Nocturnal Storm Chasers Collect “Fantastic” Data Set to Improve Forecasts
PECAN campaign ends after scientists spend more than 30 nights monitoring storms with remote sensing instruments
Starting June 1 and literally running on adrenaline until July 16, PECAN participants worked through more than 30 nights to collect data. Following directions provided by forecasters during the campaign, mobile crews often drove much of the day to set up mobile instruments in the anticipated path of large storms. Other instruments at fixed ground sites collected temperature, humidity, and wind-speed measurements throughout the boundary layer and lower troposphere at five-minute intervals, 24 hours a day. To complete the data set, three crews flew aircraft in and around storm clouds each night.
Using more than 110 instruments operated by 17 different groups, including many provided by the Atmospheric Radiation Measurement (ARM) Climate Research Facility, the collaborators represented 27 universities and 11 research laboratories, all of whom are intent on understanding the causes of MCS and how to predict what they'll do. This multi-sponsored effort was co-organized by the National Center for Atmospheric Research (NCAR), funded by the National Science Foundation (NSF), with support from the National Aeronautical and Space Administration (NASA), National Oceanic and Atmospheric Administration (NOAA), and U.S. Department of Energy’s Office of Science.
"We went home tired, took vacations and are just starting to get into the analysis, but I think we collected a fantastic data set," said David Turner, scientist at the NOAA National Severe Storms Laboratory and lead investigator for the ARM-funded support for PECAN. "We won't know exactly what we have until after we start to analyze it, but we have several hypotheses we want to evaluate and validate. These data should enable us to address a good number of those hypotheses and either support or disprove them."
The main focus of the study is to understand the initiation, growth, evolution—and unpredictability—of nighttime storms resulting from convection that isn't related to warm air rising from the ground as it is in the daytime. Nocturnal storms involve a low-level jet over a stable boundary layer, according to Bart Geerts, professor of atmospheric sciences at the University of Wyoming and a member of the PECAN steering committee.
"Low-level jets are quite shallow, occurring about 1,000 feet (300 meters) above the ground," Geerts pointed out. "Another key element is a more-shallow, stable under-layer. The jet decouples from the ground at night and the convective phenomena are not fed by the surface but rather by passing elevated air. The MCS themselves remain elevated and we may get rain but not strong winds.
"Sometimes they shift to the ground and we get strong winds," Geerts continued. "We had several national weather forecasters with us on the campaign wondering how and why there is a transition from elevated to surface-based convection. There can be a cold pool spreading away from the storm, but sometimes they remain elevated and produce bores, waves of air rising up and down. That's the reason the project was funded. The storms are complicated, but it's also because we have not done this work before."
Tammy Weckwerth, a scientist at the NCAR Earth Observing Laboratory (EOL) and PECAN steering committee member, said the goal of the project was to understand nocturnal convection, the bores or atmospheric waves, the nocturnal boundary layer, and the low-level jet—and how they interact. She noted that, during the campaign, the extreme variability of atmospheric conditions such as moisture and temperature prior to MCS formation became obvious and made forecasting more difficult.
"We expected variability in the daytime, but at night we expected it to be more uniform," Weckwerth reasoned. "Because of that variability, we were never certain whether storms would form when we arrived at a place to begin collecting data. The forecasts didn't always agree. By three o'clock in the afternoon, the forecast was often very different from the morning forecast and could even change after that. For this reason, there was a lot of last-minute relocating. A lot of the teams were actually deploying in the dark because we were using the latest and most helpful results about where to collect data."
Temperature and humidity data provided by eight Atmospheric Emitted Radiance Interferometers (AERIs), including six from the ARM Facility, in the PECAN Integrated Sounding Array (PISA) stations will help scientists better understand the conditions that lead to MCS formation, according to Turner. "The high-temporal resolution temperature and humidity data at the PISA stations are unique," he said. "Anything focused on nighttime convection is needed. In the jargon, we say that convection at night is usually not coupled with the surface, and, thus, perturbations at the surface do not influence the storm. Ultimately, the end goal is to improve our understanding of how these storms evolve so the weather service can issue a better forecast."
"People are asleep," Turner continued. "The weather service has to make a decision when they see a storm coming. Is the storm going to generate severe weather? Do they issue a warning to wake people and get them to go to their storm shelters? It's a difficult decision to make. The data we gathered during PECAN is giving us more understanding that we hope will help."
"We can improve our weather prediction models in two ways," he stated. "The first challenge is: Can you get the models to match what is happening right now? We believe the PISA data can help provide correct initial atmospheric conditions about what is happening right now to enter into the model. The second part is the physics of what's happening in the clouds and convection at night. These physical processes are represented by a set of equations in the model. If the initial conditions are correct, then we can determine whether the equations that represent how the atmosphere will evolve and predict the future are correct—and that is something we can also test with PECAN data."
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The ARM Climate Research Facility is a national scientific user facility funded through the U.S. Department of Energy's Office of Science. The ARM Facility is operated by nine Department of Energy national laboratories, including Argonne National Laboratory, which manages the ARM Southern Great Plains site in Oklahoma, where significant PECAN research occurred.