UNIVERSITY PARK, Pa. — Despite being difficult to comprehensively observe, the lowest layer of the atmosphere, known as the boundary layer, is critical to weather forecasting, according to a team of meteorology and atmospheric science researchers at Penn State. Historically, researchers have used weather balloons launched twice a day from about 100 locations across the U.S. to observe the boundary layer, but the layer changes hourly, leaving large swatches of the layer unobserved.
The National Weather Service’s Doppler radar network, however, can provide valuable information on the boundary layer, the team at Penn State recently found. They said that while the radars were designed to observe the presence and intensity of snow and rain, they can also provide a more complete analysis of the boundary layer on clear days. To expand on this work with more extensive data collection and analysis of the boundary layer, the research group in Penn State’s Department of Meteorology and Atmospheric Science received a three-year, $700,000 grant from the U.S. National Science Foundation’s (NSF) Division of Atmospheric and Geospace Sciences.
Principal investigator David Stensrud, professor of meteorology and atmospheric science, and co-principal investigator Yunji Zhang, assistant professor of meteorology and atmospheric science, spoke about this under-observed layer of the atmosphere and how the radar observations can play a role in future forecasting tools.
Q: Why are better methods to more comprehensively observe the boundary layer needed?
Zhang: Over land, the boundary layer extends from the surface of the Earth upwards to depths of a mile or two during the day and often only a thousand feet at night. Over water, it remains more constant at about a few thousand feet. The large difference in daytime and nighttime depths over land highlights how quickly the boundary layer changes.
In addition to being the space that practically everyone lives, the boundary layer also is where most severe weather is originally formed and “fueled,” and how it evolves often has great implications during the several hours before when severe weather happens. However, the twice daily weather balloons can’t see these short-term evolutions, and satellite overpasses are also not frequent enough, so we need some new strategies to fill this critical gap.
Stensrud: The boundary layer plays an outsized role in weather prediction, as it often sets the stage for severe weather events that can occur. It also can evolve quickly, making frequent boundary layer observations very beneficial to track. Launching weather balloons more than twice a day is cost-prohibitive, requiring both supplies and personnel. New radar methods that frequently observe the boundary layer using an automated approach would help to fill this observation gap.
Q: How do the boundary layer radar observations compare to weather balloon data or other observation methods?
Stensrud: Weather balloons provide point measurements of more variables, such as temperature, moisture and winds as the balloon ascends, whereas the radar provides measurements of boundary layer depth and winds as a mean over an area as the radar scans. A weather balloon provides a single vertical measurement profile for each twice daily balloon launch. In contrast, each radar scan can be processed to provide a vertical measurement profile every five to 10 minutes. While the number of variables observed by the radar is less than what is possible from a weather balloon, the radar provides at least 144 measurement profiles every day compared to two measurement profiles from weather balloons. We focus on the radar measurements observed during the daytime when the boundary layer signal is strongest.
Q: What research will you conduct with this NSF grant?
Zhang: Our previous project funded by the NSF focused on proving the accuracy and usability of these novel radar observations of the boundary layer, and this new project will connect what we see from these radar observations to different conditions of the boundary layer. We will release small weather balloons from the roof of the Walker Building at University Park and will compare what we see from the radars to what the weather balloons register about the structures of the boundary layer and the atmosphere. There are several Penn State undergraduate and graduate students already working with us on this project.
Stensrud: The new project will push the boundaries of how much information we can obtain from the radar observations within the boundary layer, with the goal to maximize the value of these observations to improve weather predictions and to understand boundary layer evolution.