Great Lakes Basin Snow-Cover Ablation and Synoptic-Scale Circulation

Abstract

Synoptic-scale atmospheric conditions play a critical role in determining the frequency and intensity of snow-cover-ablation events. Using a synoptic weather-classification technique, distinct regional circulation patterns influencing the Great Lakes basin of North America are identified and examined in conjunction with daily snow-ablation events from 1960 to 2009. An ablation event is considered in this study to be an interdiurnal decrease in areal-weighted average snow depth of greater than 2.54 cm in magnitude over the entire Great Lakes basin. General meteorological characteristics associated with ablation-causing synoptic types are examined, and three individual case studies from prominent synoptic types are presented to understand the diversity of meteorological influences on regional snow ablation. Results indicate that a variety of synoptic weather conditions lead to snow ablation in the Great Lakes basin. The 10 most common synoptic types accounted for 66% of the 349 ablation events detected from 1960 to 2009. Snow ablation in the Great Lakes basin most commonly occurs when there is advection of warm and moist air into the region to provide the sensible and latent heat fluxes that are needed for melt, but ablation frequently occurs during rain-on-snow events and in instances of high pressure overhead. Ablation magnitude is highest during rain-on-snow synoptic types, and the interannual frequency of these types significantly decreased by 37% over 1960–2009. Conversely, the frequency of high-pressure-overhead synoptic types significantly increased by more than 30% from 1960 to 2009. Such changes may influence the hydrologic impact of these synoptic types on ablation over time.

Synoptic and meteorological conditions during extreme snow cover ablation events in the Great Lakes Basin

“…A snow ablation event is defined in this study as an interdiurnal decrease in GLB‐wide areal‐weighted snow depth, in excess of 2.54 cm (Suriano & Leathers, ). Following similar studies, ablation events are analysed only under situations when the maximum daily surface air temperature on the second day of the interdiurnal snow depth decrease is at least 0°C and when no more than 2.54 cm of new snowfall accumulates on any of the previous 3 days before the event (Suriano & Leathers, ). The 2.54‐cm snow depth change threshold is utilized as it represents the minimum value of nontrace snow depth recorded in the Cooperative Observer Network in the United States.…”

“…The 2.54‐cm snow depth change threshold is utilized as it represents the minimum value of nontrace snow depth recorded in the Cooperative Observer Network in the United States. By restricting events to only those with maximum temperatures in excess of 0°C, and in which a large snow accumulation event does not occur in the previous three days, it is assumed the portion of snow depth change due to compaction is minimized (Dyer & Mote, ; Suriano & Leathers, ). Under these temperature conditions, the snowpack can be assumed to be relatively isothermal and mature; thus, minimal compaction will occur (Dyer & Mote, ).…”

“…Under these temperature conditions, the snowpack can be assumed to be relatively isothermal and mature; thus, minimal compaction will occur (Dyer & Mote, ). Similarly, the 3‐day snowfall accumulation threshold was selected based on an estimated snow‐density calculation (Anderson, ), which indicates that after this 3‐day period, the impact of compaction should be nominal (Suriano & Leathers, ). Daily snow depth for the GLB is calculated using the daily areal‐weighted average depth for the 57 grid cells within the basin (Suriano & Leathers, ).…”

“…Other work in the GLB details the role of three general synoptic‐scale weather types that lead to the majority of ablation: rain‐on‐snow, southerly flow, and high‐pressure overhead patterns. Each of these weather types provides unique meteorological conditions that can be linked to the turbulent and radiative fluxes that ablate snow (Suriano, ; Suriano & Leathers, ). Resolving the meteorological conditions and energy fluxes during ablation has proven useful in understanding the processes leading to ablation.…”

“…In ablation research in the GLB (Suriano, ; Suriano & Leathers, ), along with similar work focused over the Northern Great Plains (Grundstein & Leathers, ; Grundstein & Leathers, ), case studies are used to examine the meteorological conditions associated with ablation‐inducing atmospheric types. These case studies are selected specifically to represent “typical” conditions associated with ablation‐inducing atmospheric types.…”