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

Abstract:The summit of Mount Washington is warming slower than the surrounding lower elevations, contrary to other mountain ranges which are experiencing higher warming trends. This also contradicts globalclimate modelprojectionswhich suggesthigher elevations should be warming faster than nearby lower elevations. The unusual elevation-dependent warming trends at Mount Washington could be driven by the summit being exposed to the free troposphere for approximately 50% of the year. As a first step toward testing thishypothesis, the impacts of free tropospheric exposure at the summit must be understood. The impacts of vertical changes in air masses at the summit of Mount Washington on climate variables during ridge passages were analyzed during the fall of 2017.Summit variables, such as temperature, dewpoint wind speed and wind direction from Mount Washington Observatory, were used to identify vertical air mass changes. Characteristics, such as a rapid change in dewpoint or wind speed, assisted in determining times of transitions. The NOAA ESRL-PSD snow-level radar at Plymouth State University, which can detect turbulence at the top of the boundary layer, andradiosonde data from the National Weather Service station at Gray, Maine help to supportthe height of the boundary layer in between them at Mount Washington under a synoptic scale ridge.

A total of 25 cases of ridge passages were identifiedby summit station pressure and Weather Prediction Center surface analysis maps. Of these 25 cases, 19 were associated with an air mass transition at the summit, 6had no transition, and. Dewpoint, correlation between dewpoint and temperature, and wind speed were found to be the most effective variables during the fall season to determine a transition. A negative correlation of dewpoint and temperature was particularly indicative of a transition from moist boundary layer and dry free tropospheric air. This analysis suggests that most transitions occurred because upslope winds decreased with the approach of a ridge and the weak winds could no longer push boundary layer air up on the summit, thus exposing it to the free troposphere. Frequently, temperature increased by >1°C and dewpoint decreased by over 10°C.These results suggest that mountain slopes sufficiently high above their surrounding lower elevations may be experiencinga significantly differentclimate because of exposure to the free troposphere than they would if they were always in the boundary layer.