Forest fires, especially in southern Europe, play a significant role from an environmental point of view. In recent decades this problem has been analyzed in a synoptic sense by Brotak and Refsnyder (1977) and Brotak (1980) deducing that particular synoptic situations are associated with the fire activity. In an operative sense, there are risk indices to describe the possibility of ignition (Palmieri and Cozzi 1983). Also, there are bibliography models such as Lower Atmospheric Severity Index (Haines 1988) or BEHAVE (Andrews and Bradshaw 1991) that describe the fire development. As a first scheme, two problems or processes must be considered: ignition and development. For a given zone, the problem of ignition may be analyzed in terms of forecasting of the daily number of forest fires (E. García Diez et al. 1994; A. García Diez et al. 1995, 1996). This prediction is more useful than other qualitative indices. On the other hand, the fire development is meteorologically described on the basis of wind behavior. It is accepted by the scientific community that fire ignition is associated with temperature, humidity, and/or other parameters, and fire development is associated with wind. To obtain a risk scale, in an earlier paper (García Diez et al. 1993) a classification in four types of day was established on the basis of atmospheric stability and humidity in lowest atmospheric layer. Particularly, the stability in the lower layers is a good parameter for detecting possible upward motions. If other parameters are fixed, the heat released by a fire implies an upward movement or vertical divergence of the air that itself organizes, according to the equation of continuity, a horizontal convergence of air below. Evidently, this convergence isa wind that could play a more important role than the synoptic wind. For this reason, the fire development would be explained in terms of the stability in the local atmospheric column. This hypothesis will be analyzed here.
In this study, fire and meteorological data are analyzed from the northwest region of Spain, that includes the provinces of Asturias, Cantabria, La Coruña, Lugo, Orense, Pontevedra, León, and Zamora. In these areas, the problem of forest fires requires a statistical study due to the very high number of forest fires registered per day. For each year only the months of July, August, and September are considered because they are the ones with the most fire activity.
A critical point in this study is to investigate, on the basis of a model that explains the ignition for all fires (the burned area is not included), how the type of day is meteorologically important to the development of large fires. As will be seen, large fires occur principally during days with high risk. Consequently, the meteorological conditions associated with development (essentially the synoptic wind) are coupled with the meteorological conditions for ignition.
Stability in the 700–850-hPa layer
If the atmosphere is incompressible then ∇·v = 0, so the flux is not divergent. Because the problem is defined in three dimensions, horizontal convergence forces vertical divergence and reciprocally. If this process happens in the lowest atmospheric level, then the vertical divergence is an upward movement (Fig. 2). Generally speaking, the atmosphere presents stability in all layers, particularly in the one considered (850–700 hPa).
If one is able to demonstrate that e and e′ are the same, the only difference between them is a constant (ΔP), and then the stability criterion given in (4) for e′ is the same for e. So, when the defined value of the stability (e) is low, the energy required to raise the particles obviously decreases.
Thus, when the air is heated by fire at the initial instant, this energy is transferred to the overlaying atmosphere and transformed into vertical kinetic energy. The maximum height of vertical motion depends inversely upon e. According to the continuity Eq. (6), the elevation of the air mass implies the convergence of air over the soil. This convergence, for a given fire, depends only on the value of e, and the higher the velocity of the bubble is, the stronger the convergence in low levels will be. This implies, in an operative sense, that during days with low e values the flames will be higher and the wind stronger due to surface convergence.
In conclusion, the ignition forces an upward motion, which will be stronger when the lowest atmospheric layer (i.e., 850–700 hPa) is unstable or presents low stability (low e values).
Humidity at low levels
Clearly, a day with a highD value will be more adventageous to fire than a day with a low D value. Experimentally, as can be easily verified, hot months have more insolation and evaporation and they are less stable than cold months (a very hot soil destabilizes the atmospheric column above it). Therefore, these months present more fire activity, and the antifire campaigns start in summertime. Sometimes, the conditions described here are not present altogether in summer. There are some locations in the north of Spain such as Guipúzcoa in which these conditions occur in months like February and March because of its local Monzonic circulation. In winter the sea surface temperature (SST) is higher than land temperature and, consequently, this thermal gradient implies a low-level land–sea circulation that introduces dry air over area. This is a good example for demonstrating that the temperature alone does not determine fire risk.
Notice that these two parameters, e and D, are defined homogeneously in energy per mass (kJ kg−1). This is an important property, because if other parameters (wind, solar radiation, etc.) were considered, these new parameters could be easily introduced: in terms of energy per unit of mass.
Categorization of days and DFR
Statistically, when several summer periods are considered, the average values obtained for (e, D) are included in the intervals 6.1–7.1 and 11.4–13.8, respectively. Because the error in radiosonde record is higher, more precision in this point would not be useful. On other hand, the values e = 6 kJ kg−1 and D = 12 kJ kg−1 are very coherent physically with the results for the area of the U.S. standard atmosphere (NOAA-NASA-USAF 1976).
In this way, four types of days are determined, as seen in Table 1.
Therefore, at the beginning of the day, each day can be easily classified: if it is a type I day the risk is “very high”; a type III day, the risk is “high”; a type IV day, “low”; a type II day, “very low.”
Categorization of days: Ignition and development
These NDFR values must be interpreted in a relative sense: a type I day tends to present four times more fire activity than a type II, and so on. This is a very important point in statistical partitioning processes. The four types of day that have been defined present the same “risk interval.” Consequently, the partition established in the (e, D) plain seems to be a good one.
Forest fire size is now taken into consideration. Several series Ck (92 days, from 1 July to 30 September fork years) are considered and assigned to each element of another series Fk (number of registered fires per day during year k). Each series Ck is divided in four classes (types of days) and for each class a property (DFRk) is defined. This property becomes an ordered relation (15), and it is well defined if all days and all registered fires each day are considered for any year. Thus, the NDFR is defined as a universal property of each type of day.
The next question arises as follows. If an unusual and restrictive selective criterion is introduced (burned surface) over the number of registered fires, will the order of the relationship for the DFR be maintained? The following different possibilities must be considered.
If the unusual criterion applied on Ck and Fk selects all elements (as an example: let the fires with a burnt surface be greater than or equal to 0), the relational order is obviously conserved.
If the unusual criterion selects a few elements of each type (5%–10%)—that is, when only large fires are considered—the order relationship for DFR must break down.
Therefore, when some forest fires are chosen with a very restrictive criterion, the most likely situation is that the order between the DFRs may be different than (15). If this relation is not verified, it must be deduced that the daily fire risk describes well all the fires (ignition) but not the large ones. Consequently, the meteorological conditions considered in the definition of DFR describe the ignition well but do not describe correctly the development of fires.
If (15) holds, it may be deduced that the days with a higher number of fires are also those that register the largest ones. This means that DFR is a good way to analyze simultaneously ignitions and development of fires; that is, the meteorological conditions of ignition and development are not very different.
However, the question arises about what happens if the relation of DFR for large fires is stronger than the one for all fires? As will be seen later, this is the principal result of this paper. A strong connection between ignition and development conditions is found and, more precisely, the classification established for the meteorological conditions in the model are better associated with fire development than with ignition.
The provinces considered in this work have two main characteristics in terms of fire activity: (i) They typically have a high number of forest fires per day and (ii) the burned surfaces are small (<104 m2). For this last reason, a realistic classification for large fire may be those fires with burned surface areas of at least 10 ha. Other studies were conducted on the basis of burned surfaces of at least 15 ha and 20 ha.
The results obtained are shown in Table 2 in which the DFRs appear for the respective subseries of big fires (a, c, and e) versus the DFRs for the respective complementary subseries—that is, for fires less than these in size (b, d, and f).
Using the definition (16), the NDFRs are calculated, obtaining the results in Table 2 for all the years and each size of fire.
Graphically, this result is shown in Fig. 4. Thus, the meteorological conditions corresponding to I and III days, which are the most favorable for ignition, also are more favorable for development. And, in contrast, the conditions corresponding to days II and IV, that are the less favorable to the ignition, are even less favorable for large fire development.
The most important difference between classes I and III, and between II and IV, respectively, is humidity. When a fire has been ignited, it generates its own circulation and the stability of the initial atmospheric column becomes less important: the humidity becomes the most important parameter for development.
Consequently, the development process is better described than ignition by the DFR. Physically, it means that the meteorological conditions that favor the ignition of forest fires are not very different from the conditions that favor the development.
In this paper the dualism between ignition and development of forest fires has been analyzed. On the basis of statistical considerations, a parallel between meteorological conditions for both processes is shown. More precisely, DFR defined by considering the ignition risk appears to be a very good indicator of the development risk. Consequently, the colloquial opinion in both the operational and scientific communities that “days with a high number of fires are also those that contribute to more burned area,” is shown to be true for fires and meteorology investigated in northwestern Spain.
We wish to acknowledge the Spanish Government for the financial support under the project CICYT AMB 94-0701.
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Types of days.
DFR for the different sizes of fires.