How Often Vertical Air Mass Transitions Occur in Association with Ridges on Mount Washington During 2017-2018 Winter

Climate models show that warming rates in montane regions should be greater than those in nearby low elevation sites, a trend observed at most mountain ranges. However, the summit of Mount Washington in New Hampshire is an exception to this prediction, with slower warming rates than lower regions of the Northeast. Vertical changes in air masses at the summit, including boundary layer and free tropospheric air masses, is an overlooked potential factor that may affect the rate of warming at high elevations. This analysis describes the prevalence of vertical air mass transitions at the summit associated with anticyclonic systems passing over the region during the 2017-2018 winter season. Changes in variables, including temperature, dewpoint, wind speed, and wind direction, were investigated to identify transitions linked to the ridge passage. Comparisons among the summit variable data, snow level radar images from Plymouth State University, and radiosonde profiles from National Weather Service - Gray, ME (KGYX) verified vertical shifts in air masses at the summit. The impacts of the air mass transitions on the maximum and minimum temperature and dewpoint were determined by finding the recorded maxima and minima before and after transitions. Summit variables at the times of transitions were further studied to determine the conditions in which they would occur.

Out of the 30 analyzed cases, 20 included observed transitions and 2 were unable to be categorized. All transitions were from the boundary layer to the free troposphere. The cause of most transitions was decreasing winds associated with the ridges resulting in the cessation of orographically forced upslope winds moving boundary layer air up to the summit. Varying correlation between dewpoint and temperature were sound indicators of air mass transitions (e.g., the correlation shifts negative when the summit is in the entrainment zone), although other meteorological signals emerged and were dependent on the cause of transition.