Beth L. Hall

Climate, Santa Ana Winds and autumn wildfires in southern California

Wildfires periodically burn large areas of chaparral and adjacent woodlands in autumn and winter in southern California. These fires often occur in conjunction with Santa Ana weather events, which combine high winds and low humidity, and tend to follow a wet winter rainy season. Because conditions fostering large fall and winter wildfires in California are the result of large-scale patterns in atmospheric circulation, the same dangerous conditions are likely to occur over a wide area at the same time. Furthermore, over a century of watershed reserve management and fire suppression have promoted fuel accumulations, helping to shape one of the most conflagration-prone environments in the world [Pyne, 1997]. Combined with a complex topography and a large human population, southern Californian ecology and climate pose a considerable physical and societal challenge to fire management.

The Use of t Values in Climatological Composite Analyses

Composite maps have been used in climatology for many years. Typically their formulation has involved identifying a group of years with similar characteristics (e.g., warm anomaly, positive height anomaly), calculating the overall mean of those years using a gridded map dataset, and finally, presenting a map of the composite means with the key spatial characteristics highlighted. This is often done for anomalies, as shown in Fig. 1 (Cayan 1996). Here, 700-mb composite height anomalies associated with April snow water equivalent are related to precipitation and temperature patterns. It is not uncommon to see composite maps given as means and with some form of statistical significance test to assess chance occurrence of the pattern. The shaded areas in Fig. 1 show those regions meeting a particular parametric significance test requirement. The use of the simple arithmetic mean in composites has at least two shortcomings. These are 1) the mean as a measure of location is not resistant to outliers, especially in small samples, and 2) it does not account for the associated variance. In order to overcome these drawbacks, we recommend a form of scaled mean (following Iglewicz 1983) for the construction of composites. This form is particularly useful for small samples, which often are used for composites. It is also useful when the data distribution is unknown, or known to be non-Gaussian. Finally, it is easy to calculate. Given a population mean m and population variance s 2 , the scaled mean has the form

Precipitation Associated With Lightning-Ignited Wildfires in Arizona and New Mexico

From 1990 to 1998, over 17000 naturally ignited wildfires were observed in Arizona and New Mexico on US federal land during the fire season of April through October. Lightning strikes associated with these fires accounted for less than 0.35% of all recorded cloud-to-ground lightning strikes that occurred during the fire season during that time. Given the high aridity of this region, why do some lightning strikes ignite fires and others not? Natural wildfire ignitions in this region are often attributed to what is referred to as ‘dry’ lightning, or lightning with little or no precipitation. This study used daily and hourly gridded precipitation derived from historical gauge data to compare the amount of precipitation associated with natural wildfires and the amount of precipitation associated with lightning strikes that were not associated with natural wildfire events. Climatology of natural ignitions tend to peak before the time of the seasonal maximum lightning flashes and precipitation, suggesting ‘dry’ ignitions in the early part (e.g. late June, early July) of the wildfire season. Observed natural wildfires were more often associated with conditions of <2 mm of precipitation on the day of the event than were cases without an associated ignition. The majority of lightning flashes that did not cause a discovered wildfire were associated with a higher precipitation rate after the timing of the event, which suggests the possibility of the precipitation extinguishing the wildfires before discovery. Most lightning flashes that were not associated with a discovered ignition had twice as many hours with some measurable precipitation than where there were discovered ignitions, which eludes to the longer-term impact of preceding precipitation on the fuels than the lightning storm-associated precipitation. These results can be applied to gridded forecasts of amount of precipitation to indicate areas where there is an increased probability of natural wildfire ignition, assuming the other factors of ignition source and fuel availability are present.

Climate Warming and 21st‐Century Drought in Southwestern North America

Since 2000, southwestern North America has experienced widespread drought. Lakes Powell and Mead are now at less than 50% of their reservoir capacity, and drought or fire-related states of emergency were declared this past summer by governors in six western states. As with other prolonged droughts, such as the Dust Bowl during the 1930s, aridity has at times extended from northern Mexico to the southern Canadian prairies. A synthesis of climatological and paleoclimatological studies suggests that a transition to a more arid climate may be occurring due to global warming, with the prospect of sustained droughts being exacerbated by the potential reaction of the Pacific Ocean to warming.

Fire ignitions related to radar reflectivity patterns in Arizona and New Mexico

Over 5400 lightning-ignited wildfires were detected on federal land in Arizona and New Mexico from 1996 through 1998 during the fire season of May through September. The non-uniform and sporadic spatial nature of precipitation events in this region makes the use of rain gauge data a limited means of assessing when and where a cloud-to-ground lightning strike might have ignited a wildfire due to dry lightning. By analysing weather radar reflectivity data with lightning and wildfire data, characteristics of radar reflectivity can be used by fire weather forecasters to identify regions of increased ignition potential. Critical ranges of reflectivity, life span of a reflectivity cell, and storm movement are characteristics of radar reflectivity that are examined in this analysis. The results of this type of analysis can help focus attention of wildfire personnel to particular locations where there is known to be cloud-to-ground lightning in conjunction with radar reflectivity patterns that have been historically associated with wildfire ignition. Results from the analysis show that wildfire ignitions typically occur near the perimeter of a radar echo. The reflectivity values at the ignition location are less than the highest reflectivity located within the echo, and often magnitudes are sufficiently low to suggest that the precipitation is not reaching the ground in this dry region with high cloud bases. Interpretation of the duration, size and level of lightning activity of the radar echo associated with the ignition indicate that ignitions tend to occur in the early stages of a radar echo. However, there are often multiple storm cells having isolated areas of higher reflectivity within a radar echo at the time of ignition. Nearly two-thirds of radar echoes associated with wildfire ignitions moved more than 50 km throughout the echo’s lifetime. These moving storm systems often propagated in a northerly or easterly direction, and ignitions occurred on the leading edge of the storm in over half of the cases that propagated in the same direction. Forecasters can use results from this study to determine where there is an increased potential of wildfire ignitions when similar radar patterns appear in conjunction with lightning activity in the future.

Vulnerability of grain crops and croplands in the Midwest to climatic variability and adaptation strategies

Maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) are the dominant grain crops across the Midwest and are grown on 75% of the arable land with small but economically important crops of wheat (Triticum aestivum L.) and oats (Avena sativa L.) but economically important crops. Historically, there have been variations in annual yields for maize and soybean related to the seasonal weather patterns. Key concerns are the impacts of future climate change on maize and soybean production and their vulnerability to future climate changes. To evaluate these, we analyzed the yield gaps as the difference between the attainable and actual yield at the county level and observed meteorological data to determine which seasonal meteorological variables were dominant in quantifying the actual/attainable yields. July maximum temperatures, August minimum temperatures, and July–August total precipitation were found to be the significant factors affecting the yield gap. These relationships were used to estimate the change in the yield gap through 2100 using both the RCP 4.5 and 8.5 climate scenarios for these variables for selected counties across the Midwest. Yield gaps increased with time for maize across the Midwest with the largest increases in the southern portion of the Corn Belt showing a large north-south gradient in the increase of the yield gap and minimal east-west gradient. Soybean was not as sensitive as maize because the projected temperatures do not exceed optimum temperature ranges for growth and reductions in production that are more sensitive to precipitation changes during the reproductive stages. Adaptation strategies for maize and soybean will require more innovation than simple agronomic management and require the linkage between geneticists, agronomists, and agricultural meteorologists to develop innovative strategies to preserve production in the Midwest.