New method to determine planetary boundary layer depth

Combining the strengths of existing techniques, new algorithm could help improve climate models.

sally-wanda-method-to-determine-planetary-boundary-layer-large.jpg — Image courtesy of the Atmospheric Radiation Measurement (ARM) Climate Research Facility via a Creative Commons license.

The Science

U.S. Department of Energy investigators have developed a new method to determine the depth of the planetary boundary layer (PBL), the lowest part of the atmosphere that is affected most directly by surface processes. As a key factor in many atmospheric processes (e.g., cloud formation, aerosol mixing, and transport), PBL depth and its temporal evolution have important effects on weather, air quality, and climate. This depth evolves throughout the day because of several factors, including large-scale air motions, cloudiness, and the daily cycle of solar radiation. Measurements of temperature and moisture from radiosonde profiles are the most reliable method for determining PBL depth, but radiosondes are generally launched only two to four times per day. To understand the temporal evolution of atmospheric thermodynamics and evaluate model representations, continuous monitoring of PBL variation from remote-sensing measurements is highly desired.

The Impact

Combining the strengths of two existing gradient methods, the newly developed algorithm can synthesize data from three instruments: radiosondes, a micro-pulse lidar (MPL), and an atmospheric emitted radiance interferometer (AERI). The algorithm was applied to measurements acquired at the Atmospheric Radiation Measurement (ARM) Climate Research Facility’s Southern Great Plains site from 1996–2004 to produce a time series of PBL depth. This more detailed view of PBL variation over time from AERI and MPL can capture details of the diurnal cycle, knowledge useful for evaluating PBL simulations in climate models.

Summary

By enabling intercomparison of measurements from three instruments operating at the same site over 8 years, the novel algorithm provides a new continuous PBL dataset. Comparisons of the three PBL products (which were made for different times of day, seasons, and sky conditions) demonstrate the algorithm’s robustness. Although considerable uncertainties exist in PBL detection using all three types of measurements, agreement among the PBL products is promising under certain conditions, and the different measurements have complementary advantages. For example, comparisons of the seasonal and diurnal cycles among the three instruments revealed that results are more reliable in winter than in summer. The instruments also produce results with better agreement during daylight hours than at night and at times of day when the PBL is mature rather than collapsing or developing. While PBL depth cannot be detected from AERI data if clouds are present, or from MPL data if the boundary layer is shallower than 600 m, both instruments have much higher temporal resolution than radiosondes.

Contact

Zhanqing Li
College of Global Change and Earth System Science, State Laboratory of Earth Surface Process and Resource Ecology, Beijing Normal University, China.
zli@atmos.umd.edu

Funding

Data were obtained from the Atmospheric Radiation Measurement (ARM) Climate Research Facility program sponsored by the Climate and Environmental Sciences Division of the Office of Biological and Environmental Research within the U.S. Department of Energy’s (DOE) Office of Science. Authors gratefully acknowledge early discussions with Dr. Ellsworth Judd Welton of NASA’s Goddard Space Flight Center and the funding support of the National Basic Research Program (2013CB955804), DOE’s Atmospheric System Research (ER65319) program, and the National Science Foundation (1118325).

Publications

Sawyer, V., and Z. Li. “Detection, variations and intercomparison of the planetary boundary layer depth from radiosonde, lidar and infrared spectrometer,” Atmos. Environ. 79, 518–528 2013. [DOI: 10.1016/j.atmosenv.2013.07.019].