images
3.3. Synoptic situation for fog/stratus days 3.3.1. Northern coast of Brazil 3.3.1.1. Fog days. Fog was registered only during two days on the northern coast of Brazil in June and July (Table 1). The variations of conventional meteorological parameters for the day of more intensive fog are presented in Fig. 5. Thick lines display the time period of the fog presence (with relative humidity 100%). These data show that the air temperature rises during a period of fog (Fig. 5A). A minimum visibility of 200 m was observed during the absence of rain. The wind velocity was about 1–2 ms− 1 during the night of the first fog day and it was calm during the second fog day. The wind direction of the first fog day slowly changed from East to North–North-West before the fog and changed rapidly to the initial position after the phenomenon (Fig. 5B). It was proposed that the additional humidity from Mundau River (located North–North-West of Maceio) helps in the fog formation. In both cases fog was formed on WDTWs, localized on the northwest periphery of the South Atlantic subtropical High and on the southern periphery of the
Fig. 5. Conventional meteorological parameters variations for the intensive fog at the Maceio Airport meteorological station from 1200 15 June 1996 to 2400 16 June 1996 (local time): A) relative humidity (RH, %, solid line) and air temperature (T, °C, dashed line); B) relative humidity and wind direction (D; 0°−360°, dashed line). Thick parts of the lines indicate fog presence.
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Fig. 6. Maps of different meteorological parameters near the surface level (1000 hPa) for a fog formation day in wave disturbance in the trade winds (WDTW) on the northern coast of Brazil 0600 UTC 16 June 1996: A) stream lines and wind velocity (m s− 1, numbers are the maximum wind velocity in the nucleus; a white dotted line shows the WDTW trough axis); B) omega (m s− 1, numbers are the maximum omega in the nucleus). The arrow shows the location of Maceio.
were used to identify the synoptic scale process and have shown the presence of a frontal cloud band above the Atlantic Ocean and frontal periphery above Alagoas. Some convection was registered by the same image above the ocean and in the western region of Alagoas. The stratus clouds were observed together with altostratus and cumulus clouds at the Maceio Airport (eastern coastal region), as already mentioned in Section 3.2.2. In the second case, on 21 January 1996 (Fig. 7B), stratus clouds were localized under Upper Tropospheric Cyclonic Vortices (UTCV). The UTCV is a very typical synoptic process during the summer season for northeastern Brazil (NEB) and it is localized at the high levels (therefore, stream lines at 200 hPa provide the best information for UTCV identification). The UTCV is one of the most important precipitation production systems on NEB (Kousky and Gan, 1981). The UTCV central region is usually associated with a descendent movement in the middle and high levels and, therefore, promotes stratus cloud formation. In the case presented in Fig. 7B, the UTCV centre was registered near the eastern coast in the city of Natal (5°S), near Alagoas, and the descendent movements were identified at the high and middle levels. The third case presented the most intensive stratus cloud event and was associated with a Easterly wave on 28 April 1996. The stratus cloud's duration was 6 h and the cloud's base height was 120 m during the entire period. The minimal visibility registered was 1200 m
and a visibility lower than 2000 m was registered during 5 h. Satellite infrared images were used, as usual, for Easterly wave identification and show the synoptic scale cloud system with intensive convection to the north, east and southern directions of Maceio (around Maceio airport). Cumulus, Altostratus and Stratocumulus clouds were also registered at the airport meteorological station during the entire day. The association of stratus clouds on the northern coast with other clouds was mentioned before in Section 3.2.2. Generally, low levels troughs of easterly waves are associated with convective activity (Vasquez, 2000). In the case presented in this investigation, convective clouds from the satellite image were associated with a trough at the northern periphery of the Subtropical South Atlantic High. This trough was more intensive at the 925 hPa level. The weak ascendant movement at low levels (Fig. 8) and high humidity on the easterly wave area were similar to the WDTW area for the fog days. 3.3.2. South coast of Brazil The association of fog formation with synoptic scale processes, such as High and frontal zones, was described in various synoptic research papers, for example Petterssen (1956), Byers (1959), Guseva and Razimor (1986). Some specific synoptic situations associated with fog formation on the southern coast of Brazil are presented below.
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Fig. 7. Typical synoptic situations of stratus clouds formation for the northern coast of Brazil for winter (A) and summer (B) seasons: A) satellite infrared image 00 UTC 23 April 1996 of the cold front periphery cloudiness (dotted line shows the frontal zone position) and B) Upper Tropospheric Cyclonic Vortex (UTCV), identified by stream lines at the 200 hPa, 0600 UTC 21 January 1996 (black dot shows the UTCV centre). The arrow shows the location of Maceio in both images.
On the southern coast, fog was usually observed in two of the following synoptic conditions: 1) High movement from South-West to East/North-East of South America and at the same time barotropic Low (trough) presence in northern Argentina and 2) baroclinic Low with a frontal zone passage through the South-East of South America. High and Low were observed together in the first synoptic process. The typical positions of High and Low for fog days are presented in Fig. 9. According to a classical and universal methodology for High/Low identification, the surface pressure map was used to identify positions of High and Low. All observed trajectories of Highs were typical for cold air intrusion to the south of Brazil (Satyamurty and Lima, 1994). Although, not all Highs were associated with fog formation, predominately moderated ones (in 82% of cases) with intensified, or constant, pressure in the centre. A cyclone is a warm core barotropic Low (thermal Low) and is more typical for that region in the warm season (Seluchi et al., 2003). Fog was formed between two of these
synoptic systems and more frequently was associated with a circulation Low (41% of all fog events) than with a High (34%, Table 3).
Fig. 8. Map of omega (m s− 1, numbers are the maximum omega in the nucleus) at the level 925 hPa for a more intensive stratus cloud event 1800 UTC 28 April 1996. The black dot shows the location of Maceio.
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Table 4 Frequency of fog days (number of fog days and percentage) on the southern coast associated with different Low/High parts (center—C, north—N, northeast—NE, east—E, southeast—SE, south—S, west— W, northwest—NW) and frontal types (cold, warm)
Fig. 9. Typical High (H) and Low (L) positions for fog days on the southern coast; example of fog event 1800 UTC 30 May 1995. The black dot shows the location of Pelotas.
Detailed analyses of these Highs showed that, more frequently, fog formation was detected in the western part of a High (72%) than in any other part of a High (Table 4). Fog was less often formed in the northwestern part of a High (18%) and near a High's center (10%). No other High's part was associated with fog formation. Research of these barotropic Low positions show that fog formation occurs more frequently in the east/southeast Low periphery (34 and 32%, respectively, Table 4). More rarely, fog was formed in the south, northeast and northern Low peripheries (16, 10 and 8%, respectively). Fog formation was never registered in any other part of the Low. The second principal synoptic process indicated above is a frontal zone influence. Fog was associated with the frontal zone passing in 25% of all fog events (Table 3) and predominately with a cold front (96% of all frontal events, Table 4). It should be noted that warm fronts are rare synoptic systems in this region (Fedorova
Table 3 Frequency of fog days (number of fog days and percentage) on the southern coast, associated with different synoptic systems (Low, High and Front) Synoptic system
Days
%
Low High Front ∑
38 32 23 93
41 34 25 100
Synoptic system part
Days
%
Low N NE E SE S ∑
3 4 13 12 6 38
8 10 34 32 16 100
High NW W C ∑
6 23 3 32
18 72 10 100
Front Cold Warm ∑
22 1 23
96 4 100
and Carvalho, 2000). The frequency of fog formation before and after a cold front's passage was approximately 48 and 52%, respectively. 4. Conclusions A comparison of vertical structure and synoptic situations shows different processes for stratus clouds and fog formation on the southern and northern coasts of Brazil. Fog events are more frequent on southern coasts than on the northern coasts (93 days and 2 days of fog per year, respectively). Stratus clouds were observed during 61 and 33 days per year for these regions, respectively. The greatest stratus cloud occurrences were at the beginning and end of the cold period and were similar for both coasts. Vertical profiles for fog/stratus days are different for the southern and northern coasts. The temperature inversion and isothermal layers are associated with fog/stratus on the southern coast and were not registered on the northern coast. The absence of inversion layers and weak ascendant motion are typical for fog/stratus formation days on the northern coast. The union of stratus clouds with altostratus and cirrus clouds was observed in the south during the frontal zone passages and with altostratus and cumulus clouds in the north for all observed days. The base of CAPE positive level for these cases was higher in the south (750 hPa) than in the north (950 hPa).
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A high relative humidity at a superficial level of up to 850 hPa, formed by the influence of weak ascendant movement in Trade Winds, is the main process of fog/stratus formation on the northern coast. On the other hand, the accumulation of humidity under the stable layers (under isothermal/inversion layers) provoked fog/stratus formation on the southern coast. On the northern coast, fog and stratus clouds were associated with WDTW at low levels, which were localized at the southern periphery of the ITCZ and on the northwestern periphery of the South Atlantic subtropical High. Also, stratus clouds were observed on the cold front periphery, the easterly wave and under the Upper Tropospheric Cyclonic Vortex. The principal synoptic processes of fog formation on the southern coast are: a High dislocation from the South to the East/North-East of South America along the eastern coast and a warm core barotropic Low occurrence in northern Argentina. Fog formation initiated between two of these synoptic systems, predominately on the east and south-eastern Low periphery and at the western part of the High. It was associated with Low circulation more frequently than with High. More rarely, fog was associated with a cold frontal passage. In these cases it was formed before and after the front with the same probability. Acknowledgments Partial financial support was provided by FAPEAL (Research Support Foundation of the Alagoas State), CNPq (National Council of Scientific and Technological Development) and CAPES (Coordination of the Level Superior Personal Improvement). References Araújo, G.P., Freitas, E.D., Gonçalves, F.L.T., 2001. Climatological analysis and numerical modeling to the fog events at Sao Paulo metropolitan area. 2nd International Conference on Fog and Fog Collection. SBMET, San Paulo, pp. 417–420. Byers, H.R., 1959. Fog. General Meteorology. McGraw Hill Book Company, New York, pp. 480–510. Djuric, D., 1994. Weather Analysis. Prentice Hall, New Jersey. Doty, B., 2001. The Grid analysis and display system. Version 1.8SL11, http://grads.igds.org/grads/.
Fedorova, N., Carvalho, M.H., 2000. Processos sinóticos em anos de La Niña e de El Niño. Parte II: Zonas frontais. Revista Brasileira de Meteorologia 15 (2), 57–72. Fedorova, N., Carvalho, M.H., Nunes, F.C., 2002. The first results about stratus clouds formation on South of Brazil. The XII Brazilian Congress of the Meteorology. SBMET, San Paulo, CD. Gemiacki, L., 2005. Frontal zone on North-east of Brazil. MSc. Thesis, Federal University of Alagoas, Maceio, Brazil. Guseva, N.N., Razimor, M.Y., 1986. Fog and visibility forecasting. Short-term Weather Forecasting. Hydrometeoisdat, Leningrad (in Russian). Kousky, V.E., 1979. Frontal influences on northeast Brazil. Monthly Weather Review 107, 1140–1153. Kousky, V.E., Gan, M.A., 1981. Upper tropospheric cyclonic vortices in the tropical South Atlantic. Tellus 33, 538–551. Lima, J.S., 1982. Previsão da ocorrência de nevoeiro em Porto Alegre: método objetivo. The Second Brazilian Congress of Meteorology. SBMET, San Paulo, pp. 275–292. Molion, L.C.B., Bernardo, S.O., 2000. Dinâmica das chuvas no Nordeste Brasileiro. The XI Brazilian Congress of the Meteorology. SBMET, San Paulo, CD. Oliveira, V.M., 1998. Condições para a formação de nevoeiro em Pelotas. MSc. Thesis, Federal University of Pelotas, Pelotas, Brazil. Oliveira, V.M., Fedorova, N., 1998. Condition for fog formation in the South coast of Brazil. First International Conference on Fog and Fog Collection. International Development Research Centre (IDRC), Ottawa, Canada, pp. 233–236. Petterssen, S., 1956. Motion and motion systems. weather and weather systems. In: Weather Analysis and Forecasting, vol.1, vol. 2. McGraw-Hill, New York, Toronto, London. 266p. Piva, E.D., Fedorova, N., 1999. Um Estudo sobre a formação de nevoeiro de radiação em Porto Alegre. Revista Brasileira de Meteorologia 14 (2), 47–62. Ratisbona, L.R., 1976. The climate of Brazil. Chapter 5. In: Schwerdtfeger, W. (Ed.), Climates of Central and South America. Elsevier Scientific Publishing Company, Oxford, pp. 219–269. Satyamurty, P., Lima, L.C.E., 1994. Movimento e intensificação de anticiclones extratropicais na região sul americana. The VIII Brazilian Congress of the Meteorology, vol. 2. SBMET, San Paulo, pp. 75–77. Seluchi, M.E., Saulo, A.C., Nicolini, M., Satyamurty, P., 2003. The northwestern Agentinean Low: a study of two typical events. Monthly Weather Review 131, 2361–2378. Silveira, P.S., 2003. Analysis of the fog cases and Stratus clouds in Airport of Maceio. MSc. Thesis, Federal University of Alagoas, Maceio, Brazil. Vasquez, T., 2000. Weather Forecasting Handbook. Weather Graphics Technologies, Garland, Texas. Zverev, A.S., 1968. Synoptic meteorology and numerical weather forecasting. Hydrometeoisdat, Leningrad. (in Russian).