The Role of Airmass Types and Surface Energy Fluxes in Snow Cover Ablation in the Central Appalachians

Abstract

A one-dimensional snowpack model, a unique airmass identification scheme, and surface weather observations are used to investigate large ablation events in the central Appalachian Mountains of North America. Data from cooperative observing stations are used to identify large ablation events within a 1° latitude × 1° longitude grid box that covers the majority of the Lycoming Creek basin in northern Pennsylvania. All 1-day ablation events greater than or equal to 7.6 cm (3 in.) are identified for the period of 1950 through 2001. Seventy-one events are identified, and these days are matched with a daily airmass type derived using the Spatial Synoptic Classification technique. Average meteorological characteristics on ablation days of each airmass type are calculated in an effort to understand the diverse meteorological influences that led to the large ablation events. A one-dimensional mass and energy balance snowpack model (“SNTHERM”) is used to calculate surface/atmosphere energy fluxes responsible for ablation under each airmass type. Results indicate that large ablation events take place under diverse airmass/synoptic conditions in the central Appalachians. Five airmass types account for the 71 large ablation events over the 52-yr period. Forty-three of the events occurred under “moist” airmass types and 28 under “dry” airmass conditions. Large ablation events under dry airmass types are driven primarily by daytime net radiation receipt, especially net solar radiation. These events tend to occur early and late in the snow cover season when solar radiation receipt is highest and are characterized by relatively clear skies, warm daytime temperatures, and low dewpoint temperatures. Moist airmass types are characterized by cloudy, windy conditions with higher dewpoint temperatures and often with liquid precipitation. During these events sensible heat flux is most often the dominant energy flux to the snowpack during ablation episodes. However, in many cases there is also a significant input of energy to the snowpack associated with condensation. Combinations of high sensible and latent heat fluxes often result in extreme ablation episodes, similar to those witnessed in this area in January 1996.

Trends in snow ablation over North America

“…Snow melt is known to be influenced by the radiative balance , and variations in air temperature (Karl et al, 1993;Brown and Goodison, 1996), as well as surface energy fluxes (Leathers et al, 2004;Dyer and Mote, 2002;Kuusisto, 1986); therefore, with a shallower winter snow cover, associated changes in surface albedo and thermal properties of the snowpack could lead to an increased sensitivity to any or all of these mechanisms, increasing the rate of snow melt and the rate of decrease in SCE. This suggests that the decrease in seasonal snow cover may be because of an increase in the frequency and/or intensity of snow ablation (decreases in snow mass through melt and/or sublimation), which could be augmented by changes in the meteorological conditions related to snow melt.…”

Now You See It Now You Don’t: A Case Study of Ephemeral Snowpacks in the Great Basin U.S.A.

“…Reduction in snow duration can also be caused by the melt of snowpack (Mote, 2006) and losses from sublimation (Harpold et al, 2012;Hood et al, 1999); however, much less is known about the role and distribution of these processes outside of the seasonal snowpack zone. Finally, wind scour can reduce snowpacks by redistributing it to other areas or by increasing blowing wind sublimation (Knowles et al, 2015;Leathers et al, 2004). …”

“…At rain-snow transition elevations, even small temperature variations and other atmospheric variables can alter the mixture of rainfall and snowfall (Henderson and Leathers, 2010;Jefferson, 2011;Klos et al, 2014;Regonda et al, 2005). Complete snow water 15 equivalent (SWE) removal from melt or sublimation is also another common cause of snow ephemerality (Clow, 2010;Leathers et al, 2004;Mote et al, 2005;Sospedra-Alfonso and Merryfield, 2017). Typically, physics based models overestimate modeled SWE in ephemeral snowpack, due to neglect or underestimation of ground heat flux and the challenges of tracking cold content in shallow snowpacks (Cline, 1997;Hawkins and Ellis, 2007;Kelleners et al, 2010;Kormos et al, 2014;LaMontagne, 2009;Şensoy et al, 2006).…”

“…Elevation effects likely a summation of variety of factors, including temperature controls on the rain-snow transition, longwave radiation in cloudy areas, and sensible heat 20 flux. Aspect is often a secondary control on snow distributions because it influences incoming shortwave radiation (Jost et al, 2007;Pomeroy et al, 2003) and wind patterns (Knowles et al, 2015;Leathers et al, 2004;Winstral et al, 2013). Shortwave radiation is the primary driver of ablation via melt and sublimation (Cline, 1997;Marks and Dozier, 1992).…”

An Air Mass Based Approach to the Establishment of Spring Season Synoptic Characteristics in the Northeast United States

“…The diverse applications indicate that the SSC is both convenient and applicable in a variety of atmospheric science analyses. For instance, reference [24] used the SSC to help identify days in which significant snow cover ablation was observed in the Central Appalachians. Reference [25] used the SSC to identify a relationship between particular air mass types and the number of emergency room visits for individuals suffering from asthma or other respiratory illnesses in North Carolina.…”