The Air is Different Up Here

2018-08-23 15:30:21.000 – Eric P. Kelsey, Ph.D., Director of Research

 

If you have been to the summit of Mount Washington, you likely noticed that the air was different than in the valleys: the temperature was cooler, you may have been in a cloud, and you may have noticed with your eyes and olfactory that the air was more or less hazy and polluted. If you thought that you were in a different air mass than in the valleys, you were probably right.

For many decades, we have known that the summit of Mount Washington spends roughly half of the year in the lowest layer of the atmosphere, the boundary layer, and the rest of the time in the free atmosphere or the transition zone between these two layers called the entrainment zone. Victor Conrad, a Harvard University scientist, first estimated this indirectly in 1941 when he examined the time of day that the daily maximum and minimum temperatures occurred. When the summit air temperature peaked during the afternoon and is lowest at night like it typically does in the lower elevations, the summit is likely in the boundary layer along with everyone else in the region. When the maximum and minimum temperatures occur at other times of the day, there is an increased likelihood that the summit is in the free troposphere where horizontal changes in air temperature have a bigger influence on the timing and values of the maximum and minimum temperature. Using Conrad’s methods, Grant et al. (2005) ran this analysis with more recent data and concluded similar results: the summit is dominated by boundary layer processes 30% of the time and by the free troposphere 50% of the time (and inconclusive the other 20% of the time).

 

Schematic of the daily evolution of the boundary layer under clear skies.

Because the summit is subject to different air masses and atmospheric processes than the lower elevations roughly 50% of the time, it mak

es sense that this could be contributing to the slower warming rate at the summit than the lower elevations. Furthermore, the elevational difference in warming rates is greatest in the fall and winter when the summit is more frequently exposed to the free troposphere. These observations motivate the current MWO research on boundary layer processes and height changes. In August and September 2016, a field campaign occurred to better understand the processes dominating changes in the height of the boundary layer and the resulting types of air masses along the slopes of Mount Washington link to MWO project webpage: here. MWO Director of Research Eric Kelsey, Adriana Bailey (then at Dartmouth College, now at the National Center for Atmospheric Research), and Georgia Murray (Appalachian Mountain Club) led this field campaign largely funded by Plymouth State University with contributions from AMC, Dartmouth and Heidi Asbjornsen at UNH.

Their findings have just been published in the open-access online journal Atmosphere (link to manuscript page here). The article focuses on a one-day intensive observation campaign on 19 August 2016 during which several interesting atmospheric processes were captured with MWO summit and mesonet observations (link to our mesonet page here), weather balloons launched from the base of the Auto Road, and vertical profiles of stable isotopes of water vapor and meteorological variables. We observed large scale subsidence from a high pressure system depressing the boundary layer height while sunshine was developing warm thermals that rose from the valleys to deepen the boundary layer. Observations above treeline caught this battle between these and other processes and has helped advance our understanding of how the boundary layer behaves around a mountain. These results are also guiding our approach for future research on this topic, including the processes we want to observe and the types of new instrumentation we need to observe them. In the near future, we aim to develop a statistical model based on summit weather observations and large-scale weather pattern that predicts the type of air mass at the summit. From this we will be able to reconstruct the summit air mass type going back to 1935 and understand how this variability in air masses at the summit impacts the long term slower warming trend at the summit.

 
PSU meteorology students prepare a weather balloon for launch near the Mount Washington Auto Road base. 

 

Eric P. Kelsey, Ph.D., Director of Research

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