Boundary Layer Heights in a Heterogeneous Landscape

The Planetary Boundary Layer (PBL) is the layer of the atmosphere, approximately one kilometer in height, which is directly influenced by earth's surface. At the Beltsville Center for Climate System Observation (BCCSO) in Beltsville, MD, air quality is one of the research focuses, and PBL height is one important parameter for scalar budgets and air quality forecast, for instance. The height of the PBL has been studied extensively in homogeneous environments; however, very few observational studies at a single point measurement of PBL heights in a heterogeneous landscape have been conducted. The BCCSO is an ideal location to study the height of the PBL because it is surrounded by distinct landscapes. The Lifting Condensation Level (LCL) is known to be a good proxy for the height of the PBL. In this study, the LCL is derived from surface point measurements of temperature and mixing ratio, taken from a 31-meter flux tower, while the height of the PBL is measured directly from a ceilometer using the cloud base height. The LCL is used to find PBL-bounded clouds from the ceilometer data set: only clouds within a few hundred meters of the LCL are assumed to be PBL-bounded clouds. The data provided by the tower and the ceilometer covers a time period between February 2006 and July 2012, and all data collected over that time is averaged over one-minute intervals. To study the heterogeneous landscape's effect on PBL heights, the data is divided by wind direction. The data, once divided and grouped by wind direction, shows that there are noticeable variations in PBL heights across the different landscapes. The highest PBL heights, for example, are shown to come from an industrial area to the west-northwest of the BCCSO, while the lowest PBL heights are linked to vegetation landscape that lies to the east-southeast of the research site. Additionally, trends in PBL height between seasons across the heterogeneous landscape are also indicated by the results. More research is needed to determine if these PBL height differences are due to large-scale effects or local variations in landscape.