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

Eric P Kelsey ,Adriana Bailey,Georgia Murray

doi:10.3390/atmos9080293

Abstract

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.

Highlights

Mountain meteorology and atmospheric chemistry sites are often viewed as large-scale background climate indicators, free of direct influence from land-use changes and human activity
Measurements of conserved variables from the lowest elevations above treeline closely matched summit observations during the period when the summit was within the planetary boundary layer (PBL), providing a better sense of how well-mixed the PBL truly was compared with measurements from within the low-elevation forest canopy. These findings suggest that observational programs hoping to evaluate the changing character of PBL, entrainment zone (EZ), and free troposphere (FT) exposure at higher elevations will be best served by along-slope measurements from the elevations of interest and measurements of PBL air minimally influenced by subcanopy dynamics
The intensive observation period (IOP) to study air mass transitions associated with PBL evolution at Mount Washington, New Hampshire on 19 August 2016 revealed significant differences in air mass exposure between the mountain and over the valleys as well as between leeward and windward sides of the mountain

Summary

Introduction

Mountain meteorology and atmospheric chemistry sites are often viewed as large-scale background climate indicators, free of direct influence from land-use changes and human activity. Like Mauna Loa on the Big Island of Hawaii for instance, were established in part to measure background or “baseline” atmospheric conditions of key meteorological variables and trace gases known to modify climate. These measurements require exposure to free tropospheric air masses, minimally influenced by local surface processes. Many of the same observatories report regular exposure to planetary boundary layer (PBL) air [1,2,3,4,5,6], which may be transported upwards by a variety of processes. Quantifying the degree of exposure to PBL air masses will contextualize whether or not, and to what degree, individual mountain sites should be viewed as indicators of background climate and air quality changes

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