The Climatological Rise in Winter Temperature- and Dewpoint-Based Thaw Events and Their Impact on Snow Depth on Mount Washington, New Hampshire

We analyze how winter thaw events (TE; T>0°C) are changing on the summit of Mount Washington, New Hampshire using three metrics: the number of TE, number of thaw hours, and number of thaw degree-hours for temperature and dewpoint for winters from 1935-36 to 2019-20. The impact of temperature-only-TE and dewpoint-TE on snow depth are compared to quantify the different impacts of sensible-only and sensible-and-latent heating, respectively. Results reveal that temperature- and dewpoint-TE for all metrics increased at a statistically significant rate (p<0.05) over the full time periods studied for temperature (1935-1936 to 2019-2020) and dewpoint (1939-1940 to 2019-2020). Notably around 2000-2001, the positive trends increased for most variables, including dewpoint thaw degree-hours that increased by 82.11 degree-hours decade-1 during 2000-2020 – about five times faster than the 1939-2020 rate of 17.70 degree-hours decade-1. Furthermore, a clear upward shift occurred around 1990 in the lowest winter values of thaw hours and thaw degree-hours – winters now have a higher baseline amount of thaw than before 1990. Snow depth loss during dewpoint-TE (0.36 cm hr-1) occurred more than twice as fast as temperature-only-TE (0.14 cm hr-1). With winters projected to warm throughout the 21st century in the Northeastern US, it is expected that the trends in winter thaw events, and the sensible and latent energy they bring, will continue to rise and lead to more frequent winter flooding, fewer days of good quality snow for winter recreation, and changes in ecosystem function.

Supplemental Material: Laurentide ice sheet thinning and erosive regimes at Mount Washington, New Hampshire, inferred from multiple cosmogenic nuclides

<div>Mount Washington cosmogenic nuclide data and modeled ice sheet profile</div><div><br></div>

Supplemental Material: Laurentide ice sheet thinning and erosive regimes at Mount Washington, New Hampshire, inferred from multiple cosmogenic nuclides

<div>Mount Washington cosmogenic nuclide data and modeled ice sheet profile</div><div><br></div>

The Impact of Mount Washington on the Height of the Boundary Layer and the Vertical Structure of Temperature and Moisture

Discrimination of the type of air mass along mountain slopes can be a challenge and is not commonly performed, but is critical for identifying factors responsible for influencing montane weather, climate, and air quality. A field campaign to measure air mass type and transitions on the summit of Mount Washington, New Hampshire, USA was performed on 19 August 2016. Meteorological observations were taken at the summit and at several sites along the east and west slopes. Ozone concentrations were measured at the summit and on the valley floor. Additionally, water vapor stable isotopes were measured from a truck that drove up and down the Mount Washington Auto Road concurrent with radiosonde launches that profiled the free atmosphere. This multivariate perspective revealed thermal, moisture, and air mass height differences among the free atmosphere, leeward, and windward mountain slopes. Both thermally and mechanically forced upslope flows helped shape these differences by altering the height of the boundary layer with respect to the mountain surface. Recommendations for measurement strategies hoping to develop accurate observational climatologies of air mass exposure in complex terrain are discussed and will be important for evaluating elevation-dependent warming and improving forecasting for weather and air quality.