Date of Award

12-2025

Document Type

Thesis

Degree Name

Master of Science (MS)

Degree Statement

Master of Science in Earth System Science

Department

Department of Atmospheric Sciences

Committee Chair

Lader, Richard

Committee Member

Bhatt, Uma

Committee Member

Polyakov, Igor

Table of Contents

Chapter 1: Introduction

Chapter 2: Methods 

2.1 Data

2.1.1 ERA5

2.1.2 CFFDRS from ERA5

2.2 Methods

2.2.1 Standard statistical methodology

2.2.2 Extreme analysis

2.2.3 Sensitivity study

Chapter 3: Results

3.1 Seasonal climatologies and trends

3.2 Climatological seasonal cycle

3.3 ISI extremes

3.4 Case studies

3.5 ISI sensitivity 

Chapter 4: Discussion

Chapter 5: Conclusions

Abstract

The Alaskan fire season is an increasingly consequential aspect of the state’s summer climate, with recent years frequently exceeding one million acres burned. Among the key factors influencing fire behavior, wind and moisture play a critical role in driving fire spread. The Initial Spread Index (ISI), a component of the Canadian Forest Fire Danger Rating System, quantifies the expected rate of fire spread following ignition. ISI is currently used operationally in Alaska. Despite its significance, ISI remains under-studied in terms of its historical behavior, trends, and environmental sensitivity. This study evaluates the climatology, variability, and trends of ISI across Alaska using gridded reanalysis data from 1991-2023. Results highlight the strong influence of elevation on ISI, where the highest values are found at low elevation, particularly in interior regions during the duff season (June 11th -July 20th). Decadal trends indicate increasing ISI early in the fire season, suggesting an earlier onset and possible lengthening of fire activity in several Predictive Service Areas (PSAs). Across interior and southwest Alaska values above the 85th percentile are occurring more frequently, reinforcing the growing potential for rapid fire spread under recent and projected climate conditions. Sensitivity analysis conducted at Fairbanks from three high-fire years confirms ISI’s strong dependence on moisture inputs: even modest precipitation (~5 mm) sharply reduces ISI to near-zero levels, underscoring the index’s responsiveness to relative humidity and rainfall. Case studies in boreal and tundra PSAs demonstrate a clear connection between daily ISI maxima and spikes in fire activity, including acres burned.

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