DIE ERDE 139 2008 (1-2)

Special Issue: Fog Research

pp. 45-60

• Fog – Data interpolation – Regional climatology – Mexico

Fernando García-García and Víctor Zarraluqui (Mexiko City)

A Fog Climatology for Mexico Eine Nebelklimatologie für Mexiko

With 8 Figures

Fog can be defined as a cloud in the vicinity of the earth’s surface that affects visibility. It differs from a cloud only in that the base of fog is at the surface of the earth while clouds are further above. Fog plays an important role in the hydrological cycle, mainly in the transport of water from the atmosphere to the earth’s surface through wet deposition and interception by trees and vegetation. It is considered also a natural hazard that causes low visibility (according to the international, meteorological definition, fog reduces visibility at the ground below 1 km) and is a particular danger for all varieties of air, land and water transportation. On the other hand, fog can be also considered a potential non-conventional source of water supply when removed by artificial methods for human consumption. Fogs of all types originate when the temperature and the dewpoint of the air coincide. This may occur through cooling of the air to a little beyond its dewpoint, as a result of advection, radiation or upslope movement of the air; or by adding moisture and thereby elevating the dewpoint, thus producing so-called frontal fogs. These synoptic and mesoscale mechanisms are modified by local terrain features, such as topography, land and vegetation cover and, in turn, small-scale circulation. Thus, varied climatic regimes result in different distribution patterns of fog occurrence and development. In spite of its importance, the impacts of fog formation, development and distribution have not yet been properly assessed throughout the world. In particular, in Mexico there are very few specific studies on the topic and there are none of national character known to these authors.

1. Introduction Fog plays a major role not only in the hydrological cycle but also for many human activities, such

as agriculture and land, sea and air transport. Like most hydrometeorological phenomena, fog occurrence strongly varies with geographical location at both the local and the regional scale. In addi-

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tion, fog observation and recording methods are still very dependent on direct human presence and perception, since its detection by automated instrumentation is not a widespread practice in standard weather stations. These characteristics make it difficult to develop detailed fog climatologies that cover the whole world. The case of Mexico is no exception. Mexico is a country with a great variety of climatic regimes that reflect the uneven distribution of water resources throughout its territory. This is one reason why there are no comprehensive fog climatological studies on the national scale,

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and most fog data are scattered across diverse sources. Data availability is highly variable in terms of quality and spatial and temporal coverage, and most existing studies have a regional character in addressing specific aspects of the problem for particular applications. It can be pointed out, for instance, that the National Atlas of Mexico (Coll-Oliva 2007) does not include a single map devoted to fog. The main purpose of the present study is to develop a detailed fog climatology for Mexico. The procedure takes into consideration the inhomogeneous characteristics of the available database.

Fig. 1 Map of Mexico showing the locations of the 2,888 climatological stations used in the study. Each symbol on the map represents one station. Note that the non-uniformity in the spatial distribution of the stations is related to the difference in population density. Average data density is approximately one station per 680 km2. / Karte von Mexiko mit den 2.888 Klimastationen, deren Daten in der Studie berücksichtigt wurden. Jedes Symbol stellt eine Station dar. Die ungleichmäßige räumliche Verteilung der Stationen ist auf die unterschiedliche Bevölkerungsdichte zurückzuführen. Die durchschnittliche Datendichte beträgt etwa eine Station auf 680 km².

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These results are then used to classify regions of fog incidence in terms of their main meteorological and physical formation and development mechanisms. Thus, a secondary purpose is to show the potential of the methodology used when applied to different spatial and temporal scales.

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ues were obviously dubious or questionable, they were recalculated from the original daily records also provided by the SMN (Quintas 2000). After these data quality tests the resulting database, consisting of the average monthly number of fog days from 2,888 climatological stations with a nonuniform spatial distribution over the whole country (Fig. 1), was stored in electronic spreadsheets.

2. Data and Methodology The database used for the present study was standard climate records provided by the Mexican National Meteorological Service (SMN). Standard normal values are in this context defined as averages of weather observations – precipitation, air temperature and pressure at the surface etc. – over consecutive thirty-year periods. This is the most comprehensive and up-to-date public database of atmospheric records and products available in the country and is available from the National Water Commission (CNA 2007). In particular, standard average monthly fog days (number of days with fog occurrence as reported by an observer) were used. These were calculated from daily observations in the period from 1961 to 1990 at the 3,300 climatological stations managed by the SMN (CNA 1999). Due to the nature of the data, several corroboration checks and adjustments had to be performed. First, in the case that a given observing station had not reported a full thirty-year record, provisional normal values from data for at least ten years within the study period were used. Reports for stations with less than ten years of fog records were rejected from the database. Second, occasionally it was found that some stations had either reported incorrect geographical coordinates or changed their locations during the recording period. In both these cases, station coordinates were, whenever possible, adjusted against using historical data from the National Statistics, Geography and Informatics Institute (INEGI 2008a), or the corresponding data were not considered for the study. Third, in cases when normal val-

Fog climatologies and their geographical patterns were elaborated using the commercial contouring and surface mapping program Surfer V8.01 (Golden Software, Inc, Golden, CO, U.S.A.) in the MS Windows environment. This program allows to choose from different data interpolation schemes and to calculate “best fits” statistics. Various gridding methods were used to interpolate and represent the data in digitised base-maps of the Mexican territory – administrative boundaries, general physical features, topography etc. – obtained from the National Commission for Biodiversity Knowledge and Utilisation (Conabio 2007). The tested gridding methods included Kriging with different variograms (linear, quadratic and wave hole effect), triangulation with linear interpolation, and radial basis function (multiquadratic, inverse multiquadric and thin plate spline). Kriging is one of the more flexible methods and is useful for gridding almost any type of data set. In general, Kriging with a linear variogram is quite effective for most data sets, although it can be rather slow for larger data sets. Triangulation with linear interpolation is fast but, for small data sets, generates distinct triangular faces between data points in their graphic representations. The radial basis function (RBF) is actually a diverse group of data interpolation methods. All of the RBF methods are exact interpolators that employ an equation dependent on the distance between the interpolated point and the neighbouring sampling points. In terms of the ability to fit data and produce a smooth surface, the multiquadric function is considered by many to be best since it produces a good representation of small-sized samples (see for example Hardy

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1971). The performance of the different interpolation methods mentioned above was tested and evaluated, to finally produce fog-occurrence maps for both spatial (national, regional) and temporal (yearly, seasonal, monthly) scales.

3. Results The macro-spatial, national-scale fog-occurrence maps were produced with yearly and seasonal (winter, spring, summer and autumn) time

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resolution. Given the nature of the fog database (see section 2) it was relatively easy to adapt it to regional, smaller-scale studies provided that the proper interpolation methods were used. The choice of these methods depended very much on the spatial distribution of the data and their spatial density. In all cases it was generally found that the method of radial basis function, with an inverse multiquadric kernel used to define the set of weights to be applied to the data points when interpolating a grid node, rendered the best statistical results. Both the co-

Fig. 2 Relief map depicting the main orographic systems of Mexico: I. Sierra and Peninsula of Baja California, II. Sierra Madre Occidental, III. Sierra Madre Oriental, IV. Trans-Mexican Volcanic Belt, V. Sierra Madre del Sur, and VI. Sierra of Chiapas and Sierra of Guatemala. Black bullets show the different locations mentioned in the text: 1. Mexico Basin, 2. Teziutlán, 3. Ensenada, and 4. Las Alazanas (after Conabio 2007, INEGI 2008b and USGS/EROS 2006). / Reliefkarte mit den wesentlichen orographischen Systemen von Mexiko: I. Sierra und Halbinsel Baja California, II. Sierra Madre Occidental, III. Sierra Madre Oriental, IV. Transmexikanischer Vulkangürtel, V. Sierra Madre del Sur und VI. Sierras von Chiapas und Guatemala. Die schwarzen Punkte zeigen die verschiedenen im Text erwähnten Standorte: 1. Becken von Mexico, 2. Teziutlán, 3. Ensenada und 4. Las Alazanas (nach Conabio 2007, INEGI 2008b and USGS/EROS 2006).

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Fig. 3 Fog climatology for Mexico: distribution of the annual average number of fog days in the country. Isolines (red) are drawn every 50 fog days. The maximum annual average value for a single site is about 280 fog days per year and registered in the Teziutlán region. / Nebelklimatologie für Mexiko: Verteilung der durchschnittlichen Anzahl der Nebeltage pro Jahr. Die roten Isolinien bezeichnen jeweils einen Abstand von 50 Nebeltagen. Der maximale Mittelwert tritt im Gebiet von Teziutlán auf und beträgt ungefähr 280 Nebeltage pro Jahr.

efficient of determination (R2) and the F-test statistics confirmed this. It was also found, however, that sometimes other methods such as triangulation and Kriging resulted in more appealing graphic representations. The maps representing the annual and seasonal distributions of the average number of fog days in Mexico are shown in Figures 3 and 4. Average values of fog occurrences above 50 fog days per year are found more frequently in the country’s main orographic systems (see Fig. 2), i.e.the Sierra Madre Oriental, the Sierra Madre Occidental, the Sierra Madre del Sur, the Sierra of Chiapas and the Trans-Mexican Volcanic Belt that runs from east to west across south-central

Mexico. There are also some coastal regions with moderate incidence of fog, particularly the northwest coast of the Baja California Peninsula around the Ensenada region. The maps show the general lack of fog events in the northern desert and semi-desert regions. Several representative cases in the main fog regions mentioned above were analysed for seasonal and monthly variation. For the sake of brevity, only two of these regional cases are presented and discussed in further detail in the following section: the case of the Mexico Basin, where Mexico City is located; and an area of the southern Sierra Madre Oriental, around the Teziutlán region, where fog occurrence has its maximum value in the country.

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Fig. 4

Fernando García-García and Víctor Zarraluqui

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Fog climatology for Mexico: distribution of quarterly (seasonal) average number of fog days in the country: spring (March, April, May), summer (June, July, August), autumn (September, October, November), and winter (December, January, February). Isolines (red) drawn every 10 fog days. The maximum quarterly average value for a single site is 82 fog days per season – in the summer season in the Teziutlán region.

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Verteilung der durchschnittlichen Anzahl der Nebeltage pro Jahreszeit in Mexiko: Frühling (März, April, Mai), Sommer (Juni, Juli, August), Herbst (September, Oktober, November) und Winter (Dezember, Januar, Februar). Die roten Isolinien bezeichnen jeweils einen Abstand von 10 Nebeltagen. Der maximale Durchschnittswert für ein einzelnes Gebiet beträgt 82 Nebeltage je Saison – im Sommer in der Region Teziutlán.

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4. Discussion The general synoptic features involved in the formation and development of fog in Mexico can be inferred from analysing the seasonal fog climatologies presented in Figure 4. In most of the Mexican territory the rainy season occurs during summer and fall, between May and October (see Fig. 5). Thus, it is not surprising that the least number of fog days in the year occurs in spring, with a minimum in April. The occurrence of fog increases towards the end of spring, coinciding with the beginning of the rainy and hurricane seasons in both coastal areas of the country (in May on the Pacific coast, and in June on the Atlantic-Caribbean coast). The available sources of humidity during summer (maximum incidence of hurricanes and peak of the rainy season) also coincide with the maximum seasonal frequency of fog days all over the country, except for the Baja California Peninsula that presents a Mediterranean climate and precipita-

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tion regime. Autumn marks the transition between the end of the hurricane season and the beginning of the season of cold frontal systems affecting Mexico. The frequency of fog in the northern regions diminishes whilst the seasonal maxima concentrate in the mountainous, intertropical zone towards the south and east, in particular in the southern part of the Sierra Madre Oriental and in the Sierra of Chiapas. Finally, during winter fog incidence is almost exclusively found in the higher altitudes under the influence of the lower temperatures brought about by nortes, which also advect humidity from the Gulf of Mexico towards the continent (see below). The synoptical and mesoscale characteristics which prevail in Mexico and have an influence on the development of fog during the dry and rainy seasons are described in the following. In the summer, most of the Mexican territory gets under the influence of the trade easterlies along the southwestern end of the semi-permanent Bermuda high-

Fig. 5 Winter (December, January, February) and summer (June, July, August, September) climatologies of rain for Mexico, for the period 1958-2004 (after Vázquez 2007) / Niederschlagsverteilung in Mexiko im Winter (Dezember, Januar, Februar) und im Sommer (Juni, Juli, August, September), für den Zeitraum 1958-2004 (nach Vázquez 2007)

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pressure system (Fig. 6a). These east-northwest prevailing winds gather humidity over the Gulf of Mexico and, after moving over the Gulf Coastal Plains, are orographically forced to ascend to higher altitudes, thus producing typical cases of upslope fog along the Sierra Madre Oriental. Two examples of this are Teziutlán and Las Alazanas regions (see Fig. 2). This circulation pattern is commonly observed until autumn and also reaches the central part of the territory, including the Trans-Mexican Volcanic Belt to the south and the High Plateau to the north. On the western coast the circulation pattern is very much dependent on the dynamics of the intertropical convergence zone, since the development of a warm water pool over the northeast Pacific Ocean induces a region of deep convection and propitiates the formation of hurricanes that affect the Mexican coast.

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On the other hand, in the winter the region lies in the Subtropical High and the synoptic situation is dominated by a deep trough in low latitudes that defines an elongated area of relatively low atmospheric pressure along its axis or trough line (see Fig. 6b). This large-scale trough may include one or more closed circulations of low pressure, or cyclones, that produce northeasterly cold frontal systems which blow towards the shores of the Gulf of Mexico. These so-called nortes or northerns that result from an outbreak of cold air from the north are a common disruption of the mean synoptic condition in the winter. At this time of the year it is common to observe banks of stratus clouds near the coast of the Gulf of Mexico, both over the sea and over the coastal plains, that are advected inland by the dominant winds towards the

Fig. 6 Mean circulation in the Mexico region: (a) Mean patterns of omega-vertical wind at 700 hPa (shaded areas in Pa s-1) and horizontal wind at 925 hPa (arrows in m s-1) during summer. (b) Mean patterns of surface pressure (isobars in hPa) and horizontal wind at 925 hPa (arrows in m s-1) characteristic of the passage of a norte during winter (after Magaña et al. 1999) / Vorherrschende Zirkulation im Raum Mexiko: (a) Mittlere Stärke und Richtung des Vertikalwindes bei 700 hPa (schraffierte Fläche in Pa s-1) und des Horizontalwindes bei 925 hPa (Pfeile in m s-1) während des Sommers. (b) Mittlerer Luftdruck (Isobaren in hPa) und Horizontalwind bei 925 hPa (Pfeile in m s-1), charakteristisch für den Durchzug eines „Nortes“ während des Winters (nach Magaña et al. 1999)

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mountainous, high altitude (1,500 to 2,000 m a.s.l.) areas, thus producing orographically modified coastal fogs along the southern Sierra Madre Oriental, often accompanied by drizzle and rain. These frontal systems also advect cold air towards the central Mexican Plateau, where the flow is modified by the local orography. To illustrate the influence of mesoscale and local terrain features, two regional studies are discussed in the following. The first case corresponds to the Mexico Basin (Fig. 7), located in the Trans-Mexican Volcanic Belt, covering an area of about 30 km in radius centered roughly at downtown Mexico City and at an average altitude of 2240 m a.s.l. The basin is mostly surrounded by mountains, including some of the highest peaks in the country, like the Popocatépetl (5,462 m a.s.l.), the Iztaccíchuatl (5,286 m a.s.l.) and the Ajusco (3,930 m a.s.l.) volcanoes, except to the northeast. This latter area was originally occupied by Lake Texcoco, the largest of a system of interconnected lakes in the basin that have been systematically drained since colonial times during the last four hundred years (see Fig. 7b). Fog events in the Mexico Basin are not very frequent, reaching average maximum local values of up to 7 fog days per month throughout the year, except in the summer and early fall (June to September) when monthly maxima amount to up to 12 fog days per month. However, when present, fog has important economic impacts because it interferes with the operation of the major airport in the country, especially during winter when the fog events are more persistent and take longer to dissipate. In the winter the frontal systems described above advect cold air towards the central Mexican Plateau, the flow being modified by the local orography, reaching the Mexico Basin and producing typical radiative-advective and post-frontal fog episodes with visibilities of less than 400 m in the early morning hours. As the day passes by, solar radiation warms up the lower atmospheric layers and the fog dissi-

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pates within a few hours after sunrise. It is also evident, however, that most relative fog-occurrence maxima are associated to local terrain features that modify and reinforce the synoptic and mesoscale circulation. In particular, in fog areas close to foothills there is an obvious influence of a mountain-valley circulation that drains cool, humid air to the lower lands overnight. For the airport area, the additional presence of small water bodies like the remnants of Lake Texcoco provides an additional local source of low-level atmospheric humidity. The general characteristics of fog formation and the consequences of the location of the airport near small shallow lakes (less than 10 km2 in surface area) have been discussed by Magaña et al. (2002). The abovedescribed conditions, which are responsible for the occurrence of fog episodes that force the shutdown of all operations on the airport, agree well with the general features of the fog map for January presented in Figure 7a. The second regional case study corresponds to the Teziutlán area located in the southern Sierra Madre Oriental, where the maximum annual fog occurrence in the country is observed. Fog in this region has been studied in some detail from different viewpoints that include: climate-vegetation relationship (Maderey et al. 1989; Ern 1972; Lauer 1978; Vogelmann 1973), hydrological balance (Barradas 1983), chemical characteristics and effects of fog water deposition (Báez et al. 1998), and some meteorological (Fitzjarrald 1986) and microphysical aspects (García and Montañez 1991). The incidence of fog is particularly high during autumn and winter, with a maximum local average of up to 80 fog days per season. Monthly fog maps for the Teziutlán region for October and January, considered representative of these two seasons, are shown in Figure 8. These maps adequately represent the abovedescribed synoptic-mesoscale situation for the formation of upslope fog throughout the year, reinforced by the advection of stratus clouds from the Gulf of Mexico and the coastal plains towards

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Fig. 7 a) Fog climatology for the Mexico Basin (left panel): average number of fog days in January. Each symbol on the map represents one station, with data density being approximately one station per 25 km 2. The underlying relief map indicates the approximate altitude of the basin floor (2,240 m a.s.l.) as well as the maximum elevation shown in the map of about 3,000 m a.s.l. (the Sierra de Guadalupe) to the north. b) Vegetation cover and land use in the Mexico Basin (right panel). Note that most of the region is occupied by the metropolitan area of Mexico City (after INE 2007). a) Nebelklimatologie des Mexiko-Beckens: durchschnittliche Anzahl an Nebeltagen im Januar. Jedes Symbol auf der Karte steht für eine Station mit einer Datendichte von etwa einer Station pro 25 m². Die darunterliegende Reliefkarte zeigt die ungefähre Höhe des Beckenniveaus (2.240 m ü. NN) sowie die maximalen Höhen im Norden, die ungefähr 3.000 m ü. NN erreichen (die Sierra de Guadelupe). b) Vegetationsbedeckung und Landnutzung im Mexiko-Becken. Ein Großteil der Region gehört zum Agglomerationsraum von Mexiko City (nach INE 2007).

the region in the winter. In addition, they can be easily related to modifications due to local features associated to the complex, high mountain terrain and vegetation of the region.

5. Conclusions A fog climatology developed for Mexico at various spatial and temporal scales was developed with

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Fig. 8 Fog climatology for the southern Sierra Madre Oriental region of Teziutlán: average number of fog days in: (a) October (monthly maximum local average of 24 fog days) and (b) January (monthly maximum local average of 26 fog days). The maximum local annual average is about 280 fog days. Each symbol on the map represents one station, with data density being approximately one station per 160 km 2. The underlying relief map indicates that fog tends to concentrate in the valley, the approximate altitude of Teziutlán is 1,920 m a.s.l.; it is surrounded by elevations of up to about 3,000 m a.s.l. to the west. / Nebelklimatologie für die südliche Sierra-Madre-OrientalRegion von Teziutlán: durchschnittliche Anzahl der Nebeltage in: (a) Oktober (das monatliche Maximum des örtlichen Mittels beträgt 24 Nebeltage) und (b) Januar (das monatliche Maximum des örtlichen Mittels beträgt 26 Nebeltage). Der maximale lokale jährliche Durchschnitt beträgt etwa 280 Nebeltage. Jedes Symbol auf der Karte steht für eine Station, die Datendichte beträgt etwa eine Station pro 160 km². Die darunterliegende Reliefkarte zeigt, dass Nebel zu einer Konzentration in den Tälern tendiert; die Höhenlage von Teziutlán beträgt ungefähr 1.920 m ü. NN, die Tallagen sind im Westen umgeben von Erhebungen mit bis zu 3.000 m ü. NN.

historical weather records. The corresponding database was constructed using monthly average values of daily observations acquired over a thirtyyear period at about 2,900 climatological stations of the Mexican National Meteorological Service network. Different data interpolation schemes and their performance were tested and evaluated in

order to produce fog-occurrence maps on various spatial (national, regional) and temporal (yearly, seasonal, monthly) scales. It can be concluded that the program Surfer is a powerful tool for this type of studies, provided the interpolation methods are adequately chosen given the general characteristics of the data. In particular, radial basis function

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interpolation using the multiquadric method gave good statistical results for the national scale but lacked resolution for isolated data points. For the regional, better-resolved scale, this method provided excellent results. It should be mentioned that in some cases triangulation gridding with linear interpolation resulted in more appealing graphic representations. Needless to say, quality-assurance techniques applied to the data were vital for obtaining these satisfactory results.

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5. References Barradas, V.L. 1983: Capacidad de captación de agua a partir de la niebla en Pinus montezumae Lambert, de la región de las grandes montañas del estado de Veracruz. – Biótica 8 (4): 427-431 Báez, A.P., H.G. Padilla and F. García-García 1998: Fog Water Chemistry at High Altitudes in Mexico. – In: Schemenauer, R.S. and H. Bridgman (eds.): First International Conference on Fog and Fog Collection: Proceedings. – North York, Ontario: 77-80

Major regions of fog incidence in terms of the main meteorological and physical formation and development mechanisms were identified. These results show the high variability of fog incidence over the Mexican territory, both at the spatial and temporal scales, and the difficulties that this implies for its proper and accurate handling, graphic representation and analysis. Finally, the described graphical representations were used to better understand synoptic, mesoscale and local features related to the formation and development of fog. In particular, from regional studies local characteristics related to topography and land cover, including the presence of nearby water bodies, can be inferred as modifying factors of the larger-scales atmospheric conditions.

CNA 1999: Normales Climatológicas Estándar y Provisionales 1961-1990. – Unidad del Servicio Meteorológico Nacional, Subdirección General Técnica, Comisión Nacional del Agua. – Mexico City. – available on CD

The results presented here represent the first attempt to develop a detailed national and regional fog climatology for Mexico. In the future, the methodology will be tested and may be extended to other highly localised hydrometeorological phenomena, such as frost and hail incidence, for which there is also a lack of detailed observational data.

Ern, H. 1972: Estudio de la vegetación en la parte oriental de México central. – In: Comunicaciones Proyecto Puebla-Tlaxcala 6: 1-6

Acknowledgements

Hardy, R.L. 1971: Multivariate Equations of Topography and Other Irregular Surfaces. – In: Journal of Geophysical Research 76: 1905-1915

The authors are indebted to Jorge Luis VázquezAguirre and Dr. Víctor O. Magaña-Rueda for providing the original maps presented in Figures 5 and 6. The underlying relief maps appearing in Figures 7a and 8 were drawn with digitised data provided by María de Lourdes Godínez-Calderón.

CNA 2007: Normales Climatológicas Provisionales 1971-2000. – Unidad del Servicio Meteorológico Nacional, Comisión Nacional del Agua. – Mexico City. – http://smn.cna.gob.mx/productos/normales/ estacion/normales.html Conabio 2007: Metadatos y Cartografía en Línea. – Subdirección de Sistemas de Información Geográfica, Comisión Nacional para el Conocimiento y Uso de la Biodiversidad. – Mexico City. – http:// conabioweb.conabio.gob.mx/metacarto /metadatos.pl Coll-Oliva, A. (ed.) 2007: Nuevo Atlas Nacional de México. – Instituto de Geografía, Universidad Nacional Autónoma de México. – Mexico City

Fitzjarrald, D.R. 1986: Slope Winds in Veracruz. – Journal of Climate and Applied Meteorology 25: 133-144 García-García, F. and R.A. Montañez 1991: Warm Fog in Eastern Mexico: A Case Study. – Atmósfera 4: 53-64

INE 2007: Vegetación y Uso del suelo 2000: Distrito Federal y Estado de México. – In: J.L. PérezDamián and I. Ramírez del Razo (eds.): Mapas del Medio Ambiente de México. – Dirección General de Investigación del Ordenamiento Ecológico y

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Conservación de los Ecosistemas, Instituto Nacional de Ecología, Secretaría del Medio Ambiente y Recursos Naturales. – Mexico City. – http:// www.ine.gob.mx/emapas/index.html INEGI 2008a: Archivo Histórico de Localidades. – Sistemas Nacionales Estadístico y de Información Geográfica, Instituto Nacional de Estadística, Geografía e Informática. – Mexico City. – http:// mapserver.inegi.gob.mx/dsist/ahl2003/index.cfm

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and Science Data Center, U.S. Geological Center. – Sioux Falls, SD. – http://edc.usgs.gov/products/ elevation/gtopo30/gtopo30.html Vázquez-Aguirre, J.L. 2007: Variabilidad de la Precipitación en la República Mexicana. – M. Sc. Thesis, Universidad Nacional Autónoma de México. – Mexico City. – Available from TESIUNAM Database, Clasif. 001-03060-V2-2007 at http:// www.dgbiblio.unam.mx/

INEGI 2008b: Información Geográfica: Aspectos Generales del Territorio Mexicano. – Sistemas Nacionales Estadístico y de Información Geográfica, Instituto Nacional de Estadística, Geografía e Informática. – Mexico City. – http:// www.inegi.gob.mx/inegi/default.aspx?s=geo&c=909

Vogelmann, H.W. 1973: Fog Precipitation in the Cloud Forests of Eastern Mexico. – In: BioScience 23: 96-100

Lauer, W. 1978: Tipos ecológicos del clima en la vertiente oriental de la Meseta Mexicana. – In: Comunicaciones Proyecto Puebla-Tlaxcala 15: 235-244

Summary: A Fog Climatology for Mexico

Maderey, R.L.E., H. del Castillo G. y F.J. Cruz N. 1989: Distribución del rocío y de la niebla: Fuentes de humedad para la vegetación en la República Mexicana. – Ciencia 40: 223-231 Magaña, V., J.L. Pérez, J.L. Vázquez, E. Carrisoza y J. Pérez 1999: El Niño y el Clima. – In: Magaña-Rueda, V.O. (ed.): Los Impactos de El Niño en México: 23-66. – Universidad Nacional Autónoma de México, Inter-American Institute for Global Change Research, Secretaría de Gobernación, Secretaría de Educación Pública-Consejo Nacional de Ciencia y Tecnología. – Mexico City. – online available at http://www.atmosfera. unam.mx/ editorial/libros/el_nino/ Magaña-Rueda, V.O., A. García-Reynoso, E. Caetano, A. Jazcilevich, F. García-García y L.G. Ruiz-Suárez 2002: Estudio Preliminar para Determinar el Efecto en la Formación de Niebla y en la Calidad del Aire debido a la Creación de Cuerpos de Agua en la Ubicación del Nuevo Aeropuerto Internacional de la Ciudad de México. – Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México. – Technical Report. – Mexico City Quintas, I. 2000: ERIC II: Extractor Rápido de Información Climatológica. – Instituto Mexicano de Tecnología del Agua, Comisión Nacional del Agua, México. – Version 2. – available on CD USGS/EROS 2006: Global 30 Arc Second Elevation Data GTOPO30. – Earth Resources Observation

The results of a fog climatology developed for Mexico are presented. The study is based on standard average monthly fog days calculated from daily observations acquired over the thirty-year period from 1961 to 1990, at the 3,300 climatological stations of the Mexican National Meteorological Service network. After applying corroboration checks and adjustments to the data, different interpolation schemes and their performance were tested and evaluated in order to produce fog-occurrence maps on various spatial (national, regional) and temporal (yearly, seasonal, monthly) scales, using the commercial contouring and surface mapping program Surfer. For the data interpolation, it was found that the method of radial basis function with an inverse multiquadric kernel rendered the best statistical results. These indicate that average values of fog occurrences, of more than 50 and up to 280 fog days per year, are found more frequently in the country’s main orographic systems. It is also found that the maximum seasonal frequency of fog days in the country occurs during summer, thus coinciding with the peak of the rainy season. These results are also analysed in view of the synoptical and mesoscale characteristics that prevail in Mexico during the dry and rainy seasons and then used to classify major regions of fog incidence in terms of their main meteorological and physical formation and develop-

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A Fog Climatology for Mexico

ment mechanisms. Finally, two regional case studies are presented with the aim to illustrate the influence that mesoscale and local terrain features, such as topography and vegetation and land cover, have on the formation and development of fog. This study represents the first attempt towards a comprehensive and detailed fog climatology for Mexico. It is concluded that fog incidence over the Mexican territory shows high variability at both spatial and temporal scales, showing the difficulties that this implies for its proper and accurate handling, graphic representation and analysis.

Zusammenfassung: Eine Nebelklimatologie für Mexiko Dieser Artikel beschreibt die Ergebnisse einer Studie zur Nebel-Klimatologie Mexikos. Die Studie basiert auf der Anzahl der durchschnittlichen monatlichen Nebeltage, die berechnet wurde aus Daten von täglichen Erhebungen an 3.300 Klimastationen des mexikanischen Wetterdienstes, welche über eine 30-jährige Periode, von 1961 bis 1990, durchgeführt wurden. Nach Plausibilitätsprüfungen und Anpassungen der Daten wurden verschiedene Interpolationsverfahren und deren Ergebnisse getestet und ausgewertet, um Karten zu Nebel-Häufigkeiten auf verschiedenen räumlichen (national, regional) und zeitlichen (jährlich, saisonal, monatlich) Ebenen zu erstellen. Hierzu wurde das kommerzielle Konturierungs- und Kartenoberflächenprogramm Surfer verwendet. Für die Interpolation wurde festgestellt, dass das Verfahren der radialen Basisfunktion mit einem inversen multiquadratischen Kern die besten statistischen Ergebnisse lieferte. Diese zeigen, dass die Durchschnittswerte hinsichtlich des Auftretens von Nebel mit mehr als 50 bis zu 280 Nebeltagen im Jahr häufiger in den wesentlichen Gebirgszügen des Landes liegen. Ebenfalls fand man heraus, dass die maximale saisonale Häufigkeit von Nebeltagen im Land während des Sommers auftritt und somit identisch mit dem Höchststand der Regenzeit ist. Diese Ergebnisse wurden außderdem mit Blick auf die synoptischen und mesoskalen Merkmale analysiert, die in Mexiko während der Trocken- und Regenzeit vorherr-

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schen, um die Hauptgebiete von Nebelvorkommen bezüglich ihrer vorherrschenden meteorologischen und physikalischen Konstellation sowie ihrer Entstehungsmechanismen zu typisieren. Schließlich werden zwei regionale Fallstudien präsentiert mit dem Ziel, den Einfluss der mesoskalaren und lokalen Besonderheiten des Raumes – wie Relief, Vegetation und Bodenbedeckung – auf die Bildung und Weiterentwicklung von Nebel deutlich zu machen. Die Studie stellt den ersten Versuch einer umfassenden und detaillierten Nebel-Klimatologie für Mexiko dar. Sie macht deutlich, dass das Auftreten von Nebel über Mexiko sowohl auf räumlicher wie auch auf zeitlicher Ebene eine hohe Variabilität aufweist. Ebenso zeigt sie die Schwierigkeit, diese Daten sachgerecht und genau aufzubereiten, graphisch darzustellen und zu analysieren.

Résumé: Une climatologie du brouillard pour le Mexique Les résultats d’une climatologie du brouillard développée pour le Mexique sont présentés. L’étude est basée sur le nombre mensuel moyen des jours de brouillard calculé à partir des observations quotidiennes acquises au cours des trente années de 1961 à 1990 sur 3300 stations climatologiques du réseau du Service Météorologique National du Mexique. Après l’application des contrôles de corroboration et des ajustements des données, les différents régimes d’interpolation et leurs performances ont été testés et évalués afin de produire des cartes de l’apparition du brouillard sur diverses échelles géographiques (nationale, régionale) et temporelles (annuelle, saisonnière, mensuelle), en utilisant « Surfer », le logiciel commercial de contouring et de cartographie de surface. Pour l’interpolation des données, il a été constaté que la méthode de la fonction d’une base radiale avec un noyau inverse à élévation multiple au carré rend les meilleurs résultats statistiques. Ceux-ci indiquent que ces valeurs moyennes des événements de brouillard, allant de plus de 50 jusqu’à 280 jours de brouillard par an, se retrouvent plus fréquemment dans les principaux systèmes orographiques du pays. Il est également constaté que la fréquence saisonnière maximale de jours de brouillard dans le pays se

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produit au cours de l’été, ce qui coïncide avec le maximum de la saison des pluies. Ces résultats sont également analysés en vue des caractéristiques synoptiques et à échelle moyenne qui règnent au Mexique pendant la saison sèche et la saison des pluies, et qui sont ensuite utilisés pour classer des grandes régions d’apparition de brouillard en termes de leurs mécanismes principaux de formation et de développement météorologiques et physiques. Enfin, deux études de cas régionales sont présentées dans le but d’illustrer l’influence que les traits du relief, à échelle moyenne et au niveau local, telles que la topographie, la végétation et la couverture de terre, exercent sur la formation et le développement de brouillard. Cette étude représente la première tentative d’établir un inventaire exhaustif et détaillé de la climatologie du brouillard pour le Mexique. Il est conclu que l’apparition de brouillard sur le territoire mexicain montre une variabilité élevée aux échelles spatiale ainsi que temporelle, démontrant les difficultés que cela implique pour son traitement approprié et soigneux au niveau d´une représentation graphique et de l’analyse.

Resumen: Una climatología de niebla para México Se presentan los resultados de una climatología de niebla elaborada para México. El estudio se basó en normales climatológicas mensuales de días con niebla, calculadas de observaciones diarias realizadas durante el período de treinta años 1961-1990 en las 3,300 estaciones climatológicas pertenecientes a la red del Servicio Meteorológico Nacional. Luego de aplicar varias pruebas de corroboración y ajustes a los datos, se probó y evaluó el desempeño de diferentes esquemas de interpolación de datos para así producir mapas climatológicos de ocurrencia de niebla, tanto en escala espacial (nacional, regional) como temporal (anual, estacional, mensual), mediante la utilización del programa comercial Surfer. Se encontró que el método de función radial con kernel multicuádrico da los mejores resultados para la inter-

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polación de los datos. Estos resultados se utilizaron para clasificar las principales regiones de incidencia de niebla en términos de los principales mecanismos meteorológicos y físicos para su formación y desarrollo. Estos resultados muestran que los valores promedio más grandes de ocurrencia de niebla, de entre 50 y hasta 280 días con niebla al año, se dan en los principales sistemas montañosos del país. También se observa que la máxima frecuencia de días con niebla en el ámbito nacional ocurre en el verano, coincidiendo con la temporada de lluvias. Los resultados también se analizaron en términos de las características de la circulación, tanto sinóptica como de mesoescala, prevaleciente en México durante las temporadas de secas y lluvias, para así clasificar las regiones de niebla identificadas con respecto a los principales mecanismos físicos y meteorológicos de formación de niebla. Finalmente, se presentan dos casos de estudio a nivel regional con el fin de ilustrar la influencia que la circulación de mesoescala y las características locales del terreno, tales como la topografía y la cubierta de vegetación y el suelo, tienen en la formación y el desarrollo de la niebla. El estudio representa un primer esfuerzo de obtener una climatología detallada y completa de niebla para México. Se concluye que existe una gran variabilidad en la incidencia del fenómeno sobre el territorio mexicano, tanto en escala espacial como temporal, y se muestran las dificultades que esta variabilidad acarrea para su adecuado manejo, representación gráfica y análisis.

Dr. Fernando García-García, Víctor Zarraluqui, Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica, Ciudad Universitaria, 04510 México, D.F., México, [email protected], [email protected]

Manuskripteingang: 07.01.2008 Annahme zum Druck: 21.04.2008