The climate variability of the Haines Index over the United States

The Haines Index, also known as the Lower Atmosphere Severity Index, is employed operationally as a tool to indicate the potential for large or erratic fire growth by considering the dryness and stability of air over a specific area. In this study, we have derived a 28-year (1980-2007) Haines Index (HI) climatology for North America using the North American Regional Reanalysis (NARR). NARR is a long-term (from 1979 to present) and dynamically consistent meteorology and hydrology gridded dataset with a grid spacing of 32 km and a 3-hour time interval.

Using the climatology, we have examined the spatial distribution of warm season (May through October) mean lapse rates, dewpoint depressions, factor As, and factor Bs for each of the low-, mid-, and high-elevation variants of the HI. The factor As and factor Bs are then used to compute the HI for each variant, and the mean and standard deviation of the resultant HI are investigated to establish the seasonal and inter-annual variations of the HI for various regions in the United States. The climatology illustrates the insensitivity of the index in particular regions, while highlighting the impact of mesoscale meteorological features such as the dryline, sea/lake breezes, and orographic flows.

To investigate how HI behavior responses to the climate change, the United States is further divided into 6 climatic regions wherein yearly trends in the HI are analyzed. The yearly trends demonstrate different patterns of high risk frequency across the United States. The climatological trends indicate that the frequency of high HI values in the Pacific Northwest and Pacific Southwest is almost constant during this time period, while the Midwest, Rocky Mountains, Northeast and Southeast all exhibit increases in the occurrence of high HI values.