Relationship between temperature and dry-weather jokulhlaups from South Tahoma Glacier, Mount Rainier, Washington

Abstract

Jokulhlaups, or glacial outburst floods, from South Tahoma Glacier on Mount Rainier pose a threat to park visitors, staff, and infrastructure. Jokulhlaups release without surficial precursors, and transition into debris flows as they surge downslope. Walder and Driedger (1995) calculated the relative frequency, called the conditional probability (PC), for seven dry-weather jokulhlaups between May-November from 1986 and 1992. Dry-weather jokulhlaups are defined as outburst floods that occur during hot, dry conditions. The PC is the number of jokulhlaups that occur at a maximum temperature (TMAX) divided by the number of days in the entire distribution that have a TMAX equal to or greater than the given TMAX.

Walder and Driedger (1995) used 1-day average TMAX (TMAX-1) to calculated PC, which ranged from 0.04 to 0.11 between 17?C and 28?C. I repeated their analyses with a larger data set, using TMAX-1 between June-September from 1966-2015, which contained 14 outburst floods. The PC calculated from the larger data set ranged from 0.008 to 0.023 for the same TMAX-1. This difference in the PC show there are not enough data to produce robust analyses, and so the PC should not be used to predict jˆkulhlaups because it will change as data is added.

Dry-weather outburst floods occur later in the summer compared to peak TMAX-1, so longer intervals of TMAX are required to melt enough water to produce an outburst flood. I calculated 6-week average TMAX (TMAX-42) on the days that jokulhlaups occurred. The peak in TMAX-42 aligns with the TMAX-42 of the entire distribution. The PC for TMAX-42, calculated in the same way as TMAX-1, are less than 0.01 for TMAX-42 <18.0?C. When TMAX-42 is ?18.0?C, the PC increases rapidly from 0.01 to 0.10 at 21.1?C.