ASPECAN

ARM Support for the Plains Elevated Convection at Night Experiment (PECAN)

1 June 2015 - 15 July 2015

Lead Scientist: David Turner

Observatory: SGP

The Plains Elevated Convection at Night (PECAN) Experiment was a large field campaign supported by the National Science Foundation (NSF) with contributions from the National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA), and U.S. Department of Energy (DOE). PECAN researchers sought to improve the understanding and simulation of the processes that initiate and maintain convection and convective precipitation at night over the central portion of the U.S. Great Plains. These goals were important because:

• A large fraction of the yearly precipitation in the Great Plains comes from nocturnal convection.
• Nocturnal convection in the Great Plains is most often decoupled from the ground and is thus forced by other phenomena aloft (e.g., propagating bores, frontal boundaries, low-level jets).
• There is a relative lack of understanding about how these disturbances initiate and maintain nocturnal convection.
• This lack of understanding greatly hampers the ability of numerical weather and climate models to simulate nocturnal convection well, which leads to significant uncertainties in predicting the onset, location, frequency, and intensity of convective cloud systems and associated weather hazards over the Great Plains.

The PECAN experiment was held over a large domain encompassing most of western and central Kansas, northern Oklahoma, and southern Nebraska. A unique component of the PECAN observational strategy was the concept of the PECAN Integrated Sounding Array (PISA). Each PISA had remote sensing instruments that could profile temperature, humidity, and winds throughout the boundary layer and lower troposphere at high time resolution (~5 min). PISAs also had colocated radiosonde systems that provided detailed profile information at lower temporal resolution at all levels up to the lower stratosphere. There were six fixed PISAs in the domain and four mobile PISAs, where the latter were relocated nightly to augment the domain where convection was anticipated to occur. The ARM SGP Central Facility served as the fixed PISA in the southeastern cornerstone of the PECAN domain. A large number of instruments from the NSF Lower Atmospheric Observing Facility pool, university investigators, a private company, and from other agencies such as NOAA and NASA instrumented the other PISA sites. The detailed characterization of the thermodynamic and kinematic evolution of the boundary layer across the experimental domain greatly enhanced the rest of the observational data sets, which included fixed and mobile 3-D scanning precipitation radars, mobile mesonet systems, clear air and storm penetrating aircraft, and mobile radiosonde launching systems. ARM supported PECAN by providing five atmospheric emitted radiance interferometers (AERIs), which were deployed at the fixed PISAs in the domain. These instruments, together with the AERI at the SGP Central Facility, provided a uniform and consistent thermodynamic data set across domain, which greatly improved the ability of data assimilation schemes and other analysis techniques to characterize the thermodynamic environment and its evolution before and during these nocturnal events. Many recent studies indicated that the lack of a spatial array of high temporal resolution thermodynamic profiles in the lower troposphere had been the limiting factor in understanding the processes at work in preconvective and convective situations. ARM provided a radiosonde station and consumables that were used at one of the other fixed PISAs, thereby allowing all of the fixed PISAs to have radiosonde capability. Researchers used ARM's vast array of sensitive radar and LIDAR profilers at the SGP Central Facility (e.g., Raman LIDAR, Doppler LIDAR, 915 MHz wind profilers, etc.) to investigate nocturnal convection when the convection occurred nearby. The full PISA data set will serve for many years as a leading mesoscale profiling network data set for data assimilation and prediction studies by DOE scientists and others. It also greatly augments the data set collected during the Midlatitude Continental Convective Cloud Experiment (MC3E), which focused on diurnal convection over the SGP site.