1. INTRODUCTION 

Each year, millions of dollars are spent by state and federal agencies on wildland fire suppression and prescribed burns. Real-time daily forecasts of fire weather parameters are a necessary and required component to aid in both of these activities. Atmospheric variables which are most directly related to fire include temperature, moisture, wind, and convection. Convection is a key factor as most wildland fires are started by lightning. Dry fuels, low relative humidity, and a high incident of lightning strikes can lead to a high fire season, but a season with cooler temperatures, more moisture, and less convection can considerably reduce the number of fire starts. Wildland fire activity is closely linked to interannual variability of climate. A better understanding of the synoptic patterns associated with past wildland fire starts will both improve fire weather forecasts and outlooks, and allow agency managers to allocate their resources more efficiently.

In this study, upper level fields of temperature, geopotential height, relative humidity, and u,v winds are analyzed in relationship to seasonal characteristics of natural fire starts in Nevada. These are some of the variables commonly considered in National Weather Service (NWS) operational fire weather forecasts. Previous studies have examined large-scale synoptic patterns over areas such as the southwest U.S. and central intermountain region (e.g., Schroeder et al., 1964; Hull et al., 1966). While these results provide useful information regarding general atmospheric pattern characteristics in association with fire starts, they lack the detail needed for a forecaster to apply at the state and even smaller district level. This study 1) provides analysis of upper air variables in relationship to seasonal fire starts, and 2) focuses on these relationships at the district level within Nevada. With its topographical diversity and spatial extent, different areas of Nevada are influenced by a variety of distinct synoptic patterns. The ultimate goal is to determine patterns relevant to occurrences of unusually high or low fire starts, which a fire weather forecaster would find useful in making daily forecasts.

2. DATA

The data used in this study are taken from the NOAA National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) reanalysis global data set (Kalnay et al., 1996). It consists of several derived daily variables including air temperature, geopotential height, relative humidity, and u,v winds for mandatory levels on a 2.5 degree spatial grid. While this grid would be considered coarse for daily operational forecasts, from a monthly and seasonal climate perspective it provides very useful information, owed in part to the consistent manner in which the variables were derived for the entire reanalysis period (1957-96). Our present focus is on the period 1980 -1996, which coincides with readily available fire start data for Nevada. Since much of Nevada is located at or above the 850 hPa pressure level, only levels above 850 hPa are considered (i.e., 700, 600, 500, 400, and 300 hPa). Analysis was restricted to the months of July and August, which represent over 70% of the total annual number of fire starts.

The Bureau of Land Management (BLM) provided a data set of natural fire starts that began on Nevada BLM land during 1980-1996 (Figure 1). Nearly 83% of the land in Nevada is government owned, of which 84% is controlled by BLM. So while this data set does not include occurrences on U.S. Forest Service, Bureau of Indian Affairs, military, state, and other government and public land, it does represent the majority of fire starts occurring in Nevada.

3. METHOD

Two 2.5 degree grid cells were chosen for analysis that were located in the southeastern and northwestern parts of Nevada (Figure 1). These cells approximate the Las Vegas and Winnemucca-Susanville fire districts, respectively, with some overlap along adjoining districts. July and August fire starts were totaled for all years in each grid cell. The totals were ranked by year for each month to determine the five years with the highest number of fire start s and the five years with the lowest number of fire starts. It is interesting to note that for both months and both grid cells there were no identical sets of high or low years, and that typically only one or two years overlapped both districts for a month. In one case, a year which was low in fire starts in the Winnemucca-Susanville district was high in Las Vegas. This indicates the diversity of fire seasons for different regions within the state of Nevada.

For each set of ranked years, July and August composites were generated for each reanalysis variable. This was done by simply averaging together the five years. For example, the set of high fire start years in August for the Las Vegas district is 1980, 1984, 1988, 1994, and 1996. Clearly, this is a small sample size, with the potential for one year to dominate the mean. In order to determine the representativeness of the mean, additional parameters were examined including the standard deviation, median, and pseudo-sigma. These values indicated general agreement between the classical moment measures and the resistant and robust methods. For now, we believe these patterns to be reliable, and in the future hope to increase the sample size with additional years of fire start data.

In addition to computing composite means for the various sets of five years, a 17-year (1980-96) mean for each variable was also determined. This allows anomalies to be computed for each five year set by subtracting the long-term mean from the composite average. Using composite patterns and anomalies allows for the determination of both pattern shape and magnitude differences between high and low start years and the long-term mean. This provides a fire weather forecaster with a climatological guidance tool.

There are numerous features which can be examined with this large set of variables. Composite patterns for high and low start years in a district can be compared, along with their magnitude differences. Features at each pressure level may be uniquely important (e.g., 700 hPa and 500 hPa humidity). Patterns that affect fire starts in northern Nevada may be substantially different than those in the south, but perhaps not for all circumstances. Because of limited space available, only a few relevant results will be shown.

4. RESULTS

4.1 Air Temperature

Ideally, positive surface temperature anomalies associated with dry conditions should be most conducive to high fire starts. During the month of July, air temperature anomalies over the Las Vegas district were positive through all levels during high fire start years. At the 700 hPa level (Figure 2a) a +1.0°C anomaly was noted over the district with the largest anomaly (+1.8°C) centered over western Oregon. At 500 hPa (not shown) the anomaly over the district is +0.2°C. During low fire start years, the pattern was reversed (Figure 2b) with a -0.8°C anomaly over the district and a -2.0°C anomaly centered over eastern Oregon. The 500 hPa anomaly (not shown) over the district is +0.2°C. All of these results are similar to those found when examining air temperature anomalies in association with the Winnemucca-Susanville district fire starts.

As mentioned previously, convection is a key factor in producing fire starts within Nevada. Atmospheric instability is one element that must be present in order for convection to develop. For the Las Vegas district, the lapse rates (700 hPa - 500 hPa) for high and low start years during July were -7.6°C/km and -7.1°C/km, respectively. Similar results were also found during the month of August, and for the Winnemucca-Susanville district. These lapse rates indicate that the atmosphere was conditionally unstable during both high and low start years. However, during high fire start years the atmosphere appears to be more unstable than the corresponding low start years.

4.2 Geopotential Height

Geopotential height anomalies over both districts in July are positive during high start years, and negative during low start years. The primary anomaly centers for both districts were located over the Pacific northwest (Figures 3a and b). The magnitude of the positive anomalies during July high start years increased with height, nearly doubling between 700 and 300 hPa (e.g., 12 meters at 700 hPa and 24 meters at 300 hPa over the Las Vegas district). However, negative anomalies remain nearly constant with height in both districts during low start years. As with temperature, the largest anomalies are seen over the Pacific northwest. In the Winnemucca-Susanville district, heights are positive during high start years and negative during low start years, with similar magnitudes at all levels (±9 meters). In the Las Vegas district, anomalies are positive (+9 meters) and change little with height during high start years, but are also positive and change considerably with height (+3 meters at 700 hPa and +15 meters at 300 hPa) during low start years. Both of these situations are related to a high pressure center which commonly develops over Arizona, New Mexico and west Texas during late July and August. However, there is a notable northwestward shift in the high pressure center during the high start years. This shift affects the temperature and moisture advection patterns promoting convective activity across the state of Nevada.

4.3 Relative humidity

Nevada experiences two general classes of thunderstorms - low and high based. Low based thunderstorms (~4,000 meters) typically produce precipitation, so regardless of the amount of lightning occurrence, it is often difficult to start a fire under this circumstance. High based thunderstorms (~4,500 meters) typically produce little precipitation (in part due to considerable evaporation between the cloud base and surface) but can produce considerable lightning which in turn ignites fires.

For both July and August, a clear distinction can be made between high and low fire start years in relation to humidity. At the 700 hPa level for July, low start years are drier than normal and continue to get increasingly drier with altitude (ranging from -1% at 700 hPa to -7% at 500 hPa combining both districts). July high start years have positive anomalies at all levels, but increase with height. In the Las Vegas district, anomalies range from +2% at 700 hPa to +9% at 500 hPa (+1% at 700 hPa to +6% at 500 hPa in the Winnemucca-Susanville district). This is nearly a 15% difference between the low and high start years (Figures 4a and b). August anomalies take on a different appearance, primarily due to the influence of the Southwest Monsoon. For high start years, 700 hPa has negative anomalies for both districts (-2%), but positive anomalies at 500 hPa (+5%). Humidity anomalies during low start years decrease with height in both districts, but start out as negative (-2%) at 700 hPa in the Winnemucca-Susanville district, and positive at 700 hPa (+5%) in the Las Vegas district. This latter case is likely due to low-level moisture advection related to the south-southwest monsoon flow. For the July high start years, the primary moisture source appears to be the Baja region and Pacific ocean off of the southern California coast. This may also be the primary source during August, but it is clear that the monsoon and associated circulation plays an important role this time of year.

4.4 U,V Wind  

Streamline analysis was computed using u,v wind components. Examining high start years for July and August for both districts, the flow at all levels is primarily south-southeasterly, allowing for moisture advection from the sub-tropical Pacific and Gulf of Mexico regions (Figure 5a) During low start years a low-level trough appears along the western U.S. coast with southwesterly flow aloft over Nevada which is typically a dry flow originating from the colder northern Pacific waters (Figure 5b). For both districts and months, the anticyclonic circulation centered over Arizona and New Mexico is much more pronounced during high start years than low start years.

An examination of divergence fields (not shown) indicates lower-level convergence coupled with upper-level divergence anomalies during high start years which would support upward vertical motion and thunderstorm development, in agreement with the temperature analysis noted above. The divergence patterns reverse during low start years, which would help to inhibit thunderstorm development.

5. SUMMARY 

In this study we have examined synoptic patterns associated with seasons of high and low fire start occurrences for two regions that cover portions of BLM fire districts in Nevada. High start years are associated with positive geopotential height and temperature anomalies, greater lapse rates, and upward vertical motion through the convective level likely indicating increased instability. Wind flow is generally south-southwesterly transporting warm subtropical moisture up from the Baja and coastal Pacific regions. However, the moisture increase is found primarily at 500 hPa, which is conducive to the development of high-based "dry" thunderstorms when coupled with the anomalies noted above. Increased dry thunderstorm activity produces higher incidents of lightning caused fire ignitions. In most cases, the anomaly patterns are reversed for low start years.

A better understanding of the climatology of synoptic variables associated with fire starts will strongly benefit fire weather forecasters in their daily operations, and will ultimately provide fire managers with improved information for making decisions and allocating their resources more efficiently.

6. ACKNOWLEDGEMENTS

This paper is funded in part from a subaward (UCAR S96-73662) under a cooperative agreement between the National Oceanic and Atmospheric Administration (NOAA) and the University Corporation for Atmospheric Research (UCAR). The reanalysis data was provided to us by NOAA's Climate Diagnostic Center. Graphics were generated using the Grid Analysis and Display System (GrADS).

7. REFERENCES